Causes and Mechanisms

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Chapter 19

Acute and chronic respiratory failure

SABIN OANA*AND JAYANTA MUKHERJI

Department of Anesthesiology, Loyola University Medical Center, Maywood, IL, USA

INTRODUCTION

Respiratory failure (RF) is defined as failure of oxygenation and/or carbon dioxide (CO2) elimination. Hypox- emia exists if arterial oxygen tension (PaO2) is below 60 millimeters of mercury (mmHg). Hypercapnia is present if arterial CO2tension (PaCO2) is above 50 mmHg.

There are three common mechanisms that could lead to RF: right to left shunt (cardiac or intrapulmonary), ventilation/perfusion mismatch, and hypoventilation.

In addition, low partial pressure of inspired oxygen, dif- fusion impairment, and high partial pressure of inspired CO2 have also been described as rare causes of RF (Bartter et al., 2011).

Another way of looking at the pathophysiology is to imagine the respiratory system as consisting of two parts: the lung and the pump that moves the lung (respiratory centers, spinal cord and respiratory peripheral nerves, respiratory muscles, chest wall). Failure of the lung is primarily manifested as hypoxemia (hypoxemic or type I failure; shunt physiology) and failure of the pump results mainly in hypercapnia (hypercarbic or type II failure; hypoventilation is the principal mechanism).

Recent literature mentions a type III or perioperative failure, associated mainly with lung atelectasis, and a type IV, related to hypoperfusion of respiratory muscles in shock (Kress and Hall, 2012). The majority of neurologic diseases that progress to RF are therefore type II failures and are characterized by either a reduced drive to breathe or weak respiratory muscles.

Depending on the speed of onset, RF can be classified as acute, acute on chronic, or chronic. RF usually super- venes insidiously in chronic progressive neuromuscular diseases (NMDs) such as amyotrophic lateral sclerosis (ALS). There can also be an acute superimposed factor (e.g., respiratory infection) that precipitates their

deterioration. On rare occasions, however, RF may her- ald disease onset. More commonly, there are acute conditions that are associated with RF (e.g., Guillain–Barre´

syndrome (GBS)). Other entities such as critical illness neuropathy and myopathy are often first diagnosed in the critical care unit.

The initial manifestations of RF usually appear during sleep and, as the diseases progress, they are also present during the daytime. In normal individuals, during sleep, especially in the rapid eye movement state, there is hypotonia of respiratory muscles and of the airway as well as decreased chemosensitivity to CO2. These otherwise normal changes produce an abnormal degree of hypoventilation and hypoxemia in individuals affected by NMD (Chokroverty, 2001).

Disruption of any segment of the neural pathway of respiration can produce RF. RF may be classified according to the anatomic site of origin into: central nervous system disorders, spinal diseases, peripheral nervous system dysfunction, and muscle diseases. Previous work reviewed the need for primary mechanical ventilation (MV) in patients affected by peripheral nervous system diseases (Cabrera Serrano and Rabinstein, 2010). The most frequent conditions were myasthenia gravis (MG), GBS, myopathies, and ALS, together accounting for 75% of cases. However, 55% of patients did not have a diagnosis on admission and 10% never had a final diagnosis.

GENERAL APPROACH TO DIAGNOSIS Respiratory symptoms in NMDs include dyspnea or shortness of breath with its variants orthopnea (dyspnea when supine) and platypnea (dyspnea when upright).

Very commonly, and generally early in the course of diseases, there are various sleep disturbances (restless

*Correspondence to: Sabin Oana, M.D., Assistant Professor of Anesthesiology, Loyola University Medical Center, Department of Anesthesiology, Bldg 103, Rm 3102, 2160 S 1st Ave, Maywood, IL 60153-3328, USA. Tel:þ1-708-216-8866, Fax:þ1-708-216-1249, E-mail: [email protected]

Neurologic Aspects of Systemic Disease Part I Jose Biller and Jose M. Ferro, Editors

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sleep, frequent awakenings, nightmares, and migraines) and related symptoms of daytime fatigue. Difficulty in speech and swallowing are markers of bulbar symptoms (i.e., of the cranial nerves that originate in the medulla) that predispose the patient to ineffective cough and aspiration pneumonia. Physical examination can reveal tachypnea, shallow breathing, accessory inspiratory and expiratory muscle contraction (parasternal, sternocleidomastoid, scalene, intercostal, and abdominal) and ribcage–abdominal asynchrony. Of note, the reliability and reproducibility of the clinical examination with neurologic focus of a RF patient are greatly reduced, especially in the intensive care setting.

In cases with mild hypoxia, one can encounter headaches or inattentiveness, lethargy or delirium, somnolence or anxiety, tremors or seizures that, untreated, progress toward loss of consciousness and unresponsiveness. Hypoxic-ischemic encephalopathy is a complication of prolonged hypoxia during cardiac and respiratory arrest. A special type is that encountered during birth, which can cause long-term damage including intellectual developmental disorder and cerebral palsy.

A low percentage of adults can regain consciousness after cardiopulmonary arrest but still suffer long-term neurologic sequelae, including motor, memory, or personality changes (Dreibelbis and Jozefowicz, 2010). A special type of RF occurs when the inspired oxygen is low, as, for example, during climbing at high altitudes. Acute moun- tain sickness can progress to high altitude cerebral edema and is manifested as dizziness, somnolence, confusion, and decreased consciousness.

Arterial blood gas analysis is one the first steps in the characterization and differential diagnosis of RF. It entails measuring pH, PaO2, PaCO2, and bicarbonate.

These should provide enough data to diagnose an acute versus chronic, respiratory versus metabolic acid–base disorder. Once the respiratory component is assessed, there are multiple indices of hypoxemia that can be calculated to aid in the more detailed diagnosis of RF (i.e., differentiating pulmonary versus extrapulmonary causes). Each has its own merits and limitations.

Tension-based indices are based on alveolar air equation and content-based indices are based on oxygen content equation (Siggaard-Andersen and Gothgen, 1995;

Wandrup, 1995) (seeTable 19.1).

Electrophysiologic studies consist of nerve conduction studies (motor and sensory) as well as needle electromyography (EMG). More specialized testing includes: neuromuscular junction testing with repetitive nerve stimulation, single-fiber EMG and train of four stimulation, respiratory EMG with percutaneous or needle stimulation of phrenic nerve and diaphragm, and direct muscle stimulation (Dhand, 2006). In axonal disease there is normal conduction but reduced amplitude, whereas in demyelinating disease there is slow conduction with normal potential amplitude. For needle EMG the neurogenic pattern shows spontaneous activity, large potentials with mild contraction and less than complete recruitment and interference with full contraction. The myopathic pattern might demonstrate some spontaneous activity with small potentials during mild contraction but small amplitude and full interference patterns with full stimulation.

Pulmonary function testing in NMDs encompasses spirometry, lung volume and capacity, and measure- ment of respiratory muscle strength (Aboussouan, 2005). The most common clinical picture is that of a restrictive respiratory disorder, albeit with some

Table 19.1

Oxygen parameters*

Equation Formula Normal values

Alveolar pressure of oxygen PaO₂¼FiO₂(PatmPH₂O)PaCO₂/RQ or

PaO₂700FiO₂1.2PaCO₂ Arterial oxygen content equation CaO2¼1.34HgbSaO2þ0.03PaO2

Shunt equation Qs/Qt¼(CcO₂CaO₂)(CcO₂CvO₂) or

Qs/Qt¼(1SvO2)(1SaO2)

Less than 5% (0.05)

Dead space fraction Vd/Vt¼(PaCO2PeCO2)PaCO2 Below 0.33(33%)

*Where: PaO2, alveolar partial pressure of oxygen in mmHg; FiO2, inspired fraction of dry oxygen; Patm, atmospheric pressure (760 mmHg at sea level); PH2O, partial pressure of water vapor (47 mmHg); PaCO2, alveolar partial pressure of CO2in mmHg; RQ, respiratory quotient or CO2/O2exchange ratio (approx 0.8); Hgb, hemoglobin concentration; CaO2, CcO2, CvO2are oxygen content of arterial, capillary, and mixed venous blood; SvO2and SaO2are the saturation of hemoglobin in mixed venous blood and, respectively, arterial blood; PeCO2, partial pressure of expired CO2.

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particularities. Weakness in expiratory muscles generates a decreased expiratory reserve volume but with preserved functional residual capacity. In consequence there is elevated residual volume. Associated weakness in inspiratory muscles decreases the inspiratory reserve volume. The overall result is a marked decrease in vital capacity (VC) with a preserved total lung capacity.

Diaphragmatic dysfunction has a characteristic 20–50% decrease in VC when supine. Maximal voluntary ventilation could be reduced. Strength of the respiratory muscles is assessed by measuring the maximum inspiratory and expiratory pressures (MIP and MEP). More recently, sniff inspiratory pressures (nasal (SNIP), esophageal, and gastric) have also been employed to provide data regarding the respiratory muscles. Transdiaphragmatic pressure is the difference between the esophageal and gastric pressures and is sensitive for diaphragmatic dysfunction (Steier et al., 2007).

Genetic testing is usually recommended in inherited diseases with specific presentations like familial ALS, spinal and bulbar muscular atrophy, Charcot–Marie–Tooth (CMT) 1A, myotonic dystrophy, Duchenne muscular dystrophy (DMD) (Burgunder et al., 2011), and congenital central hypoventilation syndrome (CCHS).

SPECIFIC DISEASES Stroke

RF in stroke occurs in about 2–4% of patients and is an ominous sign, traditionally being associated with high mortality. The treatment of such patients should be performed in acute stroke units, resulting in a decrease in mortality (Gattellari et al., 2009). Younger patients with posterior cir- culation involvement have better prognosis (Rabinstein and Wijdicks, 2004). Common indications for MV in stroke patients are neurologic reasons (coma, loss of brainstem reflexes, brain edema with impending herniation), general deterioration in clinical condition, cardiopulmonary causes (pneumonia, aspiration, pulmonary embolism), or elective

for neuroradiologic or neurosurgical procedures (Steiner et al., 1997). There are several patterns of impaired breathing described after stroke that could help in the localization of stroke (Brazis et al., 2011) (seeTable 19.2). Upper airway obstruction is common in stroke patients, obstructive sleep apnea (OSA) being considered both a risk and a consequence for stroke. In OSA patients with stroke, continuous positive airway pressure (CPAP) should be offered, as it reduces mortality (Martı´nez-Garcı´a et al., 2009). Dyspha- gia is very frequent and can be diagnosed early and safe with the help of flexible fiberoptic endoscopy (Warnecke et al., 2009). More recent studies found that aggressive treatments with thrombolysis, decompressive hemicra- niectomy, MV, and feeding tubes are achieving functional independence for an unexpectedly high number of patients (Seder and Mayer, 2009).

Amyotrophic lateral sclerosis

ALS is being increasingly recognized as a disease of diverse genotype, phenotype, and rate of progression (El Escorial Criteria Revisited) (Brooks et al., 2000). RF is rare at the onset, but is the most frequent cause of death. It signifies degeneration of the center and/or of the neurons of the phrenic nerve (Kiernan et al., 2011).

Respiratory symptoms in ALS are dyspnea on exertion, orthopnea, disturbed sleep, daytime somnolence, and vivid nightmares (Wijesekera and Leigh, 2009). Respira- tory signs in ALS include tachypnea, use of accessory muscles, and paradoxical movement of the abdomen.

As far as respiratory investigations are concerned, forced vital capacity (FVC) is increasingly recognized as being an insensitive marker for early RF. In order to predict the need for noninvasive ventilation (NIV), recent focus has been on measurements such as nocturnal oximetry, nocturnal desaturations less than 90% for more than 1 full minute, MIP, supine FVC, transdiaphragmatic pressure, SNIP. Common indications for NIV are: orthopnea, SNIP

<40 cm H2O, nocturnal hypoxemia, MIP<60 cm H2O

(Miller et al., 2009). Respiratory treatment is supportive

Table 19.2

Patterns of breathing after stroke

Type of respiration Localization of stroke Description

Posthyperventilation apnea Bilateral hemispheric After five deep breaths, 20–30 seconds apnea versus 10 seconds in normal

Cheyne–Stokes respiration Cortical or bilateral thalamic Periods of hyperpnea alternating with apnea Central neurogenic hyperventilation Between diencephalon and pons Prolonged periods of hyperpnea

Apneustic breathing Lower pons High inspiratory pause

Cluster breathing Low pons or high medulla Irregular cluster of breaths

Ataxic respirations Dorsomedial medulla Completely irregular breathing

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and is aimed at improving the quality of life of the patient.

Early intervention with NIV might lengthen survival, increase quality of life (Bourke et al., 2006), and slow the decline in FVC, especially in patients without bulbar dysfunction. Invasive ventilation with tracheostomy is more expensive and should be considered in patients with ALS who want long-term support. Patients can be maintained on invasive ventilation until they are

‘locked-in.’ The goal of ventilation is to provide the minimum support that provides comfort. Various methods for clearing secretions should be employed, especially for peak cough expiratory flow (PCEF) <

270 L/min, and include suction, cough-assist devices, physiotherapy, mucolytics, and expectorants (Phukan and Hardiman, 2009). Understanding that the median survival rates are 3–5 years from the onset of symptoms, advance directives regarding respiratory management and end of life care should be discussed as early as possible with patients with ALS and their caregivers. Percutaneous endoscopic gastrostomy considerations should also be made early since there is an increased risk of complications associated with placement in patients with advanced respiratory dysfunction.

Guillain–Barre´ syndrome

GBS is a common cause of neuromuscular paralysis associated in approximately 25% of cases with RF (Hughes and Cornblath, 2005). RF needs to be actively sought after in order to prevent intubations in emergent, less than ideal situations. Frequent respiratory monitoring (as often as 2–4 hours in the acute phase) and prophy- lactic admission to intensive care units has been recommended. Classically, intubation and MV has been performed based on deteriorating respiratory function as assessed by bedside respiratory tests. The 20/30/40 rule has been used (VC<20 mL/kg, MIP<30 cmH2O, MEP<40 cmH2O) (Lawn et al., 2001). Recently, a scor- ing system that takes into consideration rapidity, severity, and extent of weakness has been developed and has a good predictive value for RF (Walgaard et al., 2010).

Despite modern treatment with intravenous immuno- globulins and plasma exchange, the mean duration of intubation in GBS is still several weeks. Adding VC (mL/kg) and MIP and MEP (cmH2O) forms a pulmonary function score. If the score is worse at 12 days after intubation as compared to the day of intubation, this signals a prolonged course of the disease (Lawn and Wijdicks, 2000). For these severe cases, tracheostomy can help in the management of RF. In GBS, RF heralds a severe long-term prognosis and poor functional recovery.

Cervical spinal cord injury

Cervical spinal cord injury (SCI) is associated with various degrees of pulmonary dysfunction. The critical levels are C3–C5 since they supply innervation to the diaphragm. Injury above this level necessitates permanent MV. Below C5, respiration is severely impaired but diaphragm function is maintained. Also, the incidence of organ dysfunction is more strongly associated with American Spinal Injury Association motor scores that incorporate the severity of injury besides the level of injury (Stein et al., 2010). In the acute setting, lower than C5 cervical injuries are associated with about 70%

decreases in VC, MIP, and MEP. The physiologic expla- nation of the above findings is that in the acute setting there is flaccid paralysis of intercostal and abdominal muscles with maintained diaphragm function. In inspiration the abdomen is relaxed, so diaphragmatic excur- sion starts from a lower, more inefficient position and during its contraction the chest wall retracts instead of expanding. Also, in expiration there is decreased elastic recoil from the abdominal wall. Several months after the injury, with the onset of spasticity of respiratory muscles, the expiratory function improves minimally but VC and MIP are back to about 60% of normal (McMichan et al., 1980; Ball, 2001). More recent studies define respiratory dysfunction and failure by the PaO2/FiO2ratio and find an aggregate incidence of around 85% for respiratory dysfunction and between 40% and 70% for RF, with an emphasis on the fact that the respiratory derange- ments are progressive after the primary injury. Besides the restrictive component there is also an obstructive component, probably secondary to unopposed parasym- pathetic tone that reduces baseline airway caliber (Schilero et al., 2009). When needed, intubation should be performed by specialized personnel, preferably anes- thesiologists, with in-line stabilization or videolaryngo- scopy (rigid or fiberoptic) and aspiration prophylaxis.

Duration of ventilation is at least 3 weeks and overall outcome is highly dependent on the occurrence of respiratory complications (atelectasis, pneumonia, aspiration, and pulmonary embolism). An energetic program designed to prevent or reverse secretion retention has been proven to significantly improve outcomes. Among emerging methods to improve pulmonary function, expiratory muscle training is more effective than inspiratory muscle training and there seems to be a role for b-adrenergic agonists. The incidence of OSA seems to be much greater in tetraplegic patients. NIV, including positive pressure ventilation and mechanically assisted coughing, can be successfully employed for the longest times if there is sufficient bulbar function (Bach, 2012).

In the other patients, early tracheostomy might be beneficial. Phrenic nerve or direct diaphragmatic pacing

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are modern alternatives to long-term ventilation (Romero et al., 2012).

Spinal muscular atrophy

Spinal muscular atrophy (SMA) is the leading genetic cause of infant deaths. The diaphragm is relatively spared but there is paradoxical breathing and various degrees of RF due to inspiratory and especially expiratory muscles impairment (Lunn and Wang, 2008).

A recent consensus statement established the general frame for respiratory diagnosis and treatment in SMA. Prompt referral for comprehensive pulmonary evaluation is indicated after diagnosis and the options for respiratory management should be discussed with the families. Airway clearance and noninvasive ventilatory support are the mainstays of treatment for chronic management. Airway clearance maneuvers including cough assistance, chest physiotherapy, and postural drainage are indicated for oxygen saturation less than 94% (Schroth, 2009). Initially the hypoventilation will occur during sleep so polysomnography has an impor- tant role in diagnosis. NIV is usually initiated noctur- nally, then, as the disease progresses, is indicated for continuous use. Acute respiratory diseases necessitate a step up in respiratory support from the baseline and antibiotics are frequently needed. Perioperative care includes optimization of preoperative pulmonary status, extubation to NIV as a bridge to weaning, and optimal pain control to avoid splinting (Wang et al., 2007). The ethics of invasive long-term ventilation and tracheostomy in SMA type I are highly debated (Benson et al., 2012).

Charcot–Marie–Tooth disease

CMT disease generally spares the respiratory system but in the most severe cases can be associated with restrictive lung disease, sleep apnea, and vocal cord paralysis. Pul- monary symptoms are most likely caused by a combination of phrenic, laryngeal, and pharyngeal neuropathy.

The combination of restriction with obstruction in the pattern of the lung disease makes it more amenable to treatment with bilevel positive airway pressure (BiPAP) than with CPAP. Vocal cord dysfunction increases the risk of aspiration and various treatments should be recommended, bearing in mind the risk of progression to bilateral vocal cord dysfunction (Aboussouan et al., 2007).

Duchenne muscular dystrophy

DMD is a progressive X-linked genetic disease. There are several clinical stages from ambulatory to nonambu- latory that correspond to worsening pulmonary function

due to respiratory muscle weakness and appearance of scoliosis. Current recommendations suggest surveil- lance at regular intervals from annual to every 6 months as the disease progresses. Every clinic visit should be accompanied by several respiratory measurements including sitting FVC, pulse oximetry, MEP, MIP, PCEF. Capnography should be obtained in more advanced cases (Birnkrant et al., 2010). Immunization with pneumoccocal and influenza vaccines and aggressive use of antibiotics are beneficial. There are five stages of progressive respiratory interventions in DMD: volume recruitment with deep breathing techniques, airway clearance with manual or mechanical techniques, support of breathing during sleep, and finally daytime support of ventilation, with eventual tracheostomy. NIV is preferred at all stages. Bilevel pressure ventilators with backup rate can be used during the nighttime. Volume-cycled ventilators can be used anytime but are preferred during the daytime. Glucocor- ticoids are proven to stabilize pulmonary function and during infections treatment with antibiotics is warranted by culture or by decreased saturations of oxygen. Of note, treatment with oxygen is generally discouraged as being only symptomatic, masking hypoxemia instead of prompting efforts at clearing the secretions or treat- ing hypoventilation (Bushby et al., 2010).

Multiple sclerosis

Acute RF can accompany acute relapses or episodes of aspiration and/or pneumonia. There is also a chronic RF that is associated with advanced progressive multiple sclerosis (MS). Pulmonary dysfunction has been attrib- uted to demyelination as well as to deconditioning associated with the disease (Karpatkin, 2008). Pulmonary dysfunction in MS is characterized by respiratory muscle weakness with low MEP and MIP, while spirometry can be preserved until later stages of the disease. Clinical evaluation can elicit signs and symptoms of respiratory weakness like weak cough and inability to clear secretions as well as inability to count to 20 in a single breath after an inspiration (Smeltzer et al., 1992). Despite increased detection and various immunomodulatory treatments, recent population studies are still attributing about 50% of deaths in MS to respiratory diseases (Hirst et al., 2008). Ventilatory support consists of MV and/or tracheostomy. Indications are RF and/or airway protection for underlying causes such as aspiration pneumonia, mucous plugging, or pure mechanical failure (Pittock et al., 2004). NIV is beneficial for these patients and intubation and tracheostomy should be reserved for NIV failure and contraindications. Ventila- tory support can increase survival in chronic terminal cases for years, but complex ethical decisions have to

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be made regarding the care of such patients. In contrast, the recovery after acute relapses treated with pulses of steroids or plasmapheresis and that of infections aggres- sively treated with antibiotics can be significant.

Parkinson’s disease

Both restrictive and obstructive patterns have been described in Parkinson’s disease (PD), as well as abnor- malities in respiratory control. The restriction is due to chest wall rigidity and the obstruction to upper airway clo- sure and decreased range of motion in the neck (Jankovic, 2008). The severity of pulmonary involvement parallels the general progression of the disease. As the disease advances, the combination of weak cough and impaired swallowing predisposes patients to aspiration and pneumonia. Introduction of modern medication have greatly delayed the appearance of RF in PD. We can still see acute respiratory distress whenL-dopa is discontinued or when it is overdosed (respiratory dyskinesia). Ergot dopamine agonists are associated with pleuropulmonary fibrosis (Shill, 2005). Percutaneous gastrostomy and tracheostomy offer some protection against aspiration and facil- itate pulmonary toilet but should be reserved for nondemented patients. In multiple system atrophy, there is a definite risk of sudden death during sleep due to nocturnal stridor and central sleep apnea. The recommendation is to relieve laryngeal obstruction with tracheostomy.

CPAP may play a role in the beginning (Iranzo, 2007).

Central sleep apnea

Central sleep apnea (CSA) and OSA comprise a variety of diseases associated with impaired breathing that occurs during sleep. They are differentiated depending on the presence (OSA) or absence (CSA) of the respiratory effort. For further classification of CSA, seeTable 19.3.

In obesity hypoventilation syndrome (OHS), there is awake hypercarbia associated with obesity. The syndrome is caused by the chronic mechanical load associated with obesity (that has both restrictive and obstructive components) and/or a central resistance to leptin. There is a significant overlap with OSA and both separate and combined conditions have been described.

Definitive treatment is loss of weight, and CPAP can also be tried for the mixed conditions (Brown, 2010).

CCHS is, fortunately, a rare disease where the patient has no awareness of hypoxia or hypercarbia and no adap- tation of the ventilatory drive to various respiratory loads as during exercise or disease. Diagnosis requires genetic testing for the paired-like homeobox 2B (PHOX2B) gene. All patients require lifetime ventilatory support guided by pulse oximetry and end tidal CO2

monitoring. The goals of therapy are to maximize oxygenation and optimize cognitive development. Young children are best treated with invasive pressure ventilation while older children and adults with mild forms can be managed noninvasively with bilevel settings (Weese-Mayer et al., 2010).

Cheyne–Stokes breathing consists of cycles of hyperpnea and apnea and can be associated with heart failure.

Idiopathic central sleep apnea is somewhat similar to Cheyne–Stokes respiration but lacks the typical periodic pattern. Periodic breathing resembles Cheyne–Stokes breathing but hypopneas replace the apnea periods.

OSA, through hypoxia and sleep fragmentation, is associated with various neurologic disturbances such as fatigue, cognitive changes, decreased attention and performance, memory loss, and headaches (Broderick and Guilleminault, 2008). Glossopharyngeal neuropathy resulting in increased pharyngeal collapsibility is an intrinsic part of the vicious cycle that generates and per- petuates OSA (Saboisky et al., 2012).

Critical inflammatory neuropathy and myopathy

These diseases are the consequence of sepsis and multiple organ failure and manifest as new onset weakness in the intensive care units. These patients enter a vicious cycle where acute RF and associated sedation and/or paralysis promotes more weakness which in turn delays weaning and resolution of the critical process. On a cellular level, there is metabolic, inflammatory, and bioen- ergetic failure with consequent muscle denervation (Hermans et al., 2008). The weakness is symmetric and usually spares the facial musculature. Confirmation can be made with electrophysiologic and pathologic studies (decreased myosin/actin ratio) but is expensive and should be reserved for refractory cases. Hyperglyce- mia, corticosteroids, and neuromuscular blockers have Table 19.3

Classification of central sleep apnea (adapted fromEckert et al., 2007)

Hypercapnic while awake Won’t breathe

Congenital central hypoventilation syndrome (Ondine’s curse)

Opioid associated

Obesity hypoventilation syndrome (Pickwickian syndrome)

Can’t breathe

Neuromuscular disorders

Normocapnic or hypocapnic while awake Cheyne–Stokes breathing

Idiopathic central sleep apnea Altitude-associated periodic breathing

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been associated with pathogenesis so they are best avoided. Early mobilization of the critical patient with physical and occupational therapy might be beneficial (Griffiths and Hall, 2010).

Myasthenic crisis

Myasthenic crisis (MC), or MG class V according to the Myasthenia Gravis Foundation of America classification (Jaretzki et al., 2000), is that severe subset of myasthenia gravis that requires intubation with or without MV. It is usually precipitated by infections, emotional or physical stress, bulbar weakness and aspiration, pregnancy or cer- tain medications such as aminoglycosides and quinolones (Chaudhuri and Behan, 2009). Upper airway weakness and obstruction and respiratory muscle weakness are the immediate factors causing RF. Patients that test positive to antibodies against muscle-specific tyrosine kinase are more likely than those positive to antibodies to acetyl- cholinesterase receptor to go into recurring RF episodes (Evoli et al., 2003). The clinical picture and a proactive attitude should govern the management of these patients.

A significant proportion of patients can benefit from BiPAP ventilation despite bulbar weakness and difficult secretion management. In hypercapnic patients, noninvasive management is likely to fail, so they should be intu- bated (Rabinstein and Wijdicks, 2002). Despite modern treatments with plasma exchange, intravenous immuno- globulins and immunomodulators, duration of intubation can sometimes exceed 3 weeks (Thomas et al., 1997).

GENERAL PRINCIPLES OF MANAGEMENT IN RESPIRATORY

FAILURE

The different aspects of respiratory care including mechanical support with various modes of conventional and noninvasive ventilation, as well as complications and weaning, are discussed. Timely interventions to provide respiratory support are life-saving, although there is a lack of good evidence to choose one mode of support over another.

Mechanical ventilation

MV is the most commonly used supportive technique used in intensive care units. The airway is accessed through an endotracheal tube or tracheostomy and support provided to assist the failing ventilatory pump.

Ventilatory modalities are designed to work in syn- chrony with the patient. Modes of ventilation describe the primary means of inspiratory assistance, which may be delivery of a predetermined volume (volume control) or airway pressure (pressure control). The ventilator cycles into the expiratory phase, with either a

preset time, preset tidal volume (TV) or a preset percentage of peak flow.

VOLUME CONTROL VENTILATION

Volume control ventilation (VCV) is a volume-targeted mode where airway pressure remains the dependent variable. The parameters that are set are respiratory rate (RR), TV, inspiratory to expiratory ratio and inspiratory flow profile (Koh, 2007). The inspiratory flow profile is usually a constant square waveform but a decelerating flow–time profile may be used to lower peak inspiratory pressure and provide an even distribution of inspired air (Kallet et al., 2000).

In assist control mode the ventilator is triggered by patient effort and an assisted breath consisting of predetermined TV from the ventilator is provided. It provides the least work of breathing and is used in severe RF.

During synchronized intermittent mandatory ventilation (SIMV) in volume-controlled mode, a specified number of volume-preset breaths are delivered each minute.

Spontaneous breaths by the patient between machine breaths are augmented with pressure support (PS) breaths.

Machine breaths are delivered only after the ventilator has recognized the patient’s effort, such that ventilator and respiratory muscle activities are synchronized.

PRESSURE CONTROL VENTILATION

Pressure control ventilation (PCV) is the prototypical pressure mode and is patient- or time-triggered, pressure-limited, and time-cycled. The target inspiratory pressure, RR, and inspiratory: expiratory ratio are preset. TV is the dependent variable and will vary with the compliance of the lungs and chest wall (Tung et al., 2005). It is preferred for ventilator management in acute lung injury and adult respiratory distress syndrome (ARDS) as it is easy to control airway pressures and prevent volutrauma (alveolar overdistention). Multivariate analysis comparing PCV versus VCV modes of ventilation did not show any differ- ences in outcome (Campbell and Davis, 2002; Rose, 2010).

Mechanical ventilation in patients with chronic airflow obstruction

Patients with severe chronic obstructive pulmonary disease (COPD) and chronic asthma have chronic airflow obstruction and increased dead space. There is increased work of breathing from narrowing of airway (asthma) or reduced elastic recoil (emphysema). The respiratory muscle dysfunction is more pronounced when COPD is associated with malnutrition, acidosis electrolyte disorders, and decreased muscle perfusion from low flow states. Inappropriate ventilatory settings may trigger a panic attack during MV. Management is usually based

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on relieving anxiety, lowering airway resistance, and synchronization of mechanical support with patient’s breathing (Loring et al., 2009). MV improves pulmonary gas exchange and allows the fatigued respiratory muscles to rest. Ventilatory support may result in complications related to dynamic pulmonary hyperinflation and intrinsic positive end-expiratory pressure (auto-PEEP). Hyperinfla- tion results from insufficient exhalation time preventing the lungs from returning to its resting end-expiratory equi- librium position. Increased expiratory flow resistance, high ventilatory demands, and a shortened expiratory time predispose to hyperinflation. The presence of auto-PEEP should be suspected when airflow at end-exhalation is not zero. Strategies used to reduce auto-PEEP include reduction of minute ventilation, use of small TVs, and prolongation of the exhalation time. The use of low levels of external PEEP improves patient–ventilator interaction and helps reduce the work of breathing in patients in whom auto-PEEP is caused by expiratory flow limitation. The use of neurally adjusted ventilatory assist is a new approach to improving patient–ventilator interaction in patients with auto-PEEP (Laghi and Goyal, 2012).

Helium–oxygen mixture reduces density-dependent resistance and is reported to improve airflow obstruction in symptomatic patients with asthma and decrease air trapping in ventilated COPD patients (Chevrolet, 2001). Long-term home oxygen therapy has been shown to reduce mortality in patients with persistent hypoxemia, as has lung volume reduction therapy in highly selected patients with emphysema (Fishman et al., 2003). A recent meta-analysis showed that pulmonary rehabilitation after a COPD exacerbation reduced the risk of mortality (Halpin and Tashkin, 2009).

Ventilatory strategy in acute respiratory distress syndrome

MV is supportive allowing time for healing of underlying cause of ARDS. Acute lung inflammation produces

decreased lung compliance and altered pulmonary mechanics. The ARDS network study showed that reducing the TV from 12 mL/kg to 6 mL/kg reduced mortality by over 20% (Acute Respiratory Distress Syndrome Network, 2000). Higher TVs induce injury to lung and are mediated by high end-inspiratory stretch (measured by plateau pressures>35 cmH2O) leading to volutrauma.

Lower TVs of 6–8 mL/kg limit plateau pressures, prevent release of inflammatory mediators, and reduce ventilator- induced lung injury. Lung protective strategies in ARDS include using lower TV and higher PEEP to avoid alveolar overdistention and derecruitment (Petrucci and Iacovelli, 2007). PEEP prevents alveolar derecruitment by restoring the functional residual volume to the physiologic range.

Permissive hypercapnia is a strategy whereby lungs are protected during MV by adoption of deliberate alveolar hypoventilation. Hypercapnic acidosis is a consequence of this strategy and not a goal. Lung protective ventilation impairs CO2clearance and may lead to right heart dysfunction or increased intracranial pressure.

Extracorporeal CO2 removal has the potential to optimize lung protective ventilation by uncoupling oxygenation and CO2clearance (Cove et al., 2012).

Complications in mechanically ventilated patients

Respiratory complicationsmay result from MV itself.

A bedside evaluation of patient, airway, and ventilator settings to monitor for ventilator–patient dysynchrony, oxygenation, gas exchange and airway pressures (peak and plateau), and compliance should be undertaken (Tables 19.4 and 19.5).

Cardiovascular complications may be secondary to sepsis, or from the effects of positive pressure ventilation. Cardiac output is diminished with decreased venous return from high airway pressures, decreased intravascular volume, fluid sequestration from sepsis, and trauma.

Table 19.4

Differential diagnosis of hypoxemia and/or hypoventilation during mechanical ventilation

Patient-related Airway-related Ventilator-related

Tension pneumothorax Atelectasis

Pneumonia Bronchospasm Pleural effusion Pulmonary edema Laryngospasm Airway trauma

Right mainstem intubation

Kinked endotracheal tube, occlusion due to biting

Endotracheal tube cuff leak, herniation Inadvertent extubation with neck extension,

migration of tube

Unrecognized esophageal intubation

Ventilator circuit leak Low FiO₂

Inappropriate ventilatory settings Patient–ventilator dysynchrony

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Gastrointestinal complications such as upper GI bleed, gastric stasis, and ileus may be caused by shock, hypoxemia, drugs that inhibit gastrointestinal motility such as narcotics, antipsychotics, or hypokalemia.

Sepsis syndrome. The inflammatory response is triggered by infection and is a series of humoral and cellular cascades. The mediators of sepsis, neutrophils, cyto- kines, coagulation factor, prostaglandins, and nitric oxide, target the endothelium and increase capillary per- meability. This is expressed in the lungs as noncardiogenic pulmonary edema.

Noninvasive ventilation

NIV is largely utilized in patients with acute hypercapnic RF, acute cardiogenic pulmonary edema, and acute exacerbation of COPD. The level of success is variable in hypoxemic forms of RF and is useful in select populations with no contraindications such as multiple organ failure, loss of consciousness, or hemodynamic instability.

NIV can be used either as a primary ventilation mode for acute RF or for weaning patients from MV (Peter et al., 2002). It preserves airway defense mechanisms, lowers sedation requirements, and allows patients to speak, eat, and clear secretions. NIV unloads inspiratory muscles and reduces the work of breathing. The mask interface has to fit well and the patient should be com- fortable. Nasal masks may be used but are ineffective in patients likely to mouth breathe. The applied pressure starts at 8–12 cmH2O and is adjusted according to the patient’s tolerance and desired TV.

NIV has been associated with reduced infections (pneumonia, sinusitis) and hospital stay, when compared with MV delivered via an endotracheal tube

(Brochard, 2003). Immunocompromised patients with RF may benefit from a reduced need for intubation and reduction in mortality (Hilbert et al., 2001).

In patients with COPD exacerbation, NIV counters the effects of auto-PEEP and diminishes dyspnea and the work of breathing (Keenan et al., 2003). In cardiogenic pulmonary edema, positive pressure breathing restores functional residual capacity (FRC), improves ventilation/perfusion ratios, and reduces afterload (Vital et al., 2008).

NIV is contraindicated in patients who are unable to protect airway due to altered mental status, agitation and excessive secretions, or develop hemodynamic instability, respiratory arrest, and myocardial ischemia.

In general, younger, cooperative patients with intact dentition, having less severe illness, hypercarbia (PaCO2>45 mmHg, <92 mmHg) or acidemia (pH<7.35, >7.10), have a higher rate of success with NIV (Hill et al., 2007).

Some centers have reported the use of a pumpless extracorporeal lung-assist device for extracorporeal CO2 removal allowing them to avoid invasive MV in patients with acute hypercapnic RF not responding to NIV (Kluge et al., 2012).

Weaning from mechanical ventilation Weaning usually implies two closely related aspects of care, discontinuation of MV and removal of any artificial airway. The clinician determines when a patient is ready to resume spontaneous ventilation. Once a patient is able to sustain spontaneous breathing, a second decision is made whether the artificial airway can be removed by assessing the patient’s mental status, airway protective mechanisms, and ability to cough and clear secretions (Alia and Esteban, 2000) (see Tables 19.6 and 19.7).

Table 19.5

Diagnostic approach of hypoxemia and/or hypoventilation during mechanical ventilation

Bag ventilate with FiO2of 1.0

Auscultate and check capnogram to confirm endotracheal tube placement

Chest X-ray, CT scan to diagnose lung parenchymal conditions Arterial blood gas to determine hypoxemia, hypoventilation,

acidosis

Fiberoptic bronchoscope to ascertain endotracheal tube position, bronchoscopy for removal of mucus plugs, bronchoalveolar lavage for pneumonia

Echocardiogram to rule out intracardiac shunts Ventilator: check for disconnection, leaks or blockages Ventilator: check difference between exhaled and set TV,

airway pressure profile, peak and plateau pressures, check response to lung recruitment and PEEP

FiO2, fraction of inspired oxygen; CT, computed tomography; PEEP, positive end-expiratory pressure; TV, tidal volume.

Table 19.6

Checklist for weaning and extubation

Patient awake and responds appropriately, aspiration risks assessed

No agitation, cooperative, adequate pain control Hemodynamically stable, normal acid–base status,

no electrolyte disturbance

Pulmonary gas exchange with acceptable parameters for oxygenation and ventilation

Cardiogenic pulmonary edema (fluid overload, left ventricular dysfunction) resolving

Noncardiogenic pulmonary edema (pneumonia, acute lung injury) resolving

Respiratory mechanics optimized, abdominal distention Underlying cause for ventilator dependency resolving

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Sleep-deprived patients in the ICU manifest signs of agitation and lethargy. Establishing sleep patterns and daily orientation to the day, time, and surroundings are helpful. Daily interruption of sedative infusions is advocated to avoid prolonged sedation and increased duration of MV (Kress et al., 2000).

The patient is assessed for respiratory drive, muscle strength, and the imposed workload before attempting weaning. Muscle fatigue results from muscle atrophy or electrolyte imbalance, NMD, increased work of breathing from acute bronchospasm, pulmonary edema, or depleted respiratory drive from excessive sedatives and narcotics.

Weaning is considered successful if patients do not require resumption of ventilatory support within 48–72 hours after extubation (MacIntyre, 2004). About 20%

of patients may fail their first attempt at weaning and may require prolonged support (Rothaar and Epstein, 2003).

A somewhat particular cause of failure is due to postextubation pulmonary edema, either from loss of positive pressure ventilation with increased afterload in left ventricular dysfunction or from negative pressure pulmonary edema.

Unplanned extubation requiring reintubation is detrimental as it increases mortality and the chances of prolonged MV, and ICU stay (Seymour et al., 2004). Evidence-based practice supports early attempts at weaning in a protocol-driven fashion (Robertson et al., 2008). Prolonged MV has been associated with pneumonia, gastrointestinal bleeding, and deep venous thrombosis.

It is difficult to determine the optimal time for extu- bating patients with neurologic deficit from brain injury.

Impaired airway reflexes and inability to clear secretions creates conditions for aspiration risks. However, delay- ing extubation of patients breathing spontaneously can carry an increased risk for pneumonia.

The most effective means of weaning follows a sys- tematic approach that includes a daily assessment of weaning readiness, along with interruption of sedation infusions and spontaneous breathing trials. Evidence- based practice dictates using protocols and checklists as decision support tools. Most studies of weaning protocols applied by nonphysician healthcare providers suggest faster weaning and shorter duration of ventilation and ICU stay (Haas and Loik, 2012).

Tracheostomy should be considered in head-injured or critically ill patients as soon as the need for prolonged intubation (longer than 14 days) is identified. Mortality is not worse with tracheotomy and may be improved with earlier provision (Durbin, 2010).

WEANING PARAMETERS

No index has proven to be ideal. The work of breathing is more predictive of successful weaning outcome in long-term MV patients. Weaning criteria consist of TV, VC, RR, MIP and rapid shallow breathing index (RSBI). The sensitivity and specificity of the weaning parameters are relatively poor (El-Khatib and Bou- Khalil, 2008).

VC is measured upright rather that supine, because diaphragmatic paralysis may cause a 30% positional reduction. Patients may be too weak to sustain prolonged inspiratory effort or fail to cooperate. It is an unreliable measure of strength and fails to predict weaning outcome (threshold is 10–15 mL/kg for VC) (Yang, 1992).

MIP provides a better negative than positive predictive value and may be performed in uncooperative intu- bated patients. A MIP value exceeding 30 cm of H2O is associated with successful extubation.

RSBI is the ratio of RR/TV and is an accurate predictor of weaning outcome when lower than 105. It is not dependent on patient cooperation and effort (Chao and Scheinhorn, 2007).

WEANING MODES AND PROTOCOL-DRIVEN WEANING

The optimal mode of weaning from MV remains contro- versial. Most patients do not require progressive with- drawal of support during weaning. The commonly used techniques of weaning are T-piece, SIMV, or PS.

Spontaneous breathing trials using PS or T-piece is com- menced if RSBI is less than 107. Daily T-piece trials are equivalent to a PS mode of weaning, and are superior to the SIMV mode (Esteban et al., 1995). PS provides a progressive unloading of inspiratory muscles compared with SIMV. NIV has been used as a method to support ventilation following early extubation (Burns et al., 2010). A successful trial for duration of 30 minutes up to 2 hours assures successful extubation (Epstein, 2009).

Table 19.7

Causes of failed extubation Upper airway obstruction

Obstructive sleep apnea Tracheal stenosis Laryngeal edema

Airway compression from neck hematoma Airway trauma

Inability to clear secretions Altered mental status Poor VC

Muscle fatigue Aspiration

Emesis from tube feeds Gastroesophageal reflux

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A protocol-driven weaning is based on best evidence, and is less influenced by personal decisions, allowing a systemic approach to learning and quality monitoring.

It leads to a significant reduction in the duration of MV and complications (Blackwood et al., 2010). Proto- colized weaning may be computer driven and has reduced MV duration when compared with physician- controlled weaning (Lellouche et al., 2006).

Management of patients on prolonged mechanical ventilation Prolonged mechanical ventilation (PMV), according to the Centers for Medicare and Medicaid Services, includes patients who have required more than 6 hours of MV for greater than 21 consecutive days and who are eligible for transfer to long-term acute care (White et al., 2008). In single center studies, about 3–7% of ICU patients meet PMV criteria (MacIntyre et al., 2005). Unlike the 48–72 hour criteria used in acute RF, patients on PMV are considered weaned if liberated from MV for 7 consecutive days. Comorbidities commonly associated with PMV include include malignancy, COPD, immunosuppression, poor nutritional status, polyneuropathy, myopathy, sepsis, recurrent aspiration, sedation, sleep deprivation, and delirium. Diaphragmatic contractile dysfunction is present in PMV with as few as 18 hours of MV, resulting in diaphragmatic atrophy (Powers et al., 2009).

Managing patients on PMV involves more than ventilator weaning and has a rehabilitative focus, involving nutritional support, respiratory muscle training, physical and occupational therapy including speech therapy, psychological and social services. Long-term facilities caring for PMV patients have some environmental advantages in that they are relatively quiet, with distinct day–night cycles, and, unlike an acute ICU, they are geared towards receiving family members, encouraging mobility, providing more staff–patient interactions and counseling.

MANAGEMENT OF OROPHARYNGEAL AND

TRACHEOBRONCHIAL SECRETIONS NMDs may compromise the effectiveness of the cough by producing weakness of the involved skeletal muscles.

A weakened cough leads to inability to clear secretions and places patients at risk for atelectasis, mucus plugging, and pneumonia. A PCEF less than 2.7 L/sec produces an ineffective cough, with retention of airway secretions predisposing to pneumonia. Patients with low VC (less than 1.5 L) and vocal cord dysfunction

are susceptible to retained secretions (Kang and Bach, 2000).

Anticholinergic medications inhibit the saliva produc- tion but their use is limited by side-effects related to blurred vision and urinary retention. Injection of botulinum into salivary glands produces local functional denervation and has been successfully used to treat sia- lorrhea in stroke, parkinsonism, and ALS (Ondo et al., 2004). Patients experience relief within days and the therapeutic effect usually lasts for approximately 3–4 months, when reinjection may be required. The use of anticholinergics and botulinum toxin can produce thick, tenacious secretions, which are difficult to mobi- lize in the setting of compromised musculature. This can be minimized by maintenance of adequate hydration and by coadministration of medications that thin secretions, such as guaifenesin or N-acetylcysteine (Elman et al., 2005).

Physical modalities usually employed in handling secretions include manual suctioning, respiratory physical therapy, postural drainage, and glossopharyngeal breathing (GPB). Postural drainage allows the mobilization of tracheobronchial secretions by gravity-assist positioning, deep breathing with or without chest percus- sions. When secretions reach the upper airway they are expelled via coughing.

Incentive spirometry is commonly used postopera- tively to reduce pulmonary complications. While deep breaths taken with an incentive spirometer may help reduce atelectasis, there is no evidence that it helps in reducing pulmonary complications after upper abdominal surgery (Guimara˜es et al., 2009).

GPB is useful for airway clearance in patients with low VC, as in NMD. It is accomplished by a series of air gulping actions generated by the lips, pharynx, and palate while the larynx serves as a valve maintaining air in the lungs between gulps. Both GPB and air stacking can increase lung volumes and, thereby, cough flows.

GPB helps supplement mechanical insufflations and may be used to decrease daytime ventilator use (Bach et al., 2007).

Oscillating devices assist in clearance of airway secretions. They generate intra- or extrathoracic oscillations orally or external to the chest wall. Mechanical vibration of the chest using vests that provide high frequency chest wall oscillations is believed to aid in the mobilization of tracheobronchial secretions by producing shearing forces that thin mucus. Forced expiratory maneuvers with Flutter VRP1^® (Scandipharm Inc., Birmingham, AL) creates an oscillatory positive pressure and vibration within the airways which facilitates mucus mobilization and expulsion. There was no clear evidence that oscillation is a more or less effective intervention overall than other forms of physiotherapy (Morrison and Agnew, 2009).

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MANAGEMENT OF RESPIRATORY FAILURE IN PATIENTS WITH

NEUROMUSCULAR DISEASE Improving pulmonary gas exchange requires an assort- ment of maneuvers ranging from providing supplemental oxygen and bronchodilators, to use of pulmonary support to sustain the weak respiratory muscles. The use of CPAP or low span BiPAP (inspiratory–expiratory pressure difference <10 cmH2O) alone would be inappropriate in augmenting the weak inspiratory and expiratory muscles.

Similarly a single intervention such as oxygen therapy used for treatment of oxygen desaturation does not address the underlying problem of impaired clearance, and may be detrimental, as it delays recognition of mucus plugging and early desaturation, and promotes hypopnea and apnea during REM sleep (Smith et al., 1989).

Devices that promote inspiratory and expiratory muscle function

The intermittent abdominal pressure ventilator assists with forced exsufflation and TV augmentation (Dean and Bach, 1996) by intermittent inflation of an air bladder during the expiration phase pushing against the abdominal wall and moving the diaphragm upward. Bladder deflation returns the diaphragm to the normal position and inspiration occurs passively.

Continuous NIV can prolong life in ALS and motor neuron disease.

According to recent authors diaphragm pacing did not accord any benefit on VC or MV-free survival (Mahajan et al., 2012).

The concept of air stacking is to improve PCEF and lung compliance, and minimize atelectasis by delivering successive volumes with volume-cycled ventilator with the expiratory valve blocked. A closed glottis holds air volumes within the lung until the largest volume of air that can be held with a closed glottis is reached. Insuffla- tion of the lungs by air stacking is only possible in a cooperative patient if the glottis is competent.

Mechanical insufflation–exsufflation devices are helpful in patients with an incompetent glottis who are unable to maintain air stacking with limited PCEF. The lungs are gradually inflated then subjected to an abrupt change to negative pressure, producing a rapid exhalation (exsufflation), simulating a cough. Insufflation–

exsufflation reduces episodes of RF, particularly during upper respiratory tract infections, so patients have a better success in weaning from MV. Other mechanisms used to improve PCEF include breath stacking with volume ventilator, GPB followed by a spontaneous cough, and manually assisted cough with an abdominal thrust (Homnick, 2007).

Progression to MV is likely to occur in patients with rapid disease progression, bulbar dysfunction, bilateral facial weakness, or dysautonomia. Bulbar dysfunction, characterized by dysarthria, dysphagia, or impaired gag reflex, was the strongest predictor (Mehta, 2006).

Noninvasive ventilation in neuromuscular disease

Inspiratory failure is most commonly seen first at night because during rapid eye movement (REM) sleep, atony of muscles other than the diaphragm occurs and weakness of the diaphragm is exposed. Symptoms of nocturnal hypoventilation are subtle and include frequent nocturnal awakenings, morning headaches, fatigue, and daytime hypersomnolence. Nocturnal ventilatory support with NIV is initiated when the presence of sleep-disordered breathing is detected.

NIV is employed in patients with NMD if an adequate oral/nasal interface to prevent air leak is possible.

NIV is used when respiratory drive is blunted from sedation, oxygen therapy, and excessive hypercapnia.

It provides respiratory muscle rest and improves pulmonary compliance. Bilevel positive pressure devices (BiPAP) are the commonly used form of NIV, whereas continuous positive pressure (CPAP) ventilation is not usually helpful. A high volume (800–1500 mL) or an IPAP to EPAP span of 15–22 mm water is recommended. The evidence-based review by Miller et al.

(2009) noted that the use of NIV lengthened survival, slowed the decline of FVC, and improved lung compliance and quality of life. However, no survival benefit was seen in patients with poor bulbar function (Bourke et al., 2006).

Oximetry feedbacks have been utilized in patients with respiratory infections and inadequate ventilatory capacity. Monitoring with oximetry allows appropriate use of NIV and mechanically assisted coughing (MAC) in patients. Low oxygen saturation from retained secretions despite optimization would be an indication for tracheostomy (Bach et al., 2004).

Extubation of patients with NMD requires a planned approach. Oxygen administration is limited to keep SpO2

no higher than 95%, a high-span BiPAP is used via mouth/nasal interface along with assisted coughing during the postextubation phase (Bach et al., 2010).

NIV has advantages over tracheostomy as it allows speech, sleep, swallowing, and utilization of GPB. Both NIV and tracheostomy help prolong survival in patients with RF. Tracheostomy is preferred in complete dysfunction of bulbar innervated musculature (Bach and Hon, 2010). Invasive ventilation may be needed when

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long-term survival is the goal, and patient and family need to be appraised of the burdens and benefits.

Bach (2012)describes using NIV to support inspiratory muscles and MAC to support inspiratory and expiratory muscles in patients with SCI. The NIV and MAC have been used to extubate or decannulate “unwean- able” patients with SCI and to eliminate tracheostomy and costly electrophrenic/diaphragm pacing for most ventilator users. GPB can be resorted to in the event of ventilator failure.

Home mechanical ventilation

Economic pressures, advances in medical technology, and the ability of family members to be intimately involved in care have driven the concept of home mechanical ventilation (HMV).

NIV has been an attractive option in allowing patients to transition care to the home environment. The rapidly expanding home use of NIV using a nasal or orofacial mask allows more patients to transition to the home environment without the need of a tracheostomy tube (Clini et al., 2002). Disorders where home ventilation is commonly indicated include SCI, NMD, chronic lung disease, and bronchopulmonary dysplasia. The technology is available with portable, lightweight, single-circuit flow generators that are capable of delivering bilevel pressure support with mask leak compensation at high levels of pressure support near 30 cmH2O. Invasive HMV is one of the most complex respiratory services delivered in the home and requires much more advanced decision-making, planning, training, and oversight (Lewarski and Gay, 2007).

CONCLUSIONS

Not too long ago, therapeutic nihilism was governing the approach to the care of the patient with a NMD. Since then, advances in respiratory care, including but not limited to NIV and HMV, coupled with advances in neurologic early diagnosis and monitoring, as well as a paradigm ethical shift, have dramatically changed the picture. Evidence-based guidelines have made a great impact and we are still welcoming randomized control trials to further direct our treatments.

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