Respiratory and Ventilatory Assessment
3.4 Monitoring During Invasive Spontaneous VentilationSpontaneous Ventilation
Table 3.1 Advanced ventilatory assessment parameters
Parameter Description
Compliance respiratory system (Cpl,rs or respiratory system compliance)
Compliance is defined as the ratio between volume and pressure. Regarding the pulmonary system, TV represents the volume variable, while delta pressure (plateau pressure minus PEEP) is the real pressure generated from machine to deliver that gas volume Cpl,rs stand for the ventilated lung parenchyma
and stiffness of chest wall, and the normal physiologic values are 1.2–1.5 mL/cmH2O/kg Auto-positive
end-expiratory pressure (PEEPi)
Presence of PEEPi is due to an incomplete emptying of the lungs. PEEPi formula is total PEEP (pressure at the end of expiration—
measured by performing an expiratory pause of 3 s with patient on controlled ventilation) minus set PEEP
PEEPi can be related to clinical findings (COPD, asthma, etc.) or direct consequence of inaccurate respiratory setting by clinicians [15]
In case of flow limitation (COPD patients), PEEPi can get reduced by low levels of external PEEP. In case of flow obstruction PEEPi, (bronchial secretions, ET tube diameter size, I/E ratio reversed), the external PEEP cannot affect the PEEPi. PEEPi is strictly related to the patient’s respiratory pattern
Esophageal pressure (Pes)
A tight correlation appears to exist between pressures inside pleural space and esophageal pressure. The most common way to perform a bedside measurement is throughout an esophageal balloon filled with air, with the balloon-tipped catheter connected to a pressure transducer kit as a part of a multiparameter monitoring system (Fig. 3.9—left)
From a nursing point of view, Pes monitoring can detect patient-ventilator asynchrony in invasive or noninvasive ventilatory support [16]
(continued)
Table 3.1 (continued)
Parameter Description
Diaphragmatic function
Trigger pneumatic signal is based on airway pressure, flow, and volume. It represents a communicative link between machine and patient’s demand, able to drive, control, and synchronize both inspiratory and expiratory cycling. Inspiratory trigger commences the inspiratory phase of ventilation, while the expiratory trigger rules the expiratory one.
Lately, a new way of trigger detection appeared called (by its acronyms) NAVA (neurally adjusted ventilatory assist) [17–19]. Basically, NAVA’s triggering principle is based on the electric diaphragm activity (Edi): it’s the best electrical signal to get analyzed in order to estimate respiratory drive and trigger off and cycle off the delivery of mechanical ventilation; Edi is definitely more accurate, reliable, and faster compared to conventional signal before mentioned. Detection of signal is possible via a dedicated NG tube equipped with electrodes on its distal end. This dedicated NG tube able to detect the diaphragmatic activity and then provide the ventilatory NAVA supports can be used itself as a valid
monitoring tool for asynchronies when it’s matched with a graphic monitoring of ventilation: flow and pressure (Fig. 3.9—right) Moreover, it could be very helpful to early detect
a multifactor syndrome defined as VIDD (ventilator-induced diaphragmatic
dysfunction) [20] mainly characterized by loss of contractile force and muscular mass.
Diaphragmatic dysfunction is common in patients mechanically ventilated, and it’s one of the main reasons for weaning failure
Esophageal pressure Volume Flow Airway pressure40 cmH2O
70 It/min
-70 It/min
-50 Patient effort
Patient trigger time
400ml
Edi Volume
Flow Airway pressure40 cmH2O
70 It/min
-70 It/min
50 mV
Patient effort
Patient trigger time
400ml
Fig. 3.9 Graphic waveforms with esophageal pressure (left) and with Edi monitoring (right)
1. Predictive measurement tools for weaning
2. Trial of unassisted breathing (CPAP with tube resistance compensation or T-tube trial)
3. Trial of extubation
During pressure support ventilation (PSV), the clinical goal is to best balance the use and abuse of patient’s respiratory muscles, avoiding functional muscular failure due to excessive WOB.
The respiratory over-assistance should be prevented [22]
because the only patient-machine’s interaction is related to the trigger activation, without any other muscular effort. It’s working like a pressure-controlled ventilation mode set on patient trigger. On the contrary, inadequate PS with a full mus- cular involvement could lead to fatigue and finally muscular respiratory failure.
Therefore, different monitoring tools should be considered in order to help bedside nurse in titrating the most suitable PSV, avoiding over- and underestimated PSV, index of muscular fatigue, and detection of maximum inspiratory pressure. Analyzing all those elements the ICU nurse can manage an advanced patient’s
respiratory assessment to get the best respiratory weaning path- way, from the very beginning (VC ventilation off) until the full patient recovery (successful extubation) (Table 3.2).
Table 3.2 Weaning process advanced assessment parameters Parameter Description
P0.1 The airway occlusion pressure (P0.1) is a reliable index of patient’s breath neuromuscular driving correlated with WOB (work of breathing) as well. P0.1 is defined as the negative airway pressure generated during the first 100 ms of an occluded inspiration P0.1 is an estimate of the neuromuscular drive but as
index could be easily affected from external stimuli, especially from patient’s level of sedation: detection and measurement should be performed in a very quiet and relaxed environment. Common range of values is 1–1.5 cmH2O. Using P0.1 as lead indicator and avoiding over-assistance or under-assistance, the pressure support range of values will be settled according to a P0.1 between 2 and 4 cmH2O. On a PS ventilation P0.1 index less than 1.5 reflects patient’s respiratory muscles completely unloaded. P0.1 index, according to literature, appears to be not reliable on patients with major neurological impairment or severe muscular force deficit (MIP <10 cmH2O) Maximum
inspiratory pressure (MIP) or negative inspiratory pressure (NIF)
MIP index is a marker about the strength of inspiratory muscles, mainly the diaphragm, as reflection of negative pressure generated against an occluded airway
A bedside calculation example should consider the baseline PEEP plus any possible PEEPi
The clinical procedure for determining this marker is very dependent on patient’s efforts and his/her previous education: a maximal inspiratory effort is required from the participant, while the operator is performing a manual expiratory hold for 30 s at least. For not collaborative patients, it appears helpful to extend the apnea time in order to the maximal effort. Normal range of values is 80/100 cmH2O, but most of the pulmonary ventilators available cannot read those negative pressure, usually no more than −45 cmH2O