Aspiration Characteristics
Aspiration pneumonitis is a rare complication that is thought to be more common in pregnancy as a result of the gastrointestinal changes that predispose to regurgitation (Chapter 2). Hormonal relaxation of the lower esophageal sphincter begins in the fi rst trimester, and refl ux symptoms are very common in late pregnancy, exacerbated by the effect of the expanding abdominal contents on the angle and function of the gastro-esophageal sphincter.
Many cases of aspiration now occur outside the immediate periop-
•
erative period (e.g., in sedated or unconscious women in the inten- sive care unit, or after collapse or seizure).
The rate of regurgitation during general anesthesia for cesarean
•
delivery is 1 in 100–200.
The rate of signifi cant aspiration associated with general anesthesia
•
is very low (estimate 1 in 600–2,000) but is twice as likely at non- elective cesarean delivery. This low incidence of clinical aspiration has been attributed to:
the predominant use of regional anesthesia
•
the use of rapid sequence induction for general anesthesia
•
pharmacological aspiration prophylaxis
•
Improved critical care has led to a falling mortality rate (estimated
•
< 5 % ).
Prevention of Aspiration
A number of measures to prevent aspiration are appropriate (Table 4.10 and Figure 4.15):
Fasting
•
Guidelines such as ASA’s
• Practice Guidelines for Obstetric
Anesthesia and ACOG’s Committee Opinion on Oral Intake during Labor describe accepted practice for oral intake in parturients.
Labor is unpredictable, however, and the need to deliver takes precedence over fasting status.
Obstetric Anesthesia145 Pregnancy per se does not alter gastric emptying, but pain during
•
labor and opioid analgesia markedly impair emptying (women entering labor may retain solids for many hours).
Reduction of gastric acidity (gastric content of pH
• > 3 is much less
likely to cause lung injury)
Prevention of regurgitation into the hypopharynx on induction of
•
anesthesia by effective application of cricoid pressure.
Management of Aspiration
Whenever consciousness or airway refl exes are signifi cantly impaired or lost, airway protection by intubation is warranted. Despite a lack of
4
3
2 1
Figure 4.15 Measures directed at preventing aspiration pneumonitis associated with obstetric anesthesia. (1) Emptying of solids from the stomach and reduction of gastric fl uid volume and acidity. (2) Prevention of sudden increases in intragastric pressure. (3) Prevention of fall in lower esophageal sphincter pressure and refl ux. (4) Cricoid pressure to prevent regurgitation into the pharynx; and maintenance of active upper airway and laryngeal refl exes to prevent aspiration into the trachea. With kind permission from Springer Science: Canadian Journal of Anesthesia , The stomach: factors of importance to the anaesthetist, vol. 37, 1990, p. 902, Davies JM.
146Anesthesia for Cesarean Delivery
evidence for outcome benefi t, cricoid pressure should be used until the lungs are isolated. Both induction and extubation are times of regurgitation risk, so safe patient positioning and timing of extubation are important.
If regurgitation is witnessed and aspiration is suspected, the airway should be assessed for soiling. If possible and appropriate, suctioning of both the upper airway and tracheobronchial tree, preferably under bronchoscopic vision, should be performed prior to controlled ventilation.
The gastric fl uid pH should be checked if feasible (for prognostic
•
purposes).
Patient assessment includes physical examination of the chest and
•
observation for postoperative dyspnea, cough, and fever.
Respiratory function should be closely monitored, with assessment
•
of oxygenation (pulse oximetry, arterial blood gas analysis). A chest X-ray looking for evidence of infi ltrates or consolidation should be done (though signs may not appear for several hours).
A decision should be made as to whether controlled ventilation is
•
required and, if so, what ventilatory support strategies, increased inspired oxygen, and respiratory care are needed.
The best environment for continuing respiratory care will be depend- ent on each facility’s resources; this may be the labor ward, the post- natal unit, or the intensive care unit.
Antibiotics are given based on microbiological evidence of
•
infection.
Failed Intubation
Despite the infrequent use of general anesthesia for cesarean section, complications of airway management comprise a large proportion of “closed claims” and are the leading cause of anesthesia-related maternal death. There is some evidence that the incidence of the latter is falling, and epidemiological data shows that general anesthesia for cesarean delivery can be exceptionally safe.
Nevertheless, airway assessment and management planning are anesthetic priorities whenever professional contact is made with a pregnant woman.
During pregnancy, the incidence of diffi cult intubation is much higher because of anatomical changes (fat deposition in the face, neck and chest, mucosal edema of the upper airway and occasionally larynx, and diffi culty in optimizing the position of the patient).
Obstetric Anesthesia147 Maternal outcomes associated with diffi cult intubation are affected
•
by the parturient’s higher oxygen consumption and lower oxygen reserve, as well as concurrent medical conditions and obesity (obesity may be independently associated with a greater risk of diffi cult airway management).
General anesthesia is often required for nonelective cesarean
•
delivery when time pressure is great, and failed intubation occurs more frequently in urgent cases conducted by less experienced anesthesiologists.
Dealing safely with the airway is of primary importance. Concern has been raised that training and experience (which has fallen with the decreased use of general anesthesia in most developed countries) may be insuffi cient. This has increased the importance of simulation training and “failed intubation drills.”
Case series and cohort studies suggest that 1 in 15–20 women will be assessed as likely to be diffi cult, and 1 in 20–50 will prove diffi cult to intubate.
The incidence of abandoned (“failed”) intubation at general anesthe-
•
sia is 1 in 250–3000 (most cases being a grade 3 view at direct laryn- goscopy), the incidence varying according to the experience and familiarity of the attending anesthesiologists.
The incidence of “can’t intubate, can’t ventilate” situations during
•
general anesthesia for cesarean delivery is estimated as 1 in 5,000–
10,000.
Regurgitation may be associated with diffi cult airway manage-
•
ment and occurs in 1 in 100–200 general anesthetics for cesarean delivery.
Severe hypoxemia occurs in 1 in 50, and other airway complications
•
(bronchospasm, laryngospasm) in 1 in 100, general anesthetics.
Assessment and Planning
Management of the diffi cult airway starts with assessment (Table 4.9 ) and planning. Obstetricians must be encouraged to refer women who are of known or predicted diffi culty. Early epidural catheterization during labor is a wise strategy. When diffi culty is predicted, regional anesthesia is the best option, but preparedness for general anesthesia remains essential in case of high or failed block. Awake fi beroptic intu- bation may be required if regional anesthesia is contraindicated, and some obstetricians have the expertise to perform the surgery under local anesthesia.
148Anesthesia for Cesarean Delivery
Specifi c Equipment
An array of special devices and equipment must be available to the anesthesiologist in the operating area. This might include:
Short-handle, Bullard or videolaryngoscopes (e.g., the Pentax
•
Airway Scope AWS-S100, the Glidescope ® , the AirTraq ® ) A fi beroptic laryngoscope
•
Stylettes, bougies, small (e.g., 6.0 mm) internal diameter tracheal
•
tubes, tube exchange catheters An intubating (Fastrach
• ® ) laryngeal mask, ProSeal ® laryngeal mask, other supraglottic airway devices or a Combitube ®
Cricothyrotomy devices and percutaneous tracheostomy sets
•
A variety of awake intubation techniques have been described, but all require expertise and may be time-consuming. When possible, give pre-procedural aspiration prophylaxis and optimize topical anesthesia of the airway with lidocaine gels and spray.
Consider mild sedation (e.g., remifentanil or midazolam) and drying
•
of secretions with an anticholinergic (glycopyrrolate, which has minimal placental transfer).
Use the oral route if possible to avoid nasal trauma and epistaxis.
•
Dealing with a Diffi cult Intubation
When diffi cult intubation occurs after induction of anesthesia, the plan must focus on:
Oxygenation (oxygen desaturation will always occur before sponta-
•
neous ventilation resumes).
Prevention of airway trauma.
•
A number of practice guidelines (e.g., the ASA) exist for nonobstetric patients, but are not specifi c to obstetric patients and situations, which are complicated by the considerations pertaining to the fetus. An example of a very simplifi ed approach for a situation in which there is an immediate threat to the life of the fetus, based on two pathways, representing adequate mask ventilation versus inadequate ventilation, is outlined in Table 4.23 .
Attention to optimal patient positioning is paramount, especially in
•
obese women, including elevation of the head and shoulders above the chest (Figure 4.13 ). Also, head support is helpful to prevent displacement as a result of left table tilt.
Incorrectly applied cricoid pressure (Figure 4.12 ) can worsen
•
the laryngoscopic view; so adjust it early (e.g., by requesting less
Obstetric Anesthesia149
pressure, pushing to the right or higher on the thyroid cartilage) and release it completely if necessary — the fi rst priority is effective ventilation and oxygenation.
Call for help as soon as possible!
•
Repeat doses of succinylcholine should not usually be given —
•
maintaining spontaneous ventilation after control of a diffi cult airway is the safest option.
A second, optimal attempt at intubation can be made, provided
•
oxygenation is satisfactory and airway trauma can be avoided.
Table 4.23 Simplifi ed Unanticipated Failed Intubation Management Algorithm
Grade 3 or 4 laryngoscopic view on induction Provided patient is still well oxygenated, the most experienced anesthesiologist should make a second attempt to intubate under optimal conditions
Adjust head or neck position
•
Adjust cricoid pressure
•
Apply thyroid cartilage pressure
•
Change laryngoscope
•
Intubate with a bougie and smaller tracheal tube
•
Declare failed intubation and call for help Maintain cricoid pressure and left pelvic tilt
•
Do not give more neuromuscular blocking drug
•
Oxygenate
•
Ventilate via face mask
•
use two hands to open the airway, maximize jaw thrust and seal,
•
insert an oral airway and get a second person to “bag”
Insert and ventilate using LMA, Proseal LMA, Intubating LMA, i-gel
•
maintain cricoid pressure unless impeding placement of airway device
•
or ventilation Decide whether to:
Wake the patient (change to combined spinal epidural anesthesia or awake intubation)
or
Continue surgery (only if airway controlled and oxygenation satisfactory) Allow spontaneous ventilation to resume and maintain anesthesia with
•
rapid onset/offset inhaled anesthetic in 100 % oxygen
If unable to oxygenate / ventilate (“can’t intubate, can’t ventilate”) use preferred cricothroidotomy technique
Percutaneous needle / cannula technique with jet ventilation
•
Scalpel, fi nger and bougie / tracheal tube technique
•
Melker size 5.0 Seldinger airway technique
•
150Anesthesia for Cesarean Delivery
Nasal instrumentation and repeated impacts on the larynx are best avoided.
A supraglottic airway device, such as the laryngeal mask, may prove
•
easier to insert and allow oxygenation.
An intubating (Fastrach
• ® ) laryngeal mask allows subsequent blind or preferably fi beroptic-guided intubation.
The ProSeal
• ® laryngeal mask adds protection against aspiration and is designed to allow intermittent positive pressure ventilation.
The i-gel airway has also been used.
•
If oxygenation can be achieved, then allowing the woman to wake is the safest approach; if oxygenation can be achieved but there is imme- diate danger to the life of the fetus, then continuing the operation using face or laryngeal mask ventilation, preferably with cricoid pres- sure still applied, can be considered.
If cricothrotomy is required because of severe hypoxemia, the method most familiar to the anesthesiologist should be attempted.
Awareness
Awareness refers to explicit recall of intraoperative events. The most commonly described events are sounds, conversation and tactile sensations (e.g., intubation), with visual and emotional memories and pain less frequent. As a result of the physiological changes of preg- nancy and concern about fetal drug exposure, pregnant women having cesarean delivery represent a high-risk population (Table 4.24 ).
Awareness is reported at a rate of 1 in 150–400 (higher than the over- all incidence in anesthesia of 1 in 500–1,000). It should be specifi cally discussed when obtaining consent for general anesthesia for cesarean delivery. If awareness occurs, it is likely to cause patient dissatisfaction.
Some women have long-term psychological disturbance, with
•
symptoms such as anxiety, phobia, sleep and mood disturbance.
This “post-traumatic stress disorder” may lead to litigation.
•
Contributors to Awareness
Although awareness can occur despite the best of care, the main antecedents are:
Human error, including inadvertent failure to administer anesthetic
•
(e.g., a syringe swap of cephalosporin antibiotic for pentothal or of succinylcholine for fentanyl).
Failure to provide adequate anesthetic agent, through inappro-
•
priate dosing or failure of drug delivery (e.g., an empty or turned off vaporizer).
Obstetric Anesthesia151
Inadequate vigilance (e.g., failure to give more intravenous anes-
•
thetic during multiple attempts to establish a clear airway).
Constrained drug administration (e.g., during severe hemodynamic
•
compromise or in patients with poor cardiac reserve).
Increased patient requirements for anesthesia, which may be genetic
•
(e.g., past history of awareness during apparently uneventful anesthesia) or drug-related (e.g., chronic benzodiazepine use).
Prevention of Awareness
A number of practice changes and new technologies have been intro- duced with a view to reducing awareness. These address hastening the transition from intravenous induction to inhalational anesthetic main- tenance, or means of detecting an inadequate depth of anesthesia. For general anesthesia for cesarean delivery these strategies include:
More liberal use of opioids prior to delivery.
•
Use of rapid uptake volatile anesthetics (e.g., sevofl urane, desfl urane).
•
Rapid uptitration of volatile anesthetic using agent monitoring
•
(“overpressure” with high initial concentrations, to rapidly increase end-tidal concentration to at least 0.75 MAC after intravenous induction).
Depth of anesthesia (brain function) monitoring (e.g., bispectral
•
index [BIS]).
Pentothal (2–2.5 mg/kg) is being more widely used for intravenous
•
induction, although an argument can be made that sodium pen- tothal 4–5 mg/kg more consistently achieves an adequate depth Table 4.24 Reasons for Higher Risk of Awareness during General Anesthesia for Cesarean Delivery
High cardiac output during pregnancy
rapid redistribution of intravenous anesthetic from effect sites and slower
•
uptake of volatile anesthetic drugs
Uncertainty about intravenous drug dosing (based on pre-pregnancy or current body weight?)
Avoidance of intravenous opioids until after delivery
Higher rate of airway diffi culty and thus delayed delivery of inhalational anesthetics
Limitation of volatile anesthetic delivery based on:
concern about uterine myometrial relaxation and bleeding
•
concern about maternal hypotension (especially in presence of
•
antepartum hemorrhage or hemodynamic compromise) concern about neonatal effects
•
152Anesthesia for Cesarean Delivery
of anesthesia. End-tidal anesthetic monitoring and BIS monitoring can reduce the risk of awareness but the number-needed-to-treat is high.
To achieve BIS values within the range at which the chance of
•
consciousness is very low (i.e., < 55–60) requires, for example, an end-tidal sevofl urane concentration of 1.5 % in 50 % nitrous oxide.
Management of the Patient Reporting Awareness
A patient reporting awareness needs consultation to ascertain exactly what has been recalled, using a structured series of questions (Table 4.25 ). If the story is convincing, or lack of adequate anesthesia has clearly occurred, frank discussion is warranted and clinical psy- chology services should also be used to support and counsel the patient and to organize ongoing review.
Venous Air Embolism Mechanisms and Background
Air may enter the venous circulation whenever venous pressure at the site of surgery is near or below atmospheric pressure (gradients 2.5–5 cm water). This is the case when the surgical site is elevated compared with the heart and pertains at cesarean delivery in the supine position, when large venous sinuses are frequently open.
Small air emboli occur most commonly between uterine incision
•
and closure.
The risk of air entrainment may be increased if the uterus is
•
exteriorized (placing it well above the level of the heart).
The incidence of subclinical air microemboli is approximately
•
90 % –95 % based on changes in end-tidal nitrogen and 40 % –70 % based on Doppler changes (detection of 0.1 ml bubbles).
Table 4.25 Patient Questionnaire to Evaluate Possible Awareness
Ask:
What is the last thing you remember before going to sleep?
•
What was the fi rst thing you remembered when you woke up?
•
Can you recall anything between?
•
Did you have any dreams during your anesthetic?
•
If any evidence of awareness is encountered, obtain a narrative and decide whether awareness was unlikely, possible or probable.
Obstetric Anesthesia153 A large air embolus causing an air lock in the right heart (estimated
•
volume > 100 ml), leading to cardiovascular collapse, is exception- ally rare.
The physiologic consequences depend on the volume of air and speed of entrainment, the amount of time air is trapped within the right atrium and ventricle (which may be up to 20 minutes), the degree of obstruction to the pulmonary circulation, and the patient’s comor- bidities. The cardiovascular response to embolized air includes:
increased pulmonary and right heart pressures
•
reduced or arrested cardiac output
•
ventilation-perfusion mismatch and hypoxemia
•
coronary artery ischemia.
• Diagnosis
The detection of venous air embolism is based on monitoring and detection of its pathophysiologic consequences.
Doppler ultrasound is very sensitive but not specifi c, often detect-
•
ing turbulent fl ow. It is not clinically useful given the high frequency of microemboli.
Central venous pressure or transthoracic/transesophageal echocar-
•
diography may reveal air bubbles, but are not indicated routinely.
A small rise in ET-nitrogen concentration during gas monitoring is
•
very sensitive but not likely to be of clinical relevance.
A rapid fall in end-tidal carbon dioxide is an early feature of a large
•
embolus that mandates immediate action.
Oxygen desaturation will follow a clinically signifi cant embolus.
•
A “mill-wheel” murmur probably only develops with a large volume
• of air.
Depending on the size of the air embolus, cardiovascular changes may be absent, minor or severe (including cardiac arrest).
Chest pain, dyspnea and loss of consciousness are manifestations
•
during regional anesthesia.
Seizures may result from cerebral hypoxia.
•
Focal neurological defi cits can result from paradoxical embolism
•
through a patent foramen ovale, resulting in cerebral air emboli.
Management
The suggested plan of management of a large venous air embolus causing clinical compromise is shown in Table 4.26 . Aspiration of air through a central venous catheter should be attempted in compromised
154Anesthesia for Cesarean Delivery
patients, as the most rapid means of potentially restoring the circula- tion. The best position in which to place the patient is unknown and controversial. The lateral head-down position has been suggested to minimize pulmonary infl ow obstruction, but reverse Trendelenburg position to lower the uterus below the level of the heart, and supine or slight head-up to prevent movement of gas into the pulmonary circulation and to aid aspiration from the right heart, have also been suggested.
Further Reading
1. Benhamou D , Wong C. Neuraxial anesthesia for cesarean delivery: what criteria defi ne the “optimal” technique? (editorial) Anesth Analg . 2009 ; 109 : 1370 - 1373 .
2. Levy DM. Emergency Caesarean section: best practice . Anaesthesia . 2006 ; 61 : 786 - 791 .
3. Reynolds F , Seed PT. Anaesthesia for Caesarean section and neonatal acid-base status: a meta-analysis . Anaesthesia . 2005 ; 60 : 636 - 653 . 4. Morgan P. Spinal anaesthesia in obstetrics . Can J Anaesth . 1995 ; 42 :
1145 - 1163 .
5. Thompson KD , Paech MJ. The use of combined spinal epidural anaesthesia for elective caesarean section is a waste of time and money .
Int J Obstet Anesth . 2001 ; 10 : 30 - 35 .
6. Cyna AM , Andrew M , Emmett RS , et al. Techniques for preventing hypotension during spinal anaesthesia for caesarean section . Cochrane Database of Systematic Reviews 2006 , Issue 4 . Art. No.: CD002251. DOI:
10.1002/14651858.CD002251.pub2 .
Table 4.26 Management of Large Venous Air Embolism Stop air entrainment
Advise the obstetrician of the suspected diagnosis
•
Ask the obstetrician to fl ood the wound with saline and close any open
• vessels
Consider placing the patient slightly head up
•
Supportive cardiorespiratory therapy Give 100
• % oxygen, cease nitrous oxide and ventilate if required Administer vasoactive drugs (metaraminol, phenylephrine,
•
norepinephrine) to support the blood pressure
If circulation is present, consider placing the patient in the left lateral
•
position to fl oat the air bubble away from the pulmonary infl ow tract in the right ventricle
If circulation is inadequate or absent, commence advanced life support, in
•
particular external cardiac massage, to disperse the air lock from the right heart into the pulmonary circulation
Rapidly insert a central venous catheter
Attempt aspiration of air from the right atrium and ventricle
•