Aspiration is one of the three factors consistently associated with maternal deaths related to obstetric general anaesthesia, the others being emergency operation and difficult tracheal intubation. Often, these three factors occur together.
Problems and special considerations
Several risk factors make the pregnant woman more prone to aspiration:
• Reduced efficacy of the lower oesophageal sphincter caused by progesterone
• Reduced gastric emptying if opioids have been given, particularly if systemic
• The physical effect of the gravid uterus on the stomach
• The presence of gastric contents if the mother has eaten (see Chapter 14, Gastric function and feeding in labour)
Every mother in the third trimester should be considered at risk of aspiration, although the point during pregnancy at which increased risk occurs, and the point postpartum at which the risk returns to normal, are controversial. Many obstetric anaesthetists would consider 16-18 weeks of pregnancy as representing the onset of the ‘at-risk’ period, although an earlier cut-off point has also been suggested, especially if there are symptoms of gastrooesophageal reflux, the mother is obese, or the procedure requires her to be positioned head-down. Similarly, although hormonal profiles alter dramatically within a few hours of delivery, studies of gastric emptying have not produced consistent results, although some have suggested as little as 4-8 hours postpartum as the time required for the risk to return to normal (longer if opioids have been administered). Most would treat women as still at increased risk of gastric aspiration for 48 hours postpartum. However, other general physiological changes of pregnancy may take several weeks to disappear.
Finally, it should not be forgotten that any pregnant woman with an obtunded level of consciousness may be at risk from aspiration, for example during or after convulsions, drug overdose, anaphylaxis, etc. Thus, women identified as high risk may not only be those in whom surgical intervention is planned or expected.
Mortality or morbidity may be related to:
• Impairment of the view at laryngoscopy causing difficulty with intubation.
• Obstruction of the upper airway by solid or semi-solid matter causing complete or partial airway obstruction, hypoventilation and atelectasis.
• Chemical pneumonitis (Mendelson’s syndrome), related to the pH and volume of the aspirated material, with a continuum of risk such that smaller volumes are required if the pH is lower. The alveolar inflammatory reaction may be intense, with oedema, cellular infiltration and the features of acute lung injury. There may be associated hypotension and poor peripheral perfusion if large amounts of fluid have been transferred from the intravascular space into the alveoli. Aspiration pneumonitis may also be caused by particulate antacids, e.g. magnesium trisilicate.
• Aspiration pneumonia, in which aspirated secretions are colonised with bacteria causing infection of the respiratory tract.
Management options
Prevention
Reduction of the volume and acidity of gastric contents. This may be achieved by:
• Withholding oral intake during labour
• Administration of metoclopramide or other prokinetic drugs
• Use of antacids or acid-reducing drugs such as H2-antagonists and proton pump inhibitors
• Emptying the stomach with a stomach tube before general anaesthesia or by inducing vomiting (rarely used, although it has been suggested that a stomach tube should be routinely passed during general anaesthesia for emergency caesarean section in order to reduce the risk of aspiration after extubation)
The first three measures are used to differing extents in different situations and countries. Thus, for example, all women in a particular unit might be given regular oral antacids and ranitidine throughout labour, whereas only women identified as being at high risk of intervention might be treated in another unit. Similarly, feeding in labour occurs to different degrees on different labour wards (see Chapter 14, Gastric function and feeding in labour). Proponents of all-inclusive treatment point to the potentially devastating effect of aspiration, the relative cheapness of therapy and the difficulty of identifying women truly at risk of a general anaesthetic. Supporters of selective treatment cite the low incidence of aspiration overall, the relatively low incidence of general anaesthesia in modern obstetric practice, the cost of therapy compared with no therapy, and resistance from many women and midwives to the ‘medicalisation’ of normal labour.
A practical breakdown of commonly used pharmacological preventive measures might be as follows (although, as already mentioned, the protocol in use may vary widely between units):
• Normal (i.e. low-risk) labour. Nil, plus local unit oral intake policy, e.g. a light diet, low- fat snacks.
• High-risk labour suggesting an increased risk of surgical intervention (e.g. obstetric complications, multiple pregnancy) or high-risk parturient suggesting an increased risk of aspiration if surgical intervention occurs (e.g. obese, significant reflux). Regular oral ranitidine 150 mg 6-hourly in active labour; following administration of pethidine: ranitidine 50 mg intramuscularly 8-hourly, plus local unit oral intake policy, e.g. clear fluids.
• Emergency caesarean section. Ranitidine 50 mg ± metoclopramide 10 mg slowly intramuscularly or intravenously when the decision for surgery is made, 30 ml sodium citrate 0.3 M orally immediately before induction of general anaesthesia.
• Elective caesarean section. Oral ranitidine 150 mg the night before and repeated the morning of surgery, metoclopramide/sodium citrate as above.
Preventing regurgitation during general anaesthesia. Standard general anaesthetic practice includes a rapid-sequence induction with application of cricoid pressure, although the method of its application and the possibility that cricoid pressure might make laryngoscopy more difficult are controversial areas (see Chapter 37, Cricoid pressure). Tracheal extubation should be in the lateral position with the patient awake, following return of full protective reflexes.
Avoidance of general anaesthesia altogether by using regional anaesthesia for operative procedures. This is generally thought to be a major factor in the reduction in maternal mortality associated with anaesthesia that occurred over the 1970s-1990s, although there is no doubt that improvements in training in, and assistance and facilities for, general anaesthesia also occurred during this period.
Diagnosis
Regurgitation may be obvious, either during induction of anaesthesia or intra-/postopera- tively. It may or may not be associated with aspiration. It is also possible for aspiration to occur without obvious, massive regurgitation, for example during induction or intraopera- tively past the cuff of the tracheal tube. Features include bronchospasm, raised airway pressure, hypoxaemia, tachypnoea, tachycardia and pyrexia; these may present for the first time postoperatively following otherwise uneventful anaesthesia. A high index of suspicion is therefore required. If fluid is aspirated from the pharynx, larynx or tracheal tube, simple litmus paper is useful for identifying its acidity, although this may not always be reliable if antacid therapy has been used.
Treatment
Initial management consists of removing the regurgitated material from the airway by using pharyngeal, laryngeal and tracheal suction and maintaining oxygenation. Tracheal intubation has the advantage of securing the airway and protecting it against further aspiration, as well as allowing ready access to the tracheobronchial tree for suction. Cricoid pressure may prevent further regurgitation during intubation, though this should be released if there is any active vomiting. The head-down lateral position may be appropriate, depending on the particular circumstances of the case, in order to encourage drainage of fluid from the upper airway and discourage further aspiration should regurgitation recur. Bronchospasm may occur, requiring usual management. Although popular in the past, the use of prophylactic steroid and antibiotic therapy in cases where aspiration is suspected is no longer advocated, since this approach has not been shown to reduce mortality and may even increase it. Antibiotics should only be administered for clinical evidence of infection. Solid particles may be removable via bronchoscopy; bronchoalveolar lavage may also be used to dilute the acidic fluid aspirated.
A chest x-ray may be useful to show the presence of large amounts of aspirated material (usually in the right lower lobe if aspiration occurred in the semi-recumbent position, and in the upper lobes if aspiration occurred when supine) and as a baseline, although a normal appearance does not exclude aspiration. In 25% of cases, there are no visible changes seen on x-ray, and any changes tend to lag behind the clinical course by up to 24 hours. Patients suspected of having aspirated should be observed and monitored carefully for at least 12-24 hours, since their condition may worsen considerably during this time. If a patient is unstable, consideration should be given to a delayed extubation once she is clinically improved.
Key points
• Aspiration of gastric contents is a major factor in maternal death associated with general anaesthesia, especially related to emergency caesarean section and difficult tracheal intubation.
• Prevention includes sensible policies on feeding in labour, use of pH-raising drugs and antacids, emptying the stomach, rapid-sequence induction when general anaesthesia is used and avoidance of general anaesthesia by encouraging regional anaesthetic techniques.
• Diagnosis may not always be obvious.
• Treatment includes general supportive measures; antibiotics and steroid therapy are no longer advocated.
Further reading
Gyte GM, Richens Y. Routine prophylactic drugs in normal labour for reducing gastric aspiration and its effects. Cochrane Database Syst Rev 2006; (3): CD005298.
Mendelson CL. The aspiration of stomach contents into the lungs during obstetric anaesthesia. Am J Obstet Gynecol 1946; 52: 191-205.
Robinson M, Davidson A. Aspiration under anaesthesia: risk assessment and decision-making.
Contin Educ Anaesth Crit Care Pain 2014; 14: 171-5.