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Curtis Lester Mendelson wrote in 1946 an article entitled "The aspiration of stomach contents into the lungs during obstetric anaesthesia", in the American Journal of Obstetrics and Gynaecology. The finding of 66 cases out of 43,000 pregnancies equates to an incidence of about 1 in 660 pregnancies. Today the incidence is much lower but it still represents the most common cause of maternal anaesthetic death. Prevention of aspiration remains the most important aim and there appears to be consensus around the world that preventative measures should be used more.
If aspirated material is sufficiently acid (pH lower than 2.5) and there is sufficient volume, then the symptoms of aspiration can ensue. The chemical pneumonia produced is caused by the parenchymal inflammatory reaction mediated by cytokines.
Accurate figures for incidence are not available. In 1975 an incidence of 1 in 1,100 surgical and obstetric patients was quoted. The number of mothers who die from Mendelson's syndrome has remained fairly constant at around 10 deaths per year in England.
Patients who develop Mendelson's syndrome become very ill 30-60 minutes after aspiration. The signs and symptoms appear during or very shortly after anaesthesia. The aspiration or regurgitation may not have been noticed by the anaesthetist. There may have been difficulty with intubation. It is likely that only a small quantity of gastric juice is required to cause the syndrome. It has been postulated that as little as 25 ml of acid gastric juices may be required.
Early signs include:
- Massive pulmonary oedema.
- Bronchospasm, which occurs often (unlike with amniotic fluid embolism).
- Hypovolaemia with haemoconcentration (the reactive transudation of fluid into the lungs contributes to this).
Later, cardiac failure may develop and accompanying this there may be:
- Increased pulmonary artery pressure.
- Reduced static lung compliance.
- Falling arterial oxygen.
- Metabolic acidosis, which is severe (usually develops later).
- Infection (this is not usually a feature).
- CXR shows pulmonary oedema and patchy atelectasis (but there is often poor correlation between the extent of pulmonary damage and the radiological appearances).
- Amniotic fluid embolism.
- Pulmonary embolus (see separate article Venous Thromboembolism in Pregnancy).
- Other causes of shock and circulatory collapse include:
These may include:
- Arterial blood gases.
- Microbiology work-up (including blood culture, culture of aspirate and sputum).
- Ultrasound for effusion.
- CT scan of chest in all cases.
Other procedures which are used or may be considered are:
- Tracheal aspiration for sampling
- Pulmonary artery catheterisation
- Mechanical ventilation
Conditions or circumstances increasing risk
Any condition producing loss of consciousness prior to anaesthesia increases the risk of aspiration. Protection of the airway is essential using measures such as putting the patient in the recovery position. Nasogastric tubes increase risk of aspiration and reduce the efficacy of Sellick's manoeuvre. They should be removed prior to induction. Difficult intubations will increase the risk of aspiration. In summary, conditions which increase the risk of aspiration include:
- Gastrooesophageal reflux.
- Any condition producing loss of consciousness prior to anaesthesia (eg seizures).
- Endotracheal intubation.
- Protracted vomiting.
- Nasogastric tubes.
Smoking does not appear to be a risk factor.
The principal therapeutic measures include:
- Corticosteroid therapy. Although effectiveness has been called into question they are still used.
- Artificial ventilation. Intermittent positive pressure ventilation is essential if patients are to survive, and transfer to an intensive therapy unit for this is mandatory.
- Supportive measures. A variety of other measures may help, including:
- Good fluid management.
- Drugs: bronchodilators, diuretics, heart failure therapy, etc.
- Physiotherapy, including techniques to promote drainage.
- Treatment of other complications if and when they arise.
- Antibiotics are not routinely indicated.
Many are possible, but they include:
- Acute respiratory distress syndrome (ARDS).
- Bacterial pneumonia.
- Bronchopleural fistula.
- Diffuse interstitial pulmonary fibrosis (may occur in recovered patients and in one small study appeared to have a good prognosis).
The mortality is high with figures for mortality as high as 60%. It can lead to acute respiratory distress syndrome (ARDS) and other complications with high morbidity and mortality.
Preventative measures may be applied in labour (particularly in patients at risk of having a Caesarean section), before Caesarean section and postpartum (for example, with anaesthesia for retained placenta), and include:
- Avoidance of general anaesthesia where possible - for example, by use of regional anaesthesia, epidurals, etc.
- Oral alkalis in labour to reduce pH of stomach contents. Different drugs and preparations have been used alone or in combination, with the aim of raising pH above 2.5 and reducing volume of gastric contents below 25 ml. It is assumed that this will reduce the risk of aspiration. Drugs used include:
- Magnesium trisilicate. This was used a lot in the past but concerns were expressed about aspiration of particulate antacids and it is used less often.
- Sodium citrate is used more often in labour and before Caesarean section. It is effective at elevating gastric pH but not at reducing gastric volume.
- Ranitidine, orally or intravenously (IV). Use IV at induction, or as a premedication orally is effective. It is also effective used in combination - for example, with sodium citrate.
- Cimetidine orally, intramuscularly (IM) or IV is effective.
- Metoclopramide used IV with oral sodium citrate reduced volume of gastric contents and pH.
- Good anaesthetic technique including:
- Presence of an experienced obstetric anaesthetist at every Caesarean section or anaesthetic for retained placenta.
- Use of Sellick's manoeuvre (cricothyroid pressure) to prevent regurgitation.
- Pre-oxygenation of the patient before intubation.
- Identifying patients likely to be difficult to intubate. Patients can be identified according to certain characteristics (short neck, etc.) but also if intubation has been difficult with previous anaesthetics.
- Compliance with and training in a 'failed intubation procedure'.
- Identification of patients at risk of aspiration.
Further reading & references
- Hein C, Owen H; The effective application of cricoid pressure, 2005; Journal of Emergency Primary Health Care
- Tourtier JP, Compain M, Petitjeans F, et al; Acid aspiration prophylaxis in obstetrics in France: a comparative survey of 1998 vs. 1988 French practice. Eur J Anaesthesiol. 2004 Feb;21(2):89-94.
- Ng A, Smith G; Gastroesophageal reflux and aspiration of gastric contents in anesthetic practice. Anesth Analg. 2001 Aug;93(2):494-513.
- Roberts RB, Shirley MA; Reducing the risk of acid aspiration during cesarean section. Anesth Analg. 1974 Nov-Dec;53(6):859-68.
- Hutchinson BR, Newson AJ; Pre-operative neutralization of gastric acidity. Anaesth Intensive Care. 1975 Aug;3(3):198-203.
- Wilkins RA, De Lacey GJ, Flor R, et al; Radiology in Mendelson's syndrome. Clin Radiol. 1976 Jan;27(1):81-5.
- Downs JB, Chapman RL Jr, Modell JH, et al; An evaluation of steroid therapy in aspiration pneumonitis. Anesthesiology. 1974 Feb;40(2):129-35.
- Jorgensen NH, Byer DE, Gould AB Jr; Aspiration pneumonitis. Prevention and treatment. Minn Med. 1989 Sep;72(9):517-9, 530.
- Adelhoj B, Petring OU, Frosig F, et al; Influence of cigarette smoking on the risk of acid pulmonary aspiration. Acta Anaesthesiol Scand. 1987 Jan;31(1):7-9.
- Auboyer C, Bouletreau P, Brinquin L, et al; Mendelson's syndrome. Poumon Coeur. 1977;33(6):365-73.
- Prefaut C, Kienlen J, Lloret MC, et al; Functional sequelae of Mendelson's syndrome. Myth or reality? Poumon Coeur. 1977;33(6):391-4.
- Torrielli R, Mene JM, Noblia M; Mendelson's syndrome in obstetrics. History or fact? J Gynecol Obstet Biol Reprod (Paris). 1989;18(1):93-8.
- Tordoff SG, Sweeney BP; Acid aspiration prophylaxis in 288 obstetric anaesthetic departments in the United Kingdom. Anaesthesia. 1990 Sep;45(9):776-80.
- Vaughan GG, Grycko RJ, Montgomery MT; The prevention and treatment of aspiration of vomitus during pharmacosedation and general anesthesia. J Oral Maxillofac Surg. 1992 Aug;50(8):874-9.
- Lim SK, Elegbe EO; The use of single dose of sodium citrate as a prophylaxis against acid aspiration syndrome in obstetric patients undergoing caesarean section. Med J Malaysia. 1991 Dec;46(4):349-55.
- Lim SK, Elegbe EO; Ranitidine and sodium citrate as prophylaxis against acid aspiration syndrome in obstetric patients undergoing caesarean section. Singapore Med J. 1992 Dec;33(6):608-10.
- Johnston JR, McCaughey W, Moore J, et al; Cimetidine as an oral antacid before elective Caesarean section. Anaesthesia. 1982 Jan;37(1):26-32.
- Thorburn J, Moir DD; Antacid therapy for emergency caesarean section. Anaesthesia. 1987 Apr;42(4):352-5.
- Manchikanti L, Grow JB, Colliver JA, et al; Bicitra (sodium citrate) and metoclopramide in outpatient anesthesia for prophylaxis against aspiration pneumonitis. Anesthesiology. 1985 Oct;63(4):378-84.
- Stuart JC, Kan AF, Rowbottom SJ, et al; Acid aspiration prophylaxis for emergency Caesarean section. Anaesthesia. 1996 May;51(5):415-21.
- Pickering BG, Palahniuk RJ, Cumming M; Cimetidine premedication in elective caesarean section. Can Anaesth Soc J. 1980 Jan;27(1):33-5.
- Paranjothy S, Griffiths JD, Broughton HK, et al; Interventions at caesarean section for reducing the risk of aspiration Cochrane Database Syst Rev. 2010 Jan 20;(1):CD004943.
- Sellick RA; Cricoid pressure to control regurgitation of stomach contents during induction of anaesthesia:preliminary communication. Lancet ii 404 (1961).
- Cohen SE; The aspiration syndrome. Clin Obstet Gynaecol. 1982 Aug;9(2):235-54.
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Dr Richard Draper
Dr Richard Draper