Mendelson's Syndrome

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 one in 660 pregnancies. Today the incidence is much lower but it still represents the commonest cause of maternal anaesthetic death. Prevention of aspiration remains the most important aim and there appears to be consensus around the world that preventive measures should be used more.^1,2,3,4,5,6

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 1100 surgical and obstetric patients was quoted.⁴ 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 mls of acid gastric juices may be required.³
Early signs include:

• Cyanosis
• Tachycardia
• Massive pulmonary oedema
• Bronchospasm occurs often (unlike with amniotic fluid embolism)
• Hypotension
• 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 is severe (usually develops later)
• Infection is not usually a feature
• CXR shows pulmonary oedema and patchy atelectasis⁵ (but there is often poor correlation between the extent of pulmonary damage and the radiological appearances⁶)

• Amniotic fluid embolism
• Pulmonary embolus (see VTE in pregnancy)
• Other causes of shock and circulatory collapse include:
  • Abruptio placentae
  • Heart disease
  • Other pulmonary diseases including asthma, pneumothorax
  • Subarachnoid haemorrhage
  • Malignant hyperthermia - very rare

These may include:

• Full blood count
• Arterial blood gases
• Microbiology work up (including blood culture, culture of aspirate and sputum)
• CXR
• Ultrasound for effusion
• CT scan of chest in all cases

Other procedures which are used or may be considered are:

• Bronchoscopy⁷
• Tracheal aspiration for sampling
• Pulmonary artery catheterisation
• Mechanical ventilation

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 (GO)
• Any condition producing loss of consciousness prior to anaesthesia (e.g. seizures)
• Endotracheal intubation
• Protracted vomiting
• Nasogastric tubes

Smoking does not appear to be a risk factor.⁸

The principal therapeutic measures include:

• Corticosteroid therapy.⁹ 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 intensive therapy unit for this is mandatory.
• Supportive measures. A variety of other measures may help including:
  • Good fluid management.⁷
  • 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.⁹

Many are possible, but they include:

• Acute respiratory distress syndrome (ARDS)
• Bacterial pneumonia
• Empyema
• Bronchopleural fistula
• Diffuse interstitial pulmonary fibrosis (may occur in recovered patients and in one small study appeared to have a good prognosis)¹⁰

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.

Preventive 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.¹¹
• 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 25mls. It is assumed that this will reduce the risk of aspiration. Drugs used include:
  • Magnesium trisilicate. This was used a lot in the past¹² but concerns were expressed about aspiration of particulate antacids and it is used less often.¹³
  • Sodium citrate is used more often in labour and before caesarean section.¹² It is effective at elevating gastric pH but not at reducing gastric volume.¹⁴
  • Ranitidine, orally or intravenously. Use intravenously at induction or as a pre-medication orally is effective. It is also effective used in combination for example with sodium citrate.¹⁵
  • Cimetidine orally,¹⁶ intramuscularly¹⁷ or intravenously is effective.
  • Metoclopramide used intravenously with oral sodium citrate reduced volume of gastric contents and pH.¹⁸
  • Omeprazole.¹⁹
  There has been considerable debate about the best method to increase pH and reduce volume of gastric contents. Pregnant patients have reduced gastric emptying and high gastric volumes.²⁰ Local policies should be followed. A variety of regimens has been used. Particulate antacids are generally now avoided in favour of sodium citrate and recent surveys reflect this trend.¹²
• Good anaesthetic technique including:²
  • Presence of experienced obstetric anaesthetist at every caesarean section or anaesthetic for retained placenta.
  • Use of Sellick's manoeuvre (cricothyroid pressure) to prevent regurgitation.²¹
  • Preoxygenation of the patient before intubation.
  • Identifying patients likely to be difficult to intubate.²² 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.²²