Describe the role of aspiration prophylaxis in pregnant women undergoing surgery

Chesnuts obstetric anaesthesia

Describe the physiological changes in preg that place the mother at increased risk of aspiration

• Decreased gastric motility (Pain, opioids and obesity)

• Reduced tone of LOS (Progesterone)

• Increased intra-abdominal pressure

• Increased risk of aspiration 

  • Mechanical - Cephalad displacement of stomach reduces 

  • Hormonal - Progesterone lowers LES tone 

• Incidence 1:1 000 pregnancies

Factors that predispose to aspiration

• Emergency surgery

• Difficult/failed intubation

• Light anaesthesia

• GORD

In obstetric patients the fasting gastric volume and pH

In 30-43% of obstetric patients the fasting gastric volume is >25mL and pH <2.5

In order to minimise the risk of aspiration obstetric patients should

• Adhere to fasting guidelines when elective / not emergency (6 hours for food, 2 hours clear fluids)

• H2 receptor antagonists e.g. ranitidine 150mg Q6hrly pre-op

  • efficacious in reducing gastric acidity and volume by blocking H2 receptors on oxyntic cells and thus reduce acid production, if given orally will result in gastric pH >2.5 within 1 hour and for duration of 8 hours. Note that cimetidine reduces the rate of clearance of certain drugs (lignocaine) by binding CytP450 253

• Sodium citrate – 30mls of 0.3M solution – 30mls will neutralize 255mL of HCl with a pH of 1.0 but has a very limited duration of action (will keep pH >3.0 for ~30 minutes)

• Metoclopramide 10mg IV pre-operatively, will assist by increasing gastric emptying

• Don’t use particulate antacids as because if they are aspirated they cause pulmonary shunting & hypoxemia to a similar degree as acid aspiration