By reading this article you should be able to:Describe the electrolyte and acid-base abnormalities associated with infantile hypertrophic pyloric stenosis. Explain why these electrolyte and acid-base abnormalities need to be corrected before surgery. Describe the technique for ultrasound examination of the gastric antrum and qualitative assessment of stomach contents before induction of anaesthesia for pyloromyotomy. Discuss the options for induction of anaesthesia including choice of drugs and techniques.Pyloric stenosis is the result of hypertrophy of the smooth muscle of the pylorus, which forms the gastric outlet. Its aetiology is uncertain, although a number of environmental and hereditary contributory factors have been identified. The reported incidence varies between 0.9 and 5.1 per 1000 live births. 1 In England and Wales it is 1.5 per 1000 live births and has remained static in recent years. 2 The risk of the disease is four to five times higher in boys than girls. 3 There is a decline in risk with increasing birth order with an odds ratio of 1.9 for first-born children. 3 The genetic element is evidenced with higher rates of concordance in monozygotic than dizygotic twins and a number of susceptibility loci have been identified.Babies with infantile hypertrophic pyloric stenosis present most commonly in the 2nd and 3rd months of life. 2 However, a recent review of surgical outcomes for 9686 infants who underwent pyloromyotomy in England over a 10-year period, found that 30% of patients were aged 7e28 days at the time of surgery. 2 The classic presentation is projectile vomiting of non-bilious stomach contents, loss of weight or failure to gain weight (crossing centiles on the growth chart), and dehydration. Abdominal examination may reveal a palpable 'olive'; however, modern developments mean that the majority of patients are being diagnosed earlier with ultrasound and this physical finding is becoming less common. Pathophysiology and electrolyte changesVomiting is the principal symptom of pyloric stenosis, classically described as being projectile in nature. The vomitus in pyloric stenosis consists of gastric secretions. These secretions are high in hydrogen and chloride ions with some sodium and potassium, all of which are lost along with water. The electrolyte losses result in a hypokalaemic, hypochloraemic metabolic alkalosis. The water loss causes dehydration and a reduction in plasma volume; this results in the secretion of aldosterone. Aldosterone causes sodium and water retention Key pointsVomiting results in hypochloraemic, hypokalaemic metabolic alkalosis and dehydration; these must be corrected before general anaesthesia and surgery. The stomach should be emptied using a nasogastric or orogastric tube before induction of anaesthesia. Ultrasound can be used to identify fluid in the gastric antrum. Ensuring adequate depth of anaesthesia with complete neuromuscular block before laryngoscopy minimises the risk of regurgitation and pulmonary aspiration. Options for analgesia include rectus sheath ...