42. Infantile pyloric stenosis Flashcards
You are asked to anaesthetise a 4 kg, 5-week-old boy for pyloromyotomy
for pyloric stenosis. Blood gases reveal the following findings
pH 7.53, PCO2 5.3, BE +10, HCO3 − 32.7
Tell me about this condition? What is the incidence and sex distribution?
Infantile pyloric stenosis is the most common surgical condition presenting in the first 6 months of life.
It has an incidence of approximately 3 per 1000.
The aetiology is not fully understood. However, genetic factors are thought to be involved with a higher incidence seen in the offspring of affected
individuals.
Males are affected four times as frequently as females and Caucasians are
more frequently affected than black or Asian infants.
Do you know anything about the pathology of this condition?
Yes.
Hypertrophy of the circular pyloric muscle with spasm causes gastric
outflow obstruction.
There are multiple theories on the pathogenesis of pyloric stenosis
- but reduced or immature innervation (particularly cholinergic)
- and reduced neuronal nitric oxide synthase activity are
thought to play a role in producing hypertrophy and spasm
How does the condition present?
Presentation usually occurs between the third and fourth week of life but
may be earlier or later.
Progressive, non-bilious, projectile vomiting is the classical presenting
complaint.
Associated hunger but failure to thrive.
Dehydration will develop if the child is not treated and mild jaundice may
be present (due to starvation).
Examination may reveal a firm ‘olive’-like tumour in the right
hypochondrium, particularly after vomiting, though this is becoming less
common with earlier diagnosis.
Diagnosis is now most frequently confirmed with ultrasound.
6%–20% will have an additional abnormality. These may be gastrointestinal such as oesophageal atresia, Hirschprung’s disease, malrotation, anorectal anomalies or cardiac abnormalities.
How would you assess the degree of dehydration?
Dehydration may be defined as mild
(<5% dehydrated),
moderate (5%–10%),
or severe (>10%) based on the clinical findings.
Signs Mild <5% Moderate 5%−10% Severe >10%
Decreased UO + + +
Dry mucosa +/− + +
Decreased skin turgor − +/− +
Prolonged cap refill − +/− +
Tachypneoa − +/− +
Tachycardia − +/− +
Fluid deficit in mls = Wt × % dehydration × 10
Tell me about the biochemical abnormalities you may expect
in this baby.
The classic biochemical abnormality is hypochloraemic alkalosis with
hyponatraemia and hypokalaemia.
Gastric secretions consisting of gastric acid (HCl), water, sodium and potassium are lost through vomiting. HCO− 3 produced
during gastric acid synthesis is retained and produces progressive alkalosis.
Initially the rising HCO− 3 may overwhelm renal resorptive capacity and alkaline urine will be produced.
However, as hyponatraemia and dehydration develop,
hydrogen ions are lost in exchange for sodium producing paradoxical
aciduria.
Gastric potassium loss combined with renal exchange for sodium and
intracellular shift caused by alkalosis produce hypokalaemia.
Severe dehydration with lactic acidosis and ketosis from starvation may produce a degree of reversal of the alkalosis.
Would you anaesthetise this child now?
No! The case is urgent but not an emergency.
Correction of dehydration and biochemical abnormalities should be performed before surgery. A nasogastric tube should be inserted and significant dehydration treated with i.v. fluid
boluses (20 ml/kg normal saline or colloid).
Multiple fluid correction regimens exist but must include sodium chloride, potassium, glucose and adequate water.
The resuscitation goals should include restoration of circulating volume
with urine output >1 ml/kg per hour and normalisation of biochemistry (i.e. Cl >106, Na >135, HCO− 3 <26, and normal pH and BE.
The high-normal chloride is
required to ensure correction of the alkalaemia).
What factors must be considered when anaesthetising this infant?
The anaesthetic issues may be divided into those specific to this condition and those general to anaesthetising neonates and infants.
In addition to the pre-operative measures above, the patient should be
protected from aspiration. This may be achieved by aspiration of the NG tube with the patient in three or four positions (supine, lateral, and prone).
A rapid sequence induction will offer the safest protection against aspiration. However, inhalational induction, intravenous induction with non-depolarising neuromuscular blockade
General issues for neonates include:
Different airway anatomy.
Use of uncuffed and correctly sized endotracheal tubes (3.5mm ID with a
size each side available).
Meticulous calculation of correct fluid and drug doses.
Limited cardio-respiratory reserve with rapid development of hypoxia (low
FRC, high BMR).
Risk of bradycardia (immature sympathetic innervation).
Increased heat loss (high surface area to weight ratio).
Increased susceptibility to induction agents and opioids.
Post-operative risk of apnoeas and hypoglycaemia.
