A) Interpretation of Blood Investigation Findings

ABG Analysis — Step by Step

Diagnosis of the Acid-Base Disorder

"Metabolic alkalosis is established by an elevated arterial pH, an increase in the serum [HCO₃⁻], and an increase in PaCO₂ as a result of compensatory alveolar hypoventilation. It is often accompanied by hypochloraemia and hypokalaemia."

• Persistent vomiting → loss of HCl (hydrochloric acid) from the stomach → net gain of HCO₃⁻ in the blood → metabolic alkalosis
• High PaCO₂ = the lungs compensate by hypoventilating to retain CO₂ and buffer the rising pH (this is appropriate respiratory compensation, not a primary respiratory acidosis)
• Hypochloraemia (low Cl⁻): direct loss of Cl⁻ in gastric juice (HCl)
• Hypokalaemia (low K⁺): as the kidney tries to retain H⁺ to correct the alkalosis, it excretes K⁺ in exchange; aldosterone is also activated by volume depletion, further wasting K⁺
• Hyponatraemia (low Na⁺): due to volume depletion from ongoing vomiting; ADH release causes free water retention, diluting Na⁺
• Mild hypoxaemia (PO₂ 56): secondary to hypoventilation (compensatory suppression of respiratory drive)

B) Diagnosis

"Pyloric stenosis presents with non-bilious projectile vomiting starting between 2 and 6 weeks of age... once vomiting starts, its frequency and forcefulness increase daily."

Key Supporting Features in This Case

Mechanism of Electrolyte Disturbance in IHPS

Next Steps

• Ultrasound abdomen (gold standard): pyloric muscle thickness >4 mm, pyloric channel length >17 mm
• Electrolyte correction before surgery (operate only when HCO₃⁻ <30 mmol/L, Cl⁻ >100 mmol/L)
• Definitive treatment: Ramstedt's pyloromyotomy (open or laparoscopic)

Immediate Management of Infantile Hypertrophic Pyloric Stenosis

Key principle: IHPS is NOT a surgical emergency — it is a metabolic emergency. Correct the biochemistry first; operate second.

1. Resuscitation & Stabilisation

• IV access — insert a reliable peripheral IV cannula
• Nil by mouth (NBM) — stop all oral feeds immediately
• Nasogastric (NG) tube — insert and leave on free drainage to decompress the stomach and prevent further aspiration risk
• Oxygen — if PO₂ remains low (as in this baby at 56 mmHg), administer supplemental O₂

2. IV Fluid & Electrolyte Correction

• Monitor electrolytes and blood gas every 4–6 hours
• Urine output target: 1–2 mL/kg/hour

3. Monitoring

• Strict fluid balance (input/output chart, weigh nappies)
• Serum electrolytes, ABG — repeat regularly to track correction
• Blood glucose — neonates are prone to hypoglycaemia, especially NBM; dextrose in IV fluids addresses this
• Serum Cl⁻, K⁺, HCO₃⁻ are the key targets to normalise

4. Criteria to Proceed to Surgery

5. Confirmatory Investigation (Concurrent)

• Ultrasound abdomen (gold standard, non-invasive):
  • Pyloric muscle thickness > 4 mm
  • Pyloric channel length > 17 mm

6. Definitive Treatment

• The hypertrophied circular muscle of the pylorus is split longitudinally down to the mucosa without entering the mucosa
• Feeds are reintroduced 4–6 hours post-operatively
• Prognosis is excellent with near-zero mortality in well-corrected patients

Summary Flow

Persistent vomiting + Metabolic Alkalosis + Low K/Cl/Na
        ↓
NBM + NG tube on free drainage
        ↓
IV access → 0.9% NaCl bolus (if dehydrated) → maintenance with KCl
        ↓
Serial electrolytes + ABG every 4–6 hours
        ↓
Ultrasound confirmation of IHPS
        ↓
Await Cl⁻ >100, K⁺ >3.5, HCO₃⁻ <30
        ↓
Ramstedt's Pyloromyotomy

Renal Tubular Acidosis (RTA) — Type Comparison

Core concept: All RTAs = normal anion gap (hyperchloraemic) metabolic acidosis with impaired renal acid handling, despite normal/near-normal GFR.

Quick Comparison Table

Why No Type 3?

Memory Hooks

Urine pH Trick

• If urine pH cannot drop below 5.5 despite systemic acidosis → Type 1
• If urine pH can drop below 5.5 → Type 2 or Type 4

Naphthalene Poisoning

Clinical Features

1. Gastrointestinal (Early — within hours)

• Nausea, vomiting, abdominal pain
• Diarrhoea
• Anorexia

2. Haematological (Hallmark)

• Haemolytic anaemia — the most dangerous systemic effect
  • Caused by oxidative damage to RBC membranes
  • Particularly severe in G6PD deficiency
• Heinz body formation (denatured haemoglobin precipitates)
• Methaemoglobinaemia — cyanosis unresponsive to O₂
• Haemoglobinuria — dark/tea-coloured urine
• Jaundice (indirect hyperbilirubinaemia from haemolysis)

3. Neurological

• Headache, lethargy, confusion
• Seizures (severe toxicity)
• Coma (rare)

4. Renal

• Acute kidney injury (AKI) — secondary to haemoglobinuria and haem pigment nephropathy
• Oliguria, haematuria

5. Hepatic

• Hepatotoxicity — raised LFTs, jaundice
• Hepatomegaly

6. Ophthalmological

• Cataracts — classic with chronic/repeated exposure
• Optic neuritis (rare)

7. Inhalation-specific

• Headache, nausea
• Irritation of mucous membranes
• Chronic exposure → neurotoxicity

Investigations

Treatment

A. Immediate/General Measures

• Remove from exposure — fresh air if inhalation; remove contaminated clothing
• Do NOT induce vomiting (aspiration risk; naphthalene is a hydrocarbon)
• Activated charcoal (50g in adults, 1g/kg in children) — if ingestion within 1–2 hours and airway is protected
• Gastric lavage — consider only if large recent ingestion with protected airway
• IV access + monitoring (ECG, SpO₂, urine output)

B. Supportive Care

• IV fluids — aggressive hydration to protect kidneys from haemoglobinuria; alkalinise urine (sodium bicarbonate) to prevent tubular precipitation of haem
• Urinary alkalinisation — NaHCO₃ IV to keep urine pH >7 (reduces haem pigment nephropathy risk)

C. Haemolysis Management

• Blood transfusion — if Hb critically low or symptomatic severe anaemia
• Monitor for ongoing haemolysis with serial Hb, LDH, bilirubin

D. Methaemoglobinaemia Treatment

• Methylene blue 1–2 mg/kg IV (1% solution) over 5 minutes
  • Reduces metHb back to functional Hb
  • Avoid in G6PD deficiency — methylene blue requires G6PD to work and can worsen haemolysis
• High-flow oxygen (100% O₂) as adjunct
• Severe refractory cases: exchange transfusion

E. Renal Support

• If AKI develops: renal replacement therapy (dialysis) may be needed
• Strict fluid balance and electrolyte monitoring

F. Seizures

• IV benzodiazepines (lorazepam/diazepam)

Summary