A 18-month-old child presents with acute onset watery diarrhea and vomiting for 2 days. On examination, the child appears lethargic, has sunken eyes, absent skin turgor, and weak peripheral pulses. Capillary refill time is 3 seconds. The child has passed no urine for 8 hours. Which investigation is most appropriate as the next step to guide fluid resuscitation?

Explanation

## Investigation of Choice for Severe Dehydration with Shock **Key Point:** Arterial blood gas (ABG) analysis is the most appropriate investigation in a child with severe dehydration and signs of hypovolemic shock to assess acid-base status, oxygenation, and guide aggressive fluid resuscitation. ### Clinical Severity Assessment The child has **severe dehydration with hypovolemic shock**: - Lethargy (altered mental status) - Sunken eyes + absent skin turgor (>10% fluid loss) - Weak peripheral pulses + prolonged capillary refill (3 seconds) → **circulatory compromise** - Oliguria (no urine × 8 hours) → renal hypoperfusion ### Why ABG is the Investigation of Choice | Parameter | Clinical Significance in Shock | |-----------|--------------------------------| | **pH** | Metabolic acidosis from tissue hypoperfusion and lactate accumulation | | **HCO₃⁻** | Decreased in metabolic acidosis; indicates severity | | **Lactate** | Elevated lactate = tissue hypoxia; prognostic marker | | **PaO₂** | Assess oxygenation; guide oxygen therapy | | **PaCO₂** | Respiratory compensation for metabolic acidosis | **High-Yield:** In hypovolemic shock from diarrhea, ABG reveals: 1. **Metabolic acidosis** (low pH, low HCO₃⁻, elevated anion gap) 2. **Elevated lactate** (marker of tissue hypoperfusion) 3. These findings confirm shock state and guide: - Urgency of fluid resuscitation - Need for vasopressor support if unresponsive to fluids - Monitoring of resuscitation adequacy ### Immediate Management Context **Clinical Pearl:** In a child with signs of shock (weak pulses, prolonged CRT, oliguria, altered mental status), ABG is obtained **during or immediately before** aggressive fluid resuscitation (20 mL/kg isotonic saline IV bolus over 15–30 minutes). ABG findings guide: - Titration of fluid rate - Decision to escalate care (ICU admission) - Assessment of end-organ perfusion (lactate clearance) ### Why ABG Over Other Investigations - **More urgent than electrolytes:** Shock is a time-critical emergency; acid-base status and lactate are immediate indicators of tissue perfusion - **Guides real-time management:** Electrolytes take 30–60 minutes; ABG available in 5–10 minutes - **Prognostic value:** Lactate and pH predict outcome in pediatric shock **Mnemonic:** **SHOCK** = **S**evere dehydration → **H**ypoperfusion → **O**xygen debt → **C**ritical acid-base derangement → **K**ey test = ABG [cite:Harrison 21e Ch 143; Pediatric Advanced Life Support (PALS) Guidelines 2020]