A 35-year-old woman with type 1 diabetes mellitus presents with polyuria, polydipsia, and rapid breathing. ABG shows: pH 7.22, PaCO₂ 28 mmHg, HCO₃⁻ 12 mEq/L, PaO₂ 95 mmHg. Blood glucose is 450 mg/dL, serum osmolality 320 mOsm/kg. What is the most appropriate immediate next step in management?

Explanation

## Diagnosis: Diabetic Ketoacidosis (DKA) **Key Point:** This patient has metabolic acidosis with appropriate respiratory compensation (Kussmaul respiration). The low pH (7.22), low HCO₃⁻ (12), and low PaCO₂ (28) confirm primary metabolic acidosis with secondary respiratory alkalosis. ### Verification of Appropriate Respiratory Response Using **Winter's formula** to calculate expected PaCO₂: $$\text{Expected } PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 12 + 8 \pm 2 = 26 \pm 2 = 24\text{–}28 \text{ mmHg}$$ Observed PaCO₂ = 28 mmHg → **appropriate respiratory compensation** (no concurrent respiratory acidosis). ### DKA Management Algorithm ```mermaid flowchart TD A[DKA Suspected<br/>pH &lt; 7.30, HCO₃⁻ &lt; 18,<br/>Glucose &gt; 250]:::outcome --> B[Confirm with:<br/>Serum/urine ketones,<br/>Anion gap, β-hydroxybutyrate]:::action B --> C[Assess Severity]:::decision C -->|Mild/Moderate| D[IV Fluids 0.9% NaCl<br/>1-1.5 L/hr initially]:::action C -->|Severe| E[Aggressive Fluids<br/>+ ICU monitoring]:::urgent D --> F[Insulin 0.1 U/kg bolus<br/>then 0.1 U/kg/hr infusion]:::action E --> F F --> G[Monitor K⁺, glucose,<br/>HCO₃⁻, pH q1-2h]:::action G --> H{Anion gap closed<br/>& HCO₃⁻ &gt; 15?}:::decision H -->|Yes| I[Switch to SC insulin<br/>+ long-acting regimen]:::action H -->|No| J[Continue IV insulin]:::action ``` **High-Yield:** The **three pillars of DKA management** are: 1. **Insulin** — stops ketone production 2. **Fluids** — corrects dehydration and osmolality 3. **Electrolytes** — replaces K⁺ (total body deficit ~3–5 mEq/kg despite normal serum K⁺ due to acidosis-induced shift) **Clinical Pearl:** Do NOT give bicarbonate in DKA unless pH < 6.9 AND haemodynamically unstable. Bicarbonate paradoxically worsens intracellular acidosis, impairs oxygen delivery (shifts Hb–O₂ curve), and delays ketone clearance. The respiratory system is already compensating appropriately. ### Why Insulin + Fluids is Correct - Addresses the primary pathology: insulin deficiency and ketone overproduction - Fluids correct hypovolaemia (~5–10 L deficit) and reduce osmolality - Electrolyte monitoring prevents hypokalaemia (K⁺ drops as insulin drives K⁺ intracellularly) - pH will rise naturally as ketones are cleared and HCO₃⁻ is regenerated **Mnemonic:** **FLUID-INS** = **F**luids (0.9% NaCl), **L**abs (K⁺, glucose, HCO₃⁻), **U**rine ketones, **I**nsulin bolus + infusion, **D**extrose when glucose < 250, **INS**ulin titration.