A 42-year-old woman with type 2 diabetes mellitus presents to the emergency department with a 2-day history of severe vomiting and abdominal pain. She has been unable to keep down food or fluids. On examination, she is alert, BP 110/70 mmHg, HR 102/min, RR 18/min. Serum electrolytes: Na⁺ 138, K⁺ 3.2, Cl⁻ 88 mEq/L, HCO₃⁻ 32 mEq/L. Arterial blood gas: pH 7.48, PaCO₂ 48 mmHg, HCO₃⁻ 32 mEq/L. Urine chloride is 8 mEq/L. What is the primary acid-base disorder and the underlying cause?

Explanation

## Acid-Base Diagnosis: Metabolic Alkalosis from Vomiting ### Step-by-Step Interpretation #### 1. Identify the Primary pH Disturbance - pH 7.48 → **alkalemia** (normal 7.35–7.45) - HCO₃⁻ 32 → **elevated** (normal 22–26) - **Primary disorder: Metabolic alkalosis** #### 2. Assess Respiratory Compensation - In metabolic alkalosis, the respiratory system should hypoventilate (retain CO₂) to lower pH - Expected PaCO₂ using **respiratory compensation rule**: PaCO₂ increases by 0.7 mmHg for each 1 mEq/L increase in HCO₃⁻ above 24 - Expected PaCO₂ = 40 + 0.7 × (32 − 24) = 40 + 5.6 = **45.6 mmHg** - Actual PaCO₂ = 48 mmHg → **slightly higher than expected** (minimal concurrent respiratory acidosis, but essentially appropriate compensation) #### 3. Classify the Metabolic Alkalosis Metabolic alkalosis is classified by **urine chloride (UCl)** into two categories: | Feature | Chloride-Responsive | Chloride-Resistant | |---------|-------------------|-------------------| | **Urine Cl⁻** | < 10 mEq/L | > 20 mEq/L | | **Common Causes** | Vomiting, diuretics, nasogastric suction | Hyperaldosteronism, Cushing's, hypokalemia | | **Mechanism** | Volume depletion + H⁺ loss | Ongoing renal H⁺ secretion | | **Treatment** | Normal saline (volume repletion) | Treat underlying cause | **This patient's urine Cl⁻ = 8 mEq/L → Chloride-responsive metabolic alkalosis** ### Clinical Pathophysiology **Key Point:** Vomiting causes loss of gastric HCl, which removes H⁺ ions and Cl⁻ from the body. This generates metabolic alkalosis through two mechanisms: 1. **Direct H⁺ loss:** Gastric fluid contains ~100 mEq/L HCl 2. **Volume depletion:** Loss of gastric fluid → hypovolemia → activation of RAAS - ↑ Aldosterone → ↑ renal H⁺ secretion and K⁺ loss - ↓ GFR → ↓ HCO₃⁻ filtration → ↑ HCO₃⁻ reabsorption **Clinical Pearl:** The **low urine chloride (8 mEq/L)** is the key finding. It indicates: - Severe volume depletion (kidneys are avidly reabsorbing Na⁺ and Cl⁻ to maintain intravascular volume) - The alkalosis is **chloride-responsive** and will resolve with isotonic saline administration ### Why This Is NOT Diuretic-Induced or Hyperaldosteronism **High-Yield Distinction:** - **Loop diuretics** cause chloride-responsive alkalosis BUT the patient is not on diuretics (history does not mention them) - **Hyperaldosteronism** causes chloride-**resistant** alkalosis (urine Cl⁻ > 20 mEq/L). This patient's UCl = 8, which is chloride-responsive. ### Hypokalemia in This Case **Mnemonic: HAVCHEM** (causes of hypokalemia in alkalosis) - **H**yperaldosteronism - **A**lcalosis (causes renal K⁺ wasting) - **V**omiting (direct K⁺ loss + alkalosis) - **C**orticosteroids - **H**yperventilation - **E**tiology: diuretics, diarrhea - **M**etabolic alkalosis This patient has K⁺ 3.2 (hypokalemia) from both: 1. Direct loss in gastric fluid 2. Alkalosis-induced renal K⁺ wasting (alkalosis shifts K⁺ into cells and increases renal K⁺ excretion) ### Treatment Algorithm ```mermaid flowchart TD A[Metabolic Alkalosis]:::outcome --> B{Urine Chloride?}:::decision B -->|< 10 mEq/L<br/>Chloride-Responsive| C[Volume Depletion Present]:::outcome B -->|> 20 mEq/L<br/>Chloride-Resistant| D[Volume Expansion Present]:::outcome C --> E[Give Normal Saline<br/>+ K⁺ replacement]:::action D --> F[Treat underlying cause<br/>Spironolactone if needed]:::action E --> G[HCO₃⁻ decreases<br/>Alkalosis resolves]:::outcome F --> H[Address hyperaldosteronism<br/>or Cushing's]:::action ``` ### Why Urine Chloride Matters **High-Yield:** Urine chloride is more reliable than urine sodium for classifying metabolic alkalosis because: - Na⁺ can be reabsorbed in the distal tubule (via aldosterone) even in volume depletion - Cl⁻ reabsorption is directly linked to volume status and is not independently regulated by aldosterone - Low UCl = volume depletion = chloride-responsive alkalosis