Metabolic alkalosis anesthesia residency
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Metabolic Alkalosis: Anesthesia Residency Review
Definition & Diagnosis
Metabolic alkalosis is defined as a primary increase in plasma [HCO3-] (>26 mEq/L), resulting in arterial pH >7.45. The body compensates by hypoventilating (retaining CO2), but this compensation is limited because hypoxemia constrains the degree of hypoventilation.
Compensatory response:
- For every 1 mEq/L rise in [HCO3-], PaCO2 increases by ~0.7 mm Hg
- Expected PaCO2 = 0.7 × [HCO3-] + 21 (±2)
If the measured PaCO2 is lower than predicted, a concurrent respiratory alkalosis exists. If higher, a concurrent respiratory acidosis exists.
Classification: Chloride-Sensitive vs. Chloride-Resistant
The most clinically useful classification divides metabolic alkalosis based on urine chloride (UCl):
| Type | UCl | Causes |
|---|---|---|
| Chloride-sensitive | < 10 mEq/L | Vomiting, NG suctioning, loop/thiazide diuretics (after use stops), post-hypercapnic, low Cl intake |
| Chloride-resistant | > 20 mEq/L | Primary hyperaldosteronism, Cushing syndrome, Bartter syndrome, severe hypokalemia, licorice ingestion |
| Miscellaneous | Variable | Massive blood transfusion, citrate/acetate-containing colloids, NaHCO3 with renal impairment, high-dose penicillin, milk-alkali syndrome, bone metastases |
Pathophysiology of chloride-sensitive alkalosis: ECF volume depletion causes renal tubules to avidly reabsorb Na+. With insufficient Cl- available, electroneutrality is maintained by increased H+ secretion and HCO3- reabsorption, perpetuating alkalosis. Hypokalemia augments this by further increasing H+ secretion. Urinary Cl- is characteristically low because Cl- is being retained. - Morgan and Mikhail's Clinical Anesthesiology, 7e
Perioperative-specific causes: Vomiting/NG suctioning loses HCl (25-100 mEq/L H+, ~200 mEq/L Cl-). Massive transfusion causes alkalosis because citrate, lactate, and acetate in blood products are converted by the liver to HCO3-. Post-hypercapnic alkalosis occurs when a patient with chronic CO2 retention (COPD) is mechanically ventilated to a "normal" PaCO2 of 40 mm Hg -- the elevated baseline [HCO3-] built up over weeks cannot be excreted rapidly, producing acute metabolic alkalosis. - Morgan and Mikhail's Clinical Anesthesiology, 7e
Anesthetic Implications
1. Ventilator Management (Most Important Perioperative Issue)
Excessive mechanical ventilation causing respiratory alkalosis is a common intraoperative cause. For patients with COPD and chronic CO2 retention, ventilating to a "normal" PaCO2 induces acute metabolic alkalosis; the baseline PaCO2 goal should be estimated from the preoperative [HCO3-]:
Estimated baseline PaCO2 = ([HCO3-] - 24) × 10/3 + 40
Such patients may fail to wean from mechanical ventilation postoperatively if their PaCO2 is normalized intraoperatively. - Miller's Anesthesia, 10e
2. Cardiac Arrhythmias
The combination of alkalemia + hypokalemia is particularly dangerous and can precipitate severe atrial and ventricular arrhythmias. This is a key board point. Alkalemia itself shifts K+ intracellularly, worsening hypokalemia even if total body K+ is not severely depleted. - Morgan and Mikhail's Clinical Anesthesiology, 7e
3. Oxyhemoglobin Dissociation Curve
Alkalemia shifts the curve leftward (Bohr effect), increasing hemoglobin-oxygen affinity and impairing O2 release to tissues. This is particularly relevant in anemic or critically ill patients.
4. Hypokalemia
Alkalosis drives K+ into cells (H+ exits to buffer). Hypokalemia increases the risk of digitalis toxicity, neuromuscular block prolongation, and arrhythmias. Correct K+ before elective surgery when possible.
5. Ionized Calcium
Alkalemia increases albumin binding of calcium, reducing ionized (free) Ca2+. This can cause perioral numbness, tetany, and in severe cases cardiovascular instability.
6. Neuromuscular Blockade
Effects of alkalemia on neuromuscular blockers are inconsistent in the literature. Hypokalemia potentiates non-depolarizing blockade and reduces the effectiveness of reversal. - Morgan and Mikhail's Clinical Anesthesiology, 7e
7. Cerebral Blood Flow
Marked respiratory alkalosis reduces cerebral blood flow. If combined with hypotension, cerebral ischemia can result. - Morgan and Mikhail's Clinical Anesthesiology, 7e
8. Pyloric Stenosis (Classic Board Scenario)
Persistent vomiting causes hypochloremic, hypokalemic metabolic alkalosis. This is not a surgical emergency - it is a medical emergency. Correct the alkalosis, volume deficit, and electrolytes (IV NaCl + KCl) before induction of anesthesia. - Schwartz's Principles of Surgery, 11e
Treatment Summary
| Situation | Treatment |
|---|---|
| Chloride-sensitive | IV NaCl + KCl; H2-blockers if ongoing gastric loss |
| Chloride-resistant | Aldosterone antagonists (spironolactone) |
| Edematous patient who can't get volume | Acetazolamide (carbonic anhydrase inhibitor - reduces HCO3- reabsorption) |
| Severe alkalemia (pH >7.60) | IV HCl (0.1 mol/L), ammonium chloride (0.1 mol/L), arginine HCl, or hemodialysis |
| Controlled ventilation | Reduce minute ventilation to allow PaCO2 to rise toward patient's baseline |
| Post-hypercapnic alkalosis | Target patient-specific PaCO2, not 40 mm Hg |
HCl dose calculation: HCl deficit (mmol) = 0.2 × weight (kg) × (desired - measured [Cl-]). Infuse via central line at no more than 0.2 mEq/kg/hr.
Systematic ABG Interpretation (High-Yield for Boards)
- Look at pH - alkalemic (>7.45)?
- Look at HCO3- - elevated? → primary metabolic alkalosis
- Look at PaCO2 - is compensation appropriate? (Expected PaCO2 = 0.7 × [HCO3-] + 21 ± 2)
- Check for mixed disorder
- Look at urine Cl- to classify
Example: pH 7.54, PaCO2 48, HCO3- 40. Expected PaCO2 = (0.7 × 40) + 21 = 49. Compensation is appropriate → simple metabolic alkalosis with appropriate respiratory compensation. Check urine Cl- to guide treatment.
Perioperative Checklist Before Elective Surgery
- pH should ideally be <7.50 before elective procedures
- K+ >3.0 mEq/L (ideally >3.5)
- Ionized Ca2+ normal
- Urine Cl- to classify type
- Review medications: diuretics, steroids, mineralocorticoids
- For COPD patients: know the baseline PaCO2 and set ventilator to match it
Sources: - Miller's Anesthesia, 10e - Morgan and Mikhail's Clinical Anesthesiology, 7e - Barash, Cullen & Stoelting's Clinical Anesthesia, 9e
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