Acid-Base Disorders: Metabolic Acidosis, Alkalosis, and Mixed Disorders

Acid-base homeostasis is fundamental to cellular function, with normal arterial pH maintained between 7.35-7.45. [KEY_CONCEPT] The body maintains this narrow range through three primary mechanisms: chemical buffering (immediate), respiratory compensation (minutes to hours), and renal regulation (hours to days).

Metabolic acidosis occurs when there is either excess acid production, decreased acid excretion, or bicarbonate loss, resulting in primary decrease in serum HCO3- (<22 mEq/L) and compensatory hyperventilation. Metabolic alkalosis results from acid loss or bicarbonate retention, causing primary increase in serum HCO3- (>26 mEq/L) with compensatory hypoventilation.

[HIGH_YIELD] The anion gap is crucial for classifying metabolic acidosis:

• Normal anion gap (8-12 mEq/L): Hyperchloremic acidosis from bicarbonate loss
• High anion gap (>12 mEq/L): Unmeasured anions from organic acids

Henderson-Hasselbalch equation: pH = 6.1 + log([HCO3-]/(0.03 × PCO2))

[CLINICAL_PEARL] In chronic kidney disease, metabolic acidosis becomes increasingly common as GFR falls below 30 mL/min/1.73m², primarily due to decreased ammonia production and impaired acid excretion [1]. This contributes to bone disease, muscle wasting, and CKD progression.

Mixed disorders occur when multiple primary acid-base disturbances coexist, requiring systematic analysis of pH, PCO2, and HCO3- to identify each component. Common combinations include metabolic acidosis with respiratory alkalosis in salicylate poisoning or sepsis.

Metabolic Acidosis Symptoms:

• Respiratory: Kussmaul respirations (deep, rapid breathing)
• Cardiovascular: Hypotension, arrhythmias, decreased contractility
• Neurologic: Altered mental status, confusion, coma
• Gastrointestinal: Nausea, vomiting, abdominal pain

Metabolic Alkalosis Symptoms:

• Neurologic: Confusion, seizures, tetany, muscle cramps
• Cardiovascular: Arrhythmias (especially with hypokalemia)
• Respiratory: Hypoventilation, hypoxemia

[HIGH_YIELD] Clinical Assessment Priorities:

[CLINICAL_PEARL] Volume status assessment is critical in metabolic alkalosis:

• Saline-responsive: Volume depletion (vomiting, diuretics) - Urine Cl- <20 mEq/L
• Saline-resistant: Volume expansion (hyperaldosteronism) - Urine Cl- >40 mEq/L

Associated Findings:

• Diabetic ketoacidosis: Polyuria, polydipsia, weight loss, acetone breath
• Uremic acidosis: Fatigue, pruritus, pericarditis, bleeding
• Lactic acidosis: Hypotension, altered mental status, hyperlactatemia
• Hyperchloremic acidosis: Often associated with diarrhea or RTA

Systematic Analysis Algorithm:

1. Assess pH → Acidemia (<7.35) or Alkalemia (>7.45) ↓
2. Identify Primary Disorder • Low HCO3- → Metabolic Acidosis • High HCO3- → Metabolic Alkalosis
   • Low PCO2 → Respiratory Alkalosis • High PCO2 → Respiratory Acidosis ↓
3. Check for Appropriate Compensation • Metabolic Acidosis: Expected PCO2 = 1.5(HCO3-) + 8 ± 2 • Metabolic Alkalosis: Expected PCO2 = 0.7(HCO3-) + 20 ± 2 ↓
4. Calculate Anion Gap if Metabolic Acidosis • Normal AG (8-12): Hyperchloremic causes • High AG (>12): Organic acid accumulation ↓
5. Assess for Mixed Disorders

[HIGH_YIELD] Anion Gap Metabolic Acidosis (MUDPILES):

• Methanol, Uremia, Diabetic ketoacidosis
• Propylene glycol, Isoniazid, Lactic acidosis
• Ethylene glycol, Salicylates

Diagnostic Criteria Checklist:

□ Metabolic Acidosis:

• pH < 7.35
• HCO3- < 22 mEq/L
• Compensatory PCO2 decrease
• Calculate anion gap and osmolal gap

□ Metabolic Alkalosis:

• pH > 7.45
• HCO3- > 26 mEq/L
• Compensatory PCO2 increase
• Assess volume status and urine chloride

[CLINICAL_PEARL] Essential Laboratory Studies:

• Basic metabolic panel: Na+, Cl-, HCO3-, BUN, creatinine, glucose
• Arterial blood gas: pH, PCO2, PO2
• Additional tests: Lactate, ketones, albumin, phosphate
• Urine studies: pH, electrolytes, osmolality

Normal Anion Gap Acidosis Causes:

• Diarrhea (most common)
• Renal tubular acidosis (Types 1, 2, 4)
• Ureterosigmoidostomy
• Carbonic anhydrase inhibitor use
• Recovery phase of DKA

Treatment Algorithm by Disorder Type:

Metabolic Acidosis Management: ├── Treat Underlying Cause (Priority #1) ├── Severe Acidosis (pH < 7.1 or HCO3- < 5) │ ├── IV Sodium Bicarbonate (careful dosing) │ └── Consider Dialysis if renal failure └── Supportive Care ├── Fluid resuscitation ├── Electrolyte correction └── Respiratory support

Metabolic Alkalosis Management: ├── Saline-Responsive (Urine Cl- < 20) │ ├── Normal Saline infusion │ └── Correct hypokalemia/hypomagnesemia └── Saline-Resistant (Urine Cl- > 40) ├── Treat hyperaldosteronism └── Consider acetazolamide

[HIGH_YIELD] Bicarbonate Administration Guidelines:

• Indications: pH < 7.1, severe hyperkalemia, tricyclic overdose
• Dose calculation: HCO3- deficit = 0.5 × weight(kg) × (desired HCO3- - actual HCO3-)
• Administration: Give 1/2 calculated dose over 6-8 hours
• Monitoring: Repeat ABG in 30-60 minutes

[CLINICAL_PEARL] Bicarbonate therapy risks: Paradoxical CNS acidosis, volume overload, hypokalemia, overshoot alkalosis.

Specific Management Approaches:

[KEY_CONCEPT] Chronic Kidney Disease Management: For CKD patients with metabolic acidosis (HCO3- < 22 mEq/L), oral sodium bicarbonate 0.5-1.0 mEq/kg/day may slow CKD progression and reduce bone disease [1,6].

Emergency Interventions:

• Severe acidosis: Consider mechanical ventilation for respiratory compensation
• Severe alkalosis: Address volume depletion aggressively, correct electrolytes
• Mixed disorders: Treat each component systematically

Monitoring Parameters:

• Serial ABGs every 4-6 hours until stable
• Electrolytes (especially K+, Ca2+, Mg2+)
• Urine output and fluid balance
• Mental status and vital signs

Acute Complications of Untreated Disorders:

Severe Metabolic Acidosis (pH < 7.1):

• Cardiovascular collapse: Decreased myocardial contractility, vasodilation
• Hyperkalemia: Life-threatening arrhythmias
• Pulmonary edema: From compensatory hyperventilation and fluid shifts
• Coma and death: From severe CNS depression

Severe Metabolic Alkalosis (pH > 7.6):

• Seizures and tetany: From decreased ionized calcium
• Cardiac arrhythmias: Especially with concurrent hypokalemia
• Respiratory failure: From compensatory hypoventilation
• Paradoxical aciduria: Worsening electrolyte abnormalities

[HIGH_YIELD] Chronic Complications in CKD:

• Bone disease: Chronic acidosis promotes bone resorption and osteomalacia
• Muscle wasting: Protein catabolism and decreased protein synthesis
• Growth retardation: In pediatric patients with chronic acidosis
• CKD progression: Metabolic acidosis accelerates kidney function decline [1,6]

Prognostic Factors:

[CLINICAL_PEARL] Treatment Complications to Monitor:

• Overshoot alkalosis: From excessive bicarbonate administration
• Cerebral edema: Rapid pH correction in chronic acidosis
• Electrolyte shifts: Hypokalemia, hypophosphatemia during treatment
• Volume overload: From sodium-containing alkali therapy

Long-term Management in CKD:

• Regular monitoring of serum bicarbonate levels
• Oral alkali supplementation (sodium bicarbonate 1-3 g daily)
• Dietary protein restriction to reduce acid load
• Treatment of mineral bone disorder

Outcomes and Follow-up:

• Acute disorders: Most resolve with appropriate treatment of underlying cause
• Chronic acidosis: Requires ongoing monitoring and treatment
• CKD patients: Target serum HCO3- ≥ 22 mEq/L to slow progression [6]

[KEY_CONCEPT] Early recognition and treatment of acid-base disorders significantly improves outcomes, particularly in critically ill patients and those with advanced CKD.