How is metabolic acidosis expressed?

Metabolic Acidosis: Clinical Expression and Diagnostic Parameters

Metabolic acidosis is characterized by decreased blood pH (<7.35), reduced serum bicarbonate (<22 mmol/L), elevated blood lactate levels, and increased base deficit, which are key laboratory parameters that express the condition clinically. 1

Laboratory Parameters of Metabolic Acidosis

• Decreased blood pH (<7.35) is the primary expression of metabolic acidosis, reflecting the accumulation of acids in the bloodstream 1
• Reduced serum bicarbonate (<22 mmol/L) indicates depletion of the body's buffer system in response to acid accumulation 1, 2
• Elevated blood lactate levels (>2 mmol/L) often accompany metabolic acidosis, particularly in shock states, and correlate with mortality 1
• Increased base deficit is an independent predictor of mortality, especially in traumatic hemorrhagic shock, reflecting global tissue acidosis 1
• Anion gap calculation ([Na+] - ([HCO3-] + [Cl-])) helps classify metabolic acidosis into normal (hyperchloremic) or elevated anion gap types 3

Clinical Manifestations

• Early onset metabolic acidosis is commonly manifested by a low anaerobic threshold (AT) during cardiopulmonary exercise testing 4
• In shock states, metabolic acidosis presents with signs of tissue hypoperfusion and can be monitored through serial measurements of lactate, base deficit, and pH 1
• Severity of acidosis generally correlates with the degree of underlying condition (e.g., renal failure, shock) 5
• Plasma bicarbonate concentrations typically range from 12 to 22 mmol/L in chronic kidney disease-related acidosis 5

Diagnostic Approach

• The first step in diagnosing metabolic acidosis is determining the presence or absence of an anion gap 6
• Additional diagnostic parameters include:
  • Urine pH assessment to help differentiate causes 6
  • Serum potassium levels which may provide diagnostic clues 6
  • Osmolal gap calculation in cases of suspected toxic ingestions 6, 7
• Arterial blood gas analysis confirms the diagnosis by showing low pH with compensatory decrease in PaCO2 of approximately 1 mmHg for every 1 mmol/L fall in serum bicarbonate 3

Monitoring in Special Circumstances

• In cardiopulmonary exercise testing, when anaerobic threshold cannot be determined noninvasively, blood samples for lactate or standard bicarbonate are recommended 4
• For shock patients, the American College of Emergency Physicians recommends serial measurements of lactate, base deficit, and pH to assess severity and response to treatment 1
• In patients with chronic kidney disease, metabolic acidosis typically appears when glomerular filtration rate decreases to less than 20-25% of normal 5

Pitfalls to Avoid

• Avoid attempting full correction of low total CO2 content during the first 24 hours of therapy, as this may lead to unrecognized alkalosis due to delayed ventilatory adjustment 8
• Be cautious with rapid infusion of large quantities of bicarbonate, as bicarbonate solutions are hypertonic and may produce an undesirable rise in plasma sodium concentration 8
• Remember that anion gap can be normal or only moderately increased even with stage 4-5 chronic kidney disease 5
• Consider that metabolic acidosis categories (normal and elevated anion gap) can overlap in complex clinical scenarios 3