Why is this considered metabolic acidosis with a normal pH of 7.355, given the arterial blood gas (ABG) results showing a slightly low bicarbonate (HCO3) level of 19.8 mmol/L?

Understanding "Normal pH" Metabolic Acidosis

This represents a partially compensated metabolic acidosis where the pH remains in the lower normal range (7.355) due to respiratory compensation, but the underlying metabolic derangement is evident from the low bicarbonate (19.8 mmol/L).

Why This Is Metabolic Acidosis Despite "Normal" pH

The key to understanding this lies in recognizing that metabolic acidosis is defined by a primary reduction in serum bicarbonate (<22 mmol/L), not solely by pH below 7.35 1. Your patient's bicarbonate of 19.8 mmol/L is clearly below the normal range of 22-26 mmol/L (or 23-30 mEq/L by more recent standards) 1.

The Compensation Mechanism

• The body attempts to compensate for metabolic acidosis by increasing ventilation to eliminate CO2 1
• Your patient's PCO2 of 36.3 mmHg demonstrates this respiratory compensation—it's appropriately reduced from the normal 40 mmHg 2
• For every 1 mmol/L fall in bicarbonate, the PCO2 should decrease by approximately 1 mmHg 2
• With a bicarbonate drop of approximately 4 mmol/L (from 24 to 19.8), the expected PCO2 would be around 36 mmHg, which matches your patient's value 2

Clinical Significance

This "compensated" state does not mean the patient is fine—the underlying metabolic acidosis still requires investigation and treatment 1. The pH of 7.355 sits at the lower limit of normal (7.35-7.45), indicating the compensation is only partial and the patient remains vulnerable.

Critical Thresholds to Remember

• pH <7.35 with low bicarbonate definitively indicates metabolic acidosis 1
• pH 7.35-7.40 with low bicarbonate indicates compensated metabolic acidosis 1, 2
• The British Thoracic Society guidelines specifically note that pH <7.35 with normal or low PCO2 should prompt investigation for metabolic acidosis 3

Diagnostic Approach

Calculate the anion gap immediately as the first step in determining etiology: [Na+] - ([HCO3-] + [Cl-]) 4, 2. This distinguishes between:

• Elevated anion gap acidosis: ketoacidosis, lactic acidosis, toxins, renal failure 4, 2
• Normal anion gap (hyperchloremic) acidosis: diarrhea, renal tubular acidosis, early renal failure 4, 2

Additional Concerning Features in Your ABG

• PO2 of 23.8 mmHg is critically low (assuming this is arterial blood)—normal arterial PO2 is 80-100 mmHg
• SO2 of 37.2% indicates severe hypoxemia requiring immediate oxygen therapy 3
• These findings suggest tissue hypoxia which could be driving lactic acidosis 2

Management Priorities

Address the hypoxemia immediately while investigating the cause of metabolic acidosis 3:

1. Initiate oxygen therapy targeting SpO2 94-98% (or 88-92% if COPD risk) 3
2. Obtain complete metabolic panel, lactate, and serum ketones to determine acidosis etiology 5
3. Assess for tissue hypoperfusion: check blood pressure, heart rate, capillary refill, mental status 1

When to Treat with Bicarbonate

Bicarbonate therapy is NOT indicated at pH 7.355 5. The American Diabetes Association recommends bicarbonate only when pH falls below 6.9-7.0 5. At this pH level, focus on:

• Treating the underlying cause (fluid resuscitation if hypovolemic, insulin if DKA, addressing sepsis, etc.) 5, 1
• Metabolic acidosis typically resolves with correction of the underlying cause 1

Common Pitfall

Do not be falsely reassured by a "normal" pH—always evaluate bicarbonate and PCO2 together 1, 2. A patient can have significant metabolic derangement with life-threatening implications despite pH remaining in the 7.35-7.40 range due to respiratory compensation 2.