Acid Base (All About That Base)

Acid-base disorders become readable when you anchor on pH, PaCO2, serum bicarbonate, and the anion gap in a fixed sequence. A normal pH does not exclude serious mixed metabolic and respiratory processes, and an anion gap of 20 or more should trigger a focused search for occult acidosis.

Systematic Blood Gas Interpretation

• Primary process framing: Acidemia and alkalemia describe the net pH effect, while acidosis and alkalosis name the underlying process seen in PaCO2 and bicarbonate; keeping those terms separate prevents early misclassification.
• Four-step interpretation sequence: Start with pH, then pair PaCO2 with serum bicarbonate, then calculate the anion gap, then use the delta gap to uncover hidden mixed disorders. We walk through the sequence in the episode.
• Serum bicarbonate preference: The bicarbonate on the metabolic panel is the value to trust; blood-gas bicarbonate is calculated and can mislead the entire interpretation if you build from it.
• Mixed disorder mindset: Do not assume one primary disturbance: critically ill patients can carry as many as three simultaneous acid-base processes, especially when the pH looks deceptively normal.
• Compensation has limits: The body never fully compensates for a primary acid-base disorder, so a normal pH in the setting of an anion-gap acidosis implies at least one additional process.

Anion Gap and Hidden Acidosis

• High-gap trigger point: An anion gap of 20 or greater is a meaningful red flag for true anion-gap metabolic acidosis, not just biologic noise, and should prompt an immediate etiologic workup.
• High-yield gap labs: When the gap is elevated in a sick undifferentiated patient, the first send-outs are lactate, salicylate, acetaminophen, serum osmolality, and ketones.
• KULT differential anchor: KULT—ketones, uremia, lactate, toxins—covers the common ED causes of anion-gap acidosis and keeps the differential practical under pressure.
• Delta gap purpose: The delta gap asks whether the fall in bicarbonate matches the rise in the gap, exposing a concomitant metabolic alkalosis or non-gap acidosis without relying on gestalt alone.
• Normal pH warning: A patient can have normal pH, normal PaCO2, and normal bicarbonate yet still harbor a clinically important anion-gap acidosis; that paradox is worth hearing in the chapter.

ABG Versus VBG Pearls

• VBG usual role: A VBG is appropriate for most emergency acid-base questions because the pH and PaCO2 trend closely enough to identify the primary disorder in routine practice.
• Respiratory acidosis screen: A VBG PCO2 below 45 effectively rules out clinically important respiratory acidosis, giving you a fast bedside screen before committing to arterial sampling.
• When ABG matters: If you need an accurate PaO2 or tightly precise PaCO2, get an ABG; neurotrauma is a common setting where that extra precision changes management.

References:

1. Gabow PA, et al. Diagnostic importance of an increased serum anion gap. N Engl J Med. 1980;303(15):854-858. PMID: 6774247
2. Kelly AM. Can VBG analysis replace ABG analysis in emergency care?. Emerg Med J. 2016;33(2):152-154. PMID 25552544