Interpreting Arterial Blood Gases (ABGs)

20 vocabulary-style flashcards covering core terms and concepts required to interpret arterial blood gases as presented in the lecture.

pH (normal range)

Measure of blood acidity/alkalinity; normal is 7.35 – 7.45.

Acidosis

Condition in which blood pH falls below 7.35.

Alkalosis

Condition in which blood pH rises above 7.45.

Primary questions for ABG interpretation

1) What’s wrong? (Did pH change?) 2) What caused it? (CO₂ or HCO₃⁻?) 3) What’s the body doing about it? (Compensation).

PaCO₂ (normal range)

Partial pressure of carbon dioxide; normal is 35 – 45 mm Hg.

Inverse CO₂–pH relationship

↑ PaCO₂ → ↓ pH; ↓ PaCO₂ → ↑ pH.

HCO₃⁻ / Bicarbonate (normal range)

Renal-produced base that buffers acids; normal is 22 – 26 mEq/L.

Direct HCO₃⁻–pH relationship

↑ HCO₃⁻ → ↑ pH; ↓ HCO₃⁻ → ↓ pH.

Metabolic change

pH alteration produced primarily by bicarbonate (HCO₃⁻).

Respiratory change

pH alteration produced primarily by carbon dioxide (PaCO₂).

Metabolic acidosis

Low pH caused by decreased HCO₃⁻ (< 22 mEq/L).

Metabolic alkalosis

High pH caused by increased HCO₃⁻ (> 26 mEq/L).

Respiratory acidosis

Low pH caused by elevated PaCO₂ (> 45 mm Hg).

Respiratory alkalosis

High pH caused by decreased PaCO₂ (< 35 mm Hg).

Compensation

Physiologic attempt by the non-primary system (lungs or kidneys) to return pH toward normal.

Uncompensated state

Body’s opposing system has not corrected the pH; pH remains outside normal range.

Partially compensated state

Opposing system is changing (CO₂ or HCO₃⁻ altered) but pH is still abnormal.

Fully compensated state

Opposing system has corrected pH back to normal, though CO₂ or HCO₃⁻ remains abnormal.

“Which moved the most?” rule

The value (CO₂ or HCO₃⁻) that deviates furthest from normal is the primary cause of the pH change.

Purpose of ABG analysis

Determine acid-base status to assess organ function, guide ventilation, and monitor therapeutic response.