How to Interpret ABGs: A Practical Guide for ICU Nurses

Why ABG Interpretation Matters in the ICU

Arterial blood gases are one of the most ordered labs in the ICU — and for good reason. They give you a real-time snapshot of your patient's respiratory and metabolic status. If you can read an ABG quickly and accurately, you can anticipate interventions before the physician even calls.

The 5-Step ABG Interpretation Method

Use this systematic approach every time so you never miss a step:

Step 1: Check the pH

Normal pH is 7.35–7.45. Below 7.35 = acidosis. Above 7.45 = alkalosis. This tells you the overall state of the patient.

Step 2: Look at the PaCO2

Normal PaCO2 is 35–45 mmHg. PaCO2 is controlled by the lungs. If it's high, the patient is retaining CO2 (hypoventilating). If it's low, they're blowing off CO2 (hyperventilating).

Step 3: Check the HCO3

Normal HCO3 is 22–26 mEq/L. HCO3 is controlled by the kidneys. Low HCO3 = metabolic acidosis. High HCO3 = metabolic alkalosis.

Step 4: Determine the Primary Problem

Match the pH to either the PaCO2 or HCO3 to identify whether the primary problem is respiratory or metabolic.

  • Respiratory acidosis: pH low + PaCO2 high
  • Respiratory alkalosis: pH high + PaCO2 low
  • Metabolic acidosis: pH low + HCO3 low
  • Metabolic alkalosis: pH high + HCO3 high

Step 5: Check for Compensation

Is the body trying to correct the imbalance? If the pH is abnormal but the other system is moving in the opposite direction, compensation is occurring. Full compensation = pH back in normal range. Partial compensation = pH still abnormal but the other system is responding.

Quick Reference: Common ICU ABG Scenarios

  • COPD exacerbation: pH 7.30 / PaCO2 60 / HCO3 30 → Respiratory acidosis with metabolic compensation
  • Sepsis (early): pH 7.50 / PaCO2 28 / HCO3 22 → Respiratory alkalosis (hyperventilation)
  • DKA: pH 7.20 / PaCO2 28 / HCO3 10 → Metabolic acidosis with respiratory compensation

Build Your Critical Thinking Skills

ABG interpretation is just one piece of the puzzle. To truly think like an ICU nurse, you need to connect the lab values to your patient's clinical picture — their vitals, vent settings, medications, and history.

If you want a deeper dive into ICU critical thinking, check out Learn to Think Like an ICU Nurse — a practical handbook built for nurses who want to move from task-based care to true clinical reasoning.

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