ABG Interpretation in 4 Steps: ROME Method (NCLEX)

ABG in 4 steps: read this gas 0:00

0:00 pH, 7.26. PaCO₂, 58. Bicarb, 25. A patient after an opioid overdose is barely breathing.

0:11 What is this gas telling you? You'll read it in 4 steps. Then we'll test you on 3 gases. Ranges vary by lab, so always use your own lab's range.

Normal ABG values: pH, PaCO2, HCO3, PaO2 0:24

0:26 First, the four numbers on an arterial blood gas. pH: about 7.35 to 7.45. Below 7.35 is acidosis. Above 7.45 is alkalosis.

0:42 PaCO₂: about 35 to 45 mmHg. That's carbon dioxide, the acid the lungs control. Bicarb: about 22 to 26 mEq/L. That's the base the kidneys control.

0:59 PaO₂: about 80 to 100. That's oxygen, and we'll come back to it. The key idea: CO₂ acts as an acid, and bicarb is a base. More CO₂ pulls the pH down. More bicarb pushes it up.

ROME and the 4-step method 1:18

1:20 Now the 4 steps. Step 1: the pH. Acidosis, alkalosis, or normal? Step 2: the PaCO₂. High, low, or normal?

1:34 Step 3: the bicarb. High, low, or normal? Step 4: which one matches the pH? That's the cause.

1:44 Then check compensation. To find the match, remember ROME: Respiratory Opposite, Metabolic Equal. Respiratory opposite: in a respiratory problem, the pH and PaCO₂ move in opposite directions. CO₂ up, pH down: respiratory acidosis.

2:06 CO₂ down, pH up: respiratory alkalosis. Metabolic equal: in a metabolic problem, the pH and bicarb move in the same direction. Bicarb down, pH down: metabolic acidosis. Bicarb up, pH up: metabolic alkalosis. Shortcut: label each value acid or base, and find the one that agrees with the pH.

Compensation: uncompensated, partial, full 2:34

2:37 Then the body fights back: compensation. If the lungs caused the problem, the kidneys adjust bicarb, slowly, over days. If it's metabolic, the lungs adjust CO₂ by breathing faster or slower, and they respond faster. Uncompensated: the other value is still normal.

2:58 The body hasn't responded yet. Partially compensated: the other value has moved to fight back, but the pH is still abnormal. Fully compensated: both values are abnormal, but the pH is back in the normal range. With a normal pH, use 7.40 as the midpoint.

3:19 Below it leans acid, above it leans base, and the lean points to the original problem. In real patients, full compensation is uncommon. Most compensation brings the pH only partway back. So a normal pH with two very abnormal values can also mean two problems at once: a mixed disorder.

Causes: acidosis and alkalosis for nurses and medics 3:41

3:44 Next, the causes. Start with breathing. Respiratory acidosis: the patient isn't breathing off enough CO₂. That's hypoventilation.

3:55 Think opioid or sedative overdose, COPD, a severe asthma attack, nerve and muscle weakness, severe obesity, and sleep apnea. Respiratory alkalosis: breathing off too much CO₂. That's hyperventilation. Think anxiety or panic, pain, fever, sepsis, hypoxia, pregnancy, and aspirin overdose.

4:23 Metabolic acidosis: too much acid, or too little bicarb. Think DKA, lactic acidosis from shock or sepsis, kidney failure, severe diarrhea, and poisonings like aspirin. The lungs compensate with rapid, deep breathing to blow off CO₂. Metabolic alkalosis: too much bicarb, or acid lost.

4:48 Think vomiting, NG suction, diuretics, low potassium, and too many antacids. A memory hook: vomiting loses stomach acid, so alkalosis. Diarrhea loses bicarb, so acidosis.

Oxygenation: PaO2 and saturation 5:04

5:07 Oxygen is a separate question. pH, CO₂, and bicarb are acid and base. PaO₂ is oxygen. On room air, PaO₂ is normally about 80 to 100.

5:20 Below that is hypoxemia, and it can sit right next to a normal pH. Check the saturation too, normally about 94 to 100%, and note how much oxygen the patient was on when the gas was drawn. Some COPD patients have a lower saturation target, often 88 to 92%. Follow the order and your protocol.

ICU depth: Winter’s formula and anion gap 5:44

5:47 Optional ICU depth: two more tools. In metabolic acidosis, Winter's formula predicts the PaCO₂: 1.5 × bicarb, + 8, ± 2. If the real PaCO₂ is higher than that range, there's a respiratory acidosis too. The patient may be tiring.

6:09 If it's lower than that range, there's a respiratory alkalosis too. Second, the anion gap: sodium, minus chloride plus bicarb. A high gap points to added acid, like ketones, lactate, or kidney failure. A normal gap points to lost bicarb, like diarrhea.

Practice: 3 NCLEX-style ABGs 6:30

6:32 Now you try. Pause after each gas. Gas 1: the opioid overdose from the start. pH, 7.26. PaCO₂, 58. Bicarb, 25.

6:49 Pause the video, and try it. pH 7.26 is acidosis. PaCO₂ 58 is high, which is acid. Bicarb 25 is normal.

7:06 The PaCO₂ matches, so it's respiratory. Bicarb is still normal: uncompensated respiratory acidosis. The priority is airway and breathing, and reversing the opioid per protocol. Gas 2: type 1 diabetes, high glucose, fast deep breathing. pH, 7.29. PaCO₂, 30. Bicarb, 14.

7:34 Pause, and try it. pH 7.29 is acidosis. Bicarb 14 is low, which is acid, and it matches. So it's metabolic.

7:49 PaCO₂ 30 is low: the lungs are blowing off CO₂ to compensate. The pH is still abnormal: partially compensated metabolic acidosis. ICU check: Winter's predicts 29, ± 2. 30 fits: the lungs are keeping up.

8:09 Gas 3: long standing COPD, at baseline. pH, 7.36. PaCO₂, 58. Bicarb, 32.

8:22 Pause, and try it. pH 7.36 is normal, but below 7.40, so it leans acid. PaCO₂ 58 is high, which is acid, and matches the lean. Bicarb 32 is high: the kidneys compensating.

8:44 Normal pH, both values abnormal: fully compensated respiratory acidosis. The kidneys had time to catch up.

Recap 8:54

8:56 Recap: pH first, then PaCO₂ and bicarb. ROME finds the cause, the other value shows compensation, and oxygen gets its own look. Practice free with RN Basics, ICU Basics, and Medic Basics.