Physiological adaptation

Arterial blood gases: a three-step method that works every time

Read pH, then PaCO2, then HCO3, match the one that explains the pH, and decide whether the body is compensating. Includes the causes the exam likes.

9 min read·foundation·

Reviewed by a licensed nurse · 25 October 2026

The four basic patterns

Pattern	pH	PaCO2	HCO3	Typical causes
Respiratory acidosis	Low	High	Normal (high if compensating)	Hypoventilation: COPD, opioid or sedative overdose, neuromuscular weakness, airway obstruction
Respiratory alkalosis	High	Low	Normal (low if compensating)	Hyperventilation: anxiety, pain, fever, early pulmonary embolism, mechanical overventilation
Metabolic acidosis	Low	Normal (low if compensating)	Low	Diabetic ketoacidosis, lactic acidosis, diarrhea, renal failure
Metabolic alkalosis	High	Normal (high if compensating)	High	Vomiting, gastric suction, diuretics, excess antacids

The lungs compensate within minutes to hours. The kidneys take days.

01
Look at the pH
Below 7.35 is acidosis. Above 7.45 is alkalosis. Remember which side of 7.40 it sits on even when it is within range.
02
Find the number that explains it
In acidosis, a high PaCO2 means respiratory and a low HCO3 means metabolic. In alkalosis, a low PaCO2 means respiratory and a high HCO3 means metabolic.
03
Check the other number for compensation
If it has moved the opposite way to correct the pH, the body is compensating. If the pH is back in range, it is fully compensated. If the pH is still abnormal, it is partly compensated.
04
Ask what the client needs
Match the pattern to the client. Support the airway and breathing first, then treat the cause.

·Respiratory acidosis: drowsy, headache, shallow slow breathing. Support ventilation, reverse the sedative if prescribed.
·Respiratory alkalosis: tingling around the mouth, light-headedness, fast breathing. Coach slow breathing, treat pain and anxiety.
·Metabolic acidosis: deep, rapid breathing (Kussmaul) as the lungs blow off CO2. Treat the cause, such as insulin and fluids in ketoacidosis.
·Metabolic alkalosis: slow shallow breathing, muscle cramps, tetany. Replace losses, stop the cause.

Check yourself

1. A client has pH 7.28, PaCO2 56 mmHg and HCO3 24 mEq/L. What is the acid-base status?
2. A client with diabetic ketoacidosis has deep, rapid breathing. Which result fits?
3. A client has had continuous gastric suction for two days. Which finding is most expected?