Arterial blood gas acid-base patterns
An arterial blood gas is a blood sample from an artery, read by looking at the pH, then the carbon dioxide, then the bicarbonate, to show the acid-base status.
Remember
A low pH with raised carbon dioxide and a normal or only slightly raised bicarbonate is an acute respiratory acidosis (carbon dioxide has risen and the pH has fallen), and once the bicarbonate is clearly raised, the kidneys have had days to respond.
An arterial blood gas is a sample of blood taken from an artery, usually the radial artery at the wrist. A machine reports the pH, the partial pressure of oxygen (pO2) and of carbon dioxide (pCO2), and a bicarbonate value. Partial pressure is the pressure a dissolved gas exerts, so it shows how much of that gas is dissolved in the blood. Sampling an artery can be painful and difficult, so it is reserved for those patients who are quite unwell.
Work through the gas in order. Start with the pH: the normal range is 7.35 to 7.45, and a raised pH means the blood is too alkaline. Next look at the carbon dioxide, because it forms carbonic acid in water. In a respiratory acidosis there is a primary rise in carbon dioxide, usually from hypoventilation (breathing too little to clear it), and the pH falls before the kidneys respond. Blowing off carbon dioxide removes acid and raises the pH, giving a respiratory alkalosis, typically from hyperventilation (breathing faster than the body needs).
Then look at the bicarbonate to see whether the kidneys have had time to respond. The kidneys, which adjust bicarbonate, take days to respond, so a normal bicarbonate points to a recent change. With raised carbon dioxide, a clearly raised bicarbonate shows that the carbon dioxide has been high for longer.
- Acute respiratory acidosis: low pH, raised carbon dioxide, normal or only slightly raised bicarbonate.
- Acute respiratory alkalosis: raised pH, low carbon dioxide, normal or only slightly low bicarbonate.
- Chronic respiratory acidosis: pH near normal, raised carbon dioxide, raised bicarbonate.
- A metabolic acidosis begins with a low bicarbonate, and the lungs normally lower the carbon dioxide by breathing faster. A mixed respiratory and metabolic acidosis shows a low pH, raised carbon dioxide and reduced bicarbonate.
Key facts
- Order of reading
- pH first, then carbon dioxide, then bicarbonate.
- What a blood gas reports
- pH, pO2 and pCO2, with the bicarbonate usually worked out by the machine from the pH and pCO2.
- Acute respiratory alkalosis
- a raised pH, low pCO2 and normal or only slightly low bicarbonate, typically from hyperventilation, before the kidneys have time to respond.
- Acute respiratory acidosis
- a low pH, raised pCO2 and normal or only slightly raised bicarbonate.
- Mixed respiratory and metabolic acidosis
- a low pH, raised pCO2 and reduced bicarbonate, because both abnormalities push the pH down.
- Acute on chronic respiratory acidosis
- a low pH, raised pCO2 and raised bicarbonate suggest a long-standing problem with a recent rise in carbon dioxide.
Exam traps
- A normal bicarbonate does not fit a chronic respiratory alkalosis, because the kidneys would have had two to three days to respond and the bicarbonate would be reduced.
- A normal pH with raised carbon dioxide and raised bicarbonate is not an acute respiratory acidosis, because it points to a chronic one, with several days of kidney response.
- A reduced bicarbonate does not fit a pure respiratory acidosis, because in that disorder the bicarbonate is normal or raised, and a low bicarbonate shows a second, metabolic process.
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