LessonsPhysiological Adaptation

Reading a blood gas

Name the disturbance from a set of arterial blood gas values, say whether it is compensated, and connect each pattern to the clients who produce it.

Three numbers, two questions

Look at the pH, the carbon dioxide and the bicarbonate, and ask two questions in order.

Is the pH low or high? Below 7.35 is acidosis, above 7.45 is alkalosis. That is the direction.
Which of the other two has moved the SAME way as the pH? Carbon dioxide moving with it makes it respiratory; bicarbonate moving with it makes it metabolic.

Carbon dioxide is an acid, so a raised carbon dioxide lowers the pH. Bicarbonate is a base, so a raised bicarbonate raises it. Normal ranges: pH 7.35 to 7.45, carbon dioxide 35 to 45 mmHg, bicarbonate 22 to 26 mEq/L.

Compensation

The system that did not cause the problem tries to correct it, and how far it has got is what "compensated" describes.

Uncompensated: the other value is still normal. Nothing has responded yet.
Partially compensated: the other value has moved to oppose the change, but the pH is still outside the range.
Fully compensated: the other value has moved and the pH is back inside the range, though usually on the side it came from.

The lungs compensate within minutes; the kidneys take one to three days. So a fully compensated respiratory problem has been going on for days, and that alone tells you something about the client.

Who produces which

Respiratory acidosis: anything that stops carbon dioxide being blown off — an opioid overdose, a COPD exacerbation, atelectasis, a chest that cannot move.
Respiratory alkalosis: anything that blows off too much — anxiety and hyperventilation, pain, early sepsis, high altitude.
Metabolic acidosis: acid gained or base lost — ketoacidosis, lactic acidosis from shock, kidney failure, severe diarrhea.
Metabolic alkalosis: acid lost or base gained — prolonged vomiting, nasogastric suction, excessive antacids, diuretics.

Vomiting and diarrhea sit on opposite sides, and it is worth knowing why: vomiting loses stomach acid, while diarrhea loses the bicarbonate-rich fluid further down.

Worth remembering

The value that moved WITH the pH names the cause: carbon dioxide respiratory, bicarbonate metabolic.
Lungs compensate in minutes, kidneys in days — so full compensation means the problem is old.
Vomiting causes alkalosis; diarrhea causes acidosis.
Fully compensated still means the pH sits on the side it came from.

Practice this now Next: Recognizing shock before the pressure falls