Acid–base disorders and compensation
1Why this matters
Maya, 19, has type 1 diabetes and has missed insulin doses during a stomach bug. She arrives breathing deeply and quickly, with a fruity smell on her breath. Her arterial blood gas reads pH 7.21, PaCO2 18 mm Hg, bicarbonate 7 mmol/L. Three numbers tell you her blood is dangerously acidic, that fixed acid caused it, and that her lungs are already fighting back. This lesson teaches you to read them.
2What this builds on
3Quick check before you start
1. Bicarbonate is 12 mmol/L and PCO2 is 20 mm Hg. What is the ratio of bicarbonate to dissolved carbon dioxide?
- 20, giving pH 7.40
- 10, giving pH 7.10
- 0.6, giving a pH below 6
Show the answer
Dissolved carbon dioxide = 0.03 × 20 = 0.6 mmol/L, and 12 ÷ 0.6 = 20. pH depends on the ratio, so it is 7.40 even though both numbers are abnormal.
- Correct: 20, giving pH 7.40:
- 10, giving pH 7.10:
- 0.6, giving a pH below 6:
2. What defines hypoventilation?
- A breathing rate below 12 a minute
- Breathing less than carbon dioxide production requires, so PCO2 rises above 45 mm Hg
- Any breathing too shallow to be seen
Show the answer
Hypoventilation is defined by carbon dioxide, not by rate: alveolar ventilation too low for the carbon dioxide being made, so arterial PCO2 rises above 45 mm Hg.
- A breathing rate below 12 a minute:
- Correct: Breathing less than carbon dioxide production requires, so PCO2 rises above 45 mm Hg:
- Any breathing too shallow to be seen:
3. For every hydrogen ion a parietal cell secretes into the stomach, what enters the blood?
- A chloride ion
- A potassium ion
- A bicarbonate ion
Show the answer
Carbonic anhydrase in the parietal cell makes H+ and HCO3−. The H+ goes into the stomach and the HCO3− into the blood: the alkaline tide.
- A chloride ion:
- A potassium ion:
- Correct: A bicarbonate ion:
4How it works, step by step
- With almost no insulin, lipolysis runs unchecked and the liver makes ketone bodies faster than tissues can burn them.Ketone bodies release hydrogen ions into the blood.
- Bicarbonate takes up the hydrogen ions and is used up, while the ketone body anions stay in the plasma.Bicarbonate falls, the anion gap rises, and the bicarbonate to carbon dioxide ratio drops: a high anion gap metabolic acidosis.
- The low ratio lowers arterial pH.The carotid bodies sense the fall and drive the respiratory centers harder.
- Deep, fast Kussmaul breathing blows off carbon dioxide.PaCO2 falls in the same direction as bicarbonate, so the ratio moves back toward 20: respiratory compensation.
- Compensation cannot replace the bicarbonate the ketone bodies used up.pH stays below 7.35, partially compensated, until insulin stops the ketone body production and the kidneys rebuild bicarbonate.
5Core concepts
6A common mistake
The wrong idea: Compensation fixes an acid–base disorder, so a normal pH means the problem is over.
What actually happens: Compensation only moves the other number in the same direction as the primary one, so the ratio moves back toward 20. The primary number stays abnormal until its cause is treated, and compensation rarely brings pH all the way to normal. A pH within 7.35 to 7.45 with an abnormal PaCO2 and bicarbonate still means a disorder is present.
7Check yourself
Anything you miss goes into your review queue.
1. An arterial blood gas shows pH 7.26, PaCO2 60 mm Hg and HCO3− 26 mmol/L. How do you classify it?
- Partially compensated respiratory acidosis
- Uncompensated respiratory acidosis
- Uncompensated metabolic acidosis
- Partially compensated metabolic alkalosis
Show the answer
pH 7.26 is acidemia. PaCO2 60 is high, which pushes toward acid and matches the pH, so the disorder is respiratory acidosis. Bicarbonate 26 is still within 22 to 26, so the kidneys have not compensated yet.
- Partially compensated respiratory acidosis: Partial compensation would need bicarbonate above 26, moving up along with PaCO2. Here it is still normal.
- Correct: Uncompensated respiratory acidosis: Correct. High PaCO2 explains the low pH, and bicarbonate is still normal: uncompensated.
- Uncompensated metabolic acidosis: A metabolic acidosis needs bicarbonate below 22. Here bicarbonate is normal and PaCO2 is what changed.
- Partially compensated metabolic alkalosis: The pH is low, not high, so this cannot be an alkalosis.
2. Maya, 19, has type 1 diabetes and has missed insulin doses during a stomach bug. She is breathing deeply and fast. Her ABG: pH 7.21, PaCO2 18 mm Hg, HCO3− 7 mmol/L. How do you classify it?
- Partially compensated metabolic acidosis
- Uncompensated respiratory alkalosis
- Fully compensated metabolic acidosis
- Partially compensated respiratory acidosis
Show the answer
pH 7.21 is acidemia. Bicarbonate 7 is very low and matches the acidemia, so the primary disorder is metabolic acidosis. PaCO2 18 has fallen in the same direction, which is her lungs compensating, but pH is still below 7.35: partially compensated.
- Correct: Partially compensated metabolic acidosis: Correct. Low bicarbonate explains the low pH, and the low PaCO2 is her respiratory compensation.
- Uncompensated respiratory alkalosis: A low PaCO2 alone would raise pH. Her pH is low, so the low PaCO2 is a response, not the cause.
- Fully compensated metabolic acidosis: Full compensation would bring pH back to 7.35 to 7.45. Hers is still 7.21.
- Partially compensated respiratory acidosis: Respiratory acidosis needs a high PaCO2. Hers is low.
3. A patient with metabolic acidosis has plasma Na+ 138, Cl− 100 and HCO3− 10 mmol/L. What is the anion gap, and what does it suggest?
- 12 mmol/L: normal, so bicarbonate has been lost
- 28 mmol/L: high, so an acid has been added
- 48 mmol/L: high, so an acid has been added
- 28 mmol/L: normal for a patient in acidosis
Show the answer
Anion gap = Na+ − (Cl− + HCO3−) = 138 − (100 + 10) = 28 mmol/L, well above the normal 8 to 12. An unmeasured anion has taken bicarbonate's place, so an acid has been added, as in ketoacidosis or lactic acidosis.
- 12 mmol/L: normal, so bicarbonate has been lost: 12 would be the gap with a normal bicarbonate of 26. Here bicarbonate is 10, so the gap is 28.
- Correct: 28 mmol/L: high, so an acid has been added: Correct. 138 − 110 = 28: a high anion gap metabolic acidosis.
- 48 mmol/L: high, so an acid has been added: 48 comes from adding bicarbonate instead of subtracting it: 138 − 100 + 10.
- 28 mmol/L: normal for a patient in acidosis: An acidosis does not change the normal range. A gap of 28 is high whatever the pH.
4. Mrs. Duarte has vomited stomach contents repeatedly for three days and kept little down. Predict the change in each variable compared with before she became ill.
| Variable | Change |
|---|---|
| Plasma bicarbonate | — |
| Blood pH | — |
| Arterial PaCO2 | — |
| Plasma chloride | — |
| Plasma potassium | — |
| Blood volume | — |
| Aldosterone | — |
Show the answer
Vomiting removes acid and leaves bicarbonate behind, a metabolic alkalosis. The lungs partly compensate by breathing less. Loss of volume, chloride and potassium keeps the kidneys reclaiming bicarbonate instead of excreting it, which is why saline and potassium chloride correct it.
- Plasma bicarbonate: up. Each H+ her parietal cells secreted sent a bicarbonate into the blood. With the acid vomited, nothing neutralizes that bicarbonate, so it accumulates.
- Blood pH: up. Higher bicarbonate at first with the same PaCO2 raises the bicarbonate to carbon dioxide ratio, so pH rises.
- Arterial PaCO2: up. The higher pH slows breathing, so carbon dioxide builds up: respiratory compensation.
- Plasma chloride: down. Vomit carries hydrochloric acid, so chloride is lost with the hydrogen ions.
- Plasma potassium: down. Potassium is lost in the vomit and, as aldosterone rises, in the urine.
- Blood volume: down. She is losing fluid and taking little in.
- Aldosterone: up. Low blood volume activates the renin–angiotensin–aldosterone system.
5. Put the steps of reading an arterial blood gas in order.
- Classify the pH as acidemia, alkalemia or within range
- Decide which way PaCO2 pushes the pH
- Decide which way bicarbonate pushes the pH
- Name the primary disorder from the number that matches the pH
- Judge compensation from the other number and the pH
Show the answer
Start with the pH, because it tells you what the disorder must explain. Then check PaCO2 and bicarbonate, name the primary disorder from whichever matches the pH, and only then judge whether the other number has moved to compensate.
- Correct order: 1. Classify the pH as acidemia, alkalemia or within range 2. Decide which way PaCO2 pushes the pH 3. Decide which way bicarbonate pushes the pH 4. Name the primary disorder from the number that matches the pH 5. Judge compensation from the other number and the pH
6. Two hours after a sedative overdose, a woman's PaCO2 is 70 mm Hg, yet her bicarbonate is nearly normal. Why hasn't her bicarbonate risen to compensate?
- The kidneys need 3 to 5 days to reach full compensation
- Sedatives block carbonic anhydrase in the tubule cells
- Compensation for high PaCO2 comes from the lungs, not the kidneys
- High PaCO2 makes the kidneys excrete bicarbonate
Show the answer
Renal compensation starts within hours but builds over days, as tubule cells step up H+ secretion and ammonium production. Two hours in, only a small rise from buffering has happened.
- Correct: The kidneys need 3 to 5 days to reach full compensation: Correct. Renal compensation is slow: full effect takes 3 to 5 days.
- Sedatives block carbonic anhydrase in the tubule cells: Sedatives act on the brain's respiratory centers. They do not block carbonic anhydrase.
- Compensation for high PaCO2 comes from the lungs, not the kidneys: The lungs are the source of this problem, so they cannot correct it. Respiratory disorders are compensated by the kidneys.
- High PaCO2 makes the kidneys excrete bicarbonate: High PaCO2 makes tubule cells secrete more H+ and keep more bicarbonate, not excrete it.
7. During resuscitation from cardiac arrest, a man's ABG shows pH 7.11, PaCO2 55 mm Hg and HCO3− 17 mmol/L. What does this show?
- Partially compensated respiratory acidosis
- Partially compensated metabolic acidosis
- Two acidoses: respiratory and metabolic
- Fully compensated metabolic acidosis
Show the answer
High PaCO2 pushes toward acid, and low bicarbonate also pushes toward acid. Compensation always moves the second number in the same direction as the first; here they move in opposite directions, so both are primary. Poor breathing raises PaCO2, and lactic acid from poorly perfused tissues lowers bicarbonate.
- Partially compensated respiratory acidosis: Renal compensation for a respiratory acidosis would raise bicarbonate, not lower it.
- Partially compensated metabolic acidosis: Respiratory compensation for a metabolic acidosis would lower PaCO2, not raise it.
- Correct: Two acidoses: respiratory and metabolic: Correct. Two acidoses together, which is why the pH is so low.
- Fully compensated metabolic acidosis: Full compensation needs a pH of 7.35 to 7.45. His is 7.11.
8. A man with long-standing COPD has an ABG of pH 7.37, PaCO2 56 mm Hg and HCO3− 31 mmol/L. How do you classify it?
- Fully compensated respiratory acidosis
- No acid–base disorder
- Fully compensated metabolic alkalosis
- Partially compensated respiratory acidosis
Show the answer
The pH is within range but on the acid side of 7.40. High PaCO2 pushes toward acid and matches that side, so it is the primary disorder. Bicarbonate has risen with it, from days of renal compensation, and the pH is back in range: fully compensated respiratory acidosis.
- Correct: Fully compensated respiratory acidosis: Correct. On exams this is fully compensated; in practice, compensation this complete is uncommon, and a clinician would also consider a second disorder.
- No acid–base disorder: Both PaCO2 and bicarbonate are abnormal. A normal pH here reflects compensation, not the absence of a disorder.
- Fully compensated metabolic alkalosis: A primary metabolic alkalosis would leave the pH on the alkaline side of 7.40. His is 7.37.
- Partially compensated respiratory acidosis: Partial compensation leaves the pH outside the normal range. His is inside it.
8Summary
An arterial blood gas gives pH (7.35 to 7.45), PaCO2 (35 to 45 mm Hg, the respiratory number) and bicarbonate (22 to 26 mmol/L, the metabolic number). Respiratory acidosis comes from hypoventilation and respiratory alkalosis from hyperventilation; in both, pH and PaCO2 move in opposite directions. Metabolic acidosis is a fall in bicarbonate, from acid gained (high anion gap: diabetic ketoacidosis, lactic acidosis, kidney failure) or bicarbonate lost (normal gap: diarrhea). Metabolic alkalosis is a rise in bicarbonate, most often from vomiting or diuretics, kept going by loss of volume, chloride and potassium. The other system compensates by moving its number in the same direction as the primary one: the lungs within hours, the kidneys over 3 to 5 days. Read every gas in five steps: pH, PaCO2, bicarbonate, the primary disorder, then compensation.