Metabolic Alkalosis and the Vomiting Patient

Stomach contents are acid. Vomit for long enough, or suction a stomach for long enough, and you remove acid the body then has to replace. The pH drifts up. So does the bicarbonate.

What the gas looks like

StatPearls gives the normal arterial pH as 7.35 to 7.45. Above 7.45 is alkalemia. Normal bicarbonate on a gas is 22 to 26 mEq/L, and normal PaCO2 is 35 to 45 mmHg.

In metabolic alkalosis the pH sits above 7.45 and the bicarbonate sits above 26 mEq/L. Both move up together, which is the signature. If bicarbonate and pH point the same way, the metabolic side is driving.

Compensation here means holding on to carbon dioxide, so the PaCO2 climbs above 45 mmHg. That is a slower and less comfortable move than blowing carbon dioxide off, because breathing less has an oxygenation cost. The full reading order sits in the four-step sequence.

Why prolonged vomiting does this

Gastric fluid is rich in hydrogen and chloride ions. Losing it repeatedly strips both out of the body. What stays behind is bicarbonate, and with less acid to neutralise, its effect on pH goes unopposed.

Suction does the same thing more quietly. A nasogastric tube on continuous suction removes gastric acid hour after hour without anyone watching a basin. Stems love this, because the loss is easy to overlook in a history line.

The kidneys can correct it, but not quickly. StatPearls describes renal compensation as slow, taking days, against the minutes to hours the lungs need. That contrast is explained properly in the compensation timing post.

The electrolytes that travel with it

This is where the item usually lives. Chloride goes out with the gastric fluid, and MedlinePlus gives a normal serum chloride of 96 to 106 mmol/L. A low chloride alongside a high bicarbonate is a coherent story, not two unrelated findings.

Potassium falls too. MedlinePlus gives normal serum potassium as 3.7 to 5.2 mEq/L. Two things pull it down at once. Potassium is lost in the vomit itself, and in alkalemia potassium shifts into cells in exchange for hydrogen ions coming out.

So the picture is a high pH, a high bicarbonate, a low chloride, and a low potassium. Learn it as one patient rather than four rows. Reference ranges vary slightly between laboratories, which is why the range on your patient's own report is the one that counts.

A low potassium has its own ECG progression, and the U wave is a mandatory part of that description. It gets covered where it belongs, in the hypokalemia ECG post.

Why the shift matters as much as the loss

Two mechanisms are worth separating, because stems test them differently. Loss is what leaves the body. Shift is what moves between compartments without leaving at all.

Vomiting causes loss. Alkalemia causes shift. A patient can have both operating at once, which is why the potassium in this picture tends to be genuinely low rather than merely low-looking.

That distinction pays off later. In acidemia the shift runs the other way, and a serum potassium can read higher than the body's true store. Same principle, opposite direction, and it is the sort of symmetry worth learning once instead of twice.

What alkalemia looks like on the patient

Alkalemia does not announce itself with one dramatic sign. It shows up as muscle irritability, tingling around the mouth or in the fingers, weakness, and a patient who feels off in a way they struggle to describe.

StatPearls links perioral and distal numbness, tetany, and muscle spasm to a falling ionized calcium. Alkalemia is one of the states that lowers it, which is why the neuromuscular findings and the acid-base findings arrive together rather than separately.

So when a stem describes tingling fingers in a patient who has been vomiting for two days, that is one coherent picture. Not two coincidences.

What the nurse does first

The patient in front of you is losing fluid and electrolytes, so volume and potassium are the practical concerns.

Safety comes before chemistry. A weak, dizzy patient who has been vomiting for two days is a fall risk before they are an acid-base problem. Exam items reward candidates who see that order.

Then think about what is still being lost. If suction is running, the loss has not stopped. If vomiting is ongoing, the same. Correcting a value while the cause keeps operating is a distractor answer more often than a correct one.

How this pairs on the exam

Metabolic alkalosis is most often tested next to its opposite, so study them as a pair rather than back to back in isolation. The low pH, low bicarbonate version is handled in the metabolic acidosis post.

One more value is worth a glance if the stem offers it. Base excess supports the metabolic read and has a published normal range, which the base excess post walks through.

When you meet a stem with vomiting, gastric suction, or both, expect the alkalotic pattern before you look at the numbers. Then check whether the numbers agree. Predicting first and confirming second is a habit that survives under time pressure.

And if the numbers disagree with your prediction, that is useful rather than embarrassing. It means something else is going on in the stem, and finding out what is usually the item.

One last framing to carry. Vomiting removes acid, so the pH goes up. Chloride and potassium leave alongside it, so both go down. The kidneys can undo all of this, but not before your shift ends.

That is four facts held together by one mechanism instead of four facts held apart by nothing. Mechanism is what makes content stay put when you are tired.