Metabolic acidosis vs metabolic alkalosis: ABG patterns, causes and potassium

Start with bicarbonate moving the same way as pH

The fastest classification step is to check whether bicarbonate travels with the pH. When pH falls below the normal range and bicarbonate is also low, the primary problem is metabolic acidosis. When pH rises and bicarbonate is also high, the primary problem is metabolic alkalosis. If carbon dioxide explains the pH instead, the disturbance is respiratory and belongs to a different comparison.

Next, look at the carbon dioxide value to judge compensation. The lungs respond quickly to metabolic problems, so a low carbon dioxide alongside metabolic acidosis, or a modestly raised carbon dioxide alongside metabolic alkalosis, suggests the respiratory system is working to correct the pH. A pH back inside the normal range with both values abnormal indicates full compensation rather than a cure.

Link each disturbance to its typical causes

Metabolic acidosis arises when acid accumulates or bicarbonate is lost. Accumulation occurs in diabetic ketoacidosis, lactic acidosis from poor tissue perfusion, kidney failure and some poisonings, such as salicylate toxicity. Bicarbonate loss happens with severe diarrhoea or intestinal fistulas. Clinicians use the anion gap to separate acid gain from bicarbonate loss, but the nurse's task is recognising the clinical context.

Metabolic alkalosis develops when acid is lost or bicarbonate is gained. Prolonged vomiting and nasogastric suction remove hydrochloric acid, while loop and thiazide diuretics increase urinary acid and chloride losses. Excess sodium bicarbonate administration can also raise bicarbonate. Volume depletion and low potassium tend to keep the kidneys retaining bicarbonate, which is why the alkalosis can persist until fluid and electrolytes are replaced.

Expect different breathing patterns and potassium problems

A patient with significant metabolic acidosis often breathes deeply and rapidly as the lungs blow off carbon dioxide; in ketoacidosis this is called Kussmaul respiration. Confusion, lethargy and hypotension may accompany worsening acidosis. In metabolic alkalosis, breathing may become slower or shallower to retain carbon dioxide, and the patient may report muscle cramps, tingling or weakness linked to electrolyte changes.

Potassium deserves close attention in both. Acidaemia can shift potassium out of cells, so a normal or high serum value may hide total body depletion, and levels can fall quickly once insulin or treatment corrects the acidosis. Alkalosis is closely tied to hypokalaemia, which both causes and results from the disorder. Watch the cardiac monitor and report potassium trends promptly.

Recognise what an ABG alone cannot tell you

An ABG names the acid-base pattern but does not identify its cause. Two patients with identical metabolic acidosis values may have ketoacidosis or sepsis-related lactic acidosis, and treatment differs. Mixed disorders also exist, such as a patient with vomiting and diarrhoea, so a near-normal pH does not exclude a serious problem. Interpret results with history, vital signs and other laboratory values.

Treatment targets the underlying cause rather than the number. Acidosis care may involve fluid resuscitation, insulin, improving perfusion or dialysis, while alkalosis care often involves replacing volume, chloride and potassium, or adjusting suction and diuretics. Sodium bicarbonate is reserved for specific prescribed situations. Nurses monitor response, repeat laboratory checks as ordered and escalate deterioration according to local protocol.

Work through an original practice scenario

Imagine a hypothetical patient with continuous nasogastric suction for three days who now reports leg cramps. The ABG shows a high pH, high bicarbonate and a slightly high carbon dioxide. Options include respiratory alkalosis, metabolic acidosis, partially compensated metabolic alkalosis and fully compensated respiratory acidosis. Partially compensated metabolic alkalosis is correct because bicarbonate drives the pH and the pH remains abnormal.

The tempting distractor is respiratory acidosis, because the carbon dioxide is raised. However, a high carbon dioxide would lower the pH, not raise it, so it reflects compensation. The nursing priority is to check the potassium result, assess for dysrhythmia and report the gastric losses so that replacement fluid and electrolytes can be prescribed under the facility protocol.

Sources and further reading

Merck Manual Professional: Metabolic acidosis. Definition, high and normal anion gap causes, compensatory hyperpnea and potassium monitoring.

Merck Manual Professional: Metabolic alkalosis. Vomiting, gastric suction and diuretic causes, chloride-responsive alkalosis and the link with hypokalaemia.

Open RN Nursing Fundamentals: Acid-base balance. pH and bicarbonate moving in the same direction, compensation levels and breathing changes nurses notice first.

MSD Manual Professional: Diabetic Ketoacidosis (DKA). Serum potassium normal or raised despite total body depletion because acidosis shifts potassium out of cells, and falling potassium once insulin treatment starts.