‘Rules of 5’ for acid-base problem-solving
| 1 Determine the arterial pH | pH < 7.38 is acidemic, pH > 7.42 is alkalemica Normal pH does not rule out an acid-base disorder | |||
| 2 If the arterial pH is abnormal, determine whether the primary process is respiratory, metabolic, or both | pH | PCO2 | HCO3 | |
| Respiratory acidosisa | Low | High | — | |
| Metabolic acidosis | Low | — | Low | |
| Mixed respiratory and metabolic acidosis | Low | High | Low | |
| Respiratory alkalosis | High | Low | — | |
| Metabolic alkalosis | High | Variesb | High | |
| Mixed respiratory and metabolic alkalosis | High | Low | High | |
| 3 Calculate the anion gap | Anion gap = sodium – (chloride + bicarbonate) If serum albumin is low, add 2.5 mmol/L to the anion gap for every 1 g the serum albumin is below normal | |||
| 4 Check the degree of compensation (respiratory or metabolic) | Pco2 and bicarbonate should increase or decrease together | |||
| Normal levels: bicarbonate 25 mmol/L, PCO2 40 mm Hg | ||||
| Acute respiratory acidosis: For every 10-mm Hg increase in Pco2, bicarbonate should increase by 1 mmol/L | ||||
| Chronic respiratory acidosis (> 48 hours): For every 10-mm Hg increase in Pco2, bicarbonate should increase by 4 mmol/L | ||||
| Metabolic acidosis: For every 1-mmol/L decrease in bicarbonate, Pco2 should decrease by 1.3 mm Hg | ||||
| Acute respiratory alkalosis: For every 10-mm Hg decrease in Pco2, bicarbonate should decrease by 2 mmol/L | ||||
| Chronic respiratory alkalosis (> 48 hours): For every 10-mm Hg decrease in Pco2, bicarbonate should decrease by 5 mmol/L | ||||
| Metabolic alkalosis: For every 1-mmol/L increase in bicarbonate, Pco2 may increase by 0.6 mm Hg | ||||
| 5 If the patient has metabolic acidosis with an elevated anion gap, assess for ‘delta gap’ (whether the decrease in bicarbonate = the increase in anion gap) | Delta gap = change in anion gap / change in bicarbonate ([anion gap – 10] / [24 – bicarbonate]) | |||
| > 1: Additional metabolic alkalosis | ||||
| 1: No additional disturbance present | ||||
| < 1: Additional non-anion gap metabolic acidosis present | ||||
↵a Acidosis and alkalosis refer to acid-base disturbances with determined metabolic or respiratory etiologies. Alkalemia and acidemia refer to disturbances in blood pH prior to determination of the underlying metabolic/respiratory cause.
↵b Pco2 in metabolic alkalosis may vary depending on the clinical context, as compensation for metabolic alkalosis requires decreased respiratory drive, which may be influenced by other factors (ie, hypoxia).
Pco2 = partial pressure of carbon dioxide
Adapted from Mani S, Rutecki GW. A patient with altered mental status and an acid-base disturbance. Cleve Clin J Med 2017; 84(1):27–34. Copyright © 2017 The Cleveland ClinicFoundation. All rights reserved.