bubble_chart Overview

Mixed acid-base disturbances refer to the simultaneous presence of two or more simple acid-base imbalances. In mixed acid-base disorders, the original compensatory responses no longer exist, and the pathophysiological changes are more complex, potentially leading to atypical clinical manifestations. Therefore, a preliminary diagnosis requires a thorough medical history and careful analysis of blood gas results. Mixed acid-base disturbances can involve various combinations, but it is clearly impossible for respiratory acidosis and respiratory alkalosis to occur simultaneously. When two primary disorders shift the pH in the same direction, the deviation from normal becomes more pronounced. For example, a patient with metabolic acidosis combined with respiratory acidosis will have a lower pH than with either disorder alone. When two disorders shift the pH in opposing directions, the plasma pH depends on the dominant disorder, and the extent of the deviation is less pronounced due to the counteracting effect of the other disorder compared to a single disorder alone. If the opposing pH shifts caused by the two disorders cancel each other out, the patient's plasma pH may appear normal, such as in metabolic acidosis combined with respiratory alkalosis.

bubble_chart Type

Mixed acid-base balance disorders commonly include the following five types.

(1) Respiratory acidosis combined with metabolic acidosis

Respiratory acidosis combined with metabolic acidosis is seen in: (1) Chronic respiratory acidosis, such as obstructive pulmonary disease accompanied by toxic shock with lactic acidosis; (2) Cardiac and respiratory arrest leading to acute respiratory acidosis and lactic acidosis due to hypoxia. This mixed acid-base balance disorder can significantly decrease plasma pH, reduce plasma [HCO₃

^-], and increase Pco2. For example, a patient's plasma pH may be 7.0, Pco2 11.3 kPa (85 mmHg), [HCO₃^-] 14.4 mmol (mEq)/L, and B.E. -12 mmol (mEq)/L.

(2) Respiratory acidosis combined with metabolic alkalosis

Respiratory acidosis combined with metabolic alkalosis is seen in patients with chronic obstructive pulmonary disease who develop hypercapnia and, due to cor pulmonale with heart failure, are treated with diuretics such as furosemide or ethacrynic acid, leading to metabolic alkalosis. This is also a common scenario encountered in respiratory, cardiac, and renal departments. The patient's plasma pH may be normal, slightly elevated, or decreased, but both [HCO₃^-] and Pco2 are significantly elevated. The increase in [HCO₃^-] is characteristic of metabolic alkalosis, while the rise in Pco2 is characteristic of respiratory acidosis, yet their ratio may remain unchanged or vary only slightly. For example, a patient's plasma pH may be 7.4, Pco2 60 mmHg, plasma [HCO₃^-] 34 mEq/L, and B.E. +14 mEq/L.

(3) Respiratory alkalosis combined with metabolic acidosis

This mixed acid-base balance disorder can be seen in: (1) Patients with renal insufficiency and metabolic acidosis who also develop respiratory alkalosis due to hyperventilation caused by fever, such as acute renal failure with high fever due to Gram-negative bacterial sepsis. (2) Patients with hepatic insufficiency may hyperventilate due to stimulation by NH₃, while also developing lactic acidosis due to metabolic dysfunction. (3) Excessive doses of salicylate causing metabolic acidosis while stimulating the respiratory center, leading to hyperventilation. The plasma pH may be normal, slightly elevated, or decreased, but both plasma [HCO₃

^-] and Pco2 are significantly reduced. The decrease in [HCO₃^-] is characteristic of metabolic acidosis, while the reduction in Pco2 is characteristic of respiratory alkalosis, yet their ratio may remain unchanged or vary only slightly. For example, a patient's plasma pH may be 7.36, Pco2 20 mmHg, plasma [HCO₃^-] 14 mEq/L, and B.E. -12 mEq/L.

(4) Respiratory alkalosis combined with metabolic alkalosis

This mixed acid-base disorder can be seen in: (1) patients with fever and vomiting, who have respiratory alkalosis caused by hyperventilation and metabolic alkalosis caused by vomiting. (2) Patients with cirrhosis and ascites, who hyperventilate due to NH₃ stimulation, and when diuretics are used or vomiting occurs, the plasma pH value of this type is significantly increased, plasma [HCO₃^-] may increase, and Pco2 may decrease. The increase in [HCO₃^-] is characteristic of metabolic alkalosis, while the decrease in Pco2 is characteristic of respiratory alkalosis. For example, a patient's plasma pH is 7.68, Pco2 is 29 mmHg, plasma [HCO₃^-] is 38 mEq/L, and B.E is +14 mEq/L.

(5) Metabolic acidosis combined with metabolic alkalosis

Respiratory acid-base imbalances cannot coexist, but metabolic acid-base imbalances can occur simultaneously. For example, in patients with acute renal failure who experience vomiting or undergo gastric suction, both metabolic acidosis and metabolic alkalosis may be present as pathological processes. However, plasma pH, [HCO₃^-], and Pco2 may all remain within the normal range or be slightly elevated or decreased.

bubble_chart Diagnosis

Mixed acid-base balance disorders are relatively complex and can only be diagnosed after thorough research and analysis of the disease's progression. Nevertheless, a few mixed acid-base balance disorders remain difficult to identify. Currently, even in well-equipped hospitals domestically, about 2.2% of cases still cannot be definitively diagnosed. This indicates that further research is needed in the principles and techniques of diagnosing acid-base balance disorders.