Chapter 048. Acidosis and Alkalosis (Part 3)

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Figure 48-1 Acid-base nomogram. Shown are the 90% confidence limits (range of values) of the normal respiratory and metabolic compensations for primary acidbase disturbances. (From DuBose, used with permission.) Mixed Acid-Base Disorders Mixed acid-base disorders—defined as independently coexisting disorders, not merely compensatory responses—are often seen in patients in critical care units and can lead to dangerous extremes of pH (Table 48-2). A patient with diabetic ketoacidosis (metabolic acidosis) may develop an independent respiratory problem leading to respiratory acidosis or alkalosis. Patients with underlying pulmonary disease may not respond to metabolic acidosis with an appropriate ventilatory response because of insufficient respiratory reserve....

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Chapter 048. Acidosis and Alkalosis (Part 3) Figure 48-1
Acid-base nomogram. Shown are the 90% confidence limits (range of values) of the normal respiratory and metabolic compensations for primary acid- base disturbances. (From DuBose, used with permission.) Mixed Acid-Base Disorders Mixed acid-base disorders—defined as independently coexisting disorders, not merely compensatory responses—are often seen in patients in critical care units and can lead to dangerous extremes of pH (Table 48-2). A patient with diabetic ketoacidosis (metabolic acidosis) may develop an independent respiratory problem leading to respiratory acidosis or alkalosis. Patients with underlying pulmonary disease may not respond to metabolic acidosis with an appropriate ventilatory response because of insufficient respiratory reserve. Such imposition of respiratory acidosis on metabolic acidosis can lead to severe acidemia and a poor outcome. When metabolic acidosis and metabolic alkalosis coexist in the same patient, the pH may be normal or near normal. When the pH is normal, an elevated anion gap (AG; see below) denotes the presence of a metabolic acidosis. A discrepancy in the ∆AG (prevailing minus normal AG) and the ∆HCO3– (normal minus prevailing HCO3–) indicates the presence of a mixed high-gap acidosis— metabolic alkalosis (see example below). A diabetic patient with ketoacidosis may have renal dysfunction resulting in simultaneous metabolic acidosis. Patients who have ingested an overdose of drug combinations such as sedatives and salicylates may have mixed disturbances as a result of the acid-base response to the
individual drugs (metabolic acidosis mixed with respiratory acidosis or respiratory alkalosis, respectively). Even more complex are triple acid-base disturbances. For example, patients with metabolic acidosis due to alcoholic ketoacidosis may develop metabolic alkalosis due to vomiting and superimposed respiratory alkalosis due to the hyperventilation of hepatic dysfunction or alcohol withdrawal. Table 48-2 Examples of Mixed Acid-Base Disorders Mixed Metabolic and Respiratory Metabolic acidosis – respiratory alkalosis Key: High- or normal-AG metabolic acidosis; prevailing Pa CO2 below predicted value (Table 48-1) Example: Na+, 140; K+, 4.0; Cl–, 106; HCO3–, 14; AG, 20; PaCO2, 24; pH, 7.39 (lactic acidosis, sepsis in ICU)
Metabolic acidosis – respiratory acidosis Key: High- or normal-AG metabolic acidosis; prevailing Pa CO2 above predicted value (Table 48-1) Example: Na+, 140; K+, 4.0; Cl–, 102; HCO3–, 18; AG, 20; PaCO2, 38; pH, 7.30 (severe pneumonia, pulmonary edema) Metabolic alkalosis – respiratory alkalosis Key: PaCO2does not increase as predicted; pH higher than expected Example: Na+, 140; K+, 4.0; Cl–, 91; HCO3–, 33; AG, 16; PaCO2, 38; pH, 7.55 (liver disease and diuretics)
Metabolic alkalosis – respiratory acidosis Key: PaCO2higher than predicted; pH normal Example: Na+, 140; K+, 3.5; Cl–, 88; HCO3–, 42; AG, 10; PaCO2, 67; pH, 7.42 (COPD on diuretics) Mixed Metabolic Disorders Metabolic acidosis – metabolic alkalosis Key: Only detectable with high-AG acidosis; ∆AG >> ∆HCO3– Example: Na+, 140; K+, 3.0; Cl–, 95; HCO3–, 25; AG, 20; PaCO2, 40; pH, 7.42 (uremia with vomiting)
Metabolic acidosis – metabolic acidosis Key: Mixed high-AG – normal-AG acidosis; ∆HCO3– accounted for by combined change in ∆AG and ∆Cl– Example: Na+, 135; K+, 3.0; Cl–, 110; HCO3–, 10; AG, 15; PaCO2, 25; pH, 7.20 (diarrhea and lactic acidosis, toluene toxicity, treatment of diabetic ketoacidosis) Note: AG, anion gap; ICU, intensive care unit; COPD, chronic obstructive pulmonary disease.