Chapter 048. Acidosis and Alkalosis (Part 3)

Figure 48-1Acid-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....
Nội dung trích xuất từ tài liệu:
Chapter 048. Acidosis and Alkalosis (Part 3) Chapter 048. Acidosis and Alkalosis (Part 3)Figure 48-1 Acid-base nomogram. Shown are the 90% confidence limits (range ofvalues) 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 careunits and can lead to dangerous extremes of pH (Table 48-2). A patient withdiabetic ketoacidosis (metabolic acidosis) may develop an independent respiratoryproblem leading to respiratory acidosis or alkalosis. Patients with underlyingpulmonary disease may not respond to metabolic acidosis with an appropriateventilatory response because of insufficient respiratory reserve. Such imposition ofrespiratory acidosis on metabolic acidosis can lead to severe acidemia and a pooroutcome. When metabolic acidosis and metabolic alkalosis coexist in the samepatient, the pH may be normal or near normal. When the pH is normal, an elevatedanion gap (AG; see below) denotes the presence of a metabolic acidosis. Adiscrepancy in the ∆AG (prevailing minus normal AG) and the ∆HCO3– (normalminus prevailing HCO3–) indicates the presence of a mixed high-gap acidosis—metabolic alkalosis (see example below). A diabetic patient with ketoacidosis mayhave renal dysfunction resulting in simultaneous metabolic acidosis. Patients whohave ingested an overdose of drug combinations such as sedatives and salicylatesmay have mixed disturbances as a result of the acid-base response to theindividual drugs (metabolic acidosis mixed with respiratory acidosis or respiratoryalkalosis, respectively). Even more complex are triple acid-base disturbances. Forexample, patients with metabolic acidosis due to alcoholic ketoacidosis maydevelop metabolic alkalosis due to vomiting and superimposed respiratoryalkalosis 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 belowpredicted 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 abovepredicted 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 bycombined 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 diabeticketoacidosis) Note: AG, anion gap; ICU, intensive care unit; COPD, chronic obstructivepulmonary disease.