tailieunhanh - Chapter 048. Acidosis and Alkalosis (Part 4)

Approach to the Patient: Acid-Base Disorders A stepwise approach to the diagnosis of acid-base disorders follows (Table 48-3). Care should be taken when measuring blood gases to obtain the arterial blood sample without using excessive heparin. Blood for electrolytes and arterial blood gases should be drawn simultaneously prior to therapy, since an increase in [HCO3–] occurs with metabolic alkalosis and respiratory acidosis. Conversely, a decrease in [HCO3–] occurs in metabolic acidosis and respiratory alkalosis. In the determination of arterial blood gases by the clinical laboratory, both pH and Pa CO2 are measured, and the [HCO3–] is calculated from the Henderson-Hasselbalch. | Chapter 048. Acidosis and Alkalosis Part 4 Approach to the Patient Acid-Base Disorders A stepwise approach to the diagnosis of acid-base disorders follows Table 48-3 . Care should be taken when measuring blood gases to obtain the arterial blood sample without using excessive heparin. Blood for electrolytes and arterial blood gases should be drawn simultaneously prior to therapy since an increase in HCO3- occurs with metabolic alkalosis and respiratory acidosis. Conversely a decrease in HCO3- occurs in metabolic acidosis and respiratory alkalosis. In the determination of arterial blood gases by the clinical laboratory both pH and PaCO2 are measured and the HCO3- is calculated from the Henderson-Hasselbalch equation. This calculated value should be compared with the measured HCO3 total CO2 on the electrolyte panel. These two values should agree within 2 mmol L. If they do not the values may not have been drawn simultaneously a laboratory error may be present or an error could have been made in calculating the HCO3- . After verifying the blood acid-base values one can then identify the precise acid-base disorder. Table 48-3 Steps in Acid-Base Diagnosis 1. Obtain arterial blood gas ABG and electrolytes simultaneously. 2. Compare HCO3- on ABG and electrolytes to verify accuracy. 3. Calculate anion gap AG . 4. Know four causes of high-AG acidosis ketoacidosis lactic acid acidosis renal failure and toxins . 5. Know two causes of hyperchloremic or nongap acidosis bicarbonate loss from GI tract renal tubular acidosis . 6. Estimate compensatory response Table 48-1 . 7. Compare AAG and AHCO3-. 8. Compare change in Cl- with change in Na . Calculate the Anion Gap All evaluations of acid-base disorders should include a simple calculation of the AG it represents those unmeasured anions in plasma normally 10 to 12 mmol L and is calculated as follows AG Na - Cl- HCO3- . The unmeasured anions include anionic proteins phosphate sulfate and organic anions. When acid anions such as .