ANALYTIC TOOLS USED IN ACID-BASE CHEMISTRY

Textbooks and clinical practice have tended to overestimate the importance of isolated changes in hydrogen or bicarbonate ion concentration. An emerging body of evidence suggests that the clinical significance of acid-base perturbations is determined by the underlying cause rather than the serum concentration of hydrogen and hydroxyl ions. Nevertheless, acid-base chemistry remains a mainstay of clinical data interpretation because of ubiquitous availability, low cost, and universal acceptance. The accuracy of acid-base measurements, however, is not determined by the blood gas value alone, which measures volatile acid and pH. Rather, measurement of each of the strong and weak ions that influence water dissociation, although cumbersome, is essential.

Acid-base abnormalities are best described by alterations in PCO₂ , SID, and A_TOT . Unfortunately, many approaches used to teach and describe the acid-base status of a patient fail to adequately explain many commonly seen perioperative acid-base abnormalities (e.g., dilutional or hyperchloremic acidosis) and mislead us about the cause of the problem (i.e., dilutional acidosis is caused by reduced SID, not dilution of bicarbonate). Although it is important to consider historically used approaches, modern approaches to acid-base interrogation focus on physical chemistry to calculate the magnitude of the disturbance in the SID or recalculate the base deficit or excess (BDE)

1606

Carbon Dioxide-Bicarbonate (Boston) Approach

Schwartz, Brackett, and others,^{[21A] [21B]} at Tufts University in Boston, developed an approach to acid-base chemistry using acid-base maps and the mathematical relationship between carbon dioxide tension and serum bicarbonate (or total CO₂ ), derived from the Henderson-Hasselbalch equation, to predict the nature of acid-base disturbances (see Table 41-2 ). A number of patients with known acid-base disturbances at steady states of compensation were evaluated. The degree of compensation from what was considered normal was measured for each disease state. The investigators were able to describe six primary states of acid-base imbalance using linear equations or maps relating hydrogen ion concentration to PCO₂ for respiratory disturbances and PCO₂ to HCO₃ ^- concentration for metabolic disturbances ( Fig. 41-2 ). For any given acid-base disturbance, an expected HCO₃ ^- concentration was determined. For most simple disturbances, this is a reasonable approach.

Using these maps and equations, physicians have been able to determine the nature of most respiratory and metabolic acid-base disturbances. Although there is a mathematical relationship in place, alterations in H⁺ and HCO₃ ^- do not reflect cause and effect. For example, chronic hypoventilation is associated with an increase in

Figure 41-2 Acid-base nomogram using the Boston approach. Different acid-base disturbances can be distinguished based on the relative values of PCO₂ and HCO₃ ^- . (Adapted from Brenner BM, Rector FC: The Kidney, 3rd ed. Philadelphia, WB Saunders, 1986, p 473.)

Although the PCO₂ -HCO₃ ^- approach is relatively accurate for most disturbances, there are several inherent pitfalls, particularly in relation to the metabolic component. First, the approach is not as simple as it seems, requiring the clinician to refer to confusing maps or to learn formulas and perform mental arithmetic. Second, the system neither explains nor accounts for many of the complex acid-base abnormalities seen in perioperative and critically ill patients, such as those with acute acidosis in the setting of hypoalbuminemia, hyperchloremic acidosis, or dilutional acidosis or with lactic acidosis in the setting of chronic respiratory acidosis.