Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (616 page)

   Metabolic alkalosis patients may be volume depleted and chloride responsive or have volume expansion and be chloride resistant.

   When the urine chloride is low (<10 mmol/L) and the patient responds to chloride treatment, the cause is more likely loss of gastric juice, diuretic therapy, or rapid relief of chronic hypercapnia. Chloride replacement is completed when urine chloride remains >40 mmol/L.

   When the urine chloride is high (20 mmol/L) and the patient does not respond to NaCl treatment, the cause is more likely hyperadrenalism or severe potassium deficiency.

   Acid–base maps (Figure
13-4
) are a graphic solution of the Henderson-Hasselbalch equation, which predicts the HCO
₃
^‒
value for each set of pH/ pCO
₂
coordinates. They also allow a check of the consistency of ABG and automated analyzer determinations, since these may determine the total CO
₂
content, of which 95% is HCO
₃
⁻
.

   These maps contain bands that show the 95% probability range of values for each disorder. If the pH/pCO
₂
coordinate is outside the 95% confidence band, then the patient has at least two acid–base disturbances.

   These maps are of particular use when one of the acid–base disturbances is not suspected clinically. If the coordinates lie within a band, it is not a guarantee of a simple acid–base disturbance.

Figure 13–4
Acid–base map. The values demarcated for each disorder represent a 95% probability range for each pure disorder. Coordinates lying outside these zones suggest mixed acid–base disorders.

METABOLIC ACIDOSIS

   With Increased Anion Gap (AG >15 mmol/L)

   Lactic acidosis—most common cause of metabolic acidosis with increased AG (frequently >25 mmol/L) (see following section “Lactic Acidosis”)

   Renal failure (AG <25 mmol/L)

   Ketoacidosis

   DM (AG frequently >25 mmol/L)