Primer on clinical acid-base problem solving
William L. Whittier, MD and Gregory W. Rutecki, MD
Acid-base problem solving has been an integral part of medical practice in recent generations. Diseases discovered in the last 30-plus years, for example, Bartter syndrome and Gitelman syndrome, D-lactic acidosis, and bulimia nervosa, can be diagnosed according to characteristic acid-base ﬁndings.Accuracy in acid-base problem solving is a direct result of a reproducible, systematic approach to arterial pH, partial pressure of carbon dioxide, bicarbonate concentration, and electrolytes. The “Rules of Five” is one tool that enables clinicians to determine the cause of simple and complex disorders, even triple acid-base disturbances, with consistency. In addition, other electrolyteabnormalities that accompany acid-base disorders, such as hypokalemia, can be incorporated into algorithms that complement the Rules and contribute to efﬁcient problem solving in a wide variety of diseases. Recently urine electrolytes have also assisted clinicians in further characterizing select disturbances. Acid-base patterns, in many ways, can serve as a “common diagnostic pathway” shared by allsubspecialties in medicine. From infectious disease (eg, lactic acidemia with highly active antiviral therapy therapy) through endocrinology (eg, Conn’s syndrome, high urine chloride alkalemia) to the interface between primary care and psychiatry (eg, bulimia nervosa with multiple potential acid-base disturbances), acid-base problem solving is the key to unlocking otherwise unrelated diagnoses. Inasmuch asthe Rules are clinical tools,
Dis Mon 2004;50:117-162. 0011-5029/$ – see front matter doi:10.1016/j.disamonth.2004.01.002
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they are applied throughout this monograph to diverse pathologic conditions typical in contemporary practice.
strange thing happened to the art of acid-base problem solving in the last decade. For some, the addition of a simple tool, the pulseoximeter, or so-called ﬁfth vital sign, seemed to relegate blood gas values to unfamiliar territory. It seemed that monitoring of oxygen saturation substituted for information obtained from arterial blood gas values! In fact, since the advent of oximetry, to many senior physicians (including the second author, G.W.R.), it appears that blood gas values have been used less frequently. This primer hasbeen undertaken to prove that “reports of the demise of acid-base problem solving have been greatly exaggerated”! As important as the noninvasive monitoring of oxygen saturation is, if the partial arterial oxygen tension (PaO2) is removed from the context of acid-base physiology, the disease puzzle will not ﬁt together successfully. Fluctuation in pH and contingent compensation by the kidneys andlungs are the remaining pieces. Pulse oximetry, as important as it has been, has not obviated the contribution of acid-base problem solving. As a group, PaO2 or oxygen saturation, partial arterial carbon dioxide tension (PaCO2), bicarbonate concentration, and the many “gaps” (anion, delta or 1:1, osmotic and urinary) complement one another. The skills required to interpret blood gas values mustremain in the repertoire of practitioners everywhere, beginning with primary care and continuing throughout subspecialty medicine. The senior author (G.W.R.) had the beneﬁt of experiencing the effect of acid-base physiology on diseases that were part of his generationin-training. Phenformin-induced lactic acidemia, elevated urine chloride-metabolic alkalemia in Bartter syndrome and Gitelman syndrome,and metabolic acidemia in ethylene glycol poisoning were all entities to which the acid-base component contributed relevant information. The junior author (W.L.W.) has been trained in a similar arena, nephrology, but with the new additions of acid-base to his generation, such as lactic acidemia during highly active antiviral therapy (HAART), D-lactic acid in short gut syndromes, and the...
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