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Human PaCO2 and standard base excess compensation for acid-base imbalance

Schlichtig, Robert MD; Grogono, Alan W. MD; Severinghaus, John W. MD

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Objectives Renal and respiratory acid-base regulation systems interact with each other, one compensating (partially) for a primary defect of the other. Most investigators striving to typify compensations for abnormal acid-base balance have reported their findings in terms of arterial pH, PaCO2, and/or HCO3 -. However, pH and HCO3 - are both altered by both respiratory and metabolic changes. We sought to simplify these relations by expressing them in terms of standard base excess (SBE in mM), which quantifies the metabolic balance and is independent of PaCO2.

Design Meta-analysis.

Setting Historical synthesis developed via the Internet.

Patients Arterial pH, PaCO2, and/or HCO3 - data sets were obtained from 21 published reports of patients considered to have purely acute or chronic metabolic or respiratory acid-base problems.

Interventions We used the same data to compute the typical compensatory responses to imbalances of SBE and PaCO2. Relations were expressed as difference (Delta) from normal values for PaCO2 (40 torr [5.3 kPa]) and SBE (0 mM).

Measurements and Main Results The data of patient compensatory changes conformed to the following equations, as well as to the traditional PaCO2 vs. HCO3 - or H+ vs. PaCO2 equations: Metablic change responding to change in PaCO2: (Equation 1) Respiratory change responding to change in SBE: (Equation 2)

Conclusion Data reported by many investigators over the past 35 yrs on typical, expected, or "normal" human compensation for acid-base imbalance may be expressed in terms of the independent variables: PaCO2 (respiratory) and SBE (metabolic). (Crit Care Med 1998; 26:1173-1179)

From the Department of Research and Development (Dr. Schlichtig), Pittsburgh Veterans Affairs Medical Center, Pittsburgh, PA; the Department of Anesthesiology (Dr. Grogono), Tulane University School of Medicine, New Orleans, LA; and the Department of Anesthesiology (Dr. Severinghaus), University of California-San Francisco, San Francisco, CA.

Address requests for reprints to: Robert Schlichtig, MD, Noble Hospital, 115 West Silver Street, Westfield, MA 01086.

© Williams & Wilkins 1998. All Rights Reserved.