RIpH revisited

Log-jam in acid-base education and investigation: why make it so difficult?

Medical students frequently have difficulty in interpreting acid-base data particularly when pH values are used. The difficulty persists when students qualify and has implications for the safe management of patients who require investigation of acid-base status. Simplification of tuition is required together with a change of practice in the reporting of acid-base data by the laboratories. To improve understanding, we recommend that the teaching and reporting of acid-base status should be changed to use [H+] instead of pH, and a greater emphasis placed on the logical interpretation of primary measurements--that is [H+] and PCO2--with less reliance on derived variables.

Carbon dioxide tensions in physiological salt solutions: direct measurements

Carbon dioxide tensions were measured directly in organ baths and a tonometer aerated in parallel with 6 different gas mixtures of O2 and CO2, 3 gas flows, 3 equilibration periods, and 3 bicarbonate concentrations. The measured partial pressure of carbon dioxide differed systematically from expected values, probably due to errors in the carbon dioxide measurement system. In conclusion, carbon dioxide equilibrates with the bubbling gas in the baths as well as in the tonometer to an almost perfect equilibration.

Constancy of blood carbonic acid pK' in patients during cardiopulmonary resuscitation

Previous studies have suggested that the apparent dissociation constant of blood carbonic acid (pK') may actually vary in acutely ill patients. We prospectively compared the pK' of healthy control subjects to that of patients undergoing cardiopulmonary resuscitation (CPR). Arterial blood obtained from 20 patients undergoing CPR and from 30 healthy volunteers was analyzed for Na+, pH, PCO2, and total CO2 content (tCO2). pK' was calculated from this data, using the Henderson-Hasselbalch equation. Total CO2 was then calculated in the CPR patients, using this equation and the control pK'. Mean pK' was 6.109 +/- 0.004 (SEM) for the control group and 6.123 +/- 0.007 for the CPR group (p = NS). In the CPR group, calculated tCO2 was not significantly different from measured from tCO2, and the correlation between calculated and measured tCO2 was 0.99. In patients undergoing CPR, pK' does not differ significantly from normal, and tCO2 can be accurately estimated with the Henderson-Hasselbalch equation.

Hydrogen ion metabolism

The renal and respiratory systems regulate acid base homeostasis by modifying the bicarbonate buffer pair HCO3- and CO2; other buffer systems adjust to alterations in this pair. The pH change following addition of metabolic acid, or base, is modified initially by the body's buffers. Subsequent respiratory compensation, by retention or excretion of CO2, modifies further this change before renal corrective responses finally occur. A primary respiratory acid base change is modified initially by cellular buffers, with renal compensatory mechanisms adjusting slowly to this change. However correction of the respiratory pH disorder only occurs with correction of the primary disease process. The body's capacity, therefore, to adjust to a matabolic acid base defect, appears greater than that observed with a primary respiratory pH abnormality. Treatment in all acid base disorders focuses initially upon the primary diseases process; thereafter therapeutic manipulation of the HCO3-, CO2 buffer pair, to aid the body's compensatory processes, can be considered.

A diagram to facilitate the understanding and therapy of mixed acid base disorders

A diagram based on in-vivo relationships between arterial hydrogen ion activity (H+) and carbon dioxide tension (PCO2) in primary abnormalities of acid base homeostasis is presented. It is designed to facilitate the interpretation of pH data by including the 95% confidence limits described in patients with simple metabolic and respiratory acid base disorders. These bands have been formulated from observation of simple acid base abnormalities and indicate the appropriate respiratory or renal compensatory response to the primary pH defect. A plot which falls outside these limits therefore indicates the presence of a mixed acid base disorder. The diagram presents a physiological approach to clinical disorders of pH regulation demonstrating maintenance of intra-cellular fluid homeostasis during primary extracellular fluid disturbances. Diagnostic and therapeutic advantages are further illustrated and discussed in six case reports.