The participation of cells in disturbances of acid-base balance

Chronotropic response of isolated atria to acid base alterations

The effect of acid-base alterations on spontaneous rate was analysed using isolated atria exposed to cumulative degrees of acidosis produced either by adding HCl or by increasing PCO2 in the incubation medium. Frequncy vs. pH curves were made to assess chronotropic response to acid-base changes. Heart rate was increased in alkalosis and decreased when the pH of the medium was lowered. Both "respiratory" and "metabolic" alterations affected the contraction rate to the same extent. Decreasing pH from normal values seemed to decrease heart rate more than the enhancement produced by the same change in pH towards the alkalotic side. When frequency was plotted as a function of hydrogen ion activity (aH+) a more linear relationship was obtained, either with pure "metabolic" or with "respiratory" acid-base alterations. Increasing (aH+) from normal values seemed to decrease heart rate to the same extent (respiratory alterations) or even less (metabolic alterations) than the enhancement produced by the same change in (aH+) towards the alkalotic side. Neither the increase in rate produced by alkalosis nor the decrease induced by acidosis were prevented by blocking the neurotransmitters by atropine or propranolol.

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.