Muscle surface pH: Measurement technique and responses to acidosis and alkalosis

The technique of muscle surface pH measurement used clinically was refined for application in animal experimentation. Guidelines for electrode utilization were developed, and sources of error were investigated. In pentobarbital sodium-anesthetized rabbits, during steady-state conditions, the surface pH of the medial gastrocnemius [7.39 ± 0.05 (SD)] was equal to that of the soleus, and both were lower than femoral venous outflow pH (7.42 ± 0.04, P < 0.001), which was lower than arterial pH (7.46 ± 0.05, P << 0.001). With acid-base infusions, the same relationship found during steady-state control conditions (arterial > venous outflow > muscle surface pH) was observed despite large fluctuations in blood pH. Only during forced hypoventilation, when CO₂ transiently moved into muscle, did the relationship reverse. In all experiments, muscle surface pH followed venous outflow pH more closely than it followed arterial pH, the soleus more rapidly than the medial gastrocnemius. Results were consistent with the physiological characteristics of the two muscles and demonstrated that muscle surface pH reflects the local pH of the interstitial compartment under the site of electrode placement.