Evaluation of a miniature antimony electrode for measurement of myocardial pH

Myocardial pH reflects the metabolic status of the heart and pH monitoring is an invaluable way to monitor the efficacy of myocardial protection during cardiac surgery. We developed a miniature antimony electrode for pH measurement in the heart. We examined the sensitivity, accuracy and the effects of temperature and oxygen tension on pH readings with this electrode in standard buffers and in anaesthetized dogs. In buffers the antimony electrode exhibited a gradient of -50.3±1.8 mV pH^-1 at 25°C, close to the Nernstian slope and showed a high correlation with conventional glass electrode readings (mean difference 0.027±0.0035 pH, r²=0.97). With increasing temperature the antimony electrode pH readings increased by 0.03±0.002 pH °C^-1. With increasing PO₂ the pH reading decreased (-0.73 pH/ log PO₂ mm Hg, r²=0.96). In the dog heart the antimony electrode showed a decrease in myocardial pH with increasing PCO₂, and an increase in pH when NaHCO₃ was given intravenously. Coronary occlusion resulted in paradoxically higher pH readings with the antimony electrode due to the effect of lowered myocardial PO₂ interfering with pH measurement. The dissolution of antimony from the electrode in blood plasma was tested and found to be low. These studies suggest that antimony electrodes have low toxicity and provide accurate pH determinations under conditions of constant PO₂. For more widespread clinical application, the problem of oxygen interference needs to be solved.