Monitoring Ca²⁺, pH, and PCO₂ in Circulating Arterial Blood by Means of Ion-Selective Electrodes in a Flow Cell

Abstract

A sensor may be put into the circulation on the tip of a catheter [1]; or in an extracorporeal, vascular circuit [2]. We chose the latter, since it permits multiple sensors to be used. Such circuits are often arterio-venous shunts [2, 5, 6]. These offer a steep pressure drop, and thus brisk extracorporeal flow, but reduce peripheral resistance and systemic arterial pressure, unless extra hydrodynamic resistance is incorporated in the circuit as a variable [5] or fixed [6] luminal constriction. Also, arterial, venous, and capillary Ca²⁺ and H⁺ differ slightly [10], so bypassing the shunted capillary bed could alter measured ion levels, especially if the sought-for changes are small. Another option is an open circuit: a small amount of venous blood flows continuously out of the animal, without being reinfused, to pass through a flow cell containing appropriate sensors. Fogt et al. [4] overcame the disadvantages of this method (poor flow and the small sample volume) by placing a pump in the circuit and continuously diluting the excurrent blood with a heparin-CaCl₂ solution. However, the activity coefficient of plasma calcium varies with plasma water concentration [10], as does the the liquid junction potential at the reference electrode with changes in erythrocyte concentration [12].