Monitoring Ca2+, pH, and PCO2 in Circulating Arterial Blood by Means of Ion-Selective Electrodes in a Flow Cell
A sensor may be put into the circulation on the tip of a catheter ; or in an extracorporeal, vascular circuit . 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  or fixed  luminal constriction. Also, arterial, venous, and capillary Ca2+ and H+ differ slightly , 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.  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 , as does the the liquid junction potential at the reference electrode with changes in erythrocyte concentration .
KeywordsCatheter Citrate Polyethylene Epoxy Heparin
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