A new method for continuous measuring of respiratory quotient in a computer-assisted working heart preparation

A method was developed for continuous monitoring of the respiratory quotient (RQ) and the ratio of O2-consumption to O2-demand (EQ) in an isolated working rat heart preparation. The RQ allows to get informations about substrates actually oxidized in the myocardium. The EQ is a parameter which behaves reciprocally to the ATP/O of the oxidized substrates during steady state and additionally it allows monitoring of a transitory oxygen debt. For registration of RQ the arterio-venous O2-difference and the CO2-release are measured. As the CO2-production is very small compared with arterial CO2- and HCO-3 -concentrations when using a bicarbonate buffer for perfusion, a bicarbonate-free perfusion-fluid is used. In a portion of coronary effluent the pH is lowered to shift the CO2/HCO-3 -equilibrium to the side of dissolved CO2. Then the pCO2 is measured with a CO2-Nelectrode. Additionally, ventricular pressure, cardiac input and coronary flow are continuously registrated. All data are fed via an AD-converter into an LSI 11-computer. Heart rate, maximum ventricular pressure, dP/dtmax, oxygen demand after Bretschneider's formula, RQ and EQ are calculated. The advantages of the system described are demonstrated with two examples.