Examination of the validity of the DPTI as an estimate of myocardial oxygen supply with special reference to the DPTI/TTI ratio.

Abstract

The validity of the diastolic pressure time index (DPTI) as an estimate of myocardial O2 supply and tension time index (TTI) as an estimate of O2 demand has been examined in 10 closed-chest anesthetized dogs. We analyzed 158 steady states including maximal variation of hemodynamics and O2 consumption. For each steady state 23 hemodynamic variables were calculated. Myocardial blood flow (MBF) was directly measured in the coronary sinus with a differential pressure catheter. Oxygen consumption (MVO2) was varied by application of catecholamines, atropine, negative and positive inotropic drugs and hypo- and hypervolemia. There was a close relationship between direct measurements of O2 supply and O2 demand over a wide range of testing (r=0.93), but there was no significant correlation between the ratio of DPTI/TTI and the ratio of O2 supply/O2 demand (directly measured). With intact metabolic regulation of coronary blood flow there is no close correlation of mean diastolic aortic pressure to coronary blood flow (r=0.45) over the entire tested range, which is one reason why the DPTI does not show a correlation to the direct measurement of O2 supply (r=0.34). The tension time index bears a close relationship to myocardial oxygen consumption only under normal and moderate stimulation of inotropism (r=0.96), but there is no significant correlation under high positive inotropic stimulation (r=0.39). Our findings indicate that the ratio of DPTI/TTI should be interpreted with great care if it is generally used as an indicator of the adequacy of myocardial O2 supply.