The ratio of the area under the diastolic portion of the arterial pulse pressure trace (diastolic pressure-time index; DPTI) to the area under the systolic component of the arterial pulse pressure trace (time-tension index; TTI) has been used to predict the relationship of myocardial blood supply to oxygen demand. Since introduction of the DPTI-to-TTI ratio as a measurement of this relationship, the accepted critical DPTI:TTI value below which subendocardial ischemia may occur has decreased by almost 50%. This lower critical value has come about as more clinical experience has been gained, particularly in patients with an arterial catheter in the arm. To investigate a potential cause for this decrease, we studied a canine model by pulse transduction from the central aorta (the site traditionally used for these ratio determinations), as well as the femoral artery and the median forepaw artery. Following inotropic and ventricular loading interventions, the changes in the DPTI:TTI, calculated by a special-purpose computer, were exaggerated by approximately 25% in the peripheral arterial measurements. The peripheral arterial sites had consistently higher systolic waveforms and consistently lower, broader diastolic waveforms than those for the central aortic site. This type of configurational change is probably a major cause of the differences among ratios from the three sites in our study, and it helps to explain why the value of the critical ratio has been a subject of controversy.