Coronary Circulation During Pacing‐induced Tachycardia

Abstract. Coronary and systemic circulation has been studied in 10 patients with complete atrioventricular block and in 4 patients with sinus rhythm. Studies were performed during spontaneous rhythm, during pacing‐induced tachycardia (PIT) of varying frequency and during a combination of PIT and exercise. Coronary flow and myocardial oxygen uptake varied linearly with heart rate. If exercise was superimposed on PIT with unchanged pacing frequency both coronary flow and myocardial oxygen uptake increased further. The coronary flow increased mainly because of a decrease in coronary vascular resistance. The results were compared with data from a previous study in which the heart rate was increased with exercise in a group of 11 patients without signs of major heart disease. In the PIT study the increase in coronary flow and myocardial oxygen uptake for a given increment in heart rate was less than half of that found in the previous exercise study. The main explanation for this difference is probably on the one hand that wall tension decreased during PIT while it increased during exercise and on the other hand that increase in contractility was more pronounced during exercise. The same relation of coronary flow or myocardial oxygen uptake to the product of systolic pressure and heart rate (BP 3 × HR) was found in both groups although the values in the PIT study were in a lower range than those in the previous exercise study. This indicates that this product is a fairly good approximation of myocardial oxygen demand over a wide range of heart rates regardless of the method used for cardio‐acceleration. It is concluded from this study that PIT is less efficient than exercise in testing the coronary vascular reserve because the stress imposed on the myocardium is low during PIT even during high heart rates.