Effect on coronary flow of atrial contraction.

The transient decrease in coronary conductance and flow that is seen when the atria contract has been studied in 6 anaesthetized, open-chest dogs in heart block, paced via epicardial pacemaker leads. Left circumflex and anterior descending coronary flows were recorded via Doppler ultrasonic flow probes; aortic and left ventricular pressures and electrocardiogram were recorded simultaneously. The transient conductance and flow decrease in diastole were found from linear regression to be consistent with the left ventricular pressure rise caused by atrial contraction, in the case of the anterior descending coronary artery, which also showed no decrease in peak systolic conductance or flow when atria and ventricles contracted simultaneously. Circumflex conductance and flow were decreased by an atrial contraction to an extent greater than could be accounted for by the rise in left ventricular diastolic pressure. Peak circumflex flow and conductance were also decreased in systole, by 12 per cent, when atrial and ventricular contractions were exactly synchronous so that the atria could not discharge into the ventricles. The systolic effect and the excessive diastolic effect are ascribed to compression of atrial branches of the circumflex artery. This artrial compression mechanism accounted for 37-75 per cent of the effect of an atrial contraction on conductance in diastole. Values from 5 dogs of systolic and diastolic conductance changes due to atrial compression were linearly related. Calculation of atrial flow by applying this percentage to the transient flow decrease indicated that 17 per cent of circumflex diastolic flow was atrial, and that this percentage stayed constant throughout diastole.