With the use of ultrasound technique, segment length and diameter of ramus circumflexus (RC) and ramus interventricularis anterior (RIA) in the dog heart, placed in a bath, were monitored under constant pressure. When the left and/or right ventricular volume was increased by 150% of the normal diastolic filling, segment length of RC increased by 2.54 +/- 0.17 and 2.15 +/- 0.06%, respectively, and diameter of RC decreased by 4.72 +/- 0.20% and by 4.60 +/- 0.20%, respectively. The same filling of the left and/or right ventricle induced in the proximal third of RIA an increase in segment length by 11.40 +/- 0.55 and 12.2 +/- 0.42%, respectively, and a decrease in diameter by 9.41 +/- 0.42% and by 10.01 +/- 0.31%, respectively. The latter values of RIA were significantly higher (P < 0.001) than those registered in RC. The deformations of RIA decreased toward the periphery of the vessel. By the fact that conduit coronary artery tracks instantaneously the increase of the volume of the left and right ventricle, its contribution to the total coronary resistance increases dynamically. With the ventricular volume increased by 150%, the RC contributes to the total coronary resistance by 14% and RIA in the proximal part contributes by 23%, with values significantly higher than control (10%) (Malindzak, G. S. In: The Coronary Artery. London: Croom Helm, 1982, p. 241-267). The results imply a methodical consequence, namely that the diameter of the respective coronary artery is an index of smooth muscle activity only if both perfusion pressure and volume of the heart are maintained constant.
