Effect of descending thoracic aorta clamping and unclamping on phasic coronary blood flow

Abstract

Myocardial infarctions during aortic surgery often occur after aortic clamping and unclamping. In order to investigate the aortic blood pressure (AoBP)-coronary blood flow (CBF) relationship, hemodynamic parameters and phasic circumflex CBF in 15 anesthetized and open-chest dogs during clamping and unclamping of the thoracic descending aorta have been recorded. During clamping, mean aortic blood pressure (MAoBP) rose from 97 + 17 to 150 + 42 mm Hg ( P < 0.001), and total combined left-ventricular power (Wtc) from 692 + 232 to 923 + 402 mW ( P < 0.001) while the ascending aortic blood flow (AoBF) and heart rate did not change significantly. The mean circumflex blood flow (MCBF) increased from 67 + 30 to 88 + 30 ml/min. The increase in systolic coronary blood flow (+73%) was larger than that in diastolic coronary blood flow (DCBF) (19%). The late coronary resistances were increased 39%. Conversely, after unclamping, MAoBP decreased while AoBF and Wtc increased. During the first cardiac cycles after aortic unclamping DCBF decreased 50% with a decrease in the diastolic blood pressure (DBP). Four cycles later, DCBF reincreased while DBP kept on decreasing. This may be related to an active coronary vasodilatation. A linear relationship between DCBF and DBP has been obtained during acute change in DBP due to clamping or unclamping. From these linear relationships, the change in diastolic zero flow pressure ( P f0) has been found to be about +40 mm Hg during clamping. This change in P f0 may be responsible for the observed large fall in DCBF during unclamping. The decrease in DCBF associated with the increase in Wtc accounts for the delayed active vasodilatation.