Myocardial blood flow reflects myocardial oxygenation in healthy swine

Abstract

Myocardial auto-regulation secures a near constant myocardial blood flow through a wide range of systemic blood pressures. However, it is not clear if coronary auto-regulation can maintain myocardial oxygenation in the face of increasing peripheral resistance and cardiac workload. Oxygenation-sensitive (OS) cardiovascular magnetic resonance (CMR) can detect changes in myocardial oxygenation. The aim of this study was to assess myocardial oxygenation during and beyond the myocardial auto-regulation range. Five anaesthetized swine underwent a left-sided thoracotomy to install a flow probe around the proximal left descending coronary artery (LAD). Blood pressure was manipulated from mean arterial pressures (MAP) of 40-180mmHg by continuous administration of Phenylephrine (16-500µg/min). Lower blood pressures were achieved by deep anaesthesia and additional α1-blockade with Urapidil. In a 3T MRI scanner, OS-images were acquired at 10-15mmHg increments in a mid left ventricular short axis slice and expressed as %change in signal intensity (SI) from a defined 70mmHg baseline. Blood pressure and coronary artery blood flow were continuously recorded. The auto-regulation range in these animals was visible with MAP between 40-110mmHg (grey). OS-SI changes (black) showed a good correlation to LAD blood flow (R=0.5, p<0.01), a strong correlation with MAP (R=0.6, p=0.001) and a weak correlation to the rate pressure product (R=0.3, p<0.05). Myocardial oxygenation parallels LAD blood flow in healthy swine. There is no evidence for a compromise in myocardial oxygenation with increased cardiac afterload at higher blood pressures beyond the auto-regulation range