Autoregulation and heart rate.

A utoregulation is an important physiological mechanism whereby blood flow to an organ is kept relatively constant as perfusing pressure changes. For all organs except the heart, autoregulation can be examined by changing aortic pressure and measuring the resulting organ flow. In the left ventricle, however, changing aortic pressure alters myocardial oxygen consumption, which itself alters myocardial blood flow. Therefore, autoregulation in the left ventricle can be tested only by cannulating the left coronary artery so that coronary perfusing pressure can be changed while aortic pressure (and myocardial oxygen consumption) remain constant. If this is done and coronary perfusing pressure is suddenly changed, coronary blood flow initially changes transiently in the direction of the pressure change (decreasing when pressure is lowered and increasing when pressure is raised) and then returns towards its control value.1-5 Autoregulation takes place in all layers of the left ventricle, thus preserving the approximate equality of flows per gram across the ventricular wall as coronary perfusing pressures change.6,7 There is a limit to how low coronary perfusing pressure can decrease before autoregulation fails to keep coronary flow relatively constant. This limit is