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Changes in myocardial blood volume over a wide range of coronary driving pressures: role of capillaries beyond the autoregulatory range
  1. D E Le,
  2. A R Jayaweera,
  3. K Wei,
  4. M P Coggins,
  5. J R Lindner,
  6. S Kaul
  1. The Cardiovascular Imaging Center, the Cardiovascular Division, University of Virginia School of Medicine, Charlottesville, Virginia, USA
  1. Correspondence to:
    Dr Sanjiv Kaul
    University of Virginia Health System, Cardiovascular Division, Box 800158, Charlottesville, VA 22908-0158, USA;


Objective: To determine whether, when the vasomotor capacity of the coronary arterioles is exhausted at rest, myocardial blood volume decreases in order to maintain a normal capillary hydrostatic pressure, even at the expense of myocardial oxygen delivery.

Methods: 18 dogs were studied. In group 1 (n  =  9), coronary driving pressure (CDP) was reduced by 10–80 mm Hg below normal by a stenosis; in group 2 (n  =  9), it was increased 20–80 mm Hg above baseline by increasing aortic pressure with phenylephrine. Myocardial contrast echocardiography (MCE) was undertaken to measure the myocardial blood volume fraction and myocardial blood flow (MBF).

Results: In group 1 dogs, as CDP was reduced, both coronary blood flow (CBF) and MBF decreased. Myocardial blood volume fraction also decreased and myocardial vascular resistance increased, while coronary sinus Po2 decreased. In group 2 dogs, as CDP was increased, epicardial CBF increased but MBF remained unchanged because of a decrease in myocardial blood volume fraction. Myocardial vascular resistance decreased, however, implying the presence of coronary arteriovenous shunting, which was supported by a progressive increase in the coronary sinus Po2.

Conclusions: When arteriolar tone is exhausted so that CBF becomes dependent on CDP, myocardial blood volume decreases in order to maintain a constant capillary hydrostatic pressure, which takes precedence over myocardial oxygen delivery. These novel findings implicate capillaries in the regulation of CBF beyond the autoregulatory range.

  • capBV, capillary blood volume
  • capHP, capillary hydrostatic pressure
  • CBF, coronary blood flow
  • CDP, coronary driving pressure
  • CVR, coronary vascular resistance
  • MBF, myocardial blood flow
  • MBV, myocardial blood volume
  • MCE, myocardial contrast echocardiography
  • MV˙o2, myocardial oxygen consumption
  • MVR, myocardial vascular resistance. NOTE: “capillary” refers to only capillaries within the myocardium, “coronary” refers to epicardial coronary arteries, and “myocardial” refers to the entire microvasculature within the myocardium, which includes arterioles, capillaries, and venules
  • coronary physiology
  • capillaries
  • myocardial contrast echocardiography

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