title: Changes in myocardial blood volume over a wide range of coronary driving pressures: role of capillaries beyond the autoregulatory range.
abstract: 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.
authors.affiliation: The Cardiovascular Imaging Center, the Cardiovascular Division, University of Virginia School of Medicine, Charlottesville, Virginia, USA.
authors.forename: D E
authors.initials: DE
authors.lastname: Le
authors.forename: A R
authors.initials: AR
authors.lastname: Jayaweera
authors.forename: K
authors.initials: K
authors.lastname: Wei
authors.forename: M P
authors.initials: MP
authors.lastname: Coggins
authors.forename: J R
authors.initials: JR
authors.lastname: Lindner
authors.forename: S
authors.initials: S
authors.lastname: Kaul
mesh_terms: D000818:Animals; D001160:Arterioles; D001794:Blood Pressure; D001810:Blood Volume; D002196:Capillaries; D003326:Coronary Circulation; D023921:Coronary Stenosis / Q000378:metabolism / Q000503:physiopathology*; D004285:Dogs; D009206:Myocardium / Q000378:metabolism*; D010101:Oxygen Consumption; D012039:Regional Blood Flow; D014655:Vascular Resistance