title: Arterial pressure-flow relationship in patients undergoing cardiopulmonary bypass.
abstract: We determined the arterial pressure-flow relationship experimentally by means of step changes of blood flow in 30 adult patients undergoing cardiopulmonary bypass (CPB). Anesthesia technique was uniform. CPB was nonpulsatile; hypothermia to 25-28 degrees C, and hemodilution to 18%-25% hematocrit were used. During stable bypass, mean arterial pressure was recorded first with blood flow 2.2 L.min-1.m-2. Flow was then increased to 2.9 L.min-1.m-2 for 10 s and reverted to baseline for 1 min. Then it was decreased to 1.45 L.min-1.m-2 for 10 s, and reverted to baseline for 1 min. Subsequently, it was decreased to 0.73 L.min-1.m-2 for 10 s and then reverted to baseline. Similar sets of measurements were repeated after 0.25 mg of phenylephrine and once the patient was rewarmed. The pressure-flow function was individually determined by regression, and the critical pressure estimated by extrapolation to zero flow. All patients had zero-flow critical pressure during hypothermia, with a mean value of 21.8 +/- 6.4 mm Hg (range 8.8-38.9). It increased after 0.25 mg phenylephrine to 25.4 +/- 7.2 mm Hg (range 12.2-43.9, P < 0.001). During normothermia, critical pressure was 21.2 +/- 5 mm Hg (range 13.4-30.9), not significantly different from hypothermia. During hypothermia, the slope of the pressure-flow function (i.e., resistance) was 14.9 +/- 3.5 mm Hg.L-1.min-1.m-2 (range 7.6-22.1). It increased significantly (P < 0.001) after phenylephrine, to 19.7 +/- 6.2 mm Hg.L-1.min-1.m-2 (range 11.4-40.5), and returned to 15.4 +/- 3.4 mm Hg.L-1.min-1.m-2 (range 10.1-24.2) during normothermic bypass. Systemic vascular resistance appeared to vary reciprocally with blood flow, although this finding may represent a mathematical artifact, which can be avoided by using zero-flow critical pressure in the vascular resistance equation.
authors.affiliation: Department of Anesthesiology, Catholic University of Chile School of Medicine, Santiago, Chile.
authors.forename: J
authors.initials: J
authors.lastname: Urzua
authors.forename: G
authors.initials: G
authors.lastname: Meneses
authors.forename: C
authors.initials: C
authors.lastname: Fajardo
authors.forename: G
authors.initials: G
authors.lastname: Lema
authors.forename: R
authors.initials: R
authors.lastname: Canessa
authors.forename: C M
authors.initials: CM
authors.lastname: Sacco
authors.forename: J
authors.initials: J
authors.lastname: Medel
authors.forename: M E
authors.initials: ME
authors.lastname: Vergara
authors.forename: M
authors.initials: M
authors.lastname: Irarrazaval
authors.forename: S
authors.initials: S
authors.lastname: Moran
mesh_terms: D000328:Adult; D000368:Aged; D001783:Blood Flow Velocity; D001794:Blood Pressure; D006348:Cardiac Surgical Procedures; D002315:Cardiopulmonary Bypass; D005260:Female; D006801:Humans; D007036:Hypothermia, Induced; D008297:Male; D008875:Middle Aged; D014655:Vascular Resistance