Paradoxical hypotension following increased hematocrit and blood viscosity.
Hematocrit (Hct) of awake hamsters and CD-1 mice was acutely increased by isovolemic exchange transfusion of packed red blood cells (RBCs) to assess the relation between Hct and blood pressure. Increasing Hct 7-13% of baseline decreased mean arterial blood pressure (MAP) by 13 mmHg. Increasing Hct above 19% reversed this trend and caused MAP to rise above baseline. This relationship is described by a parabolic function (R2 = 0.57 and P < 0.05). Hamsters pretreated with the nitric oxide (NO) synthase (NOS) inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) and endothelial NOS-deficient mice showed no change in MAP when Hct was increased by <19%. Nitrate/nitrite plasma levels of Hct-augmented hamsters increased relative to control and L-NAME treated animals. The blood pressure effect was stable 2 h after exchange transfusion. These findings suggest that increasing Hct increases blood viscosity, shear stress, and NO production, leading to vasodilation and mild hypotension. This was corroborated by measuring A1 arteriolar diameters (55.0 +/- 21.5 microm) and blood flow in the hamster window chamber preparation, which showed statistically significant increased vessel diameter (1.04 +/- 0.1 relative to baseline) and microcirculatory blood flow (1.39 +/- 0.68 relative to baseline) after exchange transfusion with packed RBCs. Larger increases of Hct (>19% of baseline) led blood viscosity to increase >50%, overwhelming the NO effect through a significant viscosity-dependent increase in vascular resistance, causing MAP to rise above baseline values.
Martini
Judith
J
Dept. of Bioengineering, Univ. of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0412, USA. jmartini@bioeng.ucsd.edu
Carpentier
Benoît
B
Negrete
Adolfo Chávez
AC
Frangos
John A
JA
Intaglietta
Marcos
M
D000818:Animals; D001794:Blood Pressure / Q000502:physiology; D001809:Blood Viscosity / Q000502:physiology*; D006224:Cricetinae; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004912:Erythrocytes / Q000502:physiology*; D005078:Exchange Transfusion, Whole Blood; D006339:Heart Rate / Q000502:physiology; D006400:Hematocrit; D007022:Hypotension / Q000209:etiology* / Q000503:physiopathology*; D008647:Mesocricetus; D051379:Mice; D018345:Mice, Knockout; D008833:Microcirculation; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology; D009569:Nitric Oxide / Q000097:blood / Q000502:physiology; D033081:Shear Strength; D013314:Stress, Mechanical
D004791:Enzyme Inhibitors; D009569:Nitric Oxide; D019331:NG-Nitroarginine Methyl Ester