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Erythrocyte-associated transients in capillary PO₂: an isovolemic hemodilution study in the rat spinotrapezius muscle

Department of Physiology, Medical College of Virginia Campus, Virginia Commonwealth University, Richmond, Virginia

Submitted 23 August 2006 ; accepted in final form 23 January 2007

Mathematical simulations of oxygen delivery to tissue from capillaries that take into account the particulate nature of blood flow predict the existence of oxygen tension (PO₂) gradients between erythrocytes (RBCs). As RBCs and plasma alternately pass an observation point, these gradients are manifested as rapid fluctuations in PO₂, also known as erythrocyte-associated transients (EATs). The impact of hemodilution on EATs and oxygen delivery at the capillary level of the microcirculation has yet to be elucidated. Therefore, in the present study, phosphorescence quenching microscopy was used to measure EATs and PO₂ in capillaries of the rat spinotrapezius muscle at the following systemic hematocrits (Hct_sys): normal (39%) and after moderate (HES1; 27%) or severe (HES2; 15%) isovolemic hemodilution using a 6% hetastarch solution. A 532-nm laser, generating 10-µs pulses concentrated onto a 0.9-µm spot, was used to obtain plasma PO₂ values 100 times/s at points along surface capillaries of the muscle. Mean capillary PO₂ (Pc; means ± SE) significantly decreased between conditions (normal: 56 ± 2 mmHg, n = 45; HES1: 47 ± 2 mmHg, n = 62; HES2: 27 ± 2 mmHg, n = 52, where n = capillary number). In addition, the magnitude of PO₂ transients (PO₂) significantly decreased with hemodilution (normal: 19 ± 1 mmHg, n = 45; HES1: 11 ± 1 mmHg, n = 62; HES2: 6 ± 1 mmHg, n = 52). Results suggest that the decrease in Pc and PO₂ with hemodilution is primarily dependent on Hct_sys and subsequent microvascular compensations.

microcirculation; oxygen transport; erythrocytes; oxygen tension gradients; phosphorescence quenching microscopy