Erythrocyte-associated transients in capillary P[O.sub.2]: an isovolemic hemodilution study in the rat spinotrapezius muscle

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 (P[O.sub.2]) gradients between erythrocytes (RBCs). As RBCs and plasma alternately pass an observation point, these gradients are manifested as rapid fluctuations in P[O.sub.2], 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 P[O.sub.2] in capillaries of the rat spinotrapezius muscle at the following systemic hematocrits ([Hct.sub.sys]): normal (39%) and after moderate (HES 1; 27%) or severe (HES2; 15%) isovolemic hemodilution using a 6% hetastarch solution. A 532-nm laser, generating 10-[micro]s pulses concentrated onto a 0.9-[micro]m spot, was used to obtain plasma P[O.sub.2] values 100 times/s at points along surface capillaries of the muscle. Mean capillary P[O.sub.2] (PC[O.sub.2]; means [+ or -] SE) significantly decreased between conditions (normal: 56 [+ or -] 2 mmHg, n = 45; HESI: 47 [+ or -] 2 mmHg, n = 62; HES2: 27 [+ or -] 2 mmHg, n = 52, where n = capillary number). In addition, the magnitude of P[O.sub.2] transients ([DELTA]P[O.sub.2]) significantly decreased with hemodilution (normal: 19 [+ or -] 1 mmHg, n = 45; HES1: 11 [+ or -] 1 mmHg, n = 62; HES2: 6 [+ or -] 1 mmHg, n = 52). Results suggest that the decrease in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] and [DELTA]P[O.sub.2] with hemodilution is primarily dependent on [Hct.sub.sys] and subsequent microvascular compensations. microcirculation; oxygen transport; erythrocytes; oxygen tension gradients; phosphorescence quenching microscopy doi:10.1152/ajpheart.00915.2006.