Erythrocyte‐associated transients (EATs) in capillary PO 2 : an isovolemic hemodilution study in the rat spinotrapezius muscle

The discrete nature of blood flow in capillaries permits the measurement of PO 2 gradients between erythrocytes (RBCs), manifest as rapid fluctuations in PO 2 known as EATs. Mathematical models have predicted that RBC spacing in capillaries is a critical determinant of EAT magnitude. Purpose and Methods: In the present study, phosphorescence quenching microscopy was used to measure capillary plasma PO 2 (P c O 2 ) 100 times/s with 532 nm laser excitation, 10 μs pulse duration, and a 0.9 μm excitation spot at stationary positions along surface capillaries of the rat spinotrapezius muscle. Systemic Hct (Hct s ) was varied from Normal (39%) to either a moderate (HES1, 27%) or severe level (HES2, 15%) via isovolemic hemodilution using 6% hetastarch with the expectation of reducing capillary Hct (Hct c ). Results: As Hct s was reduced, Hct c did not significantly change between conditions, while P c O 2 (shown as mean ± SE in mmHg; where n = number of capillaries) significantly decreased (Normal = 56 ± 2, n = 45; HES1 = 47 ± 2, n = 62; HES2 = 27 ± 2, n = 52). In addition, the magnitude of PO 2 transients (ΔPO 2 = PO 2 Max – PO 2 Min ) significantly decreased with hemodilution (Normal = 19 ± 1, HES1 = 11 ± 1, HES2 = 6 ± 1). Conclusion: Results suggest that the decrease in P c O 2 and ΔPO 2 with hemodilution is primarily dependent on the Hct s and subsequent microvascular compensations. Supported by NIH grant HL18292 and AHA grant 065449U .