Vasomotion enhanced by normovolemic hemodilution in rat diaphragmatic microcirculation

Since vasomotion--the rhythmic contraction and dilatation of arterioles--is frequently observed during homeostatic challenge, it has been assumed that vasomotion may cause the vulnerable portions of tissue to become intermittently oxygenated rather than permanently anoxic. This study used laser-Doppler flowmetry to investigate the effect of normovolemic hemodilution on the incidence, frequency and relative amplitude of vasomotion in rat diaphragm microcirculation.

Blood was substituted normovolemically with equal volumes of a 6% solution of dextran 70 (molecular weight 70,000). Infrared light (wavelength 780 nm) was delivered to and detected from diaphragm tissue area approximately 1 mm in diameter and 0.5 mm in depth. The light scattered by a moving red blood cell results in a Doppler frequency shift. Vasomotion was analyzed with Fourier transformation and autoregressive modeling.

Following hemodilution, systemic hematocrit showed a stepwise decrease of 4 hemodilution levels (80%, 60%, 51%, and 43% of baseline values). Vasomotion was found in 13/20 (65%) at baseline, and 20/20 (100%) at the 43% level (p < 0.05 from baseline). The median fundamental frequencies during the control state and at the 4 corresponding incremental hemodilution levels were 3.29 cycles per minute (cpm), 3.30 cpm, 3.14 cpm, 3.26 cpm, and 3.42 cpm (p < 0.05 from baseline). The median relative amplitudes at baseline and 4 hemodilution stages were 31.7%, 38.5%, 40.0% (p < 0.05 from baseline), 40.7% (p < 0.05 from baseline), and 41.6% (p < 0.05 from baseline).

This study found that an acute reduction of systemic hematocrit greatly increased the activity of vasomotion in the diaphragm microvascular bed of rats, suggesting vasomotion could be a protective mechanism against tissue hypoxia.