Quantifying the dynamics of capillary blood flow responses to acute local changes in oxygen and carbon dioxide concentrations

G.M. Russell McEvoy, Memorial University of Newfoundland

Objectives: We aimed to quantify the magnitude and time transients of capillary blood flow responses to acute changes in local oxygen concentration ([O2]), and carbon dioxide concentration ([CO2]) in skeletal muscle. Additionally, we sought to quantify the combined response to low [O2] and high [CO2] to mimic muscle microenvironment at the onset of exercise.

Methods: 13 Sprague Dawley rats were anaesthetized, instrumented for systemic monitoring, and mechanically ventilated. The extensor digitorum longus muscle was blunt dissected and reflected over a gas exchange chamber mounted in the stage of an inverted microscope. Four O2, four CO2, and a combined low O2 and high CO2 challenges were delivered to the muscle surface with simultaneous visualization of capillary blood flow responses. Offline analysis was completed using custom MATLAB software.

Results: 7–2% [O2] challenges decreased capillary red blood cell (RBC) saturation (SO2) within 2 s following the step change (46.53 ± 19.56% vs. 48.51 ± 19.02%, p <0.0001, τ=1.44s), increased RBC velocity within 3 s (228.53 ± 190.39 μm/s vs. 235.74 ± 193.52 μm/s, p <0.0003, τ=35.54s) with a 52% peak increase by the end of the challenge, RBC supply rate (SR) showed similar dynamics. 5–10% [CO2] challenges increased RBC velocity within 2 s following the step change (273.40 ± 218.06 μm/s vs. 276.75 ± 215.94 μm/s, p=0.007, τ=79.34s) with a 58% peak increase, RBC SR showing similar dynamics. Combined [O2] and [CO2] challenges resulted in additive responses to all microvascular hemodynamic measures with a 103% peak velocity increase by the end of the collection period.

Conclusion: Microvascular level changes in muscle [O2] and [CO2] provoked capillary hemodynamic responses with differing time transients. Simulating exercise via combined [O2] and [CO2] challenges demonstrated the independent and additive nature of local blood flow responses to these agents.