Skeletal muscle vascular volume changes with increased venous pressure

Area of Science:

• Physiology
• Vascular Biology
• Hemodynamics

Background:

• Understanding vascular volume dynamics is crucial for interpreting physiological responses to altered venous pressure.
• Previous studies have explored blood volume changes, but the specific contributions of active and mobilized volumes under varying perfusion conditions require further elucidation.

Purpose of the Study:

• To quantify changes in total, active, and mobilized vascular volumes in canine gracilis muscle under controlled venous pressure elevations.
• To compare volume changes under constant inflow pressure versus constant inflow perfusion.
• To investigate potential red blood cell (RBC) and plasma redistribution during venous pressure changes.

Main Methods:

• Canine gracilis muscles were perfused under constant inflow pressure or constant inflow conditions.
• Circulating blood volumes were measured using the constant infusion technique with red blood cell-51Cr (RBC-51Cr) and albumin-131I.
• Venous pressure was incrementally elevated (delta PV), and volume changes were assessed via plethysmography and changes in muscle radioactivity.

Main Results:

• Both active and total vascular volumes increased with venous pressure up to 25 mm Hg, then plateaued.
• Active vascular volume increases were significantly greater than total vascular volume increases.
• Volume changes were approximately double in constant inflow groups compared to constant pressure groups.
• Mobilized vascular volume constituted about 45% of the active vascular volume change.
• An increased ratio of RBC-51Cr to albumin-131I volume was observed with elevated venous pressure, suggesting plasma skimming reduction.

Conclusions:

• Venous pressure elevation significantly expands vascular volumes, particularly the active component.
• The constant inflow perfusion method yields greater volume changes than constant pressure.
• A substantial portion of the expanded vascular volume represents previously inaccessible blood (mobilized volume).
• Redistribution of red blood cells relative to plasma occurs, potentially due to reduced plasma skimming at higher venous pressures.