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Left ventricular volumes and hemodynamic responses at onset of dynamic exercise with reduced venous return
A C Nóbrega1, J W Williamson, J H Mitchell
1Harry S. Moss Heart Center, University of Texas Southwestern Medical Center, Dallas 75235-9034, USA.
Insights
Reducing venous return during exercise blunts initial cardiac output (CO) response. However, compensatory mechanisms can still increase mean arterial pressure (MAP) through other means, even with reduced blood flow.
Area of Science:
- Cardiovascular Physiology
- Exercise Physiology
- Hemodynamics
Background:
- Understanding the immediate hemodynamic adjustments to exercise is crucial for assessing cardiovascular function.
- Venous return significantly influences cardiac output and arterial pressure during dynamic exercise.
Purpose of the Study:
- To investigate the beat-by-beat hemodynamic effects of acutely reducing venous return at the onset of upright dynamic exercise.
- To determine the compensatory mechanisms that regulate mean arterial pressure (MAP) when cardiac output (CO) is compromised.
Main Methods:
- Ten healthy males performed 20-second upright cycling trials (30 W; 60 rpm) with and without leg venous occlusion (100 mmHg).
- Beat-by-beat measurements included mean arterial pressure (MAP), heart rate, and left ventricular end-systolic (ESV) and end-diastolic volumes (EDV) using two-dimensional echocardiography.
Main Results:
- Without venous occlusion, exercise increased MAP and CO, decreasing total peripheral resistance (TPR).
- Initial CO rise was due to heart rate, followed by increased stroke volume (decreased ESV, unchanged EDV).
- Venous occlusion blunted the CO response (decreased EDV, no stroke volume rise) and initial MAP increase, but MAP eventually rose without TPR change.
Conclusions:
- Reducing venous return impairs the immediate increase in cardiac output during mild upright exercise.
- The body employs compensatory mechanisms to increase mean arterial pressure even when cardiac output is limited by reduced venous return.
Abstract:
The hemodynamic effects of reducing venous return were assessed beat by beat at the onset of upright dynamic exercise. Mean arterial pressure (MAP), heart rate, and left ventricular end-systolic (ESV) and end-diastolic volumes (EDV; two-dimensional echocardiography) were measured in 10 healthy men during 20-s trials of upright cycling (30 W; 60 rpm). Exercise was performed either with or without venous occlusion of the legs (bilateral thigh cuffs inflated to 100 mmHg) in a random order. Without venous occlusion, MAP and cardiac output (CO) increased, and total peripheral resistance (TPR) decreased (P < 0.05) during the first approximately 10 beats after the onset of exercise. Initially, the CO response was accounted for by a rapid heart rate acceleration and, after approximately 15 cardiac cycles, by an increase in stroke volume, which occurred with a decrease in ESV and no change in EDV. With venous occlusion, EDV decreased and stroke volume did not rise during exercise. Thus the CO response was blunted by venous occlusion and MAP did not increase initially. However, after approximately 13 heart beats, MAP increased with no change in TPR. These findings suggest that compensatory mechanisms can elicit an increase in MAP at the onset of mild upright cycling when the CO response is blunted by reducing venous return.