Supine cycling plus volume loading prevent cardiovascular deconditioning during bed rest

Area of Science:

• Cardiovascular Physiology
• Space Medicine
• Exercise Physiology

Background:

• Orthostatic intolerance after bed rest or spaceflight is linked to reduced cardiac filling (hypovolemia) and cardiac atrophy.
• Preserving cardiac mechanical function and filling is crucial for maintaining upright tolerance.

Purpose of the Study:

• To investigate if exercise training and volume infusion can prevent orthostatic intolerance after 18 days of head-down tilt bed rest.
• To determine the independent and combined effects of exercise and volume loading on cardiovascular adaptation.

Main Methods:

• 18-day head-down tilt bed rest study with 21 participants (18 men, 3 women).
• Interventions included supine cycle exercise countermeasure and/or intravenous dextran (volume infusion) post-bed rest.
• Assessed orthostatic tolerance using lower body negative pressure (LBNP) and upright exercise capacity.

Main Results:

• Exercise training alone preserved left ventricular mass and distensibility but did not fully restore LBNP tolerance.
• Combined exercise training and dextran infusion maintained LBNP tolerance.
• Dextran infusion alone did not improve orthostatic tolerance or exercise capacity.

Conclusions:

• Daily supine exercise prevents cardiac atrophy but requires plasma volume restoration to fully prevent orthostatic intolerance after bed rest.
• Both cardiac atrophy and hypovolemia contribute independently to the decline in orthostatic tolerance.