In addition to the confounding effects of hypovolemia, increased venous pooling in the lower extremities when a patient assumes the upright posture during reambulation could contribute to lowered cardiac filling pressure and stroke volume during bed rest. Several observations support this notion. Despite maximal muscle pumping action on peripheral veins, increased leg venous pooling from application of lower body negative pressure caused lower central venous pressure, stroke volume, and cardiac output during exercise(40). Reduced cardiac filling and stroke volume during exercise induced by bed rest could be accentuated by increased venous pooling in the lower extremities since venous compliance of the legs increases by 20-25% with bed rest (14,15). In addition, reduction in ˙VO2max after 10 d of bed rest in the upright posture(≈17%) was more than twice in the supine posture (≈7%) in spite of the same elevation in heart rate (19) and ˙VO2 kinetics were significantly slowed in the upright compared with the supine posture (17). Taken together, these results provide evidence that orthostatic factors such as increased venous and leg compliance can contribute significantly to the reduction in post-bed rest˙VO2max. These factors must reduce cardiac filling and output since the reduction in post-bed rest maximal stroke volume was greater in the upright posture (36), whereas heart rate elevation in the upright posture was similar to that in the supine posture(16). The reduction in supine ˙VO2max supports the hypothesis that peripheral factors other than orthostatic pooling must contribute to reduced functional cardiovascular capacity following bed rest.
The cardiovascular consequences of bed rest can be divided into central(cardiac) and peripheral mechanisms that contribute to maintenance of˙VO2max (Fig. 9). An elevated maximal heart rate is associated with elevations in norepinephrine release at maximal exercise and increased cardiac β-adrenergic response in addition to lower vagal tone. These adrenergic changes could also increase cardiac contractility which would be consistent with the reported increase in ejection fraction during exercise following bed rest despite reductions in stroke volume and cardiac filling. Despite the increase in maximal heart rate and probably cardiac contractility, maximal cardiac output is dramatically reduced by an overwhelming decrease in stroke volume. Since cardiac contractility appears enhanced, the lowered stroke volume must be a result of reduced cardiac filling associated with less blood volume and lower central venous pressure. Increased compliance of the veins in the leg muscles may also contribute to the limited venous return, especially after return to the upright posture. Although arteriovenous O2 difference appears to remain constant during bed rest, reductions in capillarization and maximal blood flow in the muscle could limit oxygen delivery and utilization. The ultimate consequence of these alterations in cardiac and vascular functions resulting from bed rest confinement is the reduction in ˙VO2max.
1. Bassey, E. J., T. Bennett, A. T. Birmingham, P. D. Fentem, D. Fitton, and R. Goldsmith. Effects of surgical operation and bed rest on cardiovascular responses to exercise in hospital patients. Cardiovasc. Res.
2. Birkhead, N C., J. J. Blizzard, J. W. Daly, G. J. Haupt, B. Issekutz, R. N. Myers, and K. Rodahl. Cardiodynamic and Metabolic Effects of Prolonged Bed Rest
. Wright-Patterson Air Force Base, OH: Aerosp. Med. Res. Lab (AMRL-TDR-63-37) 1963.
3. Birkhead, N. C., J. J. Blizzard, J. W. Daly, G. J. Haupt, B. Issekutz, R. N. Myers, and K. Rodahl. Cardiodynamic and Metabolic Effects of Prolonged Bed Rest with Daily Recumbent or Sitting Exercise and with Sitting Inactivity
. Wright-Patterson Air Force Base, OH: Aerosp. Med. Res. Lab (AMRL-TDR-64-61) 1964.
4. Blamick, C. A., D. J. Goldwater, and V. A. Convertino. Leg vascular responsiveness during acute orthostasis following simulated weightlessness. Aviat. Space Environ. Med.
5. Blomqvist, G., J. H. Mitchell, and B. Saltin. Effects of bed rest on the oxygen transport system. In: Hypogravic and Hypodynamic Environments
, R. H. Murry and M. McCally (Ed.). Wash, DC: NASA Special Publication 269, 1971, pp. 171-176.
6. Cardus, D. Effects of 10 days recumbency on the response to the bicycle ergometer test. Aerospace Med.
7. Chase, G. A., C. Grave, and L. B. Rowell. Independence of changes in functional and performance capacities attending prolonged bed rest.Aerospace Med.
8. Convertino, V. A. Effect of orthostatic stress on exercise performance after bed rest: relation to inhospital rehabilitation. J. Cardiac Rehabil.
9. Convertino, V. A. Exercise responses after inactivity. In:Inactivity: Physiological Effects
, H. Sandler and J. Vernikos-Danellis (Eds.). Orlando, FL: Academic Press, 1986, pp. 149-191.
10. Convertino, V. A. Potential benefits of maximal exercise just prior to return from weightlessness. Aviat. Space Environ. Med.
11. Convertino, V. A., R. Bisson, R. Bates, D. Goldwater, and H. Sandler. Effects of antiorthostatic bed rest on the cardiorespiratory responses to exercise. Aviat. Space Environ. Med.
12. Convertino, V. A., D. F. Doerr, D. L. Eckberg, J. M. Fritsch, and J. Vernikos-Danellis. Head-down bed rest impairs vagal baroreflex responses and provokes orthostatic hypotension. J. Appl. Physiol.
13. Convertino, V. A., D. F. Doerr, D. A. Ludwig, and J. Vernikos. Effect of simulated microgravity on cardiopulmonary baroreflex control of forearm vascular resistance. Am. J. Physiol.
14. Convertino, V. A., D. F. Doerr, K. L. Mathes, S. L. Stein, and P. Buchanan. Changes in volume, muscle compartment, and compliance of the lower extremities in man following 30 days of exposure to simulated microgravity. Aviat. Space Environ. Med.
15. Convertino, V. A., D. F. Doerr, and S. F. Stein. Changes in size and compliance of the calf following 30 days of simulated microgravity. J. Appl. Physiol.
16. Convertino, V. A., D. J. Goldwater, and H. Sandler. Effect of orthostatic stress on exercise performance after bed rest.Aviat. Space Environ. Med.
17. Convertino, V. A., D. J. Goldwater, and H. Sandler.˙VO2
kinetics of constant-load exercise following bed rest-induced deconditioning. J. Appl. Physiol.
18. Convertino, V. A., D. J. Goldwater, and H. Sandler. Bed rest-induced peak ˙VO2
reduction associated with age, gender and aerobic capacity. Aviat. Space Environ. Med.
19. Convertino, V. A., J. Hung, D. J. Goldwater, and R. F. Debusk. Cardiovascular responses to exercise in middle-aged men following ten days of bed rest. Circulation
20. Convertino, V. A., G. M. Karst, S. M. Kinzer, D. A. Williams, and D. J. Goldwater. Exercise capacity following simulated weightlessness in trained and nontrained subjects (Abstract). Aviat. Space Environ. Med.
21. Convertino, V. A., G. M. Karst, C. R. Kirby, and D. J. Goldwater. Effect of simulated weightlessness on exercise-induced anaerobic threshold. Aviat. Space Environ. Med.
22. Convertino, V. A., J. L. Polet, K. A. Engelke, G. W. Hoffler, L. D. Lane, and C. G. Blomqvist. Increased β-adrenergic responsiveness induced by 14 days exposure to simulated microgravity.J. Gravitational Physiol.
23. Convertino, V. A., H. Sandler, P. Webb, and J. F. Annis. Induced venous pooling and cardiorespiratory responses to exercise after bed rest. J. Appl. Physiol.
24. Convertino, V. A., R. W. Stremel, E. M. Bernauer, and J. E. Greenleaf. Cardiorespiratory responses to exercise after bed rest in men and women. Acta Astronautica
25. Crandall, C. G., K. A. Engelke, J. A. Pawelczyk, P. B. Raven, and V. A. Convertino. Power spectral and time based analysis of heart rate variability following 15 days simulated microgravity exposure in humans.Aviat. Space Environ. Med.
26. Deitrick, J. E., G. D. Whedon, E. Shorr, V. Toscani, and V. B. Davis. Effects of immobilzation upon various metabolic and physiologic functions of normal men. Am. J. Med.
27. Ekblom, B., A. N. Goldbarg, A. Kilbom, and P.-O. Astrand. Effects of atropine and propranolol on the oxygen transport system during exercise in man. Scand. J. Clin. Lab. Invest.
28. Engelke, K. A. and V. A. Convertino. Catecholamine response to maximal exercise following 16 days of simulated microgravity.Aviat. Space Environ. Med.
29. Engelke, K. A., B. D. Levine, and V. A. Convertino. Effects of acute maximal exercise on maximal leg conductance following exposure to 16 days of simulated microgravity (Abstract). Med. Sci. Sports Exerc.
30. Friman, G. Effect of clinical bed rest for seven days on physical performance. Acta Med. Scand.
31. Georgiyevskiy, V. S., L. I. Kakurin, B. S. Katkovskii, and Y. A. Senkevich. Maximum oxygen consumption and functional state of the circulation in simulated zero gravity. In: The Oxygen Regime of the Organism and its Regulation
, N. V. Lauer and A. Z. Kilchinskaya (Eds.). Kiev: Naukova Dumka, 1966. p. 181-184.
32. Greenleaf, J. E., E. M. Bernauer, A. C. Ertl, T. S. Trowbridge, and C. E. Wade. Work capacity during 30-days of bed rest with isotonic and isokinetic exercise training. J. Appl. Physiol.
33. Greenleaf, J. E. and S. Kozlowski. Physiological consequences of reduced physical activity during bed rest. Exerc. Sport Sci. Rev.
34. Greenleaf, J. E. and S. Kozlowski. Reduction in peak oxygen uptake after prolonged bed rest. Med. Sci. Sports Exerc.
35. Hikida, R. S., P. D. Gollnick, G. A. Dudley, V. A. Convertino, and P. Buchanan. Structural and metabolic characteristics of human skeletal muscle following 30 days of simulated microgravity. Aviat. Space Environ. Med.
36. Hung, J., D. Goldwater, V. A. Convertino, J. H. Mckillop, M. L. Goris, and R. F. Debusk. Mechanisms for decreased exercise capacity following bed rest in normal middle-aged men. Am. J. Cardiol.
37. Kakurin, L. I., R. M. Akhrem-Adhremovich, Y. V. Vanyushina, et al. The influence of restricted muscular activity on man's endurance of physical stress, accelerations and orthostatics. In:Soviet Conference on Space Biology and Medicine
, Moscow, 1966, pp. 110-117.
38. Katkovskiy, B. S., G. V. Machinskiy, P. S. Toman, V. I. Danilova, and B. F. Demida. Man's physical performance after thirty-day hypokinesia with countermeasures. Kosm. Biol. Med.
39. Lamb, L. E., R. L. Johnson, P. M. Stevens, and B. E. Welch. Cardiovascular deconditioning from space cabin simulator confinement.Aerospace Med.
40. Mack, G., H. Nose, and E. R. Nadel. Role of cardiopulmonary baroreflexes during dynamic exercise. J. Appl. Physiol.
41. Meehan, J. P., J. P. Henry, S. Brunjes, and H. Devries.Investigation to determine the effects of long-term bed rest on G-tolerance and on psychomotor performance
. Los Angeles, CA: Dept. of Physiology, University of Southern California (NASA-CR-62073) 1966.
42. Rodahl, K., N. C. Birkhead, J. J. Blizzard, B. Issekutz, Jr., and E. D. R. Pruett. Physiological changes during prolonged bed rest. In:Nutrition and Physical Activity
, G. Blix. (Ed.). Uppsala: Almqvist& Wiksells, 1967, p. 107-113.
43. Saltin, B., G. Blomqvist, J. H. Mitchell, R. L. Johnson, K. Wildenthal, and C. B. Chapman. Response to exercise after bed rest and after training. Circulation
38(Suppl. 7):1-78, 1968.
44. Saltin, B. and L. B. Rowell. Functional adaptations to physical activity and inactivity. Fed. Proc.
45. Stevens, P. M., P. B. Miller, C. A. Gilbert, T. N. Lynch, R. L. Johnson, and L. E. Lamb. Influence of long-term lower body negative pressure on the circulatory function of man during prolonged bed rest. Aerospace Med.
46. Stremel, R. W., V. A. Convertino, E. M. Bernauer, and J. E. Greenleaf. Cardiorespiratory deconditioning with static and dynamic leg exercise during bed rest. J. Appl. Physiol.
47. Taylor, H. L., A. Henschel, J. Brozek, and A. Keys. Effects of bed rest on cardiovascular function and work performance. J. Appl. Physiol.
48. White, P. D., J. W. Nyberg, and W. J. White. A comparative study of the physiological effects of immersion and recombency. In: Proceedings of the 2nd Annual Biomedical Research Conference
, Houston, TX, 1966, pp. 117-166.
49. Williams, D. A. and V. A. Convertino. Circulating lactate and FFA during exercise: Effect of reduction in plasma volume following simulated microgravity. Aviat. Space Environ. Med.
HEART RATE; STROKE VOLUME; CARDIAC OUTPUT; CARDIAC FUNCTION; AUTONOMIC FUNCTION; BLOOD VOLUME; VENOUS COMPLIANCE; BLOOD FLOW; CAPILLARIZATION