Deconditioning (also called cardiovascular deconditioning) is the acclimation of the body to a less strenuous environment and the decreased ability to function properly under normal conditions. This basically means that when you have less physical stress on the body for a certain period of time, like seen in bed rest, the body adapts to that level of functioning, so when you want to engage again in normal physical activities, it is difficult for your body. Deconditioning makes multiple systems of your body less functional.
Bed rest is the typical situation associated with deconditioning. Patients on bed rest lose muscle mass and strength rapidly. 1-3% of muscle strength is lost per day, with 10-20% decrease in a week’s time. If completely immobilized for 3-5 weeks, a patient can lose up to 50% of their strength. Loss of muscle mass is also a problem. Upper legs can lose 3% mass within a week of bed rest. The lower back and weight bearing muscles in the legs are most affected by loss of mass.
Within 24 hours of bed rest, your cardiovascular system is changing. In this time, your blood volume decreases 5%. In less than a week, 10% is lost; in two weeks, 20%. Resting heart rate also increases 4-15 bpm within the first month of bed rest. Laying down for so long means that blood that is normally in the lower part of your body is moved to the trunk. This causes excretion of water and salt, resulting in less plasma and blood volume.
In healthy controls, when you change position, your body rapidly moves fluid from one part of the body to others. This phenomenon is called fluid shifting. Normally, when moving from a laying position to standing, 500-700 ml of blood are moved from the trunk to the legs. This movement of fluid is called “functional hemorrhage”. Special nerve clusters called baroreceptors (which measure pressure in the blood vessels) tell the nervous system that there is less blood in the chest. Your body then increases the heart rate, the force with which your heart beats, tightens up vessels so that they are less “leaky” and tells your body not to make urine temporarily. All of these functions allow your body to keep a normal blood pressure and adequate blood supply despite this large movement of fluid.
In healthy controls, when you lay down after standing, the reverse happens. 500-700 ml of fluid is rapidly transferred from the lower body to the trunk. This is called a “central shift”. This increase in fluid in the chest results in the veins returning more blood to the heart, increasing blood pressure. When the baroreceptors feel more pressure than usual from this added fluid, the heart rate and force with which the heart beats decrease, the vessels are relaxed so that fluids can move out of them more freely and your body begins to make urine again.
When you are deconditioned, your body does not make these changes correctly when you change position. The hallmark of deconditioning is reduced orthostatic tolerance. This means that when you change position, your body does not compensate correctly to maintain necessary blood pressure and adequate blood supply to the brain. Deconditioned patients often do not have sufficient blood volume to maintain blood pressure when standing. When they stand, their heart pumps out less blood than normal, so the heart starts beating faster to compensate. When it beats too fast, it is called tachycardia.
In addition to inability to maintain blood pressure correctly when changing positions, deconditioned patients also exhibit decreased blood volume pumped out by the heart, atrophy of heart muscle and decreased maximum oxygen consumption. These patients often have other forms of vascular dysfunction, diminished neurologic reflexes and reduced ability to exercise. A number of other systems are affected by deconditioning.
Though prolonged bed rest is the model with which deconditioning is most often associated, there is significant evidence that chronically ill patients may often be deconditioned, including those with chronic lower back pain, chronic fatigue syndrome, and rheumatoid arthritis.
Munsterman et al. Are persons with rheumatoid arthritis deconditioned? A review of physical activity and aerobic capacity. BMC Musculoskeletal Disorders 2012, 13:202
Eric J. Bousema, Jeanine A. Verbunt, Henk A.M. Seelen, Johan W.S. Vlaeyen, J. Andre Knottnerus. Disuse and physical deconditioning in the first year after the onset of back pain. Pain 130 (2007) 279–286.
De Lorenzo, H. Xiao, M. Mukherjee, J. Harcup, S. Suleiman, Z. Kadziola and V.V. Kakkar. Chronic fatigue syndrome: physical and cardiovascular deconditioning. Q J Med 1998; 91:475–481.
Hasser, E. M. And Moffitt, J. A. (2001), Regulation of Sympathetic Nervous System Function after Cardiovascular Deconditioning. Annals of the New York Academy of Sciences, 940: 454–468.