Decrease in plasma/blood volume

How soon does it change: This happens within the first 2 days of bedrest. During the first few weeks, the volume of blood continues to decrease up to 15%.

What changes in the body: Within the first 24 hours, there is a shift in fluid from the tissues in you body, especially your legs. The fluid moves into the blood vessels. The body responds to this increase in volume in the blood vessels. There is a change in the hormone system that regulates sodium (salt) and water, the renin-angiotensin-aldosterone regulation system. The result is a release of salt and water through the kidneys, as urine^1,2 The volume of blood in the blood stream is reduced to "normal" - but the body has less fluid in it now.

How this affects standing up If the body has 6 liters of blood, the blood volume would decrease by about 900cc. This decrease in volume means there is less blood for the heart to pump around.²

Lee has postulated that the decrease in blood volume might also have a negative effect on the ability to deliver oxygen and nutrition to the muscles and take away waste. ¹

To learn more about the effect this has on the body and symptoms with standing up, go to How the Body Works When You Stand Up

Muscles in the body respond to activity or work that needs to be done.
    •  If there is more work done, the muscle increases in size & strength. This is what happens with strength training.
    •  If there is less work to be done, the muscle also changes, it decreases in size and gets weaker. The decrease in         the size is called muscle atrophy.

Lack of activity or not using muscles results in muscle atrophy. This starts to happen pretty soon after a person becomes less active. Older people (e.g. over 60) ten to lose a lot more than younger people.

What causes the loss of muscle: The loss of muscle is due to 2 things - decrease in the amount of protein the body creates and increased breakdown of protein (muscle tissue).

How soon does it start: : It is measurable within 3-5 days of bed rest in healthy people. It is not know how the person's illness increases the loss. Critically ill people lose almost 1% of their lean body mass each day, which is a greater loss than one gets with just bed rest. ^1,5

 

Changes in the heart and blood vessels (cardiovascular system)

Heart:   When the body is lying down, there is less work for the heart to pump blood around to all the body organs. It doesn't have to work against gravity. When you stand up, blood is moved to the legs and feet and the heart has to work harder to move the blood around to the head, arms and chest.

It starts to affect the heart within 2-3 weeks².

There is a decrease in the size of the left ventricle. after 6 weeks of bedrest. The left ventricle is the side of the heart that pumps the blood out to the rest of the body. See "Orthostatic Instability" for more about the impact of atrophy of the heart muscle on how the body responds to standing up.

Heart Rate:   In health adults, after 7 to 14 days in bed, the heart rate goes up over 10 beats per minute. There is also "heart rate variability" - meaning the heart rate changes more often. There is an increase in norepinephrine and epinephrine levels after 14 days of bed rest.¹

 

Changes in other muscles of the body

Muscle Atrophy: The muscles are not all affected at the same time. There is a pattern to the changes.

    Pattern of change:
    •  The muscle used for posture - sitting up and standing - are lost early. They have more extensive atrophy.
    •   Legs - the muscles used to extend (straighten) the leg have the earliest and greatest changes followed by
        lower leg flexors (bend the leg). In the BedRest Study#2, they found significant atrophy of the muscles of the hip
        (the postero-lateral muscles) after 56 days of bedrest. ^1,6
    •  Next - the back muscles.
    •  Next - arm muscles. ⁷
        The arm muscles take more than 60 days of bed rest before they start to atrophy in healthy people

 

Change in muscle metabolism and other factors

Metabolism/Nutrition: When a person becomes ill and does not eat enough food, the body has to use the energy it has stored to keep the body functioning. It will use any glucose or stored glucose (glycogen) that is has first. Muscles are made up of protein, which are combinations of amino acids (a little biochemistry info). The muscle protein becomes an important source of energy for the cells of the body.

People with less muscle mass to start will be affected more by this change. Muscle amount (mass) is lower in older people, they have been found to have 10-20%less muscle mass and fewer days of reserve than younger people. The muscles are not all affected at the same time. ^1,8

Inflammation and oxidative stress: These are thought to increase when a person is immobile. Both of these decrease how well muscles work and their size. Some studies have linked them to muscle loss.

 

Differences between men and women

Women lose more mass and strength with bed rest. It is not known why this occurs. ^1,9 They may be the result of the difference in the nerves and muscles in women: each nerve connects to fewer motor units in the muscle in women. There may also be other differences in how muscles change and their metabolism that is not known.

 

Back - Spine/Vertebral Column

Back pain is common in people who are on bedrest. It is more common in the first week but can continue after. It is thought to be do to a decrease in the amplitude of the movements of the spine. That means, when we are doing normal daily activities, our spine moves a lot especially up and down - bending over, reaching. When we are in bed, the spine doesn't move much even when we roll over.

Its the bigger movements of the spine that let the intervertebral disks between the vertebra expand or open up.¹ The spinal muscles can spasm when the disks are not opened with the frequent changes in position. The result is back pain. The spasms can be helped if there are regular, slow, large amplitude (large movements of the body) including repositioning.

Changes that occur in the back with bedrest include muscle atrophy of back muscles (multifidus, erector spinae, quadratus lumborum and psoas), flattening of the curvature of the lower spine (lordosis curve), changes in the disk shape/size (increase d intervertebral disc height and area and altered composition) and lengthening of the spine.^{17, 19} While some muscles get smaller (atrophy), others may get larger if they are doing more.

Some of the changes last longer than others once a person is up and walking. The psoas muscle doesn't change much with bed rest and it returned to pre-bed-rest size in 28 days; the multifidus (muscles along side the spinal cord that straighten it out and allow you to bend backwards). These muscles had more atrophy than the erector spinae. There were still changes in the multifidus muscles after 90 days up walking and returning to normal activity. ¹⁹ Changes in the lumbar-pelvic muscles were still present up to 1 year after bed rest.¹⁹ For example the flexor muscles of the abdomen increase in size (those are the muscles that do a sit up).

Studies trying to prevent the changes found that having people do specific exercises for 5-6 minutes, 3 days a week with high-load resistance was not enough to prevent all the changes. The exercises did partially decrease the muscle loss but it did not prevent the changes in the disk, the length of the spine or the lordosis shape.¹⁷

The changes in intervertebral disc volume, anterior and posterior disc height, and intervertebral length seen after 21 days of bed rest did not return to pre-bed-rest values 153 days later.¹⁸

There will be more about the back and recovery, in the section- Move Into Health. Sign up to be notified when it is posted.

Tendons & Joints

At first, tendons get less stiff. This means they are more flexible. When they are more flexible, the tendon does not do as well holding a joint in the position you want.

Joints can also become contracted when they have less movement, stimulation and range of motion (ability to move in all the directions they are supposed to). The joint contracture is due to shortening of the connective tissues and the muscles.

Normally, when we are in bed, we change our position about every 12 minutes. This stimulates the joints. When a person is in the ICU, especially on a ventilator, they do not change their position that often. In one study, up to 34% (almost 1 out of every 3 patients) in the ICU who have been in the ICU more than 14 days have joint contractures. When the joints of the legs are contracted, it can interfere with walking.

 

Nerves & Muscles

There is less activity between the nerves to the muscle and the muscle when muscle atrophy is present. The speed that messages are sent through the nerves and the muscle units are stimulated are decreased. Studies have suggested that this results in loss of strength and power as well as decreases voluntary movement and physical activity. ^1,10

When the changes happen

First few days:  There is a rapid decrease in the ability to do aerobic activity, like walking and running, within the first few days of bed rest. (This is also called the 'aerobic capacity')

After 14 days:   The decrease in ability to do aerobic activities continues to decrease but it gets less drastic as the bed rest continues.¹  

After 2 weeks of bed rest, a bed rest person can do moderate intensity exercise, which is up to 65% of their pre-bedrest ability.  They can do it for about 20-40 minutes.

Doing more intense exercise for longer times seems to be affected by the body's ability to regulate its temperature (heat).¹

During the first 30 days of bed rest:   The aerobic capacity decreases at rate of about 0.8 – 0.9% each day.¹

What causes the decrease ability to do aerobic activities:

During the first 1 to 2 wk: The reduced aerobic capacity of bed rest generally is associated with the decrease in blood volume and the decrease in cardiac output.

After the first 2 weeks: The rate of decrease with longer time on bed rest is related to changes the body makes in the heart, blood vessels, and muscles. These changes include changes in the heart muscle, the blood vessels, decreased muscle strength, and reduced oxidative enzyme activity.

The rate of decline in aerobic capacity becomes progressively smaller as the duration of bed rest increases. It is thought that it reaches the point that it has the amount of aerobic capacity required to perform daily functions during bed rest ( 16 ).

Aerobic exercise may also be influenced by the oxygen carrying capacity of blood, particularly during long duration bed rest¹.

Recovering aerobic capacity following bed rest

This is related to the how long the person has been on bed rest and what the level of fitness was before bed rest started.

   •   After short time of bed rest (2 wk), the recovery process may be complete within 1 wk,

   •   Following longer period of bed rest, it may take 2 to 4 wk for full recovery.

Changes can happen with the brain and spine (the central nervous system) and the nerves that go out to the body (the peripheral nervous system).

 

Changes in Nerve Sensation

There is a decrease in the sense of position, vibration and touch, especially in women who are not active.^{1, 11}   A decreases in sensation has an affect on a person's sense of body position - where the body is and what it is doing. This affects a person's sense of balance. Imbalance is also called postural instability. People can decrease activity even more when they feel unstable walking and it may contribute to falls.^1,12-16

Changes in Pain Sensation

There are changes in the skin sense of pain and in nerve signals sent to muscles. These changes have been reported to persist for more than 5 years after a person has been in the ICU more than 28 days. Those contacted in follow up had severe weakness and functional problems after discharge with a prolonged recover. There were muscle strength and nerve sensation abnormalities on the physical exam in 60% of the patients. The EMG studies showed they had a preceding axonal neuropathy with chronic partial denervation present at follow-up. The EMG also was consistent with a motor neuropathy in the past.²¹

In one study 7 of 16 patients who had been in the ICU more than 30 days still had general weakness decreased exercise tolerance after 2 years.²⁰ They also had focal compressive mononeuropathies (symptoms related to sensory nerves) 2 years after discharge. ²⁰

Decrease in blood flow to the skin and change in sweat rate

After a week of bed rest, the blood vessels in the skin stay narrower and do not get larger (dilate).  Blood flow to the skin during rest is lower after bedrest.¹ There is also a change in sweating after a week of bed rest.

 

Increase in body temperature

It has been thought that the increased core body temperature during exercise may be because the body can't increase the blood flow to the skin where it can cool off easier.¹  Another reason may be the decrease in sweating> Sweating is one way the body cools itself off. As the sweat evaporates (dries up), it creates a cooling reaction.

 

Change in blood flow to the splanchnic vessels (vessels to the stomach & intestines)

The ability to increase the resistance in the splanchnic vessels is changed with longer bedrest.¹ (increasing resistance means decreasing the blood flow)

Decrease in red blood cell mass

 

There is a decrease in red blood cells with prolonged bed rest. This may affect the oxygen carrying capacity of blood.  The red blood cell mass (amount) decreases by day 14^{1, 3}.   The 'oxygen carrying capacity' means how much oxygen The red blood cells can carry from the lungs to the rest of the body. The cell size is decreased also. The smaller RBC size is associated with less oxygen uptake and release, combined it can contribute to a feeling of dyspnea (short of breath but it doesn't’t seem to have a negative effect on aerobic ability in the short run.³  

The bone marrow is replaced with fat - which decreases the formation of

During prolonged bed-rest, the oxygen-carrying capacity of blood is reduced during rest and when tested during submaximal and maximal exercise^{1, 3}

The combination of decrease in cardiac output and red cell mass can explain 70% of the decrease in aerobic capacity.^{1, 4}

 

 

Lungs

There are changes in the distribution of blood to the lungs - there is more blood flow in the lower lung than the apex compared to standing up.¹ There is a decrease in the use of oxygen by the body. There is more areas of the lungs where the areas of the lung close and air does not get in (atelectasis) and there is more risk of aspiration (food or saliva going down into the lung). The changes in the lungs result in low blood oxygen at times, reduced reserve and increased shortness of breath.

 

Kidneys

At first, the kidneys will create more urine as they handle the increase in fluid in the blood stream. Bed rest also increases the amount of calcium that is released in the urine.¹ The calcium comes from break down of the bone. There is also increase in oxalate and phosphate in the urine. With the increase in calcium, there is an increase in kidney stones of calcium oxalate and calcium phosphate. In prolonged bed rest, there is also increase in protein released in the urine.

After 90 days, there are more changes in the hormone system, in ANP and vasopressin levels.¹

These changes happen on top of any changes or injury to the kidney from the illness that has brought a person to the ICU or has caused the prolonged bedrest.¹

 

Immune System

The release of catecholamines and cortisol stimulate the release of white blood cells from the tissue and out into circulation.

 

GI system

The stomach may be distended during bedrest. When a person is lying on their right side, the connection between the stomach and the esophagus is below the top of the stomach. This makes it easier for liquids in the stomach to collect around the muscle that keeps the connection between the stomach and esophagus closed. The position increase how often that muscle (sphincter) relaxes. When it relaxes, stomach liquids (which are usually acidic) can leak into the esophagus. The stomach liquids are very irritating to the lining of the esophagus. \

The flow of stomach contents out of the stomach and into the in testing seems to be affect the emptying of the stomach.

 

Metabolic System (nutrition)

The changes in nutrition with illness affect the muscles. When the body does not get nutrition from glucose, it gets it by breaking down glycogen stored and from fatty acids in fat tissue. The skeletal muscles switch from using glucose for energy and use of another energy path, glycolysis. Glycolysis is not as effective in providing the energy the body needs to stand or sit up.

Physical inactivity also results in insulin resistance. Insulin is released in the body and is needed, like a lock and key, to let glucose into the cells. Insulin resistance means the cells don't recognize the insulin and the body releases more to get the glucose into the cells. After 5 days of bedrest, in addition to an increase in the level of insulin in the body, there is an increase in lipids. in total, the changes result in changes in the creation of protein by the body.

 

Bones

The bones depend on gravity and muscle action to stay strong. There is substantial bone loss with prolonged bed rest. This makes the bones weaker. It can lead to osteopenia.