2. Cardiovascular adaptation in health and disease

Last updated on May 24, 2019 at 11:32

Summary

During exercise the body requires more blood per second than in rest, meaning that the cardiac output to the muscles must increase. In healthy people, this is done with several mechanisms:

  • The myocardial contractility increases
  • The heart rate increases
  • The preload increases
  • The afterload decreases
  • Cardiac output is redistributed from visceral organs to the muscles

Circulating catecholamines and increased sympathetic activity is responsible for the first three. Vasodilation of the vessels in the muscles cause the total peripheral resistance and therefore the afterload to decrease.

People with heart failure cannot increase the cardiac output during exercise by these mechanisms. The myocardium is damaged, so the contractility can’t increase. These people rely on very increased heart rate to increase CO, but that is not sufficient. They have high sympathetic tone, which increases the TPR, hampering the CO.

Repetition

These notes assume you know most of the cardiac physiology from physiology. For repetition:

Preload = the extent to which the myocardium is stretched at the end of diastole, right before systole. Preload depends on the end-diastolic volume (EDV), which mostly depends on venous return to the heart.

Afterload = the force that the ventricles must eject blood against during systole. Afterload depends mostly on the pressure in the aorta, measured by the mean arterial pressure (MAP), and the total peripheral resistance (TPR).

Stroke volume (SV) = the volume of blood ejected by the heart in systole. It’s defined like this: SV = EDV – ESV

Ejection fraction (EF) = the fraction of blood that was inside the ventricle before systole that got ejected in systole. EF = SV / EDV

Frank-Starling mechanism

The Frank-Starling mechanism or law is a mechanism that is built into the heart. The mechanism adjusts stroke volume according to the venous return in order to maintain cardiac output. The result is that the stroke volume of both the left and the right ventricles remain the same.

The mechanism works like this: When venous return increases, the preload increases, which stretches the ventricles more. The ventricles respond to this increased stretch by increasing the force of contraction. This causes the stroke volume to increase. We can imagine the cardiac muscle like a spring or rubber band: the more it’s stretched, the harder it will snap back!

The result is that the when more blood enters the heart, more blood is ejected, so the amount of blood coming into the heart is always constant with the amount going out of it.

When the body needs more blood

In some cases, the body’s need for blood exceeds the 5 L/min of cardiac output that is found in rest. The best example for increased need for blood is in exercise, but it also increases when the temperature around a person is high. In healthy people, the cardiac output in exercise increases to 20-25 L/min, or even up to 40 in athletes! The heart accomplishes this is three ways:

  • Increasing the contractility of the myocardium
  • Increasing the heart rate
  • Increasing the stroke volume

Contractility ↑ -> Ejection fraction ↑ -> Cardiac output ↑

Heart rate ↑ -> Cardiac output ↑.

Increasing the heart rate is effective in increasing the cardiac output. The increase is mediated by increased sympathetic tone and circulating catecholamines. However, the downside with this is that the coronary blood flow is reduced. This is because the coronary arteries are compressed during systole, so blood only flows in those arteries during diastole. When heart rate increases, the diastolic time decreases, so that the cardiac muscle has less time to be perfused. This is made even worse by the fact that when heart rate increases, the oxygen demand of the myocardium increases as well. This means that tachycardia by itself is not a good way to increases cardiac output.

Catecholamine release -> Venous tone ↑ -> Venous return ↑ -> Preload ↑ -> Stroke volume ↑ -> Cardiac output ↑.

Stroke volume is increased by three factors:

  • Increased preload
  • Increased myocardial contractility
  • Decreased afterload

Recall that most of the blood in the body is stored in the veins. When catecholamines are released, these veins will constrict so that they will stop “storing” the blood and instead push it into the circulation. This causes more blood to return to the heart, so CO increases.

It’s important for the ventricles to have a high compliance (be distensible). When the compliance is sufficiently high (as in healthy people) the ventricles can fill with blood without increasing the pressure inside the ventricles, called end-diastolic pressure (EDp). When this pressure is low will blood easily flow from the veins into the ventricles, but if the EDp is high will this flow be more difficult.

Sympathetic activation will release norepinephrine into local nerve-endings in the myocardium, yielding a positive inotropic effect, meaning that the myocardial contractility increases.

Lastly, a decreased afterload will increase the stroke volume. In exercise the peripheral resistance will decrease because the vessels in the muscles will dilate. When the pressure in the aorta (and the rest of the systemic circulation) is low, then it is easier for the left ventricle to pump out blood into it.

Another important factor in exercise is the redistribution of blood from non-essential organs to the muscles and to the skin. Organs like the kidneys and the GI tract will receive much less blood, but the skin, coronaries and skeletal muscle will receive much more.

At rest

Organ

During moderate exercise

1300 mL

GI tract, liver

600 mL

1100 mL

Kidneys

550 mL

400 mL

Skin

1700 mL

700 mL

Brain

700 mL

200 mL

Coronaries

550 mL

750 mL

Striated muscle

8000 mL

550 mL

Bones, others

450 mL

5000 mL

Total

12 500 mL

Note how the brain perfusion is kept constant. The body will always try to maintain brain perfusion.

Exercising

The way the circulation adapts to an increased CO demand, such as what happens in physical exercise, is very different between healthy people and people with heart failure. The differences are summed up below:

Property

Healthy people

Heart patients

Resting heart rate

Around 60-70

Around 90

Dominant autonomic tone in rest

Parasympathetic tone dominates

Sympathetic tone dominates

Reserve in heart rate

Large. Can increase from 60 -> 200

Small. Can only increase from 90 -> 150

Contractility

Increases. EF increases.

Can’t increase due to myocardial injury. EF low.

How fast does HR increase?

Slowly

Rapidly

Left ventricular compliance

Good. EDp doesn’t increase when EDV increases.

Bad. EDp increases when EDV increases.

Change in afterload

Decreases due to vasodilation in muscle

Increases due to high sympathetic tone

Increase in BP

Only systolic BP increases

Both systolic and diastolic BP increases

How quickly HR normalizes after exercise

Normalizes quickly

Normalizes slowly

Fitness level

The Rock

Danny Devito


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3. Distribution of cardiac output and its disorders in the young and the elderly

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