Cardiac Cycle

The Cardiac Cycle

The Cardiac Cycle describes the electrical and mechanical events that occur with each heart beat.

Its duration is reciprocal to heart rate, i.e., an increase in heart rate decreases the duration of the cardiac cycle (in other words, the faster the heart beats, the faster each cardiac cycle completes).

Lasts approximately 800 milliseconds.

Diastole and Systole:

Diastole - The period of time when the atria or ventricles relax to passively fill with blood.

Systole - The period of time when the atria or ventricles actively contract to pump blood.

Valves

The atrioventricular and semilunar valves regulate blood flow through the heart. They do so by opening or closing in response to pressure changes within the heart and great vessels.

7-Step Diagram

Because the cardiac cycle is continuous, we could begin our diagram at any point; we begin with atrial systole.

1. Atrial Systole:

Initiated by the P wave, which triggers atrial depolarization.

The atria contract, which increases inter-chamber pressure and forces a small amount of blood into the ventricles (about 10% of total ventricular volume).

Be aware that atrial contraction is NOT the primary mechanism by which blood flows from the atria to the ventricles; by the time atrial systole occurs, passive ventricular filling has already occurred.

Atrial contraction is followed by a period of diastole, which overlaps with ventricular systole.

2. Early Ventricular Systole

Initiated by the QRS complex, which triggers ventricular depolarization.

The early phase of ventricular systole comprises isovolumetric contraction, during which the ventricles contract only enough to raise inter-chamber pressure and close the atrioventricular valves; NOT enough to force open the semilunar valves of the great vessels.

It helps to know that "iso" means equal, or same – ventricular pressure changes, but volume remains the same.

The closing of the AV valves can be heard as the first heart sound (S1) during auscultation.

As a result of AV valve closure, blood is temporarily held within the high-pressure ventricles, as it cannot move "backwards" into the atria, nor can it enter the aorta and pulmonary trunk.

3. Mid Ventricular Systole

During mid ventricular systole, the ventricular myocytes (muscle cells) forcefully contract, and increase ventricular pressure above the vascular pressure of the great vessels.

Thus, the semilunar valves are pushed open, and blood is rapidly ejected from the ventricles.

4. Late Ventricular Systole

Repolarization of the ventricles begins (reflected by the T wave on the ECG).

Ventricular pressure falls, and, as a result, blood ejection slows.

Meanwhile, venous return raises atrial pressures.

5. Early Ventricular Diastole

Reduced ventricular pressure closes the semilunar valves. Thus, this is a period of ventricular isovolumetric relaxation.

Although ventricular pressure is reduced, the volume of blood remains the same.

Left arterial pressure (LAP) continues to rise as venous return moves blood into the atria.

6. Mid Ventricular Diastole

Muscular relaxation reduces ventricular pressure enough to open the atrioventricular valves. This allows passive ventricular filling.

7. Late Ventricular Diastole

Continued ventricular filling results in increased ventricular pressure. This reduces the rate of passive filling.

Key Blood Volume Measurements

These measurements are used to determine stroke volume and cardiac work, which tell us how much work the heart has to do to supply the body tissues with oxygenated blood.

Typical End Ventricular Diastolic Volume (EDV) is approximately 120 milliliters. In other words, this is the total amount of blood after passive filling.

Typical End Systolic Volume (ESV) is approximately 50 milliliters. In other words, this is the amount of blood remaining after the ventricles have contracted and forced blood into the great vessels.

As you can see, this implies that the ventricles do NOT eject all of 120 milliliters of blood present at the end of diastole.

Thus, we need to calculate Stroke Volume, which is the volume of blood the ventricles actually moved into circulation. To do this, we subtract the end systolic volume from the end diastolic volume; a typical healthy value is approximately 70 milliliters.

Reduced stroke volume can indicate a problem with heart pumping and/or venous return. As a result, the body tissues may not receive adequate oxygen and nutrients.

Clinical Correlation

Heart Failure (aka Congestive Heart Failure) refers to the poor heart function (poor ejection fraction) that occurs when heart muscle is damaged (most often over time, but sometimes acutely) and it manifests with poor squeeze of the heart and classically leads to a back-up of fluid in the peripheral tissues (ie, peripheral edema and with changes on exam of venous stasis), pulmonary edema, and patient's complain of dyspnea (shortness of breath) on exertion.

Cardiac Cycle

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