Cardiac Output

 October 6

by Jonathan Downham

Cardiac output refers to the volume of blood pumped by the heart per minute. It is a measure of the heart's efficiency in delivering oxygen and nutrients to the body's tissues. Cardiac output is determined by two factors: heart rate and stroke volume.

Heart rate is the number of times the heart beats per minute, while stroke volume is the amount of blood pumped out of the heart with each beat. The formula for calculating cardiac output is:

Cardiac Output = Heart Rate x Stroke Volume

For example, if the heart rate is 70 beats per minute and the stroke volume is 70 milliliters per beat, the cardiac output would be:

Cardiac Output = 70 beats/minute x 70 ml/beat = 4,900 ml/minute or 4.9 liters/minute

The average resting cardiac output for a healthy adult is typically around 4 to 8 liters per minute. During exercise or periods of increased demand, cardiac output can increase significantly to meet the body's needs.

Cardiac output is an important parameter in assessing cardiovascular function and is often used to diagnose and monitor conditions such as heart failure, shock, and certain types of heart disease. Various techniques, such as echocardiography, can be employed to measure or estimate cardiac output in clinical settings.

So there are two key elements affecting cardiac output. How we influence these can be very important in how we manage our patients.

Stroke volume refers to the volume of blood pumped out of the heart with each contraction or heartbeat. It is a measure of the amount of blood ejected from the left ventricle of the heart per beat. Stroke volume is an essential component in determining cardiac output, which is the total amount of blood pumped by the heart in one minute.

The stroke volume is influenced by several factors, including preload, contractility, and afterload:

  1. Preload: Preload refers to the volume of blood in the ventricles at the end of diastole (the resting phase of the cardiac cycle) just before contraction. An increase in preload, such as from an increase in venous return, leads to a greater stretching of the ventricular walls, resulting in a larger stroke volume.
  2. Contractility: Contractility refers to the strength and efficiency of the heart's contractions. An increase in contractility, such as due to sympathetic nervous system stimulation or certain medications, can lead to a more forceful contraction and an increased stroke volume.
  3. Afterload: Afterload is the resistance that the heart must overcome to eject blood into the arteries. An increase in afterload, such as from increased systemic vascular resistance, makes it more difficult for the heart to pump blood out, reducing stroke volume.
    More about these elements in the next post.

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