# 52 Cardiac output

Learning Objectives

At the end of this section, you should be able to-

• Define cardiac output, and state its units of measurement
• Calculate cardiac output, given stroke volume and heart rate
• Predict how changes in heart rate (HR) and/or stroke volume (SV) will affect cardiac output
• Describe the concepts of ejection fraction and cardiac reserve.

Resting Cardiac Output

Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle in one minute. To calculate this value, multiply stroke volume (SV), the amount of blood pumped by each ventricle, by heart rate (HR), in contractions per minute (or beats per minute, bpm). It can be represented mathematically by the following equation:

CO = HR × SV

SV is normally measured using an echocardiogram to record EDV and ESV, and calculating the difference:

SV = EDV – ESV

SV can also be measured using a specialized catheter, but this is an invasive procedure and far more dangerous to the patient. A mean SV for a resting 70-kg (150-lb) individual would be approximately 70 mL. This is because typical EDV and ESV values are approximately 120 mL and 50 mL, respectively and 70 mL = 120 mL – 50 mL. A normal range for SV is between 55–100 mL. An average resting HR would be approximately 75 bpm, but could range from 60–100 bpm in some individuals.

Using these numbers, the mean resting CO is 5.25 L/min, with a range of 4.0–8.0 L/min.  The CO of 5.25 L/min, was calculated using the following values:

CO L/min = 75 beats/min x 0.070 L/beat (70 mL =  0.070 L)

Remember, however, that these numbers refer to CO from each ventricle separately, not the total for the heart. In a healthy heart, the CO from each ventricle is the same. CO is influenced by HR and by SV. If SV decreases, CO can be maintained by increasing HR.  HR is influenced by the autonomic nervous system, chemicals, and other factors. The factors influencing CO are summarized in Figure X.X.X

SV can also be used to calculate ejection fraction, which is the portion of the blood that is pumped or ejected from the heart with each contraction. To calculate ejection fraction, SV is divided by EDV. Despite the name, the ejection fraction is normally expressed as a percentage. Ejection fractions range from approximately 55–70 percent, with a mean of 58 percent.  For example, if the average EDV is 120 mL and the SV is 70 mL, the ejection fraction of 58% is calculated as follows:

Ejection fraction (%) = (70 mL/120 mL) x 100 = 58%

There are several important variables that can affect SV and HR, including size of the heart, physical and mental condition (via hormones and the ANS) of the individual, gender, contractility, duration of contraction, preload (or EDV), and afterload (or resistance).

# Exercise and Maximum Cardiac Output

In healthy young individuals, HR may increase to 150 bpm or higher during exercise. SV can also increase from 70 to approximately 130 mL due to increased strength of contraction. This would increase CO to approximately 19.5 L/min, 4–5 times the resting rate. Top cardiovascular athletes can achieve even higher levels. At their peak performance, they may increase resting CO by 7–8 times.

Since the heart is a muscle, exercising it increases its efficiency. The difference between maximum and resting CO is known as the cardiac reserve. It measures the residual capacity of the heart to pump blood.

Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 19

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