Cardiac Output

Cardiac Output is the volume of blood that the heart is able to pump out per
unit time, represented in Liters per minute. 

The cardiac output represents the volume of blood that is delivered to the body, and is therefore
an important factor in the determination of the effectiveness of the heart to deliver blood to the
rest of the body, (i.e., determining heart failure, inadequate circulation, etc).

By definition, two major factors contribute to the cardiac output: the heart rate, which is the
number of heartloads of blood is ejected per unit of time, and the stroke volume, which is 
effectively the volume of blood that the heart can fill with, which will be ejected upon contraction.
Increasing the heart rate effectively increases the cardiac output by increasing the number of
cardiac volumes of blood released into the system. As long as the heart is given a long enough 
diastole (resting phase), the effective volume that the heart outputs will increase. 

The stroke volume can be increased by three different factors: the preload, the afterload, and the
myocardial contractility. A high venous pressure (preload) can increase the stroke volume by causing
the heart to fill more fully during diastole. The increased blood pressure forces more blood to flow 
into the heart, down a steep pressure gradient. Similarly, a decrease in the afterload (arterial
blood pressure) can also increase cardiac output, by reducing the input resistance the heart must
overcome in order to pump blood into the vasulature. If the arterial pressure is high, the heart 
must work harder, against a steep pressure gradient in order to pump more blood into the system. If
the arterial pressure is lower, the heart need not work as hard, and more of the total heart volume
actually ends up being ejected into the vasculature. The myocardial contractility affects the 
pumping efficiency of the heart. A higher contractility allows the heart to pump out more of its 
blood during systole, due to the increased ejection force of the cardiac muscles. By allowing the 
heart to pump out more of its volume into the vasculature (high cardiac efficiency), the cardiac
output is increased.

Assesment of the cardiac output is important in determining the work that the heart is actually 
performing with respect to the rest of the cardiovascular system. If the cardiac output is too low,
then the body is not being properly supplied with blood (heart failure), which can and will lead to
life threatening problems if left unchecked. Cardiac Output is usually measured using the Fick 
Principle, which relates the cardiac output of the patient to the oxygen consumption, or by 
thermodilution, in which cold saline is injected into the right atrium and changes in the 
temperature in the pulmonary artery are recorded. These methods allow one to determine the amount 
of blood being delivered to the vasculature by the heart, and whether its output is sufficient to
sustain the health of the patient. 

Equations:
Cardiac Output = Heart Rate x Stroke Volume	
[Co = HR x SV]

Cardiac Output = Oxygen Consumption/(pulm venous O2 content - pulm arterial O2 content)
[Co = q/(O2_pv - O2_pa)] :: Fick's principle

Sample Problems:

1. At rest, a patient's stroke volume is measured to be approxamately 80mL at rest. Using
a stethescope, his heart rate was found to be 70 beats/min. What is this patient's cardiac
output at rest?

2. A patient's oxygen consumtion is found to be 275mL per minute. A physician measures both the
arterial and venous pulmonary oxygen concentrations and finds them to be .2mL O2/mL blood and 
.3 ml O2/ml blood, respectively.
Based on this information, what is this patient's cardiac output? If the physician finds that the
patient's heart rate is 80 beats/min, what is the patient's stroke volume?

Answers:

1. Cardiac Output is defined as HR x SV. Therefore 80mL/beat x 70beats/min = 5600mL/min
This patient's cardiac output is 5.6 liters per minute.

2. Using Fick's principle, one sees that Co = q/(O2_pv - O2_pa) such that Co = 275/(.3 -.2) = 2750mL
The patient's cardiac output is 2.75 liters per minute.
Knowing that Co = HR x SV, and that SV = Co/HR, one sees that 2750/80 = 34.38. The patient's stroke
volume is 34.38mL/beat.

For more information on this topic, please refer to Berne & Levy , pages 83, 195-221, and 76-78

Also, check out the following links that may be helpful:

• Cardiac Output and Venous Return
• Coupling Between the Heart and Vascular System: Cardiac Output and Function
• And, as always, if you have questions, or if you can suggest a helpful link, please e-mail me.

No!
Please don't make me go to any of those pages!
I want to go somewhere completely different!

This page was written by David Huber, a student in this course.

BME 403 Pages maintained by the T.A., Douglas Miles.