Question

What is stroke volume? How it is related to cardiac output.

Answer 1

Hint: The volume of the blood that is given out of each of the ventricles is measured and is used to calculate the cardiac output using a formula which relates the stroke volume and the output of the ventricles.

Complete Step by Step Answer:
Blood from the rest of the parts of the body enters the heart through the left and the right auricle. Then the blood descends through the tricuspid and bicuspid valves into the right and the left ventricles. Now, the stroke volume is the measure of the amount of blood that moves out of the two ventricles of the heart due to the contraction of the heart muscles. Multiple metabolic factors may affect stroke volume too. One such factor is the heart rate and the stroke volume is directly proportional to the heart rate, i.e., as the heart rate increases the stroke volume increases. The cardiac output is given by the formula that the volume of blood leaving the ventricle per heartbeat multiplied with the heart rate given in beat per minute gives the cardiac output. Stroke volume is typically 75ml per minute for a normal human but may vary from case to case. The formula to cardiac output is given as:
$Cardiac \ Output \ = \ Stroke \ Volume\left( { L}/{ beat } \right) \ \times \ Heart \ Rate\left( { beat }/{ min } \right)$

Additional Information:
- Cardiac Output for a normal and healthy patient will approximately be 5 L to 8L per min. Although the rate varies according to the condition of the subject and it tends to a much higher rate for a person suffering from an illness.
- Whenever there is an elevation IN the Heart rate there is also an abnormal stroke volume which is not maintained at regular levels.
- To completely know about a patient's condition, medical practitioners require both measures of Cardiac output and Cardiac Index. Some drug administration is also guided by the values of the Cardiac output and cardiac index, whose values are used to manage drugs like vasopressors and inotropic.

Note:
- You must take note that cardiac output and cardiac index are not the same things, although the terminologies are confusing, you must be sure not to mix the two up.
- The normal rate for cardiac output is 5L to 8L per minute hence you must remember this value too, and this may come in other questions.