Mastering hemodynamics can be tricky, but the first step is understanding the terminology. Let’s take a look at cardiac output and cardiac index – how to calculate them and why they’re important.
Cardiac Output (CO)
is the volume of blood the heart pumps per minute. Cardiac output is calculated by multiplying the stroke volume by the heart rate. Stroke volume
is determined by preload
, contractility, and afterload
. The normal range for cardiac output is about 4 to 8 L/min, but it can vary depending on the body’s metabolic needs. Cardiac output is important because it predicts oxygen delivery to cells.
Here’s an example:
If a patient's stroke volume is 75 mL with each contraction and his heart rate is 60 beats/minute, his cardiac output is 4,500 mL/minute (or 4.5 L/minute).
Cardiac Index (CI)
The cardiac index
is an assessment of the cardiac output value based on the patient’s size. To find the cardiac index, divide the cardiac output by the person’s body surface area (BSA). The normal range for CI is 2.5 to 4 L/min/m2
Here’s an example of how to calculate the cardiac index:
If a patient’s cardiac output is 4.5 L/minute and his BSA is 1.25 m2
, his CI would be 3.6 L/min/m2
. If another patient has a cardiac output of 4.5 L/minute, but he has a BSA of 2.5 m2
, his CI would be 1.8 L/min/m2
Both cardiac output and cardiac index are important to let us know if a patient’s heart is pumping enough blood and delivering enough oxygen to cells. We also use CO and CI values to manage certain drug therapy, such as inotropics and vasopressors.