6.1.14 Calculating Cardiac Output

Formula for Cardiac Output

$\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$

Where:

• Heart Rate (HR): The number of heartbeats per minute (measured in beats per minute).
• Stroke Volume (SV): The volume of blood pumped by one ventricle in a single contraction (measured in millilitres per beat).

Key Definitions

1. Heart Rate (HR):

• Can be measured manually or using electronic monitors.
• Average HR in adults is 60–100 beats per minute at rest.

2. Stroke Volume (SV):

• Depends on factors such as heart size, fitness level, and venous return.
• Average SV is approximately 70 mL per beat in healthy adults.

3. Cardiac Output (CO):

• Measured in litres per minute (L/min).
• Typical resting CO is approximately 5 L/min in adults but can increase significantly during exercise.

Worked Example

Question:

A person's heart rate is 75 beats per minute, and their stroke volume is 70 mL per beat. What is their cardiac output?

Solution:

$\text{CO} = \text{HR} \times \text{SV}$

$\text{CO} = 75 \, \text{beats/min} \times 70 \, \text{mL/beat}$

$\text{CO} = 5250 \, \text{mL/min}$

Convert to litres:

$5250 \, \text{mL/min} \div 1000 = 5.25 \, \text{L/min}$

Answer: Cardiac output is 5.25 L/min.

Factors Affecting Cardiac Output

1. Heart Rate:

• Increases during exercise, stress, or in response to adrenaline.
• Decreases during rest or sleep.

2. Stroke Volume:

• Influenced by venous return, myocardial contractility, and arterial pressure.

3. Fitness Level:

• Athletes often have higher stroke volumes and lower resting heart rates, leading to more efficient cardiac output.