Calculating Cardiac Output

Cardiac output is the heart’s ability to pump a certain amount of blood. It is the product of the heart rate and stroke volume. We measure this calculated cardiac output in litres/minutes. 

However, you do not focus simply on the heart for calculating cardiac output. One must consider the entire cardiovascular system and extracardiac variables.

What do you mean by cardiac output?

As mentioned earlier, the product of heart rate (beats per minute) and stroke volume is called the cardiac output. In other words, calculating cardiac output (‘Q’ or ‘CO’) is the amount of blood the heart expels in a minute. It pushes blood out of the left ventricle. 

Heart rate is the number of heartbeats per minute. The amount of blood the left ventricle discharges with each contraction is the stroke volume.

An adequate cardiac output is essential to maintain energy and life. The cardiac output of an adult is about 5-8 litres of blood per minute.

Calculating cardiac output formula is as follows:

Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR)

An adult’s cardiac output can increase to 25 litres per minute during heavy activity to meet the body’s oxygen and nutritional demands. 

Factors that affect cardiac output

Let us look at the factors influencing cardiac output.

• Heart rate

The most obvious measure of cardiac output is heart rate. The quicker the heartbeats, the more blood the system can pump in a given period. 

However, it is not so simple. There is a need to maintain an ideal heart rate. Extremely slow or fast heart rates can impair cardiac output. 

Some conditions of abnormal heart rate include acute supraventricular or ventricular tachycardia and bradyarrhythmia. These disorders can result in low cardiac output and even cardiogenic shock.

• Contractility

Contractility refers to the contractions of the heart muscles. The higher the heart muscle contractility, the higher will be the cardiac output. Decreased contractility diminishes cardiac output. 

Nevertheless, too much effort causes exhaustion, leading to a total collapse. This collapse will require the heart to slow down significantly or even stop. Inotropic drugs (i.e., ones that change the contractility) can be fatal.

• Stroke volume

While heart rate is undeniable in cardiac output, stroke volume is another important factor. It is the amount of blood evacuated with each beat.

Stroke volume varies in response to changes in cardiac output requirement. Changes in preload, afterload, and catecholamine release affect stroke volume. 

• Preload

Preload is the term used to describe the blood supply to the ventricle. We can define it as the volume or pressure in the ventricle at the end of diastole in medical terms.

Preload is an intrinsic property of heart cells, first demonstrated by Otto Frank and Ernest Starling. It represents that the force of myocardial contraction is proportional to the length of stretching in these cells, provided it is within limits. The greater the stretch, the higher the force of contraction. 

An increase in the ventricle’s distension increases the contraction and, in turn, the cardiac output. The heart eventually reaches a preload value at which cardiac output does not increase. 

• Afterload

The force against which the ventricles must act against a force to contract and expel blood; this force is the afterload. The vascular tone and arterial blood pressure are its determinants. 

Lowering the afterload can boost cardiac output, particularly in cases where contractility is compromised.

Conclusion

The cardiac cycle’s goal is to efficiently pump blood — both atria contract at the same time throughout a heartbeat, followed by the contraction of the ventricles. The interpretation of cardiac output is more complicated than it appears at first. 

However, analysing the intricate relationship between cardiac output and its four components can help us better grasp the concept. It includes understanding some complex pathophysiological changes in critical disease and the effects of various therapies on cardiac output. 

Most doctors are well-versed with the four determinants of calculating cardiac output: heart rate, contractility, preload, and afterload. But the application and practical importance of each of these four components are more difficult.