[Biology Class Notes] on Cardiac Output Pdf

Also known as heart output, cardiac output describes the volume of blood which is being pumped by the heart; this is done by the left ventricle and the right ventricle per one compression or unit time.

An excellent example of the human heart to exhibit its efficiency is cardiac output which is related to the amount of blood pumped by the heart per minute. Normally, a cardiac output results in pumping of about 5-6 litres of blood per minute at resting position; it can be said that it’s the normal cardiac output. We will get this figure with the help of calculation mentioned further in the article.

Cardiac Output Definition

Amount of blood ejected by each ventricle in one minute is the cardiac output. It (CO) is measured as ml/minute. 

How to Calculate Cardiac Output

Cardiac output can be calculated using two variables-

1. Heart Rate (HR) – It is measured by knowing the number of beats in one minute. And cardiac output also can be related to the heart rate. 

2. Stroke Volume (SV) – It is the volume of blood pumped by each ventricle with each beat. It is measured in ml/beat. 

So, the Cardiac Output formula goes like this-

CO = HR * SV 

(ml/min)    (beats/min)    (ml/beat)

Let’s Know About the Terms Heart Rate and Stroke Volume in More Detail.

• Heart Rate – It is how many beats per minute and factors that positively influence the heart rate (to increase the heart rate) are called positive chronotropic factors. We get over 100 beats per minute with a fast heart rate called tachycardia. Examples of positive chronotropic factors include sympathetic stimulation secreting adrenaline/noradrenaline or epinephrine/norepinephrine to increase heart rate. Other examples are hypercapnia which results in increased CO2 and drugs like atropine.

The negatively influencing factors are called negative chronotropic factors that are responsible for below 60 beats per minute called bradycardia. Examples are parasympathetic stimulation whose main neurotransmitter is acetylcholine, hypoxi that decreases oxygen and then heart rate and hypercalcemia that increases Calcium and decreases heart rate. 

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Determinants of Cardiac Output

As stroke volume influences the cardiac output, we can say that there are four determinants of cardiac output.

1. Heart Rate – As we mentioned above, increase in heart rate increases the cardiac output.

2. Preload – It is the volume of blood entering the ventricles. Increased volume of blood entering the heart results in stretch of ventricle and increase in preload takes place. It ultimately affects stroke volume and then increases cardiac output. 

According to the Frank Starling mechanism, the amount of blood that enters into the ventricles will be the amount of blood ejected from the ventricles to the body. So, end diastolic volume (the volume of blood in the ventricles prior to ejection) will be proportional to the stroke volume. 

3. Afterload – It is the resistance ventricles must overcome to circulate blood around the body. Increase in resistance means increase in afterload. Therefore, increase in afterload means decrease in stroke volume and vice versa. All this influences cardiac output.

4. Contractility – It is basically how hard the myocardium contracts for a given preload. Increase in contractility means increase in cardiac output and vice versa. 

Factors that increase contractility are positive inotropic factors like sympathetic stimulation, caffeine, hypercalcemia. Negative inotropic factors are parasympathetic stimulation, hypocalcemia, increase in potassium and myocardial hypoxia that decrease contractility.

Measurement for Cardiac Output

Cardiac output can be measured using various invasive and non-invasive methods. However, there can be merits and demerits of measuring cardiac output. 

Various invasive methods are available but may not be accurate or effective and these include doppler ultrasound, echocardiography, transcutaneous and transesophageal echocardiography, partial CO2 rebreathing, etc.

Cardiac Index

It is a haemodynamic parameter and relates the heart performance to the size of an individual. It relates the cardiac output (CO) from the left ventricle in one minute to the BSA called body surface area. So, its measurement is litres per minute per square meter and written as L/min/m². 

The normal range for cardiac index (CI) is 2.5 to 4 L/min/m2.

A lower cardiac index indicates a disturbance in cardiovascular performance  which is less than 2.5 L/min/m2.