Written By Shilpi Shikha
Last Modified 25-01-2023

Cardiac Output and ECG: Meaning, Factors, Significance

Cardiac Output and ECG: Did you know the human heart pumps about 1.5 gallons of blood every minute, which adds up to about 2,000 gallons of blood every day? The human heart is an amazing organ. It is the first organ to form during embryonic development. The human heart is a myogenic organ that pumps blood. The heart is the most important muscle in the human body, and it is necessary to keep it healthy. The study of the cardiac cycle and cardiac output is the primary determinant of a person’s heart health. Doctors recommend an electrocardiogram (ECG) to monitor cardiac health and related abnormalities in a person. Let’s learn details about these interesting things and make ourselves aware.

Cardiac Cycle

The cardiac cycle can be defined as a sequential event of alternating contraction (systole) and relaxation (diastole) of the atria and ventricles to pump blood throughout the body. One cardiac cycle equals one heartbeat. A cardiac cycle is initiated with signals from the SA node, which cause contraction or systole in the atria. Bicuspid and tricuspid valves open, and blood is pushed to the ventricles. Now, the ventricles receive the signal from the AV node and contract; the blood flows into the pulmonary artery and aorta through semilunar valves. Finally, blood flows from the pulmonary vein and vena cava to the atria, and the same process continues.

The human heart performs a cardiac cycle 72 times per minute on average, which implies that a cardiac cycle takes approximately 0.8 seconds to complete.

Stroke Volume

Stroke volume can be defined as blood pumped out by the ventricles due to the contraction of the heart muscle, which is approximately 70 ml.

Stroke volume = End Diastolic Volume (EDV) – End Systolic Volume (ESV).

Stroke Volume Index is the relationship between stroke volume to body surface area. It tells about the relation between heart performance and the size of the individual. It is measured in the units of millilitres per square metre (ml/m²). Normal values for a healthy resting individual would be approximately 35-65ml/m².

Cardiac Output

Cardiac output or heart output is defined as the volume of blood pumped out by each heart per minute. The heart pumps blood rhythmically. Cardiac output can be calculated by the following formula:

Cardiac output = Stroke volume x Number of heartbeat/ cardiac cycle per minute

So Cardiac output can be calculated as follows:

Cardiac output= 72 per minute X 70 mL = 5040 ml per minute (5 L/min).

So, the volume of blood pumped out by each ventricle per minute is approximately 5000 mL or 5 litres in a healthy individual. The body has the ability to alter the stroke volume as well as the heart rate. There are four determinants of cardiac output – heart rate, contractility, preload, and afterload. The cardiac output of an athlete is much higher than that of an ordinary man.

The cardiac index is the value of cardiac output in relation to the body surface area. The normal value of the cardiac index for a healthy individual is approximately 2.5-4.2L/m².

The cardiac reserve is defined as the difference between the rate at which a heart pumps blood at a particular time and its maximum capacity for pumping blood. Cardiac reserve tells about the chances of heart failure.

Pulse: Pulse can be defined as rhythmic contraction and relaxation in the aorta. Pulse rate is equal to heartbeat rate. The heart pumps the blood and pushes blood through the aorta; the contraction can be felt in the aorta and is referred to as pulse.

Factors affecting cardiac output:
(i) Ventricular end-diastolic volume
(ii) Venous return
(iii) Myocardial contraction strength
(iv) Blood volume
(v) Autonomic nervous stimulation
(vi) physical activity and exercise
(vii) Age
(viii) Gender
(ix) Emotional state
(x) Body temperature

Fig: Cardiac Output

Electrocardiogram (ECG)

An electrocardiogram (ECG) is a graphic record of the diagnostic procedure used to check the rhythm and electrical activity of the heart muscles. An electrocardiograph is a device used to obtain the ECG of a person. Sensors with three electrical leads (one to each wrist and to the left ankle) are connected to the machine that continuously monitors the heart activity to obtain a standard ECG for a patient. Multiple leads are attached to the chest region for a detailed evaluation of the heart’s function. Einthoven(1903) is known as the “father of electrocardiography“

Principle and Mechanism

During every contraction and relaxation, cells of the heart receive an electric impulse from the SA node and AV node. In response to the electric impulses, the cell membrane undergoes polarization, depolarization, and repolarization. The ECG detects the stimulation of a muscle as the electrical potential of the muscle fibres. Each peak in ECG represents specific electrical activity, and they are identified with a letter from P to T.

Reading the ECG

The waves on ECG are called PQRST waves. It has three main components: P wave, QRS wave complex, and T wave. Each peak can be interpreted for different information regarding the heart.

Fig: ECG: PQRST- Wave

P wave: The P wave is a small deflection wave that represents atrial depolarization that leads to a contraction in both the atria.

QRS wave complex: The three waves of the QRS complex represent depolarization in ventricles. The contraction in ventricles starts after the Q wave and marks the beginning of the systole.

T wave: T waves represent ventricular repolarization. During this interval, ventricles return from the excited state to a normal state. The end of the T wave marks the end of the systole.

Why is an ECG done?

An electrocardiogram can be used for the following:

To monitor the health of the heart and give accurate information about the normal functioning of the atria and ventricles.
To check abnormality in the functioning of heart valves.
To check the thickness of the chambers of the heart wall.
To monitor any possible medicinal side-effects.
To check the status of mechanical heart devices like pacemakers etc.

Significance of ECG

ECG has great clinical significance. The significance of ECG is listed below:

It provides the functioning of the atria and ventricles accurately.
It indicates the functioning of valves and detects abnormalities.
It can detect damaged tissue of the heart or a local outgrowth present in any chamber.

Abnormalities in ECG and their Significance

Inverted P-wave indicates failure of SA node to initiate the impulse and depolarization of atrial muscles by the AV node.
Enlarged P-wave is associated with enlargement of the atria.
Absent Q-wave is found in infants with congenital patency of the septum, known as a ventricular septal defect (VSD).
Abnormal T-wave indicates serious myocardial damage, cardiac hypoxia (disproportionate oxygen supply to the muscle cells).
Enlarged P-R interval Indicates inflammation of atria and AV nodes.
Repressed S-T segment indicates insufficient oxygen supply to cardiac muscles.

Summary

The human heart is a muscular organ. The cardiac cycle can be defined as a sequential event of alternating contraction (systole) and relaxation (diastole) of the atria and ventricles to pump blood throughout the body. The cardiac cycle involves rhythmic systole and diastole in the atria and ventricles of the heart. Cardiac output or heart output is defined as the volume of blood pumped out by each ventricle per minute. Blood pumped out by the ventricle during each cardiac cycle is called the stroke volume, which is approximately 70 ml.

The volume of blood pumped out by each ventricle per minute is approximately 5000 mL or 5 litres in a healthy individual. Pulse can be defined as rhythmic contraction and relaxation in the aorta. Pulse rate is equal to heartbeat rate. An electrocardiogram (ECG) is a diagnostic procedure used to check the rhythm and electrical activity of the heart. The graph obtained by ECG is called an electrocardiograph. The ECG detects the stimulation of a muscle as the electrical potential of the muscle fibres. Each peak in ECG represents specific electrical activity, and they are identified with a letter from PQRST. ECG can be used to detect abnormalities in the functioning of the heart.

Frequently Asked Question (FAQs) From Cardiac Output and ECG

Q.1. How do you calculate cardiac output?
Ans: Cardiac output can be calculated by multiplying stroke volume with heart rate.

Q.2. How does the ECG relate to the cardiac cycle?
Ans: ECG represents polarization, depolarization, and repolarization of heart muscle during the cardiac cycle.

Q.3. Who invented ECG?
Ans: Willem Einthoven invented ECG, and he is also known as the father of ECG.

Q.4. What is the principle of ECG?
Ans: The basic principle of the ECG is that excitation of heart muscle alters the electrical potential of the muscle fibres.

Q.5. What are normal ECG intervals?
Ans: Normal ECG intervals are given below:
PR interval: 120-200 milliseconds.
PR segment: 50-120 milliseconds.
QRS complex: 80-100 milliseconds.
ST-segment: 80-120 milliseconds.

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