Macro Level Electrical Functioning of Heart
The human heart is primarily a pump that pumps blood throughout the human body 24x7x365 days, even when a person is sleeping. In normal conditions heart pumps 60–100 times in a minute, but it also depends upon a person’s age and body to mass index (BMI), meaning if a person is obese (overweight). Before one may try to understand how the ECG machine works, which picks up the electrical signals’ progression through the muscles around the heart to make it function as a pump it is prudent to get a broad understanding of the way electrical signals progress to make the heart function as a pump. For this let us first look at the structure of the heart as shown below in Image 1.
Image 1 credit: Snapgalleria / Shutterstock
The above image shows the heart’s anatomy from the anterior and interior views. The main structure of the heart consists of four chambers and great vessels. The four chambers are the left ventricle, right ventricle, left atrium, and right atrium.
Image 2 below shows the electrical stimulant of the heart starts at its SA Node (shown as Sinoatrial Node), then moves to its AV Node (Atrioventricular Node) but lower to SA node, stimulant then moves along the white lines spreading down till the Apex (lowest tip) of the heart. These white arrows are nerves that carry the electrical signals originating at the SA node. After the AV node, the nerves are called the Bundle of HIS, then they divide into Left Bundle Branch (LBB) and Right Bundle Branch (RBB) to finally becoming the Purkinje Fibers shown in yellow lines spread across both sides.
Image 2 credit: John Hopkins Medicine Home
It is these nerves that carry the electrical signals originating first at the SA node then moving to the AV node and finally to the Apex of the heart to contract and relax the muscles of the heart alternatively as the muscles polarize and depolarize with each electric stimulant starting from the SA node. But the question may remain where do electrical charges appear or get created in the nerves. A valid point. So, kindly recall it is the +ions existing in the organic and biochemical compounds inside the nerves which make the electrical signal and thereby an electric field, that moves in the nerves and ultimately results in contraction/relaxation of the heart muscles making it function as a pump.
The obvious question which an inquisitive mind might then ask is “what and how the SA node kicks in the electrical signal at regular intervals”! For this, we recall the Vagus nerve in our body originating from our brain, which operates through the Cardiac Control Centre located in the Medulla Oblongata part of our brain. At present I will confine to say the Right Vagus Nerve supplies the SA node and slows (or increases) its pacemaker; while the Left Vagus innervates the AV node and slows its conduction of the cardiac impulse to the bundle of His. In another blog exclusively about how Cardiac Control Center functions as a mechanism to increase or decrease the rate at which electrical signals are passed through the Vagus nerve to the SA node, will be explained (so please look out for that blog). The Vagus fibers are preganglionic; they make synapses with parasympathetic neurons within the heart.
Further, to understand the spread of electric field across the human body which is captured by an ECG machine it is important to also visualize how the heart is placed inside a human body as given below in Image 3.
Position of Heart in Chest Cage
Image 3 credit: shutterstock.com
It can then be easily seen the portion of the human body above and in close vicinity of the heart in the ribs cage will relatively have higher electric field density compared to the upper and lower limbs when measured by any machine monitoring the spread of electric field on a human body. Further, as the electric signal moves from SA to AV node and then across all the muscles of the heart, such variation of the electric field on being tapped can give a very good understanding if there is any infarction or ischemia in any part of the heart muscles because the magnitude of the electrical signal in comparison to the normal values will then be different. Capturing such a variation helps cardiologists and cardiac surgeons finally treat that patient to recovery.
Since now we have understood the macro view of how the electric field gets generated to make the heart function; in the next blog I will take up the Cardiac Control Centre and movement of electric signals through the Vagus nerve and finally in a separate blog about the ECG machines’ functioning will be understood.
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July 28, 2021