Cardiovascular Pathologies — 3 (ECG-Part I)
In my Blogs 1 & 2, I introduced the basic structure of the heart and its requirement in the human body for it to function normally as a pump. Let us now shift gears and talk about pure Physics, which is rooted in Electricity and Electric Charge within the human body. Intriguing, isn’t it? How surprising that we do not feel electricity running in our human body in the normal course of our daily life, yet it forms the lifeline! So, how is that happening? Allow me to remind you that my blogs are not to discuss the molecular and microbiological details of the way cells, tissues, muscles, and organs in different parts of the human body get formed, function, and are made of. To understand their nitty-gritty, proper texts and works of experts in microbiology may be referred to.
How are compounds formed?
Kindly recall that the structure of all materials on this earth goes down to elements (Chemistry) existing in the universe today. But all those elements have their characteristics rooted in the structure of their respective atoms. For simplicity let us confine ourselves only to the macrostructure of atoms of all elements in general viz., comprising of Electrons, Protons, and Neutrons, which form the organic existence of the body’s cells as well. Physics tells us Electrons carry a negative charge, Protons a positive charge, and Neutrons have no charge at all.
Next, we know the chemical bondage between elements, say the table salt we eat every day, has atoms of two elements joining up to form a compound where electric charge within each element plays the primary role. In the case of table salt, the elements are Sodium (Na) and Chlorine (Cl) creating a NaCl bond, called Sodium Chloride in our Chemistry. Further, Sodium (Na), Potassium (K), and Calcium (Ca) play a monumental role in the biochemistry of cells and their functioning. There are other elements too like Carbon ©, Iron (Fe), Oxygen (O), Phosphorus (P), Magnesium (Mg), etc., which play a pivotal role in the normal functioning of the organic cells and thereby of the organs whose part they are in a human body.
Fast forward next to the Heart, Liver, Kidneys, Bones, Skin, etc., existing in the human body are all primarily made up of cells of a particular kind and properties having at times different chemical elements, which bond together because of electric charges. For instance, human blood gets its red color from the existence of iron (Fe) in it and how it floats as a chemical in the hemoglobin1. Recall that women on conceiving are given iron tablets to sustain the iron level in their blood for the normal growth of the conceived child till its delivery. Blogs 1 & 2 dealt with the essential requirement of the blood flow in the human body, meaning for both the mother and the growing child within her, for the healthy body of the pregnant mother.
Electric Potential in a Biological Cell
Having said that let us quickly see and visualize why is the electric potential caused by the existence of the charged ions in a cell. The cell membrane potential is due to disparities in the concentration and permeability of the ions across the membrane walls. The unequal concentrations of ions across a membrane shown in Fig 1, results in electrical potential.
In other words, as per Coulomb’s Law, the gradient of the existence of electric charge will create an electric potential (voltage), which can be calculated and measured (Physicists across the world do not accept anything unless they perform these two steps, even if it is inside a biological cell or a human body).
Factually, on average in the human body, there is 60% water. So, the body’s cells are surrounded by fluid comprising water as its basic ingredient. Hence, one can determine the electric potential across a cell membrane in resting position as shown in the schematic in Fig 2.
As a result, within the human body in the neurons and muscle cells, there exists electrical potential. In the case of resting neuron cells that form the nerves in the human body, it is of the order of -70 mV to -80 mV, and it facilitates the transmission of electrical signals across every part of the human body from head to toe.
It stands for Electrocardiography and forms the starting point in deciphering how the heart is “beating alternatively” in Systole and Diastole mode throughout one’s lifetime. My attempt here will be to explain the process in a very simple manner based on the existence of electric charges across the human body! Understanding and interpreting the trace of the ECG signal falls in the domain of Cardiologists. It requires 12 leads affixed to the human body to capture the electrical phenomenon taking place. So, to explain it in nonmedical terms for a common man will require more than one blog on ECG. Here we will start with a typical electrical potential variation across the heart muscles shown below in Fig 3.
In the schematic of the heart, how each section of the above wavefront leads to the actual functioning of the human heart muscles can be easily understood by Fig 4 below.
The P wave portion represented by the dark red color on the extreme left side of Fig 4 refers to the functioning of the Atriums (RA & LA). This electric charge movement is termed “Depolarization”. In the extreme left schematic of the heart, there are arrows emanating from a “white circular disc”. This disc symbolizes the SA Node which is responsible for initiating the process of “Depolarization” of the electric charge spread across the atrium muscles. That causes the movement of atrium muscles and hence the development of the pressure across the atrial walls, thereby opening of the TV & MV to let the blood flow from atriums into ventricles. The pressure within both the ventricle at this juncture is below the pressure existing in the atriums, which causes the TV & MV valves to open. This is the “Diastole” phase of the heartbeat. And it is also the time when the PV and AV are closed.
The SA (sinoatrial) Node is the Pacemaker of the heart. It is this node that initiates the electrical signal alternatively and as shown in the extreme left Fig 4 above, it causes the atriums to contract. The closest analogy that strikes me in Physics of such a behavior of the natural pacemaker is the “Multivibrator” explained very well in the textbook2 by Millman and Taub by which I was taught in my first master’s program in Physics.
The obvious question which may emerge at this point is: why the depolarization originates from SA Node, though reaches the TV and MV valvular annulus, it does not cross into the ventricles too? Instead, it gets momentarily stopped at the annulus around the two atrioventricular valves.
Atrioventricular Node (AVN)
Let me here make readers recall the analogy of the RLC (Resistance, Inductance, and Current) circuit experiment one did in secondary classes. In it we saw how the existence of an Inductance (L) in an electric circuit creates a “Phase Lag” between Current © and Voltage (V) flowing through it. So, as an analogy, the AVN in the heart is to create a “Phase Lag” in the depolarization that had started from the SA node in the muscles of atriums and reaches the valvular annulus. And then after a phase lag has been created, the depolarization process continues across the ventricular muscles. This short segment of time ensures the maximum possible contraction of the atriums takes place to push out as much blood from those chambers through MV and TV into the RV and LV respectively. This becomes evident in the Pulse Wave plot captured across the MV during the 4 Chamber evaluation of MV movement as per Fig 5 below through an Echocardiography procedure.
Image 5B & C show E and A waves. Here the former refers to the Rapid Filling and the latter to the Atrial Contraction phases of atriums’ functionality on account of the depolarization process.
The above image of the heart and beside it of the PQRST wave’s segments captured by the ECG tell a great deal about what happens once the process of depolarization is initiated by the AVN node. It then passes through the Bundle of HIS, Left, and Right Bundle Branches and finally reaches the Purkinje Fibers. It is in this manner the electrical phenomenon controls the 3D contraction of the ventricle muscles as schematically depicted in Fig 3 of my Blog 2 on CVDs. This is termed the Systole phase making the oxygenated blood reach the human body and enriching its cells with oxygen and nutrients for their sustenance and growth, as well as the lungs to get oxygenated and return to the heart.
So, till now we have understood not only the diastole and systole processes of the heart’s functioning but also how the electric charge plays a pivotal role in it which results in the pumping of the blood across the human body. It is time to take a break.
When we return to Blog 4, we will then talk about the abnormalities which arise if the process of depolarization itself develops erratic behavior. What may happen if the SA node (which is the pacemaker of the heart) develops problems and what kind of backups nature has created within the heart and the human body to make every effort that life is sustained through internally created mechanisms? The attempt will be to describe all that in layman’s language.
Finally, I leave it to the reader to think the contraction of LV & RV muscles starts first from the Base of the heart or from its Apex due to depolarization. I will address its answer in my next Blog.
2. Pulse, Digital, and Switching Waveforms, by Millman and Taub