Blog 2 Mar 31, 2024
Basics of cardiac mechanics
In this post:
- 1.
- 2.
- 3.
- 4.
My story
In July 2022, I had a near-fainting episode. By that time, for many years I had noticed my heart beating hard even when performing mild tasks (e.g. walking at normal speed, and sometimes even at rest). I was walking to work on this warm summer day, and after about 5 mins of walking, my vision started to black out.
I stopped immediately and took support of a nearby wall. I did not faint, nor did I fall. But I abandoned my plan to walk further and took a bus to work. As soon as I reached work, I called my general practitioner (GP), who agreed to see me the same day. The GP was puzzled because my blood tests had just come out to be healthy. But at the last minute he thought to hear my heartbeat, and immediately detected a “murmur”, i.e. an abnormal acoustic signature in the beating of the heart. (He suspected aortic valvular stenosis, i.e. an obstruction in one of the heart valves, and that caused me to plan my end-of-life because I read on some websites that the life expectancy in serious cases could be 1-2 years. In reality, with valve reconstruction or replacement surgery, the prognosis is quite good. Now I laugh about it, but this apparent near-death experience changed my outlook on life – more on mental health in a future post.) The GP immediately referred me to a cardiologist, who used an echocardiogram to immediately determine that I am suffering from Hypertrophic Obstructive Cardiomyopathy (HOCM). (There is also a version without the obstruction called Hypertrophic Cardiomyopathy abbreviated as HCM.)
Symptoms of HOCM that I experienced were shortness of breath (dyspnea) under physical activity or even at rest, chest pain (angina), near-fainting (presyncope), and nearly perpetual lightheadedness or dizziness. I also experienced distortions and abberration in vision, that accompanied these symptoms (but my eye test indicated healthy eyes). These are not uncommon. The one symptom of HOCM that I was reluctant to acknowledge until recently recently is fatigue, which I have experienced at least since 2017 (but since there was no diagnosis of cause at the time, I am speculating on the connection with HOCM).
My symptoms, and as I learned later the underlying disease, worsened with time over the period of about one year. What I thought could be managed through medication and lifestyle choices became unmanageable. Ultimately, I underwent a kind of surgery called the septal myectomy to address the disease. I am currently recovering from this surgery. Read all about HCM, HOCM and my experience in this and subsequent chapters.
Before we proceed any further with our discussion of HCM, it is important that we are on the same page about some aspects of how the heart functions. If you are unfamiliar with the structure of the human heart, now is a good time to brush up on the details. If you are an expert, let me know of any inaccuracies in the text.
| (a) | (b) |
.svg){kind=link}
2.1 Anatomy of the human heart
Fig. 2.1(a) shows a schematic of the human heart. The heart is divided into two halves – the right half and the left half. Each half consists of two chambers – an atrium and a ventricle. Thus, the heart has four chambers – the right atrium, the right ventricle, the left atrium and the left ventricle. As these chambers contract, the blood inside the chamber is forced to flow out causing the pumping action attributed to the heart. Separating the two chambers in each half is a valve that ensures blood only flows from the atrium towards the ventricle and not the other way around. The valve in the right half is called the tricuspid valve (label 12) and the one in the left half is called the mitral valve (or the bicuspid valve, label 4). More on structure of the mitral valve later. The two halves are separated from each other by a partition made of cardiac muscle called the septum. In addition to the tricuspid and the mitral valves, there are two more at the outlet of the two ventricles – the aortic valve (label 5) and the pulmonary valve (label 11). The aortic valve ensures that blood from the aorta does not flow back to the left ventricle, and the pulmonary valve does the same between the pulmonary artery and the right ventricle. (The septum and the mitral valve play an important part in this story.)
The heart chambers alternate between expansion and contraction. The contraction of the heart is termed systole and the expansion is termed diastole. The heart chambers fill up with blood during diastole and pump the blood out during systole.
2.2 Basics of muscle contraction
The walls of the heart chambers are made of cardiac muscles, which cause the contraction. Like every muscle, the building block of the cardiac muscle is an individual cell, called a sarcomere, which is replicated throughout the corpus of the muscle in a regular periodic manner. This sarcomere has the ability to contract, which drives the contraction of the whole muscle. The contraction of each sarcomere is powered by molecular motors, i.e. proteins that drive mechanical motion. Two main proteins constituting the molecular motor that are instrumental in the contraction are called actin and myosin. The action of these two protiens can be understood using an analogy with the game of tug-of-war. Imagine two populations that participates in the tug-of-war against each other. At any given time there are only fixed sites on either side of the rope where people can hold and pull. Those people pulling, however, get tired just after pulling once and let go. Depending on how many sites are available for pulling and unoccupied, someone fresh from the team joins in. The rope is pulled, on average, because there are always some number of people holding the rope. The number of available sites depends on the activation level of the muscle (i.e. on the “strength” of the electrical signal the muscle receives for contraction). The average force of the tug depends on the average number of active pullers. The protein called actin is like the rope and the one called myosin is like the people pulling on it from both sides. This is the mechanism of active contraction within the muscle.
A muscle can modulate the force of contraction by varying the number of sites available for tugging on the rope in this tug-of-war analogy. A muscle can turn off, and thus stop contracting, by having no available active sites for tugging. But a muscle cannot expand on its own. An external force must cause it to expand. For the heart, this is caused by the blood pressure within the body. During systole, the heart muscle contraction and pumping inflates the arteries in the body like a balloon, and raises the blood pressure in the body to the systolic blood pressure. For this to happen, the contraction of the heart must raise the left ventricular pressure above the systolic blood pressure. 11 1 A principle of fluid mechanics that applies here is that blood flows from a region of high pressure to a region of low pressure. This influence of pressure on the flow is a little simplified, but adequate for a basic understanding. Also, in this description, I am ignoring the pulmonary branch of the vascular circuit for the sake of explaining the essential mechanism. Once the heart muscles relax (nearly completely) in diastole, the left atrial and ventricular pressure drop nearly to zero, and the inflated arteries starts to deflate and fill the heart back with blood through the veins. This expands the heart chambers back ready for the next systole.
2.3 Path of blood flow through the heart
During the diastole as the heart chambers expand, the blood enters the heart from the rest of the body through the superior vena cava (label 1) and inferior vena cava (label 13) into the right atrium, and then via the tricuspid valve into the right ventricle. Blood oxygenated in the lungs during the previous beats of the heart also flows back during diastole to the left half of the heart through the pulmonary vein (label 3) into the left atrium. The mitral valve then allows the blood to flow from the left atrium to the left ventricle (but not the other way around in a healthy heart). During systole, the chambers of the heart contract. With the contraction of the right ventricle, the blood is forced to flow through the pulmonary valve (label 11) into the pulmonary artery (label 2), because the tricuspid valve closes. Contraction of the left ventricle at the same time forces the mitral valve to close, opens the aortic valve (label 5) and pumps the blood to the rest of the body through the artery known as the aorta (label 10). This completes one heart beat.
2.4 Structure of cardiac valves
The mitral valve is going to feature heavily in our discussion of HOCM. Its structure and function shares many features with the other cardiac valves, so it is useful to discuss them all. At the core, the cardiac valves function similar to a one-way pet door, e.g. a cat flap. The flap remains closed until it is pushed by the pet to open. In this analogy, the blood is analogous to the cat, and the valve is to the flap. There are three differences between to the pet door and the cardiac valves for this analogy to apply. The first one is that the cardiac valves have three flaps (except the mitral valve and sometimes the aortic valve which has two flaps) whereas the pet door has only one flap. The mitral valve flaps, known as leaflets, are like the two panes of a french window. The second difference is that while a (functional) pet door opens both ways, the cardiac valve opens only one way – from the atrium to the ventricle, and from the ventricle to the artery. The third difference applies only to the mitral valve and the tricuspid valve between the right atrium and ventricle. It is that the valve “door” is made of an extremely flexible flap. The advantage of this is that the pet (i.e. the blood) doesn’t have to push too hard to open the door (i.e. the valve). But this also has a disadvantage that by itself, the valve leaflet would not only open towards the ventricle, as they should, but also open towards the atrium, as they should not. (For the two arterial valves, i.e. the pulmonary valve – label 11 and the aortic valve – label 5, the valve material is sufficiently stiff that the flaps of the valve jam against each other and close.) To prevent the valve leaflets from prolapsing, i.e. opening towards the atrium, the edge of the mitral and tricuspid valve leaflets have chordae tendineae (i.e. tendinous chords, see Fig. 2.1(a), label 16). In systole, the papillary muscles pull on these chordae tendineae and prevent the leaflet edge from migrating too far away, and thus facilitate their effective closure.
Technical details![[Uncaptioned image]](Figures/lightbulb.png){kind=small}
The cardiac cycle
The pressure in the cardiovascular system varies cyclically, and because of it the volume of the cardiac chambers, the arteries, and the veins also pulsates. (It is for this reason that we can feel a pulse on the arteries.) Each chamber in the heart has an associated pressure-volume cycle, as shown in the adjoining figure. The labels a, b, c and d in this figure are specific timestamps in the cardiac cycle. Let us starting from label a, the beginning of diastole, when the chamber muscles have relaxed and the chamber pressure is essentially zero. This is followed by the diastole, when the blood from upstream (which should be at higher pressure) fills the chamber and increases its volume up to point b, the end of diastole. The vertical segment bc represents the transition from end-diastole to begin-systole, when the chamber muscles begin contraction and the chamber pressure rises. As this happens, the upstream valve closes when the chamber pressure rises above the upstream pressure. At or near label c, the downstream valve opens and the chamber starts to pump blood. This begins the systole for this chamber. During systole, i.e. segment cd, the chamber pressure remains higher than the downstream pressure and this the chamber continues to pump blood. At label d, systole ends and the muscles begin to relax and the downstream valve closes. On the segment da, the chamber muscles relax and the chamber pressure falls while its volume essentially remains unchanged. The upstream valve opens again at label a, and the cardiac cycle repeats.
Not only does the four chambers of the heart have this pressure-volume cycle, but a similar cycle also exists for the arteries and the veins. The cycle of each element in the cardiovascular system is offset from the other in time. The timing of the whole cardiovascular circuit is organized such that the upstream pressure is higher than the downstream pressure at the right time for the one-way valves to open and let the blood through. The pressure in the pulmonary vein must be higher than the left atrium during left atrial diastole for the left atrium to fill up. Then as the left atrium enters systole, the left ventricle remains in diastole, so that pressure builds up in the left atrium, the mitral valve leaflets open and the blood fills the left ventricle. And finally, the left ventricle enters systole, raising the chamber pressure higher than the diastolic aortic pressure to open the aortic valve and push blood into the aorta. Ultimately, during systole, the pressure in the aorta rises up to the systolic arterial pressure, ready for the next heartbeat. And this is timing of the beat in the left half of the heart; a similar timing holds in the right half. In this manner, the periodic beating of the heart pumps blood.
2.5 Conclusion
Undoubtedly, the human heart is a highly evolved piece of biomechanical organ. I do not know to what extent it is optimized, and if it is then for what function or purpose. Even the reason for the structure of the cardiac valves is evident at some level and puzzling at another. (E.g. why does the mitral valve, and sometimes the aortic valve have two leaflets, but the other valves have three? Why the two sides of the heart and a separate pulmonary vascular circuit? Have these questions been answered and/or do these questions even have answers?) A post on our understanding of the factors underlying this particular structure of the human heart (and the large variety of designs used in the extant and historic animal kingdom) is very tempting but must wait for the future. But it must be clear from even a superficial understanding of the structure and operation of the heart that it is a piece of sophisticated machinery to convert biochemical energy into fluid circulation. Scientific research continues to better understand its structure, function and evolution. Cardiomyopathy is a medical term for disease or disorder of the heart which changes the size and shape of the cardiac muscle. Such a change may either have little effect on its ability to pump blood or a profound one. In HCM, the cardiac muscles thicken. This is the topic of the next post – so stay tuned.