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Posted on Friday November 1, 2024 in Naked Heart
A technical article written by Dr Edward Leatham, Consultant Cardiologist
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As the heart opens and relaxes, a fascinating sequence of events unfolds, orchestrated to ensure the continuous circulation of blood throughout the body. This relaxation phase is termed diastole, during which the heart chambers refill with blood, preparing for the next contraction. Diastole is as vital as contraction, or systole, because the filling of the heart with blood is what ultimately ensures an adequate supply of oxygenated blood to the tissues.
The heart’s relaxation involves a suction mechanism, an active process in healthy individuals. The heart muscle’s elastic fibres and intrinsic energy gradients create a negative pressure, drawing the mitral valve open. The mitral valve, situated between the left atrium and left ventricle, allows blood to flow from the left atrium into the left ventricle during this phase. Blood from the left atrium enters the ventricle, filling it to prepare for the ejection phase.
Meanwhile, blood from the pulmonary veins carry freshly oxygenated blood from the lungs, where gas exchange occurs. The pulmonary veins connect to the alveolar spaces, tiny air sacs within the lungs, where oxygen from inhaled air diffuses into the bloodstream, and carbon dioxide diffuses out, to be exhaled. The alveolar spaces thus play an essential role in ensuring the blood that flows through the pulmonary veins is oxygen-rich, ready to be circulated to the body after it passes through the left side of the heart.
In a healthy heart, a substantial portion of left ventricular filling happens due to the suction action of the heart muscle, which actively draws blood from the atrium. This suction filling mechanism is energy-dependent, requiring ATP energy (created by mitochondria) for the ventricles to actively relax to create the negative pressure. As a result, this phase of left ventricular filling is usually efficient and smooth. By the time the left atrium contracts most of the blood has already entered the left ventricle. Consequently, the atrial contraction provides a final “top-up,” accounting for roughly 10% of the total blood volume that enters the ventricle before systole begins.
The contraction of the atrium, known as the atrial kick, is the final component of ventricular filling. This atrial contraction contributes about 10% to 20% of the total ventricular filling, giving the left ventricle an added boost just before it contracts. This step becomes even more critical in certain situations, such as when the heart rate is elevated, or in patients with heart conditions that compromise ventricular filling, like diastolic dysfunction. In these cases, the contribution of atrial contraction to ventricular filling becomes more pronounced.
The next phase in the cardiac cycle begins with the passage of electrical energy through the atrioventricular (AV) node and into the His-Purkinje system, a network of fibers that conduct electrical impulses throughout the heart muscle. This system ensures that the ventricles contract in a coordinated manner, starting from the apex (the tip of the heart) and moving upwards toward the aortic valve. This bottom-to-top contraction is crucial for efficient blood ejection because it pushes blood toward the valve at the top of the ventricle, where it can then enter the systemic circulation.
During systole, the ventricular muscle fibres contract, generating the force needed to propel blood out of the heart. This contraction closes the mitral valve, preventing backflow of blood into the left atrium. The rising pressure in the left ventricle pushes open the aortic valve, allowing blood to exit the heart and enter the aorta, the body’s main artery. On average, each heartbeat results in the ejection of about 50 to 100 millilitres of blood through the aortic valve, depending on factors such as body size, physical condition, and age.
The heart pumps blood at an average rate of about 5 litres per minute at rest, a value known as the cardiac output. Cardiac output can increase several times over during exercise or stress to meet the body’s heightened demands for oxygen and nutrients. This impressive ability of the heart to adjust its output according to the body’s needs is key to maintaining balance and function within the body. Over a lifetime, the heart can beat around a billion times, pumping over 200 million litres of blood—a testament to its resilience and essential role in sustaining life.
This entire cycle—from filling to ejection and relaxation—repeats with each heartbeat, typically 60 to 100 times per minute in a healthy adult at rest. Each phase of the cycle, though brief, plays an integral role in maintaining blood flow and ensuring oxygen delivery to tissues. Disruptions in any phase, whether from disease, injury, or ageing, can impact overall heart function and health.
In clinical settings, we often assess heart function by examining both diastolic and systolic phases. Diastolic dysfunction, which occurs when the heart muscle becomes stiff and has difficulty relaxing, impairs the heart’s ability to fill adequately during diastole. This condition often leads to heart failure with preserved ejection fraction (HFpEF), a type of heart failure where the heart can still pump blood but struggles with filling.
On the other hand, systolic dysfunction refers to a weakened heart muscle that cannot contract effectively, resulting in heart failure with reduced ejection fraction (HFrEF). This form of heart failure is associated with a reduced amount of blood ejected with each beat and often requires medications and lifestyle modifications to manage.
Whether a heart is stressed by its inefficiency in squeezing or pumping blood out (HFrEF) or difficulties in filling adequately (HFpEF) the consequences can be very similar. The cardiovascular system goes into ‘battle mode’ or ‘fight and flight’ with rise in adrenaline that increases heart rate and contractility, an increase in renal hormones to retain salt and water, a primitive reflex presumably developed in case of dangerous drop in blood pressure (shock) caused by sepsis or haemorrhage from injury through battle, with peripheral constriction to divert arterial blood to the vital organs brain, heart and kidneys. There may also be a cortisol-mediated rise in blood glucose. These reflexes evolved to protect mammals from shock, but pretty well all lead to worsening symptoms in heart failure states because in many patients there is an increase in Brain Natriuretic Peptide (BNP), salt and water retention which lead to ankle swelling, lung flooding and symptoms of breathlessness and peripheral vasoconstriction which increases the load on both ventricles.
The Importance of Atrial Function
The atrium’s role in the cardiac cycle, while sometimes understated, becomes especially significant in certain conditions. Atrial fibrillation (AF), for instance, is a common arrhythmia where the atria quiver instead of contracting effectively. This irregular activity compromises the atrial kick, thereby reducing the amount of blood entering the ventricle before systole. In patients with AF, reduced ventricular filling may contribute to symptoms such as fatigue and exercise intolerance, and can increase the risk of blood clots, which may lead to stroke.
Cardiac Function in Ageing and Disease
As we age, changes in cardiac structure and function are inevitable. The heart muscle may become stiffer, reducing its ability to relax efficiently. This increased stiffness contributes to higher filling pressures and may lead to conditions like diastolic heart failure. Age-related changes also affect the conduction system, sometimes leading to arrhythmias or blocks, such as Mobitz II or Wenckebach heart block, which can slow down or disrupt the transmission of electrical impulses through the heart.
In disease states such as coronary artery disease (CAD), blood flow to the heart muscle itself can become compromised due to a buildup of plaque in the coronary arteries. When the heart’s oxygen supply is reduced, it can lead to angina (chest pain) or, in severe cases, a heart attack (myocardial infarction). Early diagnosis and treatment of CAD are essential to prevent progression and improve outcomes.
Conclusion
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The Naked Heart is an educational project owned and operated by Dr Edward Leatham. It comprises a series of blog articles, videos and reels distributed on Tiktok, Youtube and Instagram aimed to help educate both patients and healthcare professionals about cardiology related issues.
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Since 2016, cardiologists have been utilising the HeartFlow Analysis in conjunction with cardiac CT scans to enhance the accuracy and predictive value of noninvasive coronary imaging. This technology allows many patients to avoid unnecessary invasive angiography, reducing the associated risks and stress.
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