Umair Gaines
The cardiac cycle involves four major stages: isovolumic relaxation, inflow, isovolumic contraction, and ejection. Systole is the phase of the cardiac cycle in which the heart muscles contract, and it can be divided into atrial systole and ventricular systole. Electrical systole is the nerve impulse that signals the heart muscles to contract. It is represented by the P wave of the ECG and occurs during the atrial systole.
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What You'll Learn
Electrical systole and the cardiac cycle
The cardiac cycle refers to the sequence of events that occur as the heart beats and pumps blood around the body. The cycle can be divided into two main phases: diastole and systole. Diastole is the period during which the heart relaxes and refills with blood, while systole is the period during which the heart contracts and ejects blood. Electrical systole is the nerve impulse that signals the heart muscles to contract.
During diastole, the ventricles relax and expand, allowing blood to flow from the circulatory system through the mitral and tricuspid valves (also known as the atrioventricular or AV valves) into the ventricles. This phase is known as ventricular diastole, and it includes a sub-period known as atrial systole or atrial contraction, during which the atria contract, forcing a final volume of blood into the ventricles under pressure. Diastole ends and ventricular systole begins when the ventricles start contracting, prompted by electrical signals from the sinoatrial node (the heart's natural pacemaker). As the back pressure against the ventricles increases, the AV valves are forced to close, which stops the blood flow in or out of the ventricles. This is known as the isovolumic contraction stage.
During ventricular systole, the ventricles contract, causing a rapid increase in pressure within the ventricles. When the pressure exceeds that of the blood vessels, the requisite valves (the aortic and pulmonary valves) open, and blood is ejected from the ventricles into the aorta and arteries. This is the ejection stage of the cardiac cycle. After ventricular pressures fall below their peak and below those in the aorta and pulmonary arteries, the aortic and pulmonary valves close again. Next is the isovolumic relaxation stage, during which the pressure within the ventricles decreases significantly, and the mitral and tricuspid valves open again, allowing blood to flow into the atria and refill the ventricles.
Atrial fibrillation is an electrical disorder in the heart that can occur during atrial systole. It is caused by an ectopic focus that competes with the sinoatrial node for electrical control of the atrial chambers, resulting in a loss of coordinated generation of pressure in the two atrial chambers. This can lead to a decreased performance of the heart, with the ejection fraction deteriorating by up to thirty percent. However, if the resulting high heart rate can be slowed to a normal range, the longer fill-time within the cardiac cycle can restore or improve the pumping capability of the heart.
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Atrial systole
During ventricular diastole, the heart relaxes and expands while receiving blood into both ventricles through both atria. Near the end of ventricular diastole, the two atria begin to contract (atrial systole), and each atrium pumps blood into the ventricle below it. This is known as the "atrial kick", contributing 20-30% of ventricular filling. Atrial systole lasts approximately 100 ms and ends before ventricular systole, as the atrial muscle returns to diastole.
The sinoatrial node (SA node) is the heart's natural pacemaker, issuing electrical signals that travel through the heart muscle, causing it to contract repeatedly in a cycle. It is situated at the top of the right atrium, adjacent to the junction with the superior vena cava. The SA node contains two types of cells: small, round P cells, and slender, elongated transitional cells. The SA node is essential for the ordered, sinoatrial control of atrial electrical activity. If this is disrupted, as seen in atrial fibrillation, atrial flutter, or complete heart block, normal atrial systole may be eliminated.
The cardiac cycle involves four major stages: isovolumic relaxation, inflow, isovolumic contraction, and ejection. Atrial systole occurs during the inflow stage, which is part of the ventricular diastole period. This is when blood returning to the heart flows through the atria into the relaxed ventricles. The precise coordination of the cardiac cycle ensures that blood is efficiently collected and circulated throughout the body.
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$110
Ventricular systole
The cardiac cycle can be divided into two phases: diastole and systole. Diastole is the period of relaxation, during which the chambers of the heart fill with blood. Systole, on the other hand, is the period of contraction, during which the heart pumps blood into circulation. Both the atria and the ventricles undergo these two phases.
The beginning of ventricular systole is marked by the closure of the mitral valve (or tricuspid valve), which occurs when the pressure within the ventricles exceeds that of the blood vessels. This is followed by the isovolumic contraction period, during which pressure continues to build within the ventricles but the blood does not leave. Eventually, the pressure within the ventricles becomes greater than that of the adjacent blood vessels, causing the aortic valve to open and initiating the rapid ventricular ejection phase. A healthy ventricle ejects more than 60% of its volume during this phase. The ejection of blood from the ventricles results in the characteristic first heart sound, denoted as S1, which is produced by the mitral and tricuspid valves.
The duration of ventricular systole can be measured using an electrocardiogram (ECG), which captures the electrical activity of the heart. Ventricular systole follows the depolarization of the ventricles and is represented by the QRS complex in the ECG. It can be conveniently divided into two phases, lasting a total of approximately 270 ms. The time variables of the ventricular systoles are defined as follows: for the right ventricle, from pulmonary valve opening to closure; and for the left ventricle, from aortic valve opening to closure.
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Diastole
During diastole, the aortic valve (or pulmonic) closes, and the mitral valve (or tricuspid) opens, allowing blood to flow from the atria into the ventricles. This is known as ventricular filling. Diastole ends when the mitral valve closes, and the aortic valve opens, and the cycle begins again with systole.
Diastolic heart failure occurs when there is sufficient contraction but poor distention of the myocardium. This lack of lusitropy (the rate of relaxation) is often due to a thickened myocardium.
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Systolic heart failure
The normal, healthy range for ejection fraction (EF) measurement is between 55% and 70%. An EF under 40% may indicate systolic heart failure. Systolic heart failure can be caused by various factors, including high blood pressure, coronary artery disease, previous heart attack, abnormal heart rhythm, and alcohol use disorder. High blood pressure, for instance, increases the workload on the heart, causing the heart muscle to thicken and function less efficiently.
The symptoms of systolic heart failure include chest pain, coughing or wheezing, fatigue, weakness, dizziness, nausea, rapid or irregular heartbeat, and shortness of breath during exertion or when lying flat. Swelling in the belly, feet, or legs may also occur due to the body's inability to get rid of sodium and water effectively, leading to increased blood volume and tissue swelling.
Diagnosis of systolic heart failure involves a comprehensive approach, including a physical examination, blood tests, chest X-rays, and cardiac imaging. Blood tests can reveal abnormal levels of substances indicating strain on organs due to heart failure. Chest X-rays help detect an enlarged heart or congestion, while cardiac imaging, such as echocardiograms, provides detailed visuals of the heart's structure and function.
Treatment for systolic heart failure focuses on addressing the underlying causes and improving heart function. This may include a combination of medications, lifestyle changes, and, in severe cases, surgery or the implantation of devices like defibrillators or left ventricular assist devices (LVADs). Beta-blockers, for instance, are commonly used to slow the heart rate, reduce workload, and control blood pressure. Lifestyle changes, such as limiting salt intake, maintaining a healthy weight, and reducing stress, are also crucial in managing systolic heart failure.
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Frequently asked questions
Electrical systole is the nerve impulse that signals the heart muscles to contract.
Electrical systole is the nerve impulse that signals the heart muscles to contract, while mechanical systole is the actual contraction of the muscle fibres of the heart tissue.
In an electrocardiogram (ECG), electrical systole is seen as the P wave deflection of a steady signal, which initiates the atrial systole. The QRS peaks on the ECG represent the start of systole.
Electrical systole initiates atrial systole, which is the contraction of the atria that forces blood into the ventricles.
Electrical systole is the nerve impulse that signals the ventricular systole, which is the contraction of the ventricles that forces blood out of the heart.
