The Cardiac Cycle: Relaxation to Systemic Circulation - Essay Sample

Introduction

Blood from the superior and inferior vena cava get into the heart in the right atrium. The blood then flows through the open tricuspid valve into the right ventricle. From the right ventricle, it flows to the lungs through the pulmonary artery. Blood leaves the lungs through the pulmonary vein into the left atrium, the left ventricle and finally through the aorta into the systemic circulation.

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A cardiac cycle involves all the events that occur in a single heartbeat which include relaxation and contraction of both atria and ventricles. Relaxation constitutes the diastolic phase while contraction constitutes the systolic phase. Both the atria and the ventricles remain relaxed at the start of the cardiac cycle (diastole). In describing the cardiac cycle, we shall relate it to the EKG patterns. The phases of the cardiac cycle will include atrial systole and diastole, ventricular systole, and ventricular diastole (Pollock & Makaryus, 2018).

Atrial Systole and Diastole

Contraction of the atrium represented on the EKG by the P wave occurs after depolarization. Contraction is from the upper part of the atrium inferiorly towards the atrioventricular septum and creates a buildup of pressure in the atrium that allows the pumping of blood into the ventricles. Blood flows through the open atrioventricular valves, the tricuspid and bicuspid valves into the ventricles. Atrial contraction contributes about 20-30% in filling the ventricles since most of the times they are already filled up to about 70% from a previous phase of diastole. Atrial systole ends before ventricular systole as the muscle relaxes returning to diastole.

Ventricular Systole

Ventricular systole represented by the QRS complex on the EKG occurs after depolarization of the ventricles. At the end of the systolic atrial phase and just before the contraction of the atrium, the ventricle contains typically a total volume of 130ml of blood in a resting standing adult, and this is what forms the end diastolic volume also referred to as the preload. At the start of the ventricular systole, the contraction of the ventricles does not provide enough buildup of pressure to open the semilunar valves and allow the flow of blood out of the ventricles. However, as the contraction continues, there is a buildup of pressure more than that in the atria that have relaxed which forces blood to flow back towards the atria causing the closing of the atrioventricular valves. The early stage of ventricular systole is the isovolumic contraction of the ventricles (Marcondes et al., 2015).

In the late stage of ventricular systole also referred to as the ventricular ejection stage, continued contraction leads to a buildup of pressure in the ventricles higher than that in the pulmonary trunk and aorta. Blood therefore profoundly gets pumped out of the ventricles through the now open pulmonary and aortic valves. The volume of blood pumped out is what constitutes the stroke volume whose normal range is between 70 and 80mls. After ventricular systole, there remains an end systolic volume of between 50 and 60mls of blood.

Ventricular Diastole

Ventricular diastole represented by the T wave on the EKG follows the repolarization of the ventricles. During the early phase of ventricular relaxation, the pressures inside begin to fall to less than that in the pulmonary trunk and aorta, and thus allows blood to flow back to the heart. There is the closure of the semilunar valves to prevent the backflow of blood into the heart. As a result, there is no change in the volume of blood in the ventricles, and this is the isovolumic ventricular relaxation phase.

In the late phase of ventricular diastole, the blood pressure continues to fall due to ventricular muscle relaxation and falls below that of the atria. Blood, therefore, flows from the atria into the ventricles pushing open the atrioventricular valves. Blood continues to flow from the major veins to the atria and into the ventricles. As a result, there is the completion of the cardiac cycle. The figure below is a representation of the cardiac cycle and the corresponding EKG patterns.

Role of the EKG

An electrocardiogram records the electrical activity of the heart at rest. It is significant in identifying abnormalities in the electrical activity of the heart causing pathology in the heart. An EKG has component tracings which correspond to a particular phase in the cardiac cycle as already described above. The component tracings are the P, Q, R, S, T, and U and are present in the figure above. The P wave indicates the contraction of the atria and is the first short upward movement of the EKG tracing. The QRS complex represents ventricular depolarization and contraction. The PR interval indicates the time taken for an electrical signal to travel from the sinus node to the ventricles, and finally, the T waves represent ventricular repolarization (Farrag, 2017).

Conduction disorders of the heart may easily be picked out from an EKG. The three common conduction disorders include bundle branch block, heart block and long QT syndrome. In bundle branch block, there is a delay in the electric signals reaching one of the ventricles, and as a result, arrhythmias result. An EKG will show a longer than the standard duration of the QRS complex. In a heart block, there is a block to the conduction of signals from the atria to the ventricles in the heart. It could either present as a first, second or third heart degree. In long QT syndrome, the ventricles take a longer time to contract, and there is, therefore, a prolonged cardiac cycle. The disease also presents with arrhythmias.

Besides conduction disorders, other heart conditions such as myocardial infarction can also get detected from an EKG. Myocardial infarction may present either with an ST elevation or ST depression depicted from an EKG. An EKG is, therefore, a necessary tool in the diagnosis and management of cardiovascular abnormalities (Acharya et al., 2016).

Heart Valve Malfunctions

Mitral Stenosis

In most cases due to rheumatic heart disease from the progressive fibrosis and calcification of the heart valves. There is an impairment flow of blood to the left ventricle; pressure builds up in the left atrium resulting in pulmonary congestion from backflow of blood to the lungs (Otto, Gaasch, & Yeon, 2018). An individual may present with breathlessness, easy fatigability, edema and ascites from right heart failure, cough from pulmonary edema and chest pain from pulmonary hypertension. Presenting signs include atrial fibrillation, mitral facies, a mid-diastolic murmur and crepitation from pulmonary edema. A chest X-ray, an EKG and an echocardiogram are essential investigative tools for the disease.

Mitral Regurgitation

Causes may include mitral valve prolapse, rheumatic heart disease and ischemia of the papillary muscles. It causes left atrium dilatation with less increase in pressure. When chronic, it presents as mitral stenosis but sudden onset mitral regurgitation present with signs and symptoms of acute pulmonary edema such as cough. A chest X-ray, an EKG and an echocardiogram are essential investigative tools for the disease.

Aortic Stenosis and Aortic Regurgitation

Cause of aortic stenosis may include congenital causes, rheumatic heart disease or a degenerative disorder in the elderly. There is hypertrophy of the left ventricle with inadequate coronary blood flow. Individuals mostly present with exertional dyspnea, angina and syncope. In aortic regurgitation, the cause is either congenital or from rheumatic heart disease and infective endocarditis. Individuals may have a bounding and a collapsing pulse in addition to difficulty in breathing (Thubrikar, 2018). Patients may also have an early diastolic murmur and a systolic murmur due to an increase in the stroke volume of the heart. In the old people, aortic stenosis is the primary cause of syncope, angina and heart failure. A chest X-ray, an EKG and an echocardiogram are essential investigative tools for the disease.

Tricuspid Regurgitation and Stenosis

The leading cause of both tricuspid regurgitation and stenosis is rheumatic heart disease and infective endocarditis commonly in intravenous drug users. Both result in right atrium hypertrophy and dilatation and may also cause pulmonary congestion and hypertension. Problems with the tricuspid valve may cause right heart failure symptoms such as hepatic discomfort, ascites and peripheral edema. A chest X-ray, an EKG and an echocardiogram are essential investigative tools for the disease.

Pulmonary Stenosis and Pulmonary Regurgitation

Pulmonary stenosis is a congenital disorder in most cases and may be associated with other abnormalities such as tetralogy of Fallot. It can also occur in carcinoid syndrome. Pulmonary regurgitation, on the other hand, is a rare heart valve problem associated with pulmonary artery dilatation due to pulmonary hypertension. It may present as a minor problem in healthy individuals with no clinical significance. In pulmonary stenosis, there is a systolic ejection murmur usually preceded by an ejection click. The primary organisms implicated are the viridans group of streptococci including streptococcus mitis and Streptococcus sanguis. A chest X-ray, an EKG and an echocardiogram are essential investigative tools for the disease.

References

Acharya, U. R., Fujita, H., Sudarshan, V. K., Oh, S. L., Adam, M., Koh, J. E., ... & Poo, C. K. (2016). Automated detection and localization of myocardial infarction using electrocardiogram: a comparative study of different leads. Knowledge-Based Systems, 99, 146-156.

Farrag, S. (2017). EKG Interpretation. Urgent Procedures in Medical Practice, 129.

Marcondes, F. K., Moura, M. J., Sanches, A., Costa, R., de Lima, P. O., Groppo, F. C., ... & Montrezor, L. H. (2015). A puzzle used to teach the cardiac cycle. Advances in physiology education, 39(1), 27-31.

Otto, C. M., Gaasch, W. H., & Yeon, S. B. (2018). Clinical Manifestations and diagnosis of rheumatic mitral stenosis. UpToDate, Post, TW.

Pollock, J. D., & Makaryus, A. N. (2018). Physiology, Cardiac Cycle. In StatPearls [Internet]. StatPearls Publishing.

Thubrikar, M. (2018). The aortic valve. Routledge.