Abnormalities in the cardiac cycle

The heart is evolutionary masterpiece. Yet like everything else in nature, it is not perfect and problems may arise. However before its flaws can be mentioned and analysed. It would be prudent to understand how the heart functions in the first place before going in depth into the various problems that might arise within the cardiac cycle. Of which a large number are covered under the various Arrhythmias, which can be divided into two major groups, ventricular arrythythmias and supraventricular arrythythmias. I.e., arrythythmias that occur in the ventricles and arrythythmias that occur above them. Subdivision is possible, with arrhythmias further divided into Disorders of impulse conduction and disorders of impulse generation.

The various and detailed events that take place with every heart beat is referred to as the “cardiac cycle”. The top chambers of the heart are called the atria and the bottom chambers, the ventricles. A normal heart beat begins when the Sino-Atrial Node, located on the posterior wall of the right atrium. The Sino-Atrial Node initiates the “wave of contraction” which results in the contraction of the atria. The Atrio-Ventricular Node delays the signal initiates by the Sino-Atrial node for about 0.15 seconds, permitting full atrial systole before ventricular systole. This ensures that the ventricles are filled before the ventricles contract. When the “wave of contraction” reaches the apex of the ventricles, ventricular contraction begins. As the pressure in the ventricles increases, the AV valves close. This prevents the backward flow of blood. With continued contraction of the ventricles, blood leaves the heart via the aortic and pulmonary valves into the aorta and pulmonary artery respectively. At the end of the cardiac cycle, all the chambers relax, the aortic and pulmonary valves close, preventing the blood from flowing back into the heart. Atria begin to fill up with blood once more to prepare for the start of the next cycle.

The cardiac cycle as seen above is definitely not a simple process. In its complexity a number of problems may arise, of which Arrhythmias constitute a relatively large proportion. Arrhythmias are defined as any deviation from the normal, steady beating rhythm of the heart. Small, minor irregularities in the heartbeat are common, but more serious arrhythmias can lead to fainting, angina or even heart attack. Arrhythmias are generally caused by damage to the heart muscle or to the Sino-Atrial node. They can be diagnosed primarily with the electrocardiograph, this records the electrical impulses made by the beating of the heart. An arrhythmic heart will produce an uneven pattern as compared to regular peaks and valleys of a normal heartbeat. Occasionally, an arrhythmia is so mild that not much treatment is required. However more often than not, most arrhythmia are treated with medication, defibrilators, or pacemakers. These serve to steady the irregularities and maintain a healthy steady heartbeat.

A very common form of Arrhythmias is Sinus Bradycardia. This essentially means that the Sino-Atrial node, which normally controls the heart rate, is generating electrical signals that are generally slower than they normally would be. By definition, sinus Bradycardia is present in a person if his/her sinus node produces less than 60 electrical signals per minute. Sinus Bradycardia is not necessarily abnormal. Young, healthy people, especially athletes will usually have heart rates less than 60 because their hearts become very efficient at pumping. When Sinus Bradycardia does cause problems, it is often in the elderly, This arrhythmia can be the result of medications taken for high blood pressure, angina or tachycardia. If there are no symptoms due to a Sinus Bradycardia then nothing needs to be done. However it can cause serious problems if not investigated. Such a problem would be the “Sick Sinus Syndrome”

Sick Sinus Syndrome is the name given to the medical condition in which a diseased sinus node results in the severe slowing of the heart beat. Such a condition would include the afore mentioned Sinus Bradycardia. Sometimes the Sino-Atrial node ceases firing temporarily and long pauses in the cardiac cycle would result. This condition causes symptoms of fatigue, light-headedness, shortness of breath and even fainting. It can occur at any age over 30, although it is usually in patients above the age of 50 years. Intrinsic and extrinsic problems may contribute to causing the Sick Sinus Syndrome. Intrinsic problems include the natural process of aging, ischmia, scarring or physical damage. Extrinsic problems include medications, hormonal conditions (an under active thyroid gland for example) or neurological imbalances. This condition can be corrected using a simple Pacemaker, as the problem originates from the Sino-Atrial node. Overriding the Sino-Atrial node’s impulses using the Pacemaker would subsequently eliminate all the symptoms of the Sick Sinus Syndrome.

Moving further down, we come to the Atrio-Ventricular Node. Here, a condition may arise known as AV Block. Unlike the obstruction in the coronary arteries due to atherosclerosis, AV Block is an electrical block, akin to an “open circuit” in the “wires” of the heart. AV Block occurs when the Atrio-Ventricular Node fails to properly conduct the impulses from the atria to the ventricles. The severity of AV block varies from slight to life-threatening. First degree AV block means that there is nothing more than an excessive delay in the conduction of the impulse from the atria to the ventricles, but all the impulses still get through. This is a benign condition not requiring treatment. This level of AV block may be due to disease in the AV node or it may be cause by certain medications such as calcium-channel blockers or beta blockers. Both of which slow down nervous impulses. Second degree AV block occurs when some impulses originating from the Sino-Atrial node fail to pass through the AB node, so the ventricle does not always receive its prompt to beat.

This can cause an irregular heart beat and awkward pauses in the heart system. Depending on the specific location of the block (within the AV node itself or below the AV node in the His bundle) the condition could range from being benign to rather serious. Patients suffering from Second degree AV block usually have a permanent pacemaker implanted to prevent future problems. Third degree AV block, also known as Complete Heart Block, is a condition in which no impulses can find their way through, probably due to severe disease in the conduction system. This generally causes severe bradycardia and on occasion the heart can actually cease beating. Complete heart block is a medical emergency requiring the temporary insertion of a pacemaker wire. This temporary wire is used to keep the heart beating until a permanent pacemaker can be implanted via surgery.

In the Atrium, a number of problems may occur due to abnormalities in the cardiac cycle. For instance, Atrial Fibrillation. It is a condition in which the electrical signals sent to the atrium becomes extremely rapid and chaotic. Instead of the Sino-Atrial node providing the normal electrical signals to the atrium, rapid circulating waves of abnormal electrical signals perpetually stimulating the atrium. The atrial rate can exceed 400 beats per minute, causing extreme strain to the atrium. When Atrial Fibrillation has been present for more than 6 months, it is hard to restore a normal system. Such a condition become known as Permanent Atrial Fibrillation. These signals then bombard the ventricles which then beat rapidly and irregularly. In fact, the overall rate of the ventricles vary so widely that the ventricles are placed under large levels of strain. Often, the only way to cause the disorganised activity in the atrium and subsequently the ventricles to cease is to get a large portion of all the atrial tissue to fire at the same time. This is usually done by sending an electrical shock across the chest in a procedure called “electrical cardioversion” The shock stabilises the signals and allows the normal rhythm to resume. This is usually done with deep sedation so that the patient will not remember the experience.

Of all the Arrhythmia, the most devastating heartbeat irregularity is called Ventricular Fibrillation. This occurs when the usually steady pumping of the heart is reduced to a useless quivering mass of cardiac tissue. Naturally this prevents the heart from pumping sufficient blood throughout the body. Due to the immediate collapse of the cardiovascular system it is a frequent cause of sudden cardiac death. Ventricular Fibrillation often occurs in the presence of significant heart disease, after a heart attack or some other serious injury such as a very severe electric shock. If somebody is lucky enough to be resuscitated from cardiac arrest due to Ventricular Fibrillation, he or she is considered to be at a very high risk of recurrence.

A complete electrophysiologic evaluation of the patient would be required. If Ventricular Fibrillation is caused by blockages in the coronary arteries, this can be corrected with bypass surgery or balloon angioplasty, thus decreasing the chances for recurrence. However, as Ventricular Fibrillation is a result of short circuits within the heart, it is usually not corrected by either of the aforementioned procedures. A recent study revealed improved survival rates in patients with Ventricular Fibrillation who received an implantable cardioverter-defibrillator as compared to patients placed on medical therapy. Thus problems resulting from Ventricular Fibrillation, assuming one survives in the first place, would be the very high chance of Ventricular Fibrillation striking again at anytime.

Naturally not all problems in the cardiac cycle stem from arrhythmias. For example Atrial Septal Defects and Ventrical Septal Defects (hole in the heart) are examples of other conditions that may cause problems to the patient. This defect is characterised by the incomplete closure between the two atrium and two ventricals respectively. This results in the mixing of oxygenated and deoxygenated blood. This results in less efficient pumping of the heart. As a result the heart is placed under strain as it has to pump harder and more frequently. This may result in the heart dilating and the muscles become more weak. Other conditions that may arise include increasing levels of pressure in the pulmonary arteries. If the defect is not corrected, then the pressures in the pulmonary arteries may become so high and changes in the arteries would be induced such that the closure of the defect would not improve the patient.

In conclusion, whilst the heart remains a miracle of nature, its complexity naturally contributes to the wide range of problems that may stem from abnormalities within the cardiac cycle. Fortunately, with advanced technology such as pacemakers, electrical cardioversion and cardioverter-defibrillators, coupled with ample knowledge of the various symptoms, lives can be saved even with problems as severe as that of the heart.