Heart palpitations - part 11
Palpitations are defined as a conscious awareness of the beating of ones own heart. Most of the time we are completely unaware of the beating of the heart but sometimes that unawareness is punctured by a sensation of skipped beats that can be felt in the chest or sometimes in the throat. The palpitation may be experienced either as a fleeting sensation or it may be more persistent.
In that latter situation the person may be conscious of either a regular or irregular pounding in the chest. In a previous article on the subject of palpitations I wrote about the palpitation as a common event that is experienced by all of us. In this article I will address the subject of the palpitation as a pathological event and will describe the various abnormal conditions that give rise to palpitations.
However, before we consider these various conditions it would be useful to first describe the electrical circuitry of the heart. The rate and rhythm of the heartbeat is controlled by its own intrinsic pacemaker, which is called the SA (sinoatrial) node. The SA node is a small bundle of tissue situated in the right atrium, which is one of the four chambers of the heart. This highly specialised bundle of tissue discharges electricity spontaneously, which is then conducted throughout the heart muscle through a special conduction circuit. These electrical impulses are intermittent and rhythmic and stimulate the heart muscle to contract.
When an electrical impulse is discharged from the SA node it then spreads throughout both the right and left atrium causing the muscle in the wall of these chambers to contract thereby pumping the blood into the ventricles. The electrical impulse then passes through the AV (atrioventricular) node, which acts as an electrical relay station that relays the electrical signal into the right and left ventricle. The impulse must pass through the AV node in order to reach the two ventricles. When the electrical impulse reaches the ventricles the muscle in these chambers contracts and blood is pumped into the lungs via the pulmonary artery and into the general circulation via the aorta.
If the SA node discharges in a chaotic fashion the heart muscle contracts irregularly. Rhythm disturbances are referred to as arrhythmias. Arrhythmias frequently produce the sensation we describe as a palpitation. Arrhythmias can be slow, fast or irregular. The symptom of palpitation associated with this abnormal rhythm can similarly be slow, fast or irregular; although the person may not be aware of the arrhythmia for all the time that it is present. It is also important to stress that arrhythmias can be intermittent, which means that the heartbeat can return to a normal rhythm and rate between bursts of arrhythmia. That is the reason why arrhythmias can be missed because the person may have presented to their doctor for evaluation at a time when the heart has reverted to a normal pattern of beating.
A rapid arrhythmia is called a tachycardia (heart rate greater than 100 beats per minute) whereas a slow arrhythmia is called a bradycardia (less than 60 beats per minute). Sometimes a single heartbeat occurs a split second earlier than normal and this is referred to as a premature contraction. The term fibrillation is used to describe rhythms that are irregular and these irregularities can occur in either the atria or ventricles. All of these various types of rhythm disturbance can arise from disorders anywhere along the electrical circuitry of the heart. Therefore abnormalities in the SA node, the atria, the AV node or the ventricles can lead to arrhythmias, which may be manifest to the person as palpitations.
Feeling the pulse
If we use the preceding terms in conjunction with the site of origin of the rhythm disturbance we can give a descriptive label to virtually any arrhythmia. Thus we can have atrial tachycardias and premature atrial contractions. Similarly we can have ventricular tachycardias and premature ventricular contractions. Fibrillations can be either atrial or ventricular. Atrial fibrillation is the commonest arrhythmia that we encounter in general practice. Also, if a bradycardia arises because of slow electrical discharge from the SA node we refer to this rhythm disturbance as a sinus bradycardia.
Feeling the pulse may identify many of the rhythm disturbances I have described but they all have their own individual electrical pattern that is usually readily identifiable on a standard ECG. If the arrhythmia has reverted to normal rhythm at the time of the ECG it may be necessary to proceed to Holter monitoring, which is a special form of continuous ECG that is conducted over a 24-hour period of time.
So, what causes these various disturbances of rhythm? Arrhythmias can be caused by coronary artery disease, diseases of the heart muscle or even by diseases affecting the heart valves. Sometimes the cardiac muscle, valves and coronary arteries may be relatively healthy but the conduction system may be at fault. Such situations can be considered as being a form of electrical fault in the heart.
Sometimes medication can give rise to arrhythmias. Paradoxically, digoxin, which is frequently used for treating certain arrhythmias, can cause an irregular heartbeat. This occurs if the level of digoxin in the bloodstream is too high, a situation that is referred to as 'dig toxicity'. Disorders of the thyroid gland can also change the rate and rhythm of the heartbeat. For example, people with hyperthyroidism (over activity of the thyroid gland) often experience palpitations before presenting for treatment and subsequent stabilisation of their condition. People with chronic lung disease can also develop arrhythmias because of the low levels of oxygen circulating in their blood, which in turn can impact on the activity is the SA node.
It is worth noting that excessive alcohol consumption can cause arrhythmias and the heart muscle and electrical conducting system can be permanently damaged by excessive consumption that takes place over an extended period of time. In other words the alcohol slowly poisons the heart. It is not too strong a statement to say that this situation is like pickling the heart in alcohol.
Symptoms to watch out for
Some people can be totally unaware of the presence of an arrhythmia because not every person with an arrhythmia experiences palpitations. However, if a person is experiencing palpitations there are some additional symptoms that should prompt the person to seek medical assistance. Palpitations accompanied by tightness in the chest or throat should not be ignored, even if the discomfort is slight. Shortness of breath is another significant accompanying symptom. A history of fainting or even dizziness at the time of a burst of palpitations may also be very significant. Weakness is a vague symptom but if it occurs in association with palpitations this should not be ignored and shrugged off.
As mentioned in my previous article on palpitations most people experience palpitations from time to time and such experiences are not indicative of underlying disease. However, for some people this common symptom can be a harbinger of more serious trouble especially if it is accompanied by any of the symptoms mentioned in the preceding paragraph. In such situations it would be sensible to attend your GP for medical assessment.
Dr Leonard Condren is the medical editor of irishhealth.com
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