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By: Zayd A. Eldadah, MD, PhD
The heart is an extraordinary machine. Physically situated in the core of the human body, it is truly at the center of life. From the earliest stages of embryonic development, the heart begins a life-long, relentless, and magnificent mission of maintaining the circulation of blood throughout the body. In doing so, it ensures that we stay awake, alive and healthy. The heart pumps essential nutrients and life-sustaining oxygen to our cells and tissues, and it ensures that toxins and waste products can be processed and expelled. It is this perpetual circulation that enables our eyes to see, our brains to learn, our legs to run, our arms to embrace, our skin to heal and every one of the countless other functions that our extraordinarily complex bodies perform daily. Without a heartbeat, all of this comes to a halt. Indeed, within just seconds of a heart stopping, organ injury ensues. After mere minutes, irreversible damage sets in, and if the heart does not restart, the result is death.
The heart truly sets the rhythm of life. In beating about a billion times in a typical human lifetime, the heart maintains a rhythm that sets the pace of life. This rhythm originates in the sinoatrial (SA) node, a quarter-sized area of cells in the upper part of the right atrium (Figure 1).
These cells are the heart’s natural pacemaker and rhythmically initiate every normal heart beat by launching a wave of electricity. This impulse spreads through both atria en route to the “junction box” of the heart, the centrally located atrioventricular (AV) node, where the electrical impulse is processed before being relayed rapidly down the bundle branches and into the heart’s final electrical conductive meshwork, the Purkinje fibers, deep within the thick muscle of the ventricles. When electricity passes through muscle, the muscle contracts. In the case of the heart, this rhythmic impulse progression—from top to middle to bottom of the heart—triggers the upper chambers to squeeze blood into the lower chambers, which in turn squeeze and propel blood out of the heart.
When it comes to heart rhythm, timing is everything. Electrical conduction and mechanical contraction are coupled, and when the rhythm is in tune, pumping is optimized. Conversely, when the rhythm is off, trouble arises. A normal heart paces and pumps at a rate that is just right for the body’s minute-to-minute needs—slowly when we sleep or relax, and quickly when we run or react to stress. Good timing also means that blood moves through the heart chambers optimally. In fact, the heart chambers don’t just push blood along, they actually simultaneously squeeze and twist to propel blood as efficiently and effectively as possible. Being synchronized is a particularly important part of an effective “squeeze and twist.” Like a water balloon that best expels its contents when squeezed from both sides at once, the heart pumps best when its right and left sides contract at the same time.
Arrhythmia, meaning abnormal heart rhythm, is any deviation from this ideal cardiac choreography. Arrhythmias are typically divided into two broad categories: slow rhythm (bradycardia) or fast rhythm (tachycardia).
Slow heart rhythms generally result from either a malfunctioning sinoatrial node, which is unable to initiate a sufficiently rapid heart rate, or from slowing or blocked conduction of the electrical impulse at some point during its journey from the SA node to the Purkinje fibers. Aging is a leading cause of sinus node dysfunction, which often presents as fatigue, weakness, dizziness, and even passing out. Bradycardia due to heart conduction slowing or conduction block can also be caused by aging, but more commonly, it is the result of insults, such as drugs that affect the conduction system, cardiac injury (e.g., myocardial infarction), or derangements in circulating electrolytes (e.g., high potassium). Treatment of slow heart rhythm starts by removing reversible causes if known, and when this is not possible, by implanting a cardiac pacemaker (Figure 2).
Pacemakers are half dollar-sized devices that are implanted beneath the skin and are connected to wires that are threaded within a vein and secured to the heart muscle itself. These devices are able to sense when the upper and/or lower chambers are beating too slowly. In response, they initiate an electrical impulse in the atria and/or ventricles which maintains a healthy rhythm. Modern pacemakers even have sensors that detect physical activity and speed up the rhythm when needed to meet the body’s increased demand.
Abnormally fast heart rhythms have multiple potential causes. An electrical “short circuit” within the upper chambers of the heart or within the AV node can cause supraventricular tachycardia (SVT), an abnormally rapid rhythm in which the heart races. The pulse in SVT can be as fast as 200 or more beats per minute, causing palpitations, lightheadedness, shortness of breath, and/or chest pain. Often, these arrhythmias can be cured by advancing a catheter to the heart—usually via the large vein in the thigh—directing it precisely to the site of the abnormal circuit, and eliminating this incorrect electrical pathway by burning or freezing it. When abnormally fast rhythms originate in the ventricles, more ominous possibilities arise. These rhythms are usually the result of disease in the ventricles—for example, scar tissue caused by a prior heart attack. When healthy heart muscle is replaced by scar, the potential for a rapid short circuit develops (Figure 3).
Ventricular tachycardia (VT), which often degenerates into a chaotic and disorganized ventricular fibrillation, is a classic example of this phenomenon in which electricity courses through the ventricles so rapidly that the chambers quiver rather than beat. This can be deadly because quivering ventricles cannot effectively deliver blood to the body. Symptoms include dizziness and loss of consciousness. If normal rhythm is not restored promptly, death may result. Fast heart rhythms originating from the ventricles are treated first by identifying and treating any potential reversible causes—for example, a sudden blocked coronary artery. When no such reversibility is possible, implantable cardiac defibrillators (ICDs) are the answer. These devices are slightly larger than pacemakers and offer virtually all of the features of pacemakers (i.e., they can fully treat slow heart rhythms by pacing the heart). But ICDs offer the additional critical feature of recognizing and treating rapid, dangerous ventricular arrhythmias by either pacing the heart out of the rapid rhythm or delivering a life-saving shock to the heart.
Cardiac electrophysiology is the study and practice of managing heart rhythm and is a medical subspecialty that offers advanced therapies to treat cardiac arrhythmias. Life depends on this magnificent pump—and on its rhythm. The physicians, nurses, technologists, and medical device manufacturers who help patients with heart rhythm disorders are dedicated professionals whose prime directive is restoring the rhythm of life.
Dr. Zayd Eldadah is a clinical cardiac electrophysiologist practicing at Washington Hospital Center. His areas of expertise are Cardiac Defibrillators, Cardiac Pacemakers, and Catheter Ablation. Dr. Eldadah is fluent in Arabic and French. He spearheaded community events to improve awareness in the highly diverse DC community by leveraging ambassadors to become leaders in heart health education.
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