100,000 beats daily — and getting it right almost every time
Can you imagine doing the same thing over and over again, 100,000 times each day for decades? And getting it right almost every time? That’s what the human heart does. Your heart beats regularly several billion times over a normal lifespan in order to pump and circulate 2,000 gallons of blood every day.
Amazingly, your heart does this without fancy electronic or mechanical controls. The signal to contract arises at regular intervals from a small cluster of cells called the sino-atrial (SA) node. The SA node is located in the upper chambers (atria) of the heart. The electrical impulse causes the atria to contract, pumping blood to the lower heart chambers (ventricles). The electrical signal travels in special conducting fibers to the atrioventricular (AV) node. The AV node is another cluster of specialized cells that delays the signal a bit so that it will reach the ventricles after the atria have contracted and filled them with blood. After the passing through the AV node, the electrical signal continues to the ventricles. The ventricles then contract, so that the left ventricle can send blood to the body and the right ventricle can pump blood to the lungs.
The description above is how things are supposed to work, and most of the time that’s the way things happen. But sometimes the system runs awry, and the heart rhythm is disrupted. That’s called an arrhythmia.
Types of Arrhythmias
• Premature beats — The heart may throw in an early beat, before the next regular beat is due. This premature beat may arise from either the atria or the ventricles. Premature beats are the most common type of arrhythmia. The vast majority of the time they are not dangerous, and often go unnoticed. Stress and caffeine both increase the occurrence of premature beats.
• Atrial fibrillation — This abnormal rhythm affects 3-5 percent of all Americans. It’s the most common type of medically dangerous arrhythmia. In atrial fibrillation, the atria cease beating regularly and instead quiver ineffectively. Blood may pool in the atria, leading to blood clots. These blood clots can break loose, potentially traveling to the arteries in the brain. Blockage of brain arteries can cause a stroke. The quivering atria don’t fill the ventricles properly, potentially causing insufficient blood to be pumped to the body and lungs. And the lack of an effective pacemaking impulse from the SA node causes the ventricles to contract irregularly, often too fast, too slow, or alternating between too fast and too slow.
• Bradycardia — Bradycardia means that the heart beats too slowly. It may be normal and cause no symptoms in a well-conditioned athlete. Or it can cause problems due to lack of sufficient blood circulation. Certain types of medicines can cause bradycardia.
• Tachycardia — Tachycardia means that the heart beats too quickly. It some cases it is a physiological response to fever, blood loss or dehydration. It can arise from an overactive thyroid. Or it can be due to a diseased cardiac electrical system.
• Ventricular arrhythmias — Normally the ventricles don’t contract until they receive a signal conducted from the upper portion of the heart. But sometimes the ventricles begin beating on their own. This may be ventricular tachycardia, a very fast rhythm that is seldom stable for long. Or there may be ventricular fibrillation, a potentially lethal arrhythmia where the ventricles quiver ineffectively and don’t circulate the blood at all. If prompt CPR and electrical defibrillation do not restore circulation, irreversible brain damage begins in 3-5 minutes.
Symptoms—Arrhythmias may cause palpitations — the sensation of the heart beating too quickly or irregularly. If an arrhythmia compromises blood circulation, a patient can feel short of breath, weak, lighted headed, or dizzy. There may be chest pain. Serious arrhythmias can cause loss of consciousness ranging from a brief faint to cardiac arrest.
If you experience an occasional brief palpitation with no associated symptoms, see your doctor soon to be checked. But for anything more serious, call 9-1-1 and head to the emergency room by ambulance. Don’t drive yourself or have friends and family drive you when you are experiencing the symptoms of potentially serious heart disease!
Diagnosis—An exam and EKG identify many heart rhythm disturbances. But many arrhythmias come and go, and may disappear before you see a physician. In that case you may need a Holter monitor, which observes the heart rhythm for 24 (or more) hours. For infrequent problems, you may need an event monitor, a device that the patient wears for days and activates when the symptoms occur. Certain arrhythmias are evaluated by an electrophysiological (EPS) study, where a cardiologist inserts electrodes into the heart to observe the electrical activity.
Treatment—The treatment of cardiac arrhythmias has involved tremendously since 1950, due to improved medications, new electrical devices, and invasive procedures.
Frequently used medicines include:
• Warfarin: a blood thinner that decreases the risk of blood clots in atrial fibrillation
• Antiarrhythmics: help maintain the heart’s normal rhythm. Examples are amiodorone and sotolol.
• Beta-blockers: slow the heart rate by limiting the effect of adrenaline on the heart.
• Calcium channel blockers: slow the heart rate and suppress tachycardia.
Devices that help treat arrhythmias include pacemakers and implantable cardiverter/defibrillator (ICD). An ICD detects life-threatening arrhythmias and delivers a shock to terminate the rhythm before the rhythm terminates the patient.
An example of an invasive procedure is cardiac ablation. Catheters are inserted into the heart to map the source and electrical route of the abnormal rhythm. If the abnormality is identified, radiofrequency energy can be delivered through the catheter to target a select group of hear muscle cells. A scar forms, blocking the abnormal electrical pathway.
Here’s a final tidbit. It turns out that Monday is the most common day for arrhythmias to occur. Scientists think this is because of stress. Interestingly enough, the Monday association occurs even in retired people!
Eva Briggs, a board-certified physician, works on the staff at Cayuga Medical Center in Ithaca, in its two urgent care centers: one in Ithaca, and the other in Cortland.