Heart rate is controlled by electrical impulses sent throughout the heart muscle. Disruption to the electrical conduction system can alter the rate at which the heart beats. Tachycardia and bradycardia are abnormal heart rates.
Normal Heart Rate
The electrical impulse that generates a heart beat originates at the sinoatrial node, located in the right atrium. This node consists of the heart's pacemaker cells. These cells initiate a heartbeat at a rate of approximately 60 to 100 beats per minute when a person is at rest.
Bradycardia
Bradycardia occurs when the heart is unable to maintain a resting rate of at least 60 beats per minute. The heart's major function is to deliver oxygen and nutrients throughout the body. A slow heart rate limits this delivery capability and reduces oxygen transport. Because the body, and especially the brain, relies on a constant supply of oxygen for proper functioning, the Mayo Clinic says bradycardia can cause many symptoms. Dizziness, faintness, fatigue, shortness of breath and chest pain can all result from a slow heart rate.
Bradycardia is common in aging people, those with known heart disease, and in people on certain medications, such as beta-blockers. In these people, the bradycardia is considered a pathological complication, meaning it results from a disease state, and therefore requires treatment. However, some people incur bradycardia due to the positive adaptation of the heart to exercise training. This type of bradycardia is termed physiological. Physiological bradycardia enhances oxygen delivery and indicates the heart has become a more efficient, stronger pump.
Tachycardia
The term tachycardia refers to a resting heart rate greater than 100 beats per minute. Tachycardia is identified at rest, because an elevated heart rate is a normal response to exercise.
Tachycardia causes a quivering of muscle in the atria or the ventricles rather than a strong, synchronized contraction. This quivering reduces the amount of blood ejected from each chamber per beat. The quivering associated with atrial tachycardia reduces the amount of blood sent to the ventricles, and could potentially stimulate the ventricles into a fast rhythm.
Ventricular tachycardia is potentially fatal. It can rapidly progress to sudden cardiac arrest. If the ventricles quiver rather than contract uniformly, they're unable to deliver oxygen-rich blood to the body. The lack of oxygen causes cells to die off, including those of the heart muscle, and sudden cardiac arrest ensues.
Diagnosis
According to the text "ECG Interpretation for the Clinical Exercise Physiologist," the electrocardiogram (ECG) is the initial diagnostic tool used to identify bradycardia and tachycardia. In the event that a person doesn't have an abnormal heart rate at all times, a portable ECG is applied to the person, and heart rate recordings are monitored over a period of days.
Additional testing to identify the cause, specific triggers and severity of the abnormal rhythm might be necessary. These can include tilt table testing, exercise stress testing and electrophysiological testing.
Treatment
In 2008, the American College of Cardiology published guidelines for the use of pacemakers in patients with bradycardia. The pacemaker prevents the heart rate from dropping below a designated lower limit by taking over the job of the pacemaker cells if they fail to maintain the necessary rate for adequate oxygen delivery.
The same guidelines indicate the use of a defibrillator for tachycardia patients. Defibrillators deliver an electrical shock that jolts a fast heart rate back to normal. It's used in the prevention of sudden cardiac arrest.
References
- Mayo Clinic: Bradycardia
- "ECG Interpretation for the Clinical Exercise Physiologist"; Christopher Dunbar, M.D.; 2009
- "Journal of Invasive Cardiology"; ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities; Stephen C Vlay, M.D.; 2009
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