The timing of your heartbeat is controlled by electrical impulses generated in your upper heart. If these impulses are delayed in any way, you can develop significant disturbances in your normal heart function. Doctors use a measurement called the P-R interval to help assess the accuracy of your heart's electrical signals. In some cases, this assessment takes place during an active procedure called an exercise stress test.
P-R Interval
Electrical signals that control your heartbeat pass from the upper chambers of your heart, called the atria, to the lower chambers of your heart, called the ventricles. Doctors use the P-R interval to measure the amount of time it takes for a signal from your atria to pass downward to your ventricles. Typically, this interval lasts anywhere from 0.12 to 0.20 seconds. If a signal takes more than 0.20 seconds to pass from your upper heart to your lower heart, you may have a disorder called an AV conduction block or first- or second-degree heart block.
Significance
If you have first-degree heart block, the P-R interval in your heartbeat is longer than normal, but the signal still eventually travels from your atria to your ventricles. Typically, this disorder produces no symptoms and poses no real threats to your heart health. If you have second-degree AV heart block, your P-R interval may increase until the signal from your atria no longer reaches your ventricles. In other second-degree cases, an unusually long P-R interval may hold steady, but your lower heart will intermittently fail to receive a proper signal. These types of heart block can be serious or even life-threatening.
Exercise Stress Test
Your doctor can measure your P-R interval with a procedure called an electrocardiogram, or EKG, which uses electrodes attached to your body to relay electrical information about your heart to a recording device called an EKG machine. If you have symptoms of heart disease, need to determine your heart attack risks, or need to determine how hard you can safely exercise, your doctor may choose to perform an electrocardiogram while you stress your heart with exercise. This procedure is commonly called a treadmill stress test or exercise stress test.
Exercise Options
In addition to treadmill-based exercises, your doctor can perform a stress test while you exercise on a stationary bike or use your upper body to exercise on a bicycle-like hand crank. If you have significant exercise-related health problems, he can also forgo physical activity altogether and simulate exercise's effects on your heart with special medications. During and after testing, your doctor will analyze the readout from the EKG machine and look for signs of P-R interval abnormalities or other potential problems. He will base any further actions on these findings.
References
- Cardiovascular Physiology Concepts: Electrocardiogram (EKG, ECG); Richard E. Klabunde, Ph.D.; April 2007
- National Heart Lung and Blood Institute: What to Expect During Stress Testing
- American Heart Association: Exercise Stress Test
- The Merck Manual For Healthcare Professionals: Atrioventricular Block; January 2008
