A heart monitor, often called an electrocardiogram (ECG or EKG), visualizes the heart’s electrical activity as a continuous tracing. Understanding these lines helps medical professionals assess heart function. This article explains their names and what they represent in the heart’s electrical cycle.
The Fundamental Waves
The tracing produced by a heart monitor features several distinct deflections, or “waves,” which are fundamental to interpreting cardiac activity. The first of these is the P wave, appearing as a small, rounded upward bump on the tracing. It is a smooth, positive deflection that occurs before the larger complexes.
Following the P wave is the QRS complex, which is a sharp, prominent series of deflections. This complex can include a small downward deflection (Q wave), a tall upward spike (R wave), and a subsequent downward deflection (S wave). The final distinct wave is the T wave, which appears as another rounded upward bump, broader and taller than the P wave, and occurs after the QRS complex.
Understanding the Intervals and Segments
Beyond the individual waves, a heart monitor tracing also displays “intervals” and “segments,” which are specific measurements of time or baseline periods. An interval includes at least one wave, while a segment refers to the flat line between waves.
The PR interval measures the time from the beginning of the P wave to the start of the QRS complex. The ST segment is the flat, often isoelectric (zero voltage) line found between the end of the S wave and the beginning of the T wave.
Lastly, the QT interval encompasses the entire duration from the beginning of the QRS complex to the end of the T wave. This interval measures a longer period of electrical activity, covering the large ventricular deflections and the subsequent recovery wave.
What the Lines Reveal About Heart Function
Each specific wave, interval, and segment on a heart monitor tracing corresponds to a particular electrical event within the heart, reflecting its pumping action. The P wave signifies the electrical activation, known as depolarization, of the atria, the heart’s upper chambers. This electrical signal spreads across the atria, causing them to contract and pump blood into the lower chambers.
The QRS complex represents the depolarization of the ventricles, the heart’s powerful lower chambers. This rapid electrical event causes the ventricles to contract forcefully, expelling blood to the lungs and the rest of the body.
The T wave indicates the repolarization, or electrical recovery, of the ventricles. During this phase, the ventricular muscle cells reset electrically, preparing for the next heartbeat.
The PR interval reflects the time it takes for the electrical impulse to travel from the atria, through a specialized relay station called the atrioventricular (AV) node, and into the ventricles. A normal PR interval indicates proper conduction speed.
The ST segment represents the period when the ventricles are fully depolarized but have not yet begun to repolarize. This segment corresponds to a phase where the ventricular muscle is contracting, before its electrical recovery begins.
Finally, the QT interval measures the total time required for both ventricular depolarization and repolarization. This interval reflects the complete electrical cycle of the ventricles, from their contraction to their full recovery.