An electrocardiogram (EKG or ECG) is a common, non-invasive diagnostic tool used to record the electrical activity of the heart. The procedure involves placing electrodes on the skin to detect the electrical currents generated by the heart muscle as it beats. The EKG produces a visual tracing of these electrical signals, offering a real-time snapshot of the heart’s rhythm and overall function. By analyzing the timing, direction, and magnitude of these electrical “blips,” doctors can assess the health of the cardiac system and identify potential problems.
How the Heart Generates Electrical Signals
The heart’s rhythmic beating is controlled by its own internal electrical system, which functions like a natural pacemaker. This process begins with the Sinoatrial (SA) node, a cluster of specialized cells located in the upper right chamber of the heart. The SA node spontaneously generates a regular electrical impulse that sets the pace for the entire heart.
This electrical signal spreads rapidly across the upper chambers, causing them to contract and push blood into the lower chambers. The impulse then reaches the Atrioventricular (AV) node, which acts as a gatekeeper. The AV node briefly delays the signal, ensuring the ventricles have enough time to fill completely with blood before they contract.
After this delay, the signal travels down the Bundle of His, which quickly divides into specialized conduction pathways called the Purkinje fibers. These fibers distribute the electrical impulse throughout the muscular walls of the lower chambers, or ventricles. The rapid spread of this electrical wave causes the simultaneous contraction of the ventricles, ejecting blood out to the rest of the body. The entire sequence of depolarization (electrical activation) and repolarization (electrical recovery) creates the measurable current picked up by the EKG electrodes.
Decoding the Standard EKG Waveform
A healthy heart produces a predictable sequence of waves on the EKG tracing, each representing a specific electrical event. The first small upward deflection is the P wave, which signifies the electrical activation, or depolarization, of the atria. Following the P wave is the PR interval, which measures the brief time required for the electrical impulse to travel from the atria through the AV node and into the ventricles, typically lasting between 0.12 and 0.20 seconds.
The next complex is the QRS complex. This sharp, spiked pattern represents the electrical activation of the ventricles, the main pumping chambers of the heart. The QRS complex is usually narrow, with a duration of 0.10 seconds or less, reflecting the rapid, synchronized spread of the impulse through the Purkinje fibers. Hidden within the QRS complex is the electrical recovery of the atria, masked by the much larger ventricular signal.
The final component of the waveform is the T wave, a smooth, rounded hump that follows the QRS complex. The T wave represents the repolarization, or electrical recovery, of the ventricles as the heart muscle prepares for the next beat. The QT interval measures the total time from the start of ventricular activation to the end of its recovery. Abnormalities in its duration can indicate a risk for dangerous heart rhythms. When all these components are present and correctly timed, the heart is said to be in normal sinus rhythm.
Common Deviations and What They Indicate
When the EKG tracing deviates from the standard pattern, the resulting “blip” can signal a change in the heart’s function. Rate abnormalities include Tachycardia (a heart rate exceeding 100 beats per minute) or Bradycardia (a rate slower than 60 beats per minute). Rhythm disturbances include Premature Ventricular Contractions (PVCs), which appear as an early, wide, and disorganized QRS complex that interrupts the normal rhythm.
PVCs represent an extra beat originating from an irritable spot in the ventricles, causing a momentary feeling that the heart has “skipped a beat.” While occasional PVCs are common and often harmless, frequent or complex patterns can suggest underlying heart stress or disease. Another common rhythm deviation is Atrial Fibrillation, where the P wave is absent and replaced by chaotic, rapid electrical activity, resulting in an irregularly irregular pattern of QRS complexes.
Shape changes on the waveform are often the most significant “blips,” as they can indicate damage to the heart muscle itself. The ST segment, the line connecting the QRS complex and the T wave, is important. If this segment is elevated or depressed relative to the baseline, it can be a sign of myocardial ischemia—a lack of blood flow to the heart muscle. ST segment elevation, for example, often suggests a blockage of a coronary artery, a type of heart attack.
Pathological Q Waves
Another shape change is the presence of a wide and deep Pathological Q wave within the QRS complex, which typically does not develop until hours or days after an event. Unlike the small, normal Q waves, this deep deflection acts as a permanent electrical scar, indicating an area of heart muscle that has died due to a past heart attack. The EKG provides invaluable information, but any significant deviation requires a full medical evaluation to determine the underlying cause and appropriate treatment plan.