The heart’s electrical system drives the rhythmic pumping action required to circulate blood throughout the body. An electrocardiogram (ECG or EKG) provides a visual map of this electrical activity, translating the flow of current into a tracing of distinct peaks and valleys. This non-invasive test monitors the voltage changes across the heart muscle during each cardiac cycle. The characteristic pattern recorded on the ECG represents the precisely timed sequence of electrical events that coordinate the heart’s muscle contractions. Analyzing the shape and timing of this tracing allows healthcare professionals to assess the functional health of the heart’s conduction system.
The P Wave: Atrial Contraction
The P wave is the first small, rounded upward deflection visible on the ECG tracing, signifying the electrical activation of the heart’s upper chambers. This wave represents atrial depolarization, the spread of an electrical impulse from the sinoatrial (SA) node across the right and left atria. The SA node is often called the heart’s natural pacemaker. This electrical signal triggers the atria to contract, pushing blood down into the ventricles.
In a healthy heart, the P wave is smooth and typically lasts no more than 0.11 seconds, reflecting rapid electrical movement. The depolarization of the right atrium contributes to the initial portion of the P wave, and the left atrium’s depolarization completes the wave. Abnormalities in the P wave’s height or duration, such as an unusually tall or wide shape, can indicate physical changes to the atria, like enlargement. The brief flat line immediately following the P wave, known as the PR segment, represents the impulse pausing at the atrioventricular node before continuing to the ventricles.
The QRS Complex: Ventricular Contraction
The QRS complex is the most prominent feature on the ECG tracing, appearing as a rapid sequence of sharp, high-amplitude spikes. This complex represents ventricular depolarization, the powerful electrical signal that causes the ventricles, the heart’s main pumping chambers, to contract. Because the ventricles have a greater muscle mass than the atria, the electrical currents generated are stronger, making the QRS complex much larger than the P wave. The entire complex normally lasts between 0.06 and 0.10 seconds, reflecting the fast spread of electricity through the specialized Purkinje fibers.
The complex is composed of three potential deflections: the Q wave (first downward), the R wave (first upward), and the S wave (downward following the R wave). The R wave is the tallest component and reflects the depolarization of the large ventricular mass. During this strong electrical activity, the atria undergo their recovery phase (repolarization), but this smaller electrical event is masked by the QRS signal.
The T Wave: Ventricular Recovery
The T wave follows the QRS complex and represents the electrical recovery of the ventricles, known as ventricular repolarization. This process is when the ventricular muscle cells reset their electrical charge. Repolarization is slower than depolarization, which is why the T wave is typically a broader, more rounded wave compared to the sharp spikes of the QRS complex. The duration of the T wave is a component of the QT interval, which reflects the total time required for the ventricles to depolarize and fully repolarize.
The shape of a normal T wave is usually slightly asymmetric, with the ascending slope often less steep than the descending slope. This recovery phase corresponds to the period when the ventricles are relaxing and refilling with blood. Analyzing the T wave’s direction and shape is important, as changes can indicate issues like problems with the heart muscle’s blood supply or an imbalance in electrolytes.
Interpreting the Heartbeat: What Abnormal Waves Indicate
The precise appearance and timing of the P, QRS, and T waves are used by clinicians to diagnose a range of heart conditions. Deviations from the normal morphology, duration, and rhythm of these waves signal underlying pathology. For instance, the absence of P waves, combined with an irregularly irregular rhythm of the QRS complexes, is the hallmark of atrial fibrillation, where the atria beat chaotically.
Analyzing the duration of the QRS complex is important. A widened QRS complex, lasting longer than 0.12 seconds, often indicates a delay in electrical conduction through the ventricles, such as a bundle branch block. This suggests the impulse is traveling via a slower, abnormal pathway instead of the fast conduction system. A wide and deep Q wave can also be a sign of a previous myocardial infarction (heart attack), indicating that a section of the heart muscle has been electrically silenced by damage.
Abnormalities in the T wave often point to issues with the heart muscle’s oxygen supply. A flattened or inverted T wave may suggest myocardial ischemia (lack of blood flow). Conversely, an unusually tall and sharp T wave, known as a peaked T wave, can be a sign of hyperkalemia, an elevation of potassium levels in the blood.
A prolonged PR interval (the time from the start of the P wave to the start of the QRS complex) can indicate a first-degree atrioventricular block, where the electrical signal is delayed as it passes from the atria to the ventricles.