An electrocardiogram (EKG or ECG) is a non-invasive medical test that records the heart’s electrical activity. Electrodes placed on the skin detect the tiny electrical changes that occur with each heartbeat. An EKG assesses the heart’s electrical conduction system, providing insights into its health and function, and helps understand its rate and rhythm.
Deciphering the EKG Tracing
An EKG tracing visually represents the heart’s electrical events through waves and segments. The P wave indicates the depolarization of the atria, the heart’s upper chambers. The QRS complex represents the rapid depolarization of the ventricles, the heart’s lower chambers, leading to their contraction. This complex often consists of a downward deflection (Q wave), an upward deflection (R wave), and another downward deflection (S wave).
The T wave appears after the QRS complex and signifies the repolarization of the ventricles as they prepare for the next beat. Between these waves are segments and intervals, representing periods of electrical silence or the time electrical signals travel through the heart. The isoelectric line is the flat line on the EKG tracing, indicating no electrical activity, and serves as a reference point for measuring deviations.
Defining Normal EKG Parameters
Interpreting an EKG involves evaluating specific measurements of waves and intervals against normal ranges. A normal resting heart rate for adults typically ranges from 60 to 100 beats per minute (bpm). Normal sinus rhythm, originating from the heart’s natural pacemaker (sinoatrial node), is characterized by a regular beat with a consistent P wave preceding each QRS complex.
The PR interval, from the P wave start to the QRS complex beginning, reflects the time for the electrical impulse to travel from the atria through the AV node to the ventricles; a normal duration is usually 0.12 to 0.20 seconds (120-200 milliseconds). The QRS duration, representing ventricular depolarization, is normally 0.08 to 0.12 seconds (80-120 milliseconds). The QT interval, from the QRS complex beginning to the T wave end, signifies the total time for ventricular depolarization and repolarization. When corrected for heart rate (QTc), normal values are typically less than 0.45 seconds for men and less than 0.46 seconds for women.
The ST segment, between the S wave end and the T wave beginning, should normally be isoelectric, lying flat along the baseline. Small variations, up to 0.5 mm of elevation or depression, can be considered normal. The QRS axis indicates the general direction of the heart’s electrical activity in the frontal plane. A normal QRS axis typically falls between -30° and +90°. These parameters collectively provide a comprehensive picture of the heart’s electrical function.
Influences on EKG Interpretation
While specific numerical ranges define normal EKG parameters, several factors can influence an individual’s EKG readings within a healthy spectrum. Age can affect heart rate, with average resting rates varying across different age groups. Body build can also play a role, as the heart’s physical position can alter detected electrical signals.
Medications significantly influence EKG results. Drugs like beta-blockers can intentionally lower a person’s resting heart rate, which remains normal for someone on that medication. An individual’s physical condition is also important; highly trained athletes often have lower resting heart rates due to enhanced cardiovascular efficiency. Healthcare providers always consider these individual variations and the patient’s overall clinical presentation when interpreting EKG results.