An electrocardiogram (EKG or ECG) is a non-invasive diagnostic tool that records the electrical activity of the heart. This test helps healthcare providers assess the heart’s rhythm and function by translating its electrical impulses into a visual representation on a graph. It is a widely used and important procedure for diagnosing various heart conditions, such as abnormal heart rhythms, poor blood flow, or even heart damage. The EKG provides a snapshot of how the heart’s electrical system is performing, offering valuable insights.
The Fundamental Waves
A normal EKG tracing displays a series of distinct waves, each representing a specific electrical event in the heart. The P wave, the first small, typically positive deflection, signifies the electrical activation (depolarization) of the atria, the heart’s upper chambers. This depolarization causes the atria to contract, pushing blood into the ventricles. The P wave is usually smooth and rounded, typically less than 0.12 seconds in duration and under 2.5 mm in amplitude in the limb leads.
Following the P wave is the QRS complex, a larger and sharper deflection that represents the electrical activation (depolarization) of the ventricles, the heart’s lower chambers. This complex indicates the powerful contraction of the ventricular muscles that pumps blood out to the body and lungs. The QRS complex consists of a downward Q wave (if present), an upward R wave, and a downward S wave. Its duration in adults typically ranges from 0.08 to 0.10 seconds, with anything less than 0.12 seconds considered normal.
The final wave in a normal EKG complex is the T wave, which represents the electrical recovery (repolarization) of the ventricles. This repolarization allows the ventricular muscle cells to reset their electrical charge, preparing for the next heartbeat. A normal T wave is usually upright, smooth, and rounded, typically asymmetrical with a steeper downslope than upslope. Its amplitude is generally less than 5 mm in limb leads and less than 10 mm in precordial leads.
Key Intervals and Segments
Beyond the individual waves, a normal EKG is characterized by specific intervals and segments, which measure the timing between these electrical events. The PR interval measures the time from the beginning of the P wave to the start of the QRS complex. This interval reflects the time it takes for the electrical impulse to travel from the atria, through the atrioventricular (AV) node, and into the ventricles. A normal PR interval in adults typically ranges from 0.12 to 0.20 seconds (120 to 200 milliseconds).
The QRS duration measures the length of the QRS complex, indicating the time for ventricular depolarization. A normal QRS duration is typically between 0.08 and 0.10 seconds (80 to 100 milliseconds), with a duration of up to 0.12 seconds (120 milliseconds) often still considered within normal limits. A narrow QRS complex signifies efficient electrical conduction through the ventricles.
The ST segment is the flat, isoelectric line found between the end of the S wave and the beginning of the T wave. This segment represents the period when the ventricles are fully depolarized but have not yet begun to repolarize. In a normal EKG, the ST segment should be level with the baseline, though a slight upward concavity can be normal.
The QT interval measures the total time for ventricular depolarization and repolarization, from the beginning of the QRS complex to the end of the T wave. This interval is important as it reflects the entire electrical cycle of the ventricles. Because the QT interval can vary with heart rate, a corrected QT interval (QTc) is often calculated. Normal QTc values are generally less than 450 milliseconds for men and less than 460 milliseconds for women.
Normal Rhythm and Rate
Beyond the individual components, a normal EKG also exhibits a consistent rhythm and rate, which describe the overall pattern of the heart’s electrical activity. Normal sinus rhythm is the term for a heart rhythm where electrical impulses originate from the sinoatrial (SA) node, the heart’s natural pacemaker, and spread through the heart in a regular, organized manner. This results in a predictable and consistent sequence of P waves, QRS complexes, and T waves.
In normal sinus rhythm, the heart rate typically falls within a range of 60 to 100 beats per minute (bpm) for adults at rest. The SA node adjusts the heart rate based on the body’s needs, speeding it up during activity and slowing it down during rest.