An electrocardiogram, commonly known as an ECG or EKG, is a non-invasive medical test that records the electrical activity of the heart as it contracts and relaxes. This article explains what a “normal” ECG looks like, providing a foundational understanding of this diagnostic tool.
Understanding the Electrocardiogram Test
An ECG assesses various aspects of heart health, including the heart’s rhythm, rate, and regularity. It can detect a previous heart attack or identify the cause of symptoms like chest pain, dizziness, or shortness of breath. The test also monitors the effectiveness of certain heart medications or devices like pacemakers.
The ECG procedure is quick and painless. Electrodes are placed on the skin, typically on the chest, arms, or legs. These electrodes connect to an ECG machine, which records and displays electrical signals as wavy lines on a screen or prints them on paper. A professional ECG is conducted in a medical office, hospital, or clinic.
The Basic Waves of a Normal ECG
A normal ECG tracing displays a repeating pattern of distinct waves, each representing a specific electrical event in the heart’s cycle. The P wave is the first small, rounded, upright deflection on the tracing. It signifies atrial depolarization, the electrical activation causing the atria, the heart’s upper chambers, to contract and pump blood into the ventricles. In a normal sinus rhythm, the P wave is generally upright in most leads.
Following the P wave is the QRS complex, which appears as a sharp, tall, and narrow spike. This complex represents ventricular depolarization, the electrical activity causing the ventricles to contract and pump blood out to the body. The QRS complex consists of three deflections: the Q wave (a small downward deflection), the R wave (a large upward spike), and the S wave (a downward deflection after the R wave).
The T wave is the final prominent wave in a normal ECG cycle, appearing as a rounded, upright deflection that is broader than the P wave. This wave represents ventricular repolarization, the electrical recovery or relaxation of the ventricles after contraction. A normal T wave is generally in the same direction as the QRS complex in most leads.
Normal Timing and Rhythm on an ECG
Beyond the appearance of individual waves, the timing and regularity of these electrical events are also indicators of a normal ECG. A normal resting heart rate for adults ranges from 60 to 100 beats per minute (bpm). This rate is calculated by measuring the time between consecutive QRS complexes on the ECG tracing.
Normal sinus rhythm describes a regular heart rhythm where the electrical impulses originate from the sinoatrial (SA) node, the heart’s natural pacemaker. In this rhythm, each P wave is consistently followed by a QRS complex, indicating proper conduction from the atria to the ventricles. The PR interval, which measures the time from the beginning of the P wave to the start of the QRS complex, falls between 120 and 200 milliseconds (0.12 to 0.20 seconds).
The QT interval represents the total time for ventricular depolarization and repolarization. Its normal duration varies with heart rate, but for a heart rate of 60 bpm, it is 420 milliseconds (0.42 seconds) or less, and for males it is less than or equal to 0.40 seconds and for females, it is less than or equal to 0.44 seconds. The ST segment, the flat line between the end of the S wave and the beginning of the T wave, should be at the baseline in a normal ECG. A slight elevation of up to 2.0 mm in some precordial leads can be considered normal.
What a Normal ECG Indicates
A normal ECG tracing signifies that the heart’s electrical activity is operating within expected parameters. It indicates that the heart’s electrical signals are regular, the heart chambers are contracting in the proper sequence, and there are no immediate signs of significant electrical abnormalities or damage to the heart muscle. This includes a steady heart rate, a consistent rhythm, and waveforms that appear as expected.
While a normal ECG result is reassuring, it does not rule out all possible heart conditions. It provides a snapshot of the heart’s electrical activity at a specific moment and should be interpreted by a healthcare professional in the broader context of a patient’s medical history, physical examination, and other diagnostic tests. A normal reading suggests healthy and well-timed heart function, but further evaluation may be recommended based on individual circumstances.