An electrocardiogram (EKG) is a non-invasive tool that records the electrical signals of the heart. Electrodes placed on the skin detect the tiny electrical impulses generated as the heart muscle depolarizes and repolarizes with each beat. The EKG displays this activity as a wave pattern on graph paper. By examining the shape, timing, and rhythm of these waves, healthcare providers determine if the heart’s electrical system is functioning normally.
Understanding the EKG Waves (P, QRS, T)
A complete heartbeat on an EKG consists of three main wave components. The P wave is a small, rounded, upward deflection reflecting the initial electrical activation (depolarization) of the atria. This process must be completed rapidly; the normal P wave duration is typically less than 0.12 seconds. The electrical impulse then travels to the ventricles, triggering the next wave pattern.
The QRS complex is the central and most prominent structure, representing ventricular depolarization, which causes the ventricular muscles to contract. It is composed of three potential deflections: the Q wave (downward), the R wave (upward spike), and the S wave (final downward deflection). Because the ventricles have significantly more muscle mass, the QRS complex appears much larger than the P wave. The signal travels quickly through the specialized conduction system, making the QRS complex normally very narrow, typically lasting between 0.06 and 0.10 seconds.
The T wave follows the QRS complex and reflects ventricular repolarization, the electrical recovery phase. The normal T wave usually presents as a broad, rounded, upward-sloping hump after the QRS complex. For a tracing to be considered normal, the T wave should follow the same general direction as the preceding QRS complex in most leads. This wave completes the electrical cycle for one heart beat.
Measuring Healthy EKG Intervals and Segments
The spacing and timing between waves, measured as intervals and segments, are important for defining a normal EKG. The PR interval measures the time from the beginning of the P wave to the start of the QRS complex. This reflects the time it takes for the electrical impulse to travel from the atria, through the atrioventricular (AV) node, and into the ventricles. A healthy PR interval duration falls between 0.12 and 0.20 seconds, corresponding to three to five small squares on standard EKG graph paper.
The QRS duration is narrow in a normal heart because the signal travels extremely fast through the specialized bundle branches and Purkinje fibers. A normal duration is less than 0.12 seconds (under three small boxes). This narrow appearance confirms that ventricular depolarization is occurring rapidly along the correct, highly efficient pathway.
The QT interval measures the total time from the start of ventricular depolarization to the end of ventricular repolarization. This represents the entire period of electrical activity within the ventricles, from contraction to recovery. Because the length of the QT interval changes depending on the heart rate, it is often adjusted to a corrected value (QTc) to determine its normal length. Generally, a normal QT interval is less than 0.44 seconds in adults.
Identifying Normal Sinus Rhythm
The overall pattern and pace of the EKG tracing define the heart’s rhythm, which is termed normal sinus rhythm (NSR) when all parameters are met. NSR indicates that the electrical impulse originates correctly in the sinoatrial (SA) node, the heart’s natural pacemaker. The impulse then follows the proper conduction pathway, producing the characteristic P-QRS-T sequence.
A regular rhythm is identified when the distance between consecutive R waves (the R-R interval) is constant, indicating a steady pace. Every P wave must be followed by a QRS complex, and the PR interval must be consistent from beat to beat. This relationship confirms that the electrical signal transmits reliably from the atria to the ventricles.
The rate of a normal sinus rhythm for an adult resting heart falls within the range of 60 to 100 beats per minute (bpm). This rate is calculated by measuring the time between the R waves. Finally, the appearance of the waves must be consistent: all P waves should look similar, and all QRS complexes should share a similar, narrow morphology.