An electrocardiogram (EKG or ECG) is a non-invasive procedure that records the heart’s electrical signals. These recordings visually represent the sequence of electrical impulses driving each heartbeat. An EKG provides insights into heart function and rhythm, helping identify electrical patterns.
Components of an EKG Strip
An EKG strip is a graph plotting electrical activity against time. The background grid consists of small and large squares. Each small square typically represents 0.04 seconds horizontally and 0.1 millivolts vertically. Five small squares make up one large square, which equals 0.20 seconds horizontally and 0.5 millivolts vertically. These grid lines are used for precise measurement of intervals and amplitudes within the cardiac cycle.
The P wave is the first deflection on the strip, representing atrial depolarization. This electrical event corresponds to the contraction of the atria as the electrical impulse spreads from the sinoatrial node, the heart’s natural pacemaker, across both upper chambers. The P wave is typically a small, rounded, upright deflection.
Following the P wave is the QRS complex, which signifies ventricular depolarization. This complex reflects the rapid spread of electrical activity through the ventricles, leading to their powerful contraction and the pumping of blood out to the body and lungs. The QRS complex is usually the tallest and most prominent feature on the EKG, characterized by its sharp, pointed deflections. The R wave is the first upward deflection, often preceded by the Q wave (a small downward deflection) and followed by the S wave (another downward deflection).
The T wave marks ventricular repolarization, the electrical recovery or relaxation of the ventricles. This phase allows the ventricles to rest and refill with blood before the next heartbeat begins. The T wave is generally a rounded, upright wave that follows the QRS complex, often broader and more symmetrical than the P wave. Understanding these distinct components provides a fundamental framework for interpreting the heart’s electrical activity.
Determining Heart Rate and Rhythm
Assessing the heart’s rate and rhythm is among the first steps in interpreting an EKG strip. Heart rhythm refers to the regularity of the beats, which can be determined by examining the R-R intervals. A regular rhythm shows consistent spacing between consecutive R waves, indicating predictable electrical impulses originating from a stable pacemaker. Conversely, an irregular rhythm exhibits varying distances between R waves, suggesting an unpredictable pattern of electrical activity within the heart.
To calculate heart rate from a regular EKG strip, one common method involves counting the number of large squares between two consecutive R waves and dividing 300 by that number. For instance, if there are five large squares between R waves, the heart rate would be 60 beats per minute (300/5 = 60). This quick method provides a rapid estimation for regular rhythms, offering a general sense of the cardiac pace.
Another widely used approach for regular rhythms is the “1500 method.” Count the small squares between two R waves and divide 1500 by that count. For example, if there are 25 small squares, the rate is 60 beats per minute (1500/25 = 60). This method offers a more precise calculation for regular rhythms due to the smaller unit of measurement, providing greater accuracy.
For irregular rhythms, a different technique is often employed: the 6-second method. This involves locating a 6-second segment on the EKG strip, which corresponds to 30 large squares (6 seconds / 0.20 seconds per large square = 30 large squares). Within this 6-second interval, count the total number of QRS complexes. Multiply this count by 10 to estimate the average heart rate per minute. This method provides a reasonable approximation for both regular and irregular rhythms when a precise count using other methods is challenging due to the variability in R-R intervals.
Identifying Common EKG Variations
Once the basic components, rate, and rhythm are understood, identifying common EKG variations becomes more straightforward, allowing for basic pattern recognition. A normal sinus rhythm represents the healthy baseline for heart activity. It is characterized by a regular rhythm with a heart rate typically ranging from 60 to 100 beats per minute. Each QRS complex is consistently preceded by a P wave, and all P waves are upright and uniform in shape, indicating that the electrical impulse originates appropriately from the sinoatrial node and follows the normal conduction pathway.
Sinus bradycardia is a variation where the heart rate is slower than normal, generally below 60 beats per minute, while maintaining a regular rhythm. The P waves, QRS complexes, and T waves are all present and appear normal in their morphology and sequential relationship. The distinguishing feature is the notably increased spacing between the complexes, reflecting the slower rate of electrical impulses being generated by the sinoatrial node.
Conversely, sinus tachycardia presents as a faster-than-normal heart rate, typically above 100 beats per minute, yet it also maintains a regular rhythm. All the expected waves (P, QRS, T) are present and follow the normal sequence and shape. The primary characteristic here is the significantly reduced spacing between the complexes, indicating a more rapid firing of the sinoatrial node.
Atrial fibrillation, often referred to as AFib, is a complex variation marked by an irregularly irregular rhythm, meaning there is no discernible pattern to the irregularity. In this condition, the atria do not contract effectively; instead, they quiver rapidly and chaotically, leading to uncoordinated electrical activity. This results in the absence of distinct, organized P waves on the EKG strip, which are replaced by an erratic, wavy baseline known as fibrillatory waves. The ventricular response, as indicated by the QRS complexes, is highly unpredictable and irregularly spaced.