An electrocardiogram (EKG) provides a visual representation of the heart’s electrical activity. While the EKG machine records these signals, specialized EKG calipers offer a precise method for detailed analysis of the waveforms. Calipers enable direct measurement of intervals and complexes, enhancing EKG interpretation and understanding of cardiac function.
Decoding the EKG Graph
Understanding the EKG paper grid is foundational for accurate analysis with calipers. EKG paper is standardized graph paper with small and large squares. Each small square measures 1 millimeter (mm) by 1 mm. Horizontally, these small squares represent time, with each 1 mm square equaling 0.04 seconds (40 milliseconds) at a standard paper speed of 25 mm/second. Five small squares form a larger square, representing 0.20 seconds (200 milliseconds) horizontally.
Vertically, the EKG grid measures the amplitude, or voltage, of the heart’s electrical signals. Each small square (1 mm) vertically represents 0.1 millivolts (mV). A large square, five small squares high, signifies 0.5 mV. Standard calibration sets 10 mm (two large squares) in height as equal to 1 mV.
The EKG tracing displays distinct electrical waveforms, each corresponding to a specific event in the cardiac cycle. The P wave is the first small, positive deflection, representing atrial depolarization. The QRS complex appears as a larger deflection, signifying rapid ventricular depolarization. The T wave is a broad, rounded deflection after the QRS complex, indicating ventricular repolarization.
Measuring Key Intervals
Using EKG calipers for precise measurements involves carefully positioning the instrument on the EKG tracing. Calipers help transfer distances directly from one point on the EKG to another, allowing for accurate quantification of time intervals. To measure an interval, place one tip of the caliper at the beginning of the desired waveform or complex, and extend the other tip to its end. The distance between the tips can then be transferred to the grid to determine the duration in seconds or milliseconds.
PR Interval
The PR interval measures the time for an electrical impulse to travel from the atria to the ventricles. It is measured from the beginning of the P wave to the start of the QRS complex. Place one caliper tip at the onset of the P wave and extend the other to the first deflection of the QRS complex.
QRS Duration
The QRS duration reflects the time for the electrical impulse to spread through the ventricles. This measurement is taken from the beginning of the QRS complex to its end. Use calipers to span the width of the complex, from its first deflection to where the tracing returns to the baseline.
QT Interval
The QT interval represents the total time for ventricular depolarization and repolarization. It is measured from the beginning of the QRS complex to the end of the T wave. Place one caliper tip at the start of the QRS complex and move the other to where the T wave returns to the baseline. This measurement is sensitive to heart rate changes and is often “corrected” (QTc) to account for variations in heart rate.
RR Interval
The RR interval measures the distance between two consecutive R waves, indicating the heart’s rhythm and rate. Place one caliper tip on the peak of an R wave and the other on the peak of the subsequent R wave. This measurement is fundamental for assessing both regularity and speed.
Calculating Heart Rate and Rhythm
EKG calipers are instrumental in determining heart rate and assessing rhythm regularity.
1500 Method (Regular Rhythms)
For regular rhythms, the “1500 method” provides a precise heart rate calculation. Count the number of small squares between two consecutive R waves, then divide 1500 by that number. For example, if there are 20 small squares between R waves, the heart rate is 1500 / 20 = 75 beats per minute.
300 Method (Regular Rhythms)
Another rapid method for regular rhythms is the “300 method,” also known as the sequence method. Identify an R wave that falls on a thick grid line. Then, subsequent thick lines are assigned values: 300, 150, 100, 75, 60, 50, and so on. The heart rate is estimated by noting where the next R wave falls in relation to these numbered lines. For example, if the next R wave falls on the next thick line, the rate is 300 bpm; if it falls on the second thick line, it is 150 bpm.
6-Second Method (Irregular Rhythms)
For irregular rhythms, the “6-second method” offers a reliable heart rate estimation. Count the number of QRS complexes within a 6-second strip of the EKG tracing and then multiply that count by 10. A 6-second strip corresponds to 30 large squares on the EKG paper (30 large squares 0.20 seconds/large square = 6 seconds). This method provides an average rate for irregular rhythms, which is more appropriate than methods relying on consistent R-R intervals.
Assessing Rhythm Regularity
Calipers are also used to assess rhythm regularity by “marching out” intervals. This involves setting the calipers to the length of one RR interval and then moving them along the EKG strip to see if subsequent RR intervals are consistently the same length. If the caliper tips align with the R waves across the strip, the rhythm is considered regular. Any significant variations indicate an irregular rhythm, prompting further investigation.
Understanding Your Results
After measuring intervals and calculating heart rate with EKG calipers, understanding what these numbers signify provides crucial context.
PR Interval
The normal PR interval ranges from 120 to 200 milliseconds (0.12 to 0.20 seconds), representing the time for the electrical impulse to travel from the atria through the AV node to the ventricles. A PR interval outside this range can indicate issues with conduction through the heart’s upper chambers or the AV node.
QRS Duration
The normal QRS duration, which reflects ventricular depolarization, is between 80 and 120 milliseconds (0.08 to 0.12 seconds). A QRS complex that is too wide might suggest a delay in electrical conduction within the ventricles, potentially due to a block in one of the heart’s electrical pathways.
QT Interval
The QT interval, representing the total time for ventricular depolarization and repolarization, is less than 420 milliseconds (0.42 seconds) at a heart rate of 60 beats per minute, though values vary slightly by gender and heart rate. A prolonged QT interval can indicate a higher risk of certain abnormal heart rhythms.
Heart Rate and Rhythm
A normal resting heart rate falls between 60 and 100 beats per minute. Rates outside this range, whether too fast (tachycardia) or too slow (bradycardia), may suggest an underlying cardiac issue or other physiological changes. An irregular rhythm, identified by inconsistent RR intervals, can also point to various electrical disturbances within the heart. These interpretations highlight how caliper measurements contribute to a preliminary assessment of cardiac electrical function.