An Electrocardiogram, commonly known as an EKG or ECG, records the electrical signals of your heart. It provides a visual representation of the heart’s electrical activity, allowing healthcare professionals to assess its rate and rhythm. This non-invasive test helps understand how fast or slow the heart is beating and whether its rhythm is steady or irregular.
Understanding EKG Paper
Standard EKG graph paper is a grid used to measure time and electrical voltage. The paper moves at a speed of 25 millimeters per second (mm/s).
The grid consists of small squares, each 1 millimeter. Horizontally, each small square represents 0.04 seconds. Vertically, it represents 0.1 millivolts (mV) of electrical amplitude. Heavier lines form larger squares, each encompassing five small squares. Each large square represents 0.20 seconds horizontally and 0.5 mV vertically.
Calculating Heart Rate
Calculating heart rate from an EKG strip involves different methods depending on whether the rhythm is regular or irregular. For regular rhythms, two common methods are the “300 method” and the “1500 method”. The R wave is the tall, upright deflection in the QRS complex, representing ventricular depolarization.
The “300 method” provides a quick estimate. Locate an R wave on a thick vertical line. Count the number of large squares until the next R wave. Divide 300 by this number to get the heart rate in beats per minute (bpm). For example, if there are four large squares between two R waves, the heart rate would be 75 bpm (300/4 = 75).
For a more precise calculation, the “1500 method” is used. This method involves counting the number of small squares between two consecutive R waves. Divide 1500 by this total number of small squares to determine the heart rate. For instance, if there are 20 small squares between two R waves, the heart rate is 75 bpm (1500/20 = 75).
When the heart rhythm is irregular, the “6-second method” is more suitable because the R-R intervals vary. Identify a 6-second segment on the EKG strip, which corresponds to 30 large squares. Count the number of R waves (QRS complexes) within this 6-second strip. Multiply this count by 10 to estimate the average heart rate in beats per minute. For example, if you count eight R waves in a 6-second strip, the heart rate is approximately 80 bpm (8 x 10 = 80).
Measuring Key Intervals and Durations
Measuring specific intervals and durations on an EKG strip provides insights into the timing of electrical events within the heart. These measurements are based on the small and large squares of the EKG paper.
The PR interval represents the time it takes 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. A normal PR interval ranges from 0.12 to 0.20 seconds, corresponding to three to five small squares.
The QRS duration reflects the time it takes for the ventricles to depolarize. This duration is measured from the beginning of the Q wave (or the first downward deflection) to the end of the S wave. A normal QRS duration is between 0.06 and 0.12 seconds, which translates to 1.5 to 3 small squares.
The QT interval measures the total time from the start of ventricular depolarization to the end of ventricular repolarization, essentially covering the period of ventricular contraction and relaxation. This interval begins at the onset of the QRS complex and extends to the end of the T wave. The QT interval is affected by heart rate and other factors, and its normal range varies.
Assessing Rhythm Regularity
Determining the regularity of a heart rhythm involves examining the consistency of the R-R intervals across the EKG strip. The R-R interval is the distance between consecutive R waves, which represent successive ventricular contractions. A regular rhythm is indicated when these R-R intervals are consistent.
To assess regularity, one method involves using a piece of paper or calipers. Mark the location of several consecutive R waves on the paper. Slide the marked paper along the EKG strip to see if these marks consistently align with subsequent R waves. If the R-R intervals vary by more than 1.5 small squares, the rhythm is considered irregular. This visual comparison helps identify patterns of consistency or irregularity.