An electrocardiogram (EKG) is a non-invasive test that records the heart’s electrical activity. This recording provides a visual representation of the heart’s rhythm and electrical impulses. Interpreting EKG strips offers valuable insights into heart function, helping healthcare professionals assess cardiac health by identifying patterns and measuring specific timings.
Fundamentals of EKG Interpretation
EKG strips are printed on grid paper designed to standardize time and voltage measurements. Each small square measures 1mm by 1mm, representing 0.04 seconds horizontally and 0.1 millivolt (mV) vertically. Five small squares form one large square, signifying 0.20 seconds horizontally and 0.5 mV vertically. The standard recording speed is 25 mm per second, where 5 large squares represent 1 second.
The heart’s electrical activity translates into distinct waves on the EKG strip. The P wave, a small upward deflection, indicates atrial depolarization (electrical activation and contraction of the upper chambers). The QRS complex, a sharp, larger deflection, represents ventricular depolarization (electrical activation and contraction of the lower chambers). The T wave, a rounded wave, reflects ventricular repolarization (electrical recovery of the ventricles).
Calculating Heart Rate
Determining heart rate from an EKG strip is a fundamental step, with several methods available depending on rhythm regularity. For regular rhythms, the “300 Method” involves counting large squares between two consecutive R waves (the tall, upright deflection in the QRS complex) and dividing 300 by that number. For instance, if 4 large squares separate R waves, the heart rate is 300 / 4 = 75 beats per minute (bpm).
The “1500 Method” uses smaller grid divisions for regular rhythms. Count the small squares between two consecutive R waves and divide 1500 by that count. For example, if 20 small squares separate R waves, the heart rate is 1500 / 20 = 75 bpm. This method provides a more accurate rate for very fast regular rhythms.
For irregular rhythms, the “6-Second Method” is used. Locate a 6-second segment (30 large squares) on the EKG strip. Count the QRS complexes within this span and multiply by 10 to estimate the heart rate. For example, if 7 QRS complexes are counted, the estimated heart rate is 7 x 10 = 70 bpm.
Measuring Key EKG Intervals
Measuring EKG intervals provides details about the timing of electrical conduction through different heart parts. The PR interval represents the time for an electrical impulse to travel from the atria through the atrioventricular (AV) node to the ventricles. It is measured from the beginning of the P wave to the beginning of the QRS complex. Its duration is calculated by counting the small squares between these points and multiplying by 0.04 seconds.
The QRS duration measures the time for ventricular depolarization. It is measured from the beginning of the Q wave (or R wave if no Q wave is present) to the end of the S wave. Counting the small squares within this complex and multiplying by 0.04 seconds yields the QRS duration.
The QT interval signifies 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. While calculated by counting small squares and multiplying by 0.04 seconds, the QT interval naturally shortens at faster heart rates and lengthens at slower rates.
Assessing EKG Rhythm
Determining EKG rhythm regularity is an initial step before calculating heart rate, as it guides the choice of calculation method. A regular rhythm is characterized by consistent R-R intervals, meaning the distance between consecutive R waves remains uniform across the strip. P-P intervals (distance between P waves) should also be consistent. This uniformity suggests a predictable heartbeat pattern.
An irregular rhythm exhibits inconsistent R-R or P-P intervals, indicating variability in heartbeat timing. If the R-R interval varies by two small boxes or more, the rhythm is irregular. Visual assessment involves marking R waves and moving them along the strip to check alignment.
Rhythm identification involves checking for a P wave before every QRS complex. Consistency of the P wave’s shape (morphology) provides clues about the electrical impulse’s origin. These visual assessments determine if the heart’s electrical activity follows a typical, organized sequence.