Certified Pulmonary Function Technologist (CPFT) Practice Exam

How do you determine the rate on ECG paper?

1. Count the number of P waves

2. 300 divided by the number of small boxes between each QRS

3. Calculate the number of beats in 30 seconds

4. Count the QRS complexes in one minute

The correct answer is: 300 divided by the number of small boxes between each QRS

To determine the rate on ECG paper, the method involving dividing 300 by the number of small boxes between each QRS complex is a widely used technique, especially in the context of regular rhythms. The ECG paper is standardized, with a speed of 25 mm per second. Each large box represents 0.2 seconds and contains five small boxes, making each small box represent 0.04 seconds. Therefore, the number of small boxes between two adjacent QRS complexes can be converted into time, and since there are 300 large boxes per minute (60 seconds divided by 0.2 seconds per large box), dividing 300 by the number of small boxes between the QRS complexes gives a quick estimation of the heart rate. This method allows for fast calculations in practice, particularly when assessing patients in critical care or situations where rapid decision-making is required. It's important to note that this calculation is most accurate when the rhythm is regular; in cases of irregular rhythms, other methods may be more appropriate for determining heart rate. Other choices, such as counting the number of P waves or QRS complexes over specific times, do provide valuable information but are not as direct for calculating heart rate using the paper speed of the ECG.