THE HEART RATE: MAKING OUR FIRST MEASUREMENT
It’s finally time to start making real measurements, so let’s begin by learning how to calculate the heart rate (HR). For every ECG, you should calculate both the ventricular rate and atrial rate. We can calculate the ventricular rate by measuring the time between the QRS complexes (R-R intervals), and the atrial rate by measuring the time between the P waves (P-P intervals). In normal sinus rhythm the atrial and ventricular rates are identical. In patients with cardiac arrhythmias, the atrial and ventricular rates may be very different, something we are going to learn in subsequent chapters. For the following discussion we will concentrate on the R waves to calculate the ventricular rate, but the same principles apply when using the P waves to calculate the atrial rate. There are a number of different methods that you can use, but I will present them in the order I find most practical.
We learned earlier that ECG paper comes with 3-second markers, so two of them represent a total of 6 seconds. By counting the number of QRS complexes within a 6-second period and multiplying by 10, you derive the HR in beats per minute (bpm) (Figure 5-1). This is the universal method to calculate HR that you can use with any cardiac rhythm, regular or irregular, fast or slow, and in the presence of premature complexes. Use the following steps for this method:
Select an R wave that lines up as close as possible with the first 3-second marker. If you prefer, duplicate the 3-second markers using calipers, a marked note card, or another sheet of ECG paper to line up the first complex exactly with the first marker.
Consider the first complex as “zero,” counting the subsequent R waves until you reach the end of the 6-second mark.
Multiply this number by 10.
6-second method. Starting with the number “zero,” counting seven more complexes within a 6-second interval and multiplying by a factor of ten results in a HR of 7 x 10 = 70 bpm.
That’s pretty easy! Yes, I know that the complexes may not line up exactly on the zero and 6-second mark, but the HR you calculate is going to be very close to the actual number and any difference is not likely to be clinically relevant. If the last complex occurs just before the end of the 6 seconds, adjust the HR a little higher. Similarly, if the last complex falls just after the marker, adjust the HR rate lower. Either way, you are going to be in the neighborhood. And if your ECG paper doesn’t have 3-second markers, it’s easy to mark off the 6 seconds yourself, remembering that 5 large boxes equals 1 ...