How to tell heart rate from an ECG

How to tell heart rate from an ECG? E. S. Prakash, MD; and Madanmohan, MD Department of Physiology, Jawaharlal Institute of Postgraduate Medical Education and Research (JIPMER), Pondicherry – 605006, INDIA E-mail: dresprakash@yahoo. com

Select the single best response. l A. B. C. How do you determine heart rate (HR) from an ECG? 1500 / RR interval in mm Determine the number of RR intervals in a 10 second strip and multiply by 6 Count the number of QRS complexes in 5 small squares and multiply by 60.

Could you tell HR from this ECG?

HR is commonly derived thus: l HR = 1500 / RR in mm l HR = 60 / RR interval in seconds l HR = 300 / number of large squares between successive R waves

How did this formula come about? ECG is recorded at a speed of 25 mm/s. 2. The paper moves 60 × 25 = 1500 mm per minute 3. HR is the number of cardiac cycles (or RR intervals) per minute 4. If one RR interval measures 20 mm, then HR is 1500 / 20 = 75 BPM. What does this result represent? 1.

1500 / RR interval in mm = Instantaneous heart rate l It is the number of times the heart would beat in one minute if the duration of successive cardiac cycles were constant.

Determine HR from this ECG

. . continued l Note that RR intervals vary considerably. l What is this phenomenon called?

. . continued l What if you were to take a shorter RR interval into consideration for calculating HR? l What if you were to take a longer RR interval into consideration for calculating HR?

What information are we seeking to derive from HR? l To estimate cardiac output and to assess the relationship between the prevailing BP and HR over variable periods of time (from as short as 2 seconds to as long as a minute).

What are the various means by which HR can be determined? Auscultating the heart (heart rate) 2. Examining the pulse (pulse rate) 3. Looking at an ECG (QRS or ventricular rate) 1.

What extra information does the ECG provide that one cannot readily obtain by palpating arterial pulse or auscultating the heart? 1. Assessing cardiac rhythm & quantifying arrhythmia 2. Determining the origin of each impulse (the pacemaker for each beat)

ECG 1

In ECG 1, l successive RR intervals are fairly constant. l HR is approximately 1500 / RR in mm = 1500 / 20 = 75 BPM. l Alternatively, there are 11 RR intervals in 10 seconds, average HR = 66 BPM.

ECG 2

In ECG 2, l Each QRS complex is preceded by a P wave. PR interval is normal. All impulses originate in the SA node. l RR intervals vary considerably. l This physiologic phenomenon is called sinus arrhythmia. l Here the number of RR intervals in a 10 second strip must be multiplied by 6 to obtain HR.

ECG 3

In ECG 3, l RR interval = 6 seconds = HR of 10 BPM l We may say: “At this instant the heart has stopped”. This is called asystole (no cardiac electrical activity for > 2 seconds). l We then ask: “What is the underlying mechanism? ” l The answer may be: “a sudden increase in vagal outflow to the heart”

ECG 3 (continued) NB: the wandering baseline in this and the previous ECG is due to deep breathing at this time

ECG 3 (continued) l Further period of asystole for 3. 5 seconds (= HR of 17 BPM) l This is followed by escape beats at a rate of 30 BPM.

Key Points 1. Before you determine HR from an ECG, you must assess RR variability.

Key Points (continued) 2. Instantaneous HR is the number of times the heart would beat in one minute if the duration of successive cardiac cycles were constant. Instantaneous HR = 1500 / RR interval in mm

Key Points (continued) 3. If successive RR intervals are fairly constant, then average HR is approximately 1500 / RR in mm.

Key Points (continued) 4. If RR intervals vary, it is best to estimate HR (ventricular rate) by determining the number of RR intervals in a 10 second strip and multiplying this by six.

Key Points (continued) 5. If cardiac cycle durations change significantly and abruptly, then HR should be calculated over shorter periods of time (it may be as short as one cardiac cycle) to correctly interpret underlying physiology.

What is meant by each of the following terms? 1. 2. 3. 4. 5. 6. 7. 8. heart rate pulse rate ventricular rate instantaneous heart rate sinus arrhythmia cardiac cycle length variability atrial rate asystole

At the end of this activity, the learner should be able to: l Define and calculate instantaneous heart rates l Identify RR variability in an ECG. l Calculate HR correctly when RR variability is considerable l Determine whether to calculate average HR or instantaneous HR as appropriate.

Acknowledgment l We thank Pavithran P, predoctoral fellow in our department, for preparing the ECG in this presentation.