Transcript 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: [email protected]
Select the single best response.

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:
HR = 1500 / RR in mm
 HR = 60 / RR interval in seconds
 HR = 300 / number of large squares
between successive R waves

How did this formula come about?
1.
2.
3.
4.
ECG is recorded at a speed of 25 mm/s.
The paper moves 60 × 25 = 1500 mm
per minute
HR is the number of cardiac cycles (or
RR intervals) per minute
If one RR interval measures 20 mm, then
HR is 1500 / 20 = 75 BPM. What does
this result represent?
1500 / RR interval in mm
= Instantaneous heart rate

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
Note that RR intervals vary considerably.
 What is this phenomenon called?

..continued
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What if you were to take a shorter RR
interval into consideration for calculating
HR?

What if you were to take a longer RR
interval into consideration for calculating
HR?
What information are we seeking to
derive from HR?

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?
1.
2.
3.
Auscultating the heart (heart rate)
Examining the pulse (pulse rate)
Looking at an ECG (QRS or ventricular
rate)
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,
successive RR intervals are fairly
constant.
 HR is approximately 1500 / RR in mm =
1500 / 20 = 75 BPM.
 Alternatively, there are 11 RR intervals in
10 seconds, average HR = 66 BPM.

ECG 2
In ECG 2,
Each QRS complex is preceded by a P
wave. PR interval is normal. All impulses
originate in the SA node.
 RR intervals vary considerably.
 This physiologic phenomenon is called
sinus arrhythmia.
 Here the number of RR intervals in a 10
second strip must be multiplied by 6 to
obtain HR.

ECG 3
In ECG 3,

RR interval = 6 seconds = HR of 10 BPM
We may say: “At this instant the heart has
stopped”. This is called asystole (no
cardiac electrical activity for > 2 seconds).
 We then ask: “What is the underlying
mechanism?”
 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)
Further period of asystole for 3.5 seconds
(= HR of 17 BPM)
 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:
Define and calculate instantaneous heart
rates
 Identify RR variability in an ECG.
 Calculate HR correctly when RR variability
is considerable
 Determine whether to calculate average
HR or instantaneous HR as appropriate.

Acknowledgment

We thank Pavithran P, predoctoral fellow in
our department, for preparing the ECG in
this presentation.