1. Bronze Level Electrocardiography

2. Aims Brief summary of relevant clinical electrophysiology
Indications for taking an electrocardiogram (ECG)
How to obtain a diagnostic ECG
All Covered in Part 1
Basic ECG interpretation

3. Section 4 – basic ECG interpretation

4. Definitions
QRS
R-R interval P T
PQ interval

5. Interpretation – instantaneous heart rate 1
Use R-R interval printed at bottom of trace (in milliseconds)
There are 60 x 1000 milliseconds in one minute
Heart rate = / R-R interval(ms)
In this case / 400 = 150bpm
This is how machines such as the Cardell one used in the clinics calculate heart rate.
The R-R interval is the time between to consecutive R waves and is the time shown by the red arrow. The numbers below the 3rd line of the trace show the automated calculation of the R-R interval which in this case is around 400milliseconds (ms).
In a minute there are 60seconds and there are 1000 miliseconds in a second. Therefore there are 1000 x 60 milliseconds in a minute.
To calculate the heart rate in beats per minute (bpm) you divide the number of milliseconds in 1 minute by the R-R interval which in this example is 60000/400 = 150bpm.
400ms

6. Interpretation – instantaneous heart rate 2
Use a ECG measuring ruler similar to the one shown below
It is important to choose the same scale as the paper speed – usually 25 or 50mm/s
The arrow is placed against the peak of one R wave and the number corresponding to the next R wave shows the heart rate

7. Interpretation – instantaneous heart rate 2
Using ruler
Rate here = 150bpm

8. Interpretation – mean heart rate
Use centimetre ruler
At 25mm/s, 15cm = 6 seconds
No of R waves in 6 seconds x 10 = heart rate in bpm
Rate here = 80bpm
15cm = 6 seconds of trace

9. 15cm = 6 seconds of trace
If the paper speed is 25mm/s then the information from 1 second of the recording occupies 25mm (=2.5cm) on the paper trace.
Therefore 6 seconds of information will occupy 6 x 25mm = 150mm (=15cm) of the trace.
To convert the number of beats in 6 seconds to the number of beats in 1 minute you multiply by 10 as there are 60 seconds in 1 minute.
Therefore by counting the number of beats within a 6 second period and then multiplying by 10 will give the heart rate in beats per minute (bpm).
In this example there are 8 beats during a 6 second period and therefore 8 x 10 = 80bpm.

10. Normal heart rate
Appropriateness of heart rate depends on species, age, temperament, level of fitness and also concurrent disease especially conditions altering autonomic tone (eg respiratory, gastrointestinal, intracranial)
Dog usually bpm in consult
Cat usually bpm in consult
Neonates usually have higher heart rates

11. Heart rhythm – regular or irregular?
The ratio of P waves to QRS complexes should be 1:1
What is rhythm and also the P:QRS on traces below:
Top trace P:QRS = 1:1. The rhythm is regularly irregular . This trace shows sinus arrhythmia.
Middle – the first 3 complexes on the trace show a normal P-QRS morphology and the rhythm is regular. The 4th beat occurs after a shorter time than the usual R-R interval and therefore is termed a premature beat. This beat (and also the 7th, 10th and 13th beats) have a narrow QRS complex without an obvious P wave.
These beats have a similar QRS morphology to the sinus beats suggesting that they are conducted through the ventricles in a similar way to sinus beats and therefore originate proximal to the atrioventricular node. These beats are called supraventricular premature beats often abbreviated to SPCs.
Lower trace P: QRS = 1:1. The rhythm is regular. This trace shows sinus rhythm.
The first 3 complexes on the trace show a normal P-QRS morphology and the rhythm is regular. The 4th beat occurs after a shorter time than the usual R-R interval and therefore is termed a premature beat. This beat (and also the 7th, 10th and 13th beats) have a narrow QRS complex without an obvious P wave.
These beats have a similar QRS morphology to the sinus beats suggesting that they are conducted through the ventricles in a similar way to sinus beats and therefore originate proximal to the atrioventricular node. These beats are called supraventricular premature beats often abbreviated to SPCs.
P:QRS = 1:1. The rhythm is regularly irregular . This trace shows sinus arrhythmia.
P: QRS = 1:1. The rhythm is regular. This trace shows sinus rhythm.

12. Normal ECGs – sinus rhythm

13. Normal rhythms – sinus arrhythmia
Can be seen in normal dogs at rest. The variations in heart rate are caused by rhythmic fluctuations in vagal tone and therefore this rhythm is abolished by sympathetic stimulation (eg stress, fear, pain).
In cats in a clinic, if sinus arrhythmia is present then consider conditions altering autonomic tone especially gastrointestinal, respiratory and intracranial disease.
Can be seen in normal dogs at rest. The variations in heart rate are caused by rhythmic fluctuations in vagal tone and therefore this rhythm is abolished by sympathetic stimulation (eg stress, fear, pain).
In cats in a clinic, if sinus arrhythmia is present then consider conditions altering autonomic tone especially gastrointestinal, respiratory and intracranial disease.