Page 211 - WSAVA2018
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The cardiac rhythm is normal when these normal deflections occur at a normal rate (for the species being examined). Any divergence from this is considered abnormal.
How do you record a good quality EKG?
In order to be able to interpret an EKG it is first necessary to obtain an EKG of a good quality. A good quality EKG has the following characteristics
1. It is clearly labelled and shows both vertical and horizontal calibration. If the filter is used this should be noted. Ideally a period of 50 mm/sec should be included on the trace and a prolonged “rhythm strip” of lead II recorded at 25 mm/sec.
2. It shows the minimum artefact throughout the majority of the recording.
3. All six frontal plane leads (I, II, III, avR, avL, avF) are recorded. Chest leads may be recorded.
4. It clearly shows details of the case from which it was recorded including the species and breed.
The EKG should be recorded with the patient in right lateral recumbency being gently restrained. Cats may tolerate the recording of an EKG better if they are in sternal recumbency. If right lateral recumbency is not used then this should be noted. Recumbency will not affect the cardiac rhythm but may alter the magnitude of some deflections. Sedation should be avoided if at all possible. Most sedatives will in some way alter the cardiac rhythm. The most profound effects on cardiac rhythm are seen when the alpha-2 agonists are used.
The EKG electrodes should be placed on the patient’s limbs, on the forelimbs behind and slightly proximal to the elbow, on the hindlimbs cranial and proximal to the stifle. Good electrical contact should be ensured using the either coupling gel or surgical spirit.
The patient should be held on an electrically insulated surface and as far away from other electrical equipment as possible. Particularly avoid close proximity to strip lighting and computer screens. This should minimise the risk of 50 Hz interference.
Once the ideal recording environment has been achieved then recording can proceed.
Interpretation of the EKG trace
Basic interpretation can be achieved by asking a few simple questions when faced with the EKG trace. The most important aspects of interpretation involve the determination of the heart rhythm and assessment of whether the rhythm is normal or not. The EKG may also provide clues as to the presence of enlargement of some cardiac chambers.
Is the EKG of a diagnostic quality? i.e. free from artefact, labelled and within the boundaries of the paper.
If the EKG is of a poor quality or not properly labelled then less information can be obtained. Subtle changes can be missed when there is considerable artefact.
The time to notice artefact is when the trace is being recorded as another trace can be recorded at that time.
Interpretation of rhythm
What is the heart rate?
Many significant rhythm disturbances disturb the heart rate. Arrhythmias that lead to an increase in heart rate are described as tachycardias. Arrhythmias that lead
to a decrease in heart rate are called bradycardias. Instantaneous heart rate can be calculated on the basis of the R-R interval from one complex to the next. An average rate can be taken by counting the number of depolarisations within 6 seconds and multiplying by ten. Where the rhythm is very irregular a more accurate rate may be obtained by counting over 12 seconds.
Rates are therefore calculated as below
a) Method of calculation of average heart rate (paper speed 25 mm/sec)
A distance of 15 centimetres from one R-wave is inspected on the lead II EKG strip. The number of R-R intervals in this 15 centimetres is calculated to the nearest half interval. This number is then multiplied by ten to provide the average heart rate to the nearest five beats per minute.b) Method of calculation of the instantaneous heart rate (paper speed 25 mm/sec)
If the R-R interval is x mm the instantaneous heart rate is calculated as HR = 1500/x beats per minute.
(Where the EKG is recorded at 50 mm/sec the heart rate is calculated as HR = 3000/x beats per minute.)
Determination of Rhythm
Evaluation of the heart rhythm involves examining the EKG for evidence of the normal relationship between the P-waves and the QRS complexes. This can be done in the following way
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