ECGs are an essential part of every day veterinary practice, particularly as an part of anaesthetised patient monitoring. We all put our patients onto the multi-parameter monitors, and we make sure there’s a trace. But are we watching that trace and do we know what we are looking at? It is important that we are accurately monitoring and interpreting the ECGs so that arrhythmias can be treated in an appropriate and timely manner. Severe arrhythmias may result in haemodynamic (dynamics of blood flow) compromise which can cause clinical signs or sudden death.
Getting a good trace is very important and any movement, panting and even electrical interference can affect how accurate the trace is. Firstly its essential that the ECG leads are placed on the correct limb. If they are not on the correct limb, it will affect the accuracy of your trace and/or may even cause the trace to be inverted. The best way to ensure you are putting the correct lead on the correct limb is to check your monitors instructions as different machines use a different colour coding system. Secondly the alligator clips on the end of the leads must have a good contact with the patient. We find that by applying applying conductive gel or ‘red dots’ prior to attaching the lead ensures a good contact. Thirdly try not to place the leads across the patients thorax or across an area where the surgeon may stand alongside, as any movement of the leads will interfere with the trace.
So, what is an ECG? The ECG trace is a pattern of electrical conduction and shows the depolarisation and repolarisation of the heart tissue (cardiac muscle) by way of waveforms and intervals.
The normal sinus complex consists of a P wave, a QRS complex (which should be viewed as a whole entity) and a T wave. What this represents in real terms is depolarisation and repolarisation of the heart, shown as an electrical stimulus.
- The P wave – The sinoatrial (SA) node starts the depolarisation process and the impulse spreads from right to left across the atria. When the whole of the atria have depolarised, the electrical difference returns to baseline.
- P – R interval – The atrioventricular (AV) node slowly conducts from the atria to the ventricles to allow delay between atrial and ventricular contraction.
- Q Wave – This represents the depolarisation of the ventricular septum.
- R Wave – The large muscle mass of the ventricles are depolarised through the His-Pukinje fibre network.
- S Wave – Finally, the basal regions of the ventricles are depolarised.
- T Wave – Repolarisation of the ventricles. T wave morphology can vary largely from patient to patient and therefore is of little diagnostic utility.
Once you have an ECG trace there are 4 basic things to looking at.
What is the heart rate (HR)? A lot of machines will record the HR automatically, however this should not be relied on and should be checked via stethascope of oesophageal stethascope. If you are using a machine which you can print off a trace on ECG paper, a quick method of calculating the HR is to measure a 6 second interval (15cm at a paper speed of 25mm/s, or 30cm at a paper speed of 50mm/s), count the number of QRS complexes within this period and multiply by 10 to reach number of beats per minute.
What is the rhythm? A sinus rhythm is a normal heart rhythm where a P wave is consistently followed by a QRS-T complex. Sinus arrhythmia is regularly irregular, because it always has a P wave (although this may vary in amplitude), followed by a normal QRS complex, but the rate can vary. It is associated with high vagal tone and often corresponds with respiration, particularly in dogs. It is rarely seen in cats but occasionally can be seen in cases of upper respiratory tract obstruction or ethylene glycol poisoning.
What is the P and QRS relationship? As already discussed, every P wave should be followed by a QRS complex, and there should be a consistent relationship between them both at all times.
What is the morphology of the QRS complex (narrow and upright or wide and bizarre)? If there are any normal sinus complexes they should be examined so that any abnormal complexes can be compared to them. If there is anything that does not look like a normal sinus complex, consider in what way it is abnormal.
So, start looking at those ECG traces, pick out the P wave, the QRS complex, and the T wave. Get familiar with what a normal trace is.