An electrocardiogram (ECG) is a useful diagnostic tool in veterinary practice to assess heart rate and rhythm, but anecdotally, veterinary nurses seem reluctant to use the machine, citing two distinct challenges. First, knowing how to set up and use the machine correctly, and second, knowing what to look for when it is working. This practical article will explore both these issues, and suggest cases that might benefit from ECG monitoring.
Cardiac muscle requires an electrical stimulus to start a contraction. Figure 1 shows how specialised cells within the sino-atrial (SA) node start the conduction process, by firing an impulse that spreads across the atria depolarising (contracting) the muscle as it travels. The impulse passes through the atrioventricular (AV) node, to the ventricles using the His-Purkinje fibre network. As the impulse travels through the His-Purkinje network, the ventricles are depolarised. This depolarisation is the catalyst for the systolic and simultaneous action of oxygenated blood being pumped through the aorta, and deoxygenated blood out through the pulmonary artery. Finally, as the cardiac muscle relaxes, the ECG records the repolarisation (relaxation), as the muscle prepares for the next contraction. This process should result in a sinus complex, shown in Figure 2. Table 1 provides a step-by-step guide to each part of the P-QRS-T complex.
| Section of the complex | Event |
|---|---|
| P wave | SA node starts depolarisation process. Impulse spreads from right to left across the atria, contracting the atria in a coordinated manner. When the whole of the atria have been depolarised, the electrical difference returns to baseline |
| P – R interval | The AV node slowly conducts the impulse from the atria to the ventricles to allow coordinated ventricular contraction. No muscle is depolarised, therefore the baseline remains flat |
| Q wave | Depolarisation of the ventricular septum. |
| R wave | The large muscle mass of the ventricles is depolarised via the His-Purkinje fibre network |
| S wave | Remaining basal regions of the ventricles are depolarised. |
| T wave | Repolarisation of the ventricles. T wave morphology can vary largely from patient to patient, and are generally not of diagnostic value in small animal medicine |
How to use the machine
Cables and electrodes
There are many different makes and models of ECG machine, which adds to the confusion of using them. Some machines have three cables for electrodes, usually coloured red, yellow and green. Others have four cables, generally red, yellow, green and black. Some machines even have five cables, which include a white cable and electrode. American machines have a different colour scheme completely, so it is best to refer to the user manual for clarification. There is however a standardised protocol for electrode placement, which can be followed, no matter how many electrodes are present.
Correct ECG electrode placement
Electrodes should be placed:
If there is a fifth lead, this can be attached anywhere on the thorax. To minimise artefact, have the cable running in the same direction as the other cables. Conductive gel or spirit should be applied to the patient to improve contact.
Terminology can confuse matters further, so Box 1 provides an illustrated guide.
The machine
ECG machine configuration and functionality varies from basic machines to complicated top range models. If unsure how to use the practice machine, refer to the user manual and practice using it as much as possible. Figure 3 shows an example of an ECG machine, with a few of the basic options highlighted.
Standard ECG machine settings
| Artefact type | Example |
|---|---|
| Panting artefact |
|
| Poor contact or electrical interference (clippers, fans, etc) |
|
The patient
Ideally, the patient should be calm, and in a quiet room for the ECG recording. The patient should be lying in right lateral recumbancy on a non-conductive surface, such as a vetbed. Place the electrodes as listed above, and use either surgical spirit or gel to improve conduction. Surgical spirit is flammable, and therefore should only be used if the patient will not be defibrillated. If the patient is in respiratory distress, the ECG should be taken in sternal recumbancy. Figure 4 shows gold standard positioning, and in this picture, note how the cables come away from the patient, so that respiration does not interfere with the trace. Furthermore, the limbs are being held slightly apart to minimise muscle tremor or movement artefact.
Cases that might benefit from an ECG
An ECG is used to measure heart rate and rhythm, so any case that is critically ill, is undergoing anaesthesia, has a history of collapse, or has an auscultation abnormality would benefit from ECG monitoring. The benefit of having an ECG attached is that problems can be identified quickly and treated appropriately. Suggested cases that should have an ECG attached are seen in Box 3.
If ECG monitoring is not routinely used for anaesthetic monitoring, there are some specific surgical cases that it would be beneficial to monitor, due to the disease process, or the effect of increased vagal tone. Examples of this type of surgery include correcting gastric dilatation volvulus, ocular or adrenal surgery, or cases with sepsis, gastrointestinal disease, splenic disease, or upper respiratory tract obstruction. Please note this list is not exhaustive.
The ECG trace
Sinus rhythms and common abnormalities
Life-threatening rhythms are usually very fast or very slow, therefore determining the heart rate is a crucial first step. Some machines will record the heart rate automatically, however a quick method of calculating the heart rate is to measure a 6 second interval (15 cm at a paper speed of 25 mm/second or 30 cm at a paper speed of 50 mm/second), count the number of QRS complexes within this period and multiply by 10 to reach number of beats per minute (Willis, 2010).
Sinus rhythms
Sinus complexes are individual to each patient, so after determining the rate, it is important to assess whether each complex has a P-QRS for each and every complex (Figures 5 and 6).
Sinus tachycardia — a regular fast rhythm with a P wave for every QRS complex. Often seen in excitable or stressed patients, and is very common in cats (Figure 7).
Sinus bradycardia — this is a regular slow rhythm with a P wave for every QRS complex. It is often seen in resting dogs, but also in athletic or working dogs (Figure 8).
Sinus arrhythmia — a common arrhythmia seen in small animal practice. It always has a P wave, followed by a normal QRS complex, but the heart rate can vary. It is associated with vagal tone and often corresponds with respiration, particularly in dogs. This is a normal finding in dogs, but uncommon in cats (Figure 9).
Second degree atrioventricular (AV) block — depending on the intrinsic heart rate, second degree AV block may or may not be clinically significant (Dennis, 2010). Often seen with bradycardia and fit, healthy dogs (Figure 10).
Ventricular premature complexes (VPC) — VPCs are a common finding in dogs and cats (Martin, 2007) and can be caused by a variety of cardiac and non-cardiac causes. They can vary in morphology and size, as seen below in Figures 11–13. Common questions to consider are how frequently they occur, the underlying heart rate, and disease process.
Arrhythmias requiring attention
The arrhythmias that should cause alarm are generally either fast or slow ones. It should be repeated however, that if there is any concern whatsoever, a VS should be consulted immediately. Severe arrhythmias may result in haemodynamic compromise, and less severe arrhythmias can be an indicator for more severe arrhythmias or sudden death (Dennis, 2010).
Urgent arrhythmias
Ventricular tachycardias — these arrhythmias are unstable because they are either too fast for proper and organised ventricular contraction, or are firing from many different foci within the ventricles. Complexes can have a uniform or multiform appearance, but are usually wide and bizarre (Figure 14).
Atrial arrhythmias — these are usually associated with structural heart disease and are haemodynamically unstable when very fast (Figure 15) (Ware, 2007).
Atrioventricular block — high grade AV block (2nd or 3rd degree) can be potentially life threatening because they can be haemodynamically and electrically unstable (Figure 16).
Life threatening arrhythmias
There are three arrhythmias that are life threatening and require urgent attention. All of the arrhythmias shown below (Figures 17–19) would need immediate cardiopulmonary resuscitation (CPR) and electrical defibrillation. See Box 4 for CPR algorithm.
Checklist
It may be beneficial to have a mental checklist ready when concerned about an arrhythmia, or a particular patient. Table 2 is a suggested checklist, with issues that might need to be addressed or discussed with the VS.
| Question | Rationale | Action |
|---|---|---|
| What is the heart rate? Could the patient be haemodynamically or electrically unstable? | Life threatening arrhythmias are usually a result of tachycardia or bradycardia | Consult the veterinary surgeon (VS) Check blood pressure |
| What drugs have been used? | Drugs can have proarrhythmogenic properties | Consult the VS Refer to hospital chart/anaesthetic record |
| What is the underlying disease? | Cardiac myoctyes need constant blood supply, stable composition of interstitial fluid, constant nerve supply and structural integrity | Consult the VS to see if cardiac function could be compromised |
| What is happening to the patient? | Pain and increased vagal tone can influence heart rate and rhythm | Talk to the VS Observe the procedure |
| Is the patient receiving enough oxygen? | Hypoxia can cause arrhythmias | Talk to the VS Are the lungs compromised by disease? |
Conclusion
Using an ECG can be beneficial to all patients undergoing anaesthesia, and routine procedures will allow time for staff to become comfortable with the machine and preferred settings. Emergencies, patients in a critical condition, and high risk patients should all be monitored with an ECG to allow arrhythmias to be treated in an appropriate and timely manner. The role of the ECG should be to complement the monitoring techniques that veterinary nurses already employ, such as recording pulse rate and pulse quality, and respiratory rate and effort.
Key Points
Conflict of interest: none.