How many times have you put your patient on an ECG, seen something abnormal, and felt unsure about what to do next?

ECGs are something we nurses perform very commonly. Usually, this is when we’re monitoring our patients under sedation or anaesthesia, or when we’re dealing with cardiac patients… but not always.

There are lots of conditions where we can also see abnormal heart rhythms, including:

• Hyperkalaemia

• Hypoxaemia

• Hepatic tumours

• Splenic tumours

• Gastrointestinal disease

So how do we interpret our ECGs confidently, know exactly what the rhythms mean for our patients, and which we need to intervene in?

Over the next few weeks, we’ll be looking at exactly that - starting with a refresher on the cardiac cycle and our ECG trace. By the end of this post, you’ll know what each wave of your ECG means, and exactly how to set up your machine to get a reliable waveform.

What happens when our heart beats?

As a reminder - our heart consists of our left and right atria, and left and right ventricles.

The left side of the heart pumps oxygenated blood (received from the pulmonary veins) around the body via the aorta, whereas the right side of the heart receives deoxygenated blood from the vena cava, and pumps this to the lungs via the pulmonary artery.

This blood is then oxygenated in the lungs, before travelling back to the left side of the heart to be pumped around the body.

This process is coordinated by the electrical activity of the heart, controlling the cardiac myocytes and making the heart muscle contract.

Each phase of the cardiac cycle is associated with electrical activity spreading through different areas of the heart, allowing contraction of some chambers, and relaxation (and refilling) of others.

Atrial Systole

The SAN is an area of specialised conductive tissue, located in the cranial aspect of the right atrial wall. It functions as the pacemaker of the heart, controlling heart rate.

The SAN can spontaneously generate electrical impulses (aka action potentials) that excite the surrounding myocardial cells, spreading through the atrial tissue in a wave-like pattern.

The heart contains four fibrous rings and a membrane between the chambers of the heart - these structures are collectively known as the cardiac skeleton. This skeleton separates the electrical activity of the atria from the ventricles and diverts the electrical activity through to the atrioventricular (AV) node.

As the wave of depolarisation spreads across the atrial tissue, it contracts, ejecting blood from the atria into each ventricle, through the atrioventricular valves. This contraction is known as systole.

Ventricular Systole

The AV node is located in the right atrium, near to the intraventricular septum. Once the electrical impulse enters this node, it is conducted through specialised cardiac muscle cells to the bundle of His, a group of specialised conductive cells.

The bundle of His splits into the left and right bundle, each supplying an associated ventricle. As the electrical impulse spreads through each bundle, it reaches specialised fibres called Purkinje fibres, which transmit this impulse to the cardiac muscle cells in the ventricles.

These cardiac myocytes become excited by the impulse, which in turn causes them to excite neighbouring cells, spreading a wave of electrical impulses across the ventricles.

This ventricular excitation causes contraction, which in turn causes:

• Ejection of blood from the left ventricle into the aorta, where it supplies organs with oxygenated blood for cellular processes

• Ejection of blood from the right ventricle to the pulmonary artery, where it flows to the lungs for gaseous exchange to take place

What about Diastole?

After a systolic (contraction) phase, the heart must have time to relax and refill prior to contracting again. This period is called diastole.

Atrial diastole/repolarization (relaxation and refilling) takes place during ventricular contraction - but we don’t tend to see this on an ECG, because it’s hidden under the big surge of electrical activity that happens when our ventricles contract.

Ventricular diastole/repolarization takes place at the end of ventricular systole. This is split into two phases - early ventricular diastole and late ventricular diastole. In the early stages, the pressure in the ventricles begins to drop, causing negative pressure to open the atrioventricular valves and ‘pull’ blood into the ventricles. In late-stage ventricular diastole, the atria contract, forcibly ejecting blood from the atria into the ventricles.

This late stage is atrial systole - the two phases of the cardiac cycle (ventricular diastole and atrial systole) overlap, and the cycle then starts again.

So what does this look like on an ECG?

The sinus complex on an ECG consists of 7 key components - the P wave, P-R segment, Q wave, R wave, S wave, S-T segment and T wave.

The P Wave

The P wave is associated with the SAN firing, and that wave of depolarisation (contraction) spreading across the atrial myocardial cells.

The atria are smaller than the ventricles, so contain less muscle, and therefore less electrical activity passes through them. This means our P wave appears as a small deflection on the ECG trace.

The P-R Segment

The P-R segment is the small period between the P wave and the QRS complex. 

It correlates with the electrical impulse passing through the AV node. This happens slowly, to allow the ventricles to contract in a coordinated manner, and during this time, there is no electrical impulse passing through the myocardial cells, so the trace returns to baseline.

The Q Wave

The Q wave is associated with early electrical impulses passing through the left bundle branch, and then moving across the intraventricular septum to the right side. This causes a small negative deflection on the ECG.

The R Wave

The R wave correlates with the full activation of the bundle of His and Purkinje fibres, causing ventricular depolarization and contraction. As the ventricles contain the most muscle, there is a large amount of electrical activity - and therefore a large wave is seen on the ECG.

The S Wave

The S wave is associated with depolarization or contraction of the base of the ventricles - the region near each AV valve. This is seen as a small negative deflection on the ECG.

The S-T Segment

The S-T segment is the portion where the complex returns to baseline. It represents the interval between ventricular depolarization/contraction and repolarization.

The T Wave

The final part of our sinus complex is the T wave. At this stage, the ventricles repolarize, refilling so that they’re ready for the next contraction. This causes a small deflection on the ECG - usually similar in size, or slightly smaller than, the P wave. T waves can be positive, negative, or diphasic (both positive and negative).

Once we know what our ECG should look like, and what each portion of our sinus complex means, we can start understanding abnormal complexes. But to do that, we first need to set up our ECG correctly and know how to get the most reliable trace we can. So how do we do that?

Setting up your ECG

Good technique when reading an ECG is vital - many things can cause artefacts and make interpreting the trace more challenging, for example:

• Movement

• Poor contact between the electrodes and the patient

• Electrical items (e.g. clippers, imaging equipment, warming equipment, other monitoring equipment)

So to make sure you get the best possible results from your ECG, make sure that:

• Your patient is positioned appropriately (ideally lying in right lateral recumbency, unless this isn’t possible due to the severity of their condition - respiratory patients, for example, should have their ECG performed in sternal)

• The limbs should be gently restrained, to prevent movement causing ECG artefacts

• The electrodes/pads should be placed on the limbs appropriately - either 3 or all 4 limbs, depending on the ECG used. Most commonly this will be 3 limbs; the right forelimb, left forelimb, and left hindlimb.

• Electrode gel (this is different to ultrasound gel!) can be used to improve contact where this is poor. Alternatively, if you don’t have this, spirit works well as an alternative (as long as you’re not in a CPR situation!)

The ECG Leads

When the ECG electrodes are placed, they will create a triangle of leads that measure the electrical potential across the heart. The number of electrodes connected to the patient will dictate the number of leads that can be read by the ECG.

Positioning your electrodes

The colour and positioning of your electrodes will vary depending on your location. In the UK:

• Right Forelimb = Red Electrode

• Left Forelimb = Yellow Electrode

• Left Hindlimb = Green Electrode

• Right Hindlimb = Black Electrode (if present)

In the US, for example, the right forelimb is a white electrode; the left forelimb is a black electrode; the left hindlimb is a red electrode, and the right hindlimb is a green electrode.

To get the most accurate readings, always double-check on the cables themselves - most have ‘RA’ (right arm), ‘LA’ (left arm), ‘LL’ (left leg) etc printed on them.

You can actually place these electrodes anywhere on the limb; there is some suggestion that R-wave amplitude decreases when electrodes are attached to the footpads, rather than more proximally on the leg itself.

So what do our leads tell us?

You’ll either be measuring a 3-lead or 6-lead ECG. An ECG with 3 electrodes (one placed on each forelimb and one hindlimb) will provide a 3-lead ECG, whereas an ECG with 4 electrodes (one placed on each limb) will provide a 6-lead ECG.

If you’re monitoring a patient under anaesthesia, or a critically unwell patient on a multiparameter monitor, this will most likely be a 3-lead ECG.

If you’re helping a cardiologist with a ‘full’ ECG (these usually have a paper feed and give you a printout of your patient’s trace), there’s a good chance you’ll be running a 6-lead ECG.

What does a 3-lead ECG tell us?

A 3-lead ECG allows you to assess your patient’s cardiac activity in Leads I, II and III:

• Lead I measures the activity across the heart from the right forelimb to the left forelimb

• Lead II measures the activity across the heart from the right forelimb to the left hindlimb, and this is generally the lead we use for routine interpretation

• Lead III measures the activity across the heart from the left forelimb to the left hindlimb

And what about a 6-lead ECG?

If you’re using a 6-lead ECG, you’ll have 3 additional leads available to analyse: aVR, aVL and aVF:

• aVR measures the difference in electrical potential between the left forelimb and left hindlimb to the right hindlimb

• aVL measures the difference from the right forelimb and left hindlimb to the left forelimb

• And aVF measures the difference from the right forelimb and left forelimb to the left hindlimb.

Each of these leads allows us to assess the heart’s electrical activity from different directions, providing more information than examining one lead alone.

So now you know what you should see on an ECG, how to set one up correctly, and the common artefacts we see - as well as what each lead means.

To set yourself up for success with interpreting ECGs, make sure that you’re placing your electrodes correctly, avoiding artefacts from external electrical sources or patient movement, and considering the lead you want to evaluate your ECG in.

Lastly, don’t forget to think about the cardiac cycle and how this translates into our sinus complex - this will make understanding how different arrhythmias affect our heart, and therefore the waveform we see, much easier!

Do you perform many ECGs in practice? How confident are you at interpreting them? Drop me  DM on Instagram and let me know!

References

1. Bulmer, B. 2012. Interpreting ECGs with Confidence, Part 1 [Online] Clinician’s Brief. Available from: https://www.cliniciansbrief.com/article/interpreting-ecgs-confidence-part-1

2. Edward-Durham Jr, H. 2022. Overview of Electrocardiogram Interpretation [Online] Today’s Veterinary Nurse. Available from: https://todaysveterinarynurse.com/cardiology/veterinary-electrocardiogram-interpretation/

3. Pace, C. 2018. How ECG Monitoring Contributes to patient Care [Online] The Veterinary Nurse. Available from: https://www.theveterinarynurse.com/review/article/how-ecg-monitoring-contributes-to-patient-care

4. Pace, C. 2020. ECG Interpretation. Veterinary Nursing Journal, 35, pp. 72-75.

Image Credits (in order of appearance)

1. Tudor, 2012. https://www.petmd.com/blogs/thedailyvet/ktudor/2012/sept/canine_heart_disease_and_nutrition_part_1-27104

2. By Madhero88 (original files); Angelito7 (this SVG version); - ConductionsystemoftheheartwithouttheHeart.png & ConductionsystemoftheheartwithoutHeart.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=29922595

3. By Created by Agateller (Anthony Atkielski), converted to svg by atom. - SinusRhythmLabels.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1560893