How do electrocardiograms (ECGs) work? H&H VIP

An electrocardiogram (ECG) is used to investigate the electrical activity of the heart.

[subscriber-content] The horse has an enormous and powerful heart, which consists mainly of muscle cells that squeeze together to pump out blood. Blood carries oxygen and essential nutrition to muscles and other parts of the body. When the horse works harder, more blood is required, especially by the muscles, so the heart has to pump faster and push out more blood.

At rest, the heart of a typical horse beats about 30 times a minute and pumps one litre of blood with every beat. At peak exercise, this can increase to more than 200bpm (beats per minute) with each beat pumping up to 1.7 litres. The heart of a thoroughbred racehorse may pump 300-400 litres of blood every minute at peak exercise; exceptional athletes can pump even more.

The different areas of the heart need to be precisely coordinated. The muscle in each part must squeeze at the same time to pump blood out, and the muscle cells must relax simultaneously to allow the heart to rest between beats and fill with blood again.

The heart should beat regularly, at a speed that matches the body’s needs — whether the horse is resting or working hard. To coordinate this, the heart muscle uses electrical currents produced in specialist cells.

A vet can glean information about the heart by listening to it, to check for murmurs and to investigate its beating speed and rhythm. To find out more about the electrical currents passing through the heart, however, an ECG may be called for.

How it works To perform an ECG, four electrodes are placed on the horse’s skin — two just below the withers and two at the bottom and to the rear of the chest. These detect the electrical currents running through the heart, sending the data to a computer via a transmitter attached to the saddle or roller. The information appears as a “trace”, a line — or lines — that show how the voltage in the heart changes with each beat.

At the start of each beat, an electrical current passes through the two smallest chambers of the heart, known as the atria.

This causes the muscle to contract, and produces a double bump in the trace (called the P wave).

There is then a brief pause as the electrical current spreads through to the heart’s largest chambers, the ventricles. A rapid change in the trace (known as the QRS complex) is seen as the current spreads through these chambers, which causes them to contract and push out the blood. The muscle of the ventricles then relaxes and returns to its resting voltage (the T wave), staying this way until the next beat starts with another P wave.

An ECG shows how fast the horse’s heart is beating and how regular these beats are. It is particularly useful when the rhythm is irregular or abnormal, as it can reveal the severity of the problem and indicate the part of the heart that is causing the issue. This may determine whether the condition is treatable and if the horse is considered safe to ride.

ECGs are not only used to investigate abnormal heart rhythms that can affect a horse’s performance or leave him at risk from a sudden heart attack. They are often used to monitor the heart during general anaesthetics for surgery and when nursing horses that are critically ill.

They can play a role in monitoring fitness and planning a training schedule, by comparing the horse’s heart rate with the level of work he is performing. They may also be worn by broodmares in late pregnancy to monitor the heart rate of the unborn foal.

Modern ECG equipment is lightweight and portable, so the horse can wear it during exercise or while relaxing in his stable. And examinations can be performed either at a veterinary clinic or at the yard.

Portable exercising systems can be linked to GPS equipment, so that the speed the horse is working at can be precisely linked to his heart rate. Some systems attach to an iPhone, allowing the vet to perform simple checks, although more sophisticated equipment is needed to supply more detailed information.

Risks and limitations There are no risks to the procedure as the equipment just records what the heart is doing. Sedation is unnecessary and the procedure causes no discomfort.

In some cases, an ECG will provide an immediate and definite diagnosis — with atrial fibrillation, for example, when the heart rhythm becomes very irregular. But the ECG is not perfect in every case. It can be difficult to get good-quality traces as the horse moves at speed, or if he is overweight and his heart is situated beneath excess layers of fat.

It can also be tricky to interpret the findings, as some rhythm disturbances might only happen occasionally. An ECG performed at exercise or carried out over a 24-hour period can help to detect some of these abnormal rhythms.

Procedure preparation Minimal preparation is needed for an ECG. A good trace can usually be obtained in a horse with a fine coat without removing any hair. With a thicker coat, clipping and cleaning small areas where the electrodes are to be placed will help. And if the electrodes need to stay in place for any length of time, or during fast exercise, they can be glued to the skin.

What's the damage? Results are usually immediate, although additional time may be required in some cases to analyse the trace.

Should an ECG detect a problem, your vet may suggest further investigation using different diagnostic techniques. An ultrasound scan will enable the vet to see the different chambers of the heart and the heart valves. Damaged valves can leak and cause parts of the heart to stretch and enlarge, leaving it prone to abnormal rhythms.

Cost will depend upon the duration of the ECG and whether it is combined with other procedures, but will usually be in the region of a few hundred pounds.

Ref Horse & Hound; 25 October 2018

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How do electrocardiograms (ECGs) work? H&H VIP

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