ECG: ambulatory monitoring

i canis

Contributor(s):

Serena Brownlie

Synonym(s): Holter ECG, tracker ECG, 24 hour ECG

Introduction

Ambulatory ECG monitoring is also called 24 hour ECG or Holter monitoring.
It is a technique which allows an ECG to be continuously recorded for long periods of time, either in a hospital cage or, ideally at home, so that the dog's normal daily routine is not disturbed.
It is used in cases with intermittent clinical signs, especially syncopal episodes, when a dysrhythmia Heart: dysrhythmia is suspected but a diagnosis has not been made by routine ECG examination ECG: overview ECG: principles of interpretation.
It is particularly useful for identifying episodes of ventricular tachycardia Ventricular tachycardia or sinus arrest Heart: sinus block / arrest.

There are several different types of equipment available and many different makes, some extremely expensive and others fairly inexpensive depending on function.
All have some form of recording device with leads which are attached to the animal by electrodes.
These usually have an adhesive backing to stick to the skin.
The recorder is carried around by the animal, either attached to a harness or bandaged in position.
ECG information is recorded either on to a small tape or in a microcomputer, and the information is decoded and analyzed later by computer.
Some recorders can transmit ECG information over the telephone for interpretation.
The information is timed, therefore a detailed diary of activities should be kept.
Event recorders are designed to record ECG information only when applied to the animal and switched on. These are used by the owner at the time of a collapsing episode or when a heart rhythm disturbance is detected.
Each type of equipment has its advantages and disadvantages.

An ECG can be obtained from the whole 24 hour period, including during exercise and sleep, marked with the time at which any abnormalities occur.

Most types are used for 24 hours only and it is unlikely in most cases that clinical signs will occur while the animal is wearing the monitor!
High-speed scanning computer interpretation is intended for human cardiology, and in dogs normal sinus arrhythmia, rapid changes in heart rate due to fear or excitement, and movement artifacts such as panting may be interpreted by the computer as abnormalities. It is ideal, though tedious, if the machine will also produce a print-out of the whole 24 hours' recording so that it can be examined. However, only very expensive equipment will allow this.
The main problem is keeping the monitor and electrodes attached to the animal. Some dogs have skin reactions to the adhesive electrodes. The dog may chew the equipment unless constantly supervised, or it may fall off while the dog is exercising.
There is no way of knowing whether a readable trace has been obtained until it has been decoded, therefore the whole procedure may have to be repeated.

These do not have to be attached to the animal at all times, and therefore are easy to use.
They are less expensive and are usually kept by the owner for longer periods, making it more likely that an event will be detected.

The owner must remember to keep the monitor nearby at all times, especially when out for a walk.
The owner must learn to apply the monitor quickly, and even then basic event monitors are likely to miss the dysrhythmia by being applied too late.
Some owners forget to bring back the equipment!

Adhesive electrodes will not stick to dog skin unless it has been carefully clipped and cleaned with surgical spirit.
The site of electrode application should provide ECG complexes which have R waves greater than 1 mV, and which have R and T waves of different heights, otherwise the computer will have difficulty in interpreting the trace. A base-apex lead is preferable.
Plenty of bandaging material is required, carefully applied to prevent slipping.

See also ECG: principles of interpretation ECG: principles of interpretation.
Do not overinterpret bradycardias or sinus pauses in sleeping dogs. Heart rates of 30 bpm or less are not uncommon, as are pauses of 3 seconds or even longer.
Do not be surprised by the large numbers of premature complexes which may be recorded in 24 hours. It is doubtful whether a large number of premature complexes could ever be described as normal in the dog, but unless a more severe dysrhythmia is also present, they will not provide a diagnosis in the collapsing dog.