Choosing an ECG recorder

This advice reflects my own experience and opinion, other cardiologists may have differing opinions.

An ECG machine required for diagnostic purposes is preferably a recorder rather than a monitor. In other words using an ECG monitor (used in perianaesthestic or intensive care monitoring) is not the ideal for diagnostic ECG recordings. It is best to have ‘separates’ - a recorder for diagnostic electrocardiography and a monitor for anaesthesia.

Quality
This is the most important factor for me. Recorders should produce a good quality high resolution tracing such that no pixelation (stepping) is evident. Older computerised recordings tended to produce a pixelated print out. Occasionally ECG recorders print onto ECG tracing paper, yet the print out is pixelated. Any pixelation in the tracing will affect the ability to recognise important small deflections, in particular when hunting for P waves.

Continuous recording
The ability to record a continuous paper trace without interruption and in real time is preferred. This is not possible for example with machines and computers that print out onto sheets of A4 paper. This will mean an ECG recorder with either paper in rolls or Z-fold paper. Sometimes Z folder paper can fail to adequately allow scribing of a deflection on the fold itself; however Z-fold paper is easy for storing, compared to paper rolls.

Ease of use
The recorder is simple and intuitive to use, ie. not so complex that a detailed user’s manual is required to understand it.

Automatic labelling
It is very useful to purchase a machine which performs automatic labeling on the paper trace, ie. the calibration, paper speed and filter level. It is also best if any alternation to the settings are labeled immediately following a change rather than at fixed intervals.

Automatic v manual recording mode
My preference is manual mode. Automatic mode means that the recorder runs through each of the leads, recording a set duration for each lead, however this is not a necessity. If automatic mode is used, it is useful to know how to stop the recording before attempting to print out unattached chest leads, or when the dog moves, or how to perform a continuous lead 2 rhythm strip.

Interpretative software
This does not appear to work reliably in animals. Primarily due to attempts at interpreting baseline artifacts, particularly movement artifact. I have seen a number of owners referred on the basis of misleading or wrong computer interpretation, such that I would be of the opinion that this should be avoided, except in the hands of someone experienced (in which case, interpretation is not required!).

Multichannel v single channel
Multichannel recorders permit the simultaneous recording of more than one lead at a time, most commonly 3 leads, for example, leads 1, 2 and 3. If a leg moves during a recording, sometimes this can mimic an ectopic, but in a 3-channel recording, usually one lead will remain unaffected, and thus reveal the cardiac rhythm was normal. 

Paper width is important when opting for a multichannel recorder. Some machines print out three (and sometimes four) leads on fairly miserly paper width resulting in the top and bottoms of QRS complexes overlapping. This can make interpretation challenging. Halving the sensitivity is less than an ideal way to get round this problem on a regular basis. So if choosing a multichannel recorder chose one with respectably wide paper, for which I would suggest should be > 8cm. It is also preferable that the machine is set-up so that there is not a repeat of the lead 2 rhythm strip, creating a fourth channel and compromising space.

Precordial chest leads
Generally not required for animal use. I occasionally use a single precordial lead in search of small deflections, such as P waves. But this is rarely required and the additional leads create an added storage problem.

Calibration settings
The standard paper speed settings are 25mm/sec and 50mm/sec. Other speeds (5mm/sec, 12.5mm/sec or 100mm/sec) are sometimes additionally available – these can occasionally be useful options, but are not essential.

The standard amplitude sensitivity is 10mm/mV. A double sensitivity of 20mm/mV is useful when the complexes are quite small, such as with cats. A half sensitivity of 5mm/mV is required when the QRS complexes are very tall in some dogs that might have cardiomegaly.

A filter is often required in animals to reduce baseline artifact noise from electrical interference of muscle tremor. One filter level is usually sufficient, although some machines provide two filter levels.

Automatic centring v manual control
Control of the stylus can either be performed manually (usually older recorders) or automatically. Either option is fine, and the automatic centring in new machines usually works well, despite animal movement.

Internal battery
This is a useful feature and in most machines comes as standard.

Jack plug lead fitting
Crocodile clips and paediatric limb plates are the two most common types of electrodes used in animals, these both require jack plug type fitting. The press stud fittings are of limited use in animals, in humans they are used for attaching to circular adhesive electrodes.

Monitor screen
This is a useful optional extra on some machines. It means the ECG rhythm can be monitored for long periods without printing to paper (and thus wasting paper). Machines that have a memory function are then useful in this setting, so that an arrhythmia seen on screen can then be printed out for the records or posting for an expert interpretation

Thermal paper v ink paper
Thermal sensitive paper is more practical than ink, avoiding the need for replacement ink at difficult or busy times. Thermal paper which turns black (rather than blue) usually provides the better contrast for photography or scanning. However some papers do not have a long term retention (> 1 year) of the image recorded, which gradually fades over time – something to watch for and try to avoid.

New v second hand
Whilst a new ECG recorder is the ideal, the cost of this can prevent some practices from purchase of an ECG machine – which would be ‘the greater of two evils’! If cost is a limiting factor, then second hand machines can be a good option. In these cases it is important to have the machine serviced prior to use. If they come without leads, there are companies that can make these up.

Recommended : Small Animal ECGs: An Introductory Guide, 2nd edition. Mike Martin (due 2007/8), Blackwell.