Contributors: Serena Brownlie, Mark Oyama
Species: Feline | Classification: Miscellaneous
- 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 cat'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.
- 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, bandaged in position, or confined with the cat in a small cage or kennel.
- 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.
Ambulatory monitors - advantages
- 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.
Ambulatory monitors - disadvantages
- 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 cats normal sinus arrhythmia, rapid changes in heart rate due to fear or excitement, and movement artifacts 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, and particularly for cats, allowing them some degree of normal mobility while wearing the monitor. Some cats have skin reactions to the adhesive electrodes. The cat may chew the equipment unless constantly supervised, or it may fall off while the cat is ambulating.
- 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.
Application of electrodes
- Adhesive electrodes will not stick to cat 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 of the greatest possible amplitude, 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.
Maintaining the monitor
- Often, cats are hospitalized for the 24 hour monitoring period as they will be unwilling or unable to physically carry the monitor on their body. In these cases, the cat is confined to a small cage and the monitors wires and recorder are wrapped in bandage material and placed alongside the cat in the cage. The wires are left long enough to allow the cat to ambulate within the cage. Care should be taken to prevent the cat from entangling itself in the wires.
Notes on interpretation
- See also ECG: principles of interpretation ECG: principles of interpretation.
- The low amplitude QRS complexes in cats may make automated computer-based analysis difficult. Often, the user will have to manually scan and interpret the recording.