Holter ECG Monitors in Cats

Written by Victoria Greet BVM&S, PGDipVCP, MRCVS. European Resident in Veterinary Cardiology

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 the hospital or at ideally at home so that the cat’s normal daily routine is not disturbed.
• It is used in cases with intermittent clinical signs when an arrhythmia is suspected but a diagnosis has not been made by routine ECG examination.
• It can also be used to better characterize the frequency and complexity of ventricular or supraventricular arrhythmias.

Background

• It is often difficult to obtain normal heart rates in cats at veterinary clinics, as stress may increase their heart rate.
• Previously, Holter equipment was placed in a cage beside the cat, or sometimes attached directly to the cat. The older Holter recording systems were heavier (weighing ~ 500 g), a weight that could be considered stressful for cats and may affect heart rate and rhythm.
• Modern equipment is digital and therefore smaller, which makes Holter monitoring of cats easier to perform.
• Recent studies have shown that good quality 24-hour three-lead ECG registrations can be obtained in cats in the home environment without sedation. Holter monitoring can thus be used as a compliment to routine diagnostic methods when evaluating a cat with suspected or confirmed heart disease.

Ambulatory Holter monitors

Figure 1. Image demonstrating electrode attachment in the cat using a traditional Holter recording device (imagen taken from the Journal of Veterinary Cardiology (10.1016/j.jvc.2008.10.003)

Figure 2. Modern Holter device used for ambulatory ECG recordings in cats seen through the SCVS cardiology service

Case example:

Bobby is a 3 year 10-month old, male-neutered, domestic shorthair cat who started to have unusual ‘episodes’ at home. The episodes were becoming increasingly frequent, with 10-20 reported in a day at home.  The events varied significantly in character but usually involved Bobby ‘staring into space’ or occasionally collapsing on to his back from a standing position.

Bobby was referred to the SCVS neurology department for further investigations into suspected ‘seizure-like’ episodes. Neurological examination was unremarkable, however, auscultation of the heart during physical examination identified a murmur prompting detailed cardiac assessment.

An echocardiogram (heart scan) did not reveal any structural or functional cardiac abnormalities. Paper-trace ECG showed no evidence of cardiac arrhythmias during a 5-minute recording, but transient arrhythmias could not be excluded. Consequently, Bobby was fitted with an ambulatory Holter ECG monitor and sent home the same day.

Figures 3 and 4: Images of Bobby wearing the Holter device, secured using a comfortable jacket.

Bobby had an episode on the day of discharge whilst the Holter monitor was in position (see figure 4).  The trace shows a bradyarrhythmia (slow-heart rhythm) called third-degree atrioventricular block, in which the atria and ventricles are unable to communicate effectively due to the presence of a diseased AV node. Unfortunately, oral medications are rarely successful in treating these sorts of arrhythmias. Pacemaker implantation is considered the treatment of choice for the management of symptomatic bradyarrhythmias in cats and this procedure has been scheduled for Bobby in the near future.

Figure 6. Pulse generator used for artificial pacemaker implantation in cats with clinically significant bradyarrhythmias. The pacing lead may be placed through the vein (transvenous) or through the diaphragm (trans-diaphragmatic) depending on whether intra-cardiac or epicardial pacing is used respectively.

Conclusion:

This case illustrates that reliance on history and description of episodes to differentiate seizures (sudden uncontrolled electrical disturbances in the brain) from syncope (transient reduction in blood supply to the brain associated with loss of consciousness) can be problematic. Syncopal episodes can be associated with slow or fast heart rhythms (brady- or tachyarrhythmias) and the use of Holter monitors in cats can help us to establish a definitive diagnosis and ensure appropriate treatment. We are now routinely performing Holter ECG recordings in cats using state-of-the art equipment available within the SCVS Cardiology Service.