Indications for Holter Monitoring

Intermittent collapse is a common presenting sign in small animal practice however the investigation of this problem is notoriously difficult due to the intermittent nature of clinical signs and extensive differential diagnosis list.

A careful history is required to determine whether the animal has exhibited syncope, a seizure or has fallen over. Dogs who are syncopal due to cerebral hypoperfusion may develop a hypoxic seizure so a syncopal episode may transition into a seizure. Classical features of these three syndromes are shown in the table below however it should be noted that not all dogs will fulfill all criteria and sometimes have to be classified on the basis of “best fit”.

Syncope Seizure Falling over
Usually at exercise Usually at rest Usually at exercise
Sudden onset and sort duration Pro-dromal phase Dog slows down and then becomes recumbent
Flaccid collapse sometimes progressing to opistotonus. NB cerebral hypoxia may induce a seizure Body rigid with tonic-clonic activity Dog relaxed and conscious
Usually no urination / defaecation May have urination/ defaecation No urination / defaecation
Dog unresponsive during episode Dog unresponsive during episode Dog can respond to owner
Abrupt and complete recovery after a short time Gradual return to normal over a longer period Variable recovery
At risk breeds include Boxers, Dobermanns, Great Danes At risk breeds include GSDs, collies Any breed, typically older arthritic dogs or brachycephalic on hot day
Pale mucous membranes Mucous membranes normal Mucous membranes normal

Obviously if the dog collapses whilst wearing the monitor, it should be possible to rule in or rule out an abnormal heart rhythm as the cause of the collapsing episode. However, even if the dog does not exhibit clinical signs whilst wearing the monitor, useful information can still be obtained as many dogs with intermittent dysrhythmias require a prolonged period of the dysrhythmia to exhibit collapse but will still have short paroxysms of the dysrhythmia throughout the day.

Other indications for Holter monitoring:

  • Monitoring the efficacy of anti-arrhythmic drug treatment
  • Pre-breeding screening for pre-clinical dilated cardiomyopathy in Dobermans
  • Screening for boxer cardiomyopathy which is also known as arrhythmogenic right ventricular cardiomyopathy

The trace below shows paroxysmal ventricular tachycardia with a rate of 300bpm in a young boxer. This trace suggested that the dog was a risk of sudden death and this patient has responded well to medication.