What is a "Normal" 24 Hr Holter Anyway?

Diagnosis of DCM

Overt disease:  When the disease is in the symptomatic or overt stage, diagnosis is not difficult. Symptoms may include shortness of breath, coughing, poor appetite and weight loss, fainting episodes.   Physical examination will reveal a heart murmur, a gallop (an extra heart sound which indicates heart failure) and often irregular beats.  Chest X-ray will show an enlarged heart and fluid in the lungs.  Echocardiogram (ultrasound of the heart) will show a dilated left ventricle and atrium, with poor function and mitral valve insufficiency (leaking of the valve).  

Occult disease:  When the disease is in an asymptomatic or occult stage, the diagnosis may be suspected by testing, but at this date these criteria are not uniform from center to center.  Echocardiography of healthy Dobermans recently studied at the UDC nationals and it was found that the average fractional shortening of these healthy dogs was 26% using a short axis view, and 22.5% using a long axis view. 
 In other breeds a fractional shortening of 25% or less in the short axis view is considered abnormal.  This either indicates that a large percentage of healthy Dobermans have occult DCM or that the Doberman heart at baseline is not comparable to that of most breeds.  
As such, to diagnose occult DCM, most centers now require not only a depressed fractional shortening, but some evidence of functional impairment such as dilated heart chambers or frequent ventricular arrhythmias.  
While diagnostic criteria may vary from center to center, the largest ongoing study is being performed by Dr. Michael O'Grady at the University of Guelph, and his criteria are the best defined and studied to date.  
To diagnose occult DCM, Dr. O'Grady requires the following echocardiographic findings, measured in the parasternal long axis view/  A fractional shortening of less than 20% with the following left ventricular chamber measurements: greater than 49 mm at the end of diastole (when the ventricle is completely filled), greater than 42 mm at the end of systole (when the ventricle has completed emptying).  
This diagnosis is collaborated by a progression of 3-5 mm since a prior examination.  
Alternatively, the diagnosis can be suspected by frequent ventricular arrhythmia – the criteria currently in use by Dr. O'Grady is greater than 50 PVCs (premature ventricular contractions) per hour (1200+ per 24 hr holter), however this study is on-going.  This is measured by a Holter monitor, which is and ECG running continuously for a 24 hour period. ​

Duer et al. ( 4), who examined 50 young dogs of different breeds, reported the occurrence of II° atrio-ventricular block in one dog only. In another study performed on 44 Doberman Pinscher dogs, the only observed disturbance was a wandering sinus pacemaker in one dog (5). In 16/51 dogs (31.4%) no other arrhythmias were observed, similar to other authors’ findings ( 4- 6, 12, 17). 

In conclusion, the mean 24-h heart rate frequency in healthy dogs is 100 beats per minute (range 80-120 beats/min), the mean maximum heart rate frequency is 210 beats per minute (range 180-240 beats/min), and the mean minimum heart rate frequency is 43 beats per minute (range 33-53 beats/min). 

Healthy dogs may manifest single premature supraventricular beats up to 1,500/24 h, and single premature ventricular beats not exceeding 100/24 h. Furthermore, RR pauses not longer than 3 s may occur (Table 2). No impact of breed, age or sex on the analysed parameters was noted. ​