Holter monitoring (24-h Electrocardiography) of Holstein Calves

Abstract

Background: Twenty-four h electrocardiographic monitoring is a noninvasive method of assessing cardiac rhythm. Holter monitoring in farm animals can help assessing heart rate variability and its relationship with stress and production. Several authors have reviewed the normal cardiac rhythm of bovines, but there is little information on heart rhythm in calves. The goal of this research is to elucidate which cardiac rhythms may be considered physiological in Holstein calves, from 3 to 6 months old. Materials, Methods & Results: Twenty-four h ambulatory electrocardiography (Holter monitoring) was performed in 10 male Holstein calves, with ages ranging from 3 to 6 months old. The animals came from private farms in São Paulo state and were housed in a semi-open stall during the study. The animals had their hair clipped from the third to the fifth intercostal spaces on both sides of the chest, from the sternal region to the glenohumeral joint, and seven electrodes were positioned in a diagonal configuration. Decoding of the recordings was performed using specialized software and in order to minimize digital mistakes, all of the beats marked as ectopic by the software were manually revised by the authors. Mean heart rate was 83.2 ± 11.06 beats per min (bpm), maximum heart rate was 147.1 ± 11.05 bpm, and minimum heart rate was 53.7 ± 7.45 bpm. In 90% of the calves, normal sinus rhythm was predominant, whereas in 10%, sinus arrhythmia (SA) was prevalent. Second degree atrioventricular blocks (AV blocks) were observed in 30% of the animals and supraventricular premature beats (SPB) were detected in 50%. Holter monitoring was repeated after three months in three of the animals; two showed no arrhythmias at the second exam, and in the other, the frequency of AV blocks was decreased by 88.64%. Discussion: In the present study, the authors observed a relatively high incidence of arrhythmias in Holstein calves, although there is little information available in the literature for comparison. The use of Holter monitoring is much more sensitive in the assessment of arrhythmias than a standard electrocardiogram, since it records the cardiac rhythm for 24 to 48 h, as opposed to only 2 to 3 min. Therefore, it is possible that the arrhythmias found in the calves in the current study were physiological events otherwise unidentified by conventional electrocardiogram. The AV blocks observed in the animals of the present study were intermittent and apparently non-related to any particular event or situation, so it was not possible to demonstrate whether they disappeared after exercise. As for the supraventricular ectopic beats, they are frequent in older bovines, mainly in dairy cows, and are usually related to increased vagal tone, stress, hormonal changes, ectopic atrial focus, and peak lactation volume overload. The animals used in the present study were otherwise healthy, and did not demonstrate any clinical signs of gastrointestinal dysfunction or other disease that might have justified an increase in their vagal tone. In addition, the decrease in the number of rhythm abnormalities in the older calves that were submitted to a second Holter exam supports the hypothesis that supraventricular ectopic beats and second degree AV blocks of Mobitz type II may be normal in calves up to six months old.