Review of ECG and Holter monitoring findings in 27 horses with atrial fibrillation

Purpose of the work. With a prevalence of about 2.5% (Else and Holmes 1971) atrial fibrillation (AF) is the most important dysrhythmia affecting performance in horses (Verheyen et al. 2013). Most horses with AF have little or no underlying cardiac disease and this condition is called lone AF. In this case the horses usually have normal resting heart rates (<44 bpm), with an irregularly irregular rhythm and no S4 is produced (Reef and Marr 2010). The presence of resting tachycardia in a horse with AF should suggest the presence of underlying cardiac lesions or a disorder that increases sympathetic tone such as pain, anemia, fever or infection (Bonagura et al. 2010). Aim of the present report is to present the results of electrocardiographic evaluation (ECG) and 24 Holter monitoring (HM) pre and post treatment in 27 horses with a diagnosis of AF. Materials and used methods. The horses had a history of poor-performance (6 horses), cardiovascular disorders (19 horses), exercise induced pulmonary hemorrhage (1 horse) and collapse (1 horse). The symptoms were present for less than 1 month in 17 cases and for longer than 1 month in 10 horses. The horses were 16 trotters and 11 warmblood; 10 males, 10 females and 7 geldings with a mean body weight of 497,11±61,3 kg and a mean age of 8±6 years. All horses underwent a thorough diagnostic protocol including ECG and, only for horses treated, a 24 hours HM before and after treatment. According with the results obtained by the clinical, instrumental and laboratory examination 17 horses were treated for AF (AFt), 9 horses were not treated (NT) and 1 horse showed spontaneous resolution. In AFt horses, 22 mg/kg quinidine sulfate (QS) have been used by nasogastric tube every 2 hours until cardioversion (CV) in 15 cases; 1 horse, since QS was ineffective, received amiodarone cloridrate (Am) intravenously (De Clercq et al. 2006) until CV, and 1 AFt case received Am first and then QS. Outcomes. All horses had normal body temperature (37,8±0,3 °C); on pulse palpation 12 horses had reduced intensity and amplitude and all had irregular rhythm. Heart auscultation revealed an irregularly irregular rhythm in all horses and cardiac murmurs in 10 horses. Ambulatory ECG confirmed the AF presence and showed a mean heart rate of 43±10 bpm in AFt group and of 49±13 bpm in NT group. 24 hours HM before treatment showed a mean heart rate of 44±10 bpm; rare ventricular premature complexes (VPC) in 1 horse and frequent VPC in another one. In the 15 QS treated horses CV was achieved after different number of treatments: 2 doses in 1 case, 3 doses in 4 cases, 4 doses in 2 cases, 5 doses in 1 case and 6 doses in 5 cases, in 2 horses a double QS treatment at 1 week interval was used and CV was obtained after a total of 6+5 and 6+6 doses. In the QS+Am case CV was obtained after 26 hours of Am administration. In the Am+QS case Am was discontinued due to signs of toxicity and a subsequent QS administration was ineffective. HM for 24 hours after treatment showed a mean heart rate of 42±4 bpm; rare APC in 6 horses; sinus tachycardia in 2 horses (mean 51±5 bpm); 2nd degree atrioventricular blocks in 3 horses; permanent AF in 1 horse. Conclusions. In our review the mean heart rate in NT group was higher than in AFt group because, according to the literature, the first one was primary composed by horses with important structural cardiac disease. In contrast to that reported in the literature, AFt group has mean resting rate >44 bpm, and this finding could suggest an increased ventricular response to AF. The reduced heart rate observed during HM after treatment in our cases could be determined by the correction of the functional and electrical remodeling induced by AF. In fact, AF leads to left atrial mechanical dysfunction (Schwarzwald et al. 2007, De Clercq et al. 2008) and over time, electrical remodelling associated with a progressive shortening of the atrial effective refractory period, attenuation of the atrial refractory period rate adaptation and decrease in the AF cycle length (De Clercq et al. 2008). Finally, HM performed after CV is particularly useful to identify possible arrhythmia, such as APC, which have to be considered of prognostic value.