Echocardiographic and serum biochemistry abnormalities in 17 horses with atrial fibrillation successfully treated

Purpose of the work. Atrial fibrillation (AF) is the most common pathological cardiac arrhythmia in horses (de Clercq et al. 2007) and typically is associated with exercise intolerance at higher levels of performance (Reef et al. 1995). High resting vagal tone, large atrial mass and transient potassium (K+) depletion are predisposing factors in the development of AF in horses without structural heart disease (Reef and Marr 2010). Horses with AF usually present normal laboratory evaluation (LE), but reduced level of serum K+, fractional excretion (FE) of K+ or red blood cells (RBC) K+ could be found. In horses without structural heart disease echocardiographic evaluation (ECC) is usually normal but it is not uncommon to find a slightly reduced left ventricular shortening fraction (LVSF) that return within normal limits after treatment (Bonagura et al. 2010). Aim of the present study is to present the results of LE and ECC pre and post cardioversion (CV) in 17 horses with AF. Materials and used methods. The horses had an history of poor performance (6 cases) and cardiovascular disorders (11 cases). The symptoms were present for less than 1 month in 16 horses and for longer in 1 horse. The horses were 14 standardbreds and 3 warmblood; 7 males, 7 females and 3 geldings with a mean age of 5±3 years. All horses underwent a thorough diagnostic protocol including a complete LE (haematology, complete blood biochemistry and evaluation of electrolytes FE) and ECC before and 24 hours after treatment. CV was obtained in 15 horses with 22 mg/kg quinidine sulfate (QS) by nasogastric tube every 2 hours for a maximum of 6 doses or until CV (13/15 horses); 2 horses received two times this protocol, with an interval of one week, because the first time it was ineffective; in 1 horse, since QS was ineffective, amiodarone cloridrate (Am) was administered intravenously (De Clercq et al. 2006) until CV and in 1 horse spontaneous resolution of AF was recorded. Outcomes. All horses had normal body temperature (37,7±0,3 °C); on pulse palpation 11 had reduced intensity and/or amplitude and all had irregular rhythm. In all cases, heart auscultation revealed an irregularly irregular rhythm and murmurs were detected in 3 horses. The ECG confirmed the arrhythmia and a diagnosis of FA was made. The only alterations detected by ECC before treatment were reduced LVSF in 6 horses (from 21% to 29%) and one or more valvular insufficiencies (VI) of different gravity in 12 horses (10 grade I/IV, 2 grade II/IV, 1 grade III/IV). LE showed one or multiple electrolyte alterations such as a reduced K+ concentration ([K]) in 5 horses, an increased calcium concentration ([Ca]) in 6 horses, an increased magnesium concentration ([Mg]) in 2, a decreased [Mg] in 1 horse; a decreased phosphorus concentration ([P]) in 7 and an increased [P] in 1 horse. FE showed increased excretion of K+ (FEK) in 4 horses associated to increased FE of sodium in 1 of them, and a decrease of FEK in 2 cases. ECC after treatment showed a maintained low LVSF (24%) in 1 horse and one or more VI of different gravity in 8 horses (6 grade I/IV, 2 grade II/IV). In the 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. Two horses received a double QS treatment at 1 week interval 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. Conclusions. In our cases AF was present in 16/17 horses for less than one month. The improvement of the LVSF and the reduced incidence of VI after treatment showed by ECC could be explained by a possible correction of the atrial structural remodeling possibly induced by AF (Schwarzwald et al. 2007, De Clercq et al. 2008). Furthermore, the high rate of LVSF normalization after treatment could be due to the short period of AF presence in our horses which can limit the anatomical remodeling. According to the possible role of the reduction of and transient electrolyte disturbances in the AF pathogenesis, we found a moderate decrease of [K] in only 5/17 horses, an increased of the FE of K+ in 4/17 and variation in blood [Ca] and/or [Mg] and/or [P] in 11 horses. Non other significant abnormalities were found on LE. In conclusion we would like to underline the importance of a complete diagnostic protocol that allow to recognize all the predisposing factors for AF. Moreover, the application of the more appropriate treatment as quickly as possible in AF affected horses could reduce the atrial remodeling effects of AF and could allow to obtain a better prognosis after treatment.