Digoxin is the most commonly used cardiac glycoside [1]. It is a well-tolerated and inexpensive drug which has been in wide use for many years as a treatment for heart failure and arrhythmia. Although a number of sophisticated management options and new therapeutic agents have been developed in recent years for the treatment of patients with heart failure, digoxin is still one of the most frequently prescribed drugs and remains the first choice of treatment [2–4].

In equidae, digoxin is used in the treatment of naturally-acquired heart failure [5]. Treatment with digoxin results in an increase in cardiac contractility and a decrease in heart rate, with increased myocardial oxygen consumption, increased cardiac output and decreased cardiac size [6]. The improvement in cardiac output promotes diuresis and the reduction and elimination of oedema. The half-life of digoxin in the horse is 17–23 h [7]. The proposed therapeutic, nontoxic range of digoxin in horses is calculated as follows: IV loading 14, IV maintenance 7, oral loading 70, and oral maintenance 35 μg/kg/24 h [7].

Cardiac troponins, especially cardiac troponin I (cTnI), are highly sensitive and specific markers of myocardial injury in veterinary medicine [8–11]. In horses, cTnI is elevated in animals undergoing training and endurance exercise [12–14], horses with cardiac diseases [15], horses with muscular disease [16], and in horses with monensin intoxication [11]. Measurement of cTnI concentrations has also been used recently for prognostication in horses undergoing emergency abdominal surgery [17].

The effect of digoxin administration on the serum concentration of cTnI has not been reported to date in equidae. The present study was therefore designed to investigate the influence of the cardiac glycoside digoxin on cardiac cell damage as assessed by cTnI, using donkeys (Equus asinus) as an equine model. The second objective was to investigate the effects of digoxin injection on the acid–base and electrolyte balance and on the haematobiochemical profiles in donkeys.

To the authors’ knowledge, this is the first study to evaluate the effect of digoxin on cardiac cell damage as assessed by cTnI, acid–base and electrolyte balance and the haematobiochemical profiles in donkeys (Equus asinus) as an equine model. Before injecting digoxin, the mean serum cTnI in the donkeys was 0.004 ng/mL, a value ‘below detection limit’ of the assay and below established normal values in healthy horses (0.06-0.1 ng/mL) using this assay [11, 12, 14, 22]. However, we don’t know for sure whether we can apply the normal values of horses for donkeys. At no time point after injecting digoxin was cTnI significantly different compared to pre-injection values (P > 0.05).

Cardiac troponin I is a protein found in the myocardial cells. Its serum concentration elevates after acute myocardial injury because of leakage from the damaged myocardial cells [23, 24]. It is the ‘gold standard’ for the non-invasive diagnosis of myocardial injury in humans [25, 26]. Similarly, in animals cTnI has a high sensitivity and specificity in patients with primary or secondary cardiac disease [8, 24]. Persistently increased cTnI blood concentration suggests ongoing active and irreversible damage to cardiomyocytes [8, 24]. The degree of increase has been shown to be correlated with the extent of myocardial damage and with survival in human [27] and animal [28].

In humans, the therapeutic and toxic plasma concentrations of digoxin are usually set at 0.8 - 1.6 ng/mL and greater than 2.4 ng/mL, respectively [29]. Similar numbers have been derived for animals: e.g., digoxin plasma concentrations of 0.5 - 2.0 ng/mL are nontoxic in horses [7] and a 2.3 ng/mL concentration is nontoxic in cats [30]. In dogs, plasma digoxin concentrations up to 2.5 ng/mL are nontoxic, but concentrations of 2.5 - 3.0 ng/mL are associated with increased probability of toxicosis [31]. In donkeys, however, the therapeutic and toxic plasma concentrations of digoxin have not been reported in the literature.

Digitalis intoxication is characterised by several clinical signs varying from mild gastrointestinal upset (anorexia, vomiting, and diarrhoea) to chronic weight loss and life-threatening arrhythmias [32, 33]. Increased serum BUN and creatinine are interpreted as evidence that digoxin toxicity has compromised renal function. In this study, serum digoxin concentration had increased in the donkeys at T1 to 4.08 ± 0.12 ng/mL compared to 0.19 ± 0.02 ng/mL pre-injection. Thereafter, digoxin concentration decreased steadily to reach 1.83 ± 0.40 ng/mL 4 h post-injection. During that time and up to 72 h post-injection, none of the donkeys showed signs of digoxin intoxication, cardiac arrhythmias, ECG abnormalities or elevated serum BUN and creatinine. Therefore, it can be assumed that the toxic serum digoxin concentration in donkeys would be higher than in humans, horses, dogs and cats. Thus, another study is needed to verify the toxic serum digoxin concentration in donkeys.

The metabolic alkalosis observed 3 h after digoxin injection may have been due to decreases in PCO₂ and lactate. The decreased PCO₂ and the increased PO₂ and SO₂ after injection may have resulted from the increased respiratory rates. Lactate is known as the end product of anaerobic glycolysis, a pathway that is of key importance during normal metabolic and athletic events. Aside being a waste product during high intensity exercise, lactate is also a valuable substrate that significantly contributes to the energy production of heart, liver, kidneys, non-contracting muscles and even brain [34]. In the present study, lactate concentration decreased during the 4 h (T1-T4) post-injection of digoxin, and this may have been caused by the decrease in the metabolic index due to the inactivity of the animals during these time points. Similarly, in healthy humans, lactate concentration was reported to be stable following digoxin infusion [35]. Concerning the biochemical variables, hyperglycaemia was found in the donkeys after digoxin administration. In human medicine, it is well-known that digoxin therapy increases the adrenaline-induced, and attenuates the insulin-induced hypoglycaemia due to catecholamines release in the acute stage of the action of digitalis glycosides [36].

This study has its own limitations. One of these limitations is the small sample size; therefore the results of this study are preliminary and should be interpreted with caution. Another limitation of this study is the using of healthy rather than diseased donkeys. Braunwald [37] had reported that there is a direct correlation between the level of myocardial impairment and the effectiveness of digoxin therapy on cardiac function. Although the i- STAT^® cTnI assay was initially designed for use in humans, there is good reason to anticipate cross reactivity based on the highly conserved nature of the cardiac troponins across mammalian species [38–40]. While it may not be practical in veterinary medicine to develop species-specific assays for all relevant species, this practical assumption should come with an acknowledgement of less than perfect validation of this assay. In this study, we thought that the i- STAT^® cTnI assay would detect the donkeys’ cTnI, because, despite the lack of a known donkey cTnI amino-acid sequence, a third limitation of this study, the sequence of cTnI is highly conserved among all species. In addition, the i- STAT^® analyser has been clinically validated against reference analysers in other species [11, 41, 42]. In addition, the high homology of amino-acid sequences between mammals within the sequence regions detected by cTnI analysers allows veterinarians to measure cTnI concentrations in many animal species with the same equipment as has been developed for humans, rather than necessitating the development of species-specific technology. As all the troponin samples in this study were below the detection level of the test, therefore the assay may not have had sufficient sensitively to detect small changes in cTnI levels below the detection level. These small elevations may still be of clinical relevance.

Although the serum digoxin concentrations had increased in the donkeys to 4.08 ± 0.12 ng/mL 1 h after injection, the donkeys showed no sign of digoxin intoxication, ECG abnormalities or elevated serum BUN and creatinine. It is therefore assumed that in donkeys, the toxic serum digoxin concentration is higher than in humans, horses, dogs and cats [7, 29–33]. Consequently, another study is warranted to verify the toxic serum digoxin concentration in healthy donkeys and in those with cardiac diseases.

^aLanoxin, GlaxoSmithKline S.P.A, Parma, Italy.

^bKenz Cardico 302, Suzuken Co., Ltd., Japan.

^cVetScan i-STAT 1, Abaxis, California, USA.

^dVetScan HM5, Abaxis, California, USA.

^eVetScan VS2, Abaxis, California, USA.

^fRoche Diagnostics, Indianapolis, Indiana, USA.