dronedarone is a relatively new antiarrhythmic drug and is held to be less proarrhythmic than comparable compounds , although its proarrhythmia potential in humans has not been sufficiently evaluated . we describe a so far unreported dronedarone effect , namely a significant alteration of both the morphology and the duration of the qrs complex on electrocardiogram ( ecg ) in a 41-year old patient with symptomatic paroxysmal atrial fibrillation . dronedarone is a derivate of the potent antiarrhythmic drug amiodarone that was designed to have less toxic side effects ( 1 ) . although dronedarone was approved following reports of an improved safety profile , more recent studies have cast doubt on its safety ( 1 - 2 - 3 ) . however , dronedarone is still held to be less proarrhythmic than comparable compounds , although its proarrhythmia potential in humans has not been sufficiently evaluated ( 1 ) . we describe a hitherto unreported dronedarone effect , namely a significant alteration of both the morphology and the duration of the qrs complex on ecg . a 41-year old patient was admitted to our emergency unit because of a highly symptomatic attack of paroxysmal atrial fibrillation ( af ) that a few hours later spontaneously converted into sinus rhythm . atrial fibrillation had been first diagnosed in this patent in 2008 and the af episodes had been becoming more frequent in the months immediately preceding hospital admission . on admission , it should be noted that , during sinus rhythm , the ecg was normal showing an unremarkable qrs morphology and a qrs duration of 85 msec ( fig . clinically , the patient was in a good state of health . his risk profile consisted of grade 1 obesity ( body mass index 30.9 kg / m ) and arterial hypertension that was well controlled by a combination therapy of an ace - inhibitor and a beta - blocker . routine blood tests , including kidney , liver and thyroid function analysis , were all within normal values . coronary artery disease had been previously excluded by coronary angiography in 2011 , and a recently performed transthoracic echocardiography revealed a structurally normal heart exhibiting normal systolic and diastolic function . after giving informed consent , the patient was reluctant to undergo an invasive approach and preferred pharmacological rhythm management with dronedarone . according to our standard approach for af patients admitted to our institution , the initiation of antiarrhythmic therapy , regardless of the antiarrhythmic drug , is always performed in the hospital setting . during the hospital stay , the patients are not restricted to bed rest but are allowed to be ambulant while being continuously monitored using ecg telemetry . in addition , 12-lead standard ecg is registered in each patient on a daily basis . in the index patient , after two doses of dronedarone ( 400 mg each ) , a marked prolongation of qrs duration with a left bundle branch block pattern was documented on the 12-lead ecg ( fig . , the qrs complexes again showed a normal morphology and a normal duration of 90 msec ( fig . subsequently , the patient refused further attempts at pharmacological rhythm control , but requested a catheter ablation of atrial fibrillation that had recurred . therefore , circumferential pulmonary vein isolation was successfully performed . return to normal qrs - width after dronedarone withdrawal and self - limiting episode of atrial fibrillation . three months later , at the first follow - up visit in our rhythmology outpatient clinic , the patient was free of af and continued to show a narrow qrs - complex . dronedarone is a multichannel blocker exhibiting all of the 4 vaughan williams classes of action ( 4 ) . in fact , qt - interval prolongation , increased frequency of ventricular premature beats , and episodes of atrial flutter have all been reported as potentially proarrhythmic side effects of dronedarone ( 2 , 3 ) . mechanistically , this phenomenon may be explained by a reduction in the maximum slope of the action potential upstroke due to an increased inhibition of the fast na(+ ) inward current by dronedarone ( 4 , 5 ) . thus , a widened qrs complex may be indicative of slowed impulse conduction in the ventricles and may herald potential proarrhythmia . therefore , careful telemetry monitoring appears to be mandatory , particularly during the period following the first administration of dronedarone .
dronedarone is a relatively new antiarrhythmic drug and is held to be less proarrhythmic than comparable compounds , although its proarrhythmia potential in humans has not been sufficiently evaluated . 
 we describe a so far unreported dronedarone effect , namely a significant alteration of both the morphology and the duration of the qrs complex on the electro cardiogram in a 41-year old patient with symptomatic paroxysmal atrial fibrillation .