Driving and ICD Shocks for Ventricular Arrhythmias

Abstract & Commentary

By John P. DiMarco, MD, PhD

Source: Albert CM, et al. Driving and implantable cardioverter-defibrillator shocks for ventricular arrhythmias: Results from the TOVA study. J Am Coll Cardiol. 2007;50:2233-2240.

The triggers of ventricular arrhythmia (tova Study) was a multicenter, prospective, cohort study that examined lifestyle and psychological triggers that might result in implantable cardioverter-defibrillator (ICD) shocks for ventricular tachycardia (VT) or ventricular fibrillation (VF). The study enrolled 1188 patients who had ICDs for either primary or secondary prevention indications. At entry, and during follow-up exams, patients were asked about their driving experience. They were asked whether or not they drove a car, and to estimate how long a period they drove each week. Data on ICD discharges were collected at each study visit, and each patient was asked to report ICD shocks within 72 hours of the event. A standard post shock set of questions included data about time from last exposure to driving. Stored electrograms from shock episodes were retrieved, analyzed, and the shocks classified as either appropriate or inappropriate shocks. Using a case cross-over analysis, Albert and colleagues computed an incidence rate ratio for the association between driving and episodes of VT or VF. In this study, the period of interest included the driving time and the two subsequent 30-minute periods.

The majority of patients (80%) in TOVA reported that they drove a car at least once per week. Driving was more common among younger and better educated patients. Patients who drove were more likely to be men or Caucasian and were less likely to have diabetes, hypertension, congestive heart failure, or a left ventricular ejection fraction less than 30%. Patients with recent ICD implants drove less frequently, but even among those with recent implants, 75% reported driving at least once per week and 39% were driving more than twice per day.

During a median follow-up of 562 days, there were 324 ICD shocks for VT or VF among the 1188 patients in the study, for a shock frequency rate of 1 per 56,260 patient hours. A post-shock interview was completed for 66% of these episodes. Since some patients had multiple shocks in a single episode, there were 193 ICD shock episodes included in the final analysis. Of these, 44 shock episodes among 23 patients occurred either during, or within one hour of, driving a car. The relative risk of an ICD shock for VT or VF associated with driving was 2.24 compared to all other times during the study. Despite this increase in relative risk, the absolute risk was still low (1 episode per 25,116 person hours). The increased risk associated with driving was, however, not evident during driving itself. The highest risk time period was the 30 minutes immediately after driving. Of the 44 ICD shocks that occurred within one hour of driving, only 7 occurred while driving, 30 occurred in the 30 minutes immediately after driving, and 7 occurred in the following 30 minutes. Further examination of activities in the 30 minutes after driving revealed a possible relationship with physical exertion and anger. Examination of potential modifiers of the risk for ICD shock associated with driving was not revealing. Importantly, there was no evidence that beta blocker use significantly modified risk.

Albert et al go on to discuss potential causes for the increased risk of ICD shocks in the period immediately after driving. They cite other studies showing that exposure to particulate matter alters autonomic tone, and they postulate that this may contribute to the risks seen.

Albert et al conclude that although the risk for ICD shocks during driving is low, the period immediately after driving is associated with increased frequency of ICD shock delivery, and that the mechanism for this increased in arrhythmia risk is unknown.


The American Heart Association and the Heart Rhythm Society have recently revised their recommendations for driving done by patients with implantable defibrillators (Circulation. 2007;115:1170-1176). For patients with a history of sustained VT or VF, in whom the ICD is used for secondary prevention, the guidelines recommend that driving be restricted for 6 months after each episode. For patients who receive their ICD for primary prevention indications, the recommendation is to not restrict driving unless they develop a spontaneous arrhythmia that requires ICD therapy. At that point, the restrictions would be the same as in the secondary prevention group. Despite these guidelines, many patients regard driving as a necessity, and other studies have reported that 70%-80% of patients resume driving, with medical approval. A similar pattern was noted in the TOVA trial, which is the first study that has systematically evaluated associations between driving and ICD shock episodes. Fortunately, although there is a relationship between ICD shocks and driving, the increase in risk was low and did not occur during the act of driving itself. Why shocks should occur with an increased frequency in the period immediately after driving remains uncertain. Although the TOVA investigators describe possible relationships to anger, exertion, and changes in autonomic tone, the lack of influence of beta blocking therapy suggests that other, as yet unknown, factors may be involved.