Defibrillation and Cardioversion

THERAPEUTIC PROCEDURES IN THE EMERGENCY DEPARTMENT PATIENT WITH ACUTE MYOCARDIAL INFARCTION

Life-threatening cardiac arrhythmias and other peri-infarct complications are often unexpected and commonly present with little warning. The therapeutic procedures reviewed often require immediate implementation and should be second nature to any physician involved in the management of patients with an AMI.

Effect of application of force to self-adhesive defibrillator pads on transthoracic electrical impedance and countershock success

STUDY OBJECTIVE

To examine the effect of the application of force to self-adhesive defibrillator pads on transthoracic electrical impedance and countershock success.

METHODS

A prospective, randomized, controlled pilot study was carried out in an experimental animal laboratory, involving 32 mixed-breed swine weighing 36.5 to 55.7 kg each. Ventricular fibrillation (VF) was induced, and the animals were randomly allocated to 1 of 4 groups, with 8 animals per group. Animals in groups I and II remained in VF for 30 seconds; those in groups III and IV remained in VF for 5 minutes. At the end of the VF period, up to 3 countershocks were given. In groups I and III, countershocks were delivered through the self-adhesive defibrillator pads alone; in groups II and IV, they were delivered through the defibrillator pads with 25 lb of applied force. Any animal without return of spontaneous circulation after 3 countershocks was given epinephrine after 1 minute of CPR, followed by 1 additional minute of CPR and 1 additional countershock if required.

RESULTS

The main measurements included baseline and countershock transthoracic impedance, cumulative countershock success rate, and 30-minute survival rate. Application of 25 lb of force to the defibrillator pads significantly decreased transthoracic impedance, compared with baseline values (group II, 15. 1% decrease; group IV, 16.1% decrease). The first-shock success rate was higher when force was applied during the countershock (87.5% versus 50% for groups II and I, respectively; 62.5% and 37.5% for groups IV and III, respectively). In the animals who experienced 5 minutes of VF, there were greater rates of success after the second, third, and fourth countershocks when force was applied (group IV) than when no force was applied (group III). Groups I and II (VF for 30 seconds) demonstrated 100% survival at 30 minutes. Group IV had a higher 30-minute survival rate (3/8 animals) than did group III (1/8). However, the rates of countershock success and 30-minute survival were not statistically different among the groups.

CONCLUSION

Application of force to self-adhesive defibrillator pads decreases transthoracic impedance during countershock. This effect may contribute to improving the countershock success rate.

Anaesthesia for cardioversion

Cardioversion is a minor procedure requiring sedation and analgesia. However, it is often performed out-of-hours in remote sites by inexperienced anaesthetists. An understanding is required both of the pathophysiology underlying cardiac arrhythmias and of the technical side of defibrillation equipment, including electrical safety. Patients should have their coagulation status and electrolyte balance checked prior to the procedure to reduce the likelihood of complications. Almost all the available anaesthetic agents have been used for cardioversion in the past, with varying degrees of success. The anaesthetic agent chosen for patients undergoing cardioversion must provide analgesia and sedation, cause the least cardiovascular compromise possible and still enable rapid recovery. Propofol may be the closest anaesthetic agent to this ideal currently available, although careful titration of any agent chosen is also important. Cardioversion may be performed as an emergency, including in the pregnant patient, providing safe anaesthetic practice is followed.