1 Belfast and 2 London, UK
EditorWe read Miranda and Newtons1 case report of Successful defibrillation in the prone position with interest. We wish to add some important recommendations to those made in the discussion. As the authors emphasize, the primary objective for the treatment of ventricular fibrillation (VF) is early defibrillation. The probability of successful defibrillation and subsequent survival to hospital discharge is inversely related to the time interval between onset of VF and delivery of the first shock.2 When ventricular fibrillation occurs in the prone position, whether in the operating theatre or the intensive care unit, there are both theoretical concerns and practical strategies that may improve the success of defibrillation therapy, if applied clinically.
Defibrillation is accomplished by passage of sufficient electric current (amperes) through the heart. The energy chosen (joules) and the transthoracic impedance (ohms), or resistance to current flow, determine the current flow. Factors that determine transthoracic impedance include energy selected, electrode size, paddleskin coupling material, number of and time interval between previous shocks, phase of ventilation, distance between electrodes (size of the chest), and paddle electrode pressure.2
In theory, electrode pads (or paddles) should be positioned to minimize transthoracic impedance and incorporate as much of the fibrillating myocardium between the defibrillating electrodes as possible. This was the rationale used by Lown3 to advocate the anteriorposterior positions for transthoracic cardioversion of atrial fibrillation. Thus, a critical mass of myocardium is more likely to be defibrillated. In the prone position the heart is displaced anteriorly, and positioning electrodes in the left posterior axilla and below the right scapula may not provide an adequate vector for shock delivery. Furthermore, the transthoracic impedance will be greatly increased, especially during positive pressure ventilation with positive end-expiratory pressure.
Whilst the prone patient is not easily moved to the supine position, especially in the neurosurgical setting, it may be relatively simple to manoeuvre the patient sufficiently to allow application of electrode pads (as opposed to defibrillator paddles) to the standard apical and right infraclavicular regions. This will allow a shock to be delivered across a more appropriate vector. If this is not possible, defibrillation with an impedance compensating device (now commonplace in cardiology departments), which delivers a truncated exponential biphasic shock, would be the best option for the suboptimal prone position. Indeed, even in the out-of-hospital cardiac arrest scenario, Schneider and colleagues4 demonstrated three sequential 150 J shocks to be 98% successful in electrical cardioversion of VF using standard electrode positioning with an impedance compensated, biphasic waveform (96% were successfully defibrillated with the first shock). Defibrillation should be carried out in expiration without positive end-expiratory pressure, to further minimize transthoracic impedance. We propose that consideration should be given to the use of these strategies and also to the availability, in close proximity to prone patients, of both electrode pads in addition to standard defibrillation paddles and the modern generation of impedance-compensating defibrillators. It may also be prudent to attach electrode pads to intensive care patients, who are at a high risk of ventricular arrhythmias, before turning them prone. The eventual strategy employed by the attending physician for the treatment of VF in the prone position will be a balance between the time taken to achieve optimal defibrillation and the concerns discussed above.
S. J. Walsh
A. Bedi
Belfast, UK
EditorThank you for the opportunity to reply to Bedi and Walsh. They propose several strategies which may improve the chances of successful defibrillation in the prone position. We agree that, in the prone position, electrode pads may be easier to apply than defibrillator paddles to the standard apical and right infraclavicular regions. In the situation we described, however, electrode pads were not available. We also agree that the timing of defibrillation in relation to respiration, as well as the use of impedance compensating devices, would improve the chances of successful defibrillation by reducing thoracic impedance.
C. Miranda
London, UK
References
1 Miranda CC, Newton MC. Successful defibrillation in the prone position. Br J Anaesth 2001; 87: 9378
2 ECC Guidelines: Part 6: Advanced Cardiovascular Life Support: Section 2: Defibrillation. Circulation 2000; 102 [Suppl I]: I-86I-89
3 Lown B. Electrical reversion of cardiac arrhythmias. Br Heart J 1967; 29: 46989
4 Schneider T, Martens PR, Hans P, et al. Multicenter, randomized, controlled trial of 150-J biphasic shocks compared with 200- to 360-J monophasic shocks in the resuscitation of out-of-hospital cardiac arrest victims. Circulation 2000; 102: 17807