Factors that impact on a successful cardiac defibrillation

• Transthoracic impedance

• Energy requirements

• Biphasic waveform defibrillation

The aim of defibrillation is to interrupt the chaotic pattern of ventricular impulses and restore normal conduction across the heart by delivering a low electrical current (Harris 2002). Direct current (DC) defibrillation in the early stages of VF depolarises the whole of the myocardial muscle and conducting system. Post-shock myocardial cells will enter a period of recovery, reset themselves and allow the sinoatrial node to restore sinus rhythm activity and produce organised myocardial fibre contraction (Cooper et al. 1998, Harris 2002).

Transthoracic impedance

Transthoracic impedance (TTI) plays a part in the level of cardiac response to a defibrillatory shock. Transthoracic impedance is defined as the resistance to current flow through chest wall, lung tissue and myocardium; hence the greater the resistance, the less the flow of current (Cooper et al. 1998). There are a number of practical means by which TTI can be reduced.

• Phase of respiration - shocks should be delivered at the end of expiration when there is less air in the lungs, thus reducing the distance between electrodes and heart. Patients should be disconnected from the ventilator or bagging circuit during defibrillation (Deakin et al. 1998).

• Paddle pressure - paddles must be firmly pressed against the chest as this reduces TTI and will ensure good contact; 10-12 kg of pressure per paddle should be applied (Resuscitation Council UK 2000a).

• Paddle surface area - increased paddle size lowers impedance (Jevon 2002).

• Defibrillation gel pads - protect skin from superficial burns, help to reduce impedance between pads and skin and enhance conduction. These should be replaced after 3-6 shocks or if they appear dry (Inwood & Cull 1997).

• Interval between shocks - impedance is reduced if shocks are delivered in close succession (Cooper et al. 1998). The first three shocks in the unresponsive patient should be given within 45 seconds.

• Positioning of paddles - it is important to ensure that the paddles are placed correctly (Heames et al. 2001, Mattei et al. 2002) as this facilitates maximal depolarisation of a critical mass of myocardium.

—Apex-anterior position - is the commonly used paddle placement for defibrillation and cardioversion. The 'sternal' paddle is placed to the right of the sternum, below the right clavicle. The 'apex' paddle must lie to the left of the chest, level with the fifth intercostal space in the zone of V5 and V6 (see Figures 9.1 and 9.2a).

—Anterior-posterior position - this approach is not easy, particularly if the patient is large. It should only be used when defibrillation with standard paddle technique has been ineffective or if the patient has a internal pacemaker or cardiac defibrillator. The patient must be placed on their right side. The anterior pad should be placed to the left of the sternal border and the posterior pad should be situated just below the left scapula.

—Under breast/lateral to breast - applicable when defibrillating women with large breasts. Pagan-Carlo et al. (1996) suggest that placement of the apex paddle under the breast or left lateral to the breast is preferable as it helps decrease thoracic impedance and raises current flow (as shown in Figure 9.2b). Paddles must never be placed over a nipple, clavicle or sternum or below the bottom rib.

—Special considerations - for patients with a left-sided internal pacemaker, the paddles should be at least 12-15 cm away from the unit (Resuscitation Council UK 2000a). If pacemaker is on the right, adopt the anterior-posterior position.

Energy requirements

The current standard practice for initial shock energy levels is 200 J, followed by a further 200J. If there is no response, subsequent shocks should be at 360 J. However, should there be an episode of spontaneous circulation followed by a recurrence of VT/VF, then defibrillation should recommence with 200J (see Chapter 6). It is important to deliver the shocks within 30-45 sec; this is possible with modern defibrillators as they recharge rapidly. The introduction of biphasic waveforms allows for energies of less than 200J to be used which produce equal outcomes as standard monophasic waveforms that are typical of manual defibrillators (Wanchun et al. 2000).

Sternal gel pad (must be below the right clavicle)

Apical gel pad (located below 5th intercostal rib to the left of the thorax)

Fig. 9.1 Positioning of defibrillation gel pads.

Apical gel pad (located below 5th intercostal rib to the left of the thorax)

Fig. 9.1 Positioning of defibrillation gel pads.

Biphasic waveform defibrillation

Defibrillation waveform mapping has attracted much attention since the advent of internal cardiac defibrillators (ICD). The two most commonly described are 'monophasic' and 'biphasic' waveforms which are shaped by how they are generated and TTI.

• Currents that flow in one direction between electrodes over a specific period are referred to as monophasic and are generated by stored energy being discharged from a capacitor through the chest. Monophasic waveforms tend to be characterised by an initial positive upstroke representing the discharged energy or current. The appearance of the waveform changes when it reaches its peak level, decreasing gradually until it stops suddenly back at zero.

• Biphasic waveforms provide bidirectional flow of current in two phases, over a period of time, at a much lower energy than conventional defibrillators. The shape of the waveform is identical to a monophasic waveform but once the first phase is complete, the current then reverses and flows in a negative direction for the same period and stops abruptly (Faddy et al. 2003, Harris 2002, Tang et al. 2000).

Fig. 9.2a Apex-anterior paddle position.

Fig. 9.2b Underbreast and lateral-to-breast paddle positions (adapted from Pagan-Carlo et al. 1996).

A meta-analysis investigating the advantage of integrating biphasic waveform technology into defibrillators concluded that at lower energies, biphasic defibrillators were as efficacious at terminating VF as standard 200 J monophasic waveforms (Faddy et al. 2003). Biphasic defibrillators were also more successful when compared to the use of consecutive escalating monophasic defibrillatory shocks. The benefits of lower energy levels include:

• early restoration of spontaneous circulation;

• reduced myocardial injury;

• decreased signs of ST segment elevation;

• potential hazards can be reduced.

At present there is no agreement on the ideal energy levels for biphasic waveform defibrillators (Jevon 2002).

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