Defibrillation threshold
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Defibrillation threshold indicates the minimum amount of energy needed to return normal rhythm to a heart that is beating in a cardiac dysrhythmia. Typical examples are the minimum amount of energy, expressed in joules, delivered by external defibrillator paddles or pads, required to break atrial fibrillation and restore normal sinus rhythm. Other common scenarios are restoring normal rhythm from atrial flutter, ventricular tachycardia or ventricular fibrillation. The defibrillation threshold ranking in these settings, from lowest to highest, would be, in order, ventricular tachycardia, atrial flutter, atrial fibrillation, ventricular fibrillation. The highest amount of energy that an external defibrillator can deliver at the present time is 360 joules biphasic. In clinical practice, the real threshold can be approximated but not exactly established, since the defibrillating shock can be delivered only once. Aside from that, energy isn't directly related to stimulus strength and efficiency, which is primarily determined by the delivered charge over time in mC and not power over time or energy, which are still used due to historical reasons. Charge based thresholds are more realistic parameters for shock efficacy. Usual values delivered by biphasic defibrillators lay between 50 and 300 mC. The amount of charge needed is influenced by certain medications, in particular sotalol, tend to lower such threshold, while others, such as amiodarone, may increase it.[1]
Defibrillation threshold is a concept also applicable to internal or implantable cardiac defibrillators.[2] The test needed to establish the defibrillation threshold is often referred to as DFT.
Implantable cardiac defibrillators
The main goal of an Implantable cardiac defibrillator is to reach the appropriate defibrillation threshold needed to successfully stop an abnormal heart rhythm like ventricular fibrillation. They were developed in the 1980s with the aim to prevent sudden cardiac death. Over time, they were found to be efficacious at stopping cardiac arrythmias and became an effective way to prevent them. DFT of at least 10 J below the device maximum shock output is considered acceptable with modern devices being able to deliver shocks as high as 35 J. In clinical studies, unsuccessful defibrillation during standard testing occurs in 3% of patients and corrective strategies include repositioning the defibrillation leads, modifying the shock vector or polarity, revising the generator pocket, or adding additional defibrillation electrodes such as a subcutaneous array. A greater defibrillation threshold is usually required for patients on amiodarone, those with dilated cardiomyopathy, patients with a lower left ventricular ejection fraction, and the presence of cardiac resynchronization therapy devices. Intraoperative testing was once thought to be the gold standard for verifying ICD function, but because the procedure necessitates inducing ventricular fibrillation and carries a slight risk of complications like hemodynamic instability, stroke, or death, its routine use has become more contentious in recent years. Because of this, several facilities now save defibrillation testing for patients who are more likely to require a greater defibrillation threshold.[3]
Mechanism
The right DFT sends a short and high energy shock that depolarizes a substantial part of the myocardium.[4] The shock stops disorganized electrical activity that causes fibrillation, which lets the heart's normal conduction system take back control and start coordinated contractions again. The upper limit of ventricular vulnerability is the greatest shock strength that can cause ventricular fibrillation during cardiac repolarization. To be effective, defibrillation shock strength must be equal or greater than this level. Weak shocks may fail to stop fibrillation or temporarily stop electric activity, but fibrillation may eventually return.[5]