Defibrillators

USES

• application of an electrical current across the heart to convert VF/VT -> sinus rhythm

DESCRIPTION

• single most important modality for cardiac arrest management.
• likelihood of these rhythms reverting with defibrillation is inversely proportional to time.
• the chance of successful reversion declines at a rate of 7-10% per min from onset of VF.
• current pulse causes synchronous contraction of the heart muscle, hopefully allowing SR to occur following refractory period.
• capacitor (potential difference between plates of up to 8000V, energy released during discharge is proportional to the potential difference)
  — 360J external defib
  — 50J internal defib
• after first shock thoracic impedance is reduced -> second shock will deliver greater energy to the heart.
• energy at discharge is released in a waveform (monophasic or biphasic)

Monophasic

• voltage rises rapidly and then returns to baseline (0 -> +ve -> 0)

Biphasic

• voltage rises, then reverses its direction below baseline before returning to baseline (0 -> +ve -> 0 -> -ve -> 0)
• biphasics have been shown to defibrillate as effectively as monophasic but at a lower energy.
• smaller (more portable)
• cheaper

METHOD OF INSERTION AND/OR USE

• apply paddles to patients as directed
• charge
• ‘stand clear’
• discharge

OTHER INFORMATION

• pacemaker function
• synchronised or non-synchronised shocks

Suggested commands during CPR

• We are about to perform a 2 minute rhythm check
• Stand clear, continue compressions (charging)
• Stop CPR (rhythm check)
• Everyone clear (compression stands back with hands in air, “I’m clear”)
• (deliver shock) Shock delivered, start compressions

COMPLICATIONS

• DC more effective & less damaging than AC
• repeated shocks -> myocardial damage
• electrocution of members of resuscitation team
• biphasic less myocardial damage
• burns
• arcing to other metal (backs of GTN patches, implanted defibrillators)