Defibrillation physics

Clinical use: what matters at the bedside (physics-linked)

• Goal is to deliver sufficient myocardial current to terminate VF/pulseless VT; success relates more to delivered current through the heart than the selected energy on the machine.
  • For a given selected energy, higher transthoracic impedance (TTI) reduces peak current and total charge delivered.
• Minimise TTI: good pad contact, correct position, firm pressure (paddles), adequate gel, avoid air gaps, shave hair if needed, use large pads, avoid placing over devices/metal.
  • TTI falls with repeated shocks (skin heating, improved contact) and may be lower at end-expiration.
• Pad position aims to maximise current path through myocardium (anterolateral or anteroposterior).
  • AP placement may reduce TTI and improve current through atria/ventricles in some patients; evidence mixed but commonly used for cardioversion.
• Biphasic defibrillators compensate for impedance (adjust voltage/pulse width) and achieve similar efficacy at lower selected energies than monophasic.
  • Typical adult: biphasic 150–200 J (manufacturer-specific) vs monophasic 360 J.
• Synchronised cardioversion: shock timed to R wave to avoid delivery during vulnerable period (T wave) which can precipitate VF.
  • If sync markers unreliable (e.g., very fast AF), increase gain/lead selection; if unstable, may need unsynchronised shock.
• Safety: oxygen away from chest, no one touching patient/bed, remove transdermal GTN patches, avoid wet surfaces; consider implanted devices and pacemakers.
  • Place pads at least ~8 cm from pacemaker/ICD generator; use lowest effective energy; check device function afterwards.

Core definitions and quantities (exam essentials)

• Defibrillation: delivery of a brief high-energy electrical pulse to depolarise a critical mass of myocardium, extinguishing re-entrant wavefronts and allowing organised rhythm to resume.
• Energy (J): work done. In a capacitor discharge: E = 1/2 C V^2.
  • For a fixed capacitor C, increasing selected energy requires higher charging voltage V (since E ∝ V^2).
• Charge (Q, coulombs): Q = C V. Delivered charge depends on waveform, pulse width and impedance.
• Current (I, amps): I = V/R (Ohm’s law). In defibrillation, R is mainly transthoracic impedance (TTI).
• Power (W): rate of energy transfer. P = V I = I^2 R = V^2/R (instantaneous).
• Current density (A/m^2): current per unit electrode area; relates to skin burns and local tissue injury.
  • Larger pads reduce current density at skin and reduce TTI, improving myocardial current for a given energy.

Defibrillator circuit model (capacitor discharge through patient)

• Simplified model: capacitor (C) charged to voltage (V0) discharges through patient impedance (R) via switching circuitry → exponentially decaying current/voltage (monophasic damped sinusoid or truncated exponential depending on design).
  • Time constant τ = R C (for simple RC exponential). Higher R increases τ (slower decay) but reduces peak current (I0 = V0/R).
• Modern biphasic defibrillators: controlled truncated exponential or rectilinear biphasic waveforms; microprocessor adjusts pulse width/voltage based on measured impedance.
• Energy actually delivered to patient is less than stored energy due to internal resistance and switching losses; displayed “selected energy” refers to intended delivered energy (device-dependent).

Waveforms: monophasic vs biphasic (physics and implications)

• Monophasic: current flows in one direction only.
  • Types historically: damped sinusoidal (older) and monophasic truncated exponential (MTE).
  • Higher energies required (commonly 360 J) and more myocardial injury/skin burns compared with biphasic for similar efficacy.
• Biphasic: current reverses direction part-way through shock (two phases).
  • Mechanism: first phase depolarises; second phase helps remove residual membrane charge and reduces post-shock arrhythmogenicity, improving defibrillation threshold.
  • Lower energy for similar success; less myocardial damage; better performance across a range of impedances due to impedance compensation.
• Pulse duration matters: too short may not capture enough myocardium; too long increases energy dissipation and injury. Devices choose pulse width to optimise delivered current/charge for measured impedance.

Transthoracic impedance (TTI): determinants and consequences

• TTI is the effective resistance to current flow between electrodes across chest wall and thorax; typical adult range ~50–100 Ω (can be wider).
• Higher TTI → lower peak current and reduced probability of successful defibrillation at a given selected energy.
• Factors increasing TTI: small pads, poor contact/air gaps, dry gel, hair, obesity, emphysema/hyperinflation, incorrect placement, low paddle pressure, transdermal patches, chest wall oedema.
• Factors reducing TTI: larger pads, good conductive gel, firm pressure, correct placement, shaving hair, repeated shocks, end-expiration.

Energy selection and what the number means

• Selected energy (J) is not the same as myocardial energy; what matters is myocardial current/charge delivered during the shock.
• For a given selected energy, high impedance reduces current; biphasic devices adjust waveform to mitigate this, but very high impedance can still reduce effectiveness.
• Defibrillation threshold (DFT): minimum energy (or current) required to achieve defibrillation with a given probability; varies with patient factors, electrode position, impedance and waveform.
  • Biphasic waveforms lower DFT compared with monophasic.

Skin burns and tissue injury (physics)

• Burn risk relates to current density and contact quality: J = I/A (conceptually). Small contact area or air gaps concentrate current and heat.
• Paddles with firm pressure reduce impedance and improve contact area; adhesive pads provide consistent contact but must be applied correctly.

Synchronisation and the vulnerable period

• Unsynchronised shocks can fall on the T wave (relative refractory period) causing VF (R-on-T phenomenon).
• Synchronised cardioversion uses ECG sensing to time discharge with the R wave; requires adequate signal and correct lead selection.

Electrical safety and hazards (FRCA physics angle)

• Defibrillators are high-energy sources; risk to staff is mainly macroshock if in contact with patient/bed during discharge, particularly if wet or with invasive conductors.
• Fire risk: oxygen-enriched environment + ignition source (spark at pads/paddles) + fuel (drapes, alcohol prep). Minimise oxygen pooling; allow alcohol to dry.
• ECG leads and monitoring: ensure defib-proof leads; defib can saturate/damage equipment without proper protection.