Defibrillation waveform
Charge Stored By Capacitor
Charge = capacitance x voltage
Voltage Waveform of a capacitor discharged into a fixed-resistance load is determined by
V (t) = V(i) x e (-t/RC)
The energy associated with the waveform is given by
Energy = 0.5 CV
Mono/biphasic waveforms
Monophasic defibrillation tends to have better DFTs when the RV is the anode.
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Biphasic defibrillation studies show no difference or more success with RV as anode.
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Can result in significantly lower DFTs (30-50% lower than monophasic).
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Biphasic shocks have shown to result in faster post shock reoccurrence of sinus rhythm, and to have greater efficacy than monophasic shocks in terminating VF of long duration.
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Moreover, as DFTs are generally lower with biphasic waveforms, RV lead position is less critical.
Tilt
Because the tail of the waveform in longer pulses (>10ms) refibrillates the ventricle, truncated waveforms are used.
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These waveforms involve and initial voltage (V(i)), a final voltage (V(f)) and the pulse width or tilt.
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The tilt of the waveform is a function of the size of the capacitor used, the resistance of the leads, tissues through which current passes, and the duration of the pulse.
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Tilt is defined as the percentage decrease of initial voltage.
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•Tilt = (V(i) – V(f)) / (V(i) x 100%)
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For monophasic waveforms, the optimal tilt has been shown to be 50-80%
Phase duration & tilt
for pts with high DFTs, pulse width optimisation may be considered.
Measuring shock dose
The shape of the waveform is a function of the initial voltage, the size of the capacitor, and the resistance of the load.
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If a smaller capacitor is used to diminish device size, a larger initial voltage may be needed to deliver and equivalent amount of charge into the fibrillating tissue.
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Thus 2 waveforms may have different leading edge voltages but the same energy if there are differences in capacitance.
Shock vector
The programming of this parameter depends on the number of shocking electrodes available.
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The defibrillation shock is delivered via a dedicated lead (either single or dual coil)
•Single coil shocks are delivered between the distal coil of the right ventricular lead and the coil,
•while dual coil shocks are delivered between
1) the distal coil,
2) the proximal coil and
3) the pulse generator.
The anode is usually included between the 2 shocking electrodes and the cathode is the pulse generator.
With a dual coil electrode, the shock vector can be changed by including or excluding the SVC coil