Automated External
Defibrillators

•	Clinical studies using available biphasic waveforms have demonstrated 99-100% first shock efficacy with an average first phase current of about 15 amps in patients with average impedances of 75 ohms [1, 3].
•	Human and animal studies have shown that higher biphasic energy and higher corresponding average current (at 75 ohms, approximately 15 amps versus 12 amps) results in greater first shock efficacy [1, 2].
•	Biphasic waveforms in clinical use perform well in mid-range impedances of 50-90 ohms. [1,3] In low impedance patients, the goal is to control peak current and avoid high peak currents [4]. In high impedance patients, the goal is to provide longer first phase duration with lower tilt in order to hold average current as high as possible. [1]
•	Biphasic waveforms with a second phase to first phase charge balance ratio in the range of 0.38 +/- 0.17 have lower defibrillation threshold values. [5]

Animal Data Comparing Biphasic Waveforms
Biphasic waveforms are used extensively in human clinical applications. Access CardioSystems has demonstrated the performance of its Biphasic WaveControl™ in the animal laboratory. This trial successfully compared the defibrillation thresholds of the Access CardioSystems waveform to those of clinically used biphasic waveforms. The Access CardioSystems waveform is not compared in human clinical tests to damped sinewave waveforms, but Access CardioSystems defibrillation thresholds have been compared to damped sinewave defibrillation thresholds in animal tests.

Rationale for Animal Studies
Defibrillation waveforms have been extensively studied over many years. Monophasic waveforms, initially studied in animals, have now been used very successfully in humans for more than 25 years. Approximately 20 years ago, biphasic waveforms were developed. Animal studies compared biphasic waveforms to monophasic waveforms and demonstrated that biphasic waveforms are significantly better than monophasic waveforms. Initial animal studies were replicated in humans and demonstrated that animal data can be used to predict human results.

The justification for using animal data for waveform validations, rather than human clinical trials includes:

1.	Animal studies allow more shocks per study subject and, thus, enable the collection of larger samples of data. This provides better comparison of waveforms.
2.	Animal studies avoid putting humans at the unnecessary risk from additional episodes of ventricular fibrillation and additional defibrillation shocks.
3.	The mechanism of defibrillation in animals is the same as that in humans.

Conclusion from Animal Data:
Defibrillation threshold data suggests that the Access CardioSystems waveform is at least as effective as the tested and clinically used waveforms.

Access CardioSystems WaveControl™ Technology:
Waveform Features

            Clinical Performance:

	•	Delivers first shock with higher average current for greater defibrillation efficacy margin
	•	Delivers escalating energy with controlled waveform shape

            Waveform Features:

	•	Controls peak current for better safety margin in low impedance patients
	•	Extends pulse duration with low tilt in high impedance patients
	•	Maintains optimal phase 2 to phase 1 charge balance

Specific Waveform Characteristics

Impedance-Compensation with Optimal Average Current

Impedance (ohms)	Average Current (amps)		First Phase Duration	Second Phase Duration
	200J	360J	200J/360J	200J/360J
25	21	25	7.5 mSec	4.5 mSec
50	19	22	7.5 mSec	4.5 mSec
75	17	21	8.5 mSec	4.5 mSec
100	14	16	8.5 mSec	4.5 mSec
125	11	14	8.5 mSec	4.5 mSec
150	10	12	8.5 mSec	4.5 mSec

Effectiveness:

•	First Shock Comparison at 75 Ohms The Access CardioSystems waveform delivers 200J first shock energy and, for a patient with impedance of 75 ohms, a first shock average current of about 17 amps. For the 75 ohm patient, the Access CardioSystems waveform delivers additional average current and additional defibrillation efficacy margin compared to other biphasic waveforms in clinical use [6, 7].
•	Second Shock Escalating Energy Access CardioSystems has designed a 360J second shock selection to deliver higher average current, while maintaining nearly identical shape to its 200J first shock waveform.

Patient Safety

•	The Access CardioSystems waveform also ensures patient safety by limiting the delivered peak current to less than 30 amps.
•	This restriction on peak current limits the potential for myocardial damage and post-shock dysfunction, providing a better margin for safety. There is very little clinical data published on patients with 25 ohm impedance.

Impedance Compensation
Higher Impedance

•	Extension of first phase duration from 7.5 msec to 8.5 msec in higher impedances delivers greater charge and energy.

Charge Balance of Second Phase versus First Phase

•	Access CardioSystems WaveControl™ maintains a charge ratio demonstrated to yield a lower defibrillation threshold.

WaveControl™

•	Access CardioSystems maintains a similar waveform shape for all patient impedances by controlling the waveform tilt, limiting the peak current and extending first phase duration with increased patient impedance.
•	In a patient with higher impedance of 100 ohms or greater, the Access CardioSystems waveform shape has an extended first phase, low tilt, and more first shock current than other biphasic waveforms in current clinical use. [6,7]

Access CardioSystems WaveControl™ Technology 360 Joule Selection:

Waveform Validation Study

Purpose
The Access CardioSystems biphasic waveform was tested in the animal laboratory. Defibrillation threshold currents (DFTs) were measured for three biphasic waveforms, including two waveforms in current clinical human use. A total of 292 DFTs, nearly 100 for each waveform, were measured in random order.

Method
A swine model was used in a protocol approved by the Institutional Animal Care and Use Committee. The pigs used weighed between 38.0 and 47.0 kilograms (mean weight 43.64 kg). Monitoring was provided for ECG, temperature and pulse oximetry. A catheter was inserted in the right ventricle via the jugular vein for induction of ventricular fibrillation and for pacing if required.

Induction of fibrillation was accomplished by delivering a 50 hertz, 2 msec wide pulse for a period of 3 seconds. The ECG was monitored to confirm the presence of fibrillation. Fibrillation was sustained for a period of 10 seconds from the initiation of the induction pulses to the first shock. The maximum duration of fibrillation was limited to 30 seconds by providing a rescue shock if required. Defibrillation was delivered by a series of three increasing shock strengths and a rescue shock if required. A resting period of 2 minutes was provided if the first shock was successful, and a period of 3 minutes was provided if 2 or more shocks were required.

Threshold Determination
The animal threshold was determined by using a modified Bourland protocol. This protocol determined the 50% probability for successful defibrillation. A pair of defibrillation shocks were obtained with current levels within 10% of each other, and where one shock was successful and the other was unsuccessful in converting defibrillation into a normal rhythm.

The sequence used to determine a threshold was to induce fibrillation and deliver a first shock. If the shock fails to convert the fibrillation, a second shock was delivered with a 10% increase in the current level. If a third shock was required, the current level was increased by 20%. If a fourth shock was needed, a biphasic rescue shock using full energy was delivered. The threshold was determined by using only the first two shocks of a rescue sequence. The third shock was only used to estimate the starting level of the next shock sequence.

The summary of rules for calculating a threshold were:

1.	Successful conversion of fibrillation to a normal rhythm. Note that pacing was permitted for post-shock atrioventricular block.
2.	Threshold data included only the first or second shock in a rescue sequence
3.	Successful defibrillation thresholds were based on a current level within 10% of a failed shock.

Data Collection
Defibrillation waveform data was collected and stored using a digital storage oscilloscope. The stored data includes the waveforms for delivered current and voltage. This data allowed calculation of thoracic impedance and delivered energy. Additional data was collected and manually recorded after each resuscitation sequence. This data included: shock sequence number, time, peak voltage, peak current, calculated impedance, defibrillation success or failure, annotation of a defibrillation threshold, and comments. The comments included changes in ventilation, anesthesia, ST segment elevations, or defibrillation pad skin observations.

Results
Animal Data Findings:

1.	Extension of first phase pulse duration in a 100 µF capacitor waveform offers no benefit in defibrillation threshold current. For a 200 µF waveform, extension of first phase duration lowers defibrillation threshold, while, in a 100 µF waveform, extension of first phase duration does not improve defibrillation efficacy or lower thresholds.

	TABLE:
	Average Current Threshold (amps) Energy Delivered (Joules) First Phase Duration (msec) W3 Access* (200 µF) 13.39 ± 2.27 55.48 ± 19.23 8.5 W1 (100 µF) 14.26 ± 1.76 50.13 ± 16.98 7.5 W2 (100 µF) 15.04 ± 1.41 45.83 ± 15.72 6.0 p-value** = < 0.001 0.337 N/A
	* same characteristics as second waveform in current clinical use ** using two-way ANOVA analysis

2.	The leading edge current of a 200 µF capacitor waveform is lower than that of a 100uF waveform at the threshold for each waveform. However, at that threshold, the 200 µF waveform has the same average current as that of the 100uF capacitor waveform.

3.	The Access CardioSystems waveform maintains the same tilt and pulse duration with low thoracic impedance.

Conclusions:

1.	The Access CardioSystems waveform has statistically lower average current and similar energy delivered at 50% probability threshold compared to waveform W2, the first clinically used waveform that was tested in this study.
2.	The Access CardioSystems waveform has the same average current, energy, and duration at 50% probability threshold as the second clinically used waveform.
3.	Defibrillation threshold data suggests that the Access CardioSystems waveform is at least as effective as the tested and clinically used waveforms.

Clinical Data:
The Access WaveControl Biphasic Waveform was validated in a prospective multi-center study in the United States and Europe. In this study, sixty (60) patients undergoing ICD implant or testing had ventricular fibrillation induced. All 60 patients (100%) were successfully defibrillated on the first administered shock at the lowest energy setting (200J). The waveform maintained efficacy at the highest impedances (> 90 ohms). Peak current did not exceed 35 amps, including those patients with the lowest impedances (< 50 ohms). By comparison, monophasic waveforms, a standard benchmark, yield first shock efficacy of 86%8. No ECG or skin changes were observed at the time of patient discharge in the current study. This study confirmed the safety and high first shock efficacy of the Access WaveControl Biphasic Waveform in ventricular fibrillation.

References

1.	Higgins, S.L., et al., A comparison of biphasic and monophasic shocks for external defibrillation. Prehosp Emerg Care, 2000. 4(4): p. 305-13.
2.	Walker, R.G., et al., Comparison of Clinically Used Biphasic Waveforms for External Defibrillation. Acad Emerg Med, 2001. 8(5): p. 432-433.
3.	Mittal, S., et al., Comparison of a novel rectilinear biphasic waveform with a damped sine wave monophasic waveform for transthoracic ventricular defibrillation. J Am Coll Cardiol, 1999. 34(5): p. 1595-601.
4.	Tacker, WA. Defibrillation of the Heart. St. Louis, MO. Mosby YearBook. 1994. pgs. 288-291.
5.	Geddes, L.A. and W. Havel, Evolution of the optimum bidirectional (+/- biphasic) wave for defibrillation. Biomed Instrum Technol, 2000. 34(1): p. 39-54.
6.	Unpublished data. Access CardioSystems. 2001.
7.	Gliner, B.E., et al., Treatment of out-of-hospital cardiac arrest with a low-energy impedance- compensating biphasic waveform automatic external defibrillator. The LIFE Investigators. Biomed Instrum Technol, 1998. 32(6): p. 631-44.
8.	Bardy, G.H., et al., Multicenter comparison of truncated biphasic shocks and standard damped sine wave monophasic shocks for transthoracic ventricular defibrillation. Transthoracic Investigators. Circulation, 1996. 94(10): p. 2507-14.
9.	Biphasic WaveControl™ Technology document courtesy of Access Cardiosystems, Inc.

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