Biphasic defibrillator waveform with adjustable second phase tilt

1. An external defibrillator which delivers biphasic defibrillation pulses comprising:

a high voltage circuit;

a capacitor, coupled to the high voltage circuit which is charged by the high voltage circuit for defibrillation pulse delivery;

a pair of patient electrodes;

a plurality of switches, coupled between the capacitor and the patient electrodes, and operable to couple first and second phases of a biphasic defibrillation pulse waveform to the patient electrodes, wherein the tilt of the second phase of the waveform is controllably adjustable; and

a controllable current path which enables current to controllably bypass the patient electrodes during the second phase of the biphasic waveform,

the high voltage circuit operable to close at least one of the plurality of switches to enable the bypass current during the second phase of the biphasic waveform.

2. The external defibrillator of claim 1 , wherein the plurality of switches comprises an H-bridge with one switch closure configuration for delivery of one phase of a biphasic waveform and another switch closure configuration for delivery of an opposite phase of the biphasic waveform.

3. The external defibrillator of claim 1 , wherein the capacitor comprises a single capacitance which is used for delivery of both phases the of biphasic waveform.

4. The external defibrillator of claim 1 , wherein the capacitor further comprises a first capacitor which is used for delivery of the first phase of a biphasic waveform and a second capacitor which is used for delivery of the second phase of the biphasic waveform.

5. The external defibrillator of claim 1 , wherein the plurality of switches comprises an H-bridge, and wherein the controllable current path includes a switch of the H-bridge.

6. The external defibrillator of claim 5 , wherein the H-bridge further comprises first and second switches which are closed for pulse delivery during the first phase of the biphasic waveform, and third and fourth switches which are closed for pulse delivery during the second phase of the biphasic waveform, wherein the controllable current path includes one of the first and second switches.

7. The external defibrillator of claim 6 , further comprising a first resistor coupled in series with the first and second switches when the first and second switches are closed, and a second resistor coupled in series with the third and fourth switches when the third and fourth switches are closed.

8. The external defibrillator of claim 1 , wherein the controllable current path is controlled by a pulse width modulated control signal during the second phase of the biphasic waveform.

9. The external defibrillator of claim 1 , wherein the biphasic defibrillation pulse waveform exhibits:

a first phase during which the pulse voltage increases from a reference potential to a voltage peak V 0 and declines from V 0 during the first phase of the waveform; and

a second phase which begins at an initial voltage V 2 and declines from V 2 during the second phase of the waveform to a level at or near the reference potential.

10. The external defibrillator of claim 9 , wherein voltages V 0 and V 2 are of opposite sense relative to the reference potential.

11. The external defibrillator of claim 1 , further comprising a circuit adapted to measure patient impedance, wherein parameters of the biphasic defibrillation pulse waveform are set in accordance with the measure of patient impedance.

12. The external defibrillator of claim 1 , wherein the overall tilt of the biphasic waveform is maintained at approximately 95% through control of the tilt of the second phase of the waveform.

13. The external defibrillator of claim 12 , wherein the overall tilt of the biphasic waveform is maintained at approximately 95% through an increase of the tilt of the second phase of the waveform.