One of the most common and life-threatening heart irregularities is ventricular fibrillation in which the heart is unable to pump a significant volume of blood. When such an irregularity occurs, serious brain damage and death will invariably result unless a normal heart rhythm can be restored within a few minutes. Ventricular fibrillation can occur as a result of a heart attack but may also be caused by accidental electric shock or due to severe stress, such as may accompany surgery, drowning or the like.
The most effective treatment for restoring a normal rhythm to a heart muscle experiencing ventricular fibrillation is the application of a strong electric shock to the victim using a cardiac defibrillator. Cardiac defibrillators are medical devices for producing and delivering such shocks and have been successfully used for many years.
Conventional external cardiac defibrillators accumulate an electric charge on a storage capacitor and, when a switching mechanism is closed, transfer the stored energy in the form of a large current pulse to a patient. The switching mechanisms used in most defibrillators comprise heavy-duty electro-mechanical relays. Typically, the relays are responsive to a discharge control signal that actuates the relay to complete an electrical circuit between the storage capacitor, a waveshaping network, and a pair of defibrillation electrodes attached to the patient.
The cardiac defibrillation pulse, which is delivered to the patient by an energy transfer circuit that includes a electro-mechanical relay, generally comprises a single pulse having a damped sinusoidal shape that starts when the relay closes. Alternately, a cardiac defibrillation pulse may have an exponential shape that starts when the relay contact closes and stops when the relay contact opens. However, it has been found that other types of defibrillation currents may be more effective in terminating ventricular fibrillation. For example, it may be beneficial to apply a series of cardiac defibrillation current pulses to the patient in rapid succession, or it may be beneficial to maintain the magnitude of the defibrillation current flowing through the patient at a specific level. The use of electro-mechanical relays in an energy transfer circuit do not allow such types of defibrillation currents to be applied to the patient because the relay contacts cannot be switched closed and open fast enough. This slow response time does not allow the use of feedback signals obtained from the patient to be used in controlling the amount of energy delivered. Therefore, what is needed is an energy transfer circuit that can replace the electro-mechanical relays with a fast acting solid state switching device.
The problem with using solid-state switching devices in a cardiac defibrillation circuit is that such devices allow a certain amount of leakage current to flow even when the devices are in a nonconducting state. Heretofore, safety reasons have made it impractical to use solid-state circuitry devices in a cardiac defibrillator circuit without the additional isolation of electro-mechanical relays.
Therefore, it is desirable to provide an energy transfer circuit that employs a solid state switch in combination with a current shunt which can divert the leakage current away from the patient electrodes until a defibrillation pulse is delivered to the patient.
It is also desirable to provide an energy transfer circuit including a feedback control to regulate the delivery of a cardiac defibrillation pulse to the patient.
Finally, it is desirable to provide an energy transfer circuit that can deliver predetermined defibrillation current waveforms to the patient.