Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2000-09-28
2002-08-20
Schaetzle, Kennedy (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C600S508000
Reexamination Certificate
active
06438419
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to methods and apparatus for delivering defibrillating energy to a patient and, in particular, to methods and apparatus for selectively delivering defibrillating energy to patients.
2. Description of the Prior Art
Ventricular fibrillation (VF) is the most common initial arrhythmia associated with sudden cardiac death, and is one of the most common life-threatening medical conditions that occurs with respect to the human heart. In ventricular fibrillation, the human heart's electrical activity becomes unsynchronized, which results in a loss of its ability to contract. As a result, a fibrillating heart immediately loses its ability to pump blood into the circulation system.
Electrical defibrillation remains the mainstay of therapy for VF. A common treatment for ventricular fibrillation is to apply an electric pulse to the heart that is strong enough to stop the unsynchronized electrical activity and give the heart's natural pacemaker a chance to reinitiate a synchronized rhythm. External defibrillation is the method of applying the electric pulse to the fibrillating heart through a patient's thorax. See, for example, U.S. Pat. No. 5,999,852.
Existing external cardiac defibrillators first accumulate a high-energy electric charge in an energy store, typically a capacitor. When a switching mechanism is activated, the stored energy is applied to the patient via electrodes positioned on the patient's thorax. The resultant discharge of the capacitor causes a large current pulse to be transferred through the patient.
The current practice of resuscitation relies on visual inspection of the electrocardiogram (ECG) waveform on a monitor. If the caregiver decides that it is appropriate, a defibrillation shock is administered. In many instances, this therapy is ineffective, although no known tool is available to predict a priori when defibrillation will be successful.
It has long been recognized that the efficacy of electrical defibrillation decreases with increasing duration of ischemia. See Eisenberg, M., et al., Paramedic programs and out-of-hospital cardiac arrest: I. Factors associated with successful resuscitation,
American Journal of Public Health,
1979, vol. 69, pp. 30-38; Eisenberg, M., et al., Treatment of out-of-hospital cardiac arrest with rapid defibrillation by emergency medical technicians,
New England Journal of Medicine,
1980, vol. 302, pp. 1379-83; and Roth, R., et al., Out-of-hospital cardiac arrest: factors associated with survival,
Annals of Emergency Medicine,
1984, vol. 13, pp. 237-43.
It is known that decreasing the delay to defibrillation by introduction of automatic external defibrillators (AEDs) or other modifications improves survival from cardiac arrest in certain subsets of patients. See Auble, T., et al., Effect of out-of-hospital defibrillation by basic life support providers on cardiac arrest mortality: a metaanalysis,
Annals of Emergency Medicine,
1995, vol. 25, pp. 642-47; and Stiell, I., et al., Improved out-of-hospital cardiac arrest survival through the inexpensive optimization of an existing defibrillation program: OPALS study phase II,
JAMA,
1999, vol. 281, pp. 1175-81.
However, overall recovery from cardiac arrest outside the hospital remains low, perhaps because many applications of AEDs still occur beyond the time during which defibrillation is likely to be successful. Investigators have suggested that a brief period of artificial circulation or other tailored therapy could improve outcome for selected patients in whom defibrillation is unlikely to succeed. However, identification of these subsets of patients remains problematic. See Cobb, L., et al., Influence of cardiopulmonary resuscitation prior to defibrillation in patients with out-of-hospital ventricular fibrillation,
JAMA,
1999, vol. 281, pp. 1182-88.
It is also known to employ quantitative measures of the VF waveform morphology to estimate the duration of VF and its likelihood of successful defibrillation. Callaham, M., et al.,
Annals of Emergency Medicine,
1993, vol. 22, pp. 1664-77; Weaver, W., et al.,
Annals of Internal Medicine,
1985, vol. 102, pp. 53-55; Brown, C., et al.,
Annals of Emergency Medicine,
1996, vol. 27, pp. 184-88; and Strohmenger, H., et al.,
Anesthesia Analgesia,
1994, vol. 79, pp. 434-38.
The only known prior quantitative descriptor of the ECG waveform during VF to gain widespread attention was centroid frequency—a measure derived from fast Fourier transforms. For example, a frequency measure based upon spectral analysis, the centroid frequency, varies with the duration of VF. Brown, C., et al.,
Annals of Emergency Medicine,
1989, vol. 18, pp. 1181-85. This measure has biphasic variation across time, preventing any single value of centroid frequency from predicting a unique state of the VF waveform. Because of the multiphasic profile, a particular value of the centroid frequency is not uniquely associated with a particular duration of VF.
VF is not completely random, but exhibits some deterministic organization. As shown in
FIG. 1
, early in VF, much of the power is concentrated in a narrow frequency band. Often there is a single peak in the spectral array representing a “dominant frequency.” Alternatively, the center of mass of the entire curve can be calculated as the “centroid frequency” or “median frequency.” Brown, C., et al.,
Annals of Emergency Medicine,
1989, vol. 18, pp. 1181-85. These measures change across time in a predictable fashion.
As shown in
FIG. 2
, centroid frequency (±SD) follows a predictable pattern after induction of VF in swine. Brown, C., et al., “Estimating the duration of ventricular fibrillation,”
Annals of Emergency Medicine,
1989, vol. 18, pp. 1181-85; Martin, D., et al., “Frequency analysis of the human and swine electrocardiogram during ventricular fibrillation,”
Resuscitation,
1991, vol. 22, pp. 85-91. This is, however, problematic since a given absolute value can occur at different times and the variance is substantial.
As shown in
FIG. 3
, in human VF, recorded from patients who developed VF while on Holter monitors, the initial peak of power in a narrow frequency range degenerates into a broad distribution of power among frequencies.
Unlike swine, as shown in
FIG. 4
, mean (±SD) centroid frequency from humans who developed VF while on Holter monitors has a less consistent change across time. Also, the absolute values of this measure differ between species. Martin, D., et al.,
Resuscitation,
1991, vol. 22, pp. 85-91.
FIG. 4
shows a horizontal line indicating the number of subjects upon which the average and SD are based. The first 3 minutes of data includes 7 subjects, the next 2 minutes includes only 6 of those subjects, then 5, then 4.
Other studies have found that the amplitude of the VF waveform can predict defibrillation success. Callaham, M., et al.,
Annals of Emergency Medicine,
1993, vol. 22, pp. 1664-77; Weaver, W., et al., Annals of Internal Medicine, 1985, vol. 102, pp. 53-55. However, electrode configuration, placement, body habitus, impedance and recording equipment can affect amplitude measurements.
The waveform of VF is one possible predictor for the likelihood of successful defibrillation. VF waveform analysis is motivated by the obvious visual differences between a high-amplitude, lower frequency waveform seen in early VF and a low-amplitude, higher frequency waveform seen in later VF. However, both amplitude and frequency measures are difficult to apply in practice. See Callaham, M., et al., Prehospital cardiac arrest treated by urban first-responders: profile of patient response and prediction of outcome by ventricular fibrillation waveform,
Annals of Emergency Medicine,
1993, vol. 22, pp. 1664-77 (amplitude measures); Weaver, W., et al., Amplitude of ventricular fibrillation waveform and outcome after cardiac arrest,
Annals of Internal Medicine,
1985, vol. 102, pp. 53-55 (amplitude measures); Brown, C., et al., Signal analysis of the human ECG during ventricu
Callaway Clifton W.
Sherman Lawrence D.
Droesch Kristen
Eckert Seamans Cherin & Mellott , LLC
Houser Kirk D.
Schaetzle Kennedy
Silverman Arnold B.
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