An apparatus and method for the treatment of ventricular fibrillation, ventricular tachycardia, or supraventricular tachycardia. The method being to prospectively ascertain a patient's transthoracic resistance by application of a low amplitude current via the apparatus electrodes applied to a patient's chest, charging a capacitor of the apparatus sufficiently for delivery of a minimal peak current, e.g., 25 amperes for treatment of ventricular fibrillation preselected by the operator as appropriate for attaining defibrillation, and the capacitor is discharged through the electrodes to effect the defibrillation. The apparatus can be made by modifying conventional direct current defibrillator devices.

BACKGROUND OF THE INVENTION 
Widespread use of DC defibrillators in patients suffering cardiac arrest 
has greatly increased the rate of successful resuscitation both in and out 
of hospitals over the past few decades. Defibrillation is applicable to 
life-threatening cardiac arrests resulting from ventricular fibrillation 
which occurs because of asynchronous depolarization of cardiac cells. When 
a sufficient electrical pulse is delivered to the heart from an external 
defibrillator through a set of paddles (electrodes), all cardiac cells 
briefly arrest and thereafter synchronous or normal depolarization may 
once again resume. 
The defibrillator equipment presently offered to the medical arts 
discharges the electrical energy through an RLC circuit which is manually 
triggered by the physician, and the heretofore standard quantity of the 
electrical pulse to be delivered has been calibrated in terms of joules of 
energy. The many studies reported in the medical literature of attempts to 
determine the optimal electrical strength of the pulse that should be 
delivered for defibrillation are almost invariably analyzed in terms of 
joules. Delivery of more than enough electrical energy for defibrillation 
has been associated with cardiac cell death, yet insufficient energy will 
not accomplish the desired defibrillation, resulting then in multiple 
attempts to defibrillate at ever higher energy levels. 
Previous recommendations for the "first attempt" defibrillation usually 
have been based on gross energy levels e.g., 200 joules. In fact, 
according to the STANDARDS AND GUIDELINES FOR CARDIOPULMONARY 
RESUSCITATION (CPR) AND EMERGENCY CARDIAC CARE (ECC)--published in JAMA, 
Vol. 225, pp 2942-2943, 1986, patients in ventricular fibrillation should 
receive DC countershocks of 200 joules (first shock), 200 joules (second 
shock), and 360 joules (third shock), as needed. 
Selection of energy dose level for threshold defibrillation is believed to 
be sub-optimal for several reasons. For a given pulse duration, peak 
current is a better predictor of the defibrillation threshold than 
delivered energy. Lerman et al., "Relationship between Canine 
Transthoracic Impedance and Defibrillation Threshold: Evidence for 
Current-based Defibrillation." Journal of Clinical Investigation, Vol. 80, 
pp. 797-803, Sept., 1987). Establishment of the defibrillation pulse on 
the basis of total electrical energy, as has been done by prior workers in 
the art, does not apply a consistent level of peak current (amperage) 
because, in humans as well as dogs, transthoracic resistance varies within 
large ranges from one subject to the next. The implication of these 
findings is that defibrillation doses should be calibrated in units of (or 
at least based upon) current instead of energy. 
An object of this invention is to provide a method and apparatus for 
automatically providing a preselected threshold level of peak current 
adequate for defibrillation, wherein the defibrillator capacitor is 
charged according to the transthoracic resistance of each patient in order 
to provide the selected peak current. 
Additionally, it is an object of the invention to prospectively determine 
such transthoracic resistance automatically and prior to defibrillation by 
applying to the patient a low amplitude exploration current via the 
defibrillator electrodes. 
Further objects of the invention and the advantages thereof will become 
apparent from the description which follows. 
RATIONALE OF THE INVENTION 
Although it has been customary in the defibrillator art to apply electrical 
energy as such, i.e., a pulse denominated in joules, some workers in the 
art have appreciated that delivered current is better than discharged 
energy as a measurement of a defibrillating threshold, as for example in 
U.S. Pat. No. 3,862,636, wherein the magnitude of the current delivered to 
the patient was varied in accordance with the body weight of the patient. 
Other recent art has recognized that total energy may not be the most 
adequate electrical parameter to describe the dose for defibrillation, 
urging that the peak current level per heart weight or body weight might 
be the best descriptor of the energy needed to depolarize some critical 
mass of cells and achieve successful defibrillation, (in the instance of 
canine hearts at least). See, for example, Armayor et al. "Ventricular 
Defibrillation Threshholds with Capacitor Discharge", Med. & Biol. Eng. 
and Comput. 1979, Vol. 17, pp. 435-442. 
Kerber, et al., "Automated Impedance-Based Energy Adjustment for 
Defibrillation: Experimental Studies", Circulation, Vol. 71, No. 1, 
January 1985, suggest automatic increases of energy in arbitrary amounts, 
from an operator-selected energy level, when prospected transthoracic 
resistance exceeds a mean level based on previously observed patients. 
U.S. Pat. No. 3,860,009 computes a peak defibrillation current based on 
energy and transthoracic resistance or body weight and delivers the energy 
by discharge of capacitors directly into the resistance load of the 
patient resulting in an approximately ramp voltage RC discharge pulse 
output. 
The gist of the above-noted art is that a need exists for identifying 
patients with such high transthoracic resistance that application of 
relatively low energy, e.g., 100 joules, defibrillation shock levels are 
unlikely of success. 
The inventor hereof, in U.S. Pat. No. 4,574,810, suggested that such an 
approach was too gross, and that a superior approach would be to ascertain 
a threshold level of peak current based on the requirements of each 
patient, and then to apply whatever electrical energy would result in the 
desired level of peak current. Further, a resistance measuring system was 
associated with the defibrillator circuitry and the electrical shock 
energy administered by the defibrillator was controlled according to 
measured transthoracic resistance to provide a predetermined amount of 
peak defibrillation current per calculated ohm of resistance. 
In laboratory studies using canines, defibrillation thresholds were 
determined by the inventor at two different transthoracic resistances, the 
resistance being altered by changing electrode area or by change in 
electrode force. Under the conditions of this study, it was found that 
threshold defibrillation current was independent of the transthoracic 
resistance for a given dog and was invariant for a given animal whereas, 
in contrast, energy and voltage thresholds showed large variability. These 
results suggest that redefining defibrillation threshold in terms of peak 
current rather than energy provides a superior method of defibrillation 
(Lerman, et al., "Relationship between Canine Transthoracic Impedance and 
Defibrillation Threshold: Evidence for Current-based Defibrillation," 
Journal of Clinical Investigation, Vol. 80, pp. 797-803, September, 1987). 
To test the hypothesis that a current-based defibrillation method (vide 
infra) would result in delivering less energy and peak current than the 
standard energy-based method, the inventor hereof conducted a study in 
which eighty-six (86) patients in ventricular fibrillation were 
prospectively randomized to receive either DC countershocks according to 
the above-noted energy-based guidelines (200 J, first and second shocks, 
360 J, third shock) or to receive current-based shocks of 25 amperes 
(first shock), 25 amperes (second shock), and 40 amperes (third shock), by 
a modified defibrillator, as needed. Patients randomized to each method 
were similar with respect to age, sex, cardiac diagnosis, weight, ejection 
fraction, physical parameters and transthoracic resistance. Each method 
had statistically equivalent first shock (79% current-based versus 81% 
energy-based) and cumulative shock success rates. The mean (.+-.SD) first 
shock energy for patients receiving the current-based method was 120.+-.30 
J and 200 J for patients receiving energy-based shocks, p=0.0001. The mean 
peak current was 24.+-.2.3 A and 33.+-.5.0 A, respectively, p=0.0001. 
Therefore, for equivalent first shock success rates, the energy-based 
method delivered 67% more energy and 38% more current. High transthoracic 
resistance (&gt;90 ohms) predicted first shock failure only in patients 
defibrillated by the energy-based defibrillating method. These findings 
suggest that the current-based defibrillating method precludes 
transthoracic resistance as a major determinant of defibrillation success 
and delivers significantly less current and energy than the standard 
energy-based method for an equivalent success rate. 
BRIEF STATEMENT OF THE INVENTION 
Briefly stated, the defibrillator of this invention automatically 
determines transthoracic resistance, and then uses the thus-obtained 
resistance to calculate and charge a capacitor to the level of voltage 
necessary to deliver an amount of peak current preselected by the 
operator, an inductance, resistance, capacitance (RLC) discharge pulse 
delivers that selected peak current. This method of defibrillation is 
applicable to standard defibrillators used from transthoracic 
defibrillation and to the Automatic Implantable 
Cardcoverter-Defibrillator. 
After the defibrillating electrodes are in place on a patient's chest, a 
low amplitude, sinusoidal pulse (or a rectangular pulse of low frequency, 
such as 31 kHz) is transmitted through the electrodes and a microprocessor 
is used to calculate the transthoracic resistance in order that a selected 
value of peak amperage may be delivered to this subject by the 
defibrillator. The extreme rapidity of electrical measurements, and the 
rapid response of electrical circuits to control signals are advantageous, 
since ventricular fibrillation is of life threatening urgency and brooks 
no delay. 
The selected peak defibrillation current to be applied to the patient and 
the prospected transthoracic resistance are used to control the charge 
applied to the capacitor of the defibrillator, so that upon discharge of 
the capacitor, the selected level of peak current desired for 
defibrillation will result. The charge may be calculated by second order 
source free RLC equations (Trantham et al., Journal of Clinical 
Investigation, Vol. 72, 1562-1574, 1983). 
Electrical components and circuitry known to the art may be employed in 
practice of the invention. For example, in practice of the invention, 
standard microprocessors may be adapted to calculate, transthoracic 
resistance from a preshock low amplitude current and the 
electrode-to-electrode voltage developed responsive thereto and to 
generate the selected level of peak defibrillation current. 
Suitably, the microprocessor generates a digital signal for visual readout 
and recording and, in addition, conversion to an analog form for direct 
control over the charge being placed on the capacitor of the 
defibrillator. 
Desirably, the peak current subsequently delivered to the patient by the 
defibrillator, and the voltage between the electrodes are digitized to 
generate signals which are fed into the microprocessor which, in turn, 
computes the transthoracic resistance encountered by the defibrillation 
pulse. The microprocessor then provides appropriate signals for visual 
readout and recording. If more than one shock is required during an 
episode of ventricular fibrillation, the transthoracic resistance 
determined during the immediate preceding defibrillation attempt and the 
selected peak current will be used to control the charge stored in the 
capacitor of the defibrillator, so that upon discharge of the capacitator, 
the selected level of peak current desired for defibrillation will result. 
For future defibrillation of the same and other patients, it is important 
to know the degree to which the level of peak current actually delivered 
by the defibrillator pulse relates to the previously selected peak current 
level and how transthoracic resistance during defibrillation relates to 
the patient's resistance measured by the low amplitude exploration 
current. Given sufficient experiences, a virtually exact predictability 
for delivered peak current should result, since appropriate adjustments 
can be made in multiplication factors programmed into the microprocessor. 
DISCUSSION OF THE INVENTION 
Mention has been made that defibrillation art has concerned itself with 
measurement of transthoracic resistance and, as might be expected, some 
suggestions heretofore made to the art are capable of use in practice of 
this invention, over and above the particular mode hereinafter described. 
For example, reference is made to "Determining Transthoracic Impedance, 
Delivered Energy, and Peak Current During Defibrillation Episodes" by 
Jones et al. in Medical Instrumentation, Vol. 15, No. 6, November-December 
1981, pp. 380-382, and, to Kerber et al. "Advanced Prediction of 
Transthoracic Impedance in Human Defibrillation and Cardioversion: 
Importance of Impedance in Determining the Success of Low Energy Shocks," 
Circulation, Vol. 70, pp. 303-308, 1984. 
Important to the practice of this invention, of course, is a consonance of 
the transthoracic resistance as measured by the low amplitude exploration 
pulse to the transthoracic resistance under defibrillation pulse 
circumstances. In this connection, it is noted that Kerber, et al. 
reported that their predicted resistance correlated very well with 
defibrillation pulse resistance, and such correlation resulted when 
practice of this invention advanced from animal model results obtained in 
the genesis of this invention to clinical studies. 
Through practice of this invention, the physician may apply a 
defibrillating shock which should be adequate without being excessive, 
i.e., be close to the threshold. When using defibrillators calibrated for 
energy level selections, practice of this invention will automatically 
identify patients at the extremes of the 25-150 ohm range of human 
transthoracic resistance for whom a 200 joule defibrillator shock may be 
either far too low or excessively high and automatically will cause 
calculation of the energy level needed to apply a defibrillator shock that 
delivers the selected level of peak current more appropriate to the 
patient. Preliminary data also indicate that most humans will be 
successfully defibrillated with a peak delivered current of 25 amperes. 
The conceptual framework for practice of this invention involves selecting 
a desired amount of peak delivered current, i.e., 25 amperes, by the 
operator; prospectively and automatically determining transthoracic 
resistance of each patient by application of a low energy, high frequency 
pulse; then automatically charging the defibrillator capacitor to the 
voltage level sufficient for delivery of the selected level of peak 
current transthoracically on discharge; and automatically discharging the 
capacitor for defibrillating upon attaining such voltage level. All of the 
steps--from measurement through discharge--are performed with the 
electrodes on the patient. 
Additionally, the voltage developed between electrodes applied to the chest 
of the patient during the defibrillation shock and the peak current 
supplied by discharge thereof, may be measured so as to compute and 
display the transthoracic resistance of the patient during defibrillation 
discharge and may be used to calculate the capacitor voltage necessary to 
deliver the selected peak current on subsequent shocks) if needed. 
As pointed out by Jones et al., supra, knowledge of the internal circuit 
parameters peculiar to each defibrillator mode enables normalizing of the 
peak discharge current. Although not specifically included in the 
following description of the exemplary embodiment of this invention, 
normalization for circuit components (internal resistance) in the 
defibrillator is contemplated, including normalization for add-on internal 
circuit parameters such as those of the current sensing transformers; and 
the method and apparatus of the instant invention should be considered as 
inclusive of performing such normalization whenever desirable. The details 
of normalization described by Jones et al. supra are incorporated by 
reference herein as exemplary modes of normalization contemplated herein.

DETAILED DESCRIPTION OF THE INVENTION 
As may be seen in FIG. 1, a conventional defibrillator includes hand-held 
electrode paddles 12 having switches S.sub.3 which must be simultaneously 
closed in order to apply a defibrillation energy pulse transthoracically 
to the patient whose heart is in ventricular fibrillation. With switch 
S.sub.4 closed, closing of switches S.sub.3 allows discharge of capacitor 
C and flow of defibrillating current I.sub.2 through a circuit including 
the transthoracic resistance R.sub.ext of the patient. L.sub.2 and 
R.sub.2, respectively, represent the internal inductance and resistance 
parameters of the defibrillator. Additionally, the standard defibrillator 
includes a circuit for charging capacitor C, such indicated as high 
voltage charging circuit 14 in FIG. 1. FIG. 1 also discloses a current 
sensing transformer T.sub.2 connected to sample and hold circuit 80 and an 
appropriate voltage divider so that the transthoracic resistance R.sub.ext 
2 during the shock may be computed as the quotient of the voltage V.sub.2 
between the hand-held electrode paddles 12 divided by the sampled peak 
defibrillating current I.sub.2. As may be seen in FIG. 1, A/D converter 30 
and microprocessor 40 are used to facilitate calculation of transthoracic 
resistance and delivered energy during such defibrillation, much as 
suggested by Jones et al. supra. 
According to practice of the invention, the operator will select a desired 
peak current to be delivered during defibrillation. The microprocessor 
will then ensure charging of the capacitor to a voltage sufficient to 
deliver the selected peak current, with this capacitor voltage being 
dependent on both the selected peak current and prospectively determined 
transthoracic resistance. To prospectively determine R.sub.ext 1, a low 
amplitude (approximately 0.1 milliamp) constant current I.sub.1 generator 
20 provides a pulse of current at some fixed frequency in the range of 
20/60 kHz, which by closing switches S.sub.1 and S.sub.3, is passed 
through paddles 12 via the patient's thorax prior to discharge of 
capacitor C. A response voltage V.sub.1 is developed across paddles 12 
which is proportional to the product of the transthoracic resistance 
R.sub.ext 1 and the sampled applied current I.sub.1. Sensing current 
I.sub.1 via transformer T.sub.1 and measuring the response voltage 
V.sub.1, allows a calculated transthoracic resistance R.sub.ext 1 to be 
obtained by passing the sampled sensed current and voltage through A/D 
converter 30 then to a microprocessor 40 in which the calculation is 
performed. Since transthoracic load is predominantly resistive, it may be 
appreciated that the computed or calculated transthoracic resistance 
R.sub.ext 1 may then be used, along with the selected peak current to 
compute the voltage to which the capacitor (C) is charged in order to 
deliver preselected quantity of peak current to the patient. 
With switch S.sub.2 set to AUTO (for automatic) microprocessor 40 controls 
switches S.sub.1 and S.sub.4 and high voltage charger 14 such that, upon 
placing the paddles 12 upon the chest of the patient and depressing 
switches S.sub.3, switch S.sub.1 will be closed to apply the exploration 
current I.sub.1 across the patient's chest. Prior to, during, or after 
measurement of transthoracic resistance, high voltage charger 14 commences 
to charge capacitor C. After calculating R.sub.ext 1, microprocessor 40, 
directly or indirectly, opens switch S.sub.1 and immediately controls the 
amount of voltage to which capacitor C is charged so that it will deliver 
to the patient the preselected (by the operator) peak defibrillating 
current I.sub.2. Upon capacitor C being charged to a voltage sufficient to 
provide the preselected level of peak current I.sub.2, microprocessor 40, 
directly or indirectly, will (automatically) close switch S.sub.4 for 
consequent defibrillation of the patient. The operator may wish sometimes 
to apply standard defibrillator operation, i.e., setting of a particular 
energy level, e.g., in joules, for some particular patient and such is 
permitted by setting switch S.sub.2 to manual. 
A display 60 and recording device 70 allow display and recordation of 
important defibrillation parameters such as: the transthoracic resistance 
R.sub.ext 1 calculated from the exploration current I.sub.1 ; the 
transthoracic resistance R.sub.ext 2 computed during defibrillation of the 
patient; the measured level of peak defibrillating current I.sub.2 
delivered and the delivered energy. 
FIG. 2 is a block diagram generally illustrating the add-on components used 
with a standard defibrillator 10 for practice of the present invention. 
Like numerals have been used for like components throughout the drawings. 
As can be seen, the modification requires the addition of constant current 
generator 20 and circuitry for sensing and converting exploration current 
I.sub.1, and voltage V.sub.1 as well as defibrillating current I.sub.2 and 
voltage V.sub.2 for use by microprocessor 40 which is also added. In the 
automatic mode (S.sub.2 closed), microprocessor 40 controls the operation 
of switch S.sub.1 to apply the expoloration current and through 
digital/analog converter 50 controls the charging of the defibrillator 
capacitor (not shown) to a voltage determined by the current, selected by 
S.sub.5, and the resistance calculated from V.sub.1 and I.sub.1. When the 
proper voltage is reached the defibrillating pulse is delivered under 
control of microprocessor 40. It is also contemplated that the 
microprocessor can be programmed such that the operator may choose to use 
the defibrillator 10 in its standard or energy-based mode and that the 
capacitor will be charged to the selected energy. As can be seen, the 
operation of the modified apparatus is in all respects the same as that of 
the apparatus shown in FIG. 1. 
Just as 200 joules has been used heretofore as an experience--determined 
energy level for initial defibrillator shock, approximately 25 amperes of 
peak defibrillation current has been preselected in practice of this 
invention. This value is based upon limited human patient experience and 
some change up or down therein may be required with increased human 
patient experience. As a practical matter, it is proposed that the 
operator will be able to select from approximately 1-50 peak amperes. 
Providing a range of selectable currents also permit treatment of 
arrhythmias other than ventricular fibrillation such as e.g., ventricular 
tachyardia. Thus, for patients of high transthoracic resistance, 
defibrillating with 100 joules may apply too low a level of peak 
defibrillating current. Alternatively, guideline recommended energy levels 
of 200 joules can provide unnecessarily high peak currents to patients of 
low transthoracic resistance. Application of some fixed level of peak 
current to all patients is clearly an improvement for eliminating current 
variation patient-to-patient. 
While the invention has been described with reference to particular 
embodiments, numerous variations will be obvious to those skilled in the 
art. Such variations are within the scope of the invention as defined in 
the claims appended hereto.