Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N1/00—Electrotherapy; Circuits therefor
  • A61N1/18—Applying electric currents by contact electrodes
  • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
  • A61N1/36—Applying electric currents by contact electrodes alternating or intermittent currents for stimulation
  • A61N1/362—Heart stimulators
  • A61N1/3625—External stimulators
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61N—ELECTROTHERAPY; MAGNETOTHERAPY; RADIATION THERAPY; ULTRASOUND THERAPY
  • A61N1/00—Electrotherapy; Circuits therefor
  • A61N1/18—Applying electric currents by contact electrodes
  • A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
  • A61N1/38—Applying electric currents by contact electrodes alternating or intermittent currents for producing shock effects
  • A61N1/39—Heart defibrillators
  • A61N1/3904—External heart defibrillators [EHD]

Definitions

• the invention relates generally to defibrillation and to implementation of pacemaking.
• the invention relates to the integration of defibrillation and pacemaking functions into the same device in such a way that both defibrillation and pacemaking can be carried out by the apparatus.
• Ventricular fibrillation is a chaotic heart pace on account of which the heart does not pump blood efficiently enough. This will probably result in cardiac arrest. The best way to prevent cardiac arrest is defibrillation carried out early enough. Defibrillation denotes applying a strong current pulse through the heart to terminate fibrillation. Defibriliators again are apparatus by means of which the current pulse is applied to the patient. There are both external and internal defibriliators, and various pulse forms are used therein to eliminate fibrillation.
• the pulse form employed in external defibriliators has been the monophasic pulse, which means that the current pulse applied to the patient maintains its direction during the entire duration of the pulse.
• the maximum energy applied to the patient is normally approximately 360 J. (Assuming that the impedance of the patient is about 50 . Generally, the requisite energy is proportional to the inverse of the impedance.)
• the biphasic pulse form has been introduced in external defibriliators. This pulse form has been applied already for years in internal defibrillation (i.e., applied directly to the cardiac surface).
• the biphasic pulse form has the advantage that to achieve successful defibrillation, the pulse energy need not be as great as with the monophasic pulse (in practice, the energy of the biphasic pulse is in the order of 150...250 J). On account of this, smaller patient currents, smaller component sizes and lesser disadvantage to the patient are achieved.
• FIG. 1 illustrates a typical biphasic defibrillation pulse.
• a biphasic pulse comprises two parts (phases) that are denoted with references A and B in the figure. In the latter part (B), the direction of the current is reverse to that of the first part (A). The length of the entire pulse is typically in the order of 10 ms.
• One explanation for the effectiveness of the biphasic defibrillation pulse is the fact that the latter phase strives to restore the voltage accumulated across the myocardial cell membranes back to its initial state and thereby to prevent reoccurrence of fibrillation.
• defibriliators also incorporate a synchronizing function enabling well-timed application of a current pulse in relation to the patient heart rate (electrocardiogram).
• This synchronizing feature is necessary in the treatment of other forms of cardiac arrhythmias than ventricular fibrillation, since a pulse applied at an incorrect locus of the electrocardiogram may result in ventricular fibrillation. In such cases, synchronized cardioversion is concerned.
• an external temporary pacemaker is, for its part, to assist the resuscitation of patients suffering from arrhythmia.
• a regular electrical pulse train is applied to the patient via electrodes attached to the patient's chest.
• Pacemaking differs from defibrillation in character, since the energy of the synchronizing pulse is considerably lower than that of the defibrillation pulse, and the pacemaking function is a relatively long-term operation compared to defibrillation.
• the duration of the pacemaking function can vary from several minutes up to hours.
• a modern defibrillator usually also integrates an external temporary pacemaker.
• the current implementations are typically as shown in Figure 2, i.e., they have a dedicated circuit for the defibrillation function (10) and the pacemaking function (20). These circuits usually employ common electrodes (E1 , E2) that are attached to the patient.
• the control unit CU of the apparatus is employed for selecting whether the apparatus is used in the defibrillation mode or in the pacemaking mode, and consequently one of the circuits is switched to the electrodes by means of switches (SW1...SW4).
• the implementation of the defibrillation and pacemaking functions with separate circuits is due to the fact that said functions are difficult to combine, as they are electrically very different and one may not interfere with the other so as to impair the result of the operation to be performed or to cause any other disadvantage to the patient.
• the pacemaking function which by nature is of long duration
• the electrocardiogram is measured from the same electrodes that are used for defibrillation or pacemaking.
• Polarization may also cause electrolytic irritation at the electrode-skin interface.
• the combination of defibrillator and pacemaker is quite complex and also costly to implement. The clear distinc- tiveness of the operations also adversely affects the size and weight of the apparatus.
• the objective of the present invention is to eliminate the drawbacks described above and to provide a combination of defibrillator and pacemaker that is compact in structure and more cost-effective than hitherto.
• the basic idea of the invention is to use the capacitor employed as the energy storage for the defibrillation pulse also as an energy storage for pacemaking pulses, and to switch the pacemaking pulses to the patient through at least one disconnecting or breaking element for patient leakage current, preferably through at least one other capacitor.
• Patient leakage current refers to a current leaking from the defibrillator/pacemaking apparatus through the patient to the ground or from the patient through the apparatus to the ground, said current being generated by an unintentional voltage external to the patient.
• the disconnecting or breaking element prevents the passage of such a current, or at least the DC component thereof, to the patient.
• the apparatus can also be made lighter than heretofore. Reducing the size and weight is of advantage particularly when the apparatus is being used by perambulatory paramedics, such as ambulance crew.
• both the defibrillator and the pacemaker are biphasic, and furthermore, during the latter phase of the pacemaking pulse the charge accumulated in the capacitance of said at least one other capacitor or the capacitance of a patient electrode during the first phase is discharged.
• Figure 1 illustrates a typical pulse form applied by a defibrillator to electrodes attached to a patient
• FIG. 2 shows a typical prior art defibrillator into which a pacemaking function is incorporated
• Figure 3 depicts a combination of defibrillator and pacemaker in accordance with the invention in the initial situation of the pacemaking operation
• Figure 4a shows an apparatus in accordance with Figure 3 during the first phase of the defibrillation pulse
• Figure 4b shows the apparatus of Figure 4a during the latter phase of the defibrillation pulse
• Figure 5 depicts the pulse train to be applied to the patient in the pacemaking function of the apparatus depicted in Figures 4a and 4b.
• FIG. 3 illustrates the combination of defibrillator and pacemaker in accordance with the invention in the initial state of the pacemaking function.
• the biphasic defibrillator part is imple- mented using an H-bridge connection known per se, employing one or more high-voltage capacitors C1.
• the energy of the defibrillation pulse is charged in this high-voltage capacitor whose plus terminal is connected through switch S1 and relay R2 to a first patient electrode E1 and through switch S2 and relay R2 to a second patient electrode E2.
• the minus terminal of the capacitor again is connected through switch S3 and relay R1 to the first patient electrode E1 and through switch S4 and relay R2 to the second patient electrode E2.
• the patient electrodes (which are attached to the patient) are thus disconnected from the apparatus by means of relays (or other corresponding patient disconnecting means).
• the relay terminals facing away from the patient (which in this connection are called the first relay terminals) are interconnected with a resistance Re1 , the task of which is to effectively short-circuit the leakage currents from switches S1...S4 when the switches are in non-conductive state, so that the voltage across relays R1 and R2 is always considerably less than 500 V.
• the size of the resistance is in the order of 10 kc2...1 M .
• the plus terminal of the defibrillation capacitor is further connected through switch SO to a constant-current power source CCS, whose output OP is again connected through a breaking or disconnecting element RC2 for patient leakage current to the first patient electrode E1 (i.e., to the terminal of relay R1 facing the electrode) and through switch S5 to the first terminal of relay R2.
• the disconnecting element is preferably a capacitor, but it may also be a switch or a relay.
• the constant-current power source is controlled by means of a control circuit CC by adjusting the throughgoing resistance of transistor TR (the resistance between the emitter and the collector) through transformer T1 , and thereby the current passing through the transistor.
• the constant-current power source and the control thereof are implemented in a known manner, and therefore will not be described in greater detail in this context. If the disconnecting element is a switch or a relay, it is brought to conductive state by means of the control circuit for the duration of the pacing pulse. If the disconnecting element is a capacitor, no control is needed.
• the apparatus shown in Figure 3 naturally also includes a power source by means of which the capacitor C1 is charged to the desired voltage and control means by means of which the operation of the switches is controlled. For simplicity, the power source is only depicted in Figure 3 and the control means are not shown at all. In Figure 3, the defibrillator part of the apparatus is denoted with a broken line indicated with reference DP and the pacemaking part with a broken line indicated with reference PP.
• switches S1...S5 can be implemented for example with IGBT transistors, SCR thyristors or combinations of these.
• switches SO and S5 are turned off.
• capacitor C1 is first charged to the required high voltage (typically in the order of 2.5 kV) using a suitable power source, such as a switched-mode power supply.
• a suitable power source such as a switched-mode power supply.
• Relays R1 and R2 are turned on immediately before the application of the current pulse, that is, the patient is connected to the defibrillator part immediately prior to the application of the current pulse. At other times, the relays are turned off to prevent any patient leakage current.
• switches S1 and S4 are first turned on while switches S2 and S3 are turned off.
• the switches and relays are thus in the position shown in Figure 4a.
• switches S1 and S4 are turned off and switches S2 and S3 are turned on.
• the switches and relays are thus in the position shown in Figure 4b, and thus the current passes through the patient in the reverse direction as compared to the first phase.
• relays R1 and R2 are turned off to avoid any leakage current to the patient.
• Defibriliators must also incorporate an internal discharge for the high- voltage capacitor.
• the constant-current power source CCS of the pacemaker can be used for this purpose in such a way that no energy is conducted to the patient. This is effected in such a way that the relays are turned off and only switches SO, S5 and S4 are conductive.
• the switches are initially in the position shown in Figure 3, that is, switches SO and S4 are turned on, switches S1 , S2 and S5 are turned off, relay R1 is turned off and relay R2 is turned on.
• the high-voltage capacitor C1 is charged to pacemaking voltage, which is a voltage considerably lower than the defibrillation voltage, typically in the order of 250 V.
• the voltage is generated either by charging the capacitor from the power source or discharging the defibrillation voltage, depending on whether the apparatus has been used for defibrillation previously.
• a continuous pulse train in accordance with Figure 5 is supplied to the electrodes, in which pulse train one pacemaking pulse comprises two phases similarly as the defibrillation pulse.
• the pulse frequency is typically 40...180 pulses per minute, and therefore the duration T of the phase typically varies in the range 0.33 S...1.5 s.
• the length of one biphasic pacemaking pulse is dependent on the patient but is typically between 10 and 40 ms.
• the first part (A) of the biphasic pacemaking pulse is applied to the patient through switch SO, constant-current power source CCS and disconnecting element RC2 for patient leakage current. If the disconnecting element is a capacitor, the voltage across it is 0 V in the initial state but rises linearly (constant current).
• the second part (B) of the pacemaking pulse is generated by turning switch S5 on, as a result of which capacitor RC2 is discharged through switch S5. Therefore, during the second phase (B) the current passing through the patient is reverse to that of the first phase, thus rendering the pacemaker is biphasic, like the defibrillator. If it is desired to restrict the current in the second phase, a resistance can be connected in series with switch S5. The above biphasic operation is continued at the desired frequency, the pulse train supplied to the electrodes (patient) being in accordance with Figure 5.
• the first part of the pacemaking pulse is preferably a constant-current pulse and the second part discharges the charge accumulated in capacitor RC2 or the capacitance of the electrode during the first phase.
• Capacitor RC2 will eliminate the DC component of a patient leakage current when the electrodes attached to the patient are in place for a longer period and the pacemaker is not turned on.
• an alternative way to prevent any patient leakage current from the pacemaking part is to use as the disconnecting or breaking element RC2 a relay or a switch that is only turned on for the duration of the pacemaking pulse under control of the control circuit. In that case, the pacemaking pulse is monophasic and the above stated linear voltage rise across the disconnecting element does not occur.
• a switch and a relay both the AC and the DC component of the patient leakage current can be cut off.
• a capacitor has the advantage that it does not need any control and it enables, in the simple manner described above, a biphasic pacemaker preventing the above-stated polarization of electrodes.

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• Health & Medical Sciences (AREA)
• Cardiology (AREA)
• Heart & Thoracic Surgery (AREA)
• Engineering & Computer Science (AREA)
• Biomedical Technology (AREA)
• Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
• Radiology & Medical Imaging (AREA)
• Life Sciences & Earth Sciences (AREA)
• Animal Behavior & Ethology (AREA)
• General Health & Medical Sciences (AREA)
• Public Health (AREA)
• Veterinary Medicine (AREA)
• Electrotherapy Devices (AREA)

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Cited By (3)

Citations (3)

• 1999
  • 1999-10-14 FI FI992219A patent/FI19992219A/fi unknown
• 2000
  • 2000-10-13 WO PCT/FI2000/000894 patent/WO2001026731A1/en active Application Filing
  • 2000-10-13 AU AU79267/00A patent/AU7926700A/en not_active Abandoned

Patent Citations (3)

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