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Physical principles of defibrillators
PHYSICS
Physical principles of defibrillators
Learning objectives After reading this article, you should be able to: C explain the main functional components of a defibrillator C describe different types of defibrillator waveforms and their advantages C demonstrate the safe use of a defibrillator in clinical practice
David Williams
Abstract Defibrillators use a capacitor to store a preset quantity of electrical charge and then rapidly deliver it as direct current to the myocardium to treat cardiac arrhythmias. They also incorporate an inductor to prolong the duration of the delivered current, and a rectifier to convert alternating current (AC) to direct current (DC). Modern defibrillators may be automated, and produce biphasic waveforms which increase safety and efficacy. Miniature implantable cardioverter-defibrillators (ICD) may be used in patients with recurrent life-threatening arrhythmias.
Physical principles In its most basic form, a defibrillator consists of three components: a capacitor, inductor and power supply (Figure 1). Capacitor The derived SI unit of electric charge (Q) is the coulomb, C. One coulomb is the quantity of charge transported in 1 second by a current of 1 A; equivalent to 6.24 1024 electrons. A capacitor stores energy in the form of electrical charge, and in its simplest form consists of a pair of parallel conductive plates with area of overlap (A) separated by an insulator (dielectric) of thickness (d ). Capacitance (C ) is the ability to store charge, and has derived SI units farad, F. A capacitor has 1F of capacitance if a potential difference of 1 V is present across its plates when a charge of 1 C is held by them (i.e. C ¼ Q/V). For a parallel plate capacitor C ¼ 3r A/d where 3r is the relative permittivity (dielectric constant) of the dielectric. When the switch is in position ‘1’, electrons flow from the power supply to the capacitor and a charge begins to build up on each plate, creating a potential difference. As the number of electrons on the plates increases, they increasingly oppose addition of further electrons. Plots of current flow (i) and potential difference across the capacitor (v) against time are therefore exponential curves (Figure 2a). When paddles are applied to the patient’s chest and the switch is moved to position ‘2’, current flows from the capacitor through the patient. As electrons move from the capacitor plates to the patient, both i and v fall exponentially towards zero (Figure 2b). The time constant (t) is the product of the resistance (R) and capacitance (C ) of the circuit (i.e. t ¼ R C ); and is the time taken for i or v to fall to 1/e (i.e. ¼37%) of their initial value (where e ¼ Euler’s number: 2.718.). The amount of work or energy (E ) required to move a unit of charge (Q) through a potential difference (V) is given by E ¼ Q V. However, when a capacitor is being charged, the voltage is not constant throughout the process, because the voltage is 0 at the start and V at the end. The average voltage during the charging process against which the charge has been moved will be halfway between 0 and V, i.e. V/2; and the work done in charging the capacitor is therefore given by: E ¼ Q V/2. For a perfect system with no losses, the energy released will also equal Q V/2. Substituting Q ¼ C V into the above formula gives: E ¼ C V2/2.
Keywords Capacitor; cardiac arrhythmia; cardioversion; defibrillator; inductor; inverter
Electrophysiology Fibrillation is a state of complex uncoordinated waves of myocardial depolarization resulting in decreased cardiac output. Defibrillation is the administration of a nonsynchronized direct current (DC) shock to the myocardium to terminate ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT). Cardioversion is the administration of a DC shock which is synchronized with the down stroke of the QRS complex to convert refractory tachyarrhythmias (e.g. atrial fibrillation, atrial flutter, ventricular tachycardia) to sinus rhythm. The success of defibrillation depends on: underlying cardiac condition, metabolic disturbance (e.g. hypoxia, acidosis), medications, electrode position, polarity, waveform, and transthoracic impedance (TTI). DC shocks are thought to terminate arrhythmias by extending the refractory period; after which the heart’s automaticity restores sinus rhythm. The defibrillation threshold (DFT) at which this occurs is dependent on the average current flowing through the myocardium, but for historical reasons is expressed in Joules. Excessive peak current may result in myocardial damage. The effective current delivered to the myocardium is primarily determined by the selected energy level and TTI (w50 to 150 U) due to skin contact, tissue thickness and conductivity, and lung volume. Modern defibrillators measure TTI and automatically adjust the delivered current based on the energy setting and type of arrhythmia.
Inductor The delivered current undergoes rapid exponential decay, however successful defibrillation requires current to be sustained for several milliseconds. This is achieved by a coil of wire called
David Williams FRCA DipDHM is a Consultant Anaesthetist at the Welsh Centre for Burns, Swansea, UK. Competing/conflicting interests: None to declare.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:8
384
Ó 2012 Elsevier Ltd. All rights reserved.
PHYSICS
Simplified schematic of defibrillator Switch 1
+
2 Inductor
Capacitor
+++ +++
DC power supply (~5000 V)
––– ––– Patient impedance 50–150
Figure 1
an inductor, whose net effect is analogous to the inertia of a flywheel in a mechanical system: it prevents any sudden changes in current flowing through it. When current passes through an inductor, it produces a change in the magnetic field of the coil which in turn generates a back-e.m.f. in a direction which opposes the original current flow, as predicted by Faraday’s law of electromagnetic induction. Opposition to current flow is called inductance (L), with derived SI units henry (H). A 1 H coil will induce a back-e.m.f. of 1 V when the current through it varies at the rate of 1 A/second.
the voltage delivered to the capacitor, and hence the current delivered to the patient. Many defibrillators also have internal rechargeable batteries which supply DC. This is converted to AC by an inverter, and then amplified to high voltage DC by the step-up transformer and rectifier.
Development The original defibrillator (designed by Dr W.B. Kouwenhoven, 1932) delivered current using a monophasic waveform; however most modern defibrillators employ biphasic waveforms (Figure 3), as these result in decreased peak current, reduced myocardial damage, and increased efficacy for termination of VF/VT. Early research suggested that ‘stacked’ shocks of escalating energy administered in rapid succession (e.g. 200 J, 300 J, 360 J) cause reactive cutaneous hyperaemia and oedema resulting in a progressive decrease in TTI, higher
Power supply A step-up transformer converts the mains voltage of 230 V alternating current (AC) to about 5000 V AC, which is then converted to about 5000 V DC by a rectifier. A control switch calibrated in Joules delivered (typically 2e360 J) allows the clinician to select different transformer coil windings to choose
Exponential charge (a) and discharge (b) curves for a capacitor (where Vs is the supply voltage)
100
100 Vt = Vs (1–e–t/τ)
1/3 37%
it = Vs e–t/τ 0
1
2 3 Time (τ)
4
Current (i) or voltage (v) (%)
b Current (i) or voltage (v) (%)
a
Vt = Vs e–t/τ it = Vs e–t/τ
0
5
1
2 3 Time (τ)
4
5
Figure 2
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:8
385
Ó 2012 Elsevier Ltd. All rights reserved.
PHYSICS
Examples of mono- and bi-phasic defibrillator waveforms Monophasic damped sinusoidal (MDS) (300 J at 50 )
Monophasic truncated exponential (MSE) (300 J at 50 )
Rectilinear biphasic (120 J at 50 )
0
35
Current (A)
0
Current (A)
45 Current (A)
Current (A)
45
Biphasic truncated exponential (BTE) (150 J at 50 )
0 –15
0
6 10 Time (ms)
0
6 10 Time (ms)
20
0 –15
0
6 10 Time (ms)
0
6 10 Time (ms)
Figure 3
International Electrotechnical Committee (IEC) 60601 standard symbols for ‘defibrillator safe’ equipment Type B
Type BF
Type CF
Figure 4
Safety
delivered current, and increased probability of successful defibrillation. However recent studies have shown that cutaneous blood flow during cardiopulmonary resuscitation is minimal, and no change in TTI occurs. Resuscitation Council (UK) guidelines (2010) therefore recommend single monophasic shocks of 360 J; or single biphasic shocks of at least 150 J for the first shock, followed by a fixed or escalating strategy thereafter.
The diagnosis should be confirmed. The patient and surrounding area must be clean and dry. Conductive gel pads (not liquid) are applied to the precordium across the long axis of the heart, avoiding irregular surfaces, transdermal patches and superficial (e.g. jewellery, skin clips, ECG electrodes) or implanted conductive objects. Any equipment which does not display ‘defibrillator safe’ symbols (Figure 4) may be damaged by the current, and should be disconnected from the patient before defibrillation. All sources of oxygen and flammable anaesthetic agents must be temporarily removed to prevent combustion. The paddles are firmly applied (10 kg force), and the defibrillator is charged to the appropriate energy setting. The operator must give a clear verbal warning for staff to ‘stand clear’, and ensure that no staff are in contact with the bed, patient or equipment before delivering the charge. If the charge is not required, the defibrillator is discharged by turning the control knob to zero. Paddles must never be shorted together, discharged through the defibrillator, or removed from the patient whilst charged. A
Automated external defibrillators (AEDs) analyse the patient’s heart rhythm, advise the rescuer when a shock is indicated, and can provide a series of biphasic shocks at fixed or escalating energy levels. Verbal and visual prompts instruct the rescuer to perform the appropriate steps in the resuscitation protocol. Implantable cardioverter-defibrillators (ICDs) have been commercially available since 1985 for continuous monitoring and automatic treatment of recurrent life-threatening arrhythmias. They are implanted subcutaneously in the chest wall with sensing electrodes in the right ventricle, and shocking coils in right ventricle and superior vena cava. Miniature high-capacity (Al/Al2O3) capacitors and powerful (LieAgeV2O5) batteries deliver pacing impulses or a 5e40 J DC shock directly to the myocardium as indicated.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:8
386
Ó 2012 Elsevier Ltd. All rights reserved.
PHYSICS
FURTHER READING Davey AJ, Diba A. Ward’s anaesthetic equipment. 5th edn. Elsevier, 2005. Maile KR, Warren JA. Battery System for an implantable medical device. US Patent Office May 29 2001. Patent No 6238813, 2001. Niles DE, Nishisaki A, Sutton RM, et al. Analysis of transthoracic impedance during real life cardiac arrest defibrillation attempts in older children and adolescents: are stacked shocks appropriate? Resuscitation 2010; 81: 1540e3. Resuscitation Council (UK). Adult advanced life support guidelines 2010. Scherz P. Practical electronics for inventors. 2nd edn. New York: McGraw Hill, 2007.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:8
Tooley M. Electronic circuits: fundamentals and applications. 2nd edn. Oxford: Newnes, 2001.
Acknowledgements The author would like to thank Dr A.D. Farmery, FRCA Examiner and Wellcome Senior Fellow, Nuffield Dept of Clinical Neurosciences, University of Oxford for his explanation of work done in charging a capacitor.
387
Ó 2012 Elsevier Ltd. All rights reserved.
Physical principles of defibrillators
Learning objectives After reading this article, you should be able to: C explain the main functional components of a defibrillator C describe different types of defibrillator waveforms and their advantages C demonstrate the safe use of a defibrillator in clinical practice
David Williams
Abstract Defibrillators use a capacitor to store a preset quantity of electrical charge and then rapidly deliver it as direct current to the myocardium to treat cardiac arrhythmias. They also incorporate an inductor to prolong the duration of the delivered current, and a rectifier to convert alternating current (AC) to direct current (DC). Modern defibrillators may be automated, and produce biphasic waveforms which increase safety and efficacy. Miniature implantable cardioverter-defibrillators (ICD) may be used in patients with recurrent life-threatening arrhythmias.
Physical principles In its most basic form, a defibrillator consists of three components: a capacitor, inductor and power supply (Figure 1). Capacitor The derived SI unit of electric charge (Q) is the coulomb, C. One coulomb is the quantity of charge transported in 1 second by a current of 1 A; equivalent to 6.24 1024 electrons. A capacitor stores energy in the form of electrical charge, and in its simplest form consists of a pair of parallel conductive plates with area of overlap (A) separated by an insulator (dielectric) of thickness (d ). Capacitance (C ) is the ability to store charge, and has derived SI units farad, F. A capacitor has 1F of capacitance if a potential difference of 1 V is present across its plates when a charge of 1 C is held by them (i.e. C ¼ Q/V). For a parallel plate capacitor C ¼ 3r A/d where 3r is the relative permittivity (dielectric constant) of the dielectric. When the switch is in position ‘1’, electrons flow from the power supply to the capacitor and a charge begins to build up on each plate, creating a potential difference. As the number of electrons on the plates increases, they increasingly oppose addition of further electrons. Plots of current flow (i) and potential difference across the capacitor (v) against time are therefore exponential curves (Figure 2a). When paddles are applied to the patient’s chest and the switch is moved to position ‘2’, current flows from the capacitor through the patient. As electrons move from the capacitor plates to the patient, both i and v fall exponentially towards zero (Figure 2b). The time constant (t) is the product of the resistance (R) and capacitance (C ) of the circuit (i.e. t ¼ R C ); and is the time taken for i or v to fall to 1/e (i.e. ¼37%) of their initial value (where e ¼ Euler’s number: 2.718.). The amount of work or energy (E ) required to move a unit of charge (Q) through a potential difference (V) is given by E ¼ Q V. However, when a capacitor is being charged, the voltage is not constant throughout the process, because the voltage is 0 at the start and V at the end. The average voltage during the charging process against which the charge has been moved will be halfway between 0 and V, i.e. V/2; and the work done in charging the capacitor is therefore given by: E ¼ Q V/2. For a perfect system with no losses, the energy released will also equal Q V/2. Substituting Q ¼ C V into the above formula gives: E ¼ C V2/2.
Keywords Capacitor; cardiac arrhythmia; cardioversion; defibrillator; inductor; inverter
Electrophysiology Fibrillation is a state of complex uncoordinated waves of myocardial depolarization resulting in decreased cardiac output. Defibrillation is the administration of a nonsynchronized direct current (DC) shock to the myocardium to terminate ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT). Cardioversion is the administration of a DC shock which is synchronized with the down stroke of the QRS complex to convert refractory tachyarrhythmias (e.g. atrial fibrillation, atrial flutter, ventricular tachycardia) to sinus rhythm. The success of defibrillation depends on: underlying cardiac condition, metabolic disturbance (e.g. hypoxia, acidosis), medications, electrode position, polarity, waveform, and transthoracic impedance (TTI). DC shocks are thought to terminate arrhythmias by extending the refractory period; after which the heart’s automaticity restores sinus rhythm. The defibrillation threshold (DFT) at which this occurs is dependent on the average current flowing through the myocardium, but for historical reasons is expressed in Joules. Excessive peak current may result in myocardial damage. The effective current delivered to the myocardium is primarily determined by the selected energy level and TTI (w50 to 150 U) due to skin contact, tissue thickness and conductivity, and lung volume. Modern defibrillators measure TTI and automatically adjust the delivered current based on the energy setting and type of arrhythmia.
Inductor The delivered current undergoes rapid exponential decay, however successful defibrillation requires current to be sustained for several milliseconds. This is achieved by a coil of wire called
David Williams FRCA DipDHM is a Consultant Anaesthetist at the Welsh Centre for Burns, Swansea, UK. Competing/conflicting interests: None to declare.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:8
384
Ó 2012 Elsevier Ltd. All rights reserved.
PHYSICS
Simplified schematic of defibrillator Switch 1
+
2 Inductor
Capacitor
+++ +++
DC power supply (~5000 V)
––– ––– Patient impedance 50–150
Figure 1
an inductor, whose net effect is analogous to the inertia of a flywheel in a mechanical system: it prevents any sudden changes in current flowing through it. When current passes through an inductor, it produces a change in the magnetic field of the coil which in turn generates a back-e.m.f. in a direction which opposes the original current flow, as predicted by Faraday’s law of electromagnetic induction. Opposition to current flow is called inductance (L), with derived SI units henry (H). A 1 H coil will induce a back-e.m.f. of 1 V when the current through it varies at the rate of 1 A/second.
the voltage delivered to the capacitor, and hence the current delivered to the patient. Many defibrillators also have internal rechargeable batteries which supply DC. This is converted to AC by an inverter, and then amplified to high voltage DC by the step-up transformer and rectifier.
Development The original defibrillator (designed by Dr W.B. Kouwenhoven, 1932) delivered current using a monophasic waveform; however most modern defibrillators employ biphasic waveforms (Figure 3), as these result in decreased peak current, reduced myocardial damage, and increased efficacy for termination of VF/VT. Early research suggested that ‘stacked’ shocks of escalating energy administered in rapid succession (e.g. 200 J, 300 J, 360 J) cause reactive cutaneous hyperaemia and oedema resulting in a progressive decrease in TTI, higher
Power supply A step-up transformer converts the mains voltage of 230 V alternating current (AC) to about 5000 V AC, which is then converted to about 5000 V DC by a rectifier. A control switch calibrated in Joules delivered (typically 2e360 J) allows the clinician to select different transformer coil windings to choose
Exponential charge (a) and discharge (b) curves for a capacitor (where Vs is the supply voltage)
100
100 Vt = Vs (1–e–t/τ)
1/3 37%
it = Vs e–t/τ 0
1
2 3 Time (τ)
4
Current (i) or voltage (v) (%)
b Current (i) or voltage (v) (%)
a
Vt = Vs e–t/τ it = Vs e–t/τ
0
5
1
2 3 Time (τ)
4
5
Figure 2
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:8
385
Ó 2012 Elsevier Ltd. All rights reserved.
PHYSICS
Examples of mono- and bi-phasic defibrillator waveforms Monophasic damped sinusoidal (MDS) (300 J at 50 )
Monophasic truncated exponential (MSE) (300 J at 50 )
Rectilinear biphasic (120 J at 50 )
0
35
Current (A)
0
Current (A)
45 Current (A)
Current (A)
45
Biphasic truncated exponential (BTE) (150 J at 50 )
0 –15
0
6 10 Time (ms)
0
6 10 Time (ms)
20
0 –15
0
6 10 Time (ms)
0
6 10 Time (ms)
Figure 3
International Electrotechnical Committee (IEC) 60601 standard symbols for ‘defibrillator safe’ equipment Type B
Type BF
Type CF
Figure 4
Safety
delivered current, and increased probability of successful defibrillation. However recent studies have shown that cutaneous blood flow during cardiopulmonary resuscitation is minimal, and no change in TTI occurs. Resuscitation Council (UK) guidelines (2010) therefore recommend single monophasic shocks of 360 J; or single biphasic shocks of at least 150 J for the first shock, followed by a fixed or escalating strategy thereafter.
The diagnosis should be confirmed. The patient and surrounding area must be clean and dry. Conductive gel pads (not liquid) are applied to the precordium across the long axis of the heart, avoiding irregular surfaces, transdermal patches and superficial (e.g. jewellery, skin clips, ECG electrodes) or implanted conductive objects. Any equipment which does not display ‘defibrillator safe’ symbols (Figure 4) may be damaged by the current, and should be disconnected from the patient before defibrillation. All sources of oxygen and flammable anaesthetic agents must be temporarily removed to prevent combustion. The paddles are firmly applied (10 kg force), and the defibrillator is charged to the appropriate energy setting. The operator must give a clear verbal warning for staff to ‘stand clear’, and ensure that no staff are in contact with the bed, patient or equipment before delivering the charge. If the charge is not required, the defibrillator is discharged by turning the control knob to zero. Paddles must never be shorted together, discharged through the defibrillator, or removed from the patient whilst charged. A
Automated external defibrillators (AEDs) analyse the patient’s heart rhythm, advise the rescuer when a shock is indicated, and can provide a series of biphasic shocks at fixed or escalating energy levels. Verbal and visual prompts instruct the rescuer to perform the appropriate steps in the resuscitation protocol. Implantable cardioverter-defibrillators (ICDs) have been commercially available since 1985 for continuous monitoring and automatic treatment of recurrent life-threatening arrhythmias. They are implanted subcutaneously in the chest wall with sensing electrodes in the right ventricle, and shocking coils in right ventricle and superior vena cava. Miniature high-capacity (Al/Al2O3) capacitors and powerful (LieAgeV2O5) batteries deliver pacing impulses or a 5e40 J DC shock directly to the myocardium as indicated.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:8
386
Ó 2012 Elsevier Ltd. All rights reserved.
PHYSICS
FURTHER READING Davey AJ, Diba A. Ward’s anaesthetic equipment. 5th edn. Elsevier, 2005. Maile KR, Warren JA. Battery System for an implantable medical device. US Patent Office May 29 2001. Patent No 6238813, 2001. Niles DE, Nishisaki A, Sutton RM, et al. Analysis of transthoracic impedance during real life cardiac arrest defibrillation attempts in older children and adolescents: are stacked shocks appropriate? Resuscitation 2010; 81: 1540e3. Resuscitation Council (UK). Adult advanced life support guidelines 2010. Scherz P. Practical electronics for inventors. 2nd edn. New York: McGraw Hill, 2007.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 13:8
Tooley M. Electronic circuits: fundamentals and applications. 2nd edn. Oxford: Newnes, 2001.
Acknowledgements The author would like to thank Dr A.D. Farmery, FRCA Examiner and Wellcome Senior Fellow, Nuffield Dept of Clinical Neurosciences, University of Oxford for his explanation of work done in charging a capacitor.
387
Ó 2012 Elsevier Ltd. All rights reserved.