Electric Shock Hazards In Cardiac Catheterization
By Daniel I. Weinberg, M.S.E.E., John L Art ley, D. Eng.,
Robert E. Whalen, M.D., and Henry D. Mclntosh, M.D.
• It has long been recognized that the heart
may be fibrillated as well as defibrillated by
electric shock. Burchell1 recently emphasized
the potential hazard of ventricular fibrillation
from electric shock if a pacemaker and an
electrocardiograph were connected so that a
current existed between them. It seemed possible that many of the techniques used in a
cardiac catheter laboratory, especially those
involving electrode catheters, may pose a
similar threat to patients.
The present study was designed to determine the minimum current and voltage that,
when delivered directly to the heart via intracardiac catheters, would cause fibrillation in
dogs. It was found that a very small current
or voltage could produce ventricular fibrillation. Therefore, a detailed analysis of the
equipment in a routine catheter laboratory
was undertaken to determine usually unrecognized sources of such currents. As a result
of these analyses, recommendations are made
indicating how this potential hazard can be
minimized.
Methods
ANIMAL STUDIES
Five mongrel dogs ranging from 10 to 17 Kg.
in weight were nnesthetized with pentobarbital
sodium (30 mg./Kg. body weight) administered
intravenously. After endotracheal intubation, respiration was maintained with n. positive pressure
respirator. Either no. 7 platinum-tipped electrode
From tho Cardiovnscular Laboratory, Department
of Medicine, and the Department of Electrical Engineoring, Duke University, Durham, North Carolina.
Supported in part by Grants H-4807, HTS 5369,
and H-6960 from the National Heart Institute of
tho National Institutes of Health, U. S. Public
Health Service, and grants-in-aid from the Life
1
Insurance Medical Research Fund and the American
and North Carolina Heart Associations.
This paper is based on a thesis submitted by Mr.
Weinberg, in partial fulfillment of the requirements
for the degree of Master of Science in Electrical
Engineering.
Received for publication July 9, 1962.
1004
catheters or no. 7 Lehninn catheters* were introduced into the femoral artery and/or vein and
passed under fluoroscopic control into the aorta,
right atrium, or either ventricle. These catheters
were used as electrodes and to monitor intravaseular pressures. The no. 7 Lehman catheter was
filled with either 0.85 per cent or 5 per cent sodium
chloride. Pressures were recorded by coupling
Statham strain gauge pressure transducers to a
photographic recorder-)- which wns also used for
continuous recording of the electrocardiogram.
In most cases, the current path existed between
one intrncardiac catheter nnd a 15-cm.2 metnl plate
pressed firmly against the shaven left chest wall
over the point of maximal apical impulse. Conducting paste was used to insure good electrical
conductivity between the electrode and the skin.
In some studies, the current path existed between
two intracardiac platinum-tipped electrode catheters.
The circuit diagram (fig. 1) shows the circuit
used. The transformer, T, isolates the system
from the power line ground. The variable output
transformer, e, was adjusted to supply the desired
voltage. The magnitude of the electric current
was calculated (using Ohm's law) after measuring
the voltage drop across the resistor, R,, with the
vacuum tube voltmeter, Y1. The voltage applied
across the catheter and the dog was measured with
an A.C. vacuum tube voltmeter, V2. The commercial power line frequency of 60 cycles per
second was used for these experiments because:
(1) its general availability in the United States
makes it the probable frequency used in most
catheter laboratories, and (2) it is in the range of
frequencies at which tho heart is most sensitive.2
In each study, the current magnitude was gradually increased until ventricular fibrillation developed. This phenomenon was preceded by tho
appearance of premature ventricular contractions.
When these appeared, the current was increased
more slowly. The heart was readily defibrillated
by a countershoek of 150 to 450 volts applied
across the closed chest. The position of the catheter and the number of studies carried out for each
position are listed in table 1.
PATIENT STUDIES
During normal catheterization procedures in
•U. S. Catheter Corporation, Glens Falls, New York.
(Electronics for Medicine, Tnc, ^Vhite Plains, New
York.
Circulation Research, Volume XI, December 106S
1005
ELECTRIC HAZARDS IN CATHETERIZATION
this laboratory, the patient lies on a cotton sheet
which covers an inflated vinyl air mattress on top
of n 1-inch thick foam rubber pad which is placed
on the plastic top of a fluoroscopy table. This
combination presumably isolates the patient from
the grounded table frame.
The resistance to ground was measured in 27
pntients by connecting a standard ohmmeter between an arm electrode on the patient and the
grounded table frame. This resistance was measured both beforo and after catheterization, both
with and without a connection between the patient
and the electrocardiograph. The capacitance betweon a patient on the catheter table and the table
was measured with a capncitance bridge.
Results
ANIMAL STUDIES
The results of these studies are presented
in table 1. The ventricles are clearly more
sensitive to the fibrillating effects of an electric current than is the right atrium. Ventricular fibrillation developed on the average
with a current of 170 microamperes, a voltage
of 0.2 volt, and an impedance magnitude of
the current path of 920 ohms. This voltage
and impedance were measured with platinumtipped electrode catheters in the heart. The
impedance magnitude was calculated from
simultaneous voltage and current measurements. The minimum current which produced
ventricular fibrillation was 35 microamperes
and was recorded when the current existed
between two catheters, one in each ventricle.
The minimum voltage which produced ventricular fibrillation was 0.06 volt (between
the platinum-tipped electrode catheter in the
right ventricle and the chest electrode).
PATIENT STUDIES
The resistance of patients to ground before
eatheterization, without electrocardiographic
electrodes attached to the patient, was greater
than 3 megohms. In 4 of the 27 patients, the
resistance dropped from over 3 megohms before the catheterization to an average of
37,000 ohms after the catheterization. These
measurements (both before and after catheterization) were made when the electrocardiographic leads were not connected and when
there were no catheters in the patient. The
patient was not in direct contact with the
table. The reduction in resistance after cathCiroulation Reaearch, Volume XI, Deoembsr 106t
Electrode
Catheter
Chest
Electrode
Figure I. Equipment for applying and measuring voltage and current.
riGURE 1
Equipment for applying and measuring voltage
and current. (T) isolation transformer, (e) variable voltage transformer, (B{) ammeter shunt,
(Vj) voltmeter used to measure current, and (V,)
voltmeter used to measure applied voltage.
eterization was probably caused by the introduction of a liquid, such as perspiration,
saline, or blood, which served as a path between the patient and the table. It is apparent that the events occurring during a catheterization procedure can markedly reduce the
resistance of the patient to ground. In the
other 23 eases, the resistance remained above
3 megohms. The connection of the electrocardiographic leads provides a relatively low resistance (6,000 ohms) path to ground through
the recorder used to trace the electrocardiogram. The catheterization procedure did not
appreciably affect this resistance. The capacitance measured between the patient and the
table was 700 picofarads (700 X 10"1- farads).
Discussion
That such small currents can produce electric shock in animals came as something of a
surprise. One wonders why ventricular fibrillation caused by electric shock is not more
common in cardiac catheter laboratories. The
incidence of such catastrophes is unknown,
for one learns of such accidents only by
"word of mouth." The incidence is probably
small, however. Over 2,000 humans with intracardiac catheters connected to electronic
recording equipment have been studied in this
Cardiovascular Laboratory. There have been
four episodes of established ventricular fibrillation. In two of these studies, the catheter
1006
WEINBERG, ABTLEY, WHALEN, McINTOSH
TABLE 1
Summary of Results
(l)
Current path
[. Kight vent,
to chest
TI. Left vent,
to cliest
III. Kight atrium
to chost
IV. Right vent,
to left vont.
Y. Left vent,
to chest
VI. Left vent,
to cheat
(2)
Electrode
typo
(3)
Number
of
Dogs Fibr.
(4)
Fibr. current
(microamperes)
Mesn S.D.
Mln.
(6)
Fibr. voltage
(volts)
Min.
Mean
S.D.
(6)
Resistance
(ohms)
Mean
S.D.
Pt
0
20
60
205
158
0.06
0.18
0.1
920
860
Pt
3
16
40
162
30
0.10
0.21
0.12
910
312
Pt
O
7
7,000 •.
7,300
548
1.2
2 27
0.29
439
33
Pt
2
7
35
149
97
0.25
0.33
0.04
1,420
120
5%
NaCl
1
O
140
140
0.85%
NuCl
1
6
75
110
was in the root of the aorta at the time of
arrhythmia. At the time, none of these arrhythmias was thought to have been produced
by electric shock since other well-recognized
precipitating causes for ventricular fibrillation were present in each case. However,
these studies suggest that electric shock could
possibly have caused or contributed to the
two episodes of arrhythmia that developed
with the catheter in the ventricle. The possibility of such an accident occurring requires
that cardiac physiologists use the utmost care
in laboratory equipment and study design.
The type of catheter used in these studies
as an electrode did not appear to alter the
fibrillating current appreciably. The current
magnitude resulting in ventricular fibrillation
using the saline-filled catheter is approximately the same as that observed with the
platinum-tipped electrode catheter (table 1).
The voltage requirements, however, were considerably higher for the saline-filled catheters because of their relatively high resistance.
The resistance of the platinum-tipped electrode catheter was measured at 5 ohms; that
of the no. 7 Lehman catheter filled with 5 per
cent saline solution was 57,000 ohms, while
the catheter filled with 0.85 per cent saline
had a resistance of 350,000 ohms.
There was considerable variation in the
current magnitude required to produce ven-
86
12.0
13.0
1.0
93,000
7,000
24.0
30.0
14.0
276,000
31,600
tricular fibrillation in the same and in different dogs. This variation may be due in part
to instant-to-instant changes in the metabolic
state of the heart. More likely it was caused
by variation in the current density in the
heart muscle.
For a given current, the current density in
the muscle will tend to be higher if the two
electrodes: (1) are closer together rather than
far apart, (2) are small rather than large,
(3) are in intimate contact with the muscle
rather than having an intermediate volume
of blood, and (4) have sharp projections
rather than smooth faces. For example, it
was possible to induce ventricular fibrillation
with an otherwise subfibrillatory current if
the electrode catheter was pressed against the
heart wall. When no current was in the catheter, manipulation of the electrode against
the endocardium might produce extrasystoles
but not ventricular fibrillation. The assumption that current density is critical for
inducing ventricular fibrillation is consistent
with earlier observations by Hooker.3 Although the present studies did not establish
that ventricular fibrillation was induced when
the electrode was not touching the endocardium, it appears that contact merely facilitates this phenomenon.
ANALYSIS OF ELECTRIC SHOCK HAZARDS
This study demonstrated that a current as
low as 35 microamperes can cause ventricular
Circulation Retearch, Volume XI, December l>6t
1007
ELECTRIC HAZARDS IN CATHETERIZATION
fibrillation in dogs. A voltage from catheter
tip to chest electrode as low as 0.06 volt can
produce the same catastrophe. If a safety
factor of 10 is introduced to account for unusually sensitive conditions, a current equal
to or greater than 3.5 microamperes is a potentially dangerous current. By the same
reasoning, it might be assumed that a voltage
of 6 millivolts between the catheter tip and
the chest electrode is potentially dangerous.
In figure 2 is a diagram of a current path
which can exist with a possible connection of
equipment in normal operating conditions.
The essential features of the power supply
transformer and the input stage of the amplifier are shown. The intracardiac catheter
is connected to the grid of the first stage of
the amplifier through a coupling capacitor,
Cg. The 1-megohm grid resistor, Rg, is connected to the chassis of the amplifier, as is
the center tap of the secondary of the power
supply transformer. The case and chassis of
the amplifier and power supply in this example are not grounded. RL and CL represent,
respectively, the resistance and capacitance of
the patient to ground. RL will vary over a
wide range depending on whether an electrocardiograph is connected to the patient or
whether connection to ground is established
during catheterization through liquids spilled
on and around the patient.
For this analysis, values for RL and CL are
assumed to be 10,000 ohms and 700 picofarads, respectively. Therefore, the impedance
from patient to ground at 60 cycles per second is essentially 10,000 ohms. The circuit
diagram (fig. 2) shows the 60-cycle power
supply to the amplifier to be grounded and
the switch, S, open between ground and the
primary of the supply transformer. The capacitance Ct represents the electrostatic coupling between the primary and secondary
windings of the power supply transformer.
The capacitance is about 350 picofarads. Rt
represents leakage resistance from primary to
secondary windings and may be assumed in
this example to be 10° ohms.
An equivalent circuit of the current pathCirculatioii Research, Volumo XI. December 196t
Electrode
Catheter
Patient
Figure 2.
Current Pathways.
Patient
C
Figure 3.
Equivalent Circuit.
/^Chassis
Connection
-|f Capacitor
> Transformer
©Power
Ground
-"*- Resistor
Source
-•''-Switch
i --HEIectron Tube
FIGURES 2 and 3
Current pathtvays. (e) voltage source, (S) power
line mvitch, (C,) electrostatic coupling between
primary and secondary windings of transformer,
(Rt) leakage resistance between transfonner windings, (Cg) input coupling capacitor of amplifier,
(RB) grid-leak resistor, (RL) leakage resistance
between patient and ground, and (CL) capacitance
between patient and ground.
Equivalent circuit. Symbols are the same as in
figure 2.
way described in figure 2 is indicated in figure
3. With the impedances corresponding to the
values given in the previous paragraph (ignoring those impedances which have negligible
effect) and assuming an applied line voltage
of 120 volts, the current in the "heart" is
15.7 microamperes, which is potentially dangerous.
Occasionally, in medical electronic equipment, the power supply transformer is
shielded to eliminate appreciable capacitive
coupling between the primary and secondary
windings. If, however (fig. 3), a leakage
resistance, Rt, from the primary to secondary
1008
is of the order of 40 megohms, a potentially
dangerous current still exists. If a capacitor
of 0.01 microfarad (0.01 X 10"6 farads) is
placed from the primary winding to the
chassis (as is often done to reduce 60 cycle
noise),4 approximately 110 microamperes
would be delivered to the heart.
PRECAUTIONS AGAINST ELECTRIC SHOCK
Extrapolating results obtained from experiments on animals to man should always be
done with caution. However, if there is a difference in the ventricular fibrillation threshold level in man, the safety precautions to be
recommended are still valid. If the actual
values are larger, no harm is done by taking
these precautions, while if they are smaller,
the precautions become even more necessary.
Bach cardiac catheter laboratory has its
own characteristics. It is, therefore, important that safety practices be established under
the supervision of a qualified engineer. The
safety program should cover the three phases
of operation: (1) design and construction of
equipment, (2) operating procedure, and (3)
preventive maintenance.
The materials and components used in medical equipment should be of the highest quality to minimize the danger of breakdown and
electric shock. Where possible, hermetically
sealed components and devices should be used
to prevent the introduction of moisture and
foreign matter which can speed deterioration
of insulation and electrical characteristics.
Proper grounding of electrical equipment
not only reduces the danger to patient and
personnel, but also reduces the electrical noise
level.5 Properly connected three-prong plugs
and receptacles should be used. Such connections cause the ground side of the line to remain the same with respect to the position
of ground on the equipment. There should
be a common and single ground lead to which
all connections are made. This should be large
enough to carry ground currents without appreciable voltage drops and should be connected to ground at a point near the entrance
of the power system ground into the building.
To this common ground should be connected:
WEINBEEO, ABTLEY, WHALEN, McINTOSH
(1) all chassis and cases of amplifiers, (2) the
shield between primary and secondary power
supply transformer, (3) one side of amplifier
signal input (balanced amplifiers should be
grounded by case and chassis), (4) all conductors within reach of patients.
The ungrounded side of all circuits should
be fused at a level which permits just the
proper current required for the instrument.
AVhere it is necessary to reclose circuits
quickly after a fault, for example, x-ray
fluoroscopic equipment, defibrillator, etc.,
quickly reset circuit breakers rather than
fuses should be used.
The preventive maintenance program can
be divided into two parts: (1) the testing and
inspection of the instruments and (2) inspection of the building utilities.
The instruments must be inspected and
tested for adequate insulation resistance, good
insulation condition, proper fusing, and
proper ground connections. The building
utilities must be tested and inspected for
proper ground connections.
Insulation resistance should be tested with
a meter made for that purpose, which applies
500 volts to the insulation under test and
which is used in accordance with the meter
manufacturer's instruction. The resistance
should be measured periodically. The results
should be plotted on a graph, as a function
of time. In this way, a drop in resistance can
be detected and the cause corrected before a
dangerous condition arises.
Ground resistance should be tested with an
instrument which applies a current of several
amperes. The resistance should be low enough
so the voltage drop to the common ground
point in the room, under the maximum fault
current, will not exceed 0.01 volt.
These tests and inspections should be made
by conscientious, qualified technicians. Visual
inspections should be made daily for proper
connections, frayed insulation, etc. The resistance of the insulation and ground connections should be measured upon receipt of
equipment, whenever equipment is changed,
and monthly until a history of normal values
is obtained. After normal values are deterCirculation Reacarah, Volume XI, December 196t
1009
ELECTRIC HAZARDS IN 0ATHETERIZAT1ON
mined, measurements every three months
should suffice. The building ground should
be tested whenever there is a change in the
electrical system, as well as every three
months.
RECOMMENDED LABORATORY PROCEDURE
Before an electrode catheter is connected
to a patient, but after it is connected to its
amplifier, the potential between the catheter
and the patient and between the catheter
and ground should be measured with a sensitive, high-impedance voltmeter (A.C. and
D.C.). If a dangerous potential exists, the
catheter should not be connected to the patient until the source of the voltage has been
determined and the situation corrected. If,
after the catheter has been inserted, additional electrical connections are made to the
patient, the potential of the new lead with
respect to the patient and with respect to
ground should be similarly measured.
Summary
1. These studies demonstrate that ventricular fibrillation can be produced in anesthe-
tized, mechanically ventilated dogs by a current as low as 35 microamperes and a voltage
as low as 0.06 volt.
2. Analysis of circuits of medical electronic
equipment which is usually considered safe
indicates that such low current levels may
exist in the heart during eatheterization.
3. The safety procedures established for
our laboratory to minimize the hazard of
electric shock are presented.
References
1. BUROHELL, H. B.: Hidden hazards of enrdinc
pacemakers. Circulation 24: 161, 1961.
2.
KOUWENHOVEN,
W.
B.,
HOOKER,
D.
K.,
AND
LOTZ, E. L.: Electric shock effects of fre
quency. Electrical Engineering 55: 384, 1936.
3.
HOOKER,
D.
B.,
KOUWENHOVEN,
W.
B.,
AND
LANGWORTHY, O. R.: Effect of alternating current on the heart. Am. J. Physiol. 103: 444,
1933.
•1. GEIQES, K. S.: Measurement of electric shock
hazard in radio equipment. Underwriters Laboratories, Inc. Bull. Res., vol. 33, I!)4i5.
5. National Electrical Code, Standard of the National Board of Fire Underwriters. New York,
National Board of Fire Underwriters, 1959.
Book Review
Physiologie Hnmalne. Oellnlaire et Organique,
H. Laborit. Pnris, Masson & Cie, 1961, 585
pages, illustrated. $12.00 soft cover, $13.60 hard
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Circulation Research, Volume XI, December 196t
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