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The use of liquid-junction electrodes in recording the human electrocardiogram (EGG)
1. ELECTROCARDIOLOGY, 1 (1),51-56, 1968
The Use of Liquid-Junction Electrodes in Recording the Human Electrocardiogram (ECG)* L. A. GEDDES, M.E., PH .D.t, L. E. BAKER, PH.D.t AND A. G. MOORE§
SUMMARY The electrical behavior of liquid-junction electrodes for recording the ECG was studied in relation to electrode area and input impedance of the ECG apparatus. It was shown that electrodes of the size commonly used (10 mm) require an input impedance well abo ve' 3 megohms if amplitude loss is to be avoided.
the amplifier employed must have a value many times higher than the impedance appearing across the electrode terminals, if the events are to be reproduced faithfully. This paper reports on a study of the distortion produced in the ECG when various amplifier input impedances were used with liquid -junction electrodes of various areas, and compares the data with those obtained with standard plate electrodes.
INTRODUCTION The ability ofliquid-junction electrodes (Fig. 1) to produce clean electrocardiograms on exercising subjects has been well documented. Their high electrical stability is due ma inly to stabilization of the electrode-electrolyte interface and the associated electrical double layer of ctrarge which is responsibl e for the half-cell potential and a major portion of the impedance of the electrode. Deriving their origin from studies in which the galvanic skin reflex was measuredlr", liquid-junction electrodes were soon called into service for recording the electrocardiogramv". Although permitting acquisition of the ECG in s'ituations where standard plate electrodes would fail, the higher impedance of liquid-junction electrodes requires that special consideration be given to the type of electronic equipment used with them. Electrode impedance refers strictly to the impedance of the electrode-electrolyte junction; however, the term "electrode impedance" is often used to describe the impedance measured between the electrode terminals which consists of the impedance of both electrodes and that of the subject between them. When high impedance electrodes are employed to detect bioelectric events, the input impedance of I
* Supported in part by Grant USPH 5-TI-HE 05125 CIO.
t Professor of Physiology, Chief, Section of Biomedical Engineering, Baylor University College of Medicine, Houston, Texas77025. Requestsfor reprints should be addressed to Dr. Geddes. f Assistant Professor of Physiology, Baylor University Collegeof Medicine. § Department of Physiology, Baylor University College of Medicine.
Electrode Wire
. - . - - Electrode Insulating Washer Adhesive Washer
Fig. 1. Liquid-junction electrodes. Methods and Materials In this study electrocardiograms were made using liquid-junction electrodes of dilTercntknown areas placed over each end of the sternum of human subjects'! and secured to the skin with Stomaseal." In order to vary the area, electrodes of a special design were fabricated; Fig. 2 shows
* 3M Medical Products Co., St. Paul, Minn.
52
RELIEF HOLE FOR JElLY
1
!!
GEODES, BAKER AND MOORE
t+:tELECTRODE TERMINAL ...-"0" RING SILVER ELECTRODE (2 em diameter)
..............INTERCHANGEABLE CAP -*em.
1.5 1.0 0.5 0.2
em em em em
~ STOMASEAL
ADHESIVE WASHER
DIAMETER OF ELECTRODES
Fig. 2. Variablearea liquid-junction electrode.
the essential details. Plastic caps having different sized holes were applied as shown in Fig. 2. In each case the metal electrode (silver) remained the same and its effective area was that of the hole. The skin was first prepared by cleaning with alcohol. Then the electrode cap was applied, and the hole in the cap was filled with Burdick! electrode jelly to provide a conducting path between the electrodes and skin. The electrode was then inserted into the cap and pressed down and the excessjelly was expelled through the relief hole. The recording apparatus consisted of a high gain differential amplifier system,with an input
t BurdickCorp., Milton, \Vis.
impedance of 9 megohms and an overall time constant of 3 seconds as measured on the graphic record produced in response to an applied step voltage. The connections for recording the electrocardiogram are shown in Fig. 3. With the equipment as shown it was possible to record electrocardiograms with any chosen resistive input impedance up to 9 megohms. The electrocardiograms were also recorded on magnetic tape for replay with increased amplitude. The procedure consisted of first obtaining control records with RL = 0:), i.e., the input impedance was that of the amplifier (9 megohms). Then the input impedance was lowered in steps by connecting known loading resistors (RL) across the
RECORDING PEN
ts
PRE-AMPlIfiER
TO TAPE
RECORDER
Fig. 3. Arrangement of apparatus used to investigate the relationship between electrode area and amplifier input impedance.
53
LIQUID-JUNCTION ELECTRODES
input impedance of the amplifier and recordings were taken for chosen values of RL from 3 megohms to 1000 ohms. When the ECG began to be reduced in amplitude, the amplification of the recording system was increased by a known a mount to enable easier measurement and better comparison of the ECG's with the control records. Before and after a run with each electrode area, impedance-frequency curves were measured in the frequency range extending from 10 Hz to 20KHz. The current used to measure the impedance was 2 microamperes rms. The impedance-frequency curves presented in this paper represent the averages of the pre- and post-run measurements. To minimize the effect of changes in the skin surface due to repeated applications of electrodes, a different site was chosen for each electrode. The sites were immediately adjacent to one another. To relate the data obtained with liquid-junction electrodes to those obtained with standard plate electrodes, the same procedure was carried out with the plate electrodes connected in a standard lead II configuration. The effect of connecting different resistances across the electrode terminals was quantitated by careful measurement of the records with a magnifying glass and ruler. For each resistance value, the amplitudes of three R, Sand T waves were measured. The average R, Sand T amplitudes were then calculated. The amplitudes obtained with the 9 megohm input impedance were taken as 100% (controls) and the average amplitudes obtairred with the various values of RL were expressed as percentages of these control values. Finally, the percent amplitudes for R, Sand T for each RL were averaged. Thes e values were then plotted. Presented in this way, the data show the loss in amplitude of the ventricular components of
the electrocardiograms for various resistive input impedances. Results
Figure 4 presents the percent amplitudes of the RST complex of the electrocardiogram with the various values of resistances connected across the input terminals of the amplifier. The figures 3, 20, 80 and 180, on the curves identify the areas (in square millimeters) of the various liquid-junction electrodes. The curve labelled 2200 mm? is a plot of the data obtained with standard ECG plate electrodes connected to the same subject in lead II configuration. Figure 5 presents the impedance-frequency characteristics of the various electrodes studied. The upper four curves describe the characteristics of the liquid-junction electrodes of areas 3, 20, 80 and 180 rnm-, The lower curve is the impedancefrequency characteristic for the ECG plate electrodes (2200 mm") applied to the right arm and left leg of the same subject.
Discussion From Figure 4, it is apparent that lowering the input impedance by placing resistance across the input terminals of the amplifying apparatus diminishes the amplitude of the ECG. The severity of amplitude loss is inversely related to the area of the electrode; i.e., for a loss in amplitude to be apparent with large area electrodes, a relatively
O.:lM
CI.1M
30<
,.
" " 10 "
1::0" 1~.!"'~1'
1M
1'.'llt::,l.': ;C! ,-:,._••• ~)
Fig. 4. Average percent amplitude vs. input irnpedance for electrodes of various areas,
100 .......--+-_ _....._-+-_ '0
100
300
__--_-_---1 I I(
3<
10<
3C~
mOL
Fig. 5. Impedance-frequency characteristics of electrodes of various areas.
54
GEDDES, BAKER AND MOORE I'
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e
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C.I
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Fig. 6. The relationshipbetween electrodearea and input impedance for 5 to 20% loss in amplitude. low resistance must be connected across the input terminals. When recording the ECG, or any other bioelectric event, it is, of course, desirable to avoid a loss in signal and thereby record the true amplitude. From the data presented in Fig. 4, it is possible to specify the input impedance which will produce a known loss in amplitude with electrodes of various areas. Fig. 6 presents the relationship between input impedance and electrode area for various percentage reductions in amplitude as derived from the data obtained in this study. From this illustration it is apparent that if a liquidjunction electrode with to-mm diameter circular silver plate (approximately 80 mrn? in area) is used with an ECG amplifier with a 2.7 megohm input impedance, a 5% loss in average amplitude of the R, Sand T waves will occur. If the input impedance of the amplifier is 0.6 megohm, the average amplitude loss will be to%. Similarly, the amplitude loss with other areas and input impedances can be predicted. In Fig. 6, the data obtained with the standard plate electrodes (2200 rnm")are included. It is interesting to note that 5 % loss of amplitude is encountered with an input impedance of 25,000 ohms. These data are consistent with the impedance-frequency characteristics shown in Fig. 5 which shows that irrespective of frequency, the larger the area of the electrode, the lower is its impedance. In this study the effect of lowering input impedance was confined mainly to reduction in overall amplitude of the components of the ECG. In a previous study's in which the ECG of the dog was recorded with small area needle electrodes inserted subcutaneously, a reduction in input impedance not only reduced the overall amplitude of the ECG, but, in addition, resulted in electrical
differentiation of the P, R, Sand Twaves, causing them to become markedly biphasic when low resistance values were connected across the electrodes. The lack of prominence of this phenomenon in this study may be due in part to the difference in contour of the impedance-frequency curves. The impedance of needle electrodes decreased with increasing frequency in the range of 0-100 Hz, while the impedance of the liquid-junction electrodes was more uniform in the same frequency range; i.e., that region in which the sinusoidal frequency components of the ECG are located. The impedance-frequency curves for the needle electrodes indicate that the electrode polarization impedance was reactive which could account for the electrical differentiation encountered. On the other hand, the impedance-frequency curves of the liquid-junction electrodes indicate that the electrode polarization impedance was essentially resistive; a finding consistent with only amplitude loss with a lowering of input resistance. Current density considerations may account for the lack of prominence of electrical differentiation encountered with the liquid-junction electrodes used in this study. The geometry of these electrodes was uniform, i.e., each consisted of a plane surface separated at a constant distance (2.5 mm) from the subject. Thus with loading, the current density distribution was essentially uniform, and because the areas studied were relatively large, low current densities were encountered. With the needle electrodes reported in the previous study, the areas were smaller and the electrode surface was conical; therefore, the current density at the point of the needle was probably very high. Schwan" showed that electrode polarization impedance becomes a function of current density when values of about 1 malcm'' are exceeded. Such a condition which could well exist at the sharp points of needle electrodes, may have contributed to the distortion reported previously. From the data adduced in this investigation, it is clear that liquid-junction electrodes, such as those investigated in this study, require the use of an amplifier input impedance considerably higher than that which can be used with plate electrodes. For a to-mm diameter electrode, an input impedance in excess of 3 megohms is required if loss of amplitude is to be avoided. More important, if such electrodes are used with averaging resistors, such as those employed with the V lead or in special vectorcardiographic weighting networks,
LIQUID-JUNCTION ELECTRODES
serious loss of amplitude. can result. In circumstances such as these, or in any situation in which an amplifier input impedance cannot be made sufficiently high, the advantages ofliquid-junction electrodes can be retained if a high input impedance isolating stage, consisting of a cathode or emitter follower or a suitable circuit using a field effect transistor, is connected between the electrodes and the apparatus with which it is to be used.
REFERENCES 1. Haggard, E. A, and Gerbands, R.: An apparatus for the measurement of continuous changes in palmar skin resistance. Journ. Exp. Psychol. 37: 92-98, 1947. 2. Shackel, B.: A rubber suction cup surface electrode with high electrical stability. Journ. Appl, Physiol., 13: 153-158, 1958. 3. O'Connell, D. N., Tursky, B., and Orne, M. T.: Electrodes for recording skin potential. Arch. Gen. Psychiat., 3: 252-258, 1960. 4. O'Connell, D. N., and Tursky, B.: Special modifications of the silver-silver chloride sponge electrode for skin recording. USAF-Oflice of Scientific Research. Contract AF-49 (638)-728. 37 pp. 5. Hindler, E., and Santa Maria, L. J.: Response of subjects to some conditions of a simulated orbital flight pattern. Aerosp, Med., 32: 126-133, 1961. 6. Rowley, D. A, Glagov, S., and Stoner, P.: Fluid electrodes for monitoring the electrocardiogram
7. 8. 9. 10.
1I.
12. 13.
14. 15.
16.
55
during activity and for prolonged periods of time. Amer. Heart Journ., 62: 263-269, 1961. Roman, J.: Flight research program HI-High impedance electrode techniques. Aerosp. Med., 37: 790-795, 1966. Roman, J., and Lamb, L.: Electrocardiography in flight. Aerosp. Med., 33: 527-544, 1962. Mason, R. E., and Likar, I.: A new system for multiple lead electrocardiography. Amer. Heart Journ., 71: 196-205, 1966. Boter, J., den Hertog, A., and Kuiper, J.: Disturbance-free skin electrodes for persons during exercise. Med. and Biol, Eng., 4: 91-95, 1966. Day, J., and Lippitt, M.: A long term electrode system for electrocardiography and impedance pneumography. Psychophysiology, 1: 174-182, 1964. Lucchina, G. G., and Phipps, C. G.: An improved electrode for physiological recording. Aerosp, Med., 34: 230-231,1963. Lucchina, G. G., and Phipps, C. G.: A vectorcardiographic lead system and physiologic electrode configuration for dynamic readout. Aerosp. Med., 33: 722-729, 1962. Geddes, L. A., Partridge, M., and Hoff, H. E.: An EKG lead for exercising subjects. Journ. Appl. Physiol., 15: 311-312, 1960. Geddes, L. A, and Baker, L. E.: The relationship between input impedance and electrode area in recording the ECG. Med. and BioI. Eng. 4: 439450,1966. Schwan, H. P., and Maczuk, J. G.: Electrode polarization impedance: limits of linearity. 18th Ann. Conf. on Eng. in Med. and BioI., 7: 24. McGregor & Werner, Inc., Washington, D. C. 1965.
The Use of Liquid-Junction Electrodes in Recording the Human Electrocardiogram (ECG)* L. A. GEDDES, M.E., PH .D.t, L. E. BAKER, PH.D.t AND A. G. MOORE§
SUMMARY The electrical behavior of liquid-junction electrodes for recording the ECG was studied in relation to electrode area and input impedance of the ECG apparatus. It was shown that electrodes of the size commonly used (10 mm) require an input impedance well abo ve' 3 megohms if amplitude loss is to be avoided.
the amplifier employed must have a value many times higher than the impedance appearing across the electrode terminals, if the events are to be reproduced faithfully. This paper reports on a study of the distortion produced in the ECG when various amplifier input impedances were used with liquid -junction electrodes of various areas, and compares the data with those obtained with standard plate electrodes.
INTRODUCTION The ability ofliquid-junction electrodes (Fig. 1) to produce clean electrocardiograms on exercising subjects has been well documented. Their high electrical stability is due ma inly to stabilization of the electrode-electrolyte interface and the associated electrical double layer of ctrarge which is responsibl e for the half-cell potential and a major portion of the impedance of the electrode. Deriving their origin from studies in which the galvanic skin reflex was measuredlr", liquid-junction electrodes were soon called into service for recording the electrocardiogramv". Although permitting acquisition of the ECG in s'ituations where standard plate electrodes would fail, the higher impedance of liquid-junction electrodes requires that special consideration be given to the type of electronic equipment used with them. Electrode impedance refers strictly to the impedance of the electrode-electrolyte junction; however, the term "electrode impedance" is often used to describe the impedance measured between the electrode terminals which consists of the impedance of both electrodes and that of the subject between them. When high impedance electrodes are employed to detect bioelectric events, the input impedance of I
* Supported in part by Grant USPH 5-TI-HE 05125 CIO.
t Professor of Physiology, Chief, Section of Biomedical Engineering, Baylor University College of Medicine, Houston, Texas77025. Requestsfor reprints should be addressed to Dr. Geddes. f Assistant Professor of Physiology, Baylor University Collegeof Medicine. § Department of Physiology, Baylor University College of Medicine.
Electrode Wire
. - . - - Electrode Insulating Washer Adhesive Washer
Fig. 1. Liquid-junction electrodes. Methods and Materials In this study electrocardiograms were made using liquid-junction electrodes of dilTercntknown areas placed over each end of the sternum of human subjects'! and secured to the skin with Stomaseal." In order to vary the area, electrodes of a special design were fabricated; Fig. 2 shows
* 3M Medical Products Co., St. Paul, Minn.
52
RELIEF HOLE FOR JElLY
1
!!
GEODES, BAKER AND MOORE
t+:tELECTRODE TERMINAL ...-"0" RING SILVER ELECTRODE (2 em diameter)
..............INTERCHANGEABLE CAP -*em.
1.5 1.0 0.5 0.2
em em em em
~ STOMASEAL
ADHESIVE WASHER
DIAMETER OF ELECTRODES
Fig. 2. Variablearea liquid-junction electrode.
the essential details. Plastic caps having different sized holes were applied as shown in Fig. 2. In each case the metal electrode (silver) remained the same and its effective area was that of the hole. The skin was first prepared by cleaning with alcohol. Then the electrode cap was applied, and the hole in the cap was filled with Burdick! electrode jelly to provide a conducting path between the electrodes and skin. The electrode was then inserted into the cap and pressed down and the excessjelly was expelled through the relief hole. The recording apparatus consisted of a high gain differential amplifier system,with an input
t BurdickCorp., Milton, \Vis.
impedance of 9 megohms and an overall time constant of 3 seconds as measured on the graphic record produced in response to an applied step voltage. The connections for recording the electrocardiogram are shown in Fig. 3. With the equipment as shown it was possible to record electrocardiograms with any chosen resistive input impedance up to 9 megohms. The electrocardiograms were also recorded on magnetic tape for replay with increased amplitude. The procedure consisted of first obtaining control records with RL = 0:), i.e., the input impedance was that of the amplifier (9 megohms). Then the input impedance was lowered in steps by connecting known loading resistors (RL) across the
RECORDING PEN
ts
PRE-AMPlIfiER
TO TAPE
RECORDER
Fig. 3. Arrangement of apparatus used to investigate the relationship between electrode area and amplifier input impedance.
53
LIQUID-JUNCTION ELECTRODES
input impedance of the amplifier and recordings were taken for chosen values of RL from 3 megohms to 1000 ohms. When the ECG began to be reduced in amplitude, the amplification of the recording system was increased by a known a mount to enable easier measurement and better comparison of the ECG's with the control records. Before and after a run with each electrode area, impedance-frequency curves were measured in the frequency range extending from 10 Hz to 20KHz. The current used to measure the impedance was 2 microamperes rms. The impedance-frequency curves presented in this paper represent the averages of the pre- and post-run measurements. To minimize the effect of changes in the skin surface due to repeated applications of electrodes, a different site was chosen for each electrode. The sites were immediately adjacent to one another. To relate the data obtained with liquid-junction electrodes to those obtained with standard plate electrodes, the same procedure was carried out with the plate electrodes connected in a standard lead II configuration. The effect of connecting different resistances across the electrode terminals was quantitated by careful measurement of the records with a magnifying glass and ruler. For each resistance value, the amplitudes of three R, Sand T waves were measured. The average R, Sand T amplitudes were then calculated. The amplitudes obtained with the 9 megohm input impedance were taken as 100% (controls) and the average amplitudes obtairred with the various values of RL were expressed as percentages of these control values. Finally, the percent amplitudes for R, Sand T for each RL were averaged. Thes e values were then plotted. Presented in this way, the data show the loss in amplitude of the ventricular components of
the electrocardiograms for various resistive input impedances. Results
Figure 4 presents the percent amplitudes of the RST complex of the electrocardiogram with the various values of resistances connected across the input terminals of the amplifier. The figures 3, 20, 80 and 180, on the curves identify the areas (in square millimeters) of the various liquid-junction electrodes. The curve labelled 2200 mm? is a plot of the data obtained with standard ECG plate electrodes connected to the same subject in lead II configuration. Figure 5 presents the impedance-frequency characteristics of the various electrodes studied. The upper four curves describe the characteristics of the liquid-junction electrodes of areas 3, 20, 80 and 180 rnm-, The lower curve is the impedancefrequency characteristic for the ECG plate electrodes (2200 mm") applied to the right arm and left leg of the same subject.
Discussion From Figure 4, it is apparent that lowering the input impedance by placing resistance across the input terminals of the amplifying apparatus diminishes the amplitude of the ECG. The severity of amplitude loss is inversely related to the area of the electrode; i.e., for a loss in amplitude to be apparent with large area electrodes, a relatively
O.:lM
CI.1M
30<
,.
" " 10 "
1::0" 1~.!"'~1'
1M
1'.'llt::,l.': ;C! ,-:,._••• ~)
Fig. 4. Average percent amplitude vs. input irnpedance for electrodes of various areas,
100 .......--+-_ _....._-+-_ '0
100
300
__--_-_---1 I I(
3<
10<
3C~
mOL
Fig. 5. Impedance-frequency characteristics of electrodes of various areas.
54
GEDDES, BAKER AND MOORE I'
!?<;
e
'"!!
t.3
!{
C.I
" ~
t.;
" C." C.CI I
1:1
:,
1:-')
:!:t
1m
:xo
r.!::::::~:; J..".:A. £';'''''':.1
Fig. 6. The relationshipbetween electrodearea and input impedance for 5 to 20% loss in amplitude. low resistance must be connected across the input terminals. When recording the ECG, or any other bioelectric event, it is, of course, desirable to avoid a loss in signal and thereby record the true amplitude. From the data presented in Fig. 4, it is possible to specify the input impedance which will produce a known loss in amplitude with electrodes of various areas. Fig. 6 presents the relationship between input impedance and electrode area for various percentage reductions in amplitude as derived from the data obtained in this study. From this illustration it is apparent that if a liquidjunction electrode with to-mm diameter circular silver plate (approximately 80 mrn? in area) is used with an ECG amplifier with a 2.7 megohm input impedance, a 5% loss in average amplitude of the R, Sand T waves will occur. If the input impedance of the amplifier is 0.6 megohm, the average amplitude loss will be to%. Similarly, the amplitude loss with other areas and input impedances can be predicted. In Fig. 6, the data obtained with the standard plate electrodes (2200 rnm")are included. It is interesting to note that 5 % loss of amplitude is encountered with an input impedance of 25,000 ohms. These data are consistent with the impedance-frequency characteristics shown in Fig. 5 which shows that irrespective of frequency, the larger the area of the electrode, the lower is its impedance. In this study the effect of lowering input impedance was confined mainly to reduction in overall amplitude of the components of the ECG. In a previous study's in which the ECG of the dog was recorded with small area needle electrodes inserted subcutaneously, a reduction in input impedance not only reduced the overall amplitude of the ECG, but, in addition, resulted in electrical
differentiation of the P, R, Sand Twaves, causing them to become markedly biphasic when low resistance values were connected across the electrodes. The lack of prominence of this phenomenon in this study may be due in part to the difference in contour of the impedance-frequency curves. The impedance of needle electrodes decreased with increasing frequency in the range of 0-100 Hz, while the impedance of the liquid-junction electrodes was more uniform in the same frequency range; i.e., that region in which the sinusoidal frequency components of the ECG are located. The impedance-frequency curves for the needle electrodes indicate that the electrode polarization impedance was reactive which could account for the electrical differentiation encountered. On the other hand, the impedance-frequency curves of the liquid-junction electrodes indicate that the electrode polarization impedance was essentially resistive; a finding consistent with only amplitude loss with a lowering of input resistance. Current density considerations may account for the lack of prominence of electrical differentiation encountered with the liquid-junction electrodes used in this study. The geometry of these electrodes was uniform, i.e., each consisted of a plane surface separated at a constant distance (2.5 mm) from the subject. Thus with loading, the current density distribution was essentially uniform, and because the areas studied were relatively large, low current densities were encountered. With the needle electrodes reported in the previous study, the areas were smaller and the electrode surface was conical; therefore, the current density at the point of the needle was probably very high. Schwan" showed that electrode polarization impedance becomes a function of current density when values of about 1 malcm'' are exceeded. Such a condition which could well exist at the sharp points of needle electrodes, may have contributed to the distortion reported previously. From the data adduced in this investigation, it is clear that liquid-junction electrodes, such as those investigated in this study, require the use of an amplifier input impedance considerably higher than that which can be used with plate electrodes. For a to-mm diameter electrode, an input impedance in excess of 3 megohms is required if loss of amplitude is to be avoided. More important, if such electrodes are used with averaging resistors, such as those employed with the V lead or in special vectorcardiographic weighting networks,
LIQUID-JUNCTION ELECTRODES
serious loss of amplitude. can result. In circumstances such as these, or in any situation in which an amplifier input impedance cannot be made sufficiently high, the advantages ofliquid-junction electrodes can be retained if a high input impedance isolating stage, consisting of a cathode or emitter follower or a suitable circuit using a field effect transistor, is connected between the electrodes and the apparatus with which it is to be used.
REFERENCES 1. Haggard, E. A, and Gerbands, R.: An apparatus for the measurement of continuous changes in palmar skin resistance. Journ. Exp. Psychol. 37: 92-98, 1947. 2. Shackel, B.: A rubber suction cup surface electrode with high electrical stability. Journ. Appl, Physiol., 13: 153-158, 1958. 3. O'Connell, D. N., Tursky, B., and Orne, M. T.: Electrodes for recording skin potential. Arch. Gen. Psychiat., 3: 252-258, 1960. 4. O'Connell, D. N., and Tursky, B.: Special modifications of the silver-silver chloride sponge electrode for skin recording. USAF-Oflice of Scientific Research. Contract AF-49 (638)-728. 37 pp. 5. Hindler, E., and Santa Maria, L. J.: Response of subjects to some conditions of a simulated orbital flight pattern. Aerosp, Med., 32: 126-133, 1961. 6. Rowley, D. A, Glagov, S., and Stoner, P.: Fluid electrodes for monitoring the electrocardiogram
7. 8. 9. 10.
1I.
12. 13.
14. 15.
16.
55
during activity and for prolonged periods of time. Amer. Heart Journ., 62: 263-269, 1961. Roman, J.: Flight research program HI-High impedance electrode techniques. Aerosp. Med., 37: 790-795, 1966. Roman, J., and Lamb, L.: Electrocardiography in flight. Aerosp. Med., 33: 527-544, 1962. Mason, R. E., and Likar, I.: A new system for multiple lead electrocardiography. Amer. Heart Journ., 71: 196-205, 1966. Boter, J., den Hertog, A., and Kuiper, J.: Disturbance-free skin electrodes for persons during exercise. Med. and Biol, Eng., 4: 91-95, 1966. Day, J., and Lippitt, M.: A long term electrode system for electrocardiography and impedance pneumography. Psychophysiology, 1: 174-182, 1964. Lucchina, G. G., and Phipps, C. G.: An improved electrode for physiological recording. Aerosp, Med., 34: 230-231,1963. Lucchina, G. G., and Phipps, C. G.: A vectorcardiographic lead system and physiologic electrode configuration for dynamic readout. Aerosp. Med., 33: 722-729, 1962. Geddes, L. A., Partridge, M., and Hoff, H. E.: An EKG lead for exercising subjects. Journ. Appl. Physiol., 15: 311-312, 1960. Geddes, L. A, and Baker, L. E.: The relationship between input impedance and electrode area in recording the ECG. Med. and BioI. Eng. 4: 439450,1966. Schwan, H. P., and Maczuk, J. G.: Electrode polarization impedance: limits of linearity. 18th Ann. Conf. on Eng. in Med. and BioI., 7: 24. McGregor & Werner, Inc., Washington, D. C. 1965.