A practical approach to fetal electrocardiography

A practical approach to fetal electrocardiography GRAHAM IRVING J.

SCHULER, H.

C.

WHYTE,

Ottawa,

Ontario,

M.D.

GLASS, M.D.,

M.D.,

F.R.C.P.(C)

F.R.C.S.(C)

Canada

The technical dificulties which have prevented the widespread use of fetal electrocardiography have been largely overcome by employing a very inexpensive fetal ECG preamplifier which is simple to operate and tolerant of poor electrode technique. Designed to appeal to those who are interested, but not active in fetal electrocardiografihy, the preamfilifiet selectively amplifies the fetal ECG so that any standard electrocardiograph will display fetal signals.

THE ROLE which fetal electrocardiography may play in routine obstetrical care has yet to be determined. Potential investigators of this role have been interested by many studies which have demonstrated its value in some particular cases, and suggested its usefulness in more general circumstances. However, as these studies often require sophisticated personnel for the operation of complex and costly equipment, and often produce results of minor relevance to routine obstetrical care, there has been little fetal electrocardiographic activity at the grass-roots level. The creation of a very simple and inexpensive preamplifier which selectively amplifies fetal signals, enables an inexperienced operator to easily obtain fetal ECG’s with any standard electrocardiograph. With the removal of some major impediments to its widespread use, fetal electrocardiography may find a more useful place in obstetrics. Extensive interest has been created by the investigators who have demonstrated the immense value of the fetal electrocardiogram as a detector, an indicator, and as a trigger. From the National the Card&pulmonary of Obstetrics and C+iG Hospital.

Research Unit Gynecology,

As a detector of fetal life and multiple pregnancy, the fetal electrocardiogram has proved very useful.1-3 As an indicator, the fetal ECG has demonstrated fetal distress, congenital heart disease, fetal presentation, maturity, and response to labor.3-6 As a trigger for tachycardiographic studies an ECG has demonstrated the fetal response to labor, anoxia, and maternal hypotension.21 3l 7 As a trigger for signal integration devices, the fetal ECG has been computer-processed to remove the interference and noise associated with small signals and reveal the subtle wave-form changes of congenital heart disease and fetal distress.* Unfortunately, this interest has not lead to a general activity in fetal electrocardiography, due both to the nature of these investigations and the character of the fetal ECG signal. Investigations made by a trained staff with expensive equipment, and the frequent support of engineering personnel and large research grants appear to place fetal electrocardiography beyond average means. The resulting publications often appear as support for some technique, and rarely view with enthusiasm the universal use of fetal electrocardiography. The fetal ECG signal appearing upon the maternal abdomen characteristically has an amplitude only 2 per cent of the adult signal

Council and and Division Ottawa 546

Volule Number

Fetal electrocardiography

97 4

obtained in routine electrocardiography and as such is easily susceptible to masking by power-line interference, maternal heart and muscle potentials, and random noise within the electrode and amplifier system. The low level of fetal signals accounts for discouraging reports in which fetal life is undetectable in 10 per cent of cases examined in the latter half of gestation,2-4 so that the detection of fetal death is impossible and the determination of fetal life is haphazard. Furthermore, multiple pregnancies can be detected in only half of such cases,g and a fetal signal of sufficient quality to reveal fetal distress, maturity, or congenital heart disease is almost impossible to obtain2* 3l *O Indeed it appears that, as yet, the fetal ECG has no place in routine obstetrical care since the required investment of funds is outweighed by technical complexity and the lack of diagnostic accuracy. Diagnostic deficiencies notwithstanding, a similar critique of the diagnostic accuracy of the common stethescope as used in obstetrics and chest auscultation would reveal a similarly crude instrument, frequently noninformative and often requiring subjective interpretation, but the features of simplicity and low cost which make the stethescope invaluable are not found in current fetal electrocardiographic devices. The creation of special fetal electro-

Fig.

1. The

simple

preamplifier

and

electrodes

547

cardiograph+ 6~ I1 and preamplifiers for standard ECG recorders’, 4~6, I2 have lacked the cost or convenience advantages necessary to popularize fetal electrocardiography. If fetal electrocardiograms could be obtained for the cost and operational simplicity of the common stethescope, then the great advantage of acquiring a written record of the fetal heart rate over any period of time would result in a greater use of fetal electrocardiography. A more widespread use would remove the biggest barrier to fetal electrocardiography-that of general unfamiliarity-and finally permit an assessment of what place the fetal ECG has in the routine practice of obstetrics. It is interesting to speculate that if, like fetal electrocardiography, auscultation of the chest were first be‘gun a few years ago in a laboratory setting with unfamiliar, complicated, and expensive equipment, the simple stethescope perhaps would yet have to find its universal role as a generally useful and occasionally invaluable instrument. Apparatus In an effort to encourage the wider use of fetal electrocardiography, a very simple, low-noise preamplifier has been developed to afford the display of fetal signals with any standard ECG recorder (Fig. 1) . The low

in

relation

to a standard

electrocardiograph.

548

Schulet,

Glass,

Fig. 2. A pictorial trodes,

preamplifier

and

February 15, 1967 Am. J. Obst. & Gynec.

Whyte

diagram showing the relationship of the fetal heart, abdominal surface components, and the signal input cable of a standard electrocardiograph.

NOISERiFiRRED TOWJT: RI-IO* LOI FREOULNC”I d b POINT “16” FREOYEWI 3 db PO,“7 OPERdTlNGTFYPERAWIEIIWE POIEII ORAIN.T. 25.C

Fig. 3. The with low the sharp

u) I: -20 5

I, :: s

io ;w . 23 WI .8 er :c’ ,(I MI

circuit diagram and electrical characteristics of the fetal preamplifier noise, low cost components. The use of inductors in the filtering section cornering characteristics of the frequency response curve.

cost, small size, operation simplicity, and routine usefulness of this preamplifier are intended to parallel the features of the common stethescope. The preamplifier is a selfcontained battery-operated octave bandwidth amplifier with which the fetal ECG acquired from bipolar abdominal leads is selectively amplified to present a millivolt signal at the left and right arm leads of a standard electrocardiograph (Fig. 2). The low noise, high input impedance and good differential rejection ratio of the amplifier section provide a tolerance to the poor electrode technique of most tyro fetal electrocardiographers (Fig. 3).

elec-

constructed accounts for

As the overwhelming majority of fetal electrocardiographic examinations are undertaken to detect fetal life, multiple pregnancy, and the state of fetal health as revealed by the fetal heart rate and rhythm; and as these examinations depend solely upon the recognition of the fetal QRS patterns, the frequency response of the preamplifier has been designed to specifically augment the fetal QRS complex (Fig. 4). The ability of the preamplifier to favor the fetal QRS is explained by the character of the frequency response curve which is centered at 30 c.p.s. to capture the peak of fetal QRS energy. 3* 8 The octave band-width pre-

Volume 97 Number4

Fetal

electrocardiography

549

Fig. 4. The frequency spectrum of the maternal ECG signal, random noise, and some 60 cycle interference found at electrode sites commonly used in fetal electrocardiography. Although the exact form of the spectrum varies with the particular subject and electrode location, the preponderance of maternal ECG energy is always below 20 c.p.s. The preamplifier is especially responsive to the fetal QRS near 30 c.p.s. and relatively unresponsive to the maternal ECG, noise, and interference.

serves those characteristics of the QRS which enable recognition of small fetal signals amid noise of greater amplitude. The relative suppression of signals below 20 c.p.s. and above 40 c.p.s. attenuates the effect of power-line interference, random noise, electrode movement, and the maternal ECG and EMG. The preamplifier does not offer features that are unique in themselves but does offer usefulness at a component cost less than that of many stethescopes. Three similar electrode and lead assemblies are used for grounding the patient and providing a bipolar fetal signal. Suction cup electrodes are adequate for short term recording, while disposable adhesive electrodes soldered to 24 inches of a No. 30 gauge stranded unshielded lead afford greater subject acceptance and are therefore more suitable for examination over prolonged periods. Procedure It was felt that without preparation of the patient or electrode site this simple preamplifier could provide the fetal heart rate in over 80 per cent of cases examined in the latter half of gestation, and that this operational simplicity would commend itself to those interested in the potentialities of fetal electrocardiography, but discouraged by the techniques. To assess what results might be

obtained by the novice in his first attempts at fetal electrocardiography, an operator without previous experience was instructed to record abdominal signals from the first 10 subjects arriving in the labor area of a large hospital. The technique was to be most casual, the patients were to be examined in their labor rooms with paste smeared disposable adhesive electrodes placed in the midline pubic and fundic regions, and the subject grounding electrode applied at any convenient location. The preamplifier was then to be turned “on,” the signal output cable connected to the RA, LA, and RI, leads of a commercial ECG recorder, the recorder turned “on,” the selector switch set to Lead I, and the preamplifier signal level control advanced to obtain the most informative fetal si
550

Schuler, Glass, and Whyte

Fig. 5. The first ten traces recorded preamplifier.

February 15, 1967 Am. J. Obst. & Gynec.

by an inexperienced

lent specimen traces. Statistical studies often measure the success rate of any method in terms of the paucity of useless traces, and as such, depend both on the skill of the operator to acquire good fetal signals and the experience of the interpreter to make use of the most obscure traces. The traces of Fig. 5 are designed to be specifically useful to those interested, but not active in fetal electrocardiography, since they show what any novice might acquire employing the simplest of techniques. The effects of weak fetal signals and maternal muscular tension have produced the traces in which a fetal signal is not obvious on first inspection. An application of geometrical dividers to suspected fetal signals reveals their inherent periodicity and enables the determination of fetal heart rate with a high degree of confidence. The effect of a frequency re-

operator

using the simple fetal

sponse which best permits an identification of the fetal QRS is seen in the traces which are free of drift and minimally affected by the maternal ECG complex. A presentation of the first 100 traces would better show the range of fetal signals and the improvements resulting from operator experience. The first 10 traces are useful, however, when considering the fundamental problems of fetal electrocardiography: 1. Would a written record of fetal heart rate and rhythm be useful in the routine conduct of obstetrics? Would an ability to detect fetal life and multiple pregnancy be of value? 2. With simple means, could an operator obtain useful fetal signals in his first attempts at fetal electrocardiography? 3. How important are the limitations imposed by the low level of fetal signals found

V&me Number

97 4

upon the maternal abdomen? Is the occasional noninformative tracing not to be tolerated? Does the more frequent occurrence of ambiguous traces lead to a subjective diagnosis, having no place in obstetrical care? It is hoped that this proposed simple method will assist the potential fetal electrocardiographer in considering these questions. Appendix

The routine application of midline electrodes without preparation of the subject or electrode site should provide the fetal heart rate from over 80 per cent of the subjects examined at term. Success rates which approach 100 per cent at term13 are attained with control of the factors which govern the magnitude of fetal signal, noise, and artifact. An appreciation of these factors and their control is of use to any potential fetal electrocardiographer. Unsuccessful attempts at fetal electrocardiography are associated with one or more of three circumstances: (a) some easily corrected error in the connection or operation of the system, usually indicated by a trace without a maternal ECG signal or with excessive noise; (b) the failure of the fetal heart to supply an adequate signal to the electrodes, revealed in a tracing with a normal complement of noise and maternal signal but without a fetal signal; and (c) the presence of excessive noise, artifact, and interference which masks the fetal signal. The failure of the fetal heart to supply an adequate signal to electrodes placed in the midline position may indicate an insufficient amount of functional myocardium. More often, though, a reapplication of the electrodes to overlie the fetal cephalic and caudal areas, or over the fetal mouth and placental site, will give success in over half of those tracings which are apparently noninformative due to insufficient fetal signal.’ Noise at the electrode site is generated by a chemically contaminated skin or electrode, a poor electrical contact between the electrodes and the skin, and an unstable physical relationship between the electrode and skin.

Fetal

electrocardiography

551

The following measures will reduce noise at the electrode site: (a) nondisposable electrodes are cleaned to remove all traces of electrode paste and metallic salts and oxides; (b) the skin is cleansed with alcohol, acetone, or soap and water; (c) the skin is rubbed with a stiff brush, alcohol swab, or medium garnet paper to remove the most superficial cornified layers; (d) a high salinity electrode paste such as Cambridge or Sandborn-Redux is massaged into the skin and the surplus removed; (e) a thin film of paste is applied to the electrode surface; (f) the electrode is applied and the lead so placed that without traction upon the electrode, a stable physical state is maintained between the electrode and the skin. Artifact of maternal origin arises from muscle tension, which not only disturbs the physical relationship between the electrode and the skin, but more importantly generates electromyographic potentials which mask the fetal ECG. Inasmuch as a subject who is at ease in mind, body, and spirit will be without muscle tension, adequate subject preparation is essential. The subject must be psychologically prepared by a confident and sympathetic operator who explains the innocuous nature of the procedure which provides information of interest to the subject herself and of value to the attending physician. The subject must be physically prepared with the examination occurring in a pleasant restful place, free of distractions. Her body must be comfortable. Appropriately placed pillows and bolsters insure comfort without muscular tension. The use of mild sedation is occasionally of benefit. Interference from 60 cycle power lines is recognizable in the uniformly wide band of base-line noise created as the writing pen oscillates to and fro 60 times a second. This interference, often originating in the electric cords of lamps, radios, and patient call systems, is usually overcome by unplugging or removing all power cords in the vicinity of the subject. The removal of some electrocardiographic writers from the immediate area of the subject will lessen their radiative ef-

552

Schuler,

Glass,

and

February 15, 1967 Am. J. Obst. & Gym.

Whyte

fects. Of course, it is necessary that the power cord of the electrocardiograph should not pass near the subject. Interference from a standard electrocardiograph equipped with a two-prong (ungrounded) power plug may be reduced by grounding the device to the clean metallic surface of a water pipe or radiator. The metal frame of the stretcher or bed upon which the subject lies, may be grounded to the standard electrocardiograph, considerably reducing interference. Keeping the electrode leads to the preamplifier short

and intertwined reduces the susceptibility of the lead system to interference. Finally, a simple set of geometrical dividers is a most valuable asset in fetal electrocardiography. With their assistance, tracings which on first inspection appear hopeless are often made informative. Peaks of base-line noise, or disturbances of base-line noise, are marked with pencil. More often than not, these marks are shown with the dividers to be periodic and thus very strongly suggestive of fetal cardiac activity.

REFERENCES

1. 2. 3. 4. 5. 6. 7.

Hildebrand, W.: Int. Conf. Med. Elect. 2nd, Paris, 1959. Hon, E., and Hess, 0.: AM. J. OBST. & GYNEC. 79: 1012. 1960. Mayes, B., Bradfield, A., and Smyth, E.: M. J. Australia 2: 905, 1963. Buxton, T., Hsu, I., and Barter, R.: J. A. M. A. 185: 441, 1963. Larks, S.: Obst. & Gynec. 22: 427, 1963. Schmidt, O., Cruikshank, L., and Saunders, M.: AM. J. OBST. & GYNEC. 83: 464, 1962. Hon, E., Reid, B., and Hehre, F.: AM. J. OBST. & GYNEC. 79: 209, 1960.

8. 9. 10. 11. 12. 13.

Hon, E., and Lee, S.: M. Electronics & Biol. Engin. 2: 71, 1964. Storer, W., Johnson, F., and Stander, R.: Surg., Gynec. & Obst. 119: 1233, 1964. Lee, S., and Hon, E.: AM. J. OBST. & GYNEC. 92: 1140, 1965. Larks, S., and Larks, G.: AM. J. OBST. & GYNEC. 90: 1350. 1964. Southern, E.: AL. J. OBST. & GYNEC. 73: 233, 1957. I Kendall, J. OBST.

B., Farell, & GYNEC.

D., 83:

and Kane, 1629, 1962.

H.:

AM.