1975, British Journal of Radiology, 48, 477-480
JUNE
1975
Ultrasound recording of fetal breathing By H. B. Meire, M.B., B.S., D.Obst., R.C.O.G., D.M.R.D., F.R.C.R., P. J. Fish, B.Sc, M.Sc, and T. Wheeler, M.A., M.R.C.O.G. Departments of Radiology, Medical Physics and Obstetrics, King's College Hospital, London {Received February, 1974) ABSTRACT
Apparatus for recording fetal chest wall movements is described. Special reference is made to technical considerations which can improve the quality of records obtained. Fetal breathing was observed in 44 out of 45 recordings. Maternal respiration and pulse could give rise to potentially confusing signals which were identified by recording these parameters on a separate channel.
Dawes et al. (1970) showed that fetal lambs performed rapid irregular breathing movements and that these were normally present for 40 per cent of the time (Dawes et al., 1972). Another form of fetal respiratory activity, described as gasping, was also observed and was present for 5 per cent of the time (Dawes, 1972). Boddy and Robinson (1971) recorded these movements in the human fetus using pulsed ultrasound. This paper describes a technique based upon that of Boddy and Robinson for recording human fetal chest wall movement. The system could be used in conjunction with most commercial A and B scan machines. We present our initial results with particular attention to spurious data. MATERIALS AND METHOD
The method of recording is similar to that used to monitor the motion of heart valves (Wells, 1969). A block diagram of the apparatus used is shown in Fig. 1. A modified Kelvin-Hughes ultrasonic metal flaw detector is used as a conventional A-scan instrument. It has a pulse repetition frequency of 50 Hz, a mean acoustic power output of 0-2 mW/cm2,
| TRANSDUCER]
VARIABLE
PULSE
DELAY
GENERATOR
1*
INTERVAL TO AMPLITUDE CONVERTOR
CHART RECORDER
FIG. 1. Block diagram of apparatus.
and a 1 MHz transducer 2-8 cm in diameter. Swept gain circuitry was incorporated into the flaw detector to attenuate echoes proximal to that from the fetal chest, and the instrument was retuned to 1 MHz to match the transducer. A relatively large transducer was chosen to increase the magnitude of the echo from the fetal chest wall relative to those from the fetal heart structures and to give greater latitude in orientation of the ultrasonic beam. The transmitter pulse triggers a variable delay, the output of which is connected to a pulse generator producing a marker pulse on the displayed A scan. The delay is adjusted to position the marker pulse just proximal to the fetal chest wall echo (Fig. 2). An interval-toamplitude convertor is connected to the pulse generator and the video output, which delivers a signal corresponding to the displayed A scan. The convertor feeds a signal, proportional to the time between the marker and the first edge of the following chest wall echo, to a chart recorder. Movements of the chest wall are thus recorded. In order to minimize interference from any small echoes interposed between the marker and the chest wall echo, the interval-to-amplitude convertor is not triggered unless the echo exceeds a pre-set level. A 1 MHz signal remaining in the video output was removed by low-pass filtration. If this is not done, peaks of this signal move through the trigger level as the chest wall echo amplitude varies and give rise to abrupt changes in the apparent chest wall position. Unfortunately, increasing the filtration also increased the slope of the leading edge of the chest wall echo, thus causing variation in echo amplitude to give rise to apparent changes in the echo position. For this reason the recording may not accurately represent the chest wall excursion particularly when this is small. In practice the position of the fetal chest is identified by palpation, but in cases of doubt a conventional B scan is done or a Doppler instrument used to locate the fetal heart. The transducer is directed across the fetal chest and its position is then adjusted so that the characteristic rapidly moving echo of the anterior cusp of the mitral valve is seen on the A scan. The chest wall echoes are then identified as
477
VOL.
48, No. 570 H. B. Metre, P. J. Fish, and T. Wheeler
- \
Felal Chest ( F.C.)
FIG. 2A.
Transverse section of maternal abdomen showing the path of the ultrasound beam.
FIG. 3. Regular fetal breathing of uniform amplitude. (The calibrations shown are the same for all records except Fig. 10.)
- 1 him.
FIG. 4. Fetal breathing of variable rate.
RATET©
Position of the marker, A scan.
FIG. 5. Fetal breathing with alternate large and small movements.
the large echoes either side of the mitral valve. In our experience a greater degree of respiratory movement relative to the transducer is exhibited by the distal chest wall and the electronic marker is positioned proximal to its echo. Once a satisfactory position is found the transducer is fixed in a ball joint attached to an abdominal belt. Since the transducer position may need adjusting after periods of fetal movement all recording sessions have been supervised continuously. In addition, maternal respiration and ECG are recorded. A water-filled polyvinyl tube is placed
beneath the abdominal belt and connected to a Statham Strain gauge to record respirations and electrodes are applied to the mother's chest to record the ECG. The ECG and respiration signals are added and fed into the second channel of a two channel paper recorder (Devises Ltd.). The recordings were made from 45 patients who had been admitted for rest with mild pre-eclampsia. Fetal maturity ranged from 34 to 41 weeks. All the patients delivered normal babies in satisfactory condition at birth. Informed consent was obtained before the records were made.
FIG. 2B.
478
JUNE 1975
Ultrasound recording of fetal breathing
FIG. 6. Fetal breathing with recurrent "hiccoughs".
FIG. 8. Artefact produced by maternal respiration.
FIG. 7. Fetal breathing with an amplitude of 9 mm.
FIG. 9. Record of fetal heart movements.
-ONE MINUTE -
FIG. 10. Changes in uterine tone (note reduced scale).
RESULTS
Fetal breathing was detected in 44 of the cases examined. In the one case in which no respirations were observed the record was probably made from the posterior uterine wall. The records lasted for 45-60 minutes; all were interrupted by fetal movement but this usually lasted less than one minute and the fetus often returned to its previous position thus eliminating the need for readjustment of the transducer. Breathing was periodic and lasted for 20-120 seconds or 8-20 minutes with a "rest" period of 4-20 seconds or 2-20 minutes, the longer rest and breathing periods being associated. The rate of respiration varied from 33-75 per minute but was usually in the range 50-65, remaining fairly constant for the duration of the recording (Fig. 3). Occasionally the rate was very variable over a short period of time (Fig. 4). The rhythm was usually regular and uniform in type (Fig. 3) but, occasionally, alternating large and small movements were seen (Fig. 5). These occurred when the rate was slow and approximately
twice the maternal respiratory rate. This appearance may therefore be a recording artefact due to interaction of the two rates. In two records fetal "hiccoughs" were observed during breathing periods (Fig. 6). There was considerable variation in the recorded chest excursion from case to case but most fell between 0-5 and 3 mm. Excursions of up to 9 mm were recorded from one fetus (Fig. 7), movements of this degree may represent insuction of the lower sternum rather than chest expansion. Without certain knowledge of the point of the chest which was being monitored there is little value in comparing these measurements. A number of extraneous and potentially confusing movements were also observed. These include maternal respiration (Fig. 8) and fetal heart beats (Fig. 9). The maternal respirations were identified by reference to the separate "maternal" channel. Slow regular undulations of the base line with a periodicity of about one minute were sometimes seen running throughout the recordings: these probably represent changes in uterine tone (Fig. 10).
479
VOL.
48, No. 570 H. B. Metre, P. J. Fish, and T. Wheeler DISCUSSION
Recording from the fetal chest wall by ultrasound using the interval to amplitude convertion technique poses particular problems. We believe that the procedure is facilitated by using a wide diameter transducer and by careful signal processing before the signal is fed into the interval-to-amplitude convertor. The size and frequency of the transducer used are not necessarily optimum. Since an increase in frequency will improve the "sharpness" of the fetal chest wall echo the optimum frequency is likely to be the highest at which a distal chest wall echo can be reliably obtained on the A scan in use. The optimum size of transducer is likely to be the largest that can be easily positioned. It became clear during our records that apart from extraneous echoes the principle source of artefact was maternal respiration or heart beat. These could be picked up even when the marker was aligned correctly on the fetal chest wall. For this reason the additional record of maternal respiration and ECG was helpful. If the paper speed was less than 2-5 mra/ser 1 , it was difficult to calculate the frequency of waveforms accurately. It should be noted that an operationally easier, though more expensive, method for detecting these chest wall movements would be by the use of a fibre optic chart recorder. There is evidence that, in fetal lambs, asphyxia decreases the duration of the rapid irregular breathing periods and increases the incidence and depth of gasping (Dawes, 1972). If such a correlation can be shown to hold with the human fetus then these recordings may provide some evidence of fetal
welfare. Boddy (1974) has recently described such changes in human fetal breathing, occurring shortly before intra-uterine death (paper presented to Blair Bell Research Society 1974). We have noted depressed fetal breathing from a mother under heavy sedation. Future work will be directed towards establishing the normal patterns of respiratory activity at different periods of gestation and in establishing how long and how frequently recordings have to be made in order to be fully representative of the general pattern. CONCLUSION
Fetal breathing movements can be recorded using pulsed ultrasound. These records may prove of value in the detection and management of fetal hypoxia and in detecting the effect of drugs on fetal respiration. ACKNOWLEDGMENT
We should like to thank David Atkins for his help with construction of the apparatus. REFERENCES BODDY, K., and ROBINSON, J. S., 1971. External method
for detection of fetal breathing in utera. Lancet, 2, 1231-1233. DAWES, G. S., 1972. Fetal and Neonatal Physiology. Proceedings of the Bar croft Centenary Symposium, p. 49 (Cambridge University Press). DAWES, G. S., FOX, H. E., LEDUC, B. M., LIGGINS, G. C ,
and RICHARDS, R. T., 1970. Respiratory movements and paradoxical sleep in the foetal lamb. Journal of Physiology, 210, 47-48. 1972. Respiratory movement and rapid eye movement sleep in the foetal lamb. Journal of Physiology 220, 119-143. WELLS, P. N. T. 1969. Physical principles of ultrasonic diagnosis, p. 171 (Academic Press, London and New York).
Corrigenda The radiological diagnosis of adrenal tumours by D. Sutton (April 1975). Dr. Sutton has drawn attention to the fact that in his paper there were three misprints. On page 238, column 1, line 2 should read "may be classified as in Table II". On page 244, column 1, line 3 should read "have since diagnosed seven more cases", and on page 249, paragraph two, "1 cases" should read "18 cases".
480
JUNE
1975
Ultrasound recording of fetal breathing By H. B. Meire, M.B., B.S., D.Obst., R.C.O.G., D.M.R.D., F.R.C.R., P. J. Fish, B.Sc, M.Sc, and T. Wheeler, M.A., M.R.C.O.G. Departments of Radiology, Medical Physics and Obstetrics, King's College Hospital, London {Received February, 1974) ABSTRACT
Apparatus for recording fetal chest wall movements is described. Special reference is made to technical considerations which can improve the quality of records obtained. Fetal breathing was observed in 44 out of 45 recordings. Maternal respiration and pulse could give rise to potentially confusing signals which were identified by recording these parameters on a separate channel.
Dawes et al. (1970) showed that fetal lambs performed rapid irregular breathing movements and that these were normally present for 40 per cent of the time (Dawes et al., 1972). Another form of fetal respiratory activity, described as gasping, was also observed and was present for 5 per cent of the time (Dawes, 1972). Boddy and Robinson (1971) recorded these movements in the human fetus using pulsed ultrasound. This paper describes a technique based upon that of Boddy and Robinson for recording human fetal chest wall movement. The system could be used in conjunction with most commercial A and B scan machines. We present our initial results with particular attention to spurious data. MATERIALS AND METHOD
The method of recording is similar to that used to monitor the motion of heart valves (Wells, 1969). A block diagram of the apparatus used is shown in Fig. 1. A modified Kelvin-Hughes ultrasonic metal flaw detector is used as a conventional A-scan instrument. It has a pulse repetition frequency of 50 Hz, a mean acoustic power output of 0-2 mW/cm2,
| TRANSDUCER]
VARIABLE
PULSE
DELAY
GENERATOR
1*
INTERVAL TO AMPLITUDE CONVERTOR
CHART RECORDER
FIG. 1. Block diagram of apparatus.
and a 1 MHz transducer 2-8 cm in diameter. Swept gain circuitry was incorporated into the flaw detector to attenuate echoes proximal to that from the fetal chest, and the instrument was retuned to 1 MHz to match the transducer. A relatively large transducer was chosen to increase the magnitude of the echo from the fetal chest wall relative to those from the fetal heart structures and to give greater latitude in orientation of the ultrasonic beam. The transmitter pulse triggers a variable delay, the output of which is connected to a pulse generator producing a marker pulse on the displayed A scan. The delay is adjusted to position the marker pulse just proximal to the fetal chest wall echo (Fig. 2). An interval-toamplitude convertor is connected to the pulse generator and the video output, which delivers a signal corresponding to the displayed A scan. The convertor feeds a signal, proportional to the time between the marker and the first edge of the following chest wall echo, to a chart recorder. Movements of the chest wall are thus recorded. In order to minimize interference from any small echoes interposed between the marker and the chest wall echo, the interval-to-amplitude convertor is not triggered unless the echo exceeds a pre-set level. A 1 MHz signal remaining in the video output was removed by low-pass filtration. If this is not done, peaks of this signal move through the trigger level as the chest wall echo amplitude varies and give rise to abrupt changes in the apparent chest wall position. Unfortunately, increasing the filtration also increased the slope of the leading edge of the chest wall echo, thus causing variation in echo amplitude to give rise to apparent changes in the echo position. For this reason the recording may not accurately represent the chest wall excursion particularly when this is small. In practice the position of the fetal chest is identified by palpation, but in cases of doubt a conventional B scan is done or a Doppler instrument used to locate the fetal heart. The transducer is directed across the fetal chest and its position is then adjusted so that the characteristic rapidly moving echo of the anterior cusp of the mitral valve is seen on the A scan. The chest wall echoes are then identified as
477
VOL.
48, No. 570 H. B. Metre, P. J. Fish, and T. Wheeler
- \
Felal Chest ( F.C.)
FIG. 2A.
Transverse section of maternal abdomen showing the path of the ultrasound beam.
FIG. 3. Regular fetal breathing of uniform amplitude. (The calibrations shown are the same for all records except Fig. 10.)
- 1 him.
FIG. 4. Fetal breathing of variable rate.
RATET©
Position of the marker, A scan.
FIG. 5. Fetal breathing with alternate large and small movements.
the large echoes either side of the mitral valve. In our experience a greater degree of respiratory movement relative to the transducer is exhibited by the distal chest wall and the electronic marker is positioned proximal to its echo. Once a satisfactory position is found the transducer is fixed in a ball joint attached to an abdominal belt. Since the transducer position may need adjusting after periods of fetal movement all recording sessions have been supervised continuously. In addition, maternal respiration and ECG are recorded. A water-filled polyvinyl tube is placed
beneath the abdominal belt and connected to a Statham Strain gauge to record respirations and electrodes are applied to the mother's chest to record the ECG. The ECG and respiration signals are added and fed into the second channel of a two channel paper recorder (Devises Ltd.). The recordings were made from 45 patients who had been admitted for rest with mild pre-eclampsia. Fetal maturity ranged from 34 to 41 weeks. All the patients delivered normal babies in satisfactory condition at birth. Informed consent was obtained before the records were made.
FIG. 2B.
478
JUNE 1975
Ultrasound recording of fetal breathing
FIG. 6. Fetal breathing with recurrent "hiccoughs".
FIG. 8. Artefact produced by maternal respiration.
FIG. 7. Fetal breathing with an amplitude of 9 mm.
FIG. 9. Record of fetal heart movements.
-ONE MINUTE -
FIG. 10. Changes in uterine tone (note reduced scale).
RESULTS
Fetal breathing was detected in 44 of the cases examined. In the one case in which no respirations were observed the record was probably made from the posterior uterine wall. The records lasted for 45-60 minutes; all were interrupted by fetal movement but this usually lasted less than one minute and the fetus often returned to its previous position thus eliminating the need for readjustment of the transducer. Breathing was periodic and lasted for 20-120 seconds or 8-20 minutes with a "rest" period of 4-20 seconds or 2-20 minutes, the longer rest and breathing periods being associated. The rate of respiration varied from 33-75 per minute but was usually in the range 50-65, remaining fairly constant for the duration of the recording (Fig. 3). Occasionally the rate was very variable over a short period of time (Fig. 4). The rhythm was usually regular and uniform in type (Fig. 3) but, occasionally, alternating large and small movements were seen (Fig. 5). These occurred when the rate was slow and approximately
twice the maternal respiratory rate. This appearance may therefore be a recording artefact due to interaction of the two rates. In two records fetal "hiccoughs" were observed during breathing periods (Fig. 6). There was considerable variation in the recorded chest excursion from case to case but most fell between 0-5 and 3 mm. Excursions of up to 9 mm were recorded from one fetus (Fig. 7), movements of this degree may represent insuction of the lower sternum rather than chest expansion. Without certain knowledge of the point of the chest which was being monitored there is little value in comparing these measurements. A number of extraneous and potentially confusing movements were also observed. These include maternal respiration (Fig. 8) and fetal heart beats (Fig. 9). The maternal respirations were identified by reference to the separate "maternal" channel. Slow regular undulations of the base line with a periodicity of about one minute were sometimes seen running throughout the recordings: these probably represent changes in uterine tone (Fig. 10).
479
VOL.
48, No. 570 H. B. Metre, P. J. Fish, and T. Wheeler DISCUSSION
Recording from the fetal chest wall by ultrasound using the interval to amplitude convertion technique poses particular problems. We believe that the procedure is facilitated by using a wide diameter transducer and by careful signal processing before the signal is fed into the interval-to-amplitude convertor. The size and frequency of the transducer used are not necessarily optimum. Since an increase in frequency will improve the "sharpness" of the fetal chest wall echo the optimum frequency is likely to be the highest at which a distal chest wall echo can be reliably obtained on the A scan in use. The optimum size of transducer is likely to be the largest that can be easily positioned. It became clear during our records that apart from extraneous echoes the principle source of artefact was maternal respiration or heart beat. These could be picked up even when the marker was aligned correctly on the fetal chest wall. For this reason the additional record of maternal respiration and ECG was helpful. If the paper speed was less than 2-5 mra/ser 1 , it was difficult to calculate the frequency of waveforms accurately. It should be noted that an operationally easier, though more expensive, method for detecting these chest wall movements would be by the use of a fibre optic chart recorder. There is evidence that, in fetal lambs, asphyxia decreases the duration of the rapid irregular breathing periods and increases the incidence and depth of gasping (Dawes, 1972). If such a correlation can be shown to hold with the human fetus then these recordings may provide some evidence of fetal
welfare. Boddy (1974) has recently described such changes in human fetal breathing, occurring shortly before intra-uterine death (paper presented to Blair Bell Research Society 1974). We have noted depressed fetal breathing from a mother under heavy sedation. Future work will be directed towards establishing the normal patterns of respiratory activity at different periods of gestation and in establishing how long and how frequently recordings have to be made in order to be fully representative of the general pattern. CONCLUSION
Fetal breathing movements can be recorded using pulsed ultrasound. These records may prove of value in the detection and management of fetal hypoxia and in detecting the effect of drugs on fetal respiration. ACKNOWLEDGMENT
We should like to thank David Atkins for his help with construction of the apparatus. REFERENCES BODDY, K., and ROBINSON, J. S., 1971. External method
for detection of fetal breathing in utera. Lancet, 2, 1231-1233. DAWES, G. S., 1972. Fetal and Neonatal Physiology. Proceedings of the Bar croft Centenary Symposium, p. 49 (Cambridge University Press). DAWES, G. S., FOX, H. E., LEDUC, B. M., LIGGINS, G. C ,
and RICHARDS, R. T., 1970. Respiratory movements and paradoxical sleep in the foetal lamb. Journal of Physiology, 210, 47-48. 1972. Respiratory movement and rapid eye movement sleep in the foetal lamb. Journal of Physiology 220, 119-143. WELLS, P. N. T. 1969. Physical principles of ultrasonic diagnosis, p. 171 (Academic Press, London and New York).
Corrigenda The radiological diagnosis of adrenal tumours by D. Sutton (April 1975). Dr. Sutton has drawn attention to the fact that in his paper there were three misprints. On page 238, column 1, line 2 should read "may be classified as in Table II". On page 244, column 1, line 3 should read "have since diagnosed seven more cases", and on page 249, paragraph two, "1 cases" should read "18 cases".
480
