The significance of amniotic fluid volume during intrapartum fetal acoustic stimulation Albert P. Sarno, Jr., MD, Major, Me, USA Los Angeles, California, and Honolulu, Hawaii The relationship of intrapartum fluid volume to the fetal response to acoustic stimulation was investigated in 112 patients who were in the latent phase of labor. Amniotic fluid volume appears to play an insignificant role in the genesis of fetal heart rate decelerations that occur after fetal acoustic stimulation, regardless of the amount of amniotic fluid present. (AM J OBSTET GYNECOL 1991 ;164:1100-1.)
Key words: Amniotic fluid volume, fetal heart rate, fetal acoustic stimulation
The fetal response to externally applied sound stimuli has generated a great deal of clinical research interest recently, although many questions remain to be answered. Fetal heart rate (FHR) decelerations have been noted to follow a reactive fetal response in 20% to 25% of patients. I The cause of this pattern is uncertain but is thought to be due to transient cord compression as a result of the fetal startle response. Recently, Menashe and Sherer" have suggested that fetal acoustic stimulation may be contraindicated in cases of oligohydramnios because of an increased risk for cord compression that would lead to prolonged bradycardia. However, data that describe the significance of amniotic fluid volume during fetal acoustic stimulation have not yet been reported . This study examines the effect of amniotic fluid volume on the fetal response to acoustic stimulation in patients who are in the latent phase of labor. Patients and methods
The study population consisted of 112 patients who presented in the latent phase of labor to the labor and delivery floor of Women's Hospital at the Los Angeles County/University of Southern California Medical Center or TripIer Army Medical Center in Honolulu, Hawaii. Inclusion criteria were: (1) term gestation ~36 completed weeks, (2) singleton pregnancy, From the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Women's Hospital, Los Angeles County 1University of Southern California Medical Center, and the Department of Obstetrics and Gynecology, Tripler Army Medical Center. The opinions expressed in this manuscript are those of the authors and do not necessarily reflect those of the United States Army or the Department of Defense. Received for publication August 1, 1990; accepted December 21, 1990. Reprint requests: Albert P. Sarno, Jr., MD, Major, Department of Obstetrics and Gynecology, Tripier Army Medical Center, Honolulu, HI 96859-5000. 611 127571
1100
(3) vertex presentation, and (4) latent phase of labor (:54 em dilatation). Amniotic fluid volume was assessed with a semiquantiative technique, the amniotic fluid index.' Electronic fetal monitoring was instituted and was followed by performance of fetal acoustic stimulation with a 3-second sound pulse with a fetal acoustic stimulator (model 146, Corometrics Medical Systems, Inc., Wallingford, Conn.), sound level 82 dB at 1 m in air. If no acceleration of the FHR was noted within 1 minute, an additional pulse was administered to a maximum of three pulses, each 1 minute apart. A reactive response was defined as one or more accelerations of the FHR that were 15 beats/min from baseline and persisted for IS seconds. An FHR deceleration was defined as a drop in FHR of at least IS beats/min that persisted for 15 seconds or more within I minute of sound stimulation. A nonreactive response was defined as failure to elicit a qualifying acceleration after any of three separate stimuli and for 10 minutes after the last stimulus. Care was taken to avoid performance of acoustic stimulation during or immediately after a uterine contraction to avoid periods of transient fetal hypoxia and to standardize the technique. Data were collected and analyzed by X2 analysis or Fisher's exact test, with statistical significance at p < 0.05. Results
Fig. 1 shows the incidence of various FHR responses in relation to differing amounts of amniotic fluid. Twenty-nine patients (25.8%) were found to have oligohydramnios (amniotic fluid index :5 5.0 em), 30 patients (26.8%) had borderline fluid volume (amniotic fluid index 5.1 to 8.0 cm), and 53 patients (47.3%) had normal fluid volume (amniotic fluid index> 8.0 cm). The range of amniotic fluid index values was from 0.0 cm to 27 .5 cm. There was no significant difference in the incidence of FHR decelerations after a sound stim-
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Fig. 1. FHR response to fetal acoustic stimulation grouped by amniotic fluid index. R, Reactive; RD, reactive FHR with deceleration; NR, nonreactive; AFI, amniotic fluid index in centimeters.
ulus in any subgroup for which amniotic fluid volume was determined. In the presence of oligohydramnios, however, significantly more patients exhibited non· reactive responses when compared with patients who had normal fluid volumes (10.3 % vs 0.0%, P = 0.04).
Comment Recently, Menashe and Sherer" recommended caution in the use of sound stimulation in cases of oligohydramnios because of the anecdotal experience that variable deceleration of the FHR was more common in this setting. Therefore before acoustic stimulation is widely applied, the concern that is raised in this anecdotal report regarding oligohydramnios must be addressed. This study addresses the role of amniotic fluid volume during acoustic stimulation. The findings suggest that amniotic fluid volume appears to play an insignificant role in the genesis of FHR decelerations that occur after fetal acoustic stimulation. Regardless of amniotic fluid volume, approximately 25% of patients exhibited
variable decelerations. The vigorous fetal startle reflex that is associated with sound stimulation most likely results in transient cord compression that is independent of amniotic fluid volume. Therefore it does not appear necessary to withhold fetal acoustic stimulation in the presence of oligohydramnios. In conclusion, amniotic fluid volume seems to play an insignificant role in the genesis of FHR decelerations that occur after fetal acoustic stimulation. REFERENCES
I. Sarno AP, Ahn MO, Phelan JP, Paul RH. Fetal acoustic stimulation in the early intrapartum period as a predictor of subsequent fetal condition. AM J OBSTET GVN ECOL 1990; 162:762-7. 2. Menashe M, Sherer DM. Caution regarding fetal acoustic stimulation in oligohydramnios [Letter]. AM] OBSTET GvNECOL 1990;162:603. 3. Phelan ]P, Ahn MO, Smith CV, et al. Amniotic fluid index measurements during pregnancy. J Reprod Med 1987; 632:601-4.
Key words: Amniotic fluid volume, fetal heart rate, fetal acoustic stimulation
The fetal response to externally applied sound stimuli has generated a great deal of clinical research interest recently, although many questions remain to be answered. Fetal heart rate (FHR) decelerations have been noted to follow a reactive fetal response in 20% to 25% of patients. I The cause of this pattern is uncertain but is thought to be due to transient cord compression as a result of the fetal startle response. Recently, Menashe and Sherer" have suggested that fetal acoustic stimulation may be contraindicated in cases of oligohydramnios because of an increased risk for cord compression that would lead to prolonged bradycardia. However, data that describe the significance of amniotic fluid volume during fetal acoustic stimulation have not yet been reported . This study examines the effect of amniotic fluid volume on the fetal response to acoustic stimulation in patients who are in the latent phase of labor. Patients and methods
The study population consisted of 112 patients who presented in the latent phase of labor to the labor and delivery floor of Women's Hospital at the Los Angeles County/University of Southern California Medical Center or TripIer Army Medical Center in Honolulu, Hawaii. Inclusion criteria were: (1) term gestation ~36 completed weeks, (2) singleton pregnancy, From the Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Women's Hospital, Los Angeles County 1University of Southern California Medical Center, and the Department of Obstetrics and Gynecology, Tripler Army Medical Center. The opinions expressed in this manuscript are those of the authors and do not necessarily reflect those of the United States Army or the Department of Defense. Received for publication August 1, 1990; accepted December 21, 1990. Reprint requests: Albert P. Sarno, Jr., MD, Major, Department of Obstetrics and Gynecology, Tripier Army Medical Center, Honolulu, HI 96859-5000. 611 127571
1100
(3) vertex presentation, and (4) latent phase of labor (:54 em dilatation). Amniotic fluid volume was assessed with a semiquantiative technique, the amniotic fluid index.' Electronic fetal monitoring was instituted and was followed by performance of fetal acoustic stimulation with a 3-second sound pulse with a fetal acoustic stimulator (model 146, Corometrics Medical Systems, Inc., Wallingford, Conn.), sound level 82 dB at 1 m in air. If no acceleration of the FHR was noted within 1 minute, an additional pulse was administered to a maximum of three pulses, each 1 minute apart. A reactive response was defined as one or more accelerations of the FHR that were 15 beats/min from baseline and persisted for IS seconds. An FHR deceleration was defined as a drop in FHR of at least IS beats/min that persisted for 15 seconds or more within I minute of sound stimulation. A nonreactive response was defined as failure to elicit a qualifying acceleration after any of three separate stimuli and for 10 minutes after the last stimulus. Care was taken to avoid performance of acoustic stimulation during or immediately after a uterine contraction to avoid periods of transient fetal hypoxia and to standardize the technique. Data were collected and analyzed by X2 analysis or Fisher's exact test, with statistical significance at p < 0.05. Results
Fig. 1 shows the incidence of various FHR responses in relation to differing amounts of amniotic fluid. Twenty-nine patients (25.8%) were found to have oligohydramnios (amniotic fluid index :5 5.0 em), 30 patients (26.8%) had borderline fluid volume (amniotic fluid index 5.1 to 8.0 cm), and 53 patients (47.3%) had normal fluid volume (amniotic fluid index> 8.0 cm). The range of amniotic fluid index values was from 0.0 cm to 27 .5 cm. There was no significant difference in the incidence of FHR decelerations after a sound stim-
Amniotic fluid during fetal acoustic stimulation
Volume 164 Number 4
1101
80
60
%40
20
0.0
o
NR AFI 8.0
P 80
Fig. 1. FHR response to fetal acoustic stimulation grouped by amniotic fluid index. R, Reactive; RD, reactive FHR with deceleration; NR, nonreactive; AFI, amniotic fluid index in centimeters.
ulus in any subgroup for which amniotic fluid volume was determined. In the presence of oligohydramnios, however, significantly more patients exhibited non· reactive responses when compared with patients who had normal fluid volumes (10.3 % vs 0.0%, P = 0.04).
Comment Recently, Menashe and Sherer" recommended caution in the use of sound stimulation in cases of oligohydramnios because of the anecdotal experience that variable deceleration of the FHR was more common in this setting. Therefore before acoustic stimulation is widely applied, the concern that is raised in this anecdotal report regarding oligohydramnios must be addressed. This study addresses the role of amniotic fluid volume during acoustic stimulation. The findings suggest that amniotic fluid volume appears to play an insignificant role in the genesis of FHR decelerations that occur after fetal acoustic stimulation. Regardless of amniotic fluid volume, approximately 25% of patients exhibited
variable decelerations. The vigorous fetal startle reflex that is associated with sound stimulation most likely results in transient cord compression that is independent of amniotic fluid volume. Therefore it does not appear necessary to withhold fetal acoustic stimulation in the presence of oligohydramnios. In conclusion, amniotic fluid volume seems to play an insignificant role in the genesis of FHR decelerations that occur after fetal acoustic stimulation. REFERENCES
I. Sarno AP, Ahn MO, Phelan JP, Paul RH. Fetal acoustic stimulation in the early intrapartum period as a predictor of subsequent fetal condition. AM J OBSTET GVN ECOL 1990; 162:762-7. 2. Menashe M, Sherer DM. Caution regarding fetal acoustic stimulation in oligohydramnios [Letter]. AM] OBSTET GvNECOL 1990;162:603. 3. Phelan ]P, Ahn MO, Smith CV, et al. Amniotic fluid index measurements during pregnancy. J Reprod Med 1987; 632:601-4.
