In:. J. .LLrs. Stud.. Vol. 27. No. 3. pp. 213-221. Prmcd in Great Britain.
1990 2
0020.7189/90 13 al + 0.00 1990 Pergamon Press plc
Sensory thresholds during the an tepartum, in trapartum and postpartum periods BEVERLY WHIPPLE,
Ph.D., R.N.*
Associate Professor, College of Nursing, Rutgers, The State University of New Jersey, Newark, NJ 07102, U.S.A.
JOHN B. JOSIMOVICH,
M.D.
Professor of Obstetrics and Gynecology and in Pathology, VMDNJ-NJ Medical School, Ne work, NJ 07103, U.S.A.
and BARRY R. KOMISARUK, Rutgers,
Ph.D.
Professor II Institute of Animal Behavior The State University of New Jersey, Newark, NJ 07102, U.S.A.
Abstract-The present exploratory study measured pain and tactile thresholds in response to mechanical stimulation of the hand before labor, during labor, and after parturition in women. In women who had Lamaze childbirth preparation (but not in women who did not have childbirth preparation), pain thresholds were significantly higher during labor (determined up to 8 cm cervical dilatation) than prior to labor and 24 hours postpartum. Tactile thresholds did not change during any of these conditions. These findings support earlier findings in this laboratory that vaginocervical mechanostimulation elevated pain thresholds in human and animal subjects, and more recent findings that pain thresholds increased in rats during delivery of individual young. The present findings suggest that an endogenous process that attenuates the pain of parturition is activated when the cervix dilates during labor. *Author to whom correspondence
should be addressed. 213
214
BE VERL Y WHIPPLE
et al.
Introduction
Melzack (1984) reported that the pain of childbirth was one of the most severe forms of pain that has been measured by the McGill Pain Questionnaire. Subjective reports of labor pain have been correlated with intensity, magnitude, and duration of uterine contractions (Corli et al., 1986) and with dilatation of the cervix (Bonnel and Boureau, 1985). The existence of a mechanism that could ameliorate or reduce perception of pain during labor was suggested by experiments in our laboratory in which mechanostimulation (with the plunger of a 1 cm3 syringe) of the vagina and cervix in rats, inhibited withdrawal responses to cutaneous noxious pressure (Komisaruk and Larsson, 1971; Komisaruk et al., 1976) or heat (Steinman et al., 1983) stimulation, and inhibited thalamic sensory neuronal responses to noxious, but not innocuous cutaneous pressure stimulation (Komisaruk and Wallman, 1977). In order to determine whether vaginal self-stimulation 0%) suppresses pain in humans, Whipple and Komisaruk (1985, 1988) measured the effect of pressure self-applied to the vaginal wall (using a force-calibrated transducer with digital readout) on pain thresholds and tactile thresholds in non-pregnant women. A significant increase in pain detection thresholds and pain tolerance thresholds but not tactile thresholds (all thresholds measured on the hand) was observed in response to VS. In a study in rats of whether distentional stimulation produced by the fetuses during parturition may invoke a natural analgesia in the mother, Toniolo et al. (1987) found that pain thresholds were significantly higher while fetuses were emerging from the birth canal than just after they emerged. In women, there are two studies that report an increase in pain thresholds during pregnancy (Rust et al., 1985; Cogan and Spinnato, 1986), one report of no change (Sengupta and Nielsen, 1984), and one report of a decrease in pain thresholds during pregnancy (Goolkasian and Rimer, 1984). During labor, intensity of pain has been reported to increase in correlation with uterine contractions (Corli ef al., 1986) and dilatation of the cervix (Bonnel and Boureau, 1985). The cervix is innervated by the pelvic and hypogastric nerves (Peters et al., 1987; Komisaruk et al., 1972). transection of which in rats significantly decreased analgesia produced by vaginocervical stimulation (Cunningham er al., 1989). The analgesia that normally occurs during pregnancy in rats (Gintzler, 1980) is significantly attenuated by transection of the hypogastric nerve (Gintzler ef al., 1983). W’ehypothesized that a condition in which cervical mechanostimulation occurs naturally is labor and delivery. Consequently in the present study, we measured pain thresholds during labor up to 8 cm cervical dilatation (measurements during later stages of labor and delivery uere contraindicated in order to avoid interference with the activities of the delivery room personnel). This study employed the same instrumentation used in three previous studies in women in our laboratory (Whipple and Komisaruk, 1985, 1988; Komisaruk and Whipple, 1986). We recorded endogenous pain intensity reported before, during and after labor, and concurrently measured pain thresholds and tactile thresholds in response to exogenously applied mechanostimulation. Thus, the present study ascertained the intensity of endogenous pain as distinct from the pain threshold to exogenously-applied noxious stimulation, since these may be independent phenomena. Five of the volunteer subjects had no childbirth education preparation and five of the volunteer subjects had attended Lamaze childbirth education classes.
SENSORY
THRESHOLDS
215
Methods Subjects Subjects for this exploratory study were volunteer patients from the Prenatal Clinic of the University Hospital of the University of Medicine and Dentistry of New Jersey (UMDNJ) who did not have hypertension or diabetes. The mean age of the subjects was 25.5 years (range: 20-34 years). All subjects were naive as to the hypotheses of the study. The subjects in Experiment 1 (n = 5, mean age 28) had no childbirth education classes. Three of the subjects were multipara and two had not given birth before. All of the subjects in Experiment 2 (n = 5, mean age 23) had not given birth before and all attended Lamaze classes. A questionnaire that included demographic items was omitted from the study upon the request of the Institutional Review Board for the protection of human subjects of UMDNJ. However, all subjects were volunteers who attended the same prenatal clinic, and thus were from the same geographic area and probably a similar socioeconomic level. None of these volunteers was under the care of a private physician. Instrumentation As in our previous studies in women (Whipple and Komisaruk, 1985, 1988), pain thresholds were determined by applying a gradually increasing force to each finger using a Ugo Basile analgesia meter (Milan, Italy). During each testing condition, the subject placed her finger on the 1 mm diameter point of the analgesia meter as a controlled increasing force was applied from 0 g up to a maximum of 1 kg over a 26 set period. The subject reported by saying “now”, when finger pain was first perceived (defined as ‘pain detection threshold’) and by saying “stop” when finger pain became too uncomfortable to continue (defined as ‘pain tolerance threshold’). Tactile thresholds were determined by applying a graded series of calibrated von Frey fibers (Stoelting, Inc., Chicago, II) to the dorsal surface of the hand. The tactile threshold is defined as the minimal force to bend the fiber during which rhe subject states that she feels the tip of the fiber three out of three times using an ascending/descending method of limits. Endogenous pain was determined with a ‘Visual Analog Scale for the Subjective Report of Pain due to Pregnancy’ (adapted from Raisanen et al., 1984). Subjects rated their endogenous pain from 0 (no pain) to 10 (excruciating pain). Procedure Permission to perform the study was obtained from the Institutional Review Board for the Protection of Human Subjects in Research (IRB) of Rutgers University and the IRB of UMDNJ-NJ Medical School. The subjects (whose expected date of parturition was no more than four weeks later) read and signed the informed consent form and had prior approval of their physician to participate in the study. The subjects were given copies of the informed consent form. The subject was instructed to page or call the investigator as soon as there was any indication of the onset of labor. It was noted if the subject was attending or had attended Lamaze classes. Immediately after this instruction, the subject was asked to recline on an examination table. She was again shown the visual analog scale and asked about endogenous pain. Once her response was recorded, she was positioned for subsequent testing; her left hand rested at her side, palm down. Her head turned away, the subject was instructed to close her
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eyes, and to respond “now” whenever she felt the von Frey tactile fibers applied to the dorsal surface of her left hand. The pain detection and pain tolerance thresholds were determined immediately after the tactile test. The subject remained supine while the investigator again demonstrated on herself the operation of the Ugo-Basile analgesia meter, and instructed the subject to say “now” when pain was first detected, and “stop” when the pain became too uncomfortable to continue. The index finger of the subject’s left hand was placed on the Ugo-Basile analgesia meter, and the subject was again reminded to say “now” and “stop” at the appropriate time, and to make sure she looked away and kept her eyes closed. As the weight moved out along the beam, the investigator made note of the pain detection threshold (i.e. when the subject said “now”) and the pain tolerance threshold (i.e. when the subject said “stop”) using the calibrations on the beam. The pressure was lifted immediately after the subject said “stop”. This procedure was repeated for the other three fingers (the thumb was excluded); the duration of this initial testing of the left hand was approximately 3 min. The testing of the right hand proceeded exactly as testing of the left. The mean of the four fingers for pain detection threshold and pain tolerance threshold was calculated for each hand. This established the baseline for each hand. (At this time, both hands were tested, as there was a possibility that an intravenous line would be used during labor, and it was not known which hand would be free for testing.) There was no more testing until the onset of labor. Every effort was made to start testing as soon as labor began. The above procedure was repeated approximately every 30 min on either the left or right hand (only one hand was tested throughout labor). Testing continued until pain medication was administered or until the physician’s internal examination found cervical dilatation of 8 cm. In this latter case, testing was discontinued by stipulation of the IRB of UMDNJ-NJ Medical School, in order to avoid interference with the further activities of the delivery room personnel in managing the delivery. After testing was discontinued, the subject was offered the choice of having the investigator remain in the room throughout the remaining stages of labor and delivery. The testing was conducted on the same hand used during labor, 1 and 24 hr postpartum. Thus each subject served as her own control. The subjects remained in control of all experimental procedures at all times and were reminded by the investigator that they could terminate their participation in the study at any time. During labor, at each time point, a mean pain detection threshold and pain tolerance threshold was calculated. Each subject produced a set of mean pain detection thresholds, pain tolerance thresholds, tactile thresholds, and visual analog report of endogenous pain. The mean of these means was used for the group comparisons. Experiment
1: No childbirth
preparation
(n = 5)
Results The visual analog scaie was used to assess the intensity of the subjects’ endogenous pain. The mean intensity of pain (f s.e.m.) reported during labor (6.45 f 0.65) was significantly greater than that prior to labor (1 .O f 0.45; p = 0.001, two-tailed, correlated t-test), 1 hr postpartum (2.8 f 0.79; p = 0.006, two-tailed, correlated f-test), and 24 hr postpartum (0.4 f 0.39; p = 0.001, two-tailed, correlated f-test). The pain detection threshold (mean f s.e.m.) (179 f 14.8 g) was significantly lower during labor than prior to labor (220 f 10 g; p = 0.03, two-tailed, correlated f-test) by 19% (Fig. 1).
SENSORY
Experiment
1: No Childbirth
Pain Detection TNhresholds (SIrams)
(Mean
31:
THRESHOLDS
Preparation
Thresholds
: s.e.m.)
300
@
250
T
L
200
150
100
50
0
I Pre
+
T
!
L
-
1 Hour
Labor
Postpartum
Phase
1
24 Hour POStpartum
of Parturition
Fig. I. In experiment I, the pain detection threshold (mean * s.e.m.) was significantly lower during correlated I-test. In this and following figures, the group bearing prior to labor, p = 0.03, two-tailed, (*) is significantly different from the group(s) bearing the open circle (0).
labor than the asterisk
Neither of the above measures showed significant differences between pre-labor and postpartum conditions. The tactile thresholds prior to labor, during labor, or postpartum did not differ significantly from each other. Discussion of Experiment 1 The subjects in Experiment 1, who did not have any childbirth preparation classes, showed both an increased report of endogenous pain and an increased sensitivity to exogenous pain, during labor. The investigator collecting the data reported that all the subjects were impatient to have the testing completed as quickly as possible. This may have prompted the subjects to use a strategy of claiming that the stimulus from the analgesia meter on their finger was painful almost as soon as the stimulus intensity started to increase. This would generate a spuriously low pain threshold reading. Melzack (1984) reported that emotional stress and anxiety contribute to the pain of parturition. We therefore designed Experiment 2 to test women who had attended childbirth education classes, on the supposition that they might be less anxious during labor, a factor that would permit a more valid determination of sensory thresholds.
Experiment
2: Lamaze
childbirth
preparation
(n = 5)
Results
Using the visual analog scale, the mean ( f s.e.m.) level of endogenous pain during labor (5.98 f 0.58) was significantly greater than 1 hr postpartum (0.75 f 0.22; p = 0.009, twotailed, correlated t-test), and 24 hr postpartum labor (0.5 f 0.25; p = 0.004, two-tailed,
218
BE VERL Y WHIPPLE
ei al.
correlated f-test). All subjects reported no pain (0) prior to labor; therefore, these data were not subjected to statistical analysis. When the results of the two experiments were combined, the reported level of pain during labor (6.24 f 0.6) was significantly greater than that prior to labor (0.55 kO.39; p < 0.0001, two-tailed, correlated t-test), 1 hr postpartum (1.88 f 0.75; p = 0.001, two-tailed, correlated I-test), and 24 hr postpartum (0.22 f 0.29; p < 0.0001, two-tailed, correlated t-test) (Fig. 2). Experiments Endogenous Visual Score
1 and 2 Combined Pain Report
Analog Scale (Mean : s.e.m.)
7.5 6 4.5 3 1.5
a I-
$---J ,; ,
_&-
- 1.5 PU2
Labor
Phase
1 Hour PostDartum
24 Hour Postpartum
of Parturition
Fig. 2. Using the visual analog scale, the report of endogenous pain during labor (mean f s.e.m.) was significantly greater than prior to labor, p < 0.0001, I hr after labor, p=O.OOl, and 24 hr after labor, p < 0.0001, all twotailed, correlated f-tests.
During labor in Experiment 2, thepain detection threshold (341 f 51 g) was significantly greater (21%) than prior to labor (282 f 41.6 g; p = 0.02, two-tailed, correlated t-test). The pain detection threshold was also significantly greater during labor (341 * 5 1 g) than 24 hr postpartum, as expected (mean 257 f 47.9 g; p = 0.04, one-tailed, correlated f-test) (Fig. 3). As in Experiment 1, the tactile thresholds prior to labor, during labor, or postpartum did not differ significantly from each other. Discussion of Experiment 2 During labor, the subjects in Experiment 2 reported a significant elevation in their levels of endogenous pain using the visual analog scale, and they concurrently showed a significant elevation in their pain detection thresholds in response to exogenous pain. Their tactile thresholds did not increase, indicating that the increase in pain threshold to exogenous pain was probably not due to the distraction of labor. It is likely that in the process of dilatation of the cervix to 8 cm, afferent neural activity from the cervix is generated. In laboratory rats (i) cervical mechanostimulation has been shown to produce analgesia, (ii) cervical mechanostimulation activates axonal neural activity
SENSORY
THRESHOLDS
219
Experiment 2: Childbirth Preparation Pain Detection Thresholds Thresholds (Mean : s&m.) trams) 500
400
i
Li
300
200
100
0 1 Hour
Labor
Phase
of
24 Hour Postpartum
Parturition
Fig. 3. ln experiment 2, the pain detection threshold (mean * s.e.m.) was significantly greater during labor than prior to labor, p = 0.02, two-tailed, correlated f-test and 24 hr after labor p = 0.04, one-tailed, correlated f-test.
in pelvic and hypogastric afferents from the cervix and (iii) transection of the pelvic and hypogastric nerves virtually abolishes analgesia produced by cervical probing (see introduction). Based on those studies, the elevation in pain threshold observed in the present study may be due at least in part to activation of cervical afferents during labor.
General Discussion
The subjects’ reports of their endogenous pain, using a visual analog scale, demonstrate that they experienced pain during labor. Concurrently, the pain thresholds to exogenously applied noxious stimulation increased, while the tactile thresholds to innocuous stimulation did not change, in the subjects who had childbirth preparation but not in the subjects who did not have childbirth preparation. We conclude therefore, that in the women with childbirth preparation, an analgesic process is activated during labor. While the degree of analgesia is not sufficient to eliminate the pain of labor, the present findings imply that the pain of labor would be more severe if this mechanism did not exist. On the basis of the present findings, the discrepancies in pain thresholds reported in the literature may be due to differences in anxiety in the individual subjects because of their previous experiences and/or present expectations. This may have been a contributing factor that could account for variability in other studies as well as in this study. The subjects who had Lamaze preparation may have experienced a lower level of stress and anxiety during labor that those who did not have this preparation, which thereby enabled us to obtain a more valid determination of their pain thresholds. However, it may also be that this analgesia during labor is not a component of cervical dilatation, but a consequence of the activities used during prepared childbirth. Further testing, with measurement of state anxiety and a larger sample, would be useful to understand this
220
BEVERL Y WHIPPLE et al.
phenomenon. It would also be interesting to extend the measurement of sensory thresholds beyond 8 cm cervical dilatation and into the delivery period, since there are anecdotal reports that the level of labor pain decreases during the transition phase (Gaskin, 1978). No recommendations as to nursing interventions can be made based on this exploratory study, which is limited in terms of sample size and demographic data. The potential contribution to the findings of the subjects’ support system, education, etc., cannot be assessed but is worthy of further investigation. However, it is important to emphasize that although labor pain can indeed be severe, continued laboratory and clinical research is necessary to provide insight into mechanisms underlying the naturally triggered endogenous analgesia-producing system, an understanding of which could provide the basis for new means of augmenting the actions of such an analgesia-triggering mechanism, e.g. by new pharmacological or dietary means, in order to lead to novel methods to attenuate pain of parturition (Komisaruk and Whipple, 1988). Acknowledgements&We gratefully acknowledge the assistance of MS Marlene Toniolo, MS Lucy Buglisi, R.N., and MS Mary Ellen Affanoso, R.N. in the data collection for this study. This research was supported in part by the Office of Research and Sponsored Programs, #2-02305 (B.W.) and the Charles and Johanna Busch Foundation, (B.R.K.), both of Rutgers, The State University of New Jersey. Contribution no. 491 from the Institute of Animal Behavior.
References Bonnel, A. M. and Boureau, F. (1985). Labor pain assessment: Validity of a behavioral index. Pain 22, 81-90. Cogan, R. and Spinnato, J. A. (1986). Pain and discomfort thresholds in late pregnancy. Pain 27, 63-68. Corli, O., Grossi. E., Roma, G. and Battagliarin, G. (1986). Correlation between subjective labour pain and uterine contractions: A clinical study. Pain 27, 63-68. Cunningham, S. T., Steinman, J. L. and Komisaruk, B. R. (1989). Sot. Neurosci. Abst. 15, 848. Gaskin, I. M. (1978). Spirifual Midwifery. The Book Publisher Co., Summertown TN. Gintzler, A. R. (1980). Endorphin-mediated increases in pain threshold during pregnancy. Science210, 193-195. Gintzler, A. R., Peters, L. C. and Komisaruk, B. R. (1983). Attenuation of pregnancy-induced analgesia by hypogastic neurectomy in rats. Brain Res. 277, 186-188. Goolkasian, P. and Rimer, B. A..(1984). Pain reactions in pregnant women. Pain 20, 87-95. Komisaruk, B. R. and Larsson, K. (1971). Suppression of a spinal and a cranial nerve reflex by vaginal or rectal probing in rats. Brain Res. 35, 231-235. Komisaruk, B. R. and Wallman, J. (1977). Antinociceptive effects of vaginal stimulation in rats: Neurophysiological and behavioral studies. Brain Res. 137, 85-107. Komisaruk, B. R. and Whipple, B. (1986). Vaginal stimulation produced analgesia in rats and women. In StressInduced Analgesia, D. D. Kelly (Ed.), pp. 30-39. New York Academy of Sciences, New York. Komisaruk, B. R. and Whipple, B. (1988). The role of vaginal stimulation-produced analgesia in reproductive processes. In Pain and Reproduction, A. R. Genazzani. G. Nappi, F. Facchinetti and E. Martignoni (Eds), pp. 125-140. Parthenon, England. Komisaruk, B. R., Adler, N. and Hutchinson, J. (1972). Genital sensory field: Enlargement by estrogen treatment in female rats. Science 178, 1295-1298. Komisaruk. B. R., Ciofalo, V. and Latranyi, M. B. (1976). Stimulation of the vaginal cervix is more effective than morphine in suppressing a nociceptive response in rats. In Advances in Pain Research and Therapy, J. J. Bonica and D. Albe-Fessard (Eds), Vol. 1, pp. 439-443. Raven Press, New York. Melzack. R. (1984). The myth of painless childbirth. Pain 19, 321-337. Peters, L. C., Kristal, M. B. and Komisaruk, B. R. (1987). Sensory innervation of the external and internal genitalia of the female rat. Brain Res. 408, 199-204. Raisanen, I.. Paatero, H.. Salminen, B. A. and Laatikainen, T. (1984). Pam and plasma B-endorphin level during labor. Obstetr. Gynecol. 64, 783-786. Rust, M.. Keller, M., Gassier, M. and Zieglgansberger. W. (1985). Endorphinergic mechanisms of specific adaptation to pain during pregnancy. Abst. XI World Congress of Gynecology and Obstetrics, 452. Sengupta, P. and Nielsen, M. (1984). The effect of labour and epidural analgesia on pain threshold. Anesthesia 39, 982-986. Steinman, J. L.. Komisaruk. B. R., Yaksh, T. L. and Tyce, G. M. (1983). Spinal cord monoamines modulate the antinociceptive effects of vaginal stimulation in rats. Pain 16, 155-166.
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THRESHOLDS
Toniolo, M. V., Whipple, B. H. and Komisaruk, B. R. (1987). Spontaneous maternal analgesia during birth in rats. Proc. 1987 NIH Centennial MBRS-MARC Symposium 15, 100. Whipple. B. and Komisaruk, B. R. (1985). Elevation of pain threshold by vaginal stimulation in women. Pain 21, 357-367.
Whipple, B. and Komisaruk, B. R. (1988) Analgesia produced in women by genital self-stimulation. J. Sex Res. 24, 130-140. (Received
17 January
1989; accepted for publication
15 November 1989)
1990 2
0020.7189/90 13 al + 0.00 1990 Pergamon Press plc
Sensory thresholds during the an tepartum, in trapartum and postpartum periods BEVERLY WHIPPLE,
Ph.D., R.N.*
Associate Professor, College of Nursing, Rutgers, The State University of New Jersey, Newark, NJ 07102, U.S.A.
JOHN B. JOSIMOVICH,
M.D.
Professor of Obstetrics and Gynecology and in Pathology, VMDNJ-NJ Medical School, Ne work, NJ 07103, U.S.A.
and BARRY R. KOMISARUK, Rutgers,
Ph.D.
Professor II Institute of Animal Behavior The State University of New Jersey, Newark, NJ 07102, U.S.A.
Abstract-The present exploratory study measured pain and tactile thresholds in response to mechanical stimulation of the hand before labor, during labor, and after parturition in women. In women who had Lamaze childbirth preparation (but not in women who did not have childbirth preparation), pain thresholds were significantly higher during labor (determined up to 8 cm cervical dilatation) than prior to labor and 24 hours postpartum. Tactile thresholds did not change during any of these conditions. These findings support earlier findings in this laboratory that vaginocervical mechanostimulation elevated pain thresholds in human and animal subjects, and more recent findings that pain thresholds increased in rats during delivery of individual young. The present findings suggest that an endogenous process that attenuates the pain of parturition is activated when the cervix dilates during labor. *Author to whom correspondence
should be addressed. 213
214
BE VERL Y WHIPPLE
et al.
Introduction
Melzack (1984) reported that the pain of childbirth was one of the most severe forms of pain that has been measured by the McGill Pain Questionnaire. Subjective reports of labor pain have been correlated with intensity, magnitude, and duration of uterine contractions (Corli et al., 1986) and with dilatation of the cervix (Bonnel and Boureau, 1985). The existence of a mechanism that could ameliorate or reduce perception of pain during labor was suggested by experiments in our laboratory in which mechanostimulation (with the plunger of a 1 cm3 syringe) of the vagina and cervix in rats, inhibited withdrawal responses to cutaneous noxious pressure (Komisaruk and Larsson, 1971; Komisaruk et al., 1976) or heat (Steinman et al., 1983) stimulation, and inhibited thalamic sensory neuronal responses to noxious, but not innocuous cutaneous pressure stimulation (Komisaruk and Wallman, 1977). In order to determine whether vaginal self-stimulation 0%) suppresses pain in humans, Whipple and Komisaruk (1985, 1988) measured the effect of pressure self-applied to the vaginal wall (using a force-calibrated transducer with digital readout) on pain thresholds and tactile thresholds in non-pregnant women. A significant increase in pain detection thresholds and pain tolerance thresholds but not tactile thresholds (all thresholds measured on the hand) was observed in response to VS. In a study in rats of whether distentional stimulation produced by the fetuses during parturition may invoke a natural analgesia in the mother, Toniolo et al. (1987) found that pain thresholds were significantly higher while fetuses were emerging from the birth canal than just after they emerged. In women, there are two studies that report an increase in pain thresholds during pregnancy (Rust et al., 1985; Cogan and Spinnato, 1986), one report of no change (Sengupta and Nielsen, 1984), and one report of a decrease in pain thresholds during pregnancy (Goolkasian and Rimer, 1984). During labor, intensity of pain has been reported to increase in correlation with uterine contractions (Corli ef al., 1986) and dilatation of the cervix (Bonnel and Boureau, 1985). The cervix is innervated by the pelvic and hypogastric nerves (Peters et al., 1987; Komisaruk et al., 1972). transection of which in rats significantly decreased analgesia produced by vaginocervical stimulation (Cunningham er al., 1989). The analgesia that normally occurs during pregnancy in rats (Gintzler, 1980) is significantly attenuated by transection of the hypogastric nerve (Gintzler ef al., 1983). W’ehypothesized that a condition in which cervical mechanostimulation occurs naturally is labor and delivery. Consequently in the present study, we measured pain thresholds during labor up to 8 cm cervical dilatation (measurements during later stages of labor and delivery uere contraindicated in order to avoid interference with the activities of the delivery room personnel). This study employed the same instrumentation used in three previous studies in women in our laboratory (Whipple and Komisaruk, 1985, 1988; Komisaruk and Whipple, 1986). We recorded endogenous pain intensity reported before, during and after labor, and concurrently measured pain thresholds and tactile thresholds in response to exogenously applied mechanostimulation. Thus, the present study ascertained the intensity of endogenous pain as distinct from the pain threshold to exogenously-applied noxious stimulation, since these may be independent phenomena. Five of the volunteer subjects had no childbirth education preparation and five of the volunteer subjects had attended Lamaze childbirth education classes.
SENSORY
THRESHOLDS
215
Methods Subjects Subjects for this exploratory study were volunteer patients from the Prenatal Clinic of the University Hospital of the University of Medicine and Dentistry of New Jersey (UMDNJ) who did not have hypertension or diabetes. The mean age of the subjects was 25.5 years (range: 20-34 years). All subjects were naive as to the hypotheses of the study. The subjects in Experiment 1 (n = 5, mean age 28) had no childbirth education classes. Three of the subjects were multipara and two had not given birth before. All of the subjects in Experiment 2 (n = 5, mean age 23) had not given birth before and all attended Lamaze classes. A questionnaire that included demographic items was omitted from the study upon the request of the Institutional Review Board for the protection of human subjects of UMDNJ. However, all subjects were volunteers who attended the same prenatal clinic, and thus were from the same geographic area and probably a similar socioeconomic level. None of these volunteers was under the care of a private physician. Instrumentation As in our previous studies in women (Whipple and Komisaruk, 1985, 1988), pain thresholds were determined by applying a gradually increasing force to each finger using a Ugo Basile analgesia meter (Milan, Italy). During each testing condition, the subject placed her finger on the 1 mm diameter point of the analgesia meter as a controlled increasing force was applied from 0 g up to a maximum of 1 kg over a 26 set period. The subject reported by saying “now”, when finger pain was first perceived (defined as ‘pain detection threshold’) and by saying “stop” when finger pain became too uncomfortable to continue (defined as ‘pain tolerance threshold’). Tactile thresholds were determined by applying a graded series of calibrated von Frey fibers (Stoelting, Inc., Chicago, II) to the dorsal surface of the hand. The tactile threshold is defined as the minimal force to bend the fiber during which rhe subject states that she feels the tip of the fiber three out of three times using an ascending/descending method of limits. Endogenous pain was determined with a ‘Visual Analog Scale for the Subjective Report of Pain due to Pregnancy’ (adapted from Raisanen et al., 1984). Subjects rated their endogenous pain from 0 (no pain) to 10 (excruciating pain). Procedure Permission to perform the study was obtained from the Institutional Review Board for the Protection of Human Subjects in Research (IRB) of Rutgers University and the IRB of UMDNJ-NJ Medical School. The subjects (whose expected date of parturition was no more than four weeks later) read and signed the informed consent form and had prior approval of their physician to participate in the study. The subjects were given copies of the informed consent form. The subject was instructed to page or call the investigator as soon as there was any indication of the onset of labor. It was noted if the subject was attending or had attended Lamaze classes. Immediately after this instruction, the subject was asked to recline on an examination table. She was again shown the visual analog scale and asked about endogenous pain. Once her response was recorded, she was positioned for subsequent testing; her left hand rested at her side, palm down. Her head turned away, the subject was instructed to close her
216
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et al.
eyes, and to respond “now” whenever she felt the von Frey tactile fibers applied to the dorsal surface of her left hand. The pain detection and pain tolerance thresholds were determined immediately after the tactile test. The subject remained supine while the investigator again demonstrated on herself the operation of the Ugo-Basile analgesia meter, and instructed the subject to say “now” when pain was first detected, and “stop” when the pain became too uncomfortable to continue. The index finger of the subject’s left hand was placed on the Ugo-Basile analgesia meter, and the subject was again reminded to say “now” and “stop” at the appropriate time, and to make sure she looked away and kept her eyes closed. As the weight moved out along the beam, the investigator made note of the pain detection threshold (i.e. when the subject said “now”) and the pain tolerance threshold (i.e. when the subject said “stop”) using the calibrations on the beam. The pressure was lifted immediately after the subject said “stop”. This procedure was repeated for the other three fingers (the thumb was excluded); the duration of this initial testing of the left hand was approximately 3 min. The testing of the right hand proceeded exactly as testing of the left. The mean of the four fingers for pain detection threshold and pain tolerance threshold was calculated for each hand. This established the baseline for each hand. (At this time, both hands were tested, as there was a possibility that an intravenous line would be used during labor, and it was not known which hand would be free for testing.) There was no more testing until the onset of labor. Every effort was made to start testing as soon as labor began. The above procedure was repeated approximately every 30 min on either the left or right hand (only one hand was tested throughout labor). Testing continued until pain medication was administered or until the physician’s internal examination found cervical dilatation of 8 cm. In this latter case, testing was discontinued by stipulation of the IRB of UMDNJ-NJ Medical School, in order to avoid interference with the further activities of the delivery room personnel in managing the delivery. After testing was discontinued, the subject was offered the choice of having the investigator remain in the room throughout the remaining stages of labor and delivery. The testing was conducted on the same hand used during labor, 1 and 24 hr postpartum. Thus each subject served as her own control. The subjects remained in control of all experimental procedures at all times and were reminded by the investigator that they could terminate their participation in the study at any time. During labor, at each time point, a mean pain detection threshold and pain tolerance threshold was calculated. Each subject produced a set of mean pain detection thresholds, pain tolerance thresholds, tactile thresholds, and visual analog report of endogenous pain. The mean of these means was used for the group comparisons. Experiment
1: No childbirth
preparation
(n = 5)
Results The visual analog scaie was used to assess the intensity of the subjects’ endogenous pain. The mean intensity of pain (f s.e.m.) reported during labor (6.45 f 0.65) was significantly greater than that prior to labor (1 .O f 0.45; p = 0.001, two-tailed, correlated t-test), 1 hr postpartum (2.8 f 0.79; p = 0.006, two-tailed, correlated f-test), and 24 hr postpartum (0.4 f 0.39; p = 0.001, two-tailed, correlated f-test). The pain detection threshold (mean f s.e.m.) (179 f 14.8 g) was significantly lower during labor than prior to labor (220 f 10 g; p = 0.03, two-tailed, correlated f-test) by 19% (Fig. 1).
SENSORY
Experiment
1: No Childbirth
Pain Detection TNhresholds (SIrams)
(Mean
31:
THRESHOLDS
Preparation
Thresholds
: s.e.m.)
300
@
250
T
L
200
150
100
50
0
I Pre
+
T
!
L
-
1 Hour
Labor
Postpartum
Phase
1
24 Hour POStpartum
of Parturition
Fig. I. In experiment I, the pain detection threshold (mean * s.e.m.) was significantly lower during correlated I-test. In this and following figures, the group bearing prior to labor, p = 0.03, two-tailed, (*) is significantly different from the group(s) bearing the open circle (0).
labor than the asterisk
Neither of the above measures showed significant differences between pre-labor and postpartum conditions. The tactile thresholds prior to labor, during labor, or postpartum did not differ significantly from each other. Discussion of Experiment 1 The subjects in Experiment 1, who did not have any childbirth preparation classes, showed both an increased report of endogenous pain and an increased sensitivity to exogenous pain, during labor. The investigator collecting the data reported that all the subjects were impatient to have the testing completed as quickly as possible. This may have prompted the subjects to use a strategy of claiming that the stimulus from the analgesia meter on their finger was painful almost as soon as the stimulus intensity started to increase. This would generate a spuriously low pain threshold reading. Melzack (1984) reported that emotional stress and anxiety contribute to the pain of parturition. We therefore designed Experiment 2 to test women who had attended childbirth education classes, on the supposition that they might be less anxious during labor, a factor that would permit a more valid determination of sensory thresholds.
Experiment
2: Lamaze
childbirth
preparation
(n = 5)
Results
Using the visual analog scale, the mean ( f s.e.m.) level of endogenous pain during labor (5.98 f 0.58) was significantly greater than 1 hr postpartum (0.75 f 0.22; p = 0.009, twotailed, correlated t-test), and 24 hr postpartum labor (0.5 f 0.25; p = 0.004, two-tailed,
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correlated f-test). All subjects reported no pain (0) prior to labor; therefore, these data were not subjected to statistical analysis. When the results of the two experiments were combined, the reported level of pain during labor (6.24 f 0.6) was significantly greater than that prior to labor (0.55 kO.39; p < 0.0001, two-tailed, correlated t-test), 1 hr postpartum (1.88 f 0.75; p = 0.001, two-tailed, correlated I-test), and 24 hr postpartum (0.22 f 0.29; p < 0.0001, two-tailed, correlated t-test) (Fig. 2). Experiments Endogenous Visual Score
1 and 2 Combined Pain Report
Analog Scale (Mean : s.e.m.)
7.5 6 4.5 3 1.5
a I-
$---J ,; ,
_&-
- 1.5 PU2
Labor
Phase
1 Hour PostDartum
24 Hour Postpartum
of Parturition
Fig. 2. Using the visual analog scale, the report of endogenous pain during labor (mean f s.e.m.) was significantly greater than prior to labor, p < 0.0001, I hr after labor, p=O.OOl, and 24 hr after labor, p < 0.0001, all twotailed, correlated f-tests.
During labor in Experiment 2, thepain detection threshold (341 f 51 g) was significantly greater (21%) than prior to labor (282 f 41.6 g; p = 0.02, two-tailed, correlated t-test). The pain detection threshold was also significantly greater during labor (341 * 5 1 g) than 24 hr postpartum, as expected (mean 257 f 47.9 g; p = 0.04, one-tailed, correlated f-test) (Fig. 3). As in Experiment 1, the tactile thresholds prior to labor, during labor, or postpartum did not differ significantly from each other. Discussion of Experiment 2 During labor, the subjects in Experiment 2 reported a significant elevation in their levels of endogenous pain using the visual analog scale, and they concurrently showed a significant elevation in their pain detection thresholds in response to exogenous pain. Their tactile thresholds did not increase, indicating that the increase in pain threshold to exogenous pain was probably not due to the distraction of labor. It is likely that in the process of dilatation of the cervix to 8 cm, afferent neural activity from the cervix is generated. In laboratory rats (i) cervical mechanostimulation has been shown to produce analgesia, (ii) cervical mechanostimulation activates axonal neural activity
SENSORY
THRESHOLDS
219
Experiment 2: Childbirth Preparation Pain Detection Thresholds Thresholds (Mean : s&m.) trams) 500
400
i
Li
300
200
100
0 1 Hour
Labor
Phase
of
24 Hour Postpartum
Parturition
Fig. 3. ln experiment 2, the pain detection threshold (mean * s.e.m.) was significantly greater during labor than prior to labor, p = 0.02, two-tailed, correlated f-test and 24 hr after labor p = 0.04, one-tailed, correlated f-test.
in pelvic and hypogastric afferents from the cervix and (iii) transection of the pelvic and hypogastric nerves virtually abolishes analgesia produced by cervical probing (see introduction). Based on those studies, the elevation in pain threshold observed in the present study may be due at least in part to activation of cervical afferents during labor.
General Discussion
The subjects’ reports of their endogenous pain, using a visual analog scale, demonstrate that they experienced pain during labor. Concurrently, the pain thresholds to exogenously applied noxious stimulation increased, while the tactile thresholds to innocuous stimulation did not change, in the subjects who had childbirth preparation but not in the subjects who did not have childbirth preparation. We conclude therefore, that in the women with childbirth preparation, an analgesic process is activated during labor. While the degree of analgesia is not sufficient to eliminate the pain of labor, the present findings imply that the pain of labor would be more severe if this mechanism did not exist. On the basis of the present findings, the discrepancies in pain thresholds reported in the literature may be due to differences in anxiety in the individual subjects because of their previous experiences and/or present expectations. This may have been a contributing factor that could account for variability in other studies as well as in this study. The subjects who had Lamaze preparation may have experienced a lower level of stress and anxiety during labor that those who did not have this preparation, which thereby enabled us to obtain a more valid determination of their pain thresholds. However, it may also be that this analgesia during labor is not a component of cervical dilatation, but a consequence of the activities used during prepared childbirth. Further testing, with measurement of state anxiety and a larger sample, would be useful to understand this
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phenomenon. It would also be interesting to extend the measurement of sensory thresholds beyond 8 cm cervical dilatation and into the delivery period, since there are anecdotal reports that the level of labor pain decreases during the transition phase (Gaskin, 1978). No recommendations as to nursing interventions can be made based on this exploratory study, which is limited in terms of sample size and demographic data. The potential contribution to the findings of the subjects’ support system, education, etc., cannot be assessed but is worthy of further investigation. However, it is important to emphasize that although labor pain can indeed be severe, continued laboratory and clinical research is necessary to provide insight into mechanisms underlying the naturally triggered endogenous analgesia-producing system, an understanding of which could provide the basis for new means of augmenting the actions of such an analgesia-triggering mechanism, e.g. by new pharmacological or dietary means, in order to lead to novel methods to attenuate pain of parturition (Komisaruk and Whipple, 1988). Acknowledgements&We gratefully acknowledge the assistance of MS Marlene Toniolo, MS Lucy Buglisi, R.N., and MS Mary Ellen Affanoso, R.N. in the data collection for this study. This research was supported in part by the Office of Research and Sponsored Programs, #2-02305 (B.W.) and the Charles and Johanna Busch Foundation, (B.R.K.), both of Rutgers, The State University of New Jersey. Contribution no. 491 from the Institute of Animal Behavior.
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Toniolo, M. V., Whipple, B. H. and Komisaruk, B. R. (1987). Spontaneous maternal analgesia during birth in rats. Proc. 1987 NIH Centennial MBRS-MARC Symposium 15, 100. Whipple. B. and Komisaruk, B. R. (1985). Elevation of pain threshold by vaginal stimulation in women. Pain 21, 357-367.
Whipple, B. and Komisaruk, B. R. (1988) Analgesia produced in women by genital self-stimulation. J. Sex Res. 24, 130-140. (Received
17 January
1989; accepted for publication
15 November 1989)
