Uterine Contractile Physiology: Applications In Nursing Care and Patient Teaching

continuing education

Uterine Contractile Physiology: Applications In Nursing Care and Patient Teaching U

NANCY WHITLEY, CNM

A basic review of uterine contractile physiology, with emphasis on practical applications in nursing care and patient teaching, is presented. Specific areas reviewed are terminology, uterine activity during pregnancy and labor, conduction of contractions, and positions for labor and delivery. An understanding of uterine contractile physiology is basic to the nursing care of labor patients and also has applications in childbirth education; but in my experience, programs preparing nurses, childbirth educators, and even nursemidwives provide almost no background in uterine physiology, a problem compounded by the dearth of articles on the subject in the literature of these disciplines. Among the many investigators in this field, Caldeyro-Barcia is probably the most prominent, and much of the material in this article has been taken from his work. One of the best summaries of uterine contractile physiology, however, is in Bonica’s Obstetric Analgesia and Anesthesia, which also lists an extensive bibliography.’

Review Terminology and Methods It would be risefril here to define some terms. “Amniotic fluid pres54

sure” is risrially the same as “intervilloris fluid pressure,” and the term “intrauterine pressure” may be used for both.2 Depending on the investigator, various techniques may be used to measure intrauterine pressure. Caldeyro-Barcia introduced a polyethylene catheter through the abdominal wall into the amniotic

and propagation of uterine contractile waves. The catheters were then connected to electromanometers.‘ Reynolds, in his work, used a catheter embedded in the uterine muscle, with the tip lying in a tiny pool of heparinized saline.’ There are also other possible methods. The lowest pressure recorded between contractions is called tonus. During pregnancy, this is about 8 mmHg (millimeters of mercury), and in labor it increases to 10-12 mmHg. Values above this represent varying degrees of hypertonicity, which may

There is a belief that the more false labor a woman experiences during her pregnancy, the easier and faster her actual labor will be. This idea has some substance. sac, and then connected it to a mercury manometer. He measured placental pressure in the third stage after the clamping of the cord by inserting a trocar into the umbilical vein and connecting it to a mercury m a n ~ m e t e r Another .~ technique was to introdrice catheters with pressuresensitive microballoons at the tips into the uterine muscle at the fundas, midportion, and lower uterine segment to study the origin

be associated with such occurrences as overdistension of the uterus, as in polyhydramnios, or incoordinate uterine action.’ The frequency of contractions means the number of contractions per 10 minutes. (In South America, where much of this research was done, contractions are written “3/10” or “5/10” rather than ‘‘q3m” or “q2m” as here in the United States.) The intensity of contractions means September/October 1975 JOCN Nursing

the rise in amniotic fluid pressure, measured in millimeters of mercury, above the tonus. Uterine activity is the product of the intensity multiplied by the frequency. This is expressed in millimeters of mercury per 10 minutes, or Montevideo units' (after the Montevideo School of Obstetric Physiology, where Caldeyro-Barcia did his work). For example, contractions with an intensity of 30 mmHg occurring twice in 10 minutes would equal 60 Montevideo units.

Uterine Activity During Pregnancy For the first 30 weeks of pregnancy, uterine activity measures less than 20 Montevideo units. Mild, localized contractions occur about once a minute. They may enhance the circulation and facilitate placental function. Braxton-Hicks contractions also occur during this period at the rate of about one per hour at 30 weeks. They are of greater intensity (10-15 mmHg) and involve a larger area of the uterus.' During the last two to three weeks of pregnancy, uterine activity increases markedly. Stronger BraxtonHicks contractions, occurring irregularly once or twice in a 10minute period, replace the mild localized contractions. BraxtonHicks contractions help to differentiate the uterus into a thick upper segment, which is the contractile portion, and the thinner lower uterine segment. They also effect some ripening and effacement of the cervix This prelabor is almost indistinguishable from early labor, which occurs when uterine activity reaches 80-120 Montevideo units." September/October 1975 JOCN Nursing

In some areas, patients call Braxton-Hicks contractions priming pains," and there is a belief that the more false labor a woman experiences during her pregnancy, the easier and faster her actual labor will be. Considering the effect of Braxton-Hicks contractions on the cervix, this idea has some substance. "

Moreover, in teaching signs of labor, childbirth educators usually represent the loss of the mucus plug as a probable sign of labor.'-" While this may be accurate in many cases, its loss may also occur in conjunction with cervical effacement and dilatation at some point distant from the onset of labor. In addition, especially

Childbirth educators sometimes teach couples that contractions are strongest during transition and also closer together than during the second stage, but we see that this is not so.

Before the onset of labor, there may be more cervical dilatation than is generally appreciated. In one study,' the average dilatation in the final days of pregnancy was 1.8 cm in nulliparas and 2.2 cm in multiparas. The prelabor dilatation curve advanced very slowly in the final weeks of pregnancy, at less than onehundredth the speed of the dilatation curve in active labor. The authors presumed the onset of active labor to be at the point of intersection between the prelabor and active labor curves. Slow, progressive dilatation of the cervix in the latter part of pregnancy was apparently not associated with premature labor, as one might suppose. The authors believed a closed cervix at the onset of labor to be a rarity, possibly associated with some abnormality or perhaps an inaccurate examination. What are the applications in patient teaching? It is really not correct to teach that the cervix begins to dilate with the onset of labor.

during the third trimester, spotting may occur after coitus or a vaginal examination, and the woman may confuse this with show.

Uterine Activity During Labor As previously stated, clinical labor begins when uterine activity reaches values of 80-120 Montevideo units, and there is no clear-cut division between prelabor and early labor. As the first stage of labor progresses, the intensity of contractions increases from an average of 25-30 mmHg above the tonus at the onset to 40 mmHg during transition. During the same period of time, the frequency increases from 3 per 10 minutes in the beginning to 4 per 10 minutes near the end of the first stage.' During the second stage, the contractions are even stronger, at 50 mmHg, with a frequency of 5 per 10 minutes for uterine activity of 250 Montevideo units.' The pressure brought about by the voluntary

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forces of expulsion-the diaphragm and the abdominal musclessuperimposed on the pressure of the uterine contractions themselves, increases the intrauterine pressure to 80-110 mmHg or even higher. Childbirth educators sometimes teach coyples that contractions are strongest during transition and also closer together than during the second stage,”-” but from the uterine activity values given in the preceding paragraphs, we see that this is not so: As labor progresses to delivery, the contractions get stronger and closer together. Nevertheless, most women d o seem to have less pain during the second stage than in transition, when many are quite rincomfortable. There is no entirely satisfactory explanation for this Some numbing of the vaginal outlet occiirs late in the second stage when the presenting part of the fetus reaches the pelvic floor because the pressure causes a temporary interruption of the circulation, although this is by no means always complete; but that would not explain the apparent difference in pain perception earlier and when only the contractions themselves are considered.

tribe predominating, and spread downward at the rate of 2 cm/sec, affecting the entire uterris within 15 seconds.’ The intensity of contraction is greatest in the fundus, with reduced duration and intensity in the lower uterine segment. Fiindal dominance is necessary for normal labor and dilatation of the cervix.16

Disoders of Conduction It is possible, however, that an inversion of the normal gradient of contraction may occur, in which the contraction starts in the lower part of the iiterris and spreads upward. Although such contractions can cause marked elevations in intrauterine pressure, they are ineffective in dilating the cervix.6 Incoordination is another possible disturbance of conduction. This occurs when the areas near the fallopian tribes initiate contractions independently, or when the uterus is functionally divided into a number of zones that contract separately. In such cases, spinal conduction blocks or certain regional procedures may improve the rhythm and increase the intensity of the contractions. This includes subarachnoid, lumbar epidural, and caudal blocks, and certain regional blocks such as paracer-

uterine dysfunction, which manifests itself early in labor, mainly in nulliparas, and can be treated with sedation. It is not an innocuous pattern, since it often results in fetal distress. The pain suffered by the woman appears to be out of proportion to the strength of the contractions palpated abdominally and her general progress in 1ab0r.I~ One study found the incidence of this type of dysfrinctional labor to be quite low-only 1 case in 2,551 deliveries.” Without specific reference to this particular study, I think it likely that patients with hypertonic dysfunction are often dismissed as “crocks,” so that the diagnosis is missed. It’s something to keep in mind, especially with nulliparas in early labor who “overreact” to their contractions. Hypertonicity in the nonpregnant uterris has been implicated as a cause of primary dysmenorrhea. In one study, a sustained tonus of between 50 and 60 mmHg, beginning in the secretory phase of the cycle and extending into the early menstrual phase, was associated with the condition.l8 On the other hand, hypotonic dysfunction of labor is associated with normal wave patterns. It occiirs during the accelerated phase of labor if

During the second stage, the woman, trained or untrained, takes a more active part in her labor, and The intensity and frequency of contractions and uterine this could raise her pain threshold. Then too, the end of labor is in sight, tonus between contractions are related to the position of so her outlogk may improve. Another the woman. Contractions are more efficient for the possible explanation is that dilatation progress of labor if the woman avoids the supine position. is no longer an active process during the second stage, and the stretching of the cervix is an important probavicals.’ On the other hand, the use of the intensity of the contractions is ble cause of labor pain.“ anesthetics or heavy sedation early in less than 15 mmHg and can be Barring complications, the third normal labor will significantly treated with o x y t ~ c i n . ‘ ~ - ’ ~ The most frequent causes of both stage of labor is also relatively prolong the latent phase.le Basic painless, and yet uterine activity dur- nursing measures such as massage hyper- and hypotonic dysfunction ing this time may be greater than techniques and position changes can are minor contractions of the pelvis reduce the need for medication dur- and slight abnormalities of fetal during the first or second stage.’ ing this period. In my experience, position.” Both patterns cause a deprepared patients rarely require lay in the progress of labor. medication during the latent phase, Conduction of Impulses and I see this as one of the advanPositions of Women in Labor Normal contractions begin near tages of preparation for childbirth. the uterine end of the fallopian Incoordinate uterine contractions The intensity and frequency of tribes, with the area near the right are often associated with hypertonic contractions and uterine tonus

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September/October 1975 JOGN Nursing

between contractions are related to the position of the woman. When she lies on her back, the contractions are lower in intensity, occur more frequently, and the tonus is higher ‘O One study showed uterine activity to be greatest in the right lateral position Contractions are more efficient for the progress of labor if the woman avoids the supine position

pression of the lower aorta and common iliac arteries from the weight of the gravid uterus, compromising uterine blood flow, and is also associated with a hypotensive syndrome due to compression of the inferior vena cava. There is little to recommend it, except for the convenience of the staff in listening to fetal heart tones and performing examinations

If she prefers the side-lying position, in addition to the usual pillows under head and shoulders and between her legs, place a folded sheet under her abdomen to help support the weight of the uterus. This eliminates the pull that some women feel in the inguinal region when lying on their side

Position During the Second Stage

Another possibility is keeping the woman up and about as long as she is comfortable, providing the baby’s head has engaged and the patient has not been sedated. Myles, in her British midwifery classic, says: “Labor always seems longer and more painful to the woman who goes to bed and stays there as soon as contractions begin, waiting apprehensively for each pain ”” She also notes certain other advantages to the upright position: namely, that as the

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Along somewhat different lines, the effectiveness of pushing in the second stage may also. relate to the position of the woman. Using intravaginal balloons connected to a mercury manometer, Mengert and Murphy measured the efficiency of bearing-down efforts in terms of increased intraabdominal pressure in seven positions: lateral prone, recumbent, knee-elbow, semirecumbent, squatting, standing, and sitting. The general trend showed low averages for postures with the body in a horizontal position and high averages for those with the body in a vertical position. They concluded that their observations suggested greater rise of the sitting position during the second stage whenever it was desirable to expedite labor.“

Although there are potential practical applications of research into uterine contractile physiology and related areas, there seems to be a real problem, a serious delay, in applying certain research findings in the clinical areas, especially those related to positions for labor and delivery. baby sinks into the lower pole of the uterus, the presenting part presses on cervical nerve endings, possibly stimulating stronger contractions; and also that in the upright position there is some enlargement of the pelvic brim due to a slight degree of movement at the sacroiliac joints. There seems to be little justification for confining the woman to bed, especially during the latent phase, and even less for insisting on the supine position, as is sometimes done. This position may cause comSeptember/October 1975 JOGN Nursing

A more recent study by a nursemidwife demonstrated the duration of both the first and second stages to be significantly shorter in the upright position. In a series of 60 primigravidas, 30 in the upright position (at a 30’ angle) and 30 recumbent, the first stage was an average of 85.73 minutes shorter in the upright position, and the second stage, an average of 40.67 minutes shorter.” In most non-Western cultures, women assume the upright position for birth.26 Our own delivery tables

can be modified to support the patient at about a 45” angle, or the woman’s attendants can support her back and shoiilders Obstetric procedures like episiotomy can still be done in this position, and the patient has a greater advantage in pushing A change in position is at least worth trying when there is a delay in rotation and descent The propped position in the second stage of labor also allows the woman to take a more active and positive role in the delivery of her baby 27 For this reason, prepared patients may prefer this position, especially if they have had psychoprophylactic training One hospital has a labor-delivery bed with a n adjustable backrest for its Lamaze patients The bed is modified with stirrups, and both labor and delivery take place in the same room and bed This has the added advantage of eliminating the last-minute rush to the delivery room

Conclusion The important points for nursing care are as follows: a ) Prelabor (false labor) is almost indistinguishable from early labor, even with very sophisticated techniques. b ) A delay in the progress of labor may be related to abnormal as well as subnormal yterine contractile patterns (in addition to other possible abnormalities not germane to this discussion). c ) Abnormal contractile patterns may be associated with hypertonicity and fetal distress. d ) Contractions are more efficient for the progress of labor if the patient lies on her side or sits. e) The supine position has many disadvantages, including lowering the intensity of contractions, raising the tonus between contractions, and causing unfavorable alterations in the blood supply through compression of the aorta and inferior vena cava. f ) The propped position for delivery results in more effective pushing and also allows the woman to take a more active and positive role in the delivery of her baby.

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Three important points for patient teaching follow: 1) The cervix begins to dilate well before t h e onset of clinical labor. 2 ) The loss of the miiciis plug may or may not be a sign of early labor. 3) Contractions are closer together and stronger during the second stage than during transition. Finally, although there are potential practical applications of research into uterine contractile physiology and related areas, there seems to be a real problem, a serious delay, in applying certain research findings in the clinical area, especially those related to positions for labor a n d delivery. Many of the references given in this article are quite old, e.g.,the Mengert a n d Murphy study was published in 1933. I used them not only because they are still valid, brit also precisely because they seem to point rip this difficulty. It’s not the purpose of this review to examine the problem in detail, Nevertheless, it does exist, a n d w e n e e d s o m e thorightfril analyses of why it’s so difficult to make seemingly minor changes in patient care

5. Caldeyro-Barcia, R., S . V. Pose, and H. Alvarez: “Uterine contractility in polyhydramnios and the effects of withdrawal of the excess of amniotic fluid.” A m J Obstet Gynecot 73(6):1238-1254, June 1957 6. Caldeyro-Barcia, R., and J. J. Poseiro: Physiology of the uterine contraction.” Clin Obstet Gynecol 3(2):386408, June 1960 7. Hendricks, C. H . , W. E. Brenner, and G. Kraris: “Normal cervical dilatation pattern in late pregnancy and labor.” A m J Obstet Gynecol 106(7):1065-1082, April 1, 1970 8. Hungerford, M . J . : Childbirth Education. Springfield, IL, Charles C. Thomas, Publisher, 1972, p 99 9. Kitzinger, S.: The Experience of Birth. New York, Taplinger Piiblishirig Company, 1972, p 155 10. Sasmor, J . L.: What Every Husband Should Know About Having a Baby. Chicago, Nelson-Hall, 1972, p 102 11. Wessel, H . : Natural Childbirth and the Family. New York, Harper and Row, 1973, p 126 12. Ewy, D.: Preparation for Childbirth. Boulder, CO, Priiett Publishing Company, 1970, pp 34-37 13. Wright, E.: The New Childbirth. Acknowledgments London, Library 33 Limited, 1966; The author expresses thanks to see diagrams of labor contractions for magnitride, duration and inCarmela Cavero, C N M , S u n n y e Strickland, C N M , Philip Sumner, tervals between contractions; pp MD, Sue Lynn Adams, Lamaze 138, 139 and 154, 155 teacher, a n d Dee Robinson, secre- 14. Moir, C.: “The nature of the pain of tary, for assistance in preparing this labor.” J Obstet Cynecol Br E m p 46(3):409-425, June 1939 article. 15. Hellman, L. M . , J. Harris, and S. R. References M. Reynolds: “Characteristics of the gradients of nterine contractility 1. Bonica, J , J . : Principles and Practice during the first stage of true labor.” of Obstetric Analgesia and Brill Johns Hopkins Hosp 86:234Anesthesia. Philadelphis, F. A. Davis 248, 1950 Company, 1972, pp 53-78 16. Friedman, E. A.: Labor: Clinical 2. Hendricks, C. H., T. Eskes, and K. Evaluation and Management. New Saameli: Uterine contractility at York, Meredith Publishing Comdelivery and in the p~terperium.” pany, 1967, Chapters 34, 38, and 39 A m J Ohstel Gynecol 83(7):890-906, 17 Hellman, L. M . : “Oxytocin in April 1, 1962 dysfunctional labor.” Clin Obstet 3. Alvarcz, H., and R. Caldeyro: Gynecol 2(2):343-353, June 1959 “c;ontractility of the human uterus 18. Knapp, R. C . , and J. C. Warenski: recorded by new methods.” Surg “Clinical evaluation of dysfunctional G ? p x o l Obstet 91(1):1-13, Jrily labor.” Obstet Gynecol 21(5):6271950 631, May 1963 4 Caldeyro-Barcia, R., and J. J. Poseiro: 19. Filler, W. W., and W. C. Hall: “Oxytocin and contractility of the “Dysmenorrhea and its therapy: A pregnant hnman uterus.” Ann N Y uterine contractility study.” A m .I Acad Sci 75:813-830, 1959 “

Obstet Gynecol 106(1):104-109, January 1, 1970 20. Caldeyro-Barcia, R., et al.: “Effect of position changes on the intensity and frequency of uterine contractions during labor.” A m Obstet Gynecol 80(2):284-290, Arlgltst 1960 21. Burnhill, M. S . , J . Danzeis, and J. Cohen: “Uterine contractility during labor studied by intra-amniotic fluid pressure recordings.” Am J Obstet Gynecol 83(5):561-571, March 1, 1962 22. Myles, M . F . : Textbook for Midwives. Edinburgh and London, Churchill Livingstone. Seventh edition, 1971, p 282 23. Bond, S.: “Reevaluating positions for labor-lateral vs supine.” JOGN Niirs 2:6:29-31, November/Decemhcr 1973 24. Mengert, W. F., and D. P. Murphy: Intra-abdominal pressures created by volrintary muscular effort-Part 11. Relation to posture in labor.” Sitrg Gynecol Obstet 57:745-751, I933 25. Liri, Y . C . : “Effects of an upright position during labor.” A m J Nurs T4( 12):2202-2205, December 1974 26. Naroll, F., R. Naroll, and F. H. Howard: “Position of women in childbirth.’’ A m J Obstet Gynecol 82(4):943-954, October 1961 27. Newton, M . , and N . Newton: “The propped position for the second stage of labor.” Obstet Gynecol 15(1):28-34, January 1960 28. Sumner, P. E., J. P. Wheeler, and S C. Smith: “The labor-delivery bed -simplified obstetrics.” J Reprod Med 13(4):158-161, October 1974 “

Address requests for reprints to Nancy Whitley, CNM, 125 Pearl Street, Mount Holly, NJ 08060.

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Nancy Whitley is a staff nursemidwife at Martland Hospital, Newark, New Jersey. She has a diploma in nursing f r o m Bellevue Hospital School of Nursing in New York City (1964) and a certificate in nurse-midwifery f o r m Kings County Hospitul in Brooklyn, New Kork ( 1 9 7 1 ) . She has taught prenatal classes both here and abroad and is uia ASPO-certaj5ed Lamaze teacher. September/October 1975 JOGN Nursing