STUDIES ON PAIN: MEASUREMENTSOF PAIN INTENSITYIN CHILDBIRTH'

By JAMES D. HARDYAND CARL T. JAVERT

(From the Russell Sage Institute of Pathology, The New York Hospital and the Departmentsof Physiology, Obstetrics and Gynecology, Cornell University

Medical College, New York City)

(Received for publication July 22, 1948)

It is a common observation that patients differwidely in their reaction to childbirth, some womengiving evidence of great distress and others main-taining a high degree of equanimity throughoutlabor. These variations in reaction have led ob-stetricians to question the existence of pain in cer-tain patients (1), except for the sensations ac-companying the surgical procedures often requiredfor the delivery of the baby. In the absence of aquantitative method for measurement of pain in-tensity, the reaction of the patient has been theprincipal index of the severity of labor pains.

The introduction of dolorimetry, a laboratorymethod of measuring painfulness, by Hardy, Wolffand Goodell (2, 3), has made it possible to obtainreliable estimates of spontaneous and experimen-tally induced pain intensity. This method providesa scale based on the ability of an individual to dis-criminate differences in the intensities of pain.The technique consists essentially of comparingthe intensity of a spontaneous pain with that in-duced on the skin of the subject by thermal radia-tion. No clinical application of this method hasyet been made, and in applying it to the study ofthe pain experienced during labor, it was hopedthat the patient could make reproducible compari-sons of the labor pains with those evoked by thethermal stimulus on the skin.. If this proved pos-sible, the intensity of the painful sensations ex-perienced by a patient in the various stages of laborand by different patients, could be compared on acommon basis. This would provide a convenientmethod for the evaluation of the effectiveness andmode of action of analgesic agents administeredfor the relief of pain during labor.

It is the object of this report to present experi-mental data on pain intensity, obtained from astudy of 13 women in the various stages of labor

1 This work was supported in part by funds from theOffice of Naval Research of the United States Navy.

before the administration of any analgesic or an-esthetic agent.

METHOD

Thirteen normal young women without obvious medi-cal or obstetrical complications cooperated as subjects inthese experiments. There were ten primiparas and threemultiparas. Nine were private patients who had a defi-nite interest in the investigation as they were eitherformer nurses or wives of physicians or professional men.Special urging was unnecessary to persuade these pa-tients to participate in this study, as either curiosity ordesire to be of service caused them to volunteer readily.All patients had one motive in common, namely, to with-stand any reasonable discomfort if it would help ease thesuffering of future patients. The matter was, of course,discussed with the husband and in instances in which thepatient was the wife of a physician the husband remainedin the room while the study progressed. In two instancesthe pain studies were discontinued in the latter part ofthe first stage of labor at the patients' request. At firstprivate patients were deemed most suitable for this studyas it was not clear that discriminating differences inintensity of pain would be a simple matter during thecourse of labor. Since these patients had no difficultyin making the necessary comparisons, four clinic patientswere subsequently included in the study.

Prior to hospital admission in labor, all of the privatepatients had been given preliminary instruction in thethermal radiation technique of pain threshold measure-ment (4). The operation of the equipment was ex-plained to them, and they were introduced to the severalpersons involved in the study who would be present dur-ing the investigation. As the authors and a technicianremained with the patient during the entire period ofobservation, it was felt desirable that rapport be estab-lished between the patient and the experimental team.

During the first laboratory interview the skin of thedorsum of the right hand was blackened with India inkin four areas, and the pain threshold measured on thesetest surfaces. The pain threshold was measured by firstexposing the blackened skin for three seconds to anintensity of thermal radiation slightly above the painthreshold so that a distinct pain was felt. Then, begin-ning with a stimulus well below the pain threshold, theradiation intensity was increased through successive ex-posures until the patient reported a barely perceptibleprick at the end of the exposure. This intensity was

153

JAMES D. HARDYAD CARL T. JAVERT

taken as the pain threshold. It is of the greatest impor-tance that different areas of skin be stimulated in eachexposure, and that as few stimuli as possible be given tolocate accurately the threshold. An average of fourstimuli was usually sufficient to determine the pain thresh-old within plus-minus ten millicalories per second persquare centimeter. None of the patients studied had theslightest difficulty in recognizing the pain threshold.

During this period of instruction the difference in in-tensity of two pains was demonstrated by exposing thepatient to a stimulus evoking a two-dol 2 pain and thento one evoking a four-dol pain. This was done to im-press upon the patient the importance of the intensityaspect of pain as contrasted with its quality and duration,since the experimental pain ostensibly differed from thelabor pain in these respects. Most of the patients com-mented upon the fact that the sensation from the uteruswould be different in quality, duration and locale, and,therefore, expressed some doubt as to the possibility ofequating the two sensations in terms of intensity. Theseapprehensions were not borne out by experience as onlytwo of the patients had difficulty in making comparisons,and in these instances the degree of cooperation was notsufficient to continue the observations through the firststage of labor.

The use of the word "pain"' in connection with theuterine contraction was avoided in discussing the subjectwith the patients, and "intensity of contraction" was thedescriptive term employed. It was hoped in this way toavoid suggesting to the patient that her labor wouldbe painful.

During labor the technique for measuring pain inten-sity was as follows: Immediately following a contractionthe patient was given a three-second exposure to thethermal stimulus. She made a report as to whether ornot the sensation on the back of her hand was more orless intense than that experienced from the uterine con-traction. Depending upon this report the intensity ofthe stimulus was increased or decreased and the testrepeated in order to "bracket" the uterine sensation. Byhaving two stimuli, one lower and one higher than thespontaneous sensation, it was possible to find quickly thestimulus which the patient reported to be approximatelyof the same intensity as her labor pain. Usually threeor four stimuli were enough to locate the balance pointbetween the sensation from the uterus and that from thepain apparatus, but if more stimuli were required a restperiod of about ten minutes was allowed before resumingtests. Care was taken throughout not to overstimulateany area of blackened skin and thereby produce localhyperalgesia.

It was repeatedly observed that the intensity report wasthe same whether the comparison of sensations was madeat the-height of a contraction or in the interval betweencontractions. As there was more time available betweencontractions comparisons were usually made then, adjust-ing the thermal stimulus to produce a sensation equivalent

2 "Dol" is the term used to denote the unit of pain-fulness, and has a value of approximately one-tenth theintensity of the maximal pain.

in intensity to that evoked by the previous contraction.Two measurements were made and if the reports agreedwithin one dot they were averaged and used as the pointto be plotted. Single test reports were used only at thehighest pain intensities, because of tissue damage result-ing from the strong thermal stimuli. Large variations inreports of intensity (three to five dols) occurred whenthe patient was distracted or not cooperating, and wereobserved in both patients who requested that the study bediscontinued. This possibility had been anticipated sinceerratic reporting had been observed in a previous studyof medical students (3). In spite of the clinical proce-dures necessarily carried out during the course of thepain study, the fatigue of the patient, and other disturbingcircumstances that could not be controlled or eliminated,distraction resulting in erratic estimates was not an im-portant factor. Reproducibility of the measurementswithin plus-minus one dot was taken to be the criterionof the excellence of the data.

Continued emphasis was placed during labor upon theintensity of the sensation as distinct from its quality andduration, because the patients frequently stated that thethermal pain was not "comparable" to the labor pain.The difference between the experimental pain and thelabor pain may have caused some difficulty in makingaccurate comparisons. However, repeated comparisonsduring labor showed only variations of approximatelyplus-minus one dol, and as the limit of discrimination isplus-minus one-half dol this shows that the patients weremaking satisfactory appraisals.

It is important to keep in mind that as the severity oflabor pain increases, thermal stimuli of correspondinglyhigher intensity must be administered and tissue damagewill be produced when the comparisons extend beyondeight dols.

Pain threshold measurements were made throughoutthe entire period of study, in the intervals between con-tractions. It has been previously shown that an existingpain will cause an elevation of the pain threshold else-where in the body (4), and for this reason pain thresholdmeasurements were not made during a contraction.

As soon as the patients were admitted to the hospitalthey were placed in a private labor room for study. Oneof the experimental team remained with the patient at alltimes, and pain tests were made every half hour or whenthere was any change in the patient's obstetrical status.The majority of the measurements were made in the firststage of labor, although two patients were observed inthe second stage, and two in the fourth stage (5). Nomeasurements were made in the third stage. None ofthe patients had had any analgesic or anesthetic duringthe time they were studied, although subsequent to thepain studies nine had analgesics and 11 had anestheticsfor the delivery. If at any time during the study apatient requested an analgesic it was administered and thepain studies were terminated. This accounts for the factthat most of the patients were studied in the early andmiddle parts of the first stage of labor.

Measurements were made of the frequency and dura-tion of uterine contractions by manual palpation of the

154

MEASUREMENTSOF PAIN I

subject's abdomen. Cervical dilatation was estimated byrectal examination and in some instances confirmed bysterile vaginal examination.

In four patients records were made of the uterine con-tractions by means of a pneumograph strapped to theabdomen. It was not anticipated that the excursions ofthe pneumographic needle would be proportional to theintensity of the uterine contraction, but the duration ofcontraction and the interval between contractions couldbe more accurately determined in this way than by manualpalpation of the abdomen. The patient was given a sig-nal magnet key to press during the time she felt pain, asit was desirable to know whether or not the pain out-lasted the contraction. The patient was also asked togive an estimate of the intensity of her pain each fiveseconds during the contraction. The reports were madein terms of fractions of the intensity of a recently meas-ured uterine pain. This type of procedure for estimatingpain intensity had been proved successful in the study ofheadache and other pains (6, 7).

In the course of all studies careful notes were kept asto the patients' reactions, such as- crying, complainingsweating and degree of alertness and cooperation in thepain studies. Such notes were used to arrive at an esti-mate of the patient's reaction to her pain.

NTENSITY IN CHILDBIRTH 155

RESULTS

Figure 1 shows the results obtained on the firstpatient studied. This individual was a primipara,a professional woman of high intelligence, whowished to experience labor without analgesia, be-cause of her interest in Read's publications on nat-ural childbirth. Studies were begun five hoursafter onset of the first pains, at which time laborwas still mild, and the pain intensity averaged ap-proximately three dols. The cervix was three cen-timeters dilated; uterine contractions occurred atfive-minute intervals and lasted approximately 30seconds. During the next five hours, as the cer-vix dilated gradually, the pain increased steadily,reaching an intensity of eight dols, and the con-traction interval decreased to one minute.

The subject was calm throughout the course ofher labor except for obvious tenseness. However,after the pain had reached eight dols (a severepain) she asked for an analgesic. This was ad-

MRS. E.F.- AGE 33 - HIST. NO 471895 - PRIMI GRAVIDAE D.C APRIL IS 1947

LABOR BEGAN MARCH26 1947

NO ANALGESIACa

A%

FORCEPSDELIVERY

4 5 6 7 8

DURATION OF LABOR- HOURS

FIG. 1. PAIN CHARTOF MRS. FAbscissa: Duration of labor in hours.Ordinates: Dots represent duration of contraction in seconds; circles-contraction interval in

minutes; triangles-cervical dilatation in centimeters; and crosses-pain intensity in dols.

MIN.

00

a

3

4

5

DOLS

x2

10

8

8

4

2

0

CMS

12

10

8

6

4

2

0

060

50

40

30

20

10

86110

JAMES D. HARDYAD CARL T. JAVERT

ministered in the form of nitrous oxide during thesecond stage.

The second patient was a multipara who had hadan unhappy obstetrical history characterized bysix spontaneous abortions and only one livingchild. When the study began her pains weremoderately severe, the cervix was closed and thecontraction interval was about four minutes. Inthe next two hours the pain rose very rapidly tothe extreme range above nine dols, as shown inFigure 2. The cervix dilated rapidly, and thecontraction interval shortened in proportion. Twomeasurements were made on this patient duringthe second stage of labor, although the patient hadbeen repeatedly warned of the danger of blistering.Yet she wished to cooperate fully as an expressionof her gratitude in having a term pregnancy andinsisted on having the tests made. A pain inten-sity of 10Y2 dols was measured. This is the mostintense pain which can be experienced. Increas-ing the intensity of the thermal radiation can cause

x12

I0

8

6

4

2

A12

I0

8

6

4

2

00

060

50

21 40

3

4

5

OI 01 6

30

20

10

no further increase in pain perception (2). Sec-ond degree burns were inflicted upon the hands ofthis patient by the four tests made at levels higherthan nine dols. Delivery was accomplished undergeneral anesthesia.

Figure 3 shows a composite study of the 13 pa-tients. As each determination represents not lessthan eight tests for pain intensity, the 55 measure-ments are the result of over 400 test readings.These 55 measurements represent an average ofabout four per patient during the various stages oflabor. Since the first stage of labor had a variableduration for each patient, the time scale is ex-pressed in quarters in order to obtain a commonbasis for comparison. It will be observed thatthere was a gradual increase in pain intensity asdelivery approached, at which time pain reachednine and even 10½2 dols. The dashed line in thefigure represents an approximate average of ob-served values in the first and second stages oflabor. In the case of subject H, whose values lie

MRS. S.A. -AGE 40 -HIST. NO. 351111 - PARA2 GRAVIDA 8E.DaC. MAY 17. 1947

LABOR BEGAN MAY 8. 1947NO ANALGESIA

SPONTANEOUSDELIVERY

2 3 4 5L .I1ST STAGE 2NO

STAGEDURATION OF LABOR- HOURS

FIG. 2. PAIN CHARTOF Mis. ASymbols the same as in Figure 1.

156

MEASUREMENTSOF PAIN INTENSITY IN CHILDBIRTH

/4 34

N-H/

HC

X i t AOA-w

04 F"0 f0 it

AlF

A T T

FIRST STAGE OF L A B

FIG. 3. PLOT OF 55 DETERMINATIONSOF PAIN INIFOURTHSTA

Time scale of first stage expresse

quite far above the dashed line, it is to be remarkedthat the progress of her pains was parallel to theaverage line, but subsequent to the last determina-tion shown she received a considerable amount ofan analgesic which may have prolonged her labor.

The intensity of the after pains was measured inonly two patients who had received no analgesiain labor and no anesthesia for delivery. Thesepains were moderate, three to five dols, a halfhour after delivery, and decreased to thresholdlevels two hours later.

Figure 4 shows typical uterine contractions asrecorded on the pneumograph. "A" is the re-corded curve of contraction; line "B" shows theduration of pain as recorded by the patient whenshe pressed the key of the signal magnet; andcurve "C" is the estimated intensity of pain. Thefirst contraction was recorded in the early part ofthe first stage of labor, the second during the mid-dle of the first stage, and the third curve duringthe second stage. It will be seen that the pain ineach case was of shorter duration than the con-traction, which is in contrast with the findings in

F

F

2nd 3Fdad 4"2 R IN QUARTERS STAGE STAGES

INl HOURS

TENSITY ON 13 PATIENTS DURING FIRST, SECONDANDWGESOF LABOR:d in quarters of its total duration.

dysmenorrhea. Chassar Moir (8) has reportedthat in cases of dysmenorrhea the pain outlasts theuterine contraction and from this observation hesuggested that the pain of dysmenorrhea might beattributed to hypoxia in the uterine muscle. Itwill be noted, too, that the stronger pains repre-sent a greater proportion of the time of contrac-tion than do the weaker pains, the range being from15 per cent to 95 per cent of the total contractiontime, depending upon the intensity of the con-traction. This indicates that for weak contractionsthe pain threshold is reached more slowly thanfor stronger contractions.

The duration of contractions varied considerablyfrom time to time in the same patient in variousstages of labor, and showed no tendency to in-crease as delivery approached. This is in agree-ment with the observations of Murphy (9) whoreports a decrease in average contraction time aslabor advances.

DISCUSSION

The first questions to be answered were whetheror not the method of dolorimetry was applicable to

157

N

JAMES D. HARDYAD CARL T. JAVERT

the measurement of pain intensity in labor, and,if so, what were the limitations of the method.

The more than 400 trial tests resulting in 55 suc-cessful measurements of pain intensity on 13 pa-tients establishes the method as a practical onefor this type of investigation. However, this studydoes not attempt to be a complete survey since itdoes not include examples of the many types oflabor and dystocia that may be found in the ob-stetrician's practice. Measurements can be madeeasily on any patient who can cooperate. The ef-fects of distraction and lack of cooperation can berecognized by the large and rapid variations inestimates of pain intensity. Although the data areobtained from subjective comparisons of pain in-tensity, the test instrument affords an evaluationof the patient's report over which the patient hasno control.

The most important limitation of the method isthe inflicting of burns when high pain intensitiesare measured. A second limitation is that meas-urements cannot be made in the face of distrac-tion and lack of cooperation. The patient must bewilling and able to make comparisons of her pain-ful sensations with those induced by thermalstimuli.

1. Pain intensity

In every patient studied the pain intensity in-creased as the labor progressed (Figure 3). Al-though few observations are available during thefirst quarter of the first stage of labor it can besafely assumed that these pains were of a mild in-tensity, averaging probably not more than one ortwo dols. Pain in the second quarter of the firststage was moderate, averaging between three andfive dols. During the third quarter the pain ofmost patients averaged between five and sevendols, which is a severe pain. It was at this pointthat most patients asked for relief. In the lastquarter, the pain was between seven and ten dols.The second stage of labor was characterized by thethe most severe pains, 10Y2 dols, accompanyingthe "bearing down." After delivery the pains ob-served in the fourth stage of labor were again ofmild intensity.

An approximate correlation could be made be-tween the intensity of the pain and the extent ofthe cervical dilatation. Thus, when the patientwas experiencing pain of ten dols it was found thatthe cervix was fully dilated, and if the pain was ofone or two dols' intensity the cervix was observed

2

CM

25

0

DOLS6

-~ ~ A

. f . B~~~~~~

3

_m

/1

\x ~~C

/-x\

1xx/x/

/ \

I I

TIME I MIN I MIN

FIG. 4. PNEUMOGRAPHRECORDINGANDESTIMATED INTENSITY OF UTERINEA. Typical uterine contractions as recorded by a pneumograph strapped toB. Duration of corresponding pain recorded by signal magnet.C. Estimates of corresponding pain intensity.1. Early first stage. 2. Middle of first stage. 3. Early second stage.

4

2

0

l

.-2 MIN -

CONTRACTIONSpatient's abdomen.

158

MEASUREMENTSOF PAIN INTENSITY IN CHILDBIRTH

SENSORY CORTEXTHALAMUS

CERVICAL

THORACIC

T 6

C. ~tP. HuSACRAL~~~~~.MP

FIG. 5. DIAGRAMOF THE PAIN PATHWAYSOF IMPORTANCEIN CHILDBIRTH

First stage of labor: Visceral afferents arising in plexi enervating uterus, cervix and vagina; path-way: Frankenhouser's ganglion (F.G.) into hyrpogastric plexi (I.H.P., M.H.P., and S.H.P.) thencethrough the celiac plexus (C.P.), the superior and inferior mesenteric and aortic plexi (S.M.P.,I.M.P., and A.P.) and the sacral plexus (S.P.) into spinal outflow from T6 to S4.

Second stage of labor: Visceral afferents plus somatic afferents via the pudendal nerve.

to be almost closed. However, a much more quan-titative relationship appeared to exist between thepain intensity and the interval between contrac-tions.

The relationship observed between the contrac-tion interval and the intensity of pain provides asimple method for computing the pain from meas-urements of this interval. Thus,

Dols of pain = 10.5 - 1.5 x contraction inter-val in minutes.

This formula may not apply generally, of course,to all cases, although it has been observed to bevalid in our series of patients without analgesia.

2. Character and origin of painDuring the early part of the first stage of labor

the pain was of short duration and of a dull aching

quality, and was associated with a uterine contrac-tion of relatively low intensity (see Figure 4,Curve 1). The visceral structures involved inproducing this pain are deep and noxious impulsesprobably reach the central nervous system via thesympathetic and parasympathetic afferents (10)as diagrammed in Figure 5. As the cervix di-lated and the uterine contractions became strongerthe pain assumed a sharper quality. It had beennoticed in other studies of pain intensity thatbright, burning pain and dull, aching pain have atendency to become similar. That is, a burningpain had an aching quality and an aching pain be-came sharp (3). This was also true of laborpains.

The ceiling pain was not reached until the be-ginning of the second stage at which time the so-matic perineal structures were involved. These

159

JAMES D. HARDYAD CARL T. JAVERT

superficial structures are innervated chiefly bythe pudendal nerve (10), as shown in Figure 5,and the most intense pains are caused by thestretching and tearing of these tissues, with ac-companying stimulation of the perineal reflex andconsiderable increase in intrauterine pressure(10). Curve 3, Figure 4, shows a recording ofthe onset of the reflex which is apparently in-hibited periodically, possibly by the intense pain.Superimposed on the low uterine component ofthe pressure wave were spikes of great pressurefrom the perineal reflex. The duration of thepain at this time was almost as long as the contrac-tion.

Figure 6 shows a plot of the pain intensity indols against the uterine contraction in minutes.The connection between the contraction intervaland uterine effort has been established by thetocographic measurements of Murphy (9) andothers (11), and while it cannot be assumed thatthe tocographic measurements are necessarily rep-resentative of. intrauterine pressure, they appar-

DOLSII

Jo

9

8

7

6

5

4

3

2

01

I

./R/

R

/R

ently do represent the uterine hardness and therebyoffer a measure of the uterine spasm (12). Theuterine pressure in millimeters as estimated fromMurphy's work is plotted to the right in Figure 6,and it can be seen that the pain intensity in dols isalso proportional to the uterine pressure in milli-meters, measured on Murphy's tocograph. Thisevidence, as well as the clinical observation thatdilatation of the cervical canal causes severe pain,supports the conclusion that the pain in the firststage of labor is largely due to the distention of theinternal os and the cervical canal as a result of theincreased intrauterine pressure caused by uterinecontractions.

3. Reactions to painDuring the early hours of labor there were gen-

erally few complaints and the patients were alertand interested in the study. There was no sweat-ing. Two patients did not fall into this pattern.Mrs. 0 soon began to show hostility to the entireexperimental team in spite of expressing many

UTERINEPRESSURE

MM.

10

9

8

7

~66 5 4 3 2 1

CONTRACTIONINTERVAL - MINUTES

FIG. 6. PLOT OF ESTIMATES OF PAIN INTENSITY (DoLs) AGAINST LENGTHOF INTERVALBETWEENCONTRACTIONS(MINUTES)

Ordinate at the right of diagram represents Murphy's topographic measurements ofuterine pressure.

yA

,A II/NHZH

A H H/

o 0 0A

RB NHA H

Ho Rf AN H

N RH F

/ / ~~B rucurR/ / R

Fft R

"A

160

MEASUREMENTSOF PAIN INTENSITY IN CHILDBIRTH

times prior to admission her desire to participatein the study. Cajoling and persistence made itpossible to carry on the study and in spite of thisdifficulty the pain measurements obtained were inthe range of those for the more cooperative pa-tients. Mrs. U cried and complained with vigorfrom the outset although her pain measurementsindicated only two to four dols. This patient suc-ceeded in becoming so excited that no further ob-servations could be made in her case.

Late in the first stage 11 of the 13 patients weresweating and crying with each contraction, andwere cooperating only with difficulty. Pain wassevere at this time, seven to ten dols. One pa-tient, Mrs. A (see Figure 2), gave little evidenceof her pain. As a multipara her labor was shortand this may account for her restraint in the faceof pains measured at eight to ten dols. Fatiguewas as important as pain intensity in bringingabout a breakdown in the restraint most of the pa-tients maintained over their actions. Thus, in hershort labor, Mrs. A gave less visible evidence ofpain at eight dols than did the other patients withlonger labors at four dols.

The evidence of this study is that the reactionsof the patient depended partly on the pain and to agreater extent on fatigue, anxiety, and, in somecases, hostility, whereas the pain intensity asmeasured was related to the uterine activity anddilatation of the cervical and perineal tissues. Itseems important, therefore, to make a clear dis-tinction between the intensity of the pain perceivedby the patient and visible evidences of reaction topain, such as complaining, groaning and cryingout. These findings bear out the clinical impres-sion regarding the psychological factors affectingthe behavior of patients in labor. However, bymeasuring the intensity of the pain it is now pos-sible to evaluate more fully the effects of pre-conditioning (as in "natural childbirth") and re-assurance, as well as the action of analgesia uponpain sensation in the obstetrical patient. Furtherstudies are planned to include an appraisal of sug-gestion and hypnosis.

SUMMARYANDCONCLUSIONS

1. Four hundred test readings, resulting in 55measurements of pain intensity, were made on 13patients during the first, second and fourth stagesof labor without analgesia. The measurements

were made by comparison of the labor pains with apain of standard intensity which was produced by athree-second exposure to thermal radiation on thedorsal surface of the right hand. The pain evokedon the hand had been previously standardized into101/2 units of painfulness (10Y2 dols) between thethreshold pain and the most intense pain whichcan be experienced (ceiling pain).

2. The pain intensity was observed to changewith the progress of labor as follows:

a. First stage-First quarter-threshold to two dolsSecond quarter-three to five dolsThird quarter-five to seven dolsFourth quarter-seven to ten dols.

b. Second stage-Ceiling pain, ten to ten andone-half dols.

c. Fourth stage-Intensity from three to fivedols to near threshold value in about twohours after delivery.

3. The intensity of the pain in the first stage oflabor was roughly proportional to the extent ofcervical dilatation and inversely proportional tothe duration of the interval between uterine con-tractions. Pain intensity can be estimated approxi-mately by the formula:

Dols = 10.5 - 1.5 x contraction interval in min-utes.

4. The duration of pain was 15 per cent of thecontraction time for the weak contractions charac-teristic of early labor, and 95 per cent of the con-traction time at the beginning of the second stageof labor.

5. The pain threshold was measured in the in-terval between contractions in the first, second andfourth stages of labor. It was within the normalrange for all the patients who had received noanalgesia or anesthesia.

6. The intensity of the pain experienced by thepatient could not always be evaluated on the basisof her reaction, nor correlated with her apparentdistress. Distinguishing between the intensity ofthe pain perceived by the patient and the patient'sreaction has been made possible by measurementsof pain intensity and pain thresholds.

BIBLIOGRAPHY1. Read, G. D., Childbirth Without Fear. Harper and

Bros., New York, 1944.

161

JAMES D. HARDYAD CARL T. JAVERT

2. Hardy, J. D., Wolff, H. G., and Goodell, H., Studieson pain: discrimination of differences in intensityof a pain stimulus as a basis of a scale of painintensity. J. Clin. Invest., 1947, 26, 1152.

3. Hardy, J. D., Wolff, H. G., and Goodell, H., Studieson pain: an investigation of some quantitative as-

pects of the dol scale of pain intensity. J. Clin.Invest., 1948, 27, 380.

4. Hardy, J. D., Wolff, H. G., and Goodell, H., Studieson pain: a new method for measuring pain thresh-old: observations on spatial summation of pain.J. Clin. Invest., 1940, 19, 649.

5. Javert, C. T., The immediate postpartum period as

the fourth stage of labor. Am. J. Obst. & Gynec.,1947, 54, 1028.

6. Clark, D., Hough, H., and Wolff, H. G., Experi-mental studies on headache: observations on head-ache produced by histamine. Arch. Neurol. &Psychiat., 1936, 35, 1054.

7. Wolf, S., and Hardy, J. D., Studies on pain: observa-tions on pain due to local cooling and on factorsinvolved in the "cold pressor" effect. J. Clin.Invest., 1941, 20, 521.

8. Moir, C., Discussion on intrinsic dysmenorrhea.Proc. Royal Soc. Med., 1936, 29, 950.

9. Murphy, D. P., Uterine Contractility in Pregnancy.J. B. Lippincott Co., Philadelphia, 1947.

10. Wilson, R. B., and Mussey, R. D., Nervous path-ways involved in pelvic pain. J. A. M. A., 1947,134, 857.

11. Reynolds, S. R. M., Hellman, L. M., and Bruns, P.,Uterine contractions effective in dilating the humancervix, recorded by the multichannel strain gage

tokodynamometer. Federation Proc., 1948, 7, 101.12. Wolf, W., Zur Methode der externen Wehenmessung.

Zugleich ein Beitrag zur Kenntnis des Verhalt-nisses der Uteruswandspannung zum Innendruck.Zentralbl. f. Gynak., 1937, 61, 2418.

162