OBSTETRICS Uterine contractions in spontaneous labor A quantitative study ANDREW J. KRAPOHL, M.D.* GRANT G. MYERS, PH.D. ROBERTO CALDEYRO-BARCIA, M.D. Marinette, Wisconsin A statistical study was made of the amniotic fluid pressure wave form for each centimeter of cervical dilatation in 12 multiparous patients during spontaneous labor which produced mature term-sized infants. It was found that tonus, intensity, and frequency of contractions as well as uterine activity increase as labor progresses. No significant differences were found between average values obtained before and after rupture. The duration of contractions was found to remain fairly constant throughout labor. The regularity of the rhythm of contractions increased as labor progressed. I N T H I s P A P E R we shall Statistically stetrica, Hospital de Chnicas, Montevideo. describe amniotic fluid pressure wave forms Uruguay. The main criterion of selection for each centimeter of cervical dilatation was the normality of the cervical dilatation recorded in 12 multiparous patients during pattern of the multiparous patient, taking spontaneous labors which produced mature into account the initial cervical dilatation.F term-sized infants (i.e., over 2,500 grams). The patient was excluded from the study We shall interrelate the quantitative mea- if her cervical dilatation curve was border- surement techniques of Caldeyro-Barcia and line or exceeded the statistical limits set by Alvarez1 with the graphic description of Friedman and SachtlebenG for any of the labor using the cervical dilatation curves of segments. Friedman and SachtlebenG Patients were also selected or rejected on the basis of drugs received during the labor, Materials and methods maternal age, fetal weight, parity, clinical The records of amniotic pressure analyzed history, and associated disease processes. The were selected from those obtained during ranges of factors considered were as follows: the last 15 years by Caldeyro-Barcia and as- maternal age, 18 to 38 years; gravidity, 2 to sociates at the Servicio de Fisiologia Ob- 10; length of labor, 3 hours 10 minutes to 9 hours 50 minutes; fetal weight, of 2,575 to 3.800 grams; clinical duration of pregnancy, *Currently with Marinette Medical Clinic, Marinette, Wisconsin, 34 weeks 5 days to 42 weeks 2 days. The 378 Volume 106 Uterine contractility 379 Number 3 infant of the 34 weeks 5 days pregnancy INTERVAL mmHg weighed 2,950 grams and was mature at 4 4 60 birth. One patient (No. 1832) had myas- thenia gravis and was under treatment. She 50 received 0.5 mg. of prostigmine intrave- 10 nously at between 6 and 7 cm. of cervical 30 dilatation. Detailed analysis of this record 20 demonstrated this drug to have no effect 10 upon her amniotic fluid pressure patterns. The total number of patients studied was 12. I r I All amniotic fluid pressure tracings and 0 5 10 minutes measurements were performed according to Fig. 1. Measurement of tonus, intensity, and period the techniques of Caldeyro-Barcia and Al- of contraction in records of amniotic fluid pressure. varez.l An open-end fluid-filled catheter was placed into the amniotic cavity and con- nected to a Sanborn differential transducer. cur-y per 10 minutes) and is obtained by This was electronically zeroed to the skin multiplying the amplitude of the contrac- level at the top of the uterus, the woman tion by its frequency.2 being in supine position. The catheters were The “duration” of the contractions was checked to insure that there were no air measured by a graphic method devised by bubbles in the system. one of us (A. J. K.). A wedge of paper with The parameters studied and total number an angle of 11 degrees 30 minutes was in- of measurements made in amniotic fluid serted beneath the descending limb of a con- pressure tracings were: 899 of tonus, 1,090 traction cycle (Fig. 2). One side of the of intensity or amplitude of the contractions, wedge was horizontal; the slope of the other 1,036 of the duration of contractions, 1,085 side was compared with the changing slopes of the period between contractions (and of the descending limb. The point at which frequency of contractions), and 1,076 of both slopes were equal was arbitrarily con- uterine activity measured in Montevideo sidered as the “end point” of the contrac- Units. tion. A similar method was employed for Tonus has been defined1 as the lowest determining the ‘<starting point” of the con- pressure reading (in millimeters of mercury) traction in the ascending limb. The “dura- between contractions (Fig. 1) . Amplitude tion” of the contraction was the time inter- or intensity of a contraction is the difference val (in seconds) between “starting” and in millimeters of mercury between the peak “end” points. amniotic fluid pressure and tonus preceding This method gave reproducible results, at the contracti0n.l The period for a given least when used by the same observer contraction is measured in seconds from the (A. J. K.) . The duration was measured 2 peak of that contraction to the peak of the different times on the same group of 38 con- preceding contraction (Fig. 1). The coeffi- tractions, and there was no statistically sig- cient of variation for the period S. D./ nificant difference between the two sets of mean) is a measure of the irregularity of the results; 66 per cent of the measurements contractile rhythm and is known as index of were the same and 32.08 per cent were uterine arrhythmia.5 The smaller this index, within 6 seconds of each other. the more regular the rhythm. Frequency of In the calculation of uterine activity, contractions was obtained by dividing each there .is a statistical problem which arises value for a period into 600, and the result is when any random variable is the product of expressed as the number of contractions per two other random variables and the ex- 10 minutes. The uterine activity is expressed pected value or average of the product of in Montevideo Units (or millimeters of mer- the ran lam variables is to be determined. 380 Krapohl, Myers, and Caldeyro-Barcia February 1, 1970 Amer. J. Obstet. Gynec. Table I. Hypothetical experimental data averages (&F) only if the covariance of A and F is zero. The covariance will be zero Amplitude Frequency per only if the variables (A and F) are inde- i (mm. Hd 10 min. pendent. If the variables are dependent then 1 20 3 2 30 3 the covariance will not be zero. In this case 3 40 4 Table II demonstrates the correct choice for 4 50 4 calculating the mean of the product of two Total 140 14 dependent variables8 Average 35 3.5 The method in Table II was used to cal- Montevideo Units 35 x 3.5 = 122.5 culate uterine activity in this paper since frequency and the amplitude of contractions are dependent. Computer programs were used to calcu- late the mean, standard deviation, and stan- Table II. Hypothetical experimental data dard error of the mean for tonus, amplitude, Monteuideo frequency, duration of contractions, and i Frequency x amplitude Unit uterine activity. These statistics were calcu- lated for each patient and for the pooled : 33 x 2030 6090 data, including several patients, for each 3 4 x 40 160 centimeter of cervical dilatation both before 4 - 4 x 50 200 Total 14 140 510 and after rupture of the membranes. The 95 per cent fiducial limits were established Average 3.5 35 135 for each set of pooled data. The computer lvas also used to calculate the regression coefficients, correlation co- efficient, and standard error of the estimate For example, suppose that the data are as for various sets of variables. These sets were in Table I. The average value of the ampli- amplitude, tonus, duration, and uterine tude is activity vs. frequency, as well as uterine activity vs. amplitude, and amplitude vs. duration. The relationships of variables against frequency and duration lvere sought where Ai is the amplitude of the ith con- because these latter two may be measured traction. Similarly, the average value of the clinically. The regression lines avere calcu- frequency is lated by the technique of least squares ac- cording to the equation Y = A,-+-A,X. l/4 $ Fi = 3.5 = E i=l These statistics were also calculated for - - each patient and each set of pooled data The product AxF= 122.5 Montevideo Units. for each centimeter of cervical dilatation Another possible approach is as in Table before and after rupture of the membranes. II. The average uterine activity is Scatter diagrams were made for each set of pooled data. The statistical significance of & = l/4 i Mi = 135 Montevideo Units. each correlation coefficient was determined i=l assuming that the p of the population was From this it follows that the average value zero. of the products is not necessarily the same as Histograms were constructed from the the product of the average values. pooled data for each centimeter of cervical By use of calculus the average of the dilatation before and after rupture of the product of two random variables (‘AT) is membranes for tonus, amplitude, frequency, shown to be the same as the product of the duration, and uterine activity. From these Volume 106 Uterine contractility 381 Number 3 DURATfON OF mmHg CONTRACTION 60 - I I 1 I 1 I 0 20 LO 60 80 100 120 SECONDS Fig. 2. Graphic method for measuring duration of contractions in tracings of amniotic fluid pressure. the median and 95 percentile limits were for data obtained before and after rupture calculated. of membranes in order to ascertain the effect There was no classification of data ac- of this procedure on uterine contractility.