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OBSTETRICS

Defining uterine tachysystole: how much is too much?Robert D. Stewart, MD; April T. Bleich, MD; Julie Y. Lo, MD; James M. Alexander, MD;Donald D. McIntire, PhD; Kenneth J. Leveno, MD

OBJECTIVE: We sought to determine if uterine tachysystole, �6 con-tractions per 10 minutes, within the first 4 hours of labor induction, isassociated with adverse infant outcomes.

STUDY DESIGN: This was a prospective cohort study of 584 women�37 weeks’ gestation undergoing induction of labor with 100 �g oforal misoprostol. Fetal heart rate tracings were analyzed for contrac-tions per 10 minutes during the initial 4 hours after misoprostol admin-istration. Patients were analyzed based on the maximum number ofcontractions per 10 minutes. Infant condition at birth was assessed us-
ing the fetal vulnerability composite.
Cite this article as: Stewart RD, Bleich AT, Lo JY, et al. Defining uterine tachysystole

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Center. Written inforhttp://dx.doi.org/10.1016/j.ajog.2012.07.032

290.e1 American Journal of Obstetrics & Gynecology OCTOBER 2012

RESULTS: Adverse infant outcomes showed no association with in-creasing number of contractions per 10 minutes. Six or more contrac-tions in 10 minutes were significantly associated with fetal heart ratedecelerations (P � .001). Analysis was performed using the maximumnumber of contractions per 30 minutes with similar results.

CONCLUSION: Uterine tachysystole, as currently defined, when occurringremote from delivery is not associated with adverse infant outcomes.

Key words: abnormal labor, misoprostol, uterine hyperstimulation,
uterine tachysystole
: how much is too much? Am J Obstet Gynecol 2012;207:290.e1-6.

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In 2008, the Eunice Kennedy ShriverNational Institute of Child Health

nd Human Development (NICHD)roposed definitions for the interpreta-ion of fetal heart rate (FHR) tracings.

ithin these definitions, uterine activityas quantified as the number of contrac-

ions present in a 10-minute window, av-raged over 30 minutes. Uterine tachy-ystole was defined as �6 contractions in0 minutes. Subsequently, in July 2009,he American Congress of Obstetriciansnd Gynecologists (ACOG) affirmedhis definition of uterine tachysystole.1,2

This definition of excessive uterine ac-tivity is the standard for current clinicalpractice, despite the fact that little evi-

From the Department of Obstetrics andGynecology, University of Texas SouthwesternMedical Center, Dallas, TX.

Received April 30, 2012; revised June 21,2012; accepted July 25, 2012.

The authors report no conflict of interest.

Presented at the 32nd annual meeting of theSociety for Maternal-Fetal Medicine, Dallas, TX,Feb. 6-10, 2012.

Reprints: Robert D. Stewart, MD, Departmentof Obstetrics and Gynecology, University ofTexas Southwestern Medical Center, 5323Harry Hines Blvd., Dallas, TX [email protected].

0002-9378/$36.00© 2012 Mosby, Inc. All rights reserved.

dence exists as to the clinical utility ofthis definition. Current clinical concernsregarding uterine tachysystole includethe possibility of decreased fetal oxygen-ation due to inadequate relaxation timebetween contractions.3,4 It has been sug-ested that this decreased oxygenationuring excessive contractions would re-ult in a progressive decline in fetal oxy-enation to a critical level. Based on fetalxygenation studies some investigatorsave advocated changing the definitionf uterine tachysystole to a more restric-ive �5 contractions in a 10-minutepoch.5

The purpose of our study was to deter-mine if uterine tachysystole, as currentlydefined by ACOG, �6 contractions in 10minutes, when occurring within the first4 hours of labor induction, is associatedwith adverse infant outcomes. We alsosought to determine at what threshold ofuterine activity these adverse infant out-comes occurred.

MATERIALS AND METHODSThis is a prospective cohort study ofwomen undergoing misoprostol induc-tion of labor at Parkland Hospital fromMarch 17, 2009, through December 31,2010. This study was approved by the in-stitutional review board of the Univer-sity of Texas, Southwestern Medical

med consent was f

waived because this study was limited toobservations during standard clinicalcare.

During the study period, our standardpractice of misoprostol induction was togive 100 �g of oral misoprostol for amaximum of 2 doses, 4 hours apart.6 Allonsecutive women who qualified forisoprostol induction according to our

tandard criteria were eligible for thistudy. Those women with a fetal demiser those who had incomplete data werexcluded from study. A woman qualifiedor the initial dose of misoprostol if theregnancy was a singleton, cephalic,erm gestation (�37 weeks) without ev-dence of active labor, defined a cervicalilatation of �4 cm, with 30 minutes ofeassuring FHR tracing without deceler-tions and �6 contractions per 10 min-tes prior to receiving misoprostol. Afterhours the woman was evaluated to as-

ess if she qualified to receive the secondose of misoprostol. She did not receivehe second dose if during the preceding 4ours there were �6 contractions in any0-minute epoch, any FHR decelera-ions, or if she had progressed to activeabor. Those women who were unable toeceive a second dose of misoprostol buteeded further stimulation of labor re-eived oxytocin infusion. Misoprostolas previously been shown to be an ef-

ective induction agent, with a reported

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rate of uterine tachysystole of 25% (�6contractions in 10 minutes without de-celerations spanning 20 minutes).6 Ourtandard practice for treatment of tachy-ystole with associated prolonged decel-rations was cesarean delivery for nonre-ssuring FHR. In those women withachysystole and late decelerations, theoman was placed in the lateral decubi-

us position with administration of oxy-en. If the late decelerations persisted,esarean delivery was performed foronreassuring FHR. If the tachysystoleas associated with variable decelera-

ions, the woman was placed in the lat-ral decubitus position with oxygen ad-inistration if needed. In both of these

ircumstances, the second dose of miso-rostol would not be administered. If the

TABLE 1Demographic characteristics in584 women undergoing laborinduction

CharacteristicNo. of women,n � 584

Parity..................................................................................................

0 288 (49)..................................................................................................

1 112 (19)..................................................................................................

�2 184 (32)...........................................................................................................

Epidural 370 (63)...........................................................................................................

Maternal age, y 25.9 � 6.3...........................................................................................................

Race/ethnicity..................................................................................................

Hispanic 465 (80)..................................................................................................

African American 70 (12)..................................................................................................

White 32 (5)..................................................................................................

Other 17 (3)...........................................................................................................

Pregnancycomplications

..................................................................................................

Diabetes 63 (11)..................................................................................................

Hypertension 141 (24)..................................................................................................

Severepreeclampsia

89 (15)

..................................................................................................

Abruption 0..................................................................................................

Premature rupturedmembranes

164 (28)

..................................................................................................

Postterm induction 151 (26)...........................................................................................................

All data shown as n (%) or mean � SD.

Stewart. Defining uterine tachysystole. Am J ObstetGynecol 2012.

oman had uterine tachysystole without m

ssociated decelerations, we would con-inue to monitor for evidence of fetal dis-ress, however the second dose of miso-rostol would not be administered.At the conclusion of labor, the paper

opy of the FHR tracing was collected.he contraction patterns were recordedith tocodynamometer unless an intra-terine pressure catheter was placed forbstetrical indications. These tracingsere subsequently analyzed by visual as-

essment for the number of contractionser each 10-minute epoch during the

nitial 4 hours of misoprostol induction.n addition, for each 10-minute epochhe presence of uterine hypertonus, de-ned as a contraction lasting �120 sec-nds, was recorded. Similarly, variable,

ate, or prolonged decelerations and fetalachycardia or bradycardia were re-orded, using standard definitions asutlined by ACOG.2 The time and modef delivery was also recorded. All FHRracing analysis was conducted by 1 in-estigator (R.D.S.) who was blinded tohe infant outcomes.

Obstetric and infant clinical outcomeata were obtained using the preexistingarkland Hospital obstetric database.urses attending each delivery complete

n obstetric data sheet, and researchurses assess the data for completenessnd consistency before electronic stor-ge. Data on infant outcomes are also ab-tracted from discharge records and en-ered into a separate database. Theutcome of interest for this study was in-ant condition at birth assessed using aomposite outcome termed the fetal vul-erability composite, which included:-minute Apgar scores �3, umbilical ar-ery blood pH �7.1, intubation in theelivery room, neonatal seizures, admis-ion to intensive care, or perinatal death.

These results were electronically linkedo the previously collected FHR tracingnalyses. Patients were then divided intogroups based on the greatest number ofontractions within any 10-minute ep-ch during the initial 4 hours of labor

nduction: �4, 5, 6, �7 contractions per0 minutes. Analysis was also performedsing the maximum number of contrac-

ions per 10 minutes averaged over 30
inutes. u
OCTOBER 2012 Americ

Prior to commencing the study noweported, we assessed the rate of the pri-ary outcome composite using a pilot

tudy of 187 cases meeting the criteria forhis study now reported. The fetal vul-erability composite occurred in 5% of

he pilot cohort. These 187 cases wereot included in the analysis of outcomesow reported because inclusion of thisreviously analyzed cohort could poten-ially bias our final results.

Using 80% power for a 2-sided test of.05 significance, we estimated that 584

omen receiving misoprostol wouldeed to be examined to detect a signifi-ant difference in the fetal vulnerabilityomposite. Statistical analysis includedearson �2, Cochran-Mantel-Haenszel

�2 for trend, and analysis of variance.values � .05 were considered signifi-

cant. Analysis was performed using SAS9.2 (SAS Institute Inc, Cary, NC).

RESULTSA total of 584 women undergoing induc-tion of labor with misoprostol were ana-lyzed. Maternal demographic character-istics and pregnancy complications areshown in Table 1.

Of the women undergoing induction,253 (43%) had at least one 10-minuteepoch with �6 contractions during theinitial 4 hours of induction; howeverwhen averaged over 30 minutes, 129(22%) of the 584 women met this criteriafor uterine tachysystole. Of the 584women within the cohort, 253 (43%) re-quired oxytocin infusion. Intrauterinepressure catheters were placed in 519women (89%) during their labor. Infantoutcomes, both the composite and indi-vidual components of the composite, ac-cording to number of uterine contrac-tions per 10 minutes during the first 4hours of induction are shown in Ta-

le 2. The fetal vulnerability compositehowed no association with increasingumber of uterine contractions, and no

ndividual component was significantlyssociated with increasing number ofterine contractions. Route of deliveryimilarly showed no association with theumber of contractions per 10 minutes.hen the infant outcomes were analyzed

sing the number of contractions averaged

an Journal of Obstetrics & Gynecology 290.e2

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Research Obstetrics www.AJOG.org

over 30 minutes, there was still no signifi-cant association between an increasingnumber of contractions and infant out-come, as reflected by the fetal vulnerabilitycomposite or any individual component of

TABLE 2Infant outcomes related to contrac

Outcome

Maxi

<4,

Fetal vulnerability composite: 5 (..........................................................................................................

5-min Apgar �3 0..........................................................................................................

Umbilical artery pH �7.1 1 (..........................................................................................................

Seizures 0..........................................................................................................

Intubation at delivery 1 (..........................................................................................................

Stillborn 0..........................................................................................................

Neonatal death 0..........................................................................................................

NICU admission 5 (...................................................................................................................aMisoprostol to delivery time, h 10.3...................................................................................................................

Route of delivery..........................................................................................................

Cesarean 31 (..........................................................................................................

Vaginal 121 (...................................................................................................................

All data shown as n (%) unless otherwise indicated. P value isNICU, neonatal intensive care unit.a Kruskal-Willis test. Data shown as median [1st quartile, 3rd

Stewart. Defining uterine tachysystole. Am J Obstet Gynec

TABLE 3Infant outcomes related to contrac

Outcome

Maxi

<13,

Fetal vulnerability composite 10 (..........................................................................................................

5-min Apgar �3 1 (..........................................................................................................

Umbilical artery pH �7.1 4 (..........................................................................................................

Seizures 0..........................................................................................................

Intubation at delivery 1 (..........................................................................................................

Stillborn 0..........................................................................................................

Neonatal death 0..........................................................................................................

NICU admission 8 (...................................................................................................................aMisoprostol to delivery time, h 10.8...................................................................................................................

Route of delivery..........................................................................................................

Cesarean 65 (..........................................................................................................

Vaginal 227 (...................................................................................................................

All data shown as n (%) unless otherwise indicated. P value isNICU, neonatal intensive care unit.a Kruskal-Willis test. Data shown as median [1st quartile, 3rd

Stewart. Defining uterine tachysystole. Am J Obstet Gynecol 20

290.e3 American Journal of Obstetrics & Gynecolo

the composite (Table 3). However, ad-mission to neonatal intensive care wasassociated with decreasing uterine con-tractions when analyzed per 10 minutesor per 30 minutes (P � .03 and P � .04).

ns per 10 minutes

m contractions per 10 min

152 5, n � 179 6, n �

6 (3) 2 (1.........................................................................................................................

1 (1) 0.........................................................................................................................

4 (2) 2 (1.........................................................................................................................

0 0.........................................................................................................................

0 0.........................................................................................................................

0 0.........................................................................................................................

0 0.........................................................................................................................

3 (2) 0.........................................................................................................................

.7, 15.5] 10.6 [5.9, 15.0] 9.5

.........................................................................................................................

.........................................................................................................................

35 (19) 26 (1.........................................................................................................................

144 (81) 108 (8.........................................................................................................................

antel-Haenszel �2 for trend.

tile].

12.

ns per 30 minutes

m contractions per 30 min

292 14-16, n � 163 17-19,

3 (2) 2 (1).........................................................................................................................

0 0.........................................................................................................................

3 (2) 2 (1).........................................................................................................................

0 0.........................................................................................................................

0 0.........................................................................................................................

0 0.........................................................................................................................

0 0.........................................................................................................................

0 0.........................................................................................................................

.7, 15.7] 9.7 [5.0, 14.8] 8.8 [4.........................................................................................................................

.........................................................................................................................

26 (15) 12 (14).........................................................................................................................

137 (85) 71 (86).........................................................................................................................

antel-Haenszel �2 for trend.

tile].

12.

gy OCTOBER 2012

f those infants without uterine tachy-ystole admitted to intensive care, 1 wasue to a previously undiagnosed palatebnormality, 1 due to a hypoplastic lefteart, and 1 for observation for an at-

4 >7, n � 119 P value

6 (5) .86..................................................................................................................

0 .86..................................................................................................................

6 (5) .06..................................................................................................................

0 —..................................................................................................................

0 .11..................................................................................................................

0 —..................................................................................................................

0 —..................................................................................................................

1 (1) .03..................................................................................................................

, 15.0] 8.9 [4.3, 13.2] .18..................................................................................................................

..................................................................................................................

17 (14) .34..................................................................................................................

102 (86) .28..................................................................................................................

P value83 >20, n � 46

4 (3) .82..................................................................................................................

0 .33..................................................................................................................

4 (3) .06..................................................................................................................

0 —..................................................................................................................

0 .33..................................................................................................................

0 —..................................................................................................................

0 —..................................................................................................................

1 (2) .04..................................................................................................................

15.7] 8.8 [3.9, 11.8] .03..................................................................................................................

..................................................................................................................

6 (13) .02..................................................................................................................

40 (87) .04..................................................................................................................

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tempted maternal naproxen overdoseprior to presentation to labor and deliv-ery. The remaining cases were admittedto intensive care for unanticipated rea-sons, including sepsis evaluation and re-spiratory distress syndrome. Route ofdelivery was found to be associated withthe number of contractions per 30 min-utes, with vaginal delivery being morelikely with increasing number of con-tractions per 30 minutes (P � .04). Over-all, 109 (19%) of the women underwentcesarean delivery. The most commonreason for cesarean delivery was labordystocia (n � 54, 50%), followed by FHRabnormalities (n � 52, 47%). Time frommisoprostol administration to deliverywas significantly less with increasingnumbers of contractions per 30 minutes(P � .03).

FHR decelerations were associatedwith increasing number of contractions(Table 4). Analysis of decelerations ofany configuration in relation to �4through �7 contractions per 10 minutesshowed a significant trend. A significantsimilar trend was also observed whencontractions �13 through �20 per 30minutes were analyzed. We then reana-lyzed these data looking for a “cutpoint,”a threshold that provides the best dis-crimination for decelerations. We usedthe minimum P value method for thisanalysis.7 We then used a Bonferroniorrection to adjust the P values.8 Basedn this statistical technique, we foundhat �6 contractions per 10 minutes, asell as 17-19 per 30 minutes, were the

TABLE 4Fetal heart rate decelerations relat

Decelerations

Contractions per 10 min

<4,n � 152

5,n � 179

Any 61 (40) 88 (49)...................................................................................................................

Variable 48 (32) 78 (44)...................................................................................................................

Late 24 (16) 38 (21)...................................................................................................................

Prolonged 10 (7) 16 (9)...................................................................................................................

All data shown as n (%). P value is for Mantel-Haenszel �2 foa Cutpoint for threshold of most significant P value based on m


utpoints at which the most significant

value for all possible cutpoints wasbserved.A total of 89 women (15%) experi-

nced �1 episodes of uterine hyperto-us. Uterine hypertonus was not associ-ted with any measure of adverse infantutcome (Table 5). Those women whoad 2 episodes of uterine hypertonusithin the first 4 hour of induction were

ignificantly more likely to undergo ce-arean delivery (P � .013), and were sig-ificantly more likely to have FHR trac-

ngs with late or prolonged decelerationsP � .001).

COMMENTIncreasing uterine activity during thefirst 4 hours of labor induction, quanti-fied as the maximum number of contrac-tions in 10 minutes, or per 30 minutes,had variable effects depending upon theoutcome of interest. For example, ad-verse infant outcomes were not related tothe number of uterine contractions pertime period. On the other hand, in-creased number of contractions per 30minutes was associated with a signifi-cantly increased rate of vaginal birth, andconsequently the opposite effect on ce-sarean delivery. Time from misoprostoladministration to delivery was also sig-nificantly shorter with increased numberof contractions per 30 minutes. Simi-larly, increased uterine activity, definedas �6 contractions per 10 minutes and17-19 per 30 minutes, was associatedwith increased FHR decelerations.Lastly, uterine hypertonus was associ-

to uterine contractions

Contractio

134>7,n � 119 P value

<13,n � 292

(57)a 69 (58) � .001 125 (43).........................................................................................................................

(48)a 62 (52) � .001 102 (35).........................................................................................................................

(25)a 31 (26) .02 54 (18).........................................................................................................................

(33)a 19 (16) .017 24 (8).........................................................................................................................

d.

um P value technique and adjusted with Bonferroni technique.

12.

ated with increased late and prolonged

OCTOBER 2012 Americ

FHR decelerations, as well as cesareandelivery.

Previous reports regarding excessiveuterine activity have focused on the ef-fects of uterine contractions on fetal ox-ygenation.3-5,9 It was found that the av-erage drop in fetal oxygenation, asmeasured by pulse oximetry during uter-ine tachysystole, defined as at least 1 con-traction every 2 minutes, was 18%, froma fetal oxygen saturation of 54-36%, andthat recovery is incomplete if the con-traction interval is �2 minutes.4 Usinghanges in the concentration of oxyhe-oglobin and deoxyhemoglobin mea-

ured by infrared spectroscopy as sur-ogate markers for cerebral oxygenaturation during labor, Peebles et al3

determined, using regression line analy-sis, that a contraction interval of �2-3minutes was the threshold below whichoxyhemoglobin levels consistently fell.The authors concluded that a short con-traction interval, �2-3 minutes, was as-sociated with a decrease in fetal cerebraloxygen saturation. Expanding uponthese studies, Simpson et al5 evaluatedthe effects of uterine contractions on fe-tal oxygen saturation as measured bypulse oximetry in 56 healthy nulliparouswomen undergoing oxytocin inductionof labor. In patients with 5 contractionsper 10 minutes, a negative 20% change infetal oxygen saturation from 52-42% wasfound. In patients with �6 contractionsper 10 minutes, a negative 29% change infetal oxygenation from 52-37% wasfound. These findings led the authors toconclude that a definition of uterine

er 30 min

4-16,� 163

17-19,a

n � 83>20,n � 46 P value

4 (58) 48 (58)a 28 (61) � .001..................................................................................................................

4 (52) 41 (49)a 25 (54) � .001..................................................................................................................

8 (23) 24 (29)a 11 (24) .05..................................................................................................................

9 (12) 9 (11)a 10 (22) .03..................................................................................................................

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34 3......... .........

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minutes may be more appropriate.5 Ithas previously been found that a fetal ox-ygenation threshold of 30% would be areasonable threshold for detection of fe-tal hypoxemia, and that this thresholdwas also useful in detecting fetal com-promise.10,11 This threshold, however,

as not met in these prior studies ofterine tachysystole.3-5 Bakker et al12 ex-mined uterine contractions and theirffect on umbilical artery pH. In theirtudy of 1433 women, deliveries result-ng in an umbilical artery pH of �7.11ere compared to those resulting in anmbilical artery pH of �7.12. It was

ound that those deliveries that resultedn an umbilical artery pH �7.11 had anverage contraction frequency of 5 con-ractions per 10 minutes.12 However, no

ention was made in this study of FHRatterns, and infant outcomes were notnalyzed. In contrast, in a study of 605omen who underwent labor inductionr augmentation, 240 (40%) had exces-ive uterine contractions, defined as �6ontractions in 10 minutes. Despite ex-essive uterine contractions, the authorsound that adverse infant outcomes wereare, with only 4 neonates having an um-ilical cord pH of �7.0.13 We are of theiew that the available literature nowited is problematic as to defining a uter-

TABLE 5Selected outcomes in relation to ut

Outcome

Uterine hype

0, n � 495

Cesarean delivery 85 (17)...................................................................................................................

Decelerations..........................................................................................................

Any 237 (48)..........................................................................................................

Variable 208 (42)..........................................................................................................

Late 93 (19)..........................................................................................................

Prolonged 38 (7)...................................................................................................................

5-min Apgar �3 1 (0.2)...................................................................................................................

Umbilical artery pH �7.1 12 (2)...................................................................................................................

NICU admission 9 (2)...................................................................................................................

Intubation at delivery 1 (0.2)...................................................................................................................

All data shown as n (%). P value is for Mantel-Haenszel �2 foNICU, neonatal intensive care unit.


ne contraction frequency threshold us- d

290.e5 American Journal of Obstetrics & Gynecolo

ng adverse effects on the fetus as thendpoint.

Our results could also be viewed asroblematic, and certain caveats must beiscussed. The duration of each individ-al contraction has the potential to alter

he relaxation time, and therefore poten-ially the fetal outcomes. However, thisas not measured in our study because

he duration of contractions is not in-luded in the current national definitionf uterine tachysystole. Similarly, thetrength of contractions was not studieds this is also not included in the defini-ion of tachysystole. Importantly we an-lyzed infant outcomes that occurredany hours subsequent to the observed

terine tachysystole. It could be arguedhat the elapsed time from the incident ofachysystole to the outcome of interestould introduce multiple confoundingariables which weaken our results. Onhe other hand, our study was designedo mimic a frequently encountered labor

anagement scenario, in which uterineachysystole occurs remote from deliv-ry. We chose to analyze uterine tachy-ystole during the first 4 hours after be-inning labor induction for severaleasons. First, analyzing uterine con-ractions late in labor, ie, during theecond stage, while more proximal to

ne hypertonus events

nus events

1, n � 70 2, n � 19 P value

17 (24) 7 (37) .013..................................................................................................................

..................................................................................................................

46 (66) 12 (63) .006..................................................................................................................

38 (54) 6 (32) .54..................................................................................................................

23 (33) 11 (58) � .001..................................................................................................................

20 (29) 4 (21) � .001..................................................................................................................

0 0 .69..................................................................................................................

1 (1) 0 .69..................................................................................................................

0 0 .23..................................................................................................................

0 0 .69..................................................................................................................

d.

12.

elivery of the infant, can be techni- t

gy OCTOBER 2012

ally quite difficult without the place-ent of an intrauterine pressure cath-

ter because of the much increasedterine contraction frequency and in-

ensity. Intrauterine pressure cathetersere only placed for routine obstetrical

ndications, and therefore were notlaced in all patients. Secondly, weanted to study uterine tachysystole in

he setting of labor stimulation be-ause this is the most common clinicalcenario when attention is focused onterine tachysystole, and therefore has

he potential to alter management de-isions. Thirdly, we chose the laborattern following a single dose of mi-oprostol stimulation because we pre-umed that standardization of the mi-oprostol dose as was done would alsoerve to standardize the circumstancesnder which we were analyzing uterine

achysystole. Taken together, all theseforementioned caveats limit our con-lusion that brief periods of uterineachysystole do not harm the fetus.aid another way, our results can benterpreted to mean that self-limitedpisodes of uterine tachysystole occur-ing as a result of labor stimulation re-ote from delivery are not harmful.We cannot address the issue as tohether uterine tachysystole produced

nough intrapartum compromise to re-ult in long-term neurological compro-

ise. However, we point out that nonef the infants in our study who experi-nced episodes of tachysystole had anmbilical artery pH �7.0, which is anbligate criterion to define an acute in-rapartum hypoxic event sufficientnough to result in long-term neurologicorbidity.14

Our findings that vaginal delivery in-creased, and time to delivery decreased,with increasing uterine contractions alsosuggests that �6 contractions per 10

inutes vs �5 per 10 minutes, averagedver 30 minutes, may have an advantage.his potential advantage, however, muste tempered by the fact that increasingontractions impact the FHR pattern.

e find that �6 contractions per 10inutes, or 17-19 per 30 minutes, as well

s uterine hypertonus, have FHR conse-uences compared to less frequent con-

eri

rto

.........

.........

.........

.........

.........

.........

.........

.........

.........

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ractions or no hypertonus, and that the


number of contractions can significantlymodify the route of delivery. Based uponthese findings, we believe our results sug-gest continued use of the NICHD andACOG definitions of uterine tachysys-tole as �6 contractions in 10 minutes,averaged over 30 minutes. Our results,although showing there is no differencein infant outcomes when uterine tachy-systole occurs remote from delivery, doshow that uterine contractions of �6 per10 minutes have a discernible impact onFHR patterns, and as such justify thecontinued use of this threshold. f

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