Analyses of tractive forces during the application of

Saling et ah, Tractive forces during vacuum extraction 245

J. Pcrinat. Med.l (1973) 245

Analyses of tractive forces during the application of vacuumextraction

Erich Saling, Michael Härtung

Unit of Perinatal Medicine The Free University of Berlin,Dept. of Obstetrics and Gynecology, Berlin (West)-Neukölln,

Received July 9, 1973. Accepted August 22, 1973.

Vacuum extraction dates back to the 18thCentury [1], MALMSTRÖM first introduced a fullypracticable vacuum extractor äs an operativemethod of delivery about 20 years ago [2],Vacuum extraction has since found increasingapplication and is considered in many countriesto be äs useful äs forceps.

Numerous articles have been published concerning tech-nique and clinical application of vacuum extraction. Todate, however, there has been no direct research, äs far äswc know, about the extent of the effective tractive forcesand the length of application during the extraction. Be-cause of this lack criteria based upon subjective judgmentswere used to characterize a vacuum extraction. Suchcriteria are insufficient for objective analysis of tractiveforces.

We have therefore developed a device for therecording of tractive forces in vacuum extractionsäs presented in the last issue of this Journal [4]. Inconnection with that article we are now re-porting our first results.

1. Gase material and methods

During the period from April 1972 to February1973 we investigated 74 clinically indicatedvacuum extractions with the above mentionedapparatus. Fig. l shows an example of a record.From all 74 records we have evaluated thefollowing parameters:a) length of the whole operative measureb) total duration of the applied forcesc) magnitude of the maximum tractive force

d) number of individual tractionse) force-time-integral (FTI)For the measurement of the force-time-integral(FTI) the record was evaluated planimetricallyby means of a transparent overlay. The plani-metrically evaluated result represents the timeintegral of force = / Fdt = FTI (force-time-integral). l mm2 on the recording paper cor-responds to 3.75 kpsec (= 36.78 kgm/sec) at arecording speed of 2 cm per minute.

2. ResultsTab. I presents a survey of the indications foroperative delivery by vacuum extractor in ourmaterial. In more than 80% of the cases vacuumextraction was indicated by delay of second stageor acidity-increase determined by fetal bloodsampling.

Tab. I. Indications for vacuum extraction.

Delay of second stageAcidity increase in the fetusAcute bradycardia with silent oscillationsContinual tachycardia of more than 3 hdurationDeep transverse arrestMaternal predisposition to seizures

n = 48n = 15n= 5

n = 4n= ln = l

total = 74

All were cases of vertex presentation in whichthe major portion of vacuum extractions wasperformed with the level of the fetal head at

J. Perinat. Med. l (1973)

246 Saling et al., Tractive forces during vacuum extraction

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UApH acb=731/U A.pHqu40=730Score=10A,A=X,C=ZFig. 1. Reco±d o£ fetal heart rate (upper section), uterine contractions (middle section) and applied tractive forcesduring vacuum extraction (bottom section). One division is 5 kp, paper-speed l cm/min.

mid-pelvic-plane (n = 44) and pelvic floor(n = 22). In 6 cases the baby was extracted fromthe pelvic outlet and in only 2 cases from thepelvic inlet. We used exclusively the largestsuction cup with a diameter of 6 cm.As expected the duration of the vacuum ex-traction, the number of individual tractions,the magnitude of the maximum tractiveforce correlate with the level of the fetal head(see Tab. II). The higher the position of thefetal head at the beginning of the vacuumextraction the longer the duration of theoperative measure and the greater the numberof individual tractions and the magnitude ofthe maximum tractive force.In 22 cases the maximum tractive force was^ 20 kp. In 10 of these 22 cases the suction cupcame off. There was no coming off of the

suction cup with a maximum tractive forceof less than 20 kp. It is possible for the suctioncup to come of s well in cases of less tractiveforce. In such cases, however, the threat ofcoming off can be recognized in time by thesound of entering air, so that the tractive forcecan be interrupted early enough. In half thecases of coming off we found depressed new-borns (n = 5). The total number of depressednewborns was 11.The force-time-integrals show similar results.The force-time-integral increases correspond-ing to the level of the fetal head. Tractions withFTI-values of less than 375 kpsec where re-quired chiefly for extractions frorn the pelvicfloor and the pelvic outlet. Tractions of morethan 1500 kpsec were necessary only for ex-tractions from higher pelvic levels (Fig. 2).

J. Perinat. Med. Γ(1973)

Saling et al., Tractive forces during vacuum extraction 247

Tab. II. Number of tractions, maximum tractive force, length of the whole operative measure and total duration of theapplied tractions.

Number of Maximumtractions tractive force

[M

Length ofwhole operative

Total durationof applied

measure[min]

Pelvic inlet

Midpelvic plane

Pelvic floor

Pelvic outlet

Maximum

Minimum

(mean)

2.50 19.50 kp

2.52 17.84 kp

1.95 14.31 kp

1.66 10.33 kp

8 28 kp

1 6kp

(mean)

3min

2 min,

1 min,

1 min,

12 min,

34 sec

19 sec

10 sec

30 sec

10 sec

tractions[min]

(mean)

2 min, 37 sec

2 min, 10 sec

1 min, 17 sec

1 min, 2 sec

8 min, 40 sec

number of patients Kfl\Ln 12: % A1An

Kr 12 f19·^ 10"*

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Fig. 2. Force-Time-Integrals of vacuum extractions fromthe various pelvic planes. Number of cases is placedwithin or above the columns (kpsec = kilopondsecond).

Fig. 3 represents most clearly the relationshipbetween the extractions from the pelvic inlet andmid-pelvic plane s well s from the pelvicfloor and the pelvic outlet expressed s 2 pairs onthe graph. As the .FTI increases, the relativeportions of vacuum extractions from higherlevels correspond almost inversely proportionallyto those from lower levels. It follows that thelevel of the fetal head is the first significantParameter for the magnitude of the FTI.In examining the influence of fetal size upon theforces of extraction we compared the FTI ofinfant groups with different birth weights at

Fig. 3. Relative portions of vacuum extractions frompelvic inlet and midpelvic plane s well s from pelvic floorand pelvic outlet correlated with increasing Force-Time-Integral.

same level of the fetal head. Although thissubdivision naturally results in a smaller numberof cases, the evaluation shows a clear correlationbetween FTI and neonatal birthweight. Ascompared with the extraction of a baby weighing2900 g from the mid-pelvic plane we found theFTI doubled at a birthweight of 3700g (s.Tab. III).Therefore birthweight is the second importantparameter for the magnitude of the FTIduring vacuum extraction.



Tab. III. Force-Time Integrals äs a function of birthweight.

Level of thefetal head

Midpelvic plane

Pelvic floor

Birthweight[g]

2500—2900 g2901—3700 g3701 g and more

2500—2900 g2901 g and more

Mean FTI[kpsec]

83 kpsec130 kpsec178 kpsec

43 kpsec79 kpsec

11 (14%) o£ the 74 newborns delivered byvacuum extraction were born in a state of de-pression (2 cases C I, 5 C II, 4 C III). In 5 o£these 11 cases the suction cup came off (see above).The classifications of the clinical state are givenin Fig. 4. We published a report of this classi-

% fication System two years ago [3],

number of potients 30r20;101

number ofall patients

tne„ 375 750 1125 1875 2625 3375 4125Kpsec 1500' 2250 3000 3750

easy mildlydiff icult

difficult-H

vacuumextraction

Fig. 4. Distribution-curve of all Force-Time-Integralstogether with the corresponding number of newborns ina state of depression. Number of cases is placed within orabove the columns. Correspondence of the scoring Systems:

our systemCIVcmcnci

APGAR7—85—63—40—2

It follows from Fig. 4 that the risk of neonataldepression grows with increasing FTI. Wehave therefore, for the time .being arbitrarily,divided the degree of severity of a vacuumextraction into 3 groups:

1. Easy vacuum extraction up to 375 kpsec2. Mildly difficult väciuum extraction 376 to

1125 kpsec3. Difficult vacuum extraction 1126 and more

kpsec.

In the first group there are no cases of depressednewborns, in the second 5 (16%) and in thethird 6 (33%). The depression of the newbornmight be a resült of the vacuum extraction orcould be caused by other factors; this questionremains to be resolved. To settle this problemwe are applying the measurement of the pH-value in the blood of the umbilical artery imme-diately post partum, part of our clinical routinefor the past several years. The pH-values of twoof these 11 newborns were normal pH ^ 7.25).In two cases there was an increase in acidity(pH 7.24—7.20), in 6 newborns a slight acidosiswas found (pH 7.19—7.15) and l newborn hadan advanced acidosis (pH 7.02).This enables us to exclude fetal hypoxia äs a j

cause of depression in at least four cases, so thatwe have to consider a relation between thevacuum extraction and the depressed state ofthe newborn.

3. Discussion of the Undings

The results correspond with the common ex-pectations of experienced clinicians. The ad-vantage of our newly developed measuringdevice, however, is the possibility it presentsto register accurately and record objectivelyfor the first time the magnitude and durationof tractive forces äs routine procedure.We judge this to be an important step toward acomplete collection of data concerning the birth.The clinical assessment of a vacuum ex-traction, especially its effects upon the child,can now be based upon measurable values;subjective miscalculations can be eliminated.It would be premature to make far reachingclinical conclusions äs a resült of these findings.This problem can be first solved after furtherprospective research in connection with catam-nestic inquiries. A great number of questionsstill remain open: e. g. incidence of traumäticlesions, effects of the applied tractions upon


Saling et al., Tractive forces during vacuum extraction 249

the fetal heart rate, incidence of severe de-pression in vacuum extractions with a greatFTL

Our interest in this first analysis was centeredupon researching force-time-relationships ac-cording to difFerent parameters such äs the levelof the fetal head and birthweight.

Our findings to date lead to the followingconclusions:

1. The FTI during the vacuum extraction clearlycorresponds with the size of fetus and thelevel of the fetal head.

5. Severe depression which cannot be explainedby acidosis and/or hypoxia is more oftenfound after vacuum extractions with agreat FTI.

3. Corning off of the suction cup was notedin our studies at a tractive force of morethan 20 kp.

Summary

74 clinically indicated vacuum extractions were observedusing our newly developed device for the measurement oftractive forces during vaccum extractions [4], (Fig. 1). Thefollowing parameters were evaluated:

length of the whole operative measuretotal duration of the applied tractionsmagnitude of the maximum tractive forcenumber of individual tractionsforce-time-integral (FTI).

For extractions from the various pelvic planes the followingmean values were found (pelvic inlet = PI/mid-pelvicplane = MP/pelvic floor = PF/pelvic outlet = PO):

Number of tractions: PI:2.5; MP: 2.52; PF: 1.95;PO: 1.66.Maximum tractive force: PI: 19.50 kp; MP: 17.84 kp;PF: 14.31 kp; PO: 10.33 kp.Length of the operative measure: PI: 3min; MP:2 min, 34 sec; PF: l min, 19 sec; PO: l min, 10 sec.Total duration of the applied tractions: PI: 2min,37 sec; MP: 2 min, 10 sec; PF: l min, 17 sec; PO: l min,2 sec.

All cases investigated were vertex presentations. 44 new-borns were delivered from the midpelvic plane, 22 fromthe pelvic floor, 6 from the pelvic outlet, and 2 frompelvic inlet.Duration of the vacuum extraction, number of indi-vidual tractions and magnitude of the maximumtractive force clearly correspond with the level of thefetal head.In 22 cases the maximum tractive force was ^ 20 kp.With a force of less than 20 kp no coming off of thesuction cup was observed.The force-time-integral (FTI) of each record was evaluated

planimetrically; it presents the time integral of force. Inmore than 80% of the cases vacuum extraction was in-dicated by delay of second stage or acidity-increase in thefetus. The FTI increases in correspondence to the levelof the fetal head. Tractions with FTI-values of less than375 kpsec were required chiefly for extractions from thepelvic outlet (Fig. 2). Tractions of more than 1500 kpsecwere necessary only for extractions from higher pelviclevels. As the FTI increases, the relative portions ofvacuum extractions from higher levels correspond almostinversely proportional to those from lower levels. The FTIcorrelates to birthweight äs well. The level of the fetalhead and the birthweight are therefore the two mostimportant parameters for the FTI of a vacuum ex-traction.The risk of depression grows with an increasing FTI; 11of the 74 newborns delivered by vacuum extraction weredepressed.We have arbitrarily divided the degree of severity of a va-cuum extraction into three groups:1. Easy vacuum extraction up to 375 kpsec2. Mildly difficult vacuum extraction 376—1125 kpsec3. Difficult vacuum extraction 1126 and more kpsec.It remains to be resolved whether the neonatal depressioncan be explained äs a result of the vacuum extraction orother causes.The blood pH-measurements of the umbilical arteryenabled us to exclude a hypoxia äs a cause of depressionin at least 4 of the total 11 cases. A connection betweenvacuum extraction and neonatal depression can besuspected.We judge this method to be an important Step toward acomplete collection of data concerning the birth.Subjective miscalculations can thus be eliminated in thefuture.

Keywords: Fetus, tractive force, vacuum extraction.



Zusammenfassung

Analyse der Zugkräfte bei der VakuumextraktionAnhand der von uns entwickelten Vakuumextraktions-zugmeßausrüstung [4] (Abb. 1) wurden 74 klinisch in-dizierte Vakuumextraktionen überwacht und nach fol-genden Parametern ausgewertet:Dauer des gesamten operativen EingriffsGesamtdauer der applizierten ZügeGröße der maximalen ZugkraftAnzahl der EinzelzügeKraft-Zeit-Integral (KZI).Für Extraktionen aus den verschiedenen Beckenebenenließen sich folgende Mittelwerte errechnen (Becken-Ein-gang = BE, Becken-Mitte = BM, Becken-Boden = BB,Becken-Ausgang = BA):Zahl der Züge: BE: 2,5; BM: 2,52; BB:1,95;BA: 1,66.Maximale Zugkraft: BE: 19,50 kp; BM: 17,84 kp;BB: 14,31 kp; BA: 10,33 kp.Dauer des operativen Eingriffes: BE: 3min; BM:2 min, 34 sec; BB: l min, 19 sec; BA: l min, 10 sec.Gesamtdauer der applizierten Züge: BE: 2 min, 37 sec;BM: 2 min, 10 sec; BB: l min, 17 sec; BA: l min, 2 sec.Alle untersuchten Fälle waren Schädellagen, 44 Kinderwurden aus Beckenmitte, 22 vom Beckenboden, 6 ausBeckenausgang, 2 aus Beckeneingang entwickelt.Dauer der Vakuumextraktion, Anzahl der Einzelzüge undGröße der maximalen Zugkraft korrelieren deutlich mitdem Höhenstand des kindlichen Kopfes.22mal betrug die maximale Zugkraft ^ 20 kp. Unterhalbeiner Zugkraft von 20 kp war kein Abreißen derSaugglocke zu beobachten. Das Kraft^Zeit-Integral(KZI) eines jeden Zugmeßprotokolls wurde planimetrischbestimmt; es repräsentiert das Zeitintegral der Kraft. Inmehr als 80% der Fälle erfolgte die VE wegen Geburts-stillstandes oder einer durch Fetal blutanalysen festgestellten

Aziditätssteigerung beim Feten. Das KZI wächst inAbhängigkeit vom Höhenstand des kindlichen Kopfes.Züge mit KZI-Werten unter 375 kpsec würden vorwiegend2ur Extraktion aus Beckenboden und Beckenausgangbenötigt (Abb. 2). Züge mit mehr als 1500 kpsec warennur für Extraktionen aus höheren Beckenebenen er-forderlich. Mit steigendem KZI verhalten sich die relativenAnteile von Vakuumextraktionen aus höheren Ebenen imVergleich zu denen aus niederen Ebenen nahezu umgekehrtproportional (Abb. 3). Das KZI korreliert auch mit demkindlichen Geburtsgewicht. Der Höhenstand deskindlichen Kopfes und das Geburtsgewicht sind somitdie beiden wesentlichen Parameter für das KZI einerVakuumextraktion.Das Risiko eines Depressionszustandes nimmt mit stei-gendem KZI zu (Abb. 4); 11 der 74 durch VE entwickeltenKinder wurden in einem Depressionszustand geboren.Zur Differenzierung des Schweregrades einer Vakuum-extraktion haben wir zunächst willkürlich die VE in dreiGruppen unterteilt:1. Leichte VE bis 375 kpsec2. Mittelschwere VE 376—1125 kpsec3. Schwere VE 1126 kpsec und mehr.Es bleibt zu klären, ob der kindliche Depressionszustandals Folge der VE anzusehen oder vielmehr durch andereUrsachen erklärbar ist. Durch die Meßergebnisse derNabelarterienblut-pH-Werte unmittelbar post partumkonnte in mindestens 4 der insgesamt 11 Fälle mit De-pressionszustand eine Hypoxie als Ursache der Depressionausgeschlossen werden, so .daß an Beziehungen zwischender VE und dem kindlichen Depressionszustand gedachtwerden muß. Wir halten das Zügmeßprotokoll für einenweiteren wichtigen Schritt zur Vervollständigung wichtiger,das Kind betreffender Geburtsdaten. Subjektive FehLeinschätzungen werden hiermit in Zukunft vermieden.

Schlüsselwörter: Fetus, Vakuumextraktion, Zugkräfte.

Resume

Analyses des forces de traction durant l'application del'extraction obstetricaleA Faide des Instruments de mesure que nous avons misau point [4] (Fig. 1), nous avons observe 74 extractionsobstetricales sur indication clinique en determinant lesparametres suivants:Duree de toute l'intervention operatoireDuree totale des tractions appliqueesGrandeur de la force de traction maximaleNombre des tractions individuellesIntegrale force-temps (IFT).

Pour les extractions des divers detroits du bassin, on atrouve les valeurs moyennes .suivantes (detroit superieurdu bassin = PI, detroit moyen du bassin = MP, plancherpelvien = PF, detroit inferieur du bassin = PO):

Nombre des tractions: PI: 2,5; MP: 2,52; PF: 1,95;PO: 1,66.

Force de traction maximale: PI: 19,50 kp; MP: 17,84 kp;PF: 14,31 kp; PO: 10,33 kp.

Duree de Pintervention operatoire: PI: 3min; MP:2 min, 34 sec; PF: l min, 19 sec; PO: l min, 10 sec.


Saling et al., Tractivc forces during vacuum cxtraction 251

Dur6c totale des tractions appliquoes: PI: 2min,37 scc; MP: 2 min, 10 scc; PF: l min, 17 sec; PO: l min,2scc.Tous Ics cas obscrvcs prcscntaicnt une position du sommet.44 nouveaux-nes ont ete delivrcs du detroit moyen dubassin, 22 du planchcr pelvin, 6 du dctroit inferieur dubassin et 2 du detroit supcrieur du bassin.La duroe de Pextraction obstetricale, le nombre destractions individuelles et la grandeur de la force detraction maximale dopendem clairement du niveau dela tete du foetus.Dans 22 cas, la force de traction maximale etait ^ 20 kp.Pour une force de traction Interieure ä 20 kp, on n'aobserv£ aucun dotachement de la ventouse. L'integraleforce-temps (IFT) de chaque cas cnrcgistre a ete mesurceplanimatriquement; eile represcnte l'integrale temps de laforce. Dans plus de 80% des cas, on a du recourir ä l'ex-traction obstetricale a cause d'un retard de la deuxiemeperiode d'accouchement ou d'une augmentation d'aciditechcfc le foetus. L'IFT augmentc proportionnellement auniveau de la tete du foetus. Les tractions de valeurs IFTinferieures ä 375 kpsec furent necessitces surtout pour lesextractions du detroit inferieur du bassin et du planchcrpelvien (Fig. 2) et les tractions superieures a 1500 kpsecseulemcnt pour les extractions des detroits superieurs dubassin. Pour une IFT croissante, les portions relatives des

extractions des detroits superieurs sont prcsqu'inversementproportionnclles a celles des detroits infcrieurs (Fig. 3).L'EFT correspond aussi au poids du nouveau- . Leniveau de la tete du foetus et le poids du nouvcau-neconstituent, en conscquence, les deux paramatres essenticlspour TIFT d'une cxtraction obstetricale.Le risque d'un £tat d^pressif augmente avec une IFTcroissante (Fig. 4); il atteignit 11 des 74 nouvcaux-ncsdelivres par cxtraction. Nous avons fixe arbitrairemcnt ledegre de gravite d'unc cxtraction selon les trois groupessuivants:

1. Extraction facile jusqu'a 375 kpsec2. Extraction semi-difficile de 376—1125 kpsec3. Extraction difficile de 1126 kpsec et plus.II reste ä determincr si Tctat deprcssif neonatal resulte deTextraction obstetricale ou d'autrcs causes. Les mesurcs dupH du sang arteriel ombilical eiTcctuees aussitot postpartum ont permis d'exclure che^ au moins 4 des 11 casde dcpression Phypoxcmie comme cause de cet etat dc-pressif, cc qui amane ä suspccter une correlation entrel'extraction obstetricale et l'ctat depressif neonatal.Nous estimons quc cctte methode rcpresente un progr^simportant pour completer les donnces relatives a la nais-sancc et evitcr dans Tavcnir des crreurs d'apprcciationsubjectives.

Mots-clos: Fetus, force de traction, ventouse.

Acknowledgement: This investigation was supported by grants from the Deutsche Forschungsgemeinschaft.

Bibliography

[1] CHALMERS, J. A.: The Ventouse, thc Obstetric VacuumExtractor. Lloyd-Luke Ltd., London 1971

[2] MALMSTRÖM, T.: Vacuum-Extractor — an ObstetricInstrument. Acta obstet, gyncc. scand. 33, Suppl. 44 (1954)

[3] SALING, E., K. H. WULF: Zustandsdiagnostik beim

Neugeborenen — Gruppeneintcilung. Fortschr. Med.89 (1971) 12

[4] SALING, E., U. BLÜCHER, H. SANDER: Equipmcnt forthe rccording of tractive power in vacuum extractionsJ. Perinat. Med. l (1973) 142

Prof. Dr. E. SalingUni t of Pcrinatal MedicineMariendorfer Weg 28—38D-1000 Berlin 44/Gcrmany

Note: The completc equipment is manufactured and distributed by KNICK, Katharinenstraßc 2, D-1000 Berlin 37 (West-Berlin).

J. Pcrinat, Med, l (1973)

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