ACTA FAC. MED. NAISS. 2004; 21 (1): 15-22

Original article

    BIOPHYSICAL PROCESSES AND ANGULAR TRACTION EFFECTS DURING FETAL VACUUM     EXTRACTION

    Folić Miroslav¹, Čitaković Ivanela¹ , Jeremić Branislav²

    ¹ Obstetrics and Gynecology Clinic, Clinical Center Kragujevac
    ² Terotechnology Institute, Mechanical Engineering Faculty Kragujevac

    INTRODUCTION

    Fetal vacuum extractor is used in 0.9-5.2% of births (1,2,3). The rationale of these differences in frequency of FVE use are conditioned by the attitudes of obstetric clinics towards the completion of expulsion delivery phase during fetal head stoppage over the pelvic exit and the consequences which the chosen delivery termination method may lead to (4,5). Investigation of these situations are the part of most up-to-date multidisciplinary investigation tendencies and they connote extensive use of medical engineering. This includes the application of complex knowledge from the fields of medicine, theory of the model formation process, biophysics, electronics, mechanics, measurement theory etc. This original scientific paper is the result of complementary investigations of the Clinic of Gynecology and Obstetrics and the Institute of Terotechnology, Machine Engineering Faculty in Kragujevac, for the first time conducted in Serbia and Montenegro, with quantification of the processes of biophysical modeling during vacuum fetal extraction and simultaneous determination of the angular traction effects.

    AIM OF THE WORK

    The aim of the work is objectification of the biophysical processes occurring on the fetal head during vacuum extraction, investigation of the potential consequences of negative pressure applied on the angular surface of fetal head, determination of traction forces as well as determination of the conditions for evaluation of posttraumatic decompression effects.

    RESULTS OF THE INVESTIGATION

    Modeling of the biophysical processes during fetal vacuum extraction (FVE) connotes work in several phases. The first phase is an exact definition of anatomical characteristics of fetal scalp. Measurements were made on a representative sample - the characteristic radius values were shown in the annex figure 1 and the numerical values in table 1.

    Fetal scalp, according to these measurements, can be modeled as a geometric spherical form, with diameter of 100 mm; based on that, an investigational rubber model was made, with the attached dynamometers to measure traction force and traction momentum (demonstrated in figure 2).

Figure 2. Model of the fetal scalp with dynamometers

    Dynamometers to measure the traction force and traction momentum in FVE were made of measurement bands based on the tensoresistive effect, the phenomenon that if a metal conductor is deformed its resistance is altered too. Using the Hook’s law concerning the association of charge and deformity (s = E × e) and charge and force, s = F / A (where A stands for the surface upon which the force is applied), a relationship may be obtained between the force acting upon the conductor and alte ration of its resistance. Measurement stripes are specially devised conductors between two leafs of plastic attached to the object the deformities of which are to be measured (figure 3). Since the measurements were performed at the room temperature, there was no need to eliminate the temperature effect on the resistance alteration - measurement stripes were tied in the Witson ring half-bridge.

    On deformation and alteration of resistance of measurement stripes, the output charge of the bridge Uiz is altered. These changes represent the measurement of deformity. Dynamometers are devised and manufactured depending on the intensity and type of force to be measured. Before measurements, it is necessary to burden them with known forces and determine functional dependence of the output charge.
    Calculation of the UM charge obtained with the dynamometer in the position 3, figure 2, into the traction momentum is performed according to:

        M_t = K_M × U_M

        where K_M (Nm/V) is the proportion factor depending on the applied measurement stripes, transversal surface of the dynamometer and arms of the force applied. The charge obtained with dynamometer marked with position 2, figure 2, is expressed as:

        U₂ = k₁ M_T + k₂ × F_a

        in which k1 and k2 are influence coefficients. Finally, determination of the traction force Fa could be obtained with the expression:

    An experienced obstetrician and numerous applications of fetal vacuum extractor defined empirically the resistance values with the adjustable joint (figure 2, position 4). This adjustable joint is a tribomechanic system with the elements among which a versatile traction force is acting, depending on the magnitude of contact pressure on the elastic joint slugs. These slugs are placed in order to enable a continual regulation of the resistance simulated during the FVE process modeling.
    The magnitude of the contact pressure may be changed by the change of force of tightening the screw which keeps the slugs within the joint; it could be quantified as follows:

        F = c x,

        in which c is stiffness of the elastic slugs and x deformity of the slugs. The force of tightening the screw makes a contact pressure onto the elastic slugs according to the formula:

        where A is the contact surface between the slugs and the joint carrier.

        In the first approach the tightening force F may become equal as the normal force F_n acting in the joint onto the contact surface between the elastic slugs and the constrained joint carrier, so:

        F = F_N

        Between the contact surfaces of this tribomechanic system a traction force is acting

        F_µ=µ FN ,

        in which µ is the traction coefficient value, completely equal.

Therefore, by the contact pressure change (ie. force within the joint) altered is also the traction force an obstetrician should overcome during the traction process. Simulation of the traction momentum is performed through the M_T:

        M_T = Fr l,

        where F_r is the radial FVE force component.

The M_T momentum fetal head is brought into the flexion position defined by the negative á angle values, ie. the deflexion defined by positive á values.
    FVE device creates a vacuum, transmitted through a rubber hose to the fetal scalp. Vacuum device calotte transmits the traction force to the fetal scalp made by the physician. Maximal transmittable force is the function of the calotte surface A and the sub-pressure Dpv achieved by the vacuum device:

        F=D p_v A

        The level of vacuum in FVE conventionally does not exceed 0.9 bar. Therefore there are several sizes of vacuum device calotte which enable that the maximal force could be different. In the figure 4, some basic calotte dimensions were shown used in experimental studies, and in the figure 5 models of the reduced pressure vacuum chamber for flat, spherical and supposed deformed contours of the scalp contact surface.
    Maximal force which may be generated with the given pressure level could be calculated accor- ding to the expression F=Dp_vA and relates to the vacuum chamber with flat contact surface, while in all other supposed deformities maximal force could be calculated as follows:

        with A as the surface defined by the diameter D (figure 5).

Figure 5. Formation of the vacuum chamber: contact surface flat (a), spherical (b) and deformed (c)

        It should be emphasized that the fetal scalp modeling was performed with the part of a rubber sphere, the deformities of which could be disregarded in view of the actual scalp deformities. In real situations, a part of the fetal scalp is retracted into the FVE vacuum device calotte (figure 5c). On retraction, there is a possibility that for a short period a holding force is effectuated larger than the theoretically calculated force; to the vacuum exerted holding force a friction force (between the retracted fetal scalp and vacuum calotte) should be added.

In the table 2 some maximal, theoretical values of the vacuum holding force are given, calculated according to the above mentioned expression, as well as the maximal measured values of the holding force (with different vacuum values).

Table 2. Maximal vacuum holding force values

    In the figure 6 the forces acting upon the rubber model through the calotte are presented.

Figure 6. Forces acting through the FVE calotte onto the scalp model

    The F force with which an obstetrician retracts the calotte acts at an angle á to the axis corresponding to the calotte axis. That force can be divided into two components: Fa, traction force (acting along the calotte axis – axial force) and Fr component (acting perpendicularly to the calotte axis – transversal force). The transversal force creates a separate traction momentum calculated after the expression:

        M = M_T = F_r × l

        A measurement chain for traction force and momentum monitoring is shown in figure 7.

Figure 7. Measurement chain for traction force and momentum monitoring

       The signal from the measurement dynamometer stripes (conveying the information on the traction force and momentum) is transmitted to the two-channel bridge amplifier, where the signal is augmented to the level necessary for A/D converter function. A/D converter is built in a personal computer; its role is to convert continual electric signal into a sequence of numbers (discrete values). A signal in the form of numerical sequence is appropriate for further numerical processing. Software for A/D converter control is the LABTECH NOTEBOOK model, while for signal processing and presentation Matlab for Windows was used.

Figure 8. Delivery process according to the angular traction phases

    In figure 8, a delivery process is demonstrated by the angular traction phases.

Check-ups of the subjective status of an obstetrician which performed several hundred FVE applications was performed through the process repetition on the model, with the obstetrician who could not monitor the values of forces and FVE timing. The result of this study is a family of curves through which formation of the traction vacuum is monitored (figure 9), demonstrating very good reproducibility. Based on clustering, a representative curve was devised (figure 10).

Figure 9. Family of the curves of traction vacuum formation	Figure 10. Representative curve of the traction vacuum formation; initial vacuum formation phase(up to 50 s), revision of the angular application (up to 100 s), forced traction vacuum formation(up to 170 s) and the phase of angular traction revision and extraction (up to 220 s)

CONCLUSION

Investigation of the biophysical processes occurring on the fetal head during FVE and the effects of angular tractions confirm the following opinions:

    1. The force created (maximal theoretical is 26.24 daN, and maximal measured 22.47 daN) must not be used to resolve the problems of disproportion between the fetal head and osseous pelvic delivery route.
    2. The success of FVE placement is directly proportional (which follows from the first conclusion) to the accurately established relationships of these two parameters.
    3. FVE assisted delivery should be performed only by an experienced obstetrician (several years experience; multiple FVE applications), with the appropriate knowledge of biophysical processes and angular traction effects, and with
    4. Reliable neonatology and anesthesiology support team, well trained, equipped (laboratory and instruments/machines) and always ready to render top quality support to the mother and her newborn child.

    Future studies of the biophysical processes in FVE and consequences of angular traction should clearly define deformity processes on the fetal scalp and quantify the phenomena in the intra-cranium, especially elevation of the intracranial pressure and hemorrhages in the brain structures (above all in the ventricles), as a leading partal risk in instrumentally assisted delivery with FVE.

LITERATURA

    1. Grossman, RA. An easily made obstetrical vacuum extractor. Am Med J Obstet Gynecol 1999; 22(4):113-20.
    2. Johanson RB, Rice C, Doyle M, et al. A randomized prospective study comparing the new vacuum extractor policy with forceps delivery. Br J Obstet Gynecol, 2001; 100 (6):524-30.
    3. Lucas, M J. The role of vacuum extractor in modern obstetrics. Clin Obstet Gynecol, 2001; 37 (4): 794-805.
    4. Muise K L, Duchon M A, Brown R H. The effect of artificial caput on performance of vacuum extractors. Obstet Gynecol. 2002. 88 (6): 122-34.
    5. Vacca A. Effect of angular traction on the performance of modern vacuum extractors. Am J Obstet Gynecol. 2001; (9), 500-09.