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• W2015164093 abstract "To the Editors: While developing a computer model for shoulder dystocia (SD) is an admirable task, input for it must be based on sound principles. Using empirical clinician and uterine force values, Gonik et al1.Gonik B Zhang N Grimm M.J Defining forces that are associated with shoulder dystocia: the use of a mathematic dynamic computer model.Am J Obstet Gynecol. 2003; 188: 1068-1072Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar “objectively define” maternal and clinician contributions to forces required to overcome SD and “confirm previous laboratory results” that McRoberts' positioning reduces these forces. However, some input values and results are discordant with those of earlier investigators and with scientific principles. The current model computes a uterine force value of 400 N, or 90 lbs, “needed to accomplish delivery.” Curiously, this is less than half the value the authors claimed existed in their earlier model (authors' reference 3). Yet, even 400 N remains implausible. A basic physics model of a sealed, pressurized system demonstrates that the maximum force that can be developed is simply the product of the maximum pressure within the vessel and the area of the obstruction. Specifically, for an obstructed uterus,2.Crandall S.H Dahl N.C Lardner T.J An introduction to the mechanics of solids. McGraw-Hill, New York1972: 72-142Google Scholar the maximum uterine force is about 150 N, because the maximum uterine pressure ever measured in humans is approximately 140 mm Hg (18,660 N/m2) and the area at the cervical opening is π∗[100 cm2]/4, (0.00785 m2). This value is independent of fetal size or uterine dimension. To reach 400 N, either the pelvic outlet would have to have been 16 cm or else intrauterine pressure would have to approach 240 mm Hg. The literature on uterine mechanics, including far more complex models, confirms that about 150 N is the maximum possible force the uterus can ever generate. Buhimschi et al3.Buhimschi C.S Buhimschi I.A Malinow A Weiner C.P Use of McRoberts' position during delivery and increase in pushing efficiency.Lancet. 2001; 358: 470-471Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar and others have measured peak intra-amniotic pressures clinically reporting peak uterine force values that range from 80 N during contractions to 160 N when combined with Valsalva and McRoberts'. For clinician-applied forces, the authors input a maximum of 100 N, and they calculate only 50 N needed to accomplish delivery with McRoberts' positioning. Yet, some clinicians use more than 200 N for SD deliveries (Gonik and Allen, 1987; SPO Abstract 151), and Gonik himself has used more traction (>100 N) in simulated SD deliveries (authors' reference 7), even with McRoberts' positioning. This occurred when the bisacromial diameter exceeded the pelvic outlet dimension, as it does in the current model (14.2 vs 12.5 cm). Indeed, in the earlier model, McRoberts' positioning was only protective up to a shoulder width of 12 cm. Above this, clavicle fractures—which are not incorporated in the current simulation—occurred in lithotomy and McRoberts' positioning. Although successful for many SD deliveries, McRoberts' positioning usually relies on repeated traction to complete the delivery. The effect of the near doubling of maternal forces that occurs with McRoberts' positioning3.Buhimschi C.S Buhimschi I.A Malinow A Weiner C.P Use of McRoberts' position during delivery and increase in pushing efficiency.Lancet. 2001; 358: 470-471Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar also was not considered. If maternal forces are potentially harmful during SD, then perhaps clinical management should be altered to minimize them. In particular, Valsalva should be discouraged, and fetal manipulation maneuvers should be prioritized over maternal manipulation maneuvers such as McRoberts', which appears to have less efficacy than previously thought.4.Beall M.H Spong C.Y Ross M.G A randomized controlled trial of prophylactic maneuvers to reduce head-to-body delivery time in patients at risk for shoulder dystocia.Obstet Gynecol. 2003; 102: 31-35Crossref PubMed Scopus (36) Google Scholar" @default.
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• W2015164093 title "Computer modeling of shoulder dystocia" @default.
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• W2015164093 doi "https://doi.org/10.1016/j.ajog.2004.01.087" @default.
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