Artifactually elevated basal uterine tonus resulting from measurement of hydrostatic pressure by transducer-tipped intrauterine catheters
Ross, M.G.; Walton, J.
Journal of Perinatology Official Journal of the California Perinatal Association 14(5): 408-410
1994
ISSN/ISBN: 0743-8346 PMID: 7830158 Document Number: 428170
Accurate measurements of uterine activity are a valuable adjunct in the management of parturition. Recently transducer-tipped intrauterine pressure catheters (IUPC) have been advocated because of their accurate recording of uterine contraction pressures. We hypothesized that the transducer-tipped IUPC may record elevated basal uterine pressures because of measurement of additional hydrostatic pressures within the uterus. Eight laboring patients received intrauterine placement of a transducer-tipped IUPC. Catheters were zeroed to air pressure, electronic zero was checked, and monitors were calibrated before insertion. Catheters were not rezeroed during labor. Basal resting uterine pressures were recorded at insertion and before IUPC removal. After removal, electronic zero and catheter accuracy were tested in a specially designed pressure chamber (21 and 60 mm Hg). Electronic zero averaged -3 mm Hg before insertion and after removal. Baseline uterine resting tone averaged 15 +/- 1 mm Hg in seven patients, and one patient had a measurement of 34 mm Hg. Uterine resting tone remained stable in five patients and decreased to 20 mm Hg in the one patient with elevated basal tone. Uterine resting tone increased markedly to 36 and 41 mm Hg in two patients with normal pressures on insertion, in spite of no evidence of uterine hyperstimulation. Extrauterine pressure testing recorded pressures of 23 +/- 1 and 62 +/- 1 mm Hg, respectively. Because of the measurement of the amniotic fluid column above the catheter, transducer-tipped IUPCs may artifactually elevate uterine pressure measurements and basal levels may shift during the course of labor.