Renal vascular resistance increases during acute obstruction, particularly during the first 6-48 h (Ulrich et al. 1995). This increase is related to vasoconstriction mediated by different factors such as prostaglandins. Using these parameters, Shokeir et al. (2000) indicated that a resistivity index of at least 0.7 diagnoses obstruction, with a sensitivity of 77% and a specificity of 83 %, with 88 % sensitivity and 98 % specificity if the resistiv-ityindex's variation is greater than 0.06. This measurement's performance is flawed, however, when the measurement is taken before 6 h or after 48 h, in cases of single kidney, of pathological kidney, or when nonste-roidal anti-inflammatory drug (NSAID) treatment interfering with the metabolism of prostaglandins is used (Shokeir et al. 2000; Ulrich et al. 1995).
7.3 Diagnostic Procedures in the Pregnant Patient
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