A new 'non-invasive' method for measuring intervillous and myometrial blood flow is presented. After 2 mCi of 133Xe in physiological saline was injected intravenously the patient held her breath for 20 seconds. The tracer entered the placenta as a short bolus and its removal was followed with a scintillation detector. Intervillous and myometrial blood flow per unit volume was calculated from the two-exponential curve. The mean &SD intervillous flow in normal pregnancy was 135 f49 ml/minute/100 ml, the corresponding half time being 0 -56 &O * 16 minutes. The myometrial flow was 7.7&2.5 ml/minute/100 g. The method gave reproducible results and took only 20 minutes to perform. The dose of radiation to the mother was less than 1 mrad.
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