The biphasic pattern of end-expiratory carbon dioxide pressure: a method for identification of the fertile phase of the menstrual cycle

Objective To examine the changes of end-tidal carbon dioxide pressure (P ET CO 2 ) during the menstrual cycle. Design Controlled clinical study. Setting Department of Gynecologic Endocrinology and Reproductive Medicine, Medical University of Innsbruck, Austria. Patient(s) One hundred sixty women with a history of regular menstrual cycles were studied. Intervention(s) The P ET CO 2 was determined once daily. Monitoring of the cycles was performed by ultrasonography and determination of E 2 , P, and LH in blood or LH in urine samples. Main Outcome Measure(s) The changes in P ET CO 2 during the ovulatory and anovulatory cycles were analyzed. Result(s) The time course of P ET CO 2 followed a biphasic pattern with luteal phase values being on average 6.5 ± 2.5 mm Hg lower than during the follicular phase. The decrease of P ET CO 2 started, coincident with the increase of E 2 , 1–8 days (median: 4.5 days) before onset of the LH surge. No such changes in P ET CO 2 could be observed during the anovulatory cycles. These changes could be mimicked by administration of E 2 or P. The P ET CO 2 did not exhibit a circadian rhythm and was not influenced by normal physical activity. Conclusion(s) The P ET CO 2 changes in a characteristic way during the menstrual cycle. Determination of P ET CO 2 appears to be a promising method for the detection of the fertile phase of the menstrual cycle in the context of natural family planning.