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

ObjectiveTo examine the changes of end-tidal carbon dioxide pressure (PETCO2) during the menstrual cycle.DesignControlled clinical study.SettingDepartment 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 PETCO2 was determined once daily. Monitoring of the cycles was performed by ultrasonography and determination of E2, P, and LH in blood or LH in urine samples.Main Outcome Measure(s)The changes in PETCO2 during the ovulatory and anovulatory cycles were analyzed.Result(s)The time course of PETCO2 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 PETCO2 started, coincident with the increase of E2, 1–8 days (median: 4.5 days) before onset of the LH surge. No such changes in PETCO2 could be observed during the anovulatory cycles. These changes could be mimicked by administration of E2 or P. The PETCO2 did not exhibit a circadian rhythm and was not influenced by normal physical activity.Conclusion(s)The PETCO2 changes in a characteristic way during the menstrual cycle. Determination of PETCO2 appears to be a promising method for the detection of the fertile phase of the menstrual cycle in the context of natural family planning.