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_ETCO₂) 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_ETCO₂ 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 P_ETCO₂ during the ovulatory and anovulatory cycles were analyzed.

Result(s)

The time course of P_ETCO₂ 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_ETCO₂ started, coincident with the increase of E₂, 1–8 days (median: 4.5 days) before onset of the LH surge. No such changes in P_ETCO₂ could be observed during the anovulatory cycles. These changes could be mimicked by administration of E₂ or P. The P_ETCO₂ did not exhibit a circadian rhythm and was not influenced by normal physical activity.

Conclusion(s)

The P_ETCO₂ changes in a characteristic way during the menstrual cycle. Determination of P_ETCO₂ appears to be a promising method for the detection of the fertile phase of the menstrual cycle in the context of natural family planning.

Key Words: End-tidal pCO₂, respiration, menstrual cycle, fertile phase, natural family planning