ALVEOLAR CARBON DIOXIDE TENSION LEVELS DURING PREGNANCY AND EARLY PUERPERIUM*

Abstract

THE persistence of an elevation of the basal body temperature during the early months of pregnancy is a classic example of the continuation of a systemic response, which, in the nonpregnant state, is characterized by a cyclic ebb and flow of temperature changes during the menstrual cycle. The typical hyperthermia, which becomes apparent at about midcycle and which is maintained almost until the time of the next menstruation, is attributed to the action of progesterone (1, 2). In the normally menstruating woman the concentration of carbon dioxide in the alveoli is depressed during the postovulatory phase of the cycle (3–6). The administration of progesterone lowers the alveolar CO₂ tension (7–9), and the cyclic fluctuations of this gas are also correlated with those of progesterone production. In the event of pregnancy, as is illustrated in this presentation, the suppression of carbon dioxide tension is continued. Cyclic variations in serum sodium and chloride levels during the normal menstrual cycle have been described by Eckstein et al. (10) and Phillips et al. (11). It has been postulated that the sodium retention of pregnancy is related to the increased production of estrogen and progesterone (12). Forbes (13) has shown that plasma progesterone levels may vary during pregnancy, with peaks at average intervals of four weeks. One may speculate, therefore, that cyclic phenomena occurring in normal menstrual cycles may also occur during pregnancy. Thus, Hartman (14) has described abortive cycles of vaginal desquamation after conception in the rhesus monkey. Ball (15) confirmed this observation and also noted cyclicity of sexual responsiveness during pregnancy. It is not an uncommon clinical observation that some women have short episodes of bleeding at the time of the first missed menstrual period following conception. The fact that abortions frequently occur at times when women might be expected to menstruate forms the basis for the added precautions that are advised for these times.