(1974). Thorax, 29,[567][568][569][570] Cardiovascular dynamics in women during the menstrual cycle and oral contraceptive therapy. Measurements of pulmonary blood flow and cardiac output have been made in 31 healthy women during the follicular and luteal phases of their menstrual cycle using nitrous oxide whole body plethysmography. The women were divided into three groups: 12 women were receiving oestrogen-progestogen contraceptive pills, 8 were receiving progestogen-only pills, and 11 were using either alternative methods of contraception or none at all. There was no significant difference in the values of cardiac index, pulmonary arterial distensibility, heart rate or systemic blood pressure during the two phases of the menstrual cycle in any of the three groups. When the three groups were compared the cardiac output showed the most significant difference, being greatest in the oestrogen-progestogen group, the progestogenonly group occupying an intermediate position. An increased cardiac output might play a part in the development of systemic hypertension in some women receiving oral contraceptives and could also add a risk to women with pre-existing heart disease.An increased cardiac output has been reported in women receiving the oestrogen-progestogen contraceptive pill (Walters and Lim, 1969, 1970), while changes have been noted in both plasma and lung capillary volumes and in blood vessel distensibility during the luteal phase of the normal menstrual cycle (Seaton, 1972). We have now studied the pulmonary haemodynamics of women receiving both combined and progestogen-only contraceptive pills using nitrous oxide (N2O) body plethysmography. PATIENTS AND METHODSThirty-one healthy, normotensive women were studied: 11 (group 1) were using either no contraception (5) or methods other than the pill (6); 12 (group 2) were taking combined oestrogen-progestogen contraceptive pills; and 8 (group 3) were taking progestogen-only oral contraceptives (Table I). Patients were recruited from a local family planning clinic with the exception of five control patients who were not using any form of contraception and were recruited from the hospital staff.The three groups were studied concurrently under identical conditions. The subjects were studied on two separate occasions during their menstrual cycle--between 7 and 10 days after, and one to four days before the onset of bleeding-with the exception of four women taking the progestogen-only preparation who had grossly irregular and unpredictable bleeding and were studied at an interval of four weeks.Arterial blood pressure was measured with a mercury sphygmomanometer after the subject had been lying at rest for 10 minutes. Heart rate was determined from the electrocardiogram during plethysmography. N20 BODY PLETHYSMOGRAPHY Readings were taken with the patient in the supine position. All patients were trained to perform the respiratory manoeuvre in an identical manner and to maintain constant air flow during slow exhalation.Pulmonary capillary blood flow was measured by th...