Effects of tocolytic treatment with ritodrine on cardiovascular autonomic regulation

Objective To study the acute effects of tocolytic treatment with intravenou s ritodrine on cardiovascular autonomic regulation. Design Validated methods to assess cardiovascular autonomic nervous functio n-heart rate and blood pressure variability and vagal cardiac baroreflex se nsitivity-were measured before and during ritodrine infusion. Setting Turku University Central Hospital, Turku, Finland. Sample Twelve pregnant women admitted to hospital for threatened preterm la bour. Methods Electrocardiogram and continuous noninvasive finger blood pressure signals were recorded in each woman, resting in a supine position. Autoregr essive spectrum analysis was used to quantify short term heart rate and blo od pressure variability. Vagal cardiac baroreflex sensitivity was measured as the bradycardia response to an intravenous bolus injection of phenylephr ine. Main outcome measures Vagal cardiac baroreflex sensitivity and spectrum ana lysis indices of short term heart rate and blood pressure variability. Results Ritodrine significantly decreased vagal cardiac baroreflex sensitiv ity and as total (0.00-0.40 Hz), low frequency (0.04-0.15 Hz) and high freq uency (0.15-0.04 Hz) power bands of the heart rate variability spectrum. Ri todrine significantly increased mean heart rate and the low frequency quenc y power band of the systolic blood pressure variability spectrum. Conclusions In pregnant women with threatened preterm labour intravenous ad ministration of ritodrine decreases vagal cardiac baroreflex sensitivity an d vagal modulation of heart rate, and increases sympathetically mediated bl ood pressure variability. Decreased baroreflex sensitivity and heart rate v ariability are known to be associated with a poor prognosis in some patient groups, so the effects of ritodrine tocolysis may be unfavourable in women with impaired circulatory homeostasis.