The dose–response effects of terbutaline on the variability, approximate entropy and fractal dimension of heart rate and blood pressure

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Abstract
Aims To study the dose‐response effects of intravenous terbutaline on the cardiovascular and respiratory autonomic nervous regulation. Methods The study followed a randomized, placebo‐controlled crossover design in six healthy adult volunteers. The terbutaline dose ranged from 10 to 30 μg min−1. We continuously measured electrocardiogram, finger systolic arterial pressure (SAP) and flow‐volume spirometry in supine and upright positions at baseline and during 3 h drug infusion. The periodic variability components of R‐R intervals (time between successive heart beats) and SAP in relation to respiration were assessed using spectral analysis techniques. The regularity of the time series was assessed by approximate entropy (ApEn) and the convolutedness by fractal dimension (FD). Results Terbutaline dose‐dependently decreased total variability of R‐R intervals, low frequency (LF) variability of R‐R intervals (10 s waves), high frequency (HF) variability of R‐R intervals (respiratory variability), total variability of SAP, HF variability of SAP, baroreflex sensitivity, plasma potassium concentration, approximate entropy of R‐R interval and of SAP as well as fractal dimension of R‐R interval. Terbutaline dose‐dependently increased heart rate, LF/HF ratios of R‐R intervals and of SAP, LF variability of SAP, minute ventilation and plasma terbutaline concentration. Conclusions Terbutaline infusion decreases parasympathetic cardiovascular reactivity, baroreflex sensitivity, dimensionality of heart rate and plasma potassium concentration; it increases sympathetic dominance in cardiovascular autonomic balance, minute ventilation, and the regularity of heart rate and blood pressure time series.