The 90° head down position test is a valuable laboratory parameter to investigate the consequent alterations of cardiovascular control processes. Here we describe the wavelet analysis of both arterial blood pressure temporal series and heart rate, in human subjects. A practical outcome of this analysis could be related to the predictability of the cardiovascular parameters in subject forced to be in the 90° head down position, such as helpers involved in rescue operations in caves. After an initial 20-min period of bed rest, 8 healthy males subjects, aged 33 ± 5.7 years, were analyzed in the 90° head down position, recording the arterial pressure and the heart rate data for 25 min. According to our past work , the fluctuation within the time domain of the wavelet coefficients, namely, sigma(m), evaluated from the arterial pressure data at some particular frequency scale, m, is a measure of the neuroautonomic adaptive feedback efficiency to internal and external stimuli. In particular, the study  highlights an interesting discrimination at a very low frequency scale (corresponding to about 0.03Hz) between the sigma(m) from healthy subjects and subjects affected by a defect of the neuroautonomic regulation, eventually inducing a phenomenon known as vasovagal syncope. A closely, analogous discrimination can be traced between the sigma(m) values extracted from the blood pressure recorded data belonging to each subject in basal and 90° head down position. This remarkable analogy of sigma(m) behavior in the described cases, allows stimulating speculations on the underlying physiological mechanism responsible for it, such as the variations in the relative weights of sympathetic and parasympathetic regulation. The features of sigma (m) values in head down position could be explained by stronger hydrostatic stimulation on baroreceptors with consequent sympathetic inhibition.
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