We assessed the influence of tidal volume on RSA in 11 healthy male adult volunteers (mean age ± SEM, 20.0 ± 0.5 years range 18–22 years weight 59.5 ± 1.9 kg height 170.8 ± 1.5 cm). We assessed RSA and baroreflex sensitivity in 12 healthy adult volunteers (mean age ± SEM, 20.3 ± 0.3 years range 19–22 years weight 54.3 ± 2.0 kg height 164.3 ± 2.0 cm male and female, n = 6 each).Įxperiment 2. We tested this hypothesis by investigating the effects on heart period fluctuation and baroreflex sensitivity with and without upper torso flexion in Fowler’s position on the autonomic regulation of cardiovascular function.Įxperiment 1. We speculated that the distribution of blood volume does not predominantly change because most body segment positions remain unaltered regardless of upper torso flexion in Fowler’s position, whereas TV increases with upper torso flexion and consequently enhances cardiovagal regulation. The physiological influences of various positions should be understood to improve patient care in the clinical setting. However, to our knowledge, the effects of slight postural differences in Fowler’s position on cardiovascular regulation and hemodynamics have not been investigated. Moreover, a cross-sectional study of hypertensive patients found the same tendency. A study of young healthy individuals has shown that blood pressure values in Fowler’s position are intermediate between the seated and supine positions. reported that cardiac output is decreased in Fowler’s, compared with the supine position among patients in intensive care. Some studies have described a relationship between the angle of Fowler’s position and the accuracy of hemodynamic measurements among patients in intensive care units, and Driscoll et al. Fowler’s position is clinically applied most frequently at inclinations between 30° and 60°. It is frequently used instead of the supine position to monitor hemodynamics and facilitate breathing and daily activities such as eating or conversation in frail patients. The Fowler’s position is achieved by inclining the backrest of a bed upwards from the supine position with flexed or straight knees. The Fowler’s or semi-seated position as well as the standing and supine positions are often clinically applied. In addition, an increase in TV might enhance the vagal modulation and the cardiovagal baroreflex. Moreover, tidal volume (TV) is significantly increased in tilted, compared with the supine position via the activation of vestibular reflex and lung–thoracic wall afferents. The normal response to the above is the autonomic regulation of cardiovascular function according to changes in the distribution of blood volume. These facts were determined by analyzing heart periods, blood pressure fluctuations, and muscle sympathetic nerve activity. Conversely, sympathetic nerve activity decreases in the supine position, whereas vagus nerve activity, vagal modulation, and vagal baroreflex sensitivity increase. Generally, standing or tilting the head upwards increases sympathetic nerve activity and decreases vagus nerve activity, vagal modulation, and vagal baroreflex sensitivity. Studies on autonomic regulation of the cardiovascular system in humans with the head tilted up (HUT) and down (HDT) have found that posture influences the autonomic nervous system. These findings suggested that slight flexion of the upper torso in Fowler’s position activates respiratory function and increases the contribution of vagal nerve activity to the cardiovascular system in young participants under conditions of a fixed respiratory rate. The results of Experiment 2 showed that RSA, RSA-TF, sBRS, TV, and LV spectrum were significantly higher at 60° than at 30°, and that RRI, SBP, DBP, and the RSATF phase did not significantly differ under any condition. The results of Experiment 1 showed significantly higher RSA and sBRS at 60° and 45° than at 30°, whereas RR interval (RRI), systolic blood pressure (SBP), and diastolic blood pressure (DBP) did not differ significantly under any condition. The two positions in Experiment 2 were 30° and 60° of upper torso backrest inclination with a lower torso inclination of 30° throughout all evaluations. The three positions in Experiment 1 were 30°, 45°, and 60° of upper torso inclination with a lower torso inclination of 30° throughout all evaluations. All participants maintained respiratory frequency at 15 breaths/min. We also measured RSA, sBRS, tidal volume (TV), lung volume spectrum (LV spectrum), and transfer gain and phase between lung volume and RR interval (RSA-TF, RSATF-phase) in 11 healthy individuals in two positions (Experiment 2). Respiratory sinus arrhythmia (RSA) and sequence baroreflex sensitivity (sBRS) were measured in 12 healthy individuals in three positions (Experiment 1). The present study investigates autonomic cardiovascular regulation during postural changes while in Fowler’s position.
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