The interplay between heated environment and active standing test on cardiovascular autonomic control in healthy individuals

Abstract

Objective. To investigate the interplay between active standing and heat stress on cardiovascular autonomic modulation in healthy individuals. Approach. Blood pressure (BP) and ECG were continuously recorded during 30 min in supine (SUP) and 6 min in orthostatic position (ORT) under thermal reference (TC; ∼24 °C) or heated environment (HOT; ∼36 °C) conditions, in a randomized order. All data collection was performed during the winter and spring seasons when typical outdoor temperatures are ∼23 °C. Spectral analysis was employed by the autoregressive model of R–R and systolic blood pressure (SBP) time series and defined, within each band, in low (LF, 0.04 to 0.15 Hz) and high (0.15–0.40 Hz) frequencies. The indices of cardiac sympathetic (LF) and cardiac parasympathetic (HF) were normalized (nu) dividing each band power by the total power subtracted the very-low component (<0.04 Hz), obtaining the cardiac autonomic balance (LF/HF) modulation. The gain of the relationship between SBP and R–R variabilities within the LF band was utilized for analysis of spontaneous baroreflex sensitivity (alpha index; αLF). Nonlinear analysis was employed through symbolic dynamics of R–R, which provided the percentage of sequences of three heart periods without changes in R–R interval (0V%; cardiac sympathetic modulation) and two significant variations (2UV% and 2LV%; cardiac vagal modulation). Main results. HOT increased 0V% and HR, and decreased αLF and 2UV% during SUP compared to TC. During ORT, HOT provokes a greater increment on HR, LF/HF and 0V%, indexes compared to ORT under TC. Significance. At rest, heat stress influences both autonomic branches, increasing sympathetic and decreasing vagal modulation and spontaneous baroreflex sensitivity. The augmented HR during active standing under heat stress seems to be mediated by a greater increment in cardiac sympathetic modulation, showing an interplay between gravitational and thermal stimulus.

Cardiovascular autonomic modulation during passive heating protocols: a systematic review

“…The axillary (Tax), tympanic (Ttym), and sublingual (Tsub) temperature increased in heated conditions compared to thermal reference conditions (Yamamoto et al 2007). Ferreira et al (2021) found an increment of 1.6 °C in Tskin, and Carrillo and colleagues (2016) reported an increment of ∼0.8 °C in participants' body temperature compared to a thermal neutral condition. One study described that the internal temperature increased up to 36.8 °C and 37.1 °C after 40 and 115 min of the heating protocol, respectively (Brenner et al 1997).…”

“…head-up tilt and active standing tests) were incorporated in our assessment. Indeed, autonomic maneuvers are commonly employed to confirm the physiological meaning of HRV indexes (Montano et al 1994, Rodrigues et al 2019, which seems to clarify the selective contribution of the autonomic nervous system (ANS) to the heart during heat stress (Ferreira et al 2021).…”

“…Baroreflex sensitivity (BRS) can be assessed through spontaneous oscillations of systolic blood pressure and R-R intervals time series (Parati et al 2000). Nonetheless, the heat-stress effect on BRS remains unclear since some studies suggest that heating decreases (Crandall et al 2000, Lee et al 2003, Ferreira et al 2021 or does not change (Kinugasa and Hirayanagi 1999, Yamazaki and Sone 2000 baroreflex control.…”

“…In addition, heart rate variability (HRV) has been used to assess cardiac sympathetic and vagal modulations under heating conditions (Barbic et al 2019, Ferreira et al 2021. The heated environment seems to increase sympathetic modulation and reduce vagal modulation to the heart (Barbic et al 2019, Ferreira et al 2021.…”

“…In addition, heart rate variability (HRV) has been used to assess cardiac sympathetic and vagal modulations under heating conditions (Barbic et al 2019, Ferreira et al 2021. The heated environment seems to increase sympathetic modulation and reduce vagal modulation to the heart (Barbic et al 2019, Ferreira et al 2021. On the other hand, some studies showed no changes in cardiac autonomic control (Kataoka and Yoshida 2005), or an increment of cardiac vagal modulation (Saeki 2000) under whole-body heating and local-heating conditions, respectively.…”