(46) EFFECT OF THIRST SENSATION ON HEART RATE VARIABILITY DURING AND AFTER EXERCISE IN THE HEAT, INDEPENDENT OF HYDRATION STATUS

Abstract Details: Heart rate variability (HRV) is altered during exercise due to an imbalance between sympathetic and parasympathetic pathways, resulting in decreased HRV and increased heart rate (HR). Dehydration from exercising in the heat increases cardiovascular strain and modulates autonomic nervous system. Thirst sensation may initiate when an individual is mildly dehydrated, but thirst perception on autonomic function during and after prolonged exercise in the heat is not yet understood.

Purpose: To investigate the HRV response during and after exercise in the heat in response to thirst sensation, independent of hydration status.

Methods: Twelve males (age: 29±12 years; body mass: 74.7±7.9 kg; height: 179.4±7.0 cm; maximal oxygen consumption [VO_2max]: 49.8±6.6 ml·kg⁻¹·min⁻¹) performed steady-state cycling at 55% wVO2max for 90 minutes followed by a 12 km cycling time trial in the heat (ambient temperature: 34.9±0.6 °C; relative humidity: 30.3±0.9%; wind speed: 3.4 m×sec^-1) with two experimental conditions (thirsty [T] or non-thirsty [NT]) in random order. T experienced infusion of 5ml of isotonic saline every 5 minutes via intravenous tube, whereas NT drank 25mL of water every 5 minutes. Urine specific gravity (USG) was measured prior to exercising to ensure euhydration (USG < 1.020). Nude body mass was assessed before and after exercise to measure body mass loss. HRV was assessed before 90 min exercise (PRE) and after (MID), and following the time trial (POST) with electrocardiography. Thirst was measured every 5-minutes using a visual analog scale. Linear mixed effect models with random effects of the participant were used to determine the effects of thirst on HRV. Statistical significance was set at p< 0.05.

Results: Significant interaction of thirst sensation by time point of low frequency/high frequency ratio (LF/HF ratio) was determined. Bonferroni post hoc analyses revealed that T and NT experienced significantly different LF/HF ratio at POST (T; 9.11 ± 7.49%, NT; 5.07 ± 3.73%, p< 0.05). No significant interactions were noted between thirst sensation and time point for other HRV variables, including root mean square of successive differences between normal heart beats (RMSSD), lgRMSSD, LF, HF, standard deviation of NN intervals (SDNN), standard deviation of Poincaré plot perpendicular to (SD1) and along (SD2) the line-of-identity (p >0.05).

Conclusions: These results demonstrated the significant differences between T and NT regarding the LF/HF ratio post-exercise. The LF/HF ratio is indicative of autonomic balance between sympathetic and parasympathetic domains, with an increased ratio indicating reduced parasympathetic activation. Thus, increased thirst sensation alone may increase sympathetic activation after exercising in the heat, independent of hydration status. PRACTICAL APPLICATIONS: Exercising in the heat produces cardiovascular strain due to autonomic imbalance. However, low thirst sensation may attenuate this imbalance, specifically a reduced LF/HF ratio. Athletes and their support teams can structure planned drinking strategies to mitigate feelings of thirst, thereby reducing elevated sympathetic activity following exercise in the heat.

Acknowledgements: None