Acute haemodynamic adaptation to 6 degree head down tilt and head out water immersion

BACKGROUND AND OBJECTIVES: Exposure to microgravity stimulates an almost immediate haemodynamic change within the body due to the loss of a substantial gravitational vector and the resulting cephalad fluid shift. Due to the obvious experimental constraints associated with this habitat, analogues such as 6o head-down tilt (HDT) and water immersion (HOWI) are regularly used to mimic the cardiovascular effects observed in microgravity (Pavy Le-Traon, 2007., Prisk et al, 2002., Navasiolava et al, 2011). The aim of the current study was to compare the cardiovascular effects and time course of adaptation, in an acute 50 minute exposure to both 6o HDT and HOWI. METHODS: 15 healthy male participants (age ± SD:24 ± 2.4 years) underwent 1 hour of HDT and HOWI (34.5-35.5Oc) between 10:00 and 13:00 at Charité Space Medicine Centre, on separate days, in random order. Beat-to-beat cardiovascular parameters were measured during the interventions using bioimpedance cardiography, to obtain heart rate (HR), stroke volume (SV) and cardiac output (CO). Data was averaged during a 10 minute pre-seated phase and every ten minutes during the respective interventions. Environmental parameters were monitored to maintain constant room temperature (24-26.5oC ) and humidity (28-51%). RESULTS: Preliminary data show that HR decreased markedly during the first 10 minutes of both interventions proceeded by a relative plateau during HDT and a continual less pronounced decrease during HOWI (Fig 1A). SV increased in parallel during the initial 10 minute period of each intervention followed by a plateau during the remaining 50 minutes of HDT and a continual increase during HOWI (Fig 1B). There is an increase in CO during the initial 10 minutes of both interventions with the magnitude of increase being greater during HOWI. CO reached a plateau after approximately 30 minutes during HOWI, while CO during HDT showed a slight decrease following the initial 10 minute phase (Fig 1C). CONCLUSIONS: Both analogues stimulate an acute cardiovascular response during the first 10 minutes of exposure, followed by an adaptation. The cardiovascular parameters during HOWI appear to show a greater magnitude of response. It may be postulated that although the time course of adaptation may be similar in both analogues the magnitude of the response may differ. Further research is warranted to elucidate the kinetics of the acute response to HDT and HOWI.