ECSS Paris 2023: CP-PN28
INTRODUCTION: When cerebrovascular carbon dioxide (CO₂) reactivity (CVR) is assessed using the Douglas bag method, ventilation increases gradually and requires time to reach a steady state. We hypothesized that temporal and inter-individual differences in respiratory responses influence CVR estimation, and that accurate assessment requires attainment of a respiratory steady state during CO₂ inhalation. METHODS: To test this hypothesis, we applied an 8-minute CO₂ inhalation protocol (0%, 3.5%, and 5%) and evaluated the effects of chemoreflex-driven ventilatory changes and inter-individual variability on CVR using minute-by-minute analyses. RESULTS: Among 25 participants, 8 (32%) exhibited an overshoot in middle cerebral artery mean blood velocity (MCA Vm) immediately after CO₂ inhalation onset. CVR reached a steady state within 2–3 min on average; however, in overshoot individuals, MCA Vm and end-tidal CO₂ (PETCO₂) fluctuated until 6–7 minutes. The overshoot was associated with central respiratory chemoreflex sensitivity (r = 0.59, P = 0.019), but not with CVR sensitivity, indicating a respiratory origin of CVR variability. CONCLUSION: CVR assessment during the onset of CO₂ inhalation may be over- or underestimated due to inter-individual differences in MCA Vm responses to PETCO₂. Therefore, at least 6–7 minute of CO₂ inhalation is recommended to achieve a steady-state MCA Vm across participants.
Read CV Shigehiko OgohECSS Paris 2023: CP-PN28
INTRODUCTION: In microgravity environments, body fluids shift toward the head and may cause adverse effects such as optic disc edema and venous thrombosis. A -6° head-down tilt (HDT) is widely used as a ground-based model of microgravity. Previous studies suggest central circulatory responses during prolonged HDT exhibit rapid changes in the early phase, followed by re-equilibration. However, temporal changes in cerebral and cervical circulation during this period remain unclear. This study aimed to investigate temporal changes in cerebral and cervical vascular hemodynamics during a 2-hour -6° HDT exposure. METHODS: Thirteen healthy adults (3 females, 27 ± 4 years) participated. Measurements were performed at upright, supine, and at 5, 30, 60, 90, and 120 minutes after -6° HDT onset. Mean middle cerebral artery velocity (MCAV mean) was measured using transcranial Doppler ultrasonography. Cross-sectional area, mean blood flow velocity, and blood flow volume of the internal carotid artery (ICA), external carotid artery (ECA), and bilateral internal jugular veins (IJV) were assessed using ultrasound. Blood pressure, heart rate, end-tidal carbon dioxide pressure, and autonomic nervous system indices were also recorded. RESULTS: Postural changes are described using representative values at upright and HDT 5 min, and temporal changes during HDT are described using representative values at HDT 5 min and HDT 120 min. Postural change from upright to supine and HDT 5 minutes decreased mean arterial pressure (MAP, 90 ± 4 to 84 ± 8 mmHg, p < 0.05) and heart rate (71 ± 6 to 62 ± 9 bpm, p < 0.05), while MCAV mean increased (63.7 ± 11.6 to 67.5 ± 13.7 cm/s, p < 0.05). ICA blood flow volume did not change significantly (260.05 ± 73.26 to 258.27 ± 64.54 mL/min, p = 0.78), whereas ECA blood flow volume increased (153.17 ± 47.75 to 173.00 ± 75.26 mL/min, p < 0.05). In the IJV, cross-sectional area markedly increased (7.73 ± 6.35 to 105.11 ± 56.18 mm2, p < 0.05), while mean blood flow velocity decreased (31.48 ± 12.68 to 10.28 ± 2.52 cm/s, p < 0.05), and blood flow volume increased (161.52 ± 161.75 to 599.98 ± 256.79 mL/min, p = 0.05). During HDT, MAP increased (84 ± 8 to 90 ± 7 mmHg, p < 0.05), and MCAV mean also increased (67.5 ± 13.7 to 70.5 ± 14.2 cm/s, p < 0.05). In contrast, the ICA, ECA, and IJV showed no major temporal changes, and the dilation and low-flow state of the IJV persisted throughout HDT. CONCLUSION: The 2-hour -6° HDT exposure increased cerebral blood flow velocity and induced marked dilation of the internal jugular vein, while arterial inflow through the internal carotid artery remained stable, suggesting that cerebral autoregulation may have been maintained. Although mean arterial pressure and MCAV mean increased during HDT, cervical venous dilation and low-flow state persisted. These findings indicate sustained venous congestion during early simulated microgravity, and the relationship between this venous stasis and a potential increase in thrombotic risk warrants further investigation.
Read CV Ayumu SakuraiECSS Paris 2023: CP-PN28
INTRODUCTION: There has not been confirmed if cerebral blood flow (CBF) in the prefrontal cortex (PFC) would change during exercise according to studies using positron emission tomography (PET). Despite uncertainly, huge number of studies explored cerebral oxygenation in PFC using near-infrared spectroscopy (NIRS) to identify the role of PFC in terms of executive function and affective responses induced by exercise. Since our previous studies using time domain functional NIRS (TD-fNIRS) demonstrated that in PFC cerebral blood volume (CBV) stayed at the same level as the end of moderate-intensity continuous exercise for two minutes after the cessation, we predict that CBF may also increase. The aim of this study is to explore if intensity and duration of exercise affect changes in in CBV both during and after a bout of exercise. METHODS: Fourteen active healthy males (age, 21 ± 1 years; height, 176 ± 7 cm, weight, 71± 10 kg; maximal oxygen consumption [VO2max], 46.8 ± 8.3 mL min-1 kg-1) participated. While all participants performed cycling exercise for 15 min at 60%VO2max (ExM), seven persons performed both ExM and cycling exercise for 4 min at 80%VO2max (ExV). Cardiovascular and respiratory parameters and CBV were measured. TD-fNIRS was used to measure cerebral hemoglobin oxygen saturation (ScO2) and total hemoglobin concentration (HbT), a surrogate of CBV, in both sides of PFC. During ExH and ExV, ratings of perceived exertion (RPE) were monitored. RESULTS: At the end of exercise end-tidal CO2 (PETCO2) increased from rest both in ExM and ExV. RPE at the end was higher in ExV compared with ExM, 14.3 ± 1.4 and 13.1 ± 1.3 (p=0.03). For HbT, there was no interaction effect (module x time, p=0.17, F=1.52). In ExM, HbT increased in the left and right PFC compared with rest from 84.3 ± 9.3 to 94.2 ± 11.7 and from 78.4 ± 19.0 to 86.5 ± 20.1 μmol /L at the end of exercise (p=0.002 and 0.01) and increased to 91.5 ± 9.9 and to 85.9 ± 21.0 μmol /L at 3 min from the cessation of exercise (p= 0.006 and 0.004). In ExV, HbT did not increase at the end of exercise, from 91.6 ± 9.3 to 94.9 ± 11.4 and from 83.2 ± 15.4 to 86.0 ± 17.2 μmol /L in both PFC (p= 0.08 and 0.22) but increased to 95.2 ± 9.5 and 87.5 ± 15.0 μmol /L at 3 min from the cessation of exercise (p= 0.02). ScO2 remained throughout the course. CONCLUSION: While cardiovascular variables returned to near the resting status within two minutes after the cessation of exercise, CBV remained higher than rest for three minutes both in ExM and ExV. As we hypothesized, CBV differed by intensity and duration of exercise. Although CBF cannot be identified only by fNIRS, increases in CBV may be consistent with those in CBF. By dual measurements with TD-fNIRS and PET during exercise, the relationship between CBV and CBF during exercise could be identified and oxygen metabolism in PFC could be calculated with given CBF. This study adds useful information to the protocol of these dual measurements regarding the situation after the cessation of exercise.
Read CV Mikio HiuraECSS Paris 2023: CP-PN28