Interstitial fluid shifts in simulated long-haul flights monitored by a miniature ultrasound device

Background: Long-haul flights (LHF) are known to be connected with an increased risk of deep vein thrombosis. There is still some lack of clarity about the impact of hypoxia and lower levels of air pressure and humidity on passengers during flight. This cross-over study researched interstitial fluid shifts traced by measurements of tissue thickness (TT) under controlled simulated conditions of a LHF. Methods: There were 18 male volunteers (28.4 ± 8.1yr) who were subjected to both procedure 1 (altitude: 2500 m/humidity: 15%) and procedure 2 (altitude: 0 m/humidity: 50%), each lasting 8 h. Measurements for TT at tibia (TT-t) and forehead sites (TT-f) were made using a miniature A-mode ultrasonic device. Fluid intake, bodyweight, heart rate, oxygen saturation, and systolic and diastolic blood pressure were recorded over time. Blood samples were collected before (t₀) and after 8 h (t₈) to analyze plasma viscosity (PV). Results: We found an increased TT-t over time at an altitude of 0 m (t₀: 4.8 ± 1.2; t₈: 5.2 ± 1.3) and 2500 m (t₀: 4.7 ± 1.2; t₈: 5.3 ± 1.2), respectively, without differences between study groups. TT-f increased significantly over time at 2500 m (t₀: 4.6 ± 0.8; t₈: 4.9 ± 0.8) with significantly higher TT compared to 0 m at t₈ (4.5 ± 0.7). No further significant differences were found. Conclusion: This study suggests that the controlled variable " sitting position " seems to have the strongest influence on leg edema formation during LHF. PV appears not to be affected as long as passengers receive adequate fluid intake. Further studies should be conducted to investigate different methods of preventing venous congestion. These pre-emptive efforts could be tested easily using a miniature handheld ultrasound device.