Date of Award

1-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biomedical Engineering

First Advisor

Kouhyar Tavakolian

Abstract

Aerospace environments are physically and mentally demanding. Commercial pilots, as well as astronauts during spaceflight, experience an increasing variety of task workloads. Pilots can become fatigued, leading to human error, which is the main factor in airline accidents. Similarly, astronauts during extravehicular activity (EVA) can become fatigued, which could lead to overexertion compiled with other serious injuries. Objective measures such as cardiovascular and thermal metrics have been shown to respond to changes in stress, however, in-flight physiological measures are lacking. Both cardiovascular and thermoregulation measures can help predict early warning signs of stress due to in-flight workloads. Cardiovascular reactions fluctuate during autonomic nervous system (ANS) responses to stress, varying heart rate, and shifting blood pressure through the baroreflex. Additionally, thermoregulatory fluctuations respond to ANS activity via the hypothalamus initiating effector responses such as vasoconstriction or vasodilation. The objective of this dissertation is to ascertain the interconnection between cardiovascular and thermal responses focused on vasoactivity in stress-induced flight-like environments. First, heart rate variability (HRV) metrics were identified to determine stress responses of pilots’ tasks during flight operations. Further to define flight-like physiology responses, cardiovascular timing intervals were identified using Seismocardiography and blood pressure during prolonged head-down tilt bed rest (HDBR). Building on these metrics, historic EVA heart rate, and metabolic rates were analyzed to develop a regression model providing minute-by-minute workload determination. Techniques were further developed as a cardiothermal model in a novel approach to determine spacesuit workloads using heart rates, metabolic demand, HRV metrics, and thermal inputs. During in-flight operations, short-term HRV metrics were found to determine low, medium, and high workloads attributed to stress. Further, during prolonged HDBR, cardiovascular timing intervals decreased similar to spaceflight attributed to fluid shifts. The development of regression models showed linearity predicting energy expenditure from heart rates during historic microgravity EVA. The final cardiothermal model predicted metabolic rates, core temperature, and mean skin temperature during simulated Lunar EVA in the NASA Active Response Gravity Offload System in the Mark III spacesuit. Developed objective cardiothermal metrics show implications of identifying crew stress responses and fatigue states through modeling approaches during flight.

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