The use of cardiac autonomic and hemodynamic monitoring as a surrogate marker for sleep stability and performance in austere and extreme environments

Mission success and crewmember performance can be detrimentally affected by the effects of sleep deprivation. These effects are often compounded by the isolation, confinement and the acrimonious psychosocial interactions that can result. Project CASPER (Cardiac Adapted Sleep Parameters Electrocardiogram Recorder) was deployed across a series of extreme operational environments including spaceflight and desert, arctic and sub-aqua analogue habitats to investigate the use of cardiopulmonary coupling (CPC) as a surrogate marker for sleep stability in comparison to a variety of other traditional markers of sleep and circadian rhythm disruption. The two primary components measured on every mission for each crewmember were a single lead electrocardiogram and a subjective sleep diary that was completed both pre- and post-sleep. Project CASPER demonstrated the effective use of cardiopulmonary coupling (CPC) in a variety of extreme operational environments by demonstrating the strong correlation between traditional ECG heart rate variability (HRV) and CPC high frequency components across all missions. In contrast there was a weak correlation between ECG HRV and CPC low frequency components across all missions. This was contributed largely to the noise component in the low frequency band of ECG HRV signal. In an examination of individual nights from all missions, the ECG HRV signal measured 51 nights >0.8 (80%) in the low frequency range. In contrast, the CPC signal measured only 1 night >0.8 (80%) in the low frequency range. Correlation for the CPC signal was also much stronger to responses from the subjective sleep diary questions than traditional ECG HRV. The CPC components of eLFC/DSP, LFC/CAP and HFC/NCAP correlated to 8, 7 and 7 subjective questions respectively, whereas the ECG HRV components of ECG/LF and ECG/HF correlated to only 2 and 3 subjective questions. It is clear that that the CPC signal was far more closely aligned with the subjective feedback of the crews across all environments in regards to their sleep quality, patterns and experiences. The data also strongly indicates that CPC is a valuable and useful tool for monitoring sleep stability in extreme and operational environments. Project CASPER also resulted in the development of a non-contact, non-invasive, state-of-the-art Radio Frequency Impedance Interrogation (RFII) hemodynamic monitor. The chest mounted unit was worn on various missions as a technology demonstrator and in conjunction with traditional ECG to correlate both signals. Both the peak and period of the RFII signal were used for analysis as the signal results from hemodynamic motion as opposed to cardiac autonomic activity. The RFII signal and data correlated significantly in both the low and frequency spectrums against both CPC and traditional ECG HRV. Preliminary research was also completed using the RFII signal for unique subject identification. The results and technology development from Project CASPER and this research create groundbreaking advancements and remove many of the impediments involved in the monitoring of physiology and performance in extreme operational environments ranging from spaceflight to remote habitats.