The Effect of Body Position Change on Noninvasively Acquired Intracranial Pulse Waves

Andrea Boraschi, Andreas Spiegelberg, Fariba Karimi, Kevin Graf, Arya Fallahi, Esra Neufeld, Niels Kuster, and Vartan Kurtcuoglu, Physiological Measurement 2023, online 13 March 2023; doi: https://doi.org/10.1088/1361-6579/acc3d6

Craniospinal compliance (CC), an important metric for the characterization of space-occupying neurological pathologies, is obtained by means of invasive procedures that carry risks for the patients. Noninvasive methods for acquiring surrogates of CC, most recently based on changes in the head's dielectric properties during the cardiac cycle, have been proposed. Here, we tested whether changes in body position, which are known to influence CC, are reflected in a capacitively acquired signal (hereinafter referred to as W), as measured by the PEM1 device developed by Cephalotec (Horgen, Switzerland), originating from dynamic changes of the head's dielectric properties. The test subjects were 18 healthy volunteers under 30 years of age who, after 10 minutes in a supine position, were tilted head-up (HUT), back to 0° (horizontal, control), and then head-down (HDT). Metrics related to cardiovascular action were extracted from W, including the peak-to-valley amplitude (AMP) of the cardiac modulation of W. Computational electromagnetic simulations were performed to probe the association between intracranial volume change and W. AMP decreased during HUT (0°: 2869±597 arbitrary units (au); +75°: 2307±490 au, P=0.002) and increased during HDT (−30°: 4403±1428 au, P<0.0001). The same behavior was predicted by the electromagnetic model. Tilting affects the distribution of CC between cranial and spinal compartments. Cardiovascular action, which periodically alters the distribution of blood and cerebrospinal fluid (CSF) within the head, induces compliance-dependent oscillatory changes in the intracranial fluid composition and causes corresponding variations in the head's dielectric properties. These manifest as increasing AMP with decreasing intracranial compliance, which suggests that W may contain information related to CC, and that it might be possible to derive CC surrogates therefrom.

This study is part of an ongoing research collaboration between the IT’IS Foundation and the Institute of Physiology of the University of Zurich for establishing head capacitance measurements as a non-invasive diagnostic biomarker for intracranial-compliance-related diseases. The scientific and technical impact of the study can be summarized as:

• Tilt tests were performed on 18 healthy volunteers under 30 years of age to investigate the feasibility of extracting metrics linked to variations in CC from the W signal as measured by the PEM1 device
• The findings of the study suggest that the noninvasively acquired electric signal W may contain information related to CC, and that AMP represents a potential candidate to this end, pointing to the feasibility of extracting metrics linked to variations in CC from the W signal
• The compliance-dependent modulation due to cardiovascular action is affected by tilting, which changes the spatial distribution of CC between the cranial and spinal compartments, as reflected in an increase of AMP with decreasing intracranial compliance when moving from head-up to head-down position during tilting
• The results obtained from computational electromagnetic simulations confirm that the AMP increases with decreasing compliance
• The findings dovetail with our previous computational study, which established the theoretical and practical foundation for maximizing and interpreting the subject-specific information-content of W in terms of diagnostically valuable brain pulsation and motion features