Test-retest reliability of non-invasive P2/P1 ratio and time to peak at rest and during isometric handgrip stress

PURPOSE: This study aimed to determine the test-retest reliability of the non-invasive P2/P1 ratio, a metric derived from cranial pulsatility waveforms, in healthy young adults under both resting and physiological stress conditions. METHODS: Intracranial pulsatility waveforms were acquired from 58 healthy young adults (mean age 23.4 ± 4.0 years). The protocol involved a 5-minute baseline, 1 min of maximal isometric handgrip effort, and a 5-minute recovery period. This procedure was repeated on three separate days. Relative reliability was evaluated using the Intraclass Correlation Coefficient (ICC), and absolute reliability was assessed with the Standard Error of Measurement (SEM) and the Minimum Detectable Change (MDC). RESULTS: The P2/P1 ratio demonstrated good test-retest reliability across all conditions: baseline (ICC = 0.72), during maximal isometric effort (ICC = 0.74), and recovery (ICC = 0.72). Absolute reliability was high, with a small Standard Error of Measurement (SEM ≤ 0.1) and a Minimum Detectable Change (MDC95) of approximately 0.24 established during the effort. CONCLUSION: The non-invasively measured P2/P1 ratio is a reliable metric in healthy young adults, maintaining its consistency even during significant cardiovascular stress. This study also establishes the MDC, providing a quantitative threshold to distinguish true physiological changes from measurement error. These findings support the use of the P2/P1 ratio for monitoring cerebrovascular dynamics and provide foundational data for future studies in clinical populations, such as critically ill patients.