The lower limit margin: a determinant of autoregulatory sensitivity to changes in cerebral perfusion pressure

The Lower Limit Margin, defined as the difference between the autoregulation-informed optimal cerebral perfusion (CPP) target (CPPopt) and the lower limit of autoregulation (LLA), was recently introduced as a potential dynamic prognostic marker after traumatic brain injury (TBI). Conceptually, CPPopt marks the CPP associated with “optimal” autoregulatory function, while LLA represents the CPP at which cerebrovascular autoregulation is already impaired and deteriorates with further decreases in CPP. Based on the hypothesis that the effect of the Lower Limit Margin on outcome is critically mediated by the level of short-term CPP variability and the associated increase in time spent below the LLA, we aimed to explore the prognostic value of this novel marker. In a prospective cohort of 234 severe TBI patients receiving invasive multimodality monitoring, we evaluated the prognostic and physiological relevance of the Lower Limit Margin. Minute-by-minute CPP, CPPopt, and LLA were derived using automated, validated methods. The association between the Lower Limit Margin with 6-month Glasgow Outcome Scale, short-term CPP variability, and autoregulatory burden (time spent below LLA) was assessed using logistic regression, ordinal analyses, mixed-effects models, causal mediation analysis, and generalized additive modeling. Patients with wider Lower Limit Margins experienced significantly less time with CPP below the LLA (β = − 2.1, CI − 2.2 to − 1.9) and had decreased odds of unfavorable outcome (OR 0.59, CI 0.41–0.83, p = 0.003). Causal mediation analysis indicated that 58.9% effect of the Lower Limit Margin on outcome was mediated by time spent below the LLA. A significant interaction between Lower Limit Margin and short-term CPP variability was observed: a narrow Lower Limit Margin combined with high short-term CPP variability were associated with the worst outcomes and higher amount of time spent with CPP below LLA. Nearly 50% of hourly short-term CPP variability exceeded ± 5 mmHg. Our findings demonstrate that a narrow Lower Limit Margin is a marker of increased physiological vulnerability, associated with more frequent and severe CPP insults and worse neurological outcomes after TBI. These results support a shift away from single-threshold strategies (e.g., CPPopt or LLA alone) toward a resilience-informed approach that integrates the dynamic interplay between autoregulatory reserve and short-term CPP variability.