Craniospinal compliance depends on the frequency of volume input

<i>Objective.</i>Craniospinal compliance (CC) refers to the ability to maintain stable intracranial pressure (ICP) given changes in intracranial volume. CC can be calculated directly as the change in intracranial volume over change in ICP (Δ<i>V</i>/ΔICP). Considering the distinct spectral components of the ICP signal, it is pertinent to explore whether compliance is dependent on the frequency at which it is calculated.<i>Approach.</i>Data from 92 hydrocephalus patients who underwent computerized infusion studies was retrospectively analysed. ICP was recorded via lumbar puncture and cerebral blood flow velocity (CBFV) using transcranial Doppler ultrasonography. Compliance was calculated as Δ<i>V</i>/ΔICP, where<i>V</i>is cerebral arterial blood volume (CaBV), estimated by integrating CBFV over time. Compliance was calculated across three ICP wave frequencies: vasogenic<i>B</i>-waves, respiratory<i>R</i>-waves, and pulsatile waves.<i>Main results.</i>Compliances were significantly different (<i>p</i>< 0.001) across frequencies, and moderately correlated (<i>r</i>= 0.52 to<i>r</i>= 0.66), during baseline and plateau phases of the infusion study. Compliance decreased significantly from baseline to plateau (<i>p</i>< 0.001).<i>B</i>-wave CaBV amplitude was significantly higher than all other frequencies during both phases (<i>p</i>< 0.001), while pulsatile ICP amplitude was highest at baseline (<i>p</i>< 0.01), but tied with<i>B</i>-wave ICP amplitude during plateau (<i>p</i>= 0.10).<i>Significance.</i>The results support the notion that compliance is dependent on frequency, with higher compliances at slower frequencies. Where compliance is calculated in a clinical context, in hydrocephalus and traumatic brain injury, frequency should be considered for accurate results. Further research should explore this in a larger cohort, and in additional pathologies.