Assessment of Pressure-Volume Index During Lumbar Infusion Study: What Is the Optimal Method?

Assessment of the pressure-volume index (PVI) during lumbar infusion study (LIS) has been proposed to evaluate the overall compliance of the cranio-spinal system. It is calculated from the measurement of CSF pressure changes, ΔP from Pb to Pp, in response to repeated bolus injections of a volume (ΔV) within the lumbar subarachnoid space.

We retrospectively analyzed 18 patients who underwent LIS for suspicion of normal pressure hydrocephalus, including a series of three fast bolus injections of 3 mL of saline at different levels of CSF pressure. We compared two methods for PVI calculation: (a) PVI_slope using the slope α of a linear fit ΔP = α(Pb - P ₀), PVI = ΔV/log₁₀(α + 1); (b) PVI_mean using the PVI calculated independently for each bolus injection assuming P ₀ = 0, PVI = mean(ΔV/log₁₀(Pp_i/Pb_i))_i=1.3.

We found a significant discrepancy between the two methods: the average difference (PVI_slope - PVI_mean) was -3.93 mL (95% confidence interval [8.77; -16.64]). In the PVI_slope, method, the mean P ₀ was 2.12 mmHg (±3.41 mmHg).

The clinical reliability of PVI_mean (assuming P ₀ = 0) depends on the value of P ₀. PVI_slope provides results, independent of P ₀. Future studies should focus on determining pathological PVI range rather than fixed cut-off values.