gms | German Medical Science

Objective: The pressure-volume index (PVI) elegantly estimates a volume that is supposed to decouple some intracranial pressure. Calculating PVI is simple but results suffer from robustness, because error propagation is remarkable when using pressure readings from real-world data.

Methods: To evaluate errors in PVI calculation, a random sample of 282 routine volume-pressure testing procedures from patients with suspected CSF circulatory disorder was analysed. To simulate the influence of the main error source, i.e. the measuring chain, maximum PVI error was computed at different levels of pressure measurement error (PME, 0.5…1.5, 0.1 mmHg). By weighing syringe loadings of 1, 2, 4, and 8 ml, the accuracy of test volumes was determined bysubtracting nominal and measured values.

Results: For volumes used in diagnostic volume-pressure testing the median maximum PVI error is reciprocal when PME was reasonably set to 1 mmHg: 16.3 ml (1 ml), 12.0 ml (2 ml), 5.9 ml (4 ml, p<0.05), and 1.7 ml (8 ml, p<0.01). Most dense halves (minimum range of 50% of data) were found ranging below medians. Maximum PVI error converged to a minimum when pressure responses increase and baseline pressures decrease. Discrepancies from nominal volume were irrelevant in error computations.

Conclusions: It is amazing that such an important cerebrospinal property was assumed to be error-free. With the exception of 8 ml test volume, the inherent errors qualify the pressure-volume index to be an unstable parameter estimate at least in these patients. Alternatively, cerebrospinal elastance could be estimated by Miller’s volume-pressure response (VPR), which has the advantage of a constant maximum error being two times the PME. Both methods’ selectivity benefits from distinct pressure responses, which implies higher test volumes that were not arbitrarily applicable in patients. Hence, especially the PVI has to be taken carefully into cerebrospinal parameter considerations.