182 | Chapter 7 Figure 2. Overview of inter- and intrapatient variability of device lifetime of regular VPs in weeks. This is a representative sample of twenty regular VP-users (VPG1) For regular VP–users (VPG1) the PVPR showed that patients would need 6 additional VPs replacements per year. The median used 3.4 VPs (range 0.1-48.1), and when applying PVPR 9.4 (range 8.9-48.1).For the indication TL subgroups, in PVPR, the median number of VPs was 8.6 (range 8.1-48.1) for PG1 and 9.7 (range 9.4-21.9) for PG2, but the additional number of VPs per year was 6.1 in both subgroups. PVPR in an individual adaptive model As described under methods the timing of prophylactic replacement is chosen to prevent 70% of leakages under assumption that the lifetime of the next VP is similar to that of the previous three VPs in the same patients. In our data we found a high intra-patient variability. We calculated the Coefficient of Variation (CV = SD/mean) and found a median of 0.8 (range 0.02 – 3.01). Due to this high intra-patient variability and the fact that the first three VPs will not be prophylactically replaced, in practice the prevention of 70% of leakage events is not reached in most study patients. Only nine out of 194 patients (4.6%) reached 70% prevented leakages. The median percentage leakage prevented by this method is 36% (range 0 – 81%). In the subgroup VPG1 of patients with only regular VPs the numbers are even worse: Median 25% of leakages prevented (range 81% - 100%).The patients in whom 70% of the leakage events could be prevented were patients with a high number of used VPs (range 21 – 57) during the study period.
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