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Increasing Abdominal Pressure During Bladder Filling in Pediatric Urodynamics; is the Standard Definition of Pdet Sufficient?
Eric Nelson, MD, Gillian Wolff, MD, Katherine Herbst, M.Sc., Christine Kim, MD, John Makari, MD, Fernando Ferrer, MD, Phillip Smith, MD.
Connecticut Children's Medical Center; University of Connecticut School of Medicine, Hartford, CT, USA.

BACKGROUND:
The necessity of low-pressure urine storage is axiomatic. The conventional calculation of Pdet via Pves-Pabd assumes a constant baseline Pabd during filling. We have observed in our pediatric urodynamic practice many cases in which Pabd gradually rises during filling. By lowering the value of Pdet by standard calculation, storage pressures may be falsely reassuring. We sought to quantify this effect in our academic pediatric urology practice.
METHODS:
Consecutive electronic urodynamic files were reviewed. Age, gender, neurologic status, baseline Pves and Pabd, and Pves and Pabd in the absence of obvious detrusor contraction at the conclusion of leak-free filling, and infused volume between these points were extracted. Pdet at the end of filling was calculated by standard convention, Pves-Pabd. Modified Pdet, the difference in Pves between the end and start of filling was calculated. The difference between standard and modified Pdet was termed Gap. Standard and modified Pdet at the end of filling were compared with t-test. Correlations between age, gender, neurologic status, filling volume and Gap were sought.
RESULTS:
160 studies were reviewed; 2 were excluded due to very low filling volumes resulting from persistent leakage. 74 male, 84 female; 103 had an underlying neurologic diagnosis. Mean age was 9.4 +/- 0.42 yrs, and did not differ by gender or neurologic status. Mean standard and modified Pdet were higher in neurogenic than non-neurogenic bladders. Mean modified Pdet was significantly greater than standard Pdet (p<0.001), with a mean Gap of 9.7 (range 0 - 52) cm.w.
CONCLUSIONS:
Tonic increases in Pabd during filling may result in false depression of Pdet by standard calculation. The rise in Pves over filling provides an alternative determination of Pdet dependent only upon directly measured bladder pressure. The frequency of this effect suggests the use of Pdet by standard calculation in children might result in underestimation of upper tract risk.


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