Measurement of bladder compliance can be standardized by a dimensionless number: Theoretical perspective

Abstract

OBJECTIVES: To: (i) calculate traditional compliance (CTrad) for a normal bladder by ΔV/ΔPdet, where ΔV is the mean volumetric capacity and ΔPdet is the detrusor pressure rise; (ii) assess its usefulness; (iii) identify the variables that are necessary for correctly assessing bladder compliance; and (iv) using these variables, report a method that, because it includes the effect of patient age, sex and size, correctly assesses compliance for management strategies. METHODS: We obtained the mean volumetric capacity of a normal bladder (Vcap,NL) and the mean detrusor pressure rise (Pcap,NL) on filling a normal bladder to its volumetric capacity from our and other published work; (ii) calculated CTrad for a normal bladder; (ii) showed that the variables necessary for assessing compliance correctly are ΔV, ΔPdet, V cap,NL, and Pcap,NL; and (iii) showed that the relationship among these is the dimensionless number, NWahl-1, calculated as (ΔV/Vcap,NL)/ (ΔP det/Pcap,NL). This value for individuals with a normal bladder was calculated, tabulated and graphed. RESULTS: Because a normal individual's bladder capacity increases with age while the detrusor pressure increase does not, CTrad increases with age and therefore cannot be used for assessing compliance. Published data substantiate our result that CTrad for an individual with a normal bladder varies from 6.3 at 0.5 years old to 90 mL/cmH2O at 18 years old. NWahl-1 correctly assesses bladder compliance because it is the same for all normal cases; consequently NWahl-1 is more practical for clinical use. CONCLUSIONS: Bladder compliance is standardized using ΔV, ΔPdet, Vcap,NL, and Pcap,NL to give NWahl-1, and bladder compliance is usefully reported using this value.