Estimation of the lag time between detrusor pressure- and flow rate-signals

In a urodynamic measurement setup there is a considerable spatial separation between the uroflowmeter and the location where the detrusor pressure is measured. Therefore, a "time shift" (or lag time correction) has to be applied to one of these signals in order to align related samples in studies where pressure and flow rate are considered simultaneously (e.g., assessment of bladder contractility or bladder outlet resistance). Currently, a heuristic value for this time shift of 0.8 s is applied. In this article, we present a method to estimate the lag time directly from the measurements. Using this method we have found, amongst others, that the mean lag time in our clinic is 0.6 s for males, 0.4 s for females voiding in sitting position, and 1.1 s for females voiding in standing position using a special receptacle in video urodynamics. Furthermore, we found that sphincter/urethral activity during voiding (which causes a drop in flow rate and an accompanying increase in detrusor pressure) is associated (on average) with shorter lag times than straining (when a positive pressure rise accompanies an increase in flow rate). Additionally strong evidence is provided that lag time correction is not a major source of error in urodynamics.