Decreased maximum flow rate during intubated flow is not only due to urethral catheter in situ

Abstract

Introduction and hypothesis

We used the Valentini–Besson–Nelson (VBN) mathematical micturition model to analyze the potential obstructive effect of a 7-F transurethral catheter on the voiding process during intubated flow (IF) in women. Our hypothesis was that incomplete sphincter relaxation leads to residual sphincter pressure.

Methods

We reviewed a urodynamic database of women referred for evaluation of lower urinary tract dysfunction. Exclusion criteria were neurological disease or grade ≥2 prolapse. Eligible women underwent free uroflow (FF-1) before cystometry, an IF (7-F urethral catheter), and a second FF (FF-2) at the end of the session. Interpreted flows were restricted to voided volumes ≥100 ml and continuous flow patterns. Analysis of FF and IF was made using the VBN model.

Results

Among 472 women, 157 met the inclusion criteria. The effect of the urethral catheter was geometric only in 60 (38.2 %) patients. An additional effect, identified as incomplete sphincter relaxation, was observed in 97 (61.9 %) patients. Among this second group, the same residual sphincter excitation was found for 30 (30.97 %) patients during FF-2.

Conclusion

When comparing IF with FF with the VBN model, the decrease in maximum flow rate (Q_max) did not appear to result only from the geometric effect of the catheter but from incomplete sphincter relaxation during voiding, possibly because of patient’s anxiety or a urethral reflex induced by the presence of the catheter. These findings emphasize the need to perform an FF before the IF to strengthen the reliability of conclusions of a urodynamic investigation.

Appendices

Appendix 1

The Valentini–Besson–Nelson (VBN) mathematical model [16] is a quantitative description of the mechanistic phenomena governing micturition; these phenomena are bladder contractility, elasticity, and viscoelasticity [20]; urethral elasticity [21]; urethral compression by the sphincter; and turbulent incompressible fluid hydrodynamics [22]. Each phenomenon considered separately is accurately described in previous reports and can easily be studied. However, when combined, as during voiding, they constitute such an intricate set that to analyze individual recorded voiding and reconstruct the possible causes of dysfunction (e.g., compressive or constrictive urethral obstruction), elaborate software is needed, e.g., VBN® software. Upon gender and initial bladder volume being entered, the VBN® software allows voiding curves to be computed: flow rate and detrusor pressure vs. time. Two parameters describe the status of the urethra. The first, σ, characterized its effective cross-sectional area: a constrictive obstruction (a stricture) is characterized by σ < 1 and a gaping by σ > 1; the second parameter, γ, describes a local compressive obstruction exerted on the urethra (similar to the effect of an enlarged prostate in men). Parameter σ is without unit (multiplicative coefficient of the normal value); parameter γ represents a pressure in which the unit is centimeters of H₂O. Detrusor force is characterized by a detrusor force parameter k (a normal detrusor is associated with k = 1). Any voiding depends on both urethral and detrusor parameters and on possible circumstantial parameters (fading detrusor excitation, delayed sphincter opening or incomplete relaxation…).

Appendix 2

To analyze the geometric obstructive effect of a 7-F urethral catheter, theoretical computations were performed, using the Valentini–Besson–Nelson (VBN) model [16] in well-defined conditions. Calculations were made first for a normal urethra (no compression or constriction) (Fig. 4, left) and second for a urethra with an external compression of 15 cm H₂O (Fig. 4 right). In each situation, two values of detrusor contractility were tested: k = 1 (normal detrusor) and k = 0.3 (hypocontractile detrusor). The range of initial bladder volumes was 100–600 ml.

Fig. 4

Theoretical computations of the effect of a 7-F urethral catheter on maximum flow rate vs. initial bladder volume. Left normal urethra, normal detrusor contractility (k = 1), and decreased detrusor contractility (k = 0.3). Right urethral compression of 15 cm H₂O, normal detrusor contractility (k = 1), and decreased detrusor contractility (k = 0.3)

For V_ini = 400 ml (near the mean value in our study), the maximum decrease of Q_max was 3.4 ml/s; it was observed for a normal urethra and a normal detrusor. In all other cases, the decrease was lower.