58 CHAPTER 2 Urological investigations

Post-void residual urine volume measurement

Post-void residual urine (PVR) volume is the volume of urine remaining in the bladder at the end of micturition. In normal individuals there should be no urine remaining in the bladder at the end of micturition.

A PVR may be caused by detrusor underactivity (due to aging, as the older bladder is less able to sustain a contraction than the younger bladder, or neurological disease affecting bladder innervation), bladder outlet obstruction, or a combination of both.

In clinical practice, PVR volume is measured by ultrasound after the patient has attempted to empty their bladder. A commonly used formula for calculating bladder volume is

Bladder volume (mL) = bladder height (cm) xwidth (cm) x depth (cm) x0.7

Interpretation and misinterpretation of PVR volume

PVR volume shows considerable day-to-day variability, with volumes recorded on different days over a 3-month period varying between 150 and 670 mL.1

Clinical usefulness of PVR volume measurement

PVR volume measurement cannot predict symptomatic outcome from TURP. For these reasons, residual urine volume measurement is regarded as an optional test in the AUA guidelines, but is recommended by the Fourth International Consultation on BPH.2

Residual urine volume measurement is useful (along with measurement of serum creatinine) as a safety measure. It indicates the likelihood of back pressure on the kidneys and thus it tells the urologist whether it is safe to offer watchful waiting rather than TURP.

In men with moderate LUTS it is safe not to operate when the PVR volume is <350 mL, and this probably holds true for those with higher PVR volumes (<700 mL).3

Does an elevated residual urine volume predispose to urinary infection?

Though intuition would suggest yes, what evidence there is relating residual volume to urine infection suggests that an elevated residual urine may not, at least in the neurologically normal adult, predispose to urine infection.4,5

1 Dunsmuir WD, Feneley M, Corry DA, et al. (1996). The day-to-day variation (test–retest reliability) of residual urine measurement. Br J Urol 77:192–193.

2 Denis L (Ed.) (1997). Fourth International Consultation on Benign Prostatic Hyperplasisa (BPH),

Paris, 1997.

3 Bates TS, Sugiono M, James ED, et al. (2003). Is the conservative management of chronic retention in men ever justified? Br J Urol Int 92:581–583.

4 Riehmann M, Goetzmann B, Langer E, et al. (1994). Risk factor for bacteriuria in men. Urology 43:617–620.

5 Hampson SJ, Noble JG, Rickards D, Milroy EG (1992). Does residual urine predispose to urinary tract infection. Br J Urol 70:506–508.

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60 CHAPTER 2 Urological investigations

Cystometry, pressure-flow studies, and videocystometry

•Cystometry is the recording of bladder pressure during bladder filling.

•Pressure-flow study (PFS) is the simultaneous recording of bladder pressure during voiding.

•Videocystometry is fluoroscopy (X-ray screening) combined with PFS during voiding (see Fig. 2.8).

These techniques provide the most precise measurements of bladder and urethral sphincter behavior during bladder filling and during voiding. Cystometry precedes the pressure-flow study.

Bladder pressure (Pves, measured by a urethral or suprapubic catheter) and abdominal pressure (Pabd, measured by a pressure line inserted into the rectum) are recorded as the bladder fills (cystometric phase) and empties (voiding phase), and flow rate is simultaneously measured during the voiding phase.

The pressure developed by the detrusor (the bladder muscle), Pdet, cannot be directly measured, but it can be derived by subtracting abdominal pressure from the pressure measured within the bladder (the intravesical pressure). This allows the effect of rises in intra-abdominal pressure caused by coughing or straining to be subtracted from the total (intravesical) pressure, so that a pure detrusor pressure is obtained.

All pressures are recorded in cm H2O and flow rate is measured in mL/s. The pressure lines are small-bore, fluid-filled catheters attached to an external pressure transducer, or catheter-tip pressure transducers can be used.

A computerized printout of intravesical pressure (Pves), intra-abdom-

inal pressure (Pabd), and detrusor pressure (Pdet) and flow rate (Qmax) is obtained (Fig. 2.16).

During bladder filling, the presence of overactive bladder contractions

can be detected. During voiding, the key parameters are Qmax and the detrusor pressure at the point at which Qmax is reached, Pdet Qmax. This

pressure, relative to Qmax, can be used to define the presence of bladder outlet obstruction by using a variety of nomograms, of which the ICS nomogram is most widely used.

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