Skip to main content
Medicine LibreTexts

4.1: Essential Urodynamics

  • Page ID
    15584
  • Urodynamics

    Whole books have been written on Urodynamic practice and technique. The diagnosis in women with urinary incontinence based on clinical findings is correct in only 65% of cases. There is a large overlap between symptoms and examination and urodynamic findings. 55% of women with stress incontinence will have a mixed picture. The cystometrogram becomes essential, in a number of women, to enhance diagnostic accuracy and therefore enable us to institute treatment.


    The equipment

    The Urodynamics system comprises two catheters, one placed in the bladder and another in the rectum, a computer and the urodynamics software and pressure transducers, a pump system, and a flowmeter. The catheter that is placed in the bladder is has a double lumen, one to measure the bladder pressure (Pves) and the other lumen is used to fill the bladder with water via the pump system. Sometimes, two separate catheters are used for filling and pressure recording. The rectal probe measures the intra-abdominal pressure (Pabd) and this pressure could therefore also be obtained by inserting the line into the vagina or even into a colostomy. A Urodynamic report usually gives 3 pressure tracings: Pves (bladder pressure), Pabd (abdominal) and Pdet (detrusor pressure). The detrusor pressure is obtained by the following formula Pdet = Pves-Pabd. Urodynamics is therefore often called Subtracted Cystometry.

    The Procedure

    The test comprises three phases.

    1. Free flow phase

    The woman is asked to arrive at the investigation with a full bladder. She is then asked to void on the flowmeter, which is usually mounted on a commode, in privacy. It should be noted that this part of the test differs from the voiding cystometry, which is done after the filling phase once the bladder is full and the lines are in situ to measure the pressures.

    clipboard_e34485d9b01385ee5538aed73a3e469ae.png

    Figure \(\PageIndex{1}\): Flow Meter Commode

    2. Filling phase

    The bladder and rectal lines are inserted with the patient supine and any urinary residual is noted. The lines are flushed and the system is zeroed. The women is asked to cough to check that the Pdet measurement is correct. For example, if the Pabd is not measuring correctly, the Pdet will not be accurately calculated. If both the vesical and rectal lines are measuring appropriately, when the women coughs, there should be no deviation of the Pdet – only on the vesical line and the abdominal line since these are both under the influence of abdominal pressure. In other words, when there is a rise in abdominal pressure with coughing, the same pressure is transferred to the bladder. The Pdet will therefore be zero since Pves minus Pabd is zero and the detrusor line will be flat with deviations only in the Pabd and Pves.
    Bladder filling is commenced once the operator is satisfied that the tracing is technically correct. The patient is asked to report on her first desire as well as the moment she has a strong desire to void. Any urgency and associated incontinence is noted. Provocative measures through the filling phase include asking the woman to heel bounce, wash hands and cough. This will also hopefully elicit any stress incontinence which is usually also occasionally recorded on the trace by a flowmeter but if this modality is not available on the filling phase, is usually observed by visual inspection of the vulva. When the patient is unable to tolerate any more filling, the pump is stopped, this is the maximum cystometric capacity.

    3. Voiding Cystometry

    This is done by asking the patient to void while the pressures are recorded.

    Possible Diagnoses

    During Free Flow

    Flow rate is abbreviated as Q. A normal flow curve is bell-shaped. An obstructive pattern is flat or with intermittent sections of flow. The maximum flow is denoted as Qmax. A normal flow rate is defined as less than I5ml/s.


    During Filling phase

    Any contractions of the detrusor tracing suggest a diagnosis of detrusor overactivity (DO). One should always look at the abdominal tracing and this should be flat during a detrusor contraction to diagnose DO. If the abdominal curve is also elevated, this would suggest possible poor subtraction and a diagnosis of DO should not be made. If the Detrusor pressure curve rises slowly during the filling phase, this would suggest poor compliance. If one notes both stress incontinence and DO during filling, a diagnosis of mixed incontinence is made.


    During voiding Cystometery

    Pressures are measured during the voiding cystometry phase and therefore parameters such as PdetQmax, the detrusor pressure during maximum flow, is measured. A pressure greater than 20cmH2O would suggest an obstruction.

    • Was this article helpful?