6.2.1 Residual Urine

Post-void residual (PVR) urine is assessed immediately prior to cystometry (usually following uroflowmetry – see Chapters 4 and 5) by a dedicated bladder scanner, conventional ultrasound scanner or via inserting and draining the residual urine through the urethral filling catheter.

The recommendation of draining the PVR urine before cystometry is controversial and many investigators choose to perform the cystometrogram on top of any PVR [3].

6.2.2 Checking for Urinary Tract Infection (UTI)

Cystometry is usually postponed if the patient has a urinary tract infection (UTI) because this could influence the urodynamic findings. A symptomatic UTI may lead to increased bladder sensation and reduced cystometric capacity, and cause urinary incontinence in patients who do not normally experience these symptoms.

Testing a specimen of urine with reagent strips for nitrites and leucocytes can provide a reasonable screening tool in the urodynamics clinic, with a sensitivity of at least 96.4% and a specificity of at least 88.5% [4]. If nitrites and leucocytes are present, there is a strong possibility of a UTI and the cystometrogram should not be carried out. A specimen of urine should be sent for microscopy and any infection should be appropriately treated. If a significant residual urine is noted, this should be managed appropriately.

6.3.1 Initial Pressures – Resting Intravesical and Abdominal Pressures

Gentle flushing of both catheter channels should be performed to ensure that the catheters are not kinked or that the catheter holes are not blocked or in contact with the bladder wall prior to establishing initial resting pressures.

The values for abdominal pressure (p_abd), intravesical pressure (p_ves) and detrusor pressure (p_det) should be compared with the values shown in Table 6.1. The values of p_abd and p_ves will be at the lower end of the range if the patient is small and lying down and at the higher end of the range if the patient is large and standing up. If the values are outside these ranges, then:

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  recheck the set-up before starting to fill;

  
  
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  ensure that the external transducers are positioned at the upper level of the symphysis pubis;

  
  
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  zero external transducers to atmospheric pressure;

  
  
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  check that the pressure lines have not slipped out of position (expelled catheter); and

  
  
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  ensure that the patient is not faecally loaded.

If the problem persists, proceed with the cystometrogram only if you consider that you have a valid explanation for the baseline pressures being outside the expected range.

6.3.2 Checking for Artefacts

Before commencing filling, ask the patient to cough to check that the pressure lines are recording intravesical and abdominal pressures correctly. Ideally, the strength of the cough should induce a pressure increase of about 100 cm H₂O. When the patient coughs, there should be an equal acute rise in both the abdominal and intravesical pressure traces. The detrusor pressure trace, which is derived by subtracting the abdominal pressure trace from the intravesical pressure trace, should show little movement (Figure 6.1) [3].

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  If the rise in intravesical pressure is smaller than the rise in abdominal pressure (Figure 6.2), this indicates a problem with the intravesical pressure recording (see Chapter 9).

  
  
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  If the rise in intravesical pressure is greater than the rise in abdominal pressure (Figure 6.3), this indicates a problem with the abdominal pressure recording (see Chapter 9).

ICS 2016 describes common artefacts that may occur when setting up cystometry. These include:

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  Dead signal – a signal that does not show small pressure fluctuations and does not adequately respond to patient straining, movement or coughing.

  
  
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  Pressure drift – continuous, slow fall or rise in pressure that is physiologically inexplicable.

  
  
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  Poor pressure transmission – that occurs when the cough/effort pressure peaks of the p_ves and p_abd are not nearly equal.

A biphasic artefact on coughing in p_det (Figure 6.4) is caused by small physical differences in the two pressure lines that cause their respective transducers to respond at slightly different times to the impulse of the cough. Provided that the amplitude of the pressure rises in p_abd and p_ves is equivalent, correction is not required and the next phase of the cystometrogram can be directly carried out.

Figure 6.1 Coughing during the cystometrogram showing intravesical pressure (p_ves) and (p_abd) responding correctly

Figure 6.2 Coughing during the cystometrogram showing intravesical pressure (p_ves) not responding correctly

Figure 6.3 Coughing during the cystometrogram showing abdominal pressure (p_abd) not responding correctly

Figure 6.4 Biphasic artefact in detrusor pressure (p_det) arising from timing differences between the recording of intravesical pressure (p_ves) and detrusor pressure (p_abd)

6.3.3 Selecting the Filling Rate

There is a lack of evidence relating to the optimum filling rate. For a neurologically intact adult, current ICS recommendations report that filling rate should be standardised on each individual patient’s typical voided volumes (including an estimation of the PVR value) to prevent too fast filling [1]. A filling rate in ml/min of roughly 10% of the largest voided volume at a constant rate is recommended. For example, if the maximum functional capacity is 500 ml, a filling rate of 50 ml/min is recommended. The initial filling rate may vary between 30 and 100 ml/min.

For neurological patients or if marked detrusor overactivity is suspected, select a less provocative rate, such as 10 ml/min [3]. This gives a better chance of achieving a voiding study at the end of filling cystometry. In paediatrics, the filling rate is calculated based on the child’s age and weight.

6.3.4 Temperature of the Filling Medium

Usually 0.9% physiological saline is used (except in video urodynamics when a radio-opaque dye will be used). Low temperatures can artefactually induce detrusor overactivity, particularly at low bladder volumes. There is no evidence to show that body temperature should be preferred to room temperature. Make sure that the fluid has actually achieved at least normal room temperature (20°C) before instilling it into the patient [4].

6.3.5 Patient Position

Position can influence the outcome of cystometry [6]. It is recommended that cystometry be performed in the vertical position (standing or normally seated) whenever physically possible. The position adopted at the start of cystometry can be dependent on the degree of mobility of the patient.