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A guide to bladder ultrasound

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VOL: 96, ISSUE: 40, PAGE NO: 14

Ray Addison, BSc, RN, FETC, CertHEd, is nurse consultant, bladder and bowel dysfunction, Mayday Healthcare NHS Trust, Croydon.

Residual urine occurs when either a physical or neurological impediment obstructs the flow of urine or prevents the detrusor muscle from contracting effectively. 

Bladder ultrasound is a non-invasive and reliable method to detect this and estimate its volume. If an ultrasound machine is available, this procedure is a standard part of continence assessments and most specialist nurses in urology or continence care perform ultrasound (Addison, 1999). Yet there are no formal guides, procedures or educational programmes on bladder ultrasound for nurses in the UK.

The benefits of bladder ultrasound

In comparison with catheterisation, which is a more conventional method of determining residual volume but has a range of associated risks, bladder ultrasound has a number of benefits. These include:

- Patients do not have to expose their genital areas when a scan is performed, which reduces embarrassment and helps to maintain their dignity;

- No pain or discomfort is associated with having a bladder ultrasound, which minimises patient anxiety;

- Depending on the skill of the operator and the volume of residual urine, it is reliable and accurate;

- Ultrasound is easy to use with children;

- The procedure does not require gloves or other equipment.

Residual urine can also be caused by anxiety about the investigation, so it is important to ensure that patients feel relaxed.

Indications for use

Detecting urinary retention

If patients complain of an inability to pass urine, ultrasound can be used to check for retention. It is also helpful in the routine assessment of men with prostatic symptoms and is good practice in regard to patients who are taking anticholinergic medication, when a bladder ultrasound should be performed to ensure that drug therapy has not induced a voiding problem.

Ultrasound can also be used to identify the level of bladder sensation related to bladder volume to see if it is within normal limits.

Catheters

Bladder ultrasound can be used to:

- Assess the degree of retention before catheterisation and aid any decision to admit a patient to hospital for catheterisation and observation;

- Assess the volume of urine in the bladder if a catheter is failing to drain it. This could indicate that the catheter is blocked or that urine has not been produced;

- Check if a catheter is blocked, which will result in residual urine in the bladder;

- After a trial without a catheter, evaluate whether a patient is able to void and to what degree.

Biofeedback

Ultrasound can be used to help with bladder retraining by providing biofeedback to patients with urge incontinence. They can then be shown their bladder volume in the context of their symptoms.

Pad tests

When performing a pad test as part of a urodynamic investigation, patients need to have a bladder volume of about 300ml. This can be assessed by ultrasound, which means that they may not need to be catheterised so that sterile water can be introduced to ensure that their bladders contain the right amount of liquid before the test begins. The volume of urine can also be stated before the test. This takes less time than catheterisation and is cost-effective, less invasive and poses no risk to patients.

Patients with multiple sclerosis

All patients with multiple sclerosis should have a regular bladder ultrasound at least once a year to monitor residual urine. This is particularly important in the case of patients with progressive MS or after a relapse. Mechanical bladder emptying

Bladder ultrasound can be combined with a symptom profile to assess the need for mechanical bladder emptying.

How to perform a bladder ultrasound

Patient consent must be obtained before the procedure is carried out. Ultrasound is generally safe so there are no specific exclusions, but if the patient has a wound where the scan head is to be placed seek the advice of the local radiology department before proceeding.

The patient should be in the supine position, with the head raised on one pillow and the lower abdomen exposed (Resnick, 1995). There is no need to remove clothing.

Use transmission gel and apply a generous amount on the rounded dome of the scan head (Resnick, 1995). There are several different approaches to assessing residual urine volumes using a bladder ultrasound. These include:

- Ask the patient to void and then perform a bladder ultrasound as soon as possible. Resnick (1995) suggests that this should be done within 10-15 minutes;

- A bladder ultrasound is performed and if any urine is present the patient is asked to void into a jug and this is measured. The difference between the volume in the bladder according to the scan and the volume of voided urine gives the residual volume;

- A bladder ultrasound can be performed before and after voiding to assess the residual volume;

- If you are not sure whether you have pinpointed the patient’s bladder during the scan, additional fluids may highlight the site. Ask the patient to drink extra fluids until he or she has a strong urge to void. A bladder ultrasound is then performed before and after voiding to indicate the residual volume.

At the end of the procedure the scan head is cleaned with an alcohol wipe (Resnick, 1995) and the remaining gel is removed from the patient’s abdomen with a paper towel. The outcome of the scan and any immediate action taken, recommendations and future care must be recorded in the nursing record (Benbow, 1999).

False readings

The following factors may result in false readings when using a bladder ultrasound, depending on the type of machine used:

- Volumes over 1,000ml may not show up and those under 100ml may not be accurate (Ouslander et al, 1994);

- If a Foley catheter is in situ, there is an intravesical mass or the bladder is not a standard shape (Coombes and Millard, 1994);

- If there is a fluid-filled cyst in the bladder area or the scan has been performed with the patient on his or her side;

- Obesity (Resnick, 1995), advanced pregnancy or lower abdominal scarring over the scanning area;

- Possible altered position or shape of the bladder, for example, after surgery or if there are blood clots or stones in the bladder.

Action on residual urine

If residual urine is found an appropriate referral, usually to a consultant urologist, must be considered. But finding residual urine should not be the only factor to influence interventions such as a decision to catheterise. Renal function and lower urinary tract symptoms (LUTS) must be assessed and compared with the results of the scan before any intervention is contemplated (Box 1).

According to the International Continence Society, 100ml or above is residual urine (Andersen et al, 1992) but a residue of 100ml is not likely to be acted on. Volumes of 300ml or more may require intervention, but varying amounts of residual urine may not cause any problems in relation to LUTS and renal function.

Who should perform the scan?

According to the UKCC (1992), only a nurse who is competent in the procedure should perform a bladder ultrasound. Nurses should also ensure that their employers have given their approval. Employers should provide a policy and procedure for bladder ultrasound and should support or provide training for nurses who are not competent in the procedure.

Training should be seen as ongoing as the operators of diagnostic equipment should be regularly audited to ensure that their competence is maintained. A service agreement for the ultrasound machine should be in place to ensure that it is calibrated and accurate in its measurements.

Conclusion

All patients who are to undergo general anaesthetic should have a bladder ultrasound to ensure that they have no voiding difficulties. Any problems could then be identified, investigated and dealt with before surgery.

In patients having a hip replacement, for example, this could help to prevent unnecessary complications in the postoperative period as a result of urine retention and catheter usage. And after the removal of a catheter, particularly after surgery or a general anaesthetic, bladder ultrasound should be routinely used to assess voiding.

With more ultrasound equipment being made available to district nurses and GP surgeries, the procedure should soon become standard nursing practice.

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