A practical guide to using bladder ultrasound to diagnose urinary retention
Rigby, D., Housami, F.A. (2009) Using bladder ultrasound to detect urinary retention in patients. Nursing Times; 105: 21
Bladder ultrasound is now considered a safer alternative to catheterisation in the diagnosis of urinary retention. This article outlines how bladder ultrasound works and its practical uses.
Keywords: Bladder ultrasound, Residual urine, Procedure
* This article has been double-blind peer reviewed
Deborah Rigby, MSc, OND, RGN, is programme manager – elective care, Bristol PCT, and was formerly nursing lead, Biomed Centre, Southmead Hospital, Bristol; Fadi Al Housami, MRCS, MD, is research fellow, Bristol Urological Institute, Bristol.
Implications for practice
* Bladder scanning for estimating residual urine is faster than urethral catheterisation, carries a lower risk of infection, costs less and is less uncomfortable for patients.
* To ensure accuracy of bladder screening, staff need to be trained in use of ultrasound and how to interpret the results. At least 10% of images must be audited on an ongoing basis
Urethral catheterisation, an invasive procedure with risks of infection, used to be regarded as the gold standard for measuring residual urine volume. It has now been superseded by an ultrasound scan of the bladder estimating the post-void residual urine (Martin et al, 2006).
The use of portable ultrasound scanners is becoming routine in primary care and acute settings. The benefits include fewer invasive catheterisations and increased patient comfort and satisfaction (Choe et al, 2007; Gilbert, 2005).
How ultrasound works
Ultrasound imaging uses sound waves with a frequency above the upper limit of human hearing. They are pulsed into the body and the echoes can be recorded, measured and used to build a picture of the soft tissues in the body.
For diagnostic purposes, the frequency of ultrasound waves is measured in megahertz (MHz) and scanners used in clinical practice use a range of 2-18MHz. Lower frequencies – for example, a frequency of 3.5MHz, or 3,500,000 ‘compressions’ per second – are used to examine deep structures, such as the kidney and liver. Higher frequencies are used for superficial structures, such as the bladder, breast and muscle.
The ultrasound probe consists of a head with tiny quartz or piezoelectric crystals. When an electric current is applied to these crystals they change shape rapidly, initiating a sound wave.
When a sound wave travels through a tissue or fluid with a uniform density and encounters an interface with a different density, some of that sound wave bounces back as an echo.
Ultrasound works by pulsing a sound wave through the body, measuring the intensity of the returning echo and the time taken to receive the signal, which will be related to its depth.
The returning echo is translated into a greyscale; tissues returning a strong echo are normally visualised at the top of the greyscale (bright white), while a region without echo will be interpreted at the bottom of the greyscale (black).
Scanning the bladder
Modern ultrasound scanners emit ultrasound waves in one plane so the returning echo appears as a cross-section of the organ scanned.
The bladder is usually scanned trans-abdominally, using an ultrasound probe on the abdomen at the suprapubic region, to provide transverse and sagittal views (Fig 1).
A scan shows the bladder as a globe-like structure with the interface between the bladder wall and the urine showing a clear demarcation. The bladder wall has a symmetrical, smooth, gently curved surface. In a transverse scan, its shape can vary from almost circular to square.
In women, the bladder lies in front of and below the uterus and a small volume of residual urine can be confused with the vagina (Fig 2). A sagittal view shows the vagina as a tube-like structure while a transverse scan shows it to be circular. In men, the bladder can be partly obstructed by the pubic bone and it should be viewed at a slightly oblique angle.
Following micturition, no residual urine should be visible and in adults the bladder cannot be easily identified.
Benefits of scanning The use of bladder scanning instead of urethral catheterisation for estimating residual urine has reduced the incidence of urinary tract infections and care costs for hospitalised adults (Patraca, 2005; Moore and Edwards, 1997). It also saves a patient the discomfort of catheterisation and associated risk of developing systemic infection.
Using a bladder scan to measure post-void residual urine volume can reduce the frequency
of catheterisation, and save staff time (Teng et al, 2005).
O’Farrell et al (2001) compared 105 paired ultrasound measurements on 45 patients. The ultrasound assessment changed nursing practice in 51% of the cases; the most common change (32%) was that nurses did not catheterise the patient. Where the ultrasound assessment did not change practice, the most common reason was that it confirmed the need for catheterisation. The authors concluded that bladder ultrasound was an accurate and reliable tool that changed nursing practice in an acute neuroscience unit.
However, not all studies on bladder ultrasound have been positive. Fedorkow et al (2005) compared the pain scores recorded with urethral catheterisation and bladder scan and reported that patients found the bladder scan to be more uncomfortable. They also found that ultrasound consistently underestimated the urine volume in post-operative urogynaecology patients.
The accuracy of the portable bladder scan has also been questioned. Tan et al (2003) suggested the results were not accurate in the presence of other pelvic pathology and Alivizatos et al (2004) reported that scans were not accurate after an increased water load had caused a diuresis.
Caution has also been expressed about the use of automated bladder scanners to identify urine volumes in complex cases in neonatal ICU (Wyneski et al, 2005) and in postnatal women (Mathew et al, 2007).
Training and equipment
A simple residual bladder scan, using a scanner set up for this purpose, requires minimal technical skills but training is required to interpret the results (Lehman and Owen, 2001).
A range of portable bladder scanners are available in the UK, which can broadly be defined as:
* Real-time scanners;
* Volume mode – point and shoot (non-real time);
The latest ‘real-time’ scanners enable the nurse practitioner to freeze the image of the bladder, giving interim and final residual volume readings.
Although these images may reveal potentially serious bladder abnormalities – for example, stones, diverticulae, ovarian cysts and tumours – they are not intended for use as a diagnostic instrument for bladder pathology. In continence management, the bladder scanner’s primary role is to measure pre- and post-void residual urine, thus determining bladder volume and, potentially, identifying incomplete bladder emptying.
If the scan is unclear or inconclusive, or if an abnormality is identified, the patient should be referred for further investigation.
If a portable bladder scanner is to be used, a governance framework must be in place.
A quality assurance programme (QAP) is required to ensure safe practice to protect patients and staff from misdiagnosis and malpractice. This requires a programme of training that provides basic-level skill in the use of ultrasound, leading to clinical competency. Continuing professional development is key to delivering highly effective health care and supporting clinical governance.
Bladder scanning courses run by local specialists are recommended as a minimum training requirement. They should be recognised and accredited by the College of Radiographers and/or other relevant professional bodies and include supervised practice and clinical assessment.
Practitioners should conduct regular audits of their clinical practice and at least 10% of the images/volume measurements should be subjected to external scrutiny every year.
Healthcare professionals should have medical indemnity insurance; this is usually provided by their professional body. All employers must provide liability insurance for all staff for practice in any clinical environment. A comprehensive insurance cover for the scanner and transducers should also be in place in case of damage or theft.
Equipment should be treated with care and stored safely. The battery should always be charged and care should be taken to avoid overheating the scanner or the probe.
A maintenance programme should be in place with the manufacturers. Scanners should be checked and calibrated regularly. A log should be kept of all engineers’ tests, and any action taken.
Contingency plans must be in place in case a scanner is out of commission because of servicing, recalibration or an unforeseen fault.
Box 1. Benefits of ultrasound for measuring residual urine volume
* Gives high-resolution diagnostic images
* Results are available immediately
* Requires little preparation
* Safe to use
* A distended bladder is required for an assessment of the bladder structure but obviously not if the aim is to calculate the residual volume after voiding. The patient should ideally be supine, although, if necessary, the bladder can be viewed from the standing or sitting position.
* The environment in which the patient is to be scanned must be critically assessed, for example for safety and cleanliness. It is important to check that the procedure is appropriate for the patient. For example, the healthcare professional should check whether the patient is pregnant, or whether there is a pacemaker or a latex allergy.
* Patients should be asked for consent to the ultrasound procedure according to local protocol.
* Patients should be given an opportunity to have a chaperone during an ultrasound examination. Local protocols should be referred to.
* A full, written explanation of the procedure should be available to all patients when they book an appointment. Limitations and potential benefits of the examination should be explained.
* Healthcare professionals should provide a full verbal explanation of the procedure and its implications, whenever relevant and at appropriate times. This will allow the patient to comply fully with the examination procedure.
* A reporting protocol should be in place. Healthcare practitioners should have written information on referrals, second opinions, the reporting of results/outcomes to patients and counselling where abnormalities are detected.
* Artefacts, or misrepresentation of the image, is a risk, but an understanding of some of the causes is important for interpretation of the scan.
Nurses should not be afraid to use an ultrasound scanner but instead consider whether its use enhances the care they can provide (see Box 1).
There is a view that bladder ultrasound requires a high level of expertise. This is not the case as long as clinicians are well trained. Real-time images are easy to interpret and provide valuable results.
This evolving technology represents a significant challenge to the nursing profession and we need to ensure we are well trained and supervised to implement the change in practice.
Urethral catheterisation has been superseded by ultrasound scanning of the bladder and there is strong evidence to recommend its use as an alternative to catheterisation (Choe et al, 2007).