Measurement of bladder volume is quick and easy to perform. Point-of-care machines will usually have a built in bladder volume calculation.
With the patient supine place your transducer transversely just above the pubic symphysis. If the bladder is relatively empty you may need to angle the probe down into the pelvis. Once you have identified the bladder fan the probe back and forth until you find the largest cross-sectional area of the bladder and freeze the image.
Enter the calculation function of you machine and select bladder volume. You will find three measurements which need to be taken, length, width and height. These are multiplied together to calculate the volume.
Select width and place calipers across the maximum width of the bladder. Save the measurement. It is also good practice to save the image to demonstrate what you have measured.
Rotate the probe into a sagittal position and obtain a view of the bladder in long section.
Make sure you fan from side to side to ensure you identify the largest cross sectional area of the bladder and freeze the image.
Complete the other two measurements required for the calculation. Place one set of calipers to measure the size of the bladder from left to right and one set from top to bottom of the image.
Once the calipers have been positioned the bladder volume will be shown on the screen; at the top left corner in this case. Save the image.
A common error is to measure the bladder from top to bottom in both transverse and longitudinal view. You should ensure you take only one measurement from top to bottom of the screen and two measurement from left to right, one on each view.
If the patient is catheterised check you can see the catheter ballon within the bladder. If the bladder contains urine spotting the balloon is easy, it will appear as a round structure and you will often see the catheter extending from it. If you can’t see the ballon within the bladder this may be the cause of the bladder containing urine.
Why not just use a bladder scanner?
Bladder scanners have been show to produce an estimate of bladder volume which is highly correlated with the bladder volume measured by catheterisation (summary correlation coefficient r = 0.93 (95% CI, 0.91-0.95))1. The key advantage of a bladder scanner is that they can be used by staff with minimal training. So why might you want to use an ultrasound machine instead?
- You might be using the ultrasound to assess other areas such as the kidneys, or as part of an assessment of undifferentiated shock.
- You might need to check the location of a catheter.
- Bladder scanners can be inaccurate in some situations.
A bladder scanner automatically assesses the volume of a fluid filled space and assumes this is the bladder, based on where the transducer is placed. Fluid with the pelvis but outside the bladder, such as with ascites may cause confusion. Similarly, fluid filled structures such as large ovarian cysts or large renal cysts may mislead the scanner2. The video below is a good illustration of when an ultrasound scan will be more use.
- Does this man with lower urinary tract symptoms have bladder outlet obstruction?: The Rational Clinical Examination: a systematic review. D’Silva KA, Dahm P, Wong CL. JAMA. 2014 Aug 6;312(5):535-42. doi: 10.1001/jama.2014.5555.
- Ultrasound bladder scanner presents falsely elevated postvoid residual volumes. Alagiakrishnan K, Valpreda M. Can Fam Physician. 2009 Feb;55(2):163-4.