A 70-year-old man presents to the ED with a complaint of weakness and vague abdominal pain. His vital signs are within normal limits, his physical exam is unremarkable, but his laboratory studies are significant for markedly elevated BUN and creatinine. He begins receiving IV fluids and a bedside ultrasound is performed.
Bedside ultrasound can be helpful to emergency physicians when evaluating the patient with acute kidney injury. This is frequently the result of dehydration or medications, but post-renal obstruction remains an important etiology to consider. A focused ultrasound evaluation of the bilateral kidneys and bladder can quickly assess for signs of post-renal obstruction, enabling the EP to identify and treat a potentially reversible cause of acute kidney injury.
This exam should be performed with a low-frequency transducer to allow for optimum depth of penetration. The right kidney will be found in the anterior axillary line, at approximately the 8th to 11th rib space, while the left kidney will be found in the posterior axillary line, at approximately 6th to 9th rib space. The collecting system will normally appear as a bright hyperechoic (white) area in the center of kidney surrounded by the more hypoechoic (grey) medulla and cortex. (Image 1.) Scanning through both kidneys in the longitudinal and transverse planes will allow for a full assessment.
The collecting system will appear anechoic (black) and distended (Image 2) when obstruction and hydronephrosis are present. The residual hyperechoic rim of the calyx can be seen outlining the area of distention. Medullary pyramids may at times be confused with hydronephrosis, but these structures are typically seen toward the periphery of the kidney and lack the brightly echoic rim of hydronephrosis. The vasculature of the kidney may also be confused with hydronephrosis. Application of color flow to the area in question will rapidly clear up any confusion.
Once both kidneys have been evaluated, the bladder should also be evaluated for distention. The presence of bilateral hydronephrosis strongly suggests obstruction in the pelvis (for example, prostatic hypertrophy), and bladder distention conveys the need for urgent bladder catheterization. The bladder can easily be evaluated in either the transverse or longitudinal plane just superior to the pubic symphysis. (Image 3.) Measurements can be taken to estimate the total bladder volume, but a quick visual assessment will frequently be sufficient to enable the EP to determine whether obstruction is present, particularly in a patient unable to urinate.
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