The main questions being asked of diagnostic imaging in the critically ill patient with impaired renal function are as follows. What size are the kidneys? Can obstruction be detected or excluded with confidence? The size of the kidneys and the depth of the parenchyma will give an indication of either acute, or acute on chronic, renal failure. Ultrasound will provide this information with simple accuracy. Since the apparatus is mobile, the examination can be performed in the intensive care unit (ICU).
The exclusion of obstruction is usually, but not always, straightforward and again ultrasound is the first imaging tool to use. If the collecting system is dilated because of obstruction, it will show up clearly as an echo-free area within the parenchyma ( Fig 1). However, an acute obstruction may present with only mild dilatation, particularly when there is a small intrarenal pelvis. Whenever there is the slightest dilatation of the pelvicalyceal system in a patient with impaired renal function, obstruction must be suspected until proved otherwise. Conversely, renal pelvic cysts can easily mimic dilated pelvicalyceal systems and obstruction can be suggested by the ultrasound image (Fig 2). The limitation of ultrasound is its operator dependence. The correct settings for any given patient (obese or thin) are vital, and interpretation of the real-time images can be quite taxing.
Fig. 1 Moderately severe pelvicalyceal dilatation on sonography. The dilated renal pelvis and calyces are represented by the central echo-free (fluid-containing) zone with separation of the high echoes from the echogenic renal sinus (fat-containing). The renal parenchyma has intermediate echoes.
Fig. 2 The central echo-free zone in this case is caused by pelvic cysts and not a dilated renal pelvis or calyces. Renal cysts are an infrequent cause of a false-positive diagnosis of renal obstruction on ultrasound.
Spectral Doppler ultrasound studies of obstructed kidneys were originally pursued enthusiastically. The resistive index
^ _ peak systolic — minimal diastolic calculated from the velocity waveform of an intrarenal artery, was reported to exceed 0.7 if obstruction was present; however, a more recent study achieved a sensitivity of only 44 per cent in patients with colic (Tubin e.L&L 1994). Doppler study is not always practical in the critically ill, and these more sophisticated machines are not really mobile.
Patients with solitary kidneys (congenital or previous nephrectomy) can have many causes of obstruction which will produce renal failure. The most common cause is calculus disease, and ultrasound may detect renal pelvic stones or stones in the proximal few centimeters of the ureter. Distal ureteric stones lodged at the ureteric orifice can be seen by scanning through the bladder. In any case a plain abdominal radiograph (portable in the ICU if necessary) will be required to exclude mid-ureteric stones, which are easily missed by ultrasound because of overlying bowel.
Echogenic calculus in an obstructed pelvicalceal system. Note the typical acoustic shadow.
Patients presenting with renal failure due to bilateral renal obstruction will probably have pelvic or retroperitoneal pathology. It may be important to discover whether pelvic malignancy is the culprit and an urgent CT scan may be indicated because of its superior resolution of the retroperitoneum.
Currently, biopsy is almost always necessary in renal parenchymal disease to reach a specific tissue diagnosis, and it can be carried out in the ICU using ultrasound guidance. The possibility of renal artery stenosis is seldom of immediate concern in critical care; its recognition is still challenging and ultrasound has a limited reliability. Exclusion of renal artery or vein thrombosis, or injury (see below), may be more urgent, particularly in the transplant patient. Color Doppler ultrasound is the preferred modality. Nuclear medicine scans are cumbersome, and CT, MRI, and angiography, which is the most specific, are inconvenient. Nuclear medicine scans are uniquely informative in the assessment of the contribution of each kidney to overall renal reserve. This information can be essential in guiding the necessity and timing of intervention.
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