Bench-to-bedside review: Contrast enhanced ultrasonography - a promising technique to assess renal perfusion in the ICU

Acute kidney injury (AKI) is common and severe in critically ill patients, with a reported incidence between 1 and 30% and mortality between 28 and 90%, depending on the definition used. Uchino and colleagues [1] reported an incidence of 5.7% and a mortality of 60.3% in a recent multicenter study involving nearly 30,000 critically ill patients. In patients with multiple organ failure, AKI is an independent predictor of mortality [2,3].The pathophysiology of AKI remains poorly understood. However, as renal function is intimately linked to organ blood flow, alterations in renal perfusion are considered key to the pathophysiology of AKI. Little, however, is known about renal perfusion in critically ill patients [4] or about its association with AKI. This is because methods currently available to assess and monitor renal perfusion are either inaccurate or not rapidly applicable in routine ICU patients. The physiological gold standard to estimate renal plasma flow is the calculation of para-amino-hippurate clearance. Unfortunately, this technique is inaccurate in the presence of oliguria [4,5]. Doppler ultrasound studies have been demonstrated to be inaccurate in estimating renal blood flow [6] and only provide information about flow in main arteries. In addition, all methods only give information about global organ perfusion and not about the renal microcirculation or on the intra-renal distribution of blood flow.Imaging methods such as scintigraphy or magnetic resonance imaging (MRI) are much more accurate and can provide valuable information on kidney perfusion [7,8]. However, their use in the ICU is limited by equipment availability, costs and their requirement for extensive and prolonged patient manipulation, which is associated with risk and major logistic challenges. These techniques can be used in research protocols but are not suitable for routine use in most ICU patients and cannot be repeated several times within the same day.A method allowing reproducible ren