Role of Renal Doppler Ultrasound in Early Detection of Acute Kidney Injury in Critically Ill Patients

The incidence of Acute kidney injury in intensive care unit patients is rising due to the population age increasing, more comorbidities, and a higher prevalence of risk factors. Improved Intensive Care Unit management has, however, significantly diminished morbidity and the mortality of patients who develop Acute kidney injury I. Despite an increase in the number and severity of comorbidities, in-hospital mortality has declined, but the incidence of Acute kidney injury ,and Acute kidney injury requiring renal replacement therapy has increased over time .

There is still some dispute over the characterization of the different types of Acute kidney injury. Classically there are three types of Acute kidney injury: pre-renal, intrinsic renal, and post-renal failure. These are characterized as decreased renal blood flow (in 40-70% of the patients), direct (intrinsic) renal parenchymal damage (in 10-50% of the patients), and obstruction of urine flow which is less common in the ICU (10%), respectively..

In critically ill patients, sepsis and acute kidney injury are very common diseases and are associated with increased hospitalization and elevated in-hospital mortality rates Acute kidney injury is a dynamic process that evolves from an early reversible condition to an established disease and leads to sustained renal impairment, cell death ,and delayed renal recovery .Consequently, prompt resuscitation of the circulation and optimal perfusion pressure are the primary therapies for critically ill patients with Acute kidney injury. These methods are based principally on the appropriate management of intravenous fluid replacement and vasopressor administration under strict hemodynamic monitoring .

Mechanisms of Acute kidney injury comprise renal hypo-perfusion, intra-renal vasoconstriction, inflammation,oxidative stress and nephrotoxicity .An important pathophysiological pathway includes intra renal vasoconstriction and endothelial damage of the microvessels, leading to impaired macro- and microvascular flow, which further aggravates ischemia .

Renal vasoconstriction is an early manifestation of Acute kidney injury. More specifically, ultrasound imaging of the renal arteries can be instrumental in the early detection of Acute kidney injury. Indeed, renal blood flow is decreased at a nearly stage during acute tubular necrosis as a consequence of protracted intra renal vasoconstriction. The renal resistive index , which is used for assessing arterial pulsatility, was shown to correlate with renal vascular resistance . Renal Doppler ultrasound can measure the renal resistive index , a sonographic index that reflects alterations in blood flow profile of the intrarenal arcuate or interlobar arteries. It reflects the relation between the decline in speed loss of flow (flow velocity) between the peak of systole and the end of diastole in (renal) blood vessels: RRI = (peak systolic velocity _end diastolic velocity)/(peak systolic velocity). Normal values are reported between 0.60 and 0.70 with the difference between both kidneys mostly being less than 5% .