POCUS in Acute Kidney Injuries
By Rolando Claure Del Granado
Acute kidney injury (AKI) is a clinical syndrome caused by a multitude of hemodynamic, toxic, and structural insults to the kidney, and is associated with worse patient outcomes. Despite careful history taking, physical examination, and analysis of laboratory data, a void is evident in the diagnostic process and clinical monitoring of AKI.
Point-of-care ultrasonography (POCUS) is a limited ultrasound study performed by the clinician at bedside and compliments the physical examination. A growing body of evidence points to POCUS as a powerful tool in a variety of clinical settings and is being increasingly recognized in the field of nephrology as an adjunct to physical examination.
The kidney’s imaging provides useful information in AKI diagnosis and management. As such, a renal ultrasound is often obtained as a part of initial diagnostic work up to give information on kidney anatomy, urinary obstruction, and for differential diagnosis between AKI and chronic kidney disease (CKD). POCUS allows these questions to be answered within minutes, using a safe, non-invasive, and repeatable technique. For example, in real time B-Mode imaging has a very important clinical impact in patient management, it can diagnose CKD, urinary tract obstruction and can indicate treatment and prognosis. The kidney size immediately distinguishes AKI from CKD, in which both kidneys are small with altered structure like the presence of homogeneous parenchyma without cortico-medullary differentiation. In instances where baseline serum creatinine is not available, the likelihood of treatable disease can be deemed low if the kidney size is small and cortical echogenicity is increased.
The utility of POCUS in discerning the cause of intrinsic AKI is limited. Parameters such as cortical echogenicity, kidney size, and arterial resistive index (RI) are useful when interpreted in the right clinical context but are non-specific. In AKI, infiltrative diseases such as monoclonal gammopathy, lymphoma, or amyloidosis are suspected if the kidney’s volume is increased with globular aspect, hyperechoic cortex and rounded-hypoechoic pyramids. However, as previously mentioned, these features are not specific and they can also be observed in acute interstitial nephritis, acute proliferative glomerulonephritis with crescents, and in acute tubular necrosis.
Hemodynamic AKI is where nephrologist-performed POCUS could prove the most helpful. This type of AKI encompasses various insults that result in renal hypoperfusion (e.g., hypovolemia, systemic vasodilation, and increased intra-abdominal pressure) and renal venous congestion. POCUS aids in the objective volume status assessment by facilitating comprehensive evaluation of the hemodynamic circuit at the bedside.
One can assess the forward arterial inflow (focused cardiac US), venous outflow (inferior vena cava US and Doppler evaluation of the systemic veins), as well as tissue congestion (assessment of extravascular lung water and ascites) and instantaneously integrate this data with overall clinical picture to formulate an individualized management plan for the patient. Moreover, severe flow alterations on hepatic, portal, and renal parenchymal venous Doppler (VExUS) together with a dilatated inferior vena cava have shown to predict congestive kidney injury better than assessing the inferior vena cava alone. In addition, serial POCUS examinations can aid in assessing the efficacy and adequacy of therapy. For example, in patients with pre-renal AKI, bedside stroke volume assessment can be used to monitor improvement in hemodynamics along with laboratory results.
Color Doppler provides information on renal blood flow. Resistances indexes (RI) are calculated based on Doppler waveform in main renal artery or smaller intrarenal vessels. Measurement of intrarenal arterial RI is an attractive means to assess renal perfusion in multiple clinical scenarios including heart failure, septic shock, and hepatorenal syndrome. RI is influenced by several variables like pulse pressure, heart rate, arteriosclerosis, vasoconstriction, venous congestion, CKD, and drugs. Furthermore, the inter and intra-operator variability in reporting RI is high precluding reliable monitoring of response to therapeutic intervention and limiting its utility in the point-of-care setting.
In summary, kidney ultrasound is the most common imaging technique used in the first evaluation of patients with AKI. It is widely available, easy to use and free of complications. Multi-organ POCUS is a valuable addition to nephrologists’ toolkit, which enhances the diagnostic accuracy and guides therapy when properly integrated with clinical and laboratory parameters.(1-5)
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