Evaluation of Renal Resistive Index in Patients with Controlled Hypotension

Controlled hypotension could be defined as a reduction of the systolic blood pressure to 80-90 mmHg, a reduction of mean arterial pressure (MAP) to 50-65 mmHg, or a 30% reduction of baseline MAP (1). It is frequently applied in orthopedics, neurosurgery, and ear-nose-throat surgeries to reduce blood loss and provide a good field of view to the surgeon. However, the combination of hypotension with hypovolemia may result in postoperative acute kidney injury (AKI), especially in the elderly and in patients with hypoperfusion-sensitive disease. The situation may worsen with nephrotoxic drugs.

In patients with AKI, the length of hospital stay is prolonged, and the risk of morbidity and mortality increases, so early detection of AKI is significant. Perioperative AKI diagnosis is difficult and often delayed. Although there are various tests (cystatin-c, urea, serum creatinine, creatinine clearance, etc.), there is still no early, accurate, easy-to-use AKI marker in clinical practice.

The most commonly used guidelines for AKI classification are 2004-RIFLE (Risk, Injury, Failure, Loss of kidney function, End stage renal disease), 2007-AKIN (Acute Kidney Injury Network), and 2012-KDIGO (Kidney Disease Improving Global Outcomes). Based on the validity of the RIFLE and AKIN criteria, the KDIGO guideline was developed to diagnose AKI with a straightforward definition for clinical practice.

Doppler ultrasonography is widely used in the evaluation of chronic kidney diseases. Renal resistive index (RRI), one of the Doopler-derived indices, is calculated by imaging the intrarenal (arcuate or interlobar) artery and measuring the highest systolic and lowest end-diastolic blood flow velocity using a colored Doppler. It is generally accepted that the normal value of the RRI is 0.60 ± 0.01 (mean±SD), and there is a general opinion that the upper limit of the RRI is 0.7 (2). The renal resistive index has been used for years to diagnose and follow up on various kidney diseases (evaluation of chronic renal allograft rejection, detection and management of renal artery stenosis, and chronic differential diagnosis) (3). Recent studies have reported that RRI effectively detects postoperative AKI in the early period before the criteria for AKIN are formed (4-5).

This study aims to evaluate the preoperative and postoperative renal resistive index variation with intraoperative controlled hypotension and research the renal resistive index's utility in the early detection of renal dysfunction that may develop after surgery.

Inclusion criteria

• >18 years old
• will undergo elective arthroscopic shoulder surgery
• ASA (American Society of Anesthesiologist Classification) I-II or III

Exclusion criteria

• Cardiac arrhythmia
• Asymmetric kidney disease
• Poor echogenicity (for imaging)
• Chronic renal dysfunction (GFR<30)
• Renal artery stenosis
• Endocarditis
• Postoperative agitation or confusion
• Postoperative polypnea >35/min or respiratory failure