Use of Transesophageal Echocardiography to Measure Intraop Renal Resistive Index and Predict Acute Renal Injury

The significant morbidity and mortality associated with acute kidney injury in critical care
patients and after cardiac surgery is well-known. Studies have demonstrated between 1 to
30% postoperative mortality and even higher rates of up to 70% when patients develop renal
failure and require dialysis. Even small increases in creatinine between 0 to 0.5 mg/dl can
result in a greater than two-fold rise in 30-day mortality.1 Despite efforts to improve
outcome, there has been no proven effective pharmacological interventions to treat acute
kidney failure. Most recommendations are aimed at prevention by identifying high-risk
patients, avoiding nephrotoxic drugs and minimizing intraoperative hypotensive insults.2

There are few studies that have examined the maintenance of adequate renal perfusion by
measuring renal blood flow using transesophageal echocardiography (TEE) during
cardiopulmonary bypass.3, 4 Yang et al. examined the left renal artery of 60 patients using
TEE during cardiac surgery and evaluated the feasibility of using TEE as a method to measure
renal blood flow intraoperatively.4 Although they were only able to include 60% of the
subjects due to technical difficulties, they did demonstrate less than 10% variability
between measurements and therefore good reproducibility using TEE. However, renal blood
flow may not be the best method to predict sufficient renal perfusion. Renal autoregulation
is not preserved under general anesthesia even with the maintenance of adequate mean
arterial blood pressure and cardiac output. Renal blood flow is further worsened by
hemodilution and hypothermia. In addition, due to its pulsatile nature, the diameters of
the renal arteries vary during the cardiac cycle and are a source of calculation error when
determining renal blood flow as a function of renal blood velocity and arterial diameter.2

Renal resistive index (RI) is a measure of intrarenal hemodynamics that is calculated using
the blood flow velocities of segmental or intrarenal vessels and correlates with renal blood
flow and renovascular resistance. The renal artery is not used because the flow varies and
is inconsistent between systole and diastole. RI becomes elevated in pathological
conditions and is associated with increasing creatinine, renal injury and dysfunction.5-7
As blood flow and creatinine clearance decrease through the renal vasculature, the resistive
index increases. Resistive index may be a better gauge of renal dysfunction rather than
renal blood flow because it is easier to assess and less dependent on obtaining a Doppler
beam view that is oriented perfectly parallel to the blood flow.2 Because RI is a ratio of
the renal blood flow velocities [RI = (peak systolic velocity - peak end diastolic
velocity)/peak systolic velocity], the margin of error created by non-parallel Doppler beams
cancels out.

Traditionally, resistive index is obtained by transabdominal Doppler ultrasonography (USG)
although there have been transesophageal studies that have used RI as a secondary endpoint
when examining renal blood flow.2, 4 There is currently no technique that routinely uses TEE
to intraoperatively monitor resistive index as a determination of adequate renal blood
perfusion and an indication of renal compromise.8 In addition, while other studies have
reported RI as a secondary outcome, no studies have validated the TEE calculation of
resistive index compared to the established transabdominal doppler technique.

Inclusion Criteria:

- TEE is part of a anesthetic plan as determined by the anesthesiologist

Exclusion Criteria:

- Subjects on hemodialysis/peritoneal dialysis

- Subjects with a contraindication to use of TEE; esophageal stricture, esophageal
diverticulum, esophageal tumor, recent esophageal/gastric surgery or radiation to the
chest

- Subjects who are pregnant

- Subjects with esophageal varices

- Subjects with bleeding disorders