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Sonographic Technique/TIPS Malfunction
A low frequency transducer is typically used to evaluate most patients with a stent graft due to its’ deep location, and the high velocities through the stent. When evaluating the portal vein end of the stent graft, a high lateral intercostal or subcostal subzyphoid approach is used. When evaluating the hepatic vein end of the stent, either a high or low lateral intercostal or subcostal/subzyphoid approach is used. Prior to obtaining stent graft velocities, the stent graft should be evaluated with color Doppler to determine if there is focal aliasing; if found, the area should be carefully interrogated to determine if a stenosis is present. It is also important to obtain stent graft velocities within those portions of the stent graft that are surrounded by liver parenchyma. The velocities should be obtained at quiet respiration.
A main portal vein velocity should be obtained proximal to the stent graft, and flow direction should be determined in the portal vein branches and the draining hepatic vein into which the stent has been placed. The presence of portosystemic collaterals should also be recorded. One must also know why the stent graft was placed (bleeding esophageal varices, ascites, hydrothorax). For example, if it was placed to control refractory ascites, the amount of ascites should decrease over the first few months if the stent graft is functioning properly. An increase in ascitic or pleural fluid suggests stent graft malfunction.
There are several complications that may occur in patients with stent grafts. Stent thrombosis usually occurs in the first 2-3 weeks. Stent grafts can also migrate into the liver parenchyma though this is unusual. Uncovered stent grafts are far more prone to stenosis or obstruction than covered stents because of bile leakage into the uncovered stent, and the fibrotic response to stent graft placement.
The are several parameters that can be used to detect stent graft stenosis including stent velocity, main portal vein velocity, shunt and main portal vein pulsatility, portal vein branch flow direction, and hepatic vein flow direction. At our institution, we evaluate patients for all of the above parameters as well as for ascites, pleural effusion, and portosystemic collaterals.
A study performed at our institution several years ago established the lower limits of normal for the main portal vein velocity and the upper and lower limits of normal for stent graft velocity 11. Our data showed that the lower limit of normal for the main portal vein is 30cm/sec. We obtain 2 angle-corrected velocities within the stent graft, a mid to proximal velocity and a more distal velocity. We and other institutions have found that the upper limit of normal for stent graft velocity is approximately 190cm/sec, and the lower limit of normal is 90 cm/second11. Typically the waveform within the stent is very pulsatile; if the pulsatility diminishes, this is suggestive of a downstream stenosis12. At our institution, we keep a log of each visit so as to be able to compare changes from visit to visit.
Flow direction is also evaluated in both the right and left portal vein branches using color Doppler. The flow should be hepatofugal, that is, towards the stent although rarely a patient will have hepatopetal (into the liver) flow in both portal vein branches at the baseline study. The finding of hepatofugal flow is not sensitive for detecting stent stenosis. Many patients with a significant stent graft stenosis have hepatofugal flow in the portal vein branches. Flow direction in the hepatic vein into which the stent was placed should be toward the stent; if it reverses, this suggests a stenosis downstream. However, this sign is also very insensitive and many patients will have significant stent stenosis with the flow direction within the hepatic vein towards the stent.
At our institution, we use all of the above parameters to get an overall impression as to whether the stent graft is patent or malfunctioning; the sensitivity was 92% in our study11.
There are also complications that can occur after stent graft placement. Procedural complications occur in 10-15% of patients, and include intraabdominal bleeding, hemobilia (blood within the bile ducts), sepsis, right heart failure, and liver infarction. The latter may occur in the setting of severe celiac artery stenosis. Long-term complications include progressive liver failure which may occur in 3-8% of patients, and encephalopathy which may occur in 23-50% of patients.
As can be seen, ultrasound is a valuable tool for evaluating TIPS malfunction. The polytetrafluoroethylene covered stents have vastly improved long-term patency and patient outcome. At our institution, ultrasound will continue to play an important role in surveying these patients.