Subharmonic Aided Pressure Estimation (SHAPE)

Abstract

Portal hypertension results from obstruction of the portal blood flow. An increase of over 5 mmHg in the pressure gradient between the portal vein and the inferior vena cava (IVC) or the hepatic vein defines portal hypertension. The current clinical technique for measuring the hepatic venous pressure gradient (HVPG) is invasive. Noninvasive techniques have very poor sensitivity for portal pressure estimation and are therefore, not accurate enough to be used routinely for diagnosis of portal hypertension. Thus, an alternative accurate, noninvasive ultrasound based procedure would be a major development in the diagnosis of portal hypertension making the diagnosis safer, quicker and cheaper. Our group has proposed the use of ultrasound contrast agents as pressure sensors (i.e., Subharmonic Aided Pressure Estimation, SHAPE) for noninvasive, quantitative pressure estimation in portal hypertension. SHAPE is based on the inverse relationship between the subharmonic amplitude of the microbubbles and the ambient pressure change. A pilot study was conducted with 45 patients; the SHAPE gradient between the portal and hepatic veins was in good overall agreement with the clinical HVPG measurements (r = 0.82). Patients at increased risk for variceal hemorrhage (HVPG >12 mmHg) had a significantly higher mean subharmonic gradient than patients with lower HVPGs (1.93 dB ± 0.61 vs 21.47 dB ± 0.29; p < 0.001), with a sensitivity of 100% and a specificity of 81%, indicating that SHAPE may be a useful tool for the diagnosis of clinically important portal hypertension. A large, multi-center trial is now underway and early results appear to confirm these initial findings.