Aortic Valve Area—Technical Communication: Continuity and Gorlin Equations Revisited

Abstract

AORTIC VALVE (AV) surgery for aortic stenosis (AS) is one of the most common surgeries performed in the United States. 1 Baumgartner H Hung J Bermejo J et al. Recommendations on the echocardiographic assessment of aortic valve stenosis: A focused update from the European Association of Cardiovascular Imaging and the American Society of Echocardiography. J Am Soc Echocardiogr. 2017; 30: 372-392 Abstract Full Text Full Text PDF PubMed Scopus (516) Google Scholar In addition to symptoms of left ventricular failure, estimation of the aortic valve area (AVA) and the degree of AS are the basis of clinical decision making. There are numerous methods to measure AVA. In the surgical arena, the continuity equation (CE) is most commonly used to accurately quantify the severity of AS. 2 Mahmood F Fritsch M Maslow A Unanticipated mild-to-moderate aortic stenosis during coronary artery bypass graft surgery: Scope of the problem and its echocardiographic evaluation. J Cardiothorac Vasc Anesth. 2009; 23: 869-877 Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar Accuracy and reliability of the estimated AVA are determined using a multitude of technical and operator-related factors in the CE. With percutaneous interventions becoming more popular, there is renewed attention on the severity assessment of the AVA. Accurate measurement of the left ventricular outflow tract (LVOT) diameter, a critical component of this calculation, is considered the source of greatest error. 1 Baumgartner H Hung J Bermejo J et al. Recommendations on the echocardiographic assessment of aortic valve stenosis: A focused update from the European Association of Cardiovascular Imaging and the American Society of Echocardiography. J Am Soc Echocardiogr. 2017; 30: 372-392 Abstract Full Text Full Text PDF PubMed Scopus (516) Google Scholar Availability of 3-dimensional (3D) imaging and its quantitative application has demonstrated the heterogeneity of the LVOT and its likely effect on the severity assessment of AS. 3 Montealegre-Gallegos M Matyal R Khabbaz KR et al. Heterogeneity in the structure of the left ventricular outflow tract: A 3-dimensional transesophageal echocardiographic study. Anesth Analg. 2016; 123: 290-296 Crossref PubMed Scopus (9) Google Scholar Similarly, accuracy of Doppler velocity profiles across the LVOT and AV also can significantly affect the accuracy of the AVA. Being flow dependent, the AVA also is significantly affected by the degree of systolic dysfunction that can affect the mechanical opening of the leaflets and the peak velcoities. 4 Grayburn PA Assessment of low-gradient aortic stenosis with dobutamine. Circulation. 2006; 113: 604-606 Crossref PubMed Scopus (35) Google Scholar Therefore, it is not uncommon to encounter discordant data in the operating room with consequent dilemmas in clinical decision-making. 2 Mahmood F Fritsch M Maslow A Unanticipated mild-to-moderate aortic stenosis during coronary artery bypass graft surgery: Scope of the problem and its echocardiographic evaluation. J Cardiothorac Vasc Anesth. 2009; 23: 869-877 Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar The discordance often is attributed to differences in methodology of AVA calculation, the effective orifice area (EOA) versus geometric orifice area (GOA), and peak gradient estimation using echocardiography and cardiac catheterization. Specifically, the differences between the Gorlin-derived and CE equation-derived AVA can create challenging intraoperative dilemmas. Considering the availability of new information and its effect on the calculation of the AVA, the authors reviewed the hemodynamic principles of CE and compared them with those of the Gorlin equation 5 Gorlin R Gorlin SG Hydraulic formula for calculation of the area of the stenotic mitral valve, other cardiac valves, and central circulatory shunts. I. Am Heart J. 1951; 41: 1-29 Crossref PubMed Scopus (1144) Google Scholar and attempted to explain the most common sources of controversy and discrepancy in calculation of the AVA.