CON: Proximal Isovelocity Surface Area Should Not Be Measured Routinely in All Patients with Mitral Regurgitation

Abstract

Long-standing moderate and severe mitral regurgitation (MR) lead to remodeling of the left atrium, left ventricle, and, ultimately, the right ventricle. These changes are associated with increased morbidity and mortality. Because of the importance of the MR severity in a patient's long-term outcome, the American Society of Echocardiography has recommended that the evaluation of MR severity take the time to “integrate multiple parameters of severity assessment rather than rely on a single measurement (1).” In taking this approach to severity assessment, the inherent theoretical, technical, or measurement errors associated with each method are minimized. Color flow Doppler (CFD) measurements of maximal jet area (MJA) at an aliasing velocity of 50–60 cm/s, the ratio of jet area/left atrial area, and the vena contracta combined with pulsed wave Doppler interrogation of the pulmonary veins, in the absence of proximal isovelocity surface area (PISA), can provide the required multiple techniques. Just because one can do PISA measurements, it does not necessarily follow that they should be done routinely. Though PISA is a recognized means to assess the severity of MR, there are many reasons that its use should not be universal. Even in the most simplified ways, PISA calculations may be time consuming for novices and require focus. Thus, performance of these measurements may lead to distraction from patient care. Another reason not to perform PISA calculations intraoperatively on a routine basis is that in many, if not most, cases there are conditions that invalidate the findings (Table 1). PISA measurements assume perfectly symmetric spherical unconstrained flow acceleration. Unfortunately, in the real world of pathologic mitral valves (MV), the geometry is often irregular and not symmetric (Fig. 1). Another problem is that the measurements vary throughout the cardiac cycle. Several studies evaluated continuous PISA-regurgitant orifice area (ROA) using computational calculations of the PISA-eROA (the effective size of the regurgitant orifice) through the complete systolic period (2–4). These instantaneous calculations were computationally performed using instantaneous MR flow rates and the corresponding simultaneous mitral regurgitant velocity in patients with rheumatic, functional, and myxomatous MR. Patients with rheumatic MR demonstrated parabolic MR flow rates and MR velocities which yielded a relatively a fixed PISA-ROA. Patients with functional MR demonstrated a bifid MR flow rate and parabolic MR velocity spectral envelope yielding a bifid PISA-ROA curve that was largest in early and late systole. The ROA in MV prolapse demonstrated a late peak as a result of a parabolic MR velocity spectral envelope and a mid-to-late increase in the MR flow rate. This study suggests that the maximal instantaneous PISA-ROA may not provide a reliable reflection of the ROA throughout systole or of the average systolic ROA (4).Table 1: Assumptions Involved in Mitral Regurgitation (MR PISA Calculations) and LimitationsFigure 1.: Image of an ischemic Type IIIb restricted mitral valve. Note the irregular shape of the deformity.In patients with Carpentier Type I and III mechanisms of regurgitation, there are often multiple regurgitant sites in addition to the dynamic nature of the ROA. Using PISA in these circumstances would require an independent assessment of each regurgitant orifice. On the other hand, multiple jets do not cause a problem when using CFD MJA color or pulse-wave Doppler pulmonary vein assessment. Just as one would not try to calculate mitral valve area by the pressure half-time method in patients with significant aortic regurgitation; you should not rely on the PISA measurement in all patients. Another key point relates to patients scheduled for MV surgery. The decision regarding the severity of MR has already been made, and the most important role of the prepump transesophageal echocardiography examination is to determine the mechanism of the MR not the severity. In MV repair surgery, the surgeon needs to know exactly why and where the valve is leaking. This determination requires a careful two-dimensional analysis of all views of the valve and then CFD is used to confirm the location and mechanism. Furthermore, as it has been shown in previous work (5), due to variations in loading conditions under anesthesia, intraoperative assessment may underestimate the severity of MR using CFD and pulmonary venous flow analysis. The effects of changes in loading conditions on PISA measurements are not well characterized. In conclusion, PISA is not routinely indicated in all intraoperative TEEs. However, in those circumstances when MR severity is critical in guiding the intraoperative management of our patients, the PISA measurements should be performed. Most importantly, however, PISA findings must be 1) considered in the context of the patient's presentation and 2) integrated with other severity assessment methods in a manor that is weighted by the quantitative ability of the specific method, and the quality of the recorded data (1).