The concept of patient-prosthesis mismatch.

Abstract

Valve replacement represents the standard therapy for the surgical treatment of diseased valves that cannot be repaired. The final goal of valve replacement should be to replace the diseased valve with a competent and non-stenotic prosthesis. The concept of patient-prosthesis mismatch (PPM) was originally introduced by Rahimtoola in 1978, and defined “...to be present when the effective prosthetic valve area, after insertion into the patient, is less than that of a normal valve.” (1). The issue of choosing the most adequate valvular prosthesis in patients with a small aortic root and a large body surface area (BSA) has been widely debated, since it still represents a common problem in routine surgical procedures on the aortic valve (2-4). However, the evaluation of PPM has not been sufficiently emphasized in common practice, in spite of the fact that failure of its recognition may lead to a significant hemodynamic impairment and worsening of the clinical status in time. Postoperative echocardiographic evaluation of patients after aortic valve replacement has often demonstrated the presence of high transvalvular gradients even at rest and in patients with reduced BSA, despite normal prosthetic function (2,5,6). This finding appears to be influenced by two main factors: first, the in-vitro area of the majority of valve prostheses (especially with an internal diameter <23 mm) is less than that of the normal human valve area; and second, the in-vivo prosthetic area may be further reduced by interventricular septum hypertrophy, anatomical interactions, progressive endothelialization and tissue ingrowth. As a consequence, it is not uncommon that aortic prosthetic devices may be functionally stenotic. The choice of a reliable predictor appears, therefore, mandatory in order to identify patients at risk of developing postoperative prosthesis mismatch. The parameter often used in order to define the presence of PPM has been the geometric orifice area (GOA) that is, a measurement deriving from the internal diameter of the prosthesis and measured in vitro by the valve manufacturer. However, the GOA has been shown to overestimate the ‘functional area’ of a valve prosthesis. Following the introduction of echoDoppler studies, a more reliable parameter has been validated in clinical practice, termed the effective orifice area (EOA) (7,8). Examples of differences between the GOA (as provided by the manufacturer) and the EOA (as calculated using echocardiography) of two commonly implanted prostheses are detailed in Table I. Transvalvular gradients observed postoperatively are related not only to the EOA but also to the transvalvular flow. In turn, transvalvular flow is related to cardiac output, that at rest is primarily related to the patient’s body size. As a consequence, the most reliable parameter to estimate the hemodynamic properties of the valve is the indexed effective orifice area (iEOA) (9,10). The iEOA is obtained from the relationship (EOA/BSA), with the EOA being calculated using the continuity equation: EOA= SV/VTI, where SV is the stroke volume and VTI is the velocitytime integral of the aortic jet Doppler signal. It is accepted worldwide that moderate aortic valve stenosis is present when the iEOA is <0.9 cm2/m2, and this concept should apply also to valve prostheses. In fact, several studies have demonstrated that when a prosthetic iEOA is <0.9 cm2/m2, a significant transvalvular gradient is present at rest (10,11). It is well known that mechanical valve prostheses have a more favorable relationship between their external diameter and the EOA if compared to a stented bioprosthesis, as clearly depicted in Table I. In particular, it is clear from the data in Table II that the use Presented at The ideal human heart valve substitute: 50 years between perceptions and realities meeting, September 2003, Naples, Italy