11.6.7 ECC and post ECC

The decrease in intrinsic contractility of the LV becomes apparent after surgical correction of the valve when the ventricle is under normal loading conditions. It then has to eject against the systemic impedance without the pressure valve of the MI; wall stress increases and its ejection fraction decreases due to mismatch with the new afterload (afterload mismatch) [8]. In reality, the LV is getting a bad deal: it is exchanging a well-tolerated volume overload for a pressure overload that it cannot cope with. On the pressure-volume diagram, the upward gradient of intraventricular pressure is now vertical instead of oblique, and the isovolumetric contraction phase has been restored. Compared to the situation before ECC, the work of pressure has increased while the work of ejection has decreased; therefore, the energy yield for the LV is less good (Figure 11.82). In addition, the slope of the maximum elastance (Emax) is reduced by several phenomena: mechanical problems with the prosthesis, loss of the subvalvular apparatus, changes in the contraction of the basal segments, presence of inflammatory cardiomyopathy (ARF), reduced contractility after ECC [6]. Plasty surgery is better tolerated than valve replacement because it interferes less with functional structures. This afterload mismatch is less pronounced with percutaneous mitral valve repair without ECC [5].

 

 Figure 11.82: P/V loop of a patient with mitral regurgitation before (MI, red) and after mitral valve replacement (post-MVR, yellow and green). The prosthesis competence imposes an unknown isovolumetric contraction phase before ejection (vertical instead of oblique rise in systolic pressure gradient). Although the end-diastolic volume is increased, stroke volume and ejection fraction decrease, as indicated by the reduced surface area of the P/V loop. The slope of the maximum elastance (Emax) is lowered after ECC (E'max) because contractility is reduced. Compared to the situation before ECC, the work of pressure (area of the dotted triangle Tp) has increased, while the work of ejection (area of the green-yellow square) has decreased. For the LV, the energy yield is therefore less good and the situation very difficult: in addition to the functional handicap of ECC, there is the exchange of a volume overload (well tolerated) for a pressure overload (poorly tolerated) [according to ref 6].

 Inotropic support is almost always required for patients coming off bypass [2]. This should be provided by non-alpha amines or inodilators: dobutamine, milrinone, levosimendant in refractory cases. The addition of an arterial vasodilator may occasionally be necessary if the SAR is too high. Enlarged ventricles require large filling volumes and sustained preload, especially in non-sinus rhythms. In the presence of pulmonary hypertension, there is a risk of right ventricular decompensation at the end of ECC. Positive pressure ventilation is beneficial because it reduces effective LV afterload. Weaning the patient from positive pressure ventilation can be a particularly delicate process and should not be considered too early, but only when left ventricular function has stabilised. Management of haemodynamics after correction of MI therefore requires :

• High preload,
• Low afterload,
• High contractility.

In this context, arterial hypotension should not be treated with vasopressors alone, as this will only increase LV difficulties. Alpha agents, which increase the SAR, are only indicated to ensure coronary, cerebral and renal perfusion pressure when this is compromised. In these circumstances, the most sensible approach is to place an intra-aortic balloon pump (IABP) to relieve the LV of the increased afterload while maintaining optimal perfusion pressure.

After mitral valve replacement with an attached bioprosthesis, one of the 3 tips of the prosthesis may partially obstruct the LVOT and induce abnormal flow acceleration. After mitral valve replacement, the LV pressure chamber may be subobstructed by the distal part of the anterior leaflet of the mitral valve (SAM, systolic anterior motion), leading to obstructive cardiomyopathy (HOCM effect) (see Mitral Valve Repair). In fact, the mitral coaptation zone moves closer to the LVOT when the mitral annulus is narrowed by a prosthetic ring, because the fixed point of the mitral annulus is at the mitro-aortic junction (fibrous trine), whereas the posterior part is thin and supported only by the ventricular musculature; it will therefore move anteriorly, causing an anterior displacement of the coaptation zone (Figures 11.56 and 11.57). Excessive anterior displacement (SAM) occurs in 4-11% of mitral valve repair cases [4,7]. It is associated with four different phenomena [4].

Video: Example of anterior displacement of the mitral leaflet (SAM) obstructing the left outflow tract in mesosystole in a case of severe hypertrophic cardiomyopathy (long-axis view of the LV 140°).


Video: Example of anterior displacement of the mitral leaflet (SAM) after mitral plasty (4-cavity 0° view); the anterior leaflet bends into the outflow tract in mesosystole; plasty of the posterior leaflet gives it a very altered appearance.