Differences in mitral valve-left ventricle dimensions between a beating heart and during saline injection test
Nonaka M, Marui A, Fukuoka M, et al.
Eur J Cardiothorac Surg 2008; 34:755-59.
Reviewer: Bala Subramaniam, MD
Beth Israel Deaconess Medical Center, Boston, MA
Saline injection test (SIT) is routinely done after mitral valvuloplasty in a cardioplegic heart to check the adequacy of repair. This naked eye test may vary from the post-bypass findings from echocardiography. The null hypothesis in this study was that “there are no differences in the mitral valve-left ventricular (LV) geometry between the cardioplegic and the beating heart”. In this study, three-dimensional (3D) shapes of mitral valve components between the beating heart and the heart during SIT were compared.
Seven normal mongrel dogs were used in an established cardiac surgery model from the study group. Sonomicrometry crystals were implanted at the mitral annulus, edge of the mitral leaflets between scallops, tips of papillary muscles, and LV apex under cardio-pulmonary bypass. Geometry data of the LV and mitral valve were acquired during the SIT and in the beating heart. It is important to note that to avoid distortion by LA traction; saline was injected through a 20F catheter fixed at the LV apex. The mitral annular plane was defined as the least-squares plane fitted to the six points of the mitral annulus. The results of the comparison of the geometry were analyzed by the Wilcoxon signed rank test with StatView software (SAS Institute, Inc., Cary, NC). A pilot study revealed that a LV pressure of 40mm Hg during cardioplegia was adequate to close the mitral vale without stretching the LV.
The LV pressure during SIT was approximately 42 mm of Hg and in the beating heart was 96mm of Hg. During SIT, the commissures (20.5 vs. 17, p<0.01) were stretched out with a decreased annular height (5.5 vs. 7, p<0.05) losing the saddle shape. The middle scallop width (14 vs. 11, p<0.05) and the distance between the papillary tips (22.9 vs. 11.6, p<0.01) were significantly greater during SIT. The distance between the papillary tips and the mitral annular plane was similar (19 vs. 18.6, p=ns) in both the groups.
There was enough evidence to reject the null hypothesis. The mitral valve geometry is significantly different under SIT test during cardioplegia and in the beating heart. The saddle-shaped mitral annulus was more flattened during SIT. The distance between the tips of the papillary muscles was greater and the LV chamber was more spherical during SIT. The authors hypothesize that since the distance between the papillary tips and the mitral annular plane did not differ, SIT is best suited to evaluate chordal reconstructions. In other situations, flattening of the mitral valve might lead to inaccurate conclusions with SIT.
This study is the first to analyze the mitral valve geometry, which has direct implications on the every day practice of mitral valve repair procedures. With advancing technology (no limitations in acquiring images such as heart rate trigger), probably these findings can be replicated with real time 3D Echocardiography replacing the traditional SIT under cardioplegia. However, with no other alternatives, SIT still remains the best naked eye test to assess the mitral valve repair just like the ejection fraction assessment.