SUMMARY1. The effects of phosphate and protons on the mechanics and energetics of muscle contraction have been investigated using glycerinated rabbit psoas muscle.2. Fibres were fully activated by addition of Ca21 (pCa 4-5) at 10 'C. The velocities of contraction were measured in isotonic load clamps, and the velocities of unloaded fibres were measured by applying a series of step changes in fibre length. Fibre ATPase activity was monitored using an enzyme system to couple ADP production to reduced nicotinamide-adenine dinucleotide (NADH) and measuring the depletion of NADH by optical density.3. At pH 7-0 and 3 mM-phosphate, isometric tension (P0) was 13-2 +0-9 N/cm2 (mean+S.E.M., n = 10 observations), the maximum contraction velocity (Vmax) was 1-63 + 0-05 lengths/s (n = 5) and the ATPase activity was 1-27 + 0-12 s-' myosin head-' (n = 35). Increasing phosphate from 3 to 20 mm at pH 7 0 does not affect Vmax, causes a small decrease in the ATPase activity (15-20%) and decreases Po by approximately 20%. Changing pH from 7 to 6 at 3 mM-phosphate decreases Po by 45 % and both Vma. and ATPase activity by 25-30 %. The effects of changing both pH and phosphate were approximately additive for all parameters measured. The inhibition of these parameters by low pH and high concentration of phosphate was reversible.4. The force-velocity relation was fitted by the Hill equation using a non-linear least-squares method. The value of the parameter which describes the curvature, a/PO, was 0-20. The curvature of the force-velocity relation was not changed by addition of phosphate or by changes in pH.5. These data provide information on both the kinetics of the actomyosin interaction and on the process of muscle fatigue. The data are consistent with models of cross-bridge kinetics in which phosphate is released within the powerstroke in a step involving a rapid equilibrium between states. The inhibition by protons is more complex, and may involve less specific effects on protein structure.6. During moderate fatigue of living skeletal muscle, MgATP concentration is t To whom correspondence should be addressed.

Background-The Ross operation is an alternative to mechanical aortic valve replacement in the young. Early dilatation of the pulmonary autograft root exposed to the systemic circulation has been reported. To define the prevalence of, risk factors for, and consequences of late autograft dilatation, outcome in all consecutive patients operated since May 1994 was reviewed. Methods and Results-Ninety one patients, 77 males and 14 females, with at least 1 year of follow-up underwent cross-sectional clinical and echocardiographic examination. Age at operation was 27Ϯ10 years (range 6 to 49), and the indication was aortic regurgitation in 54 (59%) patients and bicuspid valve was present in 62 (68%). End-points of the study were freedom from autograft dilatation (root diameter Ͼ4 cm or 0.21 cm/m 2 ), from (moderate) autograft regurgitation and from reoperation. Follow-up (4.0Ϯ1.9, range 1 to 8 years) autograft root diameters were anulus, 29Ϯ4 mm (18 -39); sinus of Valsalva, 38Ϯ7 mm (24 -53); sinotubular junction, 37Ϯ6 mm (23-54); and ascending aorta, 37Ϯ5 mm (27-54). Late autograft dilatation was identified in 31 (34%) patients and regurgitation in 13 (14%), 7 of whom had autograft dilatation. At 7 years, freedom from dilatation was 42Ϯ8%, freedom from regurgitation was 75Ϯ8%, and freedom from reoperation was 85Ϯ10%. Cox proportional hazard analysis identified younger age (Pϭ0.05), preoperative sinus of Valsalva (Pϭ0.02), root replacement technique (Pϭ0.03), and absence of pericardial buttressing (Pϭ0.04) as predictive of autograft dilatation, whereas female sex (Pϭ0.002), follow-up sinus of Valsalva (Pϭ0.003), and sinotubular junction diameter (Pϭ0.02) as predictive of autograft regurgitation. Conclusions-Autograft dilatation is common late after the Ross procedure, particularly in younger patients, in those with preoperative aortic aneurysm, and those having root replacement without support of anulus and sinotubular junction. Bicuspid aortic valve is not a risk factor. Significant autograft valve dysfunction affects a minority of patients, but it is more prevalent in those with autograft dilatation.

Relief of severe subaortic stenosis during one-stage neonatal repair of aortic arch interruption and ventricular septal defect can be accomplished successfully without resection of the conal septum.

In bicuspid aortic valve (BAV) disease, the role of genetic and hemodynamic factors influencing ascending aortic pathology is controversial. To test the effect of BAV geometry on ascending aortic flow, a finite element analysis was undertaken. A surface model of aortic root and ascending aorta was obtained from magnetic resonance images of patients with BAV and tricuspid aortic valve using segmentation facilities of the image processing code Vascular Modeling Toolkit (developed at the Mario Negri Institute). Analytical models of bicuspid (antero-posterior [AP], type 1 and latero-lateral, type 2 commissures) and tricuspid orifices were mathematically defined and turned into a volumetric mesh of linear tetrahedra for computational fluid dynamics simulations. Numerical simulations were performed with the finite element code LifeV. Flow velocity fields were assessed for four levels: aortic annulus, sinus of Valsalva, sinotubular junction, and ascending aorta. Comparison of finite element analysis of bicuspid and tricuspid aortic valve showed different blood flow velocity pattern. Flow in bicuspid configurations showed asymmetrical distribution of velocity field toward the convexity of mid-ascending aorta returning symmetrical in distal ascending aorta. On the contrary, tricuspid flow was symmetrical in each aortic segment. Comparing type 1 BAV with type 2 BAV, more pronounced recirculation zones were noticed in the latter. Finally, we found that in both BAV configurations, maximum wall shear stress is highly localized at the convex portion of the mid-ascending aorta level. Comparison between models showed asymmetrical and higher flow velocity in bicuspid models, in particular in the AP configuration. Asymmetry was more pronounced at the aortic level known to be more exposed to aneurysm formation in bicuspid patients. This supports the hypothesis that hemodynamic factors may contribute to ascending aortic pathology in this subset of patients.

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