Radiology

Background Newer three-dimensional imaging technologies provide insight into cardiac shape and geometry from views previously unobtainable. Standard formulae like the continuity equation (CE) that rely on inherent assumptions about left ventricular outflow tract (LVOT) shape may need to be revisited. In the CE, small changes in LVOT diameter may significantly change calculated aortic valve area (AVA). Using 64-slice Multi-detector CT (MDCT), we performed LVOT planimetry to obviate the need for any geometric assumptions. Methods 64-slice MDCT was performed in 30 consecutive patients. The diameter-derived LVOT area (ALVOTdiam) was calculated from a view analogous to the 2D echo parasternal long axis. Direct planimetry of the LVOT (ALVOTplan) was performed just beneath the aortic valve in a plane perpendicular to the LVOT long axis. Further, assuming an ellipsoid outflow tract shape, LVOT area (ALVOTellip) was calculated using πab from the long and short diameters of the planimetered LVOT view. Eccentricity index (EI) was estimated by subtracting the ratio of shortest and longest LVOT diameters from one. Results ALVOTdiam always measured smaller than ALVOTplan (mean 3.7 ± 1.2 cm2 vs. 4.1 ± 1.3 cm2, respectively). The median EI was 0.18 (95% CI = 0.16–0.2; P = 0.0001). ALVOTellip more closely agreed with ALVOTplan (correlation = 0.96; P diam (correlation = 0.87; P Conclusion Using MDCT, the LVOT was shown to be elliptical in most patients. Applying the CE which assumes roundness of the LVOT consistently underestimated the LVOT area which may affect estimated AVA. Planimetry of the LVOT utilizing three-dimensional imaging modalities such as 3-D echocardiography, MRI, or MDCT may render a more precise AVA.

The effects of a neutral lidocaine homologue, 5-hydroxyhexano-2',6'-xylidide (5-HHX), on the kinetics and amplitude of sodium currents in voltage-clamped amphibian nerve fibers are described. 5-HHX produced two types of sodium current inhibition: (a) tonic block, in resting fibers (IC50 approximately 2 mM), and (b) phasic block, an additional, incremental inhibition, in repetitively depolarized fibers (frequency greater than 1 Hz). The kinetics of phasic block were characterized by a single-receptor, switched-affinity model, in which binding increases during a depolarizing pulse and decreases between pulses. In the presence of 4 mM 5-HHX, binding increased during pulses from -80 to 0 mV, with an apparent rate constant of 6.4 +/- 1.4 s-1. Binding decreased between pulses with an apparent rate constant of 1.1 +/- 0.3 s-1. There was little effect of extracellular pH on the kinetics of phasic block. These findings demonstrate that neither the presence of a terminal amine nor a n...