DK2985_C000 1..28 - AlSharqia Echo Club

Quantitative Echocardiography 117Hatle, Anelsen, and Tromsdal reported that PHT correlateswith the MVA and is less influenced by flow acrossthe valve. They reported a value of ,60 ms in normalmitral valves, increasing to 100 ms with mild stenosisand becoming progressively longer with increasingseverity. A PHT .220 ms was usually associated with aMVA ,1.00 cm 2 , and Eq. (5.42) was derived from thesestudies (45):MVA ¼ 220PHT ¼ 759Mdt(5:42)where MVA is the mitral valve area (in cm 2 ); PHT is thepressure half-time (in ms); and Mdt is the mitral decelerationtime (in ms).Although the MVA obtained by the PHT method isindependent of the volumetric flow across the valve asdetermined by the CO or the presence of coexistent significantmitral regurgitation (MR), it is affected by other conditionsmodifying the differential pressure between the LAand the LV. The applicability of this method is decreasedby changes in chamber compliance following percutaneousballoon mitral valvuloplasty or cardiac surgery, aswell as other conditions such as severe aortic valve regurgitationor delayed left ventricular relaxation abnormalitieswhich modify the transmitral pressure gradientdecay by changing the left ventricular diastolic pressure.These limitations are reviewed in detail in Chapter 17.Since the flow at the narrowest orifice is given by thefollowing equation:Q ¼ CSA V max (5:44)where Q is the flow rate (in mL/s); CSA is the crosssectionalarea of the narrowest orifice (in cm 2 ); and V maxis the maximal velocity across the orifice, obtained byCWD (in cm/sec).Using the continuity equation, we can obtain the CSAby computing the following equation:v rCSA ¼ 2pr 2(5:45)V maxIn MR, where the surface surrounding the regurgitantorifice is relatively planar, the effective regurgitantorifice can be calculated by the PISA method:v rERO ¼ 2pr 2(5:46)V MR maxwhere ERO is the effective regurgitant orifice (in cm 2 ); r isthe measured radius (in cm) of the hemispheric shell of thealiased velocity; v r is the aliased velocity at the radius r,identified as the Nyquist limit (in cm/sec); and V MR maxis the maximal systolic velocity across the orifice, obtainedby CWD (in cm/sec).In the case of MS or TR, rather than being planar, thesurface surrounding the narrowed orifice is funnelshapedand distorts the PISA so that correction factorsmust be applied.In the case of MS (46):F. Proximal Flow Convergence (ProximalIsovelocity Surface Area)MVA ¼ 2pr 2 v r aV MS max 180(5:47)The proximal flow convergence method is another applicationof the principle of conservation of mass (volume)using a different hydraulic equation than the circularorifice and the TVI. In the proximal isovelocity surfacearea (PISA) flow model, as the red blood cells approachand converge towards a narrowed orifice, their velocityincreases in a linear fashion, forming before the orifice aseries of concentric hemispheric shells of similar velocity(called isovelocity hemispheres). The closer the hemispheresare to the orifice and, therefore, the smaller theradius of the isovelocity hemisphere, the higher the correspondingvelocity.The flow rate at a given hemispheric shell velocity isshown by the following equation:Q ¼ 2pr 2 v r (5:43)where Q is the flow rate (in mL/s); 2pr 2 is the area of thehemispheric shell from its radius r (in cm); and v r is thevelocity (in cm/sec) at the radius r.where MVA is the mitral valve area (in cm 2 ); r is themeasured radius of the hemispheric shell of the aliasedvelocity; v r is the aliased velocity at the radius r, identifiedas the Nyquist limit (in cm/sec); V MS max is the maximaldiastolic velocity across the orifice, obtained by CWD (incm/sec); and a is the angle measured between the twomitral leaflets.In the case of tricuspid regurgitation, two correctionfactors are required (47) as mentioned in the followingequation:v rERO ¼ 2pr 2 V TR maxv rV TR maxv r a180(5:48)where ERO is the effective regurgitant orifice (in cm 2 ); r isthe measured radius (in cm) of the hemispheric shell of thealiased velocity; v r is the aliased velocity at the radius r,identified as the Nyquist limit (in cm/sec); V TR max isthe maximal systolic velocity across the orifice, obtainedby CWD (in cm/sec); and a is the angle measuredbetween the two tricuspid leaflets.