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Cardiac Cath Measurement of Stenotic Aortic Valve Area Ryan Tsuda, MD Case Report: CC: Shortness of Breath HPI: 62 y/o Caucasian male, without previous significant medical history, presents with 6-8 months of progressively worsening dyspnea. Recalls 1 month h/o new onset leg and belly swelling. Describes 2 pillow orthopnea and occasional PND. Denies CP, syncope, or lightheadedness. Case Report: PMHx: Childhood murmur Meds: None All: NKDA SHx: Denies etoh, smoking, or illicit drugs FMHx: Did not have a relationship with his family, and therefore, is was not familiar with their medical problems. Case Report: PE: 97.6 115 159/109 26 100%2L Gen: Middle aged male with mild respiratory distress Neck: Short and thick, No obvious jvd CV: Tachycardic w/ RR, nl S1 S2, +S3, 2/6 crescendo decrescendo systolic murmur at URSB Pulm: Mild bilateral base crackles Abd: Diffuse abdominal wall edema, +shifting dullness GU: +scrotal edema Ext: 3+ Bilateral pitting edema Case Report: Na 143, K 4.3, Cl 105, CO2 30, BUN 23, Cr 1.3, Glu 108……… WBC 10.6 w/ NL diff, Hg 15.8, Hct 50.8, Platelets 219,000 ……. Tprot 6.7, Alb 3.5, Ast 66, Alt 57, Alkphos 127, Tbili 1.2 UA: +protein BNP 3690 Case Report: EKG: STach 115, LVH CXR: CM, Increased PVC, Small bil pleural effusions Initial A/P: New CHF…..Started on Natrecor, Lasix, Digoxin, Captopril, and Aldactone……More to follow……………………… Cardiac Cath Measurement of Stenotic Aortic Valve Area As valvular stenosis develops, the valve orifice produces more resistance to blood flow, resulting in a pressure gradient (pressure drop) across the valve Gorlin Formula Calculates cardiac valvular orifice area from flow and pressure-gradient data Incorporates 3 preexisting formulas Gorlin Formula 1.) Torricelli’s Law (flow across a round orifice) F = AVCc F = Flow Rate A = Orifice Area V = Velocity of Flow Cc = coefficient of orifice contraction (compensates for the physical phenomenon, that except for a perfect orifice, the area of a stream flowing through an orifice will be less than the true area of the orifice) Gorlin Formula 2.) Relates pressure gradient to velocity of flow V2 = (Cv)2 x 2gh Cv = coefficient of velocity, corrects for energy loss as pressure energy is converted to kinetic energy g = acceleration due to gravity (980 cm/sec/sec) h = pressure gradient in cm H2O Gorlin Formula Combining the two equations, yields: F A = ---------------------------- (C)(44.3) (sq root of h) C = Empirical constant incorporating Cv and Cc, and accounting for h adjusted to units of mmHg, and correcting calculated valve area to actual valve area as measured at surgery or autopsy. Using this constant, the maximum derivation of calculated valve area from measured valve area was 0.2 cm2. Gorlin Formula Since antegrade aortic flow occurs only in systole, F is the total CO during which there is forward flow across the valve F = CO/(SEP)(HR) F (cm3/sec) CO (cm3/min) SEP (sec/beat) HR (beats/min) *SEP (systolic ejection period) begins with aortic valve opening and proceeds to the dicrotic notch or other evidence of valve closure. Gorlin Formula Thus, the final Gorlin equation for the calculation of valve orifice area (in cm2) is: CO/(SEP)(HR) Area = ---------------------------------------- 44.3(C)(sq rt of pressure gradient) Where C = empirical constant For MV, C = 0.85 (Derived from comparative data) For AV, TV, and PV, C = 1.0 (Not derived, is assumed based on MV data) Alternative to the Gorlin Formula *A simplified formula for the calculation of stenotic cardiac valves proposed by Hakki et al…Circulation 1981. Tested 100 patients with either AS or MS. *Based on the observation that the product of HR, SEP or DFP, and the Gorlin equation constant was nearly the same for all patients measured in the resting state (pt. not tachycardic). Values of this product were close to 1.0. *Calculations somewhat comparable……… Aortic Valve Area (cm2) Critical AS: < 0.7 Moderate AS: 0.7 – 1.5 Mild AS: 1.5 - 2.5 NL Aortic Valve: 2.5 - 3.5 *Ranges have variability based on body size (i.e. a larger person, requiring higher CO for perfusion, may become symptomatic at a larger aortic valve area) Relationship between CO and Aortic Pressure Gradient over a range of values for AV area (Based on Gorlin formula) *As HR increases (i.e. during exercise), the SEP shortens. However, SEP shortening is attenuated by increased venous return and peripheral arteriolar vasodilation. CO / (HR)(SEP) 2 Change in pressure = [ ------------------- ] (44.3)(AVA) Therefore, the increase in CO will be partially offset by the increase in (HR)(SEP), so that the gradient across the valve will not quadruple with a doubling of CO during exercise. Relationship between CO and Aortic Pressure Gradient over a range of values for AV area (Based on Gorlin formula) *As HR slows in patients with AS, the SV increases if CO remains constant. Thus, Flow across the valve increases, as does the pressure gradient. Relationship between CO and Aortic Pressure Gradient over a range of values for AV area (Based on Gorlin formula) Acquiring Hemodynamic Data Acquiring Hemodynamic Data Acquiring Hemodynamic Data Acquiring Hemodynamic Data Indicator Dilution Method (CO): *Based on the principle that a single injection of a known amount of indicator (cold/room temperature saline for thermodilution technique or indocyanine green dye) injected into the central circulation mixes completely with blood and changes concentration as it flows distally. Acquiring Hemodynamic Data Thermodilution Indicator Method: *Rapidly inject 10 cc of saline through proximal port of PA catheter. An external thermistor measures the temperature of the injectate. Complete mixing of saline with blood causes a decrease in the blood temperature, which is sensed by a distal thermistor. Computer calculates CO based on the change in indicator concentration (using temperature over time). Acquiring Hemodynamic Data *Accurate method of measuring CO, especially in patients with low cardiac output. O2 consumption measured from metabolic hood or Douglas bag; it can also be estimated as 3 ml/min/kg or 125 ml/min/m2. AVo2 difference calculated from arterial – mixed venous (pulmonary artery) O2 content, where O2 content = saturation x 1.36 x Hg *Metabolic Hood (Polaragraphic method) *Utilizes a polaragraphic oxygen sensor cell to measure oxygen content of expired air. *Room air is withdrawn at a constant rate through a plastic hood over the patient’s head. *Measures the contents of the hood (room air/expired air) through a flexible tubing that feeds to the polaragraphic oxygen sensor. *Douglas Bag *Patient is asked to breathe into a large, sealed, air-tight bag for a specific period of time. *The mouthpiece to the bag has a two-way valve. *Allows patient to inspire room air, while the expired air (pt. wears a nose clip) goes into the Douglas bag. *After the specified interval, the bag is sealed and the contents analyzed. Cardiac Output by Fick Method (example) Arterial saturation 95% Pulmonary artery saturation 65% Hg = 13 O2 consumption is 210 ml/min (3 ml/kg given a 70 kg person) Pressure Gradients *Multiple sites for recording transaortic valve gradients *Simultaneous tracings between site 1 and 3 would give the most accurate pressure gradient *Usually use sequential readings (pullback) from 1 to 3, and use simultaneous tracings at 1 + 5 *Assey et al. measured the transaortic valve gradients in 15 patients from eight different combinations of catheter locations. In some patients, the differences in gradient among the different measurement sites were as much as 45 mmHg. May then obtain mean pressure gradient across aortic valve by planimetry *In addition to time delay, peripheral artery waveforms are distorted by systolic amplification and widening of the pressure waveforms. *Errors in pressure gradient can also occur if, during pullback, the LV catheter is placed in the LV outflow tract *Alternative to measuring transaortic valve gradient using simultaneous LV and femoral artery pressures, as introduced by Krueger et al. at the University of Utah. Calculating Aortic Valve Area Calculating Aortic Valve Area (example) Mean aortic valve pressure gradient = 40 mm Hg SEP = 0.33 sec HR = 74 CO = 5000 mL/min AV constant = 44.3 Calculating Aortic Valve Area (example) CO/(SEP)(HR) A = ---------------------------------------- 44.3(C)(sq rt of pressure gradient) Assessment of Aortic Stenosis in Patients with low Cardiac Output * Valve calculations using the Gorlin formula are flow dependent. Therefore, low CO states may give an errantly low calculation of aortic valve area. * Decreased flow through the stenotic valve in conjunction with decreased LV pressure, physically opens the valve to a lesser orifice area, and thus, the valve orifice really is smaller during low flow states. * Should keep this in mind when calculating aortic valve area using standard techniques in patients with low cardiac output. Assessment of Aortic Stenosis in Patients with low Cardiac Output In patients with AS, an infusion of nitroprusside or dobutamine substantially increases forward output, and may substantially decrease the transvalvular gradient. Potentially dangerous Assessment of Aortic Stenosis in Patients with low Cardiac Output *”Valve resistance” may be an adjunct to the Gorlin equation in differentiating truly severe AS in patients with low cardiac output states. (Cannon et al….JACC 1992) (mean gradient)(SEP)(HR)(1.33) VR = ---------------------------------------- CO *Advantage of being calculated from two directly measured variables, and requires no discharge coefficient. Resistance appears to be less flow dependent than valve area. *Patients with resistance > 250 dynes sec cm -5 are more likely to have significant AS, while those with resistance < 200 dynes sec cm -5 are less so. Case Report Echo Clips… Case Report: 2D-Echo: LVEF 15-20% Severely reduced RVEF 4-Chamber DCM Abnormal LV Relaxation Severe Aortic Stenosis (PK AV Vel 4.3 m/s, Mean AV gradient 33 mmHg, AV area 1.0 cm2) Mild Aortic Insufficiency, Mild Tricuspid Regurgitation, and Mild Mitral Regurgitation Gorlin Formula LHC + RHC CO 4.2 L/min CI 2.2 L/min/m2 RA pressure 12 RV pressure 65/10-13 PA pressure 56/41 Wedge 32-35 LV pressure 200/35 Aortic pressure 150/85 Simultaneous pressure gradient 48.5 mmHg Valve Flow 178 cm3/sec Mean gradient 60 mmHg Aortic Valve Area 0.52 cm2 Distal LCX 80-90% prior to large PDA filling via right to left collaterals Case Report *LV to Aorta Pullback Case Report *Simultaneous pressure gradient Case Report *Planimetry of shaded area yields pressure gradient Case Report: Hospital Course and Discharge Plan: *Achieved adequate diuresis in the hospital *Referral to CT Surgery for possible AVR and 1V-CABG Summary: Cath measurement of aortic valve stenosis is based on the Gorlin formula. Proper calibration and procedural techniques using the catheter is important in acquiring accurate cardiac output and pressure gradients. During low cardiac output states (i.e. CHF), may need to use adjunctive techniques to acquire reliable hemodynamic data to calculate accurate aortic valve area, and in turn, make the appropriate recommendation regarding valve replacement. References Baim, Grossman. Grossman’s Cardiac Catheterization, Angiography, and Intervention, 6th Edition. 2000. pp 193- 207. Kern, Morton. The Cardiac Catheterization Handbook, 2nd Edition. 1995. pp 108-138. Braunwald. Heart Disease, 6th Edition. 2001. pp 371-385.
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