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7:00 – 8:00 pm Real-life Hemodynamic Assessment GS Hartman, MD Professor of Anesthesiology Vice-chair and Clinical Director Director of Cardiac Anesthesia Department of Anesthesiology The Geisel School of Medicine at Dartmouth Dartmouth Hitchcock Medical Center Lebanon, NH NJ Skubas, MD, FASE Associate Professor of Anesthesiology Director, Cardiac Anesthesiology Department of Anesthesiology Weill Cornell Medical College New York, NY Objectives: At the conclusion of this lecture, the participant should be able to: 1. Describe the various 2D and Doppler ultrasound methods used for hemodynamic assessment 2. Verbalize the use of Doppler in hemodynamic calculations 3. Calculate intracardiac pressures using Doppler ultrasound 4. Calculate valvular pressure gradients using Doppler ultrasound Blood flow and the VTI Flow is expressed in volume per a certain amount of time. It may be thought of as the volume that has passed a point in a certain amount of time; flow is expressed as volume (ml or L) per time unit (seconds, heart beat, etc). Commonly, intracardiac flow calculations are performed for one heart beat and the resultant assessment is multiplied by the heart rate (HR) to give the per-minute estimate of flow. For volume calculations we need to: - determine the cross sectional area (CSA) through which the flow is measured, and - measure the integral of the flow velocity (VTI, see below) Volume is then calculated as CSA (cm2) x VTI (cm), and flow (ml/min or L/min) as volume x HR. The following analogies will help to conceptualize these calculations. If one were trying to calculate the amount of water flowing in a hose he would need to know the following: - How big is the hose? - (diameter of the hose ) - How fast is the water flowing? - (velocity of flow) - How long, or for what amount of time, is the hose turned on? In another example, if I wanted to know how much money is going past a coin counter, I’d have to know if the coins were quarters, nickels or pennies as well as, how many are going by. If the geometry of the region in which we are measuring the velocity is regular, calculation of its CSA is easily accomplished. Stroke volume is commonly measured in the left ventricular outflow track (LVOT), which is assumed to take on a cylindrical shape; thus, knowing the diameter (d) permits easy calculation of the LVOT CSA from the formula π x (d/2)2. Other CSA used is the right ventricular outflow tract (RVOT). The intracardiac flow is calculated from a spectral Doppler display of the velocity of the blood passing through the CSA. The velocities are usually obtained from within the sample gate (or volume) of pulsed wave Doppler. In a Doppler velocity spectral display, the instantaneous velocities are plotted against time. The mathematical solution (integral) of a velocity vs time is distance (VTI). The VTI represents the “distance” the blood flow traveled over a heartbeat. A commonly used illustration is a basic mathematics problem: How far did the car going one hour if it went 20 MPH for 20 minutes, 40 MPH for 20 minutes and then 60 MPH for 20 minutes? The plot of the car’s speed over the hour is shown in the right. The integral of the velocity vs time plot would be the average speed (40 MPH) x one hr or 40 miles. Note that the solution is a distance. Using the coin analogy above, one can think of this as discs passing by in a certain time period. These discs are infinitely thin and thus area (in fact the cross sectional areas) are summed or stacked up over time (and thus a height or distance the stack has travelled). The solution of the Velocity Time Integral is in the units of length or distance. It can be thought of as the distance a fixed cross sectional area has moved over a time period or alternatively the number of discs one has “stacked-up” over that same time interval. Thus it is also termed the stroke-distance, or the length that particular cross-section of blood “traveled” in one heart beat. Therefore, the stroke volume through the LVOT is calculated as CSA x VTI, and VTI is the distance the blood profile traveled throughout a heart beat. (from: AC Perrino, ST Reeves. A Practical Approach to Transesophageal nd Echocardiography. 2 ed. Lippincott Williams & Wilkins. 2008) In this manner, any where within the heart that a CSA can be measured or calculated and a Doppler beam can be aligned as parallel to blood flow it is possible to measure the VTI of the passing blood. Common sites include the LVOT in the TG and dTG views, the main PA or the right ventricular outflow tract in the UE arch short axis view. Intracardiac pressure assessment and the Bernoulli equation Pressure gradients are used to estimate intracavitary pressures and to assess conditions such as valvular disease (e.g., aortic stenosis), septal defects, outflow tract obstruction, and major vessel pathology (e.g., coarctation). As blood flows across a narrowed or stenotic orifice, the blood velocity increases proportionally to the degree of narrowing. In the clinical situation, the simplified Bernoulli equation describes the relation between the increase in blood flow velocity and the pressure gradient (ΔP) across the narrowed orifice: ∆P = 4Vmax2, where ∆P (mmHg) is the pressure gradient (PG) across the narrowed orifice and Vmax4 (m/s) is the maximum velocity (Vmax) across that orifice measured by spectral Doppler. In clinical echocardiography, PG is obtained by measuring the peak velocity of blood flow across the lesion of interest. The measured Vmax is then entered into the simplified Bernoulli equation to estimate the PG. CALCULATION OF INTRACARDIAC PRESSURES PRESSURE EQUATION RVSP or PASP = 4(VTR)2 + RAP PAM = 4(Vearly PI)2 + RAP PADP = 4(Vlate PI)2 + RAP LAP = SBP - 4(VMR)2 LVEDP = DBP - 4(VAl end)2 RVSP, right ventricular systolic pressure; PASP, pulmonary artery systolic pressure; V, peak velocity; TR, tricuspid regurgitation; RAP, right atrial pressure; PAM, pulmonary artery mean pressure; Pl, pulmonic valve insufficiency; PADP, pulmonary artery diastolic pressure; LAP, left atrial pressure; SBP, systolic blood pressure; MR, mitral regurgitation; LVEDP, left ventricular end-diastolic pressure; DBP, diastolic blood pressure; Al, aortic insufficiency. However, if the velocity proximal to a narrowed orifice is >1.5 m/s, the modified Bernoulli equation should be used: ∆P = 4 x (V-distalmax2) – (V-proximalmax2). This is the case in LVOT obstruction or severe aortic regurgitation, when the LVOT velocity may be high enough so that, if not taken into consideration, the pressure gradient across the aortic valve may be overestimated. (if the Vproximal is < 1, squaring it results in an even smaller valvue hence may be ignored). For example: if the velocity across an aortic valve is 4 m/s and the LVOT is 2 m/s, then the pressure gradients will be calculated as 4 x 42 = 64 mmHg (simplified Bernoulli) and 4 x (42 – 22) = 4 x (16 – 4) = 48 mmHg (modified Bernoulli). While the first (64 mmHg) is diagnostic of severe aortic stenosis, the second (48 mmHg) is not. The major limitation when estimating an intracardiac pressure gradient is the non-parallel alignment between the Doppler beam and the direction of blood flow. If the angle is >20o the true velocity is underestimated and the pressure gradient will be lower. The American Society of Echocardiography practice guidelines do not encourage angle correction, so every effort should be made to align the Doppler beam with the blood flow. First, the direction of blood flow should be imaged with color Doppler, the Doppler beam cursor should be positioned accordingly and then the spectral Doppler (pulsed- or continuous-wave Doppler) should be activated. Continuity Equation in Aortic Stenosis The continuity equation is another expression of the principle of conservation of mass, which is simply expressed as “flow in” equals “flow out”. Specifically, the continuity equation states that stroke volume (SV2) across a stenotic orifice is equal to the stroke volume (SV1) proximal to the lesion: SV2 = SV1 CSA2 × VTI2 = CSA1 × VTI1 Thus, using the Doppler formula for stroke volume, the area of a stenotic valve (CSA2) can be calculated as: CSA2 = CSA1 × (VTI1 / VTI2) The continuity equation is used in echocardiography to determine the area of a stenotic valve (i.e., aortic valve area in aortic stenosis), or the effective regurgitant orifice area (i.e., in mitral regurgitation). In aortic stenosis, stroke volume across the aortic valve must equal the stroke volume across the LVOT. Thus, the unknown aortic valve area (AVA) may be calculated using the continuity equation as follows: AVA = CSALVOT × (VTILVOT / VTIAV) AVA (cm2) = 0.785 × DLVOT2 × (VTILVOT / VTIAV) where DLVOT (cm) is the LVOT diameter, (VTILVOT, cm) is measured using pulsed wave Doppler, and VTIAV (cm) is measured using continuous wave Doppler. Because the shapes of the VTILVOT and VTIAV Doppler profiles are similar in aortic stenosis, the ratio of the maximum velocities (VLVOT/VAV) may be substituted for the ratio of the velocity-time integrals (VTILVOT/VTIAV) without introducing significant error into the AVA calculation: AVA (cm2) = 0.785 × DLVOT2 × (VLVOT / VAV). The primary concern in determining the AVA with the continuity equation when using TEE is related to the possible underestimation of time-velocity integrals (or peak velocities) in the LVOT and/or aortic valve due to suboptimal alignment of the Doppler beam with the blood flow direction. References: Lee RT, et al. Prospective Doppler echocardiographic evaluation of pulmonary artery diastolic pressure in the medical intensive care unit. Am J Cardiol 1989;64:1366–77 Maslow AD, et al. Measurement of cardiac output by pulsed wave Doppler of the right ventricular outflow tract. Anesth Analg 1996;83:466–71 Muhiuden IA, et al. Intraoperative estimation of cardiac output by transesophageal pulsed Doppler echocardiography. Anesthesiology 1991;74:9–14 Perrino AC, et al. Intraoperative determination of cardiac output using multiplane transesophageal echocardiography: a comparison to thermodilution. Anesthesiology 1998;89:350–57 Nishimura RA and Tajik AJ. Determination of left-sided pressure gradients by utilizing Doppler aortic and mitral regurgitation signals: validation by simultaneous dual catheter and Doppler studies. J Am Coll Cardiol 1988;11:317–21 Quinones MA, et al. Recommendations for the quantification of Doppler echocardiography: a report from the Doppler Quantification Task Force of the Nomenclature and Standards Committee of the American Society of Echocardiography. J Am Soc Echocardiogr 2002;15:167–84 Yock PG and Popp RL. Noninvasive estimation of right ventricular systolic pressure by Doppler ultrasound in patients with tricuspid regurgitation. Circulation 1984;70:657–62