Download Real-life Hemodynamic Assessment

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:
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Muhiuden IA, et al. Intraoperative estimation of cardiac output by transesophageal
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Perrino AC, et al. Intraoperative determination of cardiac output using multiplane
transesophageal echocardiography: a comparison to thermodilution. Anesthesiology
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Nishimura RA and Tajik AJ. Determination of left-sided pressure gradients by utilizing
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Quinones MA, et al. Recommendations for the quantification of Doppler
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