Hemodynamic Assessment
Robert M. Savage1
Lee K. Wallace2
Michael G. Licina2
Ahmad Adi2
1OUTLINE AUTHOR
2ORIGINAL CHAPTER AUTHORS
▪ KEY POINTS
The Doppler velocity-time integral (VTI) can be used to calculate stroke volume (SV) at a specific location:
SV = CSA × VTI
Doppler-derived SVs can be used to determine cardiac output (CO), the pulmonary-to-systemic shunt ratio (Qp/Qs), and regurgitant volume (RV). However, propagation of errors may lead to a significant error in either the (Qp/Qs) or an RV calculated using Doppler SVs using transesophageal echocardiography (TEE).
The proximal flow convergence method can be used to determine regurgitant severity based on the maximal regurgitant orifice area:
EROA=6.28×r2×aliasing velocity)/VRJ
This method may overestimate the actual hemodynamic impact of the regurgitant lesion as the degree of regurgitation is not constant throughout systole in many patients. In spite of this and other limitations, the flow convergence method is useful in evaluating regurgitation as it is quantitative, relatively simple to perform, and applicable in a large number of patients with valvular regurgitation.
The continuity equation is another expression of the principle of conservation of mass (flow through the left ventricular outflow tract [LVOT] must equal flow through the aortic valve):
(AVA=CSALVOT × (VTILVOT / VTIAV)
The primary concern in determining aortic valve area with the continuity equation using TEE is related to the possible underestimation of time-velocity integrals (TVIs) (or peak velocities) in the LVOT and/or aortic valve due to inadequate beam alignment.
With valvular stenosis a longer pressure half-time (PHT) indicates more severe stenosis, whereas with valvular regurgitation a shorter PHT indicates more severe regurgitation. The PHT can be used to estimate the mitral valve area (MVA) of a stenotic native mitral valve using the following empirically derived formula:
MVA (cm2) = 220 / PHT (ms)
Estimation of an intracardiac or pulmonary pressure is possible by combining a pressure gradient calculated from a Doppler velocity using the simplified Bernoulli equation with a known or estimated pressure from a proximal or distal chamber. Example:
RVSP (mm Hg)=4 (vTR)2 + RAP (mm Hg)
I. INTRODUCTION
The quantitative hemodynamic data discussed in this chapter can be obtained with the combination of two-dimensional (2D) echocardiography and Doppler echocardiography (Table 15-1). The accuracy of many of these Doppler-derived measurements has been validated in the cardiac catheterization laboratory using transthoracic echocardiography.1,2,3,4
TABLE 15.1 HEMODYNAMIC DATA OBTAINABLE WITH 2D DOPPLER ECHOCARDIOGRAPHY | ||||||||||||||||||||||||||||||||||||||
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II. DOPPLER MEASUREMENTS OF STROKE VOLUME AND CARDIAC OUTPUT
A. Stroke volume
1. The Hydraulic orifice formula
The flow rate of a fluid through a fixed orifice is directly proportional to the product of the cross-sectional area (CSA) of the orifice and the flow velocity of the fluid within the orifice (Fig. 15-1).
Flow rate (cm3 / s)=CSA (cm2)×flow velocity (cm / s)
2. The velocity-time integral
The acceleration and deceleration of blood flow velocity during the ejection period (or filling period) provide a distinct Doppler profile for a given orifice. The summation of velocities over the entire flow period is correctly called the VTI, although it is also commonly referred to as the TVI. The VTI is equal to the area bounded by the Doppler flow velocity profile and the zero velocity baseline (Fig. 15-2).
3. Calculation of stroke volume
SV can be calculated at many different locations within the heart or great vessels by using the appropriate Doppler velocity signal to determine the VTI at the same location that 2D imaging is used to determine CSA (Fig. 15-3).
SV (cm3) = CSA (cm2)×VTI (cm)
The VTI is usually measured with pulse wave Doppler; however, continuous wave Doppler may be utilized to determine the aortic valve VTI in the absence of subvalvular or supravalvular aortic obstruction.
 ▪ FIGURE 15.1 |
 ▪ FIGURE 15.2 |
 ▪ FIGURE 15.3 |
TABLE 15.2 ASSUMPTIONS FOR ACCURATE DOPPLER STROKE VOLUME CALCULATIONS | |||||
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4. Cross-sectional area
Most often the CSA of the “orifice” to be measured is assumed to be circular and thus can be calculated using the formula for the area of a circle (of radius r) after measuring the orifice diameter (D) in cm:
5. Assumptions in Doppler SV calculation (Table 15-2)
a. Laminar blood flow with a flat flow velocity profile
Blood flow is assumed to be laminar and the spatial flow velocity profile is assumed to be flat, as is generally the case in the LVOT (Fig. 15-4). The narrow band of velocities and smooth spectral signal obtained with pulsed wave Doppler are evidence of laminar flow in the great vessels and across normal cardiac valves. A flat flow velocity profile can be demonstrated by showing uniform velocities while moving the pulsed wave Doppler sample volume from side to side within the flow of interest from two orthogonal views.
b. Simultaneous CSA and VTI measurement at the same location
CSA and VTI measurements are assumed to be made at the same time and at the same anatomic location. Diameter is measured most
accurately when the ultrasound beam is perpendicular to the blood-tissue interface, while VTI is measured most accurately when the ultrasound beam is parallel to blood flow. Thus, diameter measurements and Doppler velocity profiles are usually not recorded from the same imaging plane. Efforts should be made to perform these measurements at the same anatomic location and in close sequence in order to minimize error in the calculated SV.
accurately when the ultrasound beam is perpendicular to the blood-tissue interface, while VTI is measured most accurately when the ultrasound beam is parallel to blood flow. Thus, diameter measurements and Doppler velocity profiles are usually not recorded from the same imaging plane. Efforts should be made to perform these measurements at the same anatomic location and in close sequence in order to minimize error in the calculated SV.
 ▪ FIGURE 15.4 |
c. Average VTI measurement
The VTI used in calculating SV is assumed to represent the average VTI. Therefore, several measurements should be averaged for a patient in normal sinus rhythm, whereas 8 to 10 measurements should be averaged for a patient in atrial fibrillation, in order to most accurately estimate the average VTI.
d. Accurate CSA measurement
Changes in CSA during the flow period or deviations from an assumed geometry (usually circular) are inherent problems in Doppler SV calculations. Accurate determination of 2D measurements for calculation of CSA is essential. In the case of an assumed circular orifice, a small error in diameter measurement will result in a large error in the calculated CSA due to the quadratic relationship between the radius and area of a circle (i.e., CSA = II r2).
e. Accurate VTI measurement
The VTI is assumed to be recorded with the ultrasound beam parallel to the flow (i.e., the intercept angle θ = 0). In this case the velocities measured by Doppler are accurate based on a cosine θ = 1
in the Doppler equation (Fig. 15-5). However, as θ increases from 20 to 60 degrees, the error in the calculated Doppler velocity increases from 6% to 50% (Fig. 15-6). The highest velocity signal obtained (the loudest audio signal) will correlate with the most parallel alignment of the Doppler beam with blood flow.
in the Doppler equation (Fig. 15-5). However, as θ increases from 20 to 60 degrees, the error in the calculated Doppler velocity increases from 6% to 50% (Fig. 15-6). The highest velocity signal obtained (the loudest audio signal) will correlate with the most parallel alignment of the Doppler beam with blood flow.
 ▪ FIGURE 15.5 |
 ▪ FIGURE 15.6 |
B. Cardiac output
1. Calculation of cardiac output
CO can be estimated with 2D Doppler after determining a Doppler SV and measuring heart rate (HR)5:
CO (L / min) = SV (cm3) × (1 L / 1,000 cm3 × HR (bpm)
CO measurements performed with TEE, usually measured at the LVOT or aortic valve in the absence of aortic regurgitation, have been shown to correlate well with measurements made by thermodilution. Cardiac index (CI) can be calculated by dividing CO by body surface area (BSA):
CI (L /min /m2) = CO (L / min) / BSA (m2)
2. Preferred sites for CO calculation: LVOT and aortic valve
a. Laminar blood flow with a flat flow velocity profile
The acceleration of blood through the LVOT or aortic valve during systole favors laminar flow with a flat flow velocity profile, in contrast to the parabolic flow velocity profile present in the ascending aorta or pulmonary artery (PA).
b. Excellent views for CSA estimation
Multiplane TEE provides excellent views of the LVOT and aortic valve for accurate determinations of LVOT diameter and aortic valve CSA.
c. Little change in CSA
The LVOT is more circular and changes shape very little during the cardiac cycle when compared to the main PA or mitral valve. Measurements made at the main PA or mitral valve are less reliable than those made at the LVOT and aortic valve.6 Although the CSA of the aortic valve orifice changes dramatically throughout systole, the CSA of the aortic valve during midsystole can be used to provide a good estimate of transaortic SV by Doppler.
C. Data for LVOT stroke volume calculation
1. VTILVOT
The pulsed wave Doppler sample volume is placed in the LVOT just proximal to the aortic valve (˜1 cm) using either the transgastric long-axis view or the deep transgastric long-axis view for determination of the VTILVOT (Fig. 15-7A, B).
2. CSALVOT
The diameter (cm) of the LVOT is best obtained from the midesophageal long-axis view of the aortic valve (approximately 1 cm proximal to the valve) for determination of CSA using the formula for the area of a circle (Fig. 15-7C):
CSALVOT (cm2) = 0.785×DLVOT2
 ▪ FIGURE 15.7 |
 ▪ FIGURE 15.7 (CONTINUED) |
 ▪ FIGURE 15.8 |
 ▪ FIGURE 15.8 (CONTINUED) |
D. Data for transaortic valve stroke volume calculation
1. VTIAV
The continuous wave Doppler beam is placed through the aortic valve from either the transgastric long-axis view or the deep transgastric long-axis view for determination of the VTIAV (Fig. 15-8A, B).
2. CSAAV
Planimetry can be used to measure the area (cm2) of the aortic valve orifice during midsystole from a cine of the midesophageal short-axis view of the aortic valve (Fig. 15-8C).7 Alternatively, a cine of a “normal” aortic valve from the same midesophageal short-axis view is used to measure the side (S) in cm of the equilateral opening of the valve during midsystole (Fig. 15-8D). Several measurements may be made and then averaged in order to improve accuracy. The formula for the area of an equilateral triangle is then used to calculate the CSA of the aortic valve:
CSAAV (cm2) = 0.433×(S)2
E. Data for main PA stroke volume calculation
1. VTIPA
The pulsed wave Doppler sample volume is placed in the main PA using the upper esophageal short-axis view of the aortic arch (with the transducer rotated from 80 to 90 degrees) or the midesophageal shortaxis view of the aorta for determination of the VTIPA (Fig. 15-9A).
2. CSAPA
The diameter (cm) of the main PA is obtained from either view at the same location for determination of the CSA using the formula for the area of a circle (Fig. 15-9B, C):
CSAPV (cm2) = 0.785×DPA2
 ▪ FIGURE 15.9 |
F. Data for RVOT stroke volume calculation
1. VTIRVOT
The RVOT may be visualized using a transgastric RV inflow-outflow view with the transducer rotated from 110 to 150 degrees and the probe turned to the right. The pulsed wave Doppler sample volume is placed in the RVOT just proximal to the pulmonic valve for determination of the VTIRVOT (Fig. 15-10A).
 ▪ FIGURE 15.10 |
2. CSARVOT
The diameter (cm) of the RVOT is best obtained from the same view at the same location for determination of CSA using the formula for the area of a circle:
CSARVOT (cm2) = 0.785×DRVOT2
Alternatively, the diameter (cm) of the RVOT can be measured from the upper esophageal short-axis view of the aortic arch in some patients (Fig. 15-10B).
G. Data for transmitral stroke volume calculation
1. VTIMV
The pulsed wave Doppler sample volume is placed at the level of the mitral valve annulus using the midesophageal four-chamber view (alternatively, the midesophageal two-chamber view or midesophageal long-axis view may be used) for determination of the VTIMV (Fig. 15-11A, B).
2. CSAMV
While the mitral valve orifice is not truly elliptical during diastole, it is more elliptical than circular. The American Society of Echocardiography has concluded that assumption of a circular orifice has generally worked well for all valves other than the tricuspid in its document on Quantitation of Doppler Echocardiography.8 Nevertheless, it may be preferable to estimate the CSA of the mitral valve using the formula for an ellipse. The long and short diameters (cm) of the mitral valve annulus can be approximated using measurements from the midesophageal four-chamber and two-chamber views (Fig. 15-11C, D). The formula for an ellipse can then be used to calculate the CSA of the mitral valve:
CSAMV (cm2) = 0.785×D1 × D2
III. DOPPLER MEASUREMENT OF PULMONARY-TO-SYSTEMIC FLOW RATIO
A. Calculation of Qp/Qs
The ratio of pulmonic to systemic blood flow, Qp/Qs, usually indicates the magnitude of a shunt (i.e., atrial septal defect, ventricular septal defect, or patent pulmonary ductus arteriosus) and may be useful information in determining the need for surgery or the timing of surgery. Qp/Qs can be calculated once the systemic SV (measured at the LVOT or aortic valve) and pulmonic SV (measured at the PA or RVOT) have been determined9:
Qp / Qs = (SVPA × HR) / (SVLVOT × HR)
B. Limitations
1. Doppler SV measurement
Potential errors in the estimation of Qp/Qs are the same as for any Doppler determination of SV.
2. Propagation of errors
It should be noted that there is also the possibility of compounding calculated Doppler SV errors in the calculation of Qp/Qs with this formula (i.e., if SVPA is overestimated and SVLVOT is underestimated, then Qp/Qs may be significantly overestimated).
 ▪ FIGURE 15.11 |
 ▪ FIGURE 15.11 (CONTINUED) |
3. Aortic regurgitation
In the presence of significant aortic regurgitation this calculation is not accurate and Qp/Qs will be underestimated.
IV. DOPPLER MEASUREMENT OF REGURGITANT VOLUME AND FRACTION
A. Volumetric method
RV is the volume of blood that flows backward through a regurgitant valve during one cardiac cycle. Conservation of mass says that the
SV delivered to the systemic circulation (SVSYSTEMIC) must equal the total forward SV across a regurgitant valve (SVTOTAL) minus the RV (Fig. 15-12):
SV delivered to the systemic circulation (SVSYSTEMIC) must equal the total forward SV across a regurgitant valve (SVTOTAL) minus the RV (Fig. 15-12):
SVSYSTEMIC = SVTOTAL-RV
or
RV=SVTOTAL-SVSYSTEMIC
The regurgitant fraction (RF) for any valve is calculated as the ratio of RV to total forward flow across the regurgitant valve expressed as a percentage:
RF (%) = (RV / SVTOTAL) × 100%
B. Assessment of mitral regurgitation
1. Calculation of RVMV
In mitral regurgitation, the SVTOTAL is the mitral inflow SV and the SVSYSTEMIC is the LVOT SV. Thus, the mitral valve RV can be estimated by subtracting the LVOT SV from the mitral valve inflow SV and then the mitral RF can be calculated (Fig. 15-13).10
RVMV = SVMVI-SVLVOT
RFMV (%) = (RVMV / SVMVI) × 100%
2. Limitations
a. Doppler SV measurement
Potential errors in the estimation of RVMV are the same as for any Doppler determination of SV.
 ▪ FIGURE 15.12 |
 ▪ FIGURE 15.13 |
b. Propagation of errors
It should be noted that there is also the possibility of compounding calculated Doppler SV errors in the calculation of RVMV with this formula.
c. Aortic regurgitation
In the presence of significant aortic regurgitation this calculation is not accurate and mitral RV will be underestimated.
C. Assessment of aortic regurgitation
1. Calculation of RVAV
In aortic regurgitation, the SVTOTAL is the LVOT forward SV and the SVSYSTEMIC is the mitral valve inflow SV. Thus, the aortic valve RV can be estimated by subtracting the mitral valve inflow SV from the LVOT forward SV and then aortic RF can be calculated (Fig. 15-14).10
RVAV – SVLVOT – SVMVI
RFav (%) = (RVAV / SVLVOT) × 100%
2. Limitations
a. Doppler SV measurement
Potential errors in the estimation of RVAV are the same as for any Doppler determination of SV.
b. Propagation of errors
It should be noted that there is also the possibility of compounding calculated Doppler SV errors in the calculation of RVAV with this formula.
c. Mitral regurgitation
In the presence of significant mitral regurgitation this calculation is not accurate and aortic RV will be underestimated.
D. Proximal convergence method
1. PISA concept
As blood flows toward a regurgitant orifice (i.e., mitral regurgitation), or in some cases a stenotic orifice (i.e., mitral stenosis), blood flow velocity increases with the formation of multiple concentric “isovelocity” shells (Fig. 15-15A, B).10,11 These “isovelocity” shells can be “seen” with color flow imaging (Fig. 15-16A, B) and have been termed proximal isovelocity surface areas (PISAs). The size of a PISA proximal to a regurgitant orifice can be altered by adjusting the Nyquist limit of the color flow map. As the negative aliasing velocity is reduced (in the case of mitral regurgitation), the transition from red to blue will occur farther from the regurgitant orifice resulting in a hemispheric shell with a larger radius (r). The instantaneous velocity of blood at the PISA is the same as the aliasing velocity on the color flow map.
2. PISA flow rate
The instantaneous flow rate through a PISA that is a hemispheric shell is equal to the product of the area of the PISA and the instantaneous velocity of blood at the PISA:
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