Scholarly article on topic 'Quantification of AS and AR'

Quantification of AS and AR Academic research paper on "Medical engineering"

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Academic research paper on topic "Quantification of AS and AR"

Echo Tutorial

Quantification of AS and AR

Yatin Mehta, Rajni Singh

Department of Anaesthesia and Critical Care, Indraprastha Apollo Hospitals, New Delhi, India

ABSTRACT Trans-esophageal echocardiography (TEE) is routinely used in valvular surgery in most institutions. The popularity of TEE stems from the fact that it can supplement or confirm information gained from other methods of evaluation or make completely independant diagnoses. Quantitative and qualitative assessment permits informed decisions regarding surgical intervention, type of intervention, correction of inadequate surgical repair and re-operation for complications. This review summarizes the various methods for quantification of aortic regurgitation and stenosis on TEE. The application of Doppler echo (pulsed wave, continuous wave and color) with two-dimensional echo allows the complete evaluation of AV lesions.

Received: 19-11-08

Accepted: 24-02-09 Key words: Trans-esophageal echocardiography, aortic stenosis, aortic regurgitation, intra operative, echo

DOI: 10.4103/0971-9784.53437

INTRODUCTION

Trans-esophageal echocardiography (TEE) is used intra-operatively to evaluate aortic valve (AV) anatomy, function and hemodynamics.111 Quantitative and qualitative assessment permits informed decisions regarding surgical intervention, type of intervention, correction of inadequate surgical repair and reoperation for complications. Pre-bypass TEE evaluation also helps in identifying myocardial and valvular abnormalities associated with AV lesions and determining the size of valve to be implanted.

Inra-operative TEE among patients with with known AV disease undergoing valve surgery is used to confirm the pre-operative diagnosis and define the etiology of valve dysfunction. High resolution images owing to the close proximity of the valve and the esophagus permit accurate diagnosis of the mechanism of valve dysfunction, a key aspect of feasibility of repair vs replacement.

Post-operatively, TEE is used to evaluate the success of repair or the function of the prosthetic valve. Patients undergoing Ross procedure also require TEE for evaluation of the prosthetic pulmonary valve.

TEE views for assessment of AV

1. Mid-esophageal short axis view

2. Mid-esophageal AV long axis

3. Trans-gastric long axis

4. Deep trans-gastric long axis

Mid esophageal short axis view

It is obtained from the mid-esophageal window by advancing or withdrawing the probe until the AV comes into view and then turning the probe to centre the AV on display. The image depth is adjusted to 10-12 cm to position the AV in the middle of the display. Then the multi-plane angle is rotated to aprox. 20 to 60 degrees until a symmetrical image of all the three cusps of the AV comes into view. This cross section is the only view that provides a simultaneous image of all the three cusps of the AV.[2]

It is used to measure the area of the AV orifice by planimetry [Figure 1]. Color Flow Doppler (CFD) is applied in this cross section to detect AR and the size, shape and location of the regurgitant orifice.

Mid-esophageal long axis view

Keeping the AV in the centre of display, rotate the multi-plane angle to 120 to 160 degree until the LVOT, AV and proximal ascending

Address for corresspondance: Dr. Yatin Mehta, Department of Anaesthesia and Critical Care, Indraprastha Apollo Hospitals, New Delhi, India. E-mail: yatinmehta@hotmail.com

aorta line up in the image [Figure 2]. It is the best cross section to assess the size of aortic root by measuring the diameter of the AV annulus, sinuses of valsava, sino-tubular junction and proximal ascending aorta adjusting the probe to maximize the internal diameter of these structures.

Mid-esophageal view with CFD is used to assess flow through the LVOT, AV and proximal ascending aorta and is especially useful for detecting and quantifying AR.

Trans-gastric views

The primary purpose is to direct Doppler beam parallel to flow through the aortic valve.

Also, it may provide good images of the ventricular

aspect of the AV in some patients.[2]

Deep Transgastric long axis view is obtained by advancing the probe deep into the stomach and positioning it adjacent to the LV apex. The probe is then ante-flexed until the imaging plane is directed superiorly towards the base of the heart, developing the deep transgastric long axis view [Figure 3]. Detailed assessment of valve anatomy is difficult as LVOT and AV are far from the transducer but Doppler quantification of flow velocities is usually possible [Figure 4].

Echo assessment of aortic stenosis

A two-dimensional /Visual Assessment of AV is best done in ME AV SAX view. It gives information about: Number of cusps Calcification

Figure 1: Mid esophageal short axis view of the aortic valve used for estimation of artic valve area by planimetry. The cross section of the left coronary and right coronary artery can be seen by withdrawing the probe slightly

Figure 2: Av annulus, STJ and are aorta mean ME long axis view of the aortic root. Precise measurements of aortic valve annulus sinotubular junction and ascending aorta are important for determiing the size of prosthetic valve

Figure 3: Deep transgastric view. The aortic valve is seen in long axis at the far end of the image. It is the view used most often for Doppler interrogation of aortic valve and LVOT as it allows a near parallel alignment of the Doppler beam across the aortic root

Figure 4: CW Doppler interrogation of the aortic root in severe aortic stenosis in deep transgastric view

Planimetry of valve can be done. However, in calcified valve, proper delineation of orifice circumference is difficult and erronous.

Hemodynamic assessment parameters

1. Peak aortic velocity

2. Mean pressure gradient

3. Estimated AVA

4. Dimensionless index (LVOT VTI/AV VTI) [Table 1]

Normally a slight pressure difference (1-2 mmHg) is required between the LV and the aorta for aortic valve to open and blood to be ejected from the LV to aorta. The normal aortic blood flow is laminar and most of the red cells in the aortic root are moving at approximately same speed. Normal peak velocity across the AV rarely exceeds 1.5 m/s. When the AV is stenosed, systolic pressure in the LV must go high enough to force the blood across the obstruction into the aorta. This leads to both turbulent flow and increased velocities: Two characteristics readily detected by Doppler echocardiography.131 In severe AS, peak velocity exceeds 4 m/s. The pressure drop, as blood is forced from LV to aorta, leads to generation of pressure gradients that can exceed 100 mg/Hg in systole. Peak velocity is obtained by applying CW Doppler across the AV in deep trans-gastric long axis view. Color Doppler may be used for alignment of Doppler beam and considerable operator skill is required to acquire an adequate spectral tracing. Incompletely formed tracings are inadequate and should never be used for estimation.

Table 1: Gradient of severity of aortic stenosis on TEE parameters

Valve Peak Mean LVOT/

area velocity PG AVVTI

(cm2) (m/s) ratio

Normal 3.0-4 1.4-2.2

Mild 1.5-2.9 >2.5 <20

Moderate 1.0-1.5 3-4 30-50

Severe <1.0 >4 >50 <0.25

Differentiation of spectral profile of AS from that of MR should be done by recording the onset in relation to ECG tracing; in addition MR VTI is longer in duration.131 Careful tracing of the spectral profile gives the peak velocity, peak gradient and mean gradient. Peak gradient obtained by Doppler is higher than the 'peak to peak' gradient reported in cardiac catheterization. Therefore, mean pressure gradient is used to grade severe AS.

In patients with normal LV function, AS is severe if :

• Peak AV velocity is greater than 4.5 m/s

• Mean PG is greater than or equal to 50 mmHg

When LV function and cardiac output are abnormal, aortic stenosis may be assessed erroneously from the peak velocity and mean PG as a dysfunctional LV is able to generate the high velocities across the AV and consequently the pressure gradients are correspondingly lower.

Calculation of AVA by continuity equation overcomes this error [Figure 5].

The principle is that forward volume flow on ventricular side of the valve is the same as forward flow on the aortic side- SV(stroke volume) through LVOT = SV through AV

AVA X VTIAV = D2 x 0.785 x VTIlvot

AVA = D2 x 0.785 x VTIlvot VTI

v 1JAV

ava = svlyot

D = LVOT diameter (2 D echo)

Aj X Vj = A2 X V2

Figure 5: Continuity equation in aortic stenosis, the systolic flow passes through the LVOT at one velocity (V,) and then is rapidly accelerated to a higher velocity (V2) through the narrowed area of the stenotic orifice

VTI LVOT = velocity time integral access LVOT (PWD 1 cm proximal to AV in LVOT)

VTI AV = velocity time integral across AV (CWD through the AV)

If LVOT diameter cannot be accurately measured, ratio of VTI „.__ to VTI ... can be used

LV O! AV

In severe AS:

• AVA less than or equal to 1 cm2

• LVOT/AV VTI ratio is less than or equal to 0.25

AORTIC REGURGITATION

Assessment and quantification of AR is based on a comprehensive utilization of 2D echocardiography, color flow images, pulsed and CW Doppler techniques [Table 2].

2-D echocardiogarphy

Provides important information regarding :

• Valve anatomy and structural deformities-Bicuspid valve, dissection, endocarditis

• Presence and severity of aortic root dilatation

• Adaptation of LV to the volume over-load state.[1]

• All these indirectly indicate the severity as well as duration (Acute vs Chronic) of AR.

Doppler method

Clor flow Doppler: CF imaging directly shows the regurgitant flow through the aortic valve during diastole [Figure 6]. The regurgitant flow has 3 components that help in quantification of AR:

• Jet direction and size in the LV

• Vena contracta through the regurgitant orifice

• Flow convergence region in the aorta.

Regurgitant Jet size is used in all patients because of simplicity and real time availability, however, length of jet penetration into LV is an unsatisfactory indicator of AR severity.

Proximal jet width is the preferred assessment. It is measured immediately below the AV within one cm of

Table 2: Quantitative and qualitative parameters useful in grading aortic regurgitation

Structural parameters Mild Moderate Severe

LV size Normal Normal or Usually dilated

(Chronic AR) dilated (chronic AR)

Aortic leaflets Normal or Normal or Abnormal, flail or

abnormal abnormal wide coaptation

defect

Doppler

parameters

Jet width in Small, in Intermediate Large in central

LVOT-CFD central jets jets, variable in

ecc. jets

Jet density-CW Incomplete/ Intermediate Dense

Jet deceleration >500 500-200 <200

time(ms)-CW

Diastolic flow Brief, early Intermediate Prominent

reversa in Desc diastolic holodiastolic

Aorta- PW reversal

Quantitative

methods

VC width (cm) <0.3 0.3-0.6 >0.6

Jet width/LVOT <25 25-65 >65

width (%)

Reg vol. (ml/ <30 30-60 >60

Reg <30 30-50 >50

fraction (%)

EROA (cm2) <0.10 .10-0.3 >0.3

Figure 6: Color Doppler images of mild and severe AR. Further quantification is done by measurement of the width origin the regurgitation jet (vena contracta, the spatial orientation and flow convergence into the regurgitant orifies (PISA))

the valve and ratio to LVOT width indicates severity of AR. Regurgitant orifice should be relatively round for accurate assessment, elliptical regurgitant orifice as in bicuspid valve may lead to under-estimation; AV SAX view helps identify such cases.

Jet eccentricity: Central jets appear larger while those directed towards AML or ventricular septum are under-estimated.

6.28 x r2 x aliasing velocity Peak AR velocity

Regurgitant vol = ERO area X regurgitant VTI

PISA is not a preferred method for AR as calculations are accurate only if the region of PISA appears hemispheric and is well visualized for which sound waves should be parallel to blood flow.

Limitations of color flow method

Jet shape should be parallel to LVOT.

Eccentric jets that are directed predominantly to the AML or septum are under-estimated. Central jets may be over-estimated.

Diffuse jets are poorly evaluated.

Pulsed wave Doppler

Regurgitant volume (RV) and regurgitant fraction (RF) by continuity equation

In the presence of AR flow (stroke volume) through the affected valve is greater than through other competent valves. The difference between the two gives regurgitant volume.

In practice, assessment of AR based on jet size in LVOT is most often based on visual measurement rather than a quantitative measurement.

Jet width/LVOT width greater than 0.65 indicates severe AS Vena Contracta

It is defined as the smallest neck of the flow region at the level of the AV immediately below the flow convergence region. Vena Contracta provides an estimate of EROA. Effective Regurgitant Orifice Area

Limitation - Multiple jets, Jets with irregular shapes cannot be evaluated satisfactorily.

Thresholds of Vena Contracta width associated with severe AR are; 0.5 cm is a highly sensitive threshold, 0.7 is highly specific threshold and 0.6 cm is the threshold with best combination of sensitivity and specificity.151

Flow convergence or PISA

There is considerably less experience with PISA for AR assessment. The regurgitant flow rate across the AV is obtained from the flow rate of proximal surface area with a known flow velocity. "r" is the radius from the alias line to the orifice

Peak AR flow rate = 2 (r)2 x Aliasing velocity Peak flow rate

SV = CSA X VTI nd2

= 0.785 d2 X VTI

RV SVReg value SV normal value

In the absence of significant MR, mitral valve inflow can be used to represent systemic stroke volume

RV..... = LVOT flow - MV flow

= 0.785 X d2 X VTI (LVOTJ - 0.785 X d2 X VTI (MV)

EROA =

Peak AR velocity (CWD)

AorticRV xlQQ% LVOT SV

Aortic diastolic flow reversal

A brief reversal of flow during diastolic is normal in the aorta. With increasing AR, both the duration and velocity of reversal increase. Therefore, a holodiastolic reversal is usually a sign of at least moderate AR and is more specific if recorded from the thoracoabdominal aorta. Reduced arterial compliance increases it patients with mild and moderate AR. So AR may be overestimated by this method in advancing arteriosclerosis.

Continuous wave Doppler

Density of CW signal reflects the volume of regurgitation especially in comparison to antegrade spectral density.

It is an imperfect indicator of severity of AR. A faint AR jet indicates mild AR but there is significant overlap between mild and severe regurgitation in more dense jet recordings.[4]

Diastolic jet deceleration

Rate of deceleration: The diastolic regurgitant jet and the derived pressure half time reflect the rate of equalization of aortic and LV diastolic pressures. With increasing severity of AR, aortic diastolic pressure decreases more rapidly.

Pressure half time is easily measured if the peak diastolic velocity is appropriately recorded. A pressure half time greater than 500 ms is compatible with mild AR while PHT less than 200 ms is consistent with severe AR.[4]

Limitations

PHT will be further shortened by an elevated LV diastolic pressure (stiff ventricle, acute AR) or by vasodilator therapy which reduces AR.[5,6] PHT will be lengthened or normalized with chronic LV adaptation to severe AR.

CONCLUSION

TEE is currently being used routinely during aortic valve replacement (AVR). It provides information that can lead to modifications of anesthetic and surgical care which in turn lead to improved outcome. Numerous studies have shown that modifications in therapy occur from 10% to more than 40% of cases.[7] The impact of TEE can be divided among modifications of therapy before, during, and after

cardiopulmonary bypass. Before cardiopulmonary bypass, TEE can provide prognostic information, optimize hemodynamics, and diagnose conditions that were not appreciated before surgery, including patient—prosthesis mismatch. After bypass, TEE verifies the surgical result, rules out left and right ventricular outflow tract obstruction, and assures stable hemodynamics.

REFERENCES

1. Troianos CA. Assessment of the arotic valve. In (Eds. Savage RM, Aronson S) Comprehensive Textbook of Intraoperative Transesophageal Echocardiography. Lippincott: Williams and Wilkins; 2004. p. 205-18.

2. Shanewise JS, Cheung AT, Aronson S, Stewart WJ, Weiss RL, Mark JB, et al. ASE/SCA gudielines for performing a comprehensive intraoperative multiplaine transesophageal echocardiography examination. Recommendations of the American Society of Echocardiography council for intraoperative Echocardiography and the society. Anesth Analg 1999;89:870-84.

3. Kisslo JA, Adams DB. Valvular stenosis in Doppler evaluation ofvalvular stenosis. Available from: www.echoincontext.com

4. Tribouilloy CM, Enriquez-Sarano M, Bailey KR, Seward JB, Tajik AJ. Assessment of Severity ofAortic Regurgitation Using the Width of the Vena Contracta: A Clinical Color Doppler Imaging Study. Circulation 2000;102:558-64.

5. Teague SM, Heinsimer JA, Anderson JL, Sublett K, Olson EG, Voyles WF, et al. Quantification of aortic regurgitation utilizing continuous wave Doppler ultrasound. J Am Coll Cardiol 1986;8:592-9.

6. Padial LR, Oliver A, Vivaldi M, Sagie A, Freitas N, Weyman AE, et al. Doppler echocardiographic assessment of progression of aortic regurgitation. Am J Cardiol 1997;80:306-14.

7. Qizilbash CB. Couture P, Denaut A. Impact of perioperative transesophageal echocardiography in aortic valve replacement. Semin Cardiothorac Vasc Anesth 2007;11:288.

Cite this article as: Mehta Y, Singh R. Quantification of AS and AR. Ann Card Anaesth 2009;12:173.

Source of Support: Nil, Conflict of Interest: None declared.

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