Shunt quantification

79
CARDIAC SHUNT QUANTIFICATION

description

shunt detection and quantification various methods

Transcript of Shunt quantification

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CARDIAC SHUNT

QUANTIFICATION

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shunts are abnormal communications between the systemic circulation and pulmonary circulation.

Detection, localisation and quantification of intracardiac shunts form an integral part of the hemodynamic evaluation of patients with congenital heart disease

Cardiovascular are quantified by measuring the ratio of pulmonary blood flow (Qp) to systemic blood flow that is, Qp:Qs.

The extent of a shunt is determined by the size of the defect and the left-to-right pressure gradient.

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Indicator dilution techniques◦ Invasive cardiac catheterization – oximetry and

angiocardiography◦ Cardio green , ascorbic acid, H2 inhalation

Radionuclide methods Phase contrast MRI Echocardiography

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In the oximetry run the oxygen content or % saturation is measured in PA,RV,RA,VC.

A left-to-right shunt may be detected and localized if a significant step-up in blood oxygen saturation or content is found in one of the right heart chambers

A significant step-up is defined as an increase in blood oxygen content or saturation that exceeds the normal variability that might be observed if multiple samples were drawn from that cardiac chamber.

Oximetry run

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1. Left-to-right Intracardiac Shunts - Oximetry run

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Oxygen content The technique of the oximetry run is based on the

pioneering studies of Dexter and his associates in 1947

Oxygen content was measured by Van Slyke technique , and other manometric studies

It was found that multiple samples drawn from the right atrium could vary in oxygen content by as much as 2%.

The maximal normal variation within right ventricle was found to be 1%.

Because of more adequate mixing, a maximal variation within the pulmonary artery was found to be only 0.5%.

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Thus using Dexter Criteria a significant step up is present

at the atrial level when highest oxygen content in blood samples drawn from the right atrium exceeds the highest content in the venae cavae by 2 vol %.

at the ventricular level, if the highest right ventricular sample is 1 vol % higher than the highest right atrial sample.

at the level of the pulmonary artery if the pulmonary rtery oxygen content is more than 0.5% vol% higher than the highest right ventricular sample.

1 vol% = 1ml O2/100ml blood or 10mlO2/l

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O2 content Vs O2 saturation Dexter ‘s study described normal variability and

gave the criteria for a significant step-up only for measurement of blood oxygen content.

In recent years nearly all cardiac cath laboratories have moved toward the measurement of percentage oxygen saturation by spectrophotometric oximetry as the routine method for oximetric analysis of blood samples.

Oxygen content may then be calculated as : Hb × 1.36 (ml O2/g of hb)×10×%

saturation

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But oxygen content derived in this manner is less accurate than by Van Slyke or other direct oximetric technique.

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Antman and coworkers prospectively studied the normal variation of both oxygen content and oxygen saturation of blood in the right heart chambers

Pts. without intracardiac shunts who were undergoing diagnostic cath.

Oxygen content and Oxygen saturation was calculated Finally it was concluded that O2 sat. and O2 content

correlate well and also proposed that systemic blood flow and mixing of blood both determine step up of O2 levels.

Antman EM. Blood oxygen measurements in the assessment of intracardiac left to right shunts: a critical appraisal of methodology. Am J Cardiol 1980

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CHAMBER LEVEL

STEP UP

GARSON MOSS AND ADAMS

GROSSMAN (MEAN OF SAMPPLES) SINGLE

SAMPLEMULTIPLE SAMPLES

SVC TO RA 7 5 9 7

RA TO RV 5 3 6 5

RV TO PA 4 3 6 5

Step up values

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Procedure of oximetry run 2-mL sample from each of the following locations.1. Left and/or right pulmonary artery & Main pulmonary

artery2. Right ventricle, outflow tract, mid & tricuspid valve .3. Right atrium, low or near tricuspid valve , mid & high .4. Superior vena cava, low (near junction with right

atrium).5. Superior vena cava, high (near junction with innominate

vein).6. Inferior vena cava, high (just at or below diaphragm).7. Inferior vena cava, low (at L4-L5).8. Left ventricle.9. Aorta (distal to insertion of ductus).

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SVC sample

Taken at mid SVC level: below the innominate and above the azygous vein

A location that is too high may provide a sample from the axillary (peripheral arm) vein and give an erroneously high O2 saturation and a sample from the internal jugular vein can give an erroneously low saturation.

A sample obtained too low in the SVC (at, or close to, the superior vena cava–right atrial junction) may actually include some blood refluxing into the SVC from the right atrium.

5- 10 % lower than IVC sample (higher in GA)

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IVC Sample

True IVC sample is taken below the hepatics. Slight catheter manipulation causes significant

change in values. Greatest streamlining occurs close to IVC RA

junction. Samples close to coronary sinus are as low as 25-

40%, from close to renal veins can be as high as 90%, saturations from hepatic veins are intermediate between CS and renal vein saturations.

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Right Atrium

Right atrial sample should be taken at lateral mid atrial wall to avoid the low saturation stream from coronary sinus and to facilitate mixing from IVC and SVC streams

Moss and adams Heart disease in infants, children and adolescent 8th edition

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Procedure of oximetry run In performing the oximetry run, an end-hole

catheter (e.g., Swan-Ganz balloon flotation catheter) or one with side holes close to its tip (e.g., a Goodale-Lubin catheter) can be used.

The catheter tip position further confirmed by pressure measurements at the sites noted.

The entire procedure should take less than 7 minutes.

If a sample cannot be obtained from a specific site because of ventricular premature beats, that site should be skipped until the rest of the run has been completed.

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An alternative method for performing the oximetry run is to withdraw a fiberoptic catheter from the pulmonary artery through the right heart chambers and the inferior and superior vanae cavae.

Uses fiberoptic catheters, which work on spectrophotometric principals for analysis.

The output signal from the fiberoptic catheter is displayed as a continuous graph of the percent saturation.

This permits a continuous read out of oxygen saturation that follows detection of a step-up in oxygen content.

The greatest problem with fiberoptic catheters is the catheters themselves, as they are not suitable for easy manipulation within the heart.

Continuous fiberoptic oxymetry

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O2 saturation by spectrophotometry :◦ Based on Beers law ◦ Advantages : quick ,accurate, precise , subject to

few errors , less dependency on Hb% .◦ Disadvantages :

Inaccurate if large amounts of carboxy hemoglobin is present

Indocyanin green interfere with light source of spectrphotometry

Elevated bilirubin affect absorbtion of light

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Disadvantages of oxygen content technique 15 – 30 min for obtaining a reading Technically difficult to perform Dependency on Hb content

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Limitations of Oximetry Method

1. Antman and coworkers shows that oxygen saturation influenced by the magnitude of systemic blood flow.

◦ High levels of systemic flow tend to equalize the arterial and venous and low levels increase difference.

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Therefore, elevated systemic blood flow will cause the mixed venous oxygen saturation to be higher than normal, and interchamber variability owing to streaming will be blunted.

Even a small increase in right heart oxygen saturation might indicate presence of significant left to right shunt

Larger increase would indicate voluminous left to right shunting of blood.

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Limitations of Oximetry Method2. Antman and colleagues , the influence of blood

hemoglobin concentration may be important when blood O2 content (rather than O2 saturation) is used to detect a shunt

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A primary source of error may be the absence of steady state during the collection of blood samples. That is if oxymetry run is prolonged because of technical difficulties, if the patient is agitated, or if arrhythmias occur during the oximetry run, the data may not be consistent.

It lacks sensitivity. Small shunts are not consistently detected by this technique. Most shunts of a magnitude that would lead to recommendation for surgical closure would be detected.

Limitations

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Qualitative by oximetry and next Quantitative by flow ratio

Quantification is done by Qp , Qs , Qp/Qs , Effevtive blood flow, L-R shunt , R-L shunt .

Qp and Qs are amount of blood flowing through pulmonary and systemic vascular bed

Qef is quantity of mixed venous blood that carries desaturated blood from systemic capillaries to be oxygenated by lungs

L-R and R-L shunt are amount of blood that bypass systemic and pulmonary vascular bed .

Left-to-right Intracardiac Shunts - Flow ratio

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Qp , Qs , Qeff are based on Ficks principle for calculation of cariac output

Cardiac output = VO2 / AVO2 difference

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Qs = V O2/ m² (SA % Sat - MV % Sat) x1.36 xHb x10

Qp = V O2/ m² (PV % Sat - PA % Sat) x 1.36 x Hbx 10

In normal circulatory state mixed venous saturation is same as pul artery saturation and the saturation of pul vein is same as that of systemic arteries.

Hence calculated QP is equal to Qs

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Any shunt from saturated left side of heart to rt side causes a increase in the pul artery saturation and hence decrease in the denominator value of Qp calculation, thus resulting in a higher value of Qp, and Qp/Qs of > 1.

When the pulmonary blood flow is markedly increased (e.g., pulmonary artery saturation 89%), the difference in pulmonary vein and pulmonary artery saturation is small (e.g., 99% - 89%), so the normal error that occurs with each measurement (±2%-3%) becomes significant. Thus, when there is a large left-to-right shunt, the Qp/Qs is simply reported as greater than 3:1.

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Points of importance while calculation:1. Oxygen consumption2. Calculation of saturations3. Oxygen content

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Oxygen consumption:◦ Oxygen consumption = oxygen inspired – oxygen

expired◦ Methods for OC are the Douglass bag , the

polarographic method and paramagnetic method

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1. Hemoglobin (Hgb in g/dl)

2. Oxygen consumption (VO2 in ml/min) : Best if measured by an oxygen sensor at the time of catheterization. E.g.

a) Women: VO2 = BSA × [138.1–17.04 × ln(age) + 0.378 × HR]

(b) Men: VO2 = BSA × [138.1–11.49 × ln(age) + 0.378 × HR]

DATA REQUIRED FOR SHUNT CALCULATION

Craig Broberg et al. Appendix: Shunt Calculations. Adult Congenital Heart Disease: A Practical Guide.

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LaFarge C.G., Miettinen O.S. The estimation of oxygen consumption. Cardiovasc Res. 1970

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Calculation of saturation :◦ PAO2 and FAO2 are usually calculated by blood

samples◦ MVO2 and PVO2 calculations are most important◦ MVO2 – the key to proper management of systemic

flow in the presence of intracardiac shunt is that mixed venous oxygen content must be measured in the chamber immediately proximal to the shunt

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MVO2 at atrium level1. At rest = 3SVC + IVC / 4

Flamm's formula weights blood returning from the superior vena cava more heavily than might be expected on the basis of relative flows in the superior and inferior cavae.

2. During bicycle ergometry = SVC + 2IVC / 33. Directly taking SVC saturation as MVO2- Flamm and

associates concluded that this method was less accurate in patients without shunt or with shunt. Flamm MD, Cohn K E, Hancock EW. Measurement of systemic cardiac output at rest and exercise in patients with atrial septal defect. Am J Cardiol 1969;23:258

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Calculation of saturation PVO2◦ NOT usually entered ◦ LA vs PVO2

Assumed valve if not calculated

FA saturation≥ 95% < 95%

Take FA sat. 1. d/t R – L shunt assume 98% as PVO2

2. Not d/t R – L shunt take FA saturation

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EXAMPLE OF LEFT-TO-RIGHT SHUNT DETECTION:ATRIAL SEPTAL DEFECT

Vena Cava SPO2= (3x67.5+1x73)/4=69% Right Atrium SPO2= (74+84+79)/3=79% A significant step-up 79%-69%=10%

>=7% 84%-

68%=16%>=11% SPO2 from SVC to PA is 12%-13% i.e. >8%

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PV O2 content =1.36×10×Hb×saturation

=1.36×10×14×0.96 =183 mlO2/liter

PA O2 content

=1.36x10×14× 0.80 =152ml O2/liter

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CONTD…

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Qp = O2 consumption (ml/min)/ (PVO2 content – PAO2 content) = 240ml O2/min /(183-152) mlO2/L = 7.74 L/min Qs = 240ml O2/min/ (Systemic arterial O2 content –Mixed venous

O2 content ) = 240/(0.96-0.69)14(1.36)10 = 4.6 L/min

Qp/Qs = 7.74/4.6 = 1.68 Left-to-right Shunt=7.7- 4.6 = 3.1 L/min

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VENTRICULAR SEPTAL DEFECT

Qp = 260/(0.97-

0.885)15(1.36)10 = 15 L/min Qs= 260/(0.97-

0.66)15(1.36)10 = 4.1 L/min Qp/Qs=15/4.1=3.7 LShunt=15-4.1=10.9

L/min

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The ratio Qp/Qs gives important physiologic information about the magnitude of a left-to-right shunt.

A Qp/Qs < 1.5 signifies a small left-to-right shunt and is often felt to argue against operative correction, particularly if the patient has an uncomplicated atrial or ventricular septal defect.

A Qp/Qs between 1.5 and 2.0 are obviously intermediate in magnitude ; surgical intervention is generally recommended if operation risk is low.

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FLOW RATIO(QP/QS)

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A Qp/Qs < 1.0 indicates a net right-to-left shunt and is often a sign of the presence of irreversible pulmonary vascular disease.

A simplified formula : Qp (SAO2-MVO2) Qs (PVO2-PAO2)

CONTD…

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The quantification of net right to left and left to right shunt requires the calculation of Qeff, which is the net amount of systemic venous return going to lungs for oxygnation.

Qeff = V O2/ m²

(PV % Sat - MV % Sat) x1.36 xHb x10

Net right to left = Qs - QeffNet left to right = Qp - Qeff

Bidirectional shunt

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VO2=100 & Hb = 12Qp = 12.25 ltsQs = 2.45 lts Net left to right shunt = 9.8 lts Net Qp ( Qpa) considering shunt

only at atrial level, and thus resulting in PA saturation of 80 = 4.08 lt

Net shunt at atria Qpa – Qs = 4.08 – 2.45 = 1.63 lt Net shunt at ventricle Qp - net atrial shunt = 9.8 – 1.63 =

8.17 lts

Multiple left to right shunts

LV95

LA95

RA80

RV90

SVC70

PA90

AO95

PV95

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Many other more sensitive techniques are availaable for detecting smaller left to right shunts:

Contrast angiography Indocyanine green dye curves Radionuclide techniques Echocardiographic methods.

Other indicators

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During invasive cardiac cath anatomic delineation of shunt defects is carried out by contrast injection under fluoroscopy.

When mainly used for left to right shunts like VSD , PDA, accuracy is high.

Angiocardiography

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Exogenous indicator used for both qualitative and quantitative indicator dilution studies

Cardio green dye is non toxic and rapidly cleared from the circulation by liver

Accurate cardiac output determinations can be made.

Swan HJC, Wood EH. Localization of left to right cardiac shunt. Proc Staff Met Mayo Clin1953;28:95

Shunt calculation through cardio green dye

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If there is intracardiac or other intravascular shunting, an accurate total cardiac output cannot be determined, however, the degree of right to left or left to right shunting can be semi-quantitated using the Cardio-Green dye dilution technique.

Extremely sensitive for the detection of very minute shunts but the quantification of shunts is less accurate and more cumbersome than utilizing oxygen determinations.

The studies of Castillo and cowrkers suggest that left to right shunt as small as 25% of systemic output can be detected.

Hyman et al. A comparative study of detection shunt by oxygen analysis and indicator dilution methods. Ann Intern Med 1962;56:535

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Requires a densitometer which measures the concentration of dye in blood.

After calibrating the densitometer, a known amount of dye is injected in pulmonary artery

Through an arterial line continuous measurement of dye concentration is carried out by the densitometer.

A characteristic time/concentration output curve is inscribed on the recorder.

Method

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The height of the curve corresponds to the concentration of the dye in the blood at that particular instant.

The tailing off or flattening of the curve appears as recirculation of the same dye begins to appear and eventually creates a second, but less significant, peak.

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Quantification

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Similar to oximetry, it is invasive, and its sensitivity is only modestly better than oximetry in detecting small shunts.

Only of limited utility for left to right shunts. Quantitative evaluation is not as accurate. It is an outmoded technique, only of historical

importance.

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Echocardiographic estimation of Qp/Qs is based on doppler aided calculation of cardiac output as ◦ CO = Mean Velocity x Vessel flow area x 60 s/min

Cosine θWhere θ is the cosine of the incidence angle between Doppler beam and direction of flow

Echocardiography

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The preferred sites for determining SV and cardiac output (in descending order of preference) are as follows:

1.The LVOT tract or aortic annulus 2.The mitral annulus

3.The pulmonic annulus

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The LVOT is the most widely used site. SV is derived as:

SV = CSA ×VTI The CSA of the aortic annulus is circular, with little

variability during systole. Because the area of a circle = πr2, the area of the

aortic annulus is derived from the annulus diameter (D) measured in the parasternal long axis view as:

CSA = D2 ×π/4 = D2 × 0.785

◦ Zoghbi WA, Farmer KL, Soto JG, Nelson JG, Quiñones MA. Accurate noninvasive quantification of stenotic aortic valve area by Doppler echocardiography. Circulation 1986;73:452- 9.

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Qp = RVOT CSA× RVOT TVI

Qs = LVOT CSA × LVOT TVI

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Measurement of the annulus diameter done during early systole from the junction of the aortic leaflets with the septal endocardium, to the junction of the leaflet with the mitral valve posteriorly, using inner edge to inner edge.

The largest of 3 to 5 measurements should be taken because the inherent error of the tomographic plane is to underestimate the annulus diameter.

The LV outflow velocity is recorded from the apical 5-chamber or long-axis view, with the sample volume positioned about 5 mm proximal to the aortic valve.

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Although the mitral annulus is not perfectly circular, applying a circular geometry gives similar or better results than attempting to derive an elliptical CSA with measurements taken from multiple views.

The diameter of the mitral annulus should be measured from the base of the posterior and anterior leaflets during early to mid- diastole, 1 frame after the leaflets begin to close after its initial opening.

The sample volume is positioned so that in diastole it is at the level of the annulus.

Lewis JF, Kuo LC, Nelson JG, Limacher MC, Quiñones MA. Pulsed Doppler echocardiographic determination of stroke volume and cardiac output: clinical validation of two new methods using the apical window. Circulation 1984;70:425- 31.

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The pulmonic annulus is probably the most difficult of the 3 sites, mostly because the poor visualization of the annulus diameter limits its accuracy and the right ventricular (RV) outflow tract contracts during systole.

Measure the annulus during early ejection (2 to 3 frames after the R wave on the electrocardiogram) from the anterior corner to the junction of the posterior pulmonic leaflet with the aortic root.

Sample volume placed just proximal to the pulmonary valve.

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In patients with an atrial septal defect, QP is measured in the main pulmonary artery and

QS is quantified at the mitral valve or in the ascending aorta.

Atrial septal defect

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In patients with a ventricular septal defect or a left ventricular to right atrial shunt, QP is calculated as transmitral valve flow, QP can also be calculated in the pulmonary artery and

Qs is calculated as the aortic flow.

Ventricular septal defect

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In patients with patent ductus arteriosus, pulmonary flow (QP) is calculated as pulmonary venous return through the mitral valve orifice and as flow in the ascending aorta, and

QS is calculated as systemic venous return by measuring flow within right ventricular outflow tract in the subpulmonary region.

Patent ductus arteriosus

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Intravenous contrast injection of saline remains one of the primary diagnostic tools for detecting an atrial septal defect and, in smaller defects, may provide crucial information as to the presence of a potential shunt that is not directly visualized or has not resulted in a right ventricular volume overload

Contrast echocardiography

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In atrial septal defects with biphasic flow agitated saline bubbles will be seen immediately in left atria.

If the bubbles are seen after 3 cardiac cycles, it is diagnostic of pulmonary AV fistula.

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Error in measurement of mean velocity◦ Error in intercept angle◦ The lack of uniform velocity profile across the

vessel lumen◦ Respiratory variation ( 15% at mitral level)

Error in measurement of cross sectional area◦ Inaccurate gain settings◦ Cross sectional area of the vessel changes

throughout the cardiac cycle

Possible sources of error in echocardiographic evaluation

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Due to these limitations the calculated flow values by doppler show a lot of variability.

As such in day to day practice calculation of Qp/Qs by echocardiography is rarely performed, which is primarily used for initial evaluation of congenital cardiac defects, and invasive cardiac catheterization and oximetry being the gold standard for shunt evaluation in complex cases.

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Pulmonary and systemic flows can be measured by multiplying cross sectional area x flow velocity through that area

MRI scans, due to high resolution can measure the cross sectional area accurately and the velocity can be recorded by phase contrast MRI

Phase contrast MRI

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 For the proximal aorta, the section is cut approximately 2 to 4 cm above the aortic valve and distal to the coronary arterial ostia.

For pulmonary artery the position is distal to the pulmonic valve but proximal to the bifurcation.

For patients with PDA flow is calculated separately in LPA and RPA, and hence vessel area is also calculated separately for the LPA and RPA

Measuring Cross Sectional Area

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On the velocity map, the gray scale intensity for each pixel encodes for velocity.

For each frame of the cardiac cycle, velocity within the vessel is calculated as the average velocity for all the pixels within the lumen.

Measuring the velocity

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Flow is calculated by multiplying the cross-sectional area of the vessel lumen by the mean blood flow velocity for each frame sampled in the cardiac cycle.

Flow for the whole cardiac cycle was calculated by summation of flow per frame in the cardiac cycle.

By multiplying heart rate by the sum of the flow for all frames of the cardiac cycle, the flow per minute through the vessel is determined.

K Debl et al. Quantification of left-to-right shunting in adult congenital heart disease: phase-contrast cine MRI comparedwith invasive oximetry. The British Journal of Radiology, 82 (2009), 386–391

Flow measurement

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Irregular rhythms MRI does not allow discrimination of patients without

shunts from those with small amounts of left-to-right shunting.

Patients of pulmonic or aortic regurgitation.◦ Negative flow in the proximal great vessels during

diastole would interfere with the determination of forward flow.

Patients with aortic or pulmonic stenosis,◦ since they have turbulent, high-velocity flow jets in

the proximal great vessels.

Limitations

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Quantification of left-to-right shunting can be performed reliably and accurately by PC-MRI and provides results that correlate closely with those obtained by invasive oximetry, although there is a small overestimation of the degree of shunting.

In the clinical management of patients with left-to-right shunting, MRI can provide anatomical and functional information in a single examination and is a useful technique for the assessment of adult congenital heart disease.

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Albumin aggregates are examined microscopically to assure that particles are in the 10- 50 micron size range

The amount of albumin per test dose does not exceed 0.2 mg.

After intravenous injection, two or more scintigrams, each of 2 min duration, are taken with a gamma camera to produce a whole-body image.

% Rt to Lt shunt = total body count – total lung count total lung count◦ Gates.G.F., Orme.H.W and Dose,E.K: Measurement of Cardiac Shunting with Technetium

labeled Albumin Aggregate, J.Nuc.Med., 12:746,1971.

Rt to lt shunt calculation using albumin aggregates

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This method allows the detection of Qp/Qs as small as 1.2

Can be carried out by peripheral injection Has been mainly described for lt to rt shunt It requires the injection of a radioactive isotope