Shunt quantification
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This document discusses various methods for quantifying intracardiac shunts in patients with congenital heart lesions. It describes invasive oximetry and indicator dilution techniques as well as noninvasive Doppler echocardiography methods. For echocardiography, it outlines techniques for quantifying left-to-right shunts using pulmonary and aortic flow measurements, as well as a simplified method using diameter ratios. It also discusses limitations and sources of error for these quantification methods.
Shunt quantification
- 1. By: Dr. Ankur Gupta Resident, DM Cardiolgy
- 2. Introduction Majority of the congenital lesions are asso. with intracardiac shunts. Detection, localisation and quantification of shunts – integral part of hemodynamic evaluation of these patients. Quantification of shunts by: Invasive Noninvasive methods.
- 3. Non invasive methods Clinical evaluation CXR and ECG Echo and Doppler MRI CT Nuclear scanning
- 6. 2 –D Echocardiography: direct visualization of any communication and their shunting excellent for localization of shunt limited in the ability to quantify shunt Indirect estimations ASD with large shunting RA, RV dilated diastolic septal flattening VSD with large shunting LA, LV dilatation Systolic and diastolic septal flattening
- 7. Contrast Echo Agitated saline Detection of small R L shunts Large left to right shunts may cause a negative contrast jet in the Rt. atrium
- 8. Doppler echocardiography Most of the studies were done in ASD & VSD. Based on the measurement of flow across the valves or the defect itself. Based on the formula that flow can be calculated from CSA & VTI.
- 9. Volumetric flow measurement requires - calculation of temporal mean flow velocity and cross sectional area - velocity profile is uniform across the vessel - single sampling of the flow velocity in the center of the vessel is recorded To calculate computerised calculation of the area under the doppler curve/ flow period
- 10. For systemic flow Aortic velocity is recorded from the apical view. Vessel diameter is measured from the parasternal long axis view.
- 11. Pulmonary blood flow: Pulm. mean velocity is recorded from the parasternal short axis view In case of large shunt and a shunt located close to PA, the doppler tracing shows spectral broadening or signs of disturbed flow, in such cases uniform velocity cannot be assumed Doppler tracing cannot be used if any PS is present.
- 12. Measurement of the cross sectional area - at the valve annulus (as it is the flow limiting point of the vessel) - measurement is made from the inner edge to the inner edge. - diameter is measured at the early systole – time when vessel is at peak systolic dimension. - flow velocity should be measured at same location
- 13. Methods: 1.Qp/Qs calculation from flow velocity (velocity time integral) and cross sectional area Qp/Qs = Area of Pa x VTI Area of Aorta x VTI Sanders et al used pulmonary flow with good correlation , r=0.85. Barron et al used Mitral flow with better correlation , r= 0.9 vs 0.69.
- 14. 2. Simplified method - the square of the pulmonary to aortic diameter ratio was substituted for the area ratio and the flow peak velocity ratio for the velocity time integral. Qp/Qs = Vel. Pk PA x r2 Vel. Pk Ao x r2 r= radius / diameter square of the ratio of the pulmonary to aortic luminal diameter (or radius)is used instead of vessel areas.
- 15. 3. Qp/Qs = (pulm + mitral flow)/2 (tricuspid + aortic flow)/2 Assumption that tricuspid and aortic flow represent systemic flow and pulmonary & mitral flows represent pulmonary flow. Maximal diameter of mitral and tricuspid valve annulus is measured during diastole in an apical 4 –chamber view & area calculated assuming the annulus to be circular. Laborious & time taking. Combines measurements from all the 4 valves but was not shown to be better than the previous 2.
- 16. 4. Qp/Qs = Shunt flow + aortic flow aortic flow Shunt (VSD) flow as the product of the VTI and color-derived cross-sectional area of VSD jet measured at its narrowest point. This method does not use the pulmonary flow & outflow diameter which are more variable.
- 17. Difficulties : It is assumed that 1. The defect is circular. 2. The size does not vary during systole. 3. No angle correction is needed. 4 . Flow velocity is uniform across the orifice.
- 18. Most studies show moderate correlation of doppler techniques with oximetry data. Sabry et al, studied above 4 methods with cardiac cath calculated shunts. First 2 methods showed moderate correlation ( r=0.54 & 0.56 ). Correlation improved when large VSDs were excluded (r=0.62 & 0.66 ). 3rd method showed no better correlation & it was more time taking (r =0.57). 4th method showed best correlation (r =0.82). When large VSDs were excluded it was r = 0.9.
- 19. Sources of error Mainly from the assumptions made 1) assumption that velocity profile is uniform through out the cross sectional area 2) alignment of the doppler beam with the direction of the flow ( angle < 200 ) 3) turbulent flow in the vessels mainly in the PA. 4) inaccuracy in measurement of cross sectional area of vessels - PA dimensions vary significantly during cardiac cycle
- 20. 5) errors in calculation because of presence of additional defects e.g PDA present downstream from the PA 6) semilunar valve regurgitation results in overestimation of flow because of failure to measure the regurgitant volume 7) All these methods overestimate the shunt in large VSDs. 8) Quantification of PDA is not accurate due to the turbulent flow across the defect & difficulty in measuring the flow distal to the shunt.
- 21. Quantification of ASD TTE estimation of ASD diameters & shunt quantification by cath showed only fair correlation (r=0.56). • The size of the defect by transesophageal Doppler color flow mapping correlated fairly well with the size estimated at surgery (r = 0.73 ). Other measurements by TTE for ASD are – - RV/LV diameter (r=0.64) - Area of PA (r=0.62) - RV volume (r=o.71) - PA/Ao (r=0.89)
- 22. Doppler colour flow mapping by TEE • Area of the ASD- calculated by assuming it to be circular and taking the maximal Doppler color flow jet width at the defect site as its diameter. • The pulsed Doppler sample volume is to be placed parallel to the shunt flow direction at the defect site to obtain the mean velocity and flow duration.
- 23. • From these values, the shunt volume can calculated as a product of the defect area, mean velocity, flow duration and heart rate. • The calculated shunt flow volume obtained by transesophageal study showed a good correlation with shunt flow volume (r = 0.91) and pulmonary to systemic blood flow ratio (r = 0.84) obtained at cardiac catheterization.
- 24. R L shunt quantification Aortic flow + VSD flow = Input to RV Pulmonary flow + R L shunt = output from RV. Hence R L shunt = Aortic flow + VSD flow – Pulmonary flow. Only fair correlation found with Cath data (r = 0.61 – 0.77 )
- 25. Automated cardiac flow measurement method ACM method – using spatial and temporal integration of colour Doppler profiles . Method velocity profile in a region of interest set on the flow tract is detected in each frame , imaged & recorded on the image memory Flow volume rate is calculated by integration of the velocity profile Stroke volume is measured by temporal integration, throughout the systolic period.
- 26. ACM method: Advantages: Quick and requires only two manual procedures.- selection of systolic period in the stored image memory, positioning at the area of interest Calculations are done without tracing the Doppler wave form to measure VTI No need to measure area of outflow tract, because the edge of the color profile is detected as the width of the flow tract.
- 27. ACM method ACM uses velocity profile across the flow tract diameter, while in conventional pulsed Doppler method the spectral velocity from a centrally placed sample point is measured Flow rate at each frame is temporally integrated for the stroke volume calculation Thus this method requires fewer assumptions than pulsed Doppler method.
- 28. Martin et al found strong correlations between ACM and invasive QP/QS ratio and the agreement with invasive data was better using ACM than using conventional echocardiographic method ( r = 0.91 ) The only restriction remains to select manually the systolic period and to carefully choose the region of interest. Gain must be optimised, allowing to see only one colour throughout the outflow tract
- 29. Avoids the potential error of conventional PWD method for the calculation of pulmonary output, linked to the difficulty of measuring the exact pulmonary diameter. This method was applied to ASD & VSD with better correlation .
- 31. Principle: oxygenated blood shunted from left side of the heart to the rt cause an abnormal increase (step – up) in the oxygen content or saturation of blood in the chamber into which shunting occurs Dexter et al, max. increase in oxygen content from, RV to PA - 0.5 ml/dl RA to RV - 1.0 ml/dl SVC to RA - 2.0 ml/dl
- 32. Barratt -Boyes and Wood suggested that multiple blood samples to be obtained from each Rt. heart chamber. Data to be averaged before applying Dexter Criteria Advantages - easy to perform, results available immediately - can ascertain the site of shunt - magnitude of the shunt can be calculated
- 33. Detection of left to right shunt Level of shunt Mean O2 sat % in distal chambers (Highest value in proximal chambers) Mean O2 vol. % in proximal chambers (Highest value in distal chambers) Min Qp/Qs reqd. for detection at 3L/min/m2 Possible causes of step up Atrial (SVC/IVC to RA) ≥7 (≥11) ≥1.3 (≥2.0) 1.5 – 1.9 ASD, RSOV, VSD with TR, coronary fistula to RA, anomalous PV drainage Ventricular (RA to RV) ≥5 (≥10) ≥1.0 (≥1.7) 1.3 – 1.5 VSD, PDA with PR, Primum ASD, coronary fistula to RV
- 34. Detection of left to right shunt Level of shunt Mean O2 sat % in distal chambers (Highest value in proximal chambers) Mean O2 vol. % in proximal chambers (Highest value in distal chambers) Min Qp/Qs reqd. for detection at 3L/min/m2 Possible causes of step up Great vessel (RV to PA) ≥5 (≥5) ≥1.0 (≥1.0) ≥1.3 PDA, Aorta- pulmonic window, Aberrant coronary artery origin Any level (SVC to PA) ≥7 (≥8) ≥1.3 (≥1.5) ≥1.5 All of the above
- 35. Detection of left to right shunt Chambers sampled Single Sample Multiple samples SVC – RA 7% 5% RA – RV 5% 3% RV – PA 4% 3% PV/LA – LV/SA -3% -2%
- 36. Oximetry – limitations: It lacks sensitivity – i.e. does not allow detection of small left to right shunts Requirement of steady state during collection of blood samples Calculation of Mixed venous saturation: Normal variability of blood oxygen saturation in the Rt heart chambers is strongly influenced by the magnitude of systemic blood flow. Higher levels of systemic blood flow higher mixed venous saturation blunting of inter-chamber variability.
- 37. Step up in oxygen content of the receiving chamber depends also on the oxygen carrying capacity of blood i.e. Hb conc. Depends on the associated lesions also – like TR, PR.
- 38. Precautions Samples at multiple sites to be obtained rapidly , should take < 7 min. O2 saturation data rather than O2 content are preferable, Comparision of mean values is preferable to highest values. When the systemic blood flow at rest is low, exercise shoud be used. At higher FiO2 the dissolved O2 should also be considered.
- 39. Shunt calculation Qp = VO2 / (PVO2- PAO2) (Hb) (1.36) (10) Qs = VO2 / (AoO2 – SVCO2) (Hb) (1.36) (10) Qp eff. = VO2 / (PVO2 – SVC O2) (Hb) (1.36) (10) Qp / Qs = (AoO2 – SVC O2) / (PVO2 – PAO2) Q L R shunt = (Qp – Qp eff) Q R L = (Qs – Qs eff)
- 40. Carter formula Determines the left to right shunting. Best used when there is a smooth downstroke They can detect a minimum shunt of 25 – 30%. Simplified formula of Victoria & Gessner is used when there is no smooth transition.
- 41. Calculation of Rt to Lt shunt BT1 T2
- 42. %Qs = BT1 x P1x 100 BT1xP1 + 0.44xT2xP2 %Qs = percentage of systemic blood flow due to Rt to Lt shunt. PC1 = height of first peak. BT1 = bulid up time from appearance to first peak. T2 = time from injection to P2.