1. Understanding Intracardiac Shunts
Denise Joffe, MD

2. Objectives To explain intracardiac shunting using illustrations
To describe the hemodynamic and physiologic consequences of intracardiac shunting

3. Outline Describe simple (unidirectional) shunts
Describe complex (bidirectional) shunts
Describe the pathological consequences of shunting
Describe how anesthetic management is effected by intracardiac shunts
Case presentations

4. Qp/Qs Differs from Qs/Qt (intrapulmonary shunt)
Is a result of abnormal intracardiac communications (see below for exception)
What’s yours?
Qs/Qt is the intrapulmonary shunt fraction and refers to the blood flow that does not get oxygenated because of pulmonary pathology (Q shunt or Qs). Divided by the total pulmonary blood flow (Qt). Qs/Qt is a result of lung disease and has nothing to do with cardiac disease. The low pulmonary venous saturation results in a decreased arterial SaO2.
Qp/Qs is the ratio of pulmonary to systemic blood flow (Q=flow). A normal Qp/Qs is close to 1:1. Blood flows ”in series” from the pulmonary artery (PA) to the aorta.
In patients with cyanotic CHD an abnormal Qs/Qt can only be demonstrated if the saturation in the pulmonary veins is not normal. In other words, in patients with cyanotic CHD, it is impossible to tell the contribution of intracardiac or intrapulmonary shunt to arterial desaturation without knowing the pulmonary venous saturation.
Not all sources of cardiac shunting are from intracardiac communications. For example, lesions such as patent ductus arteriosus, and aortopulmonary collaterals, cause shunting but are NOT the result of intracardiac shunting since the mixing occurs outside the heart.

5. In order to understand intracardiac shunts you must understand Qp/Qs (ratio of pulmonary to systemic blood flow)…
In patients with congenital heart disease (CHD) they are frequently different. It is CRUCIAL to use the appropriate flow when performing calculations such as pulmonary vascular resistance and systemic vascular resistance, i.e. Qs for systemic cardiac output and Qp for pulmonary cardiac output.

6. Understanding Simple Intracardiac Shunts
Lesions with isolated L →R or R →L shunts
Eg. atrial septal defect (ASD), ventricular septal defect (VSD), atrioventricular canal (AVC), PDA (patent ductus arteriosus) tetralogy of Fallot (TOF)
In patients with an ASD or VSD, there may be a small R➔L shunt during part of the cardiac cycle but the predominant direction is L➔R.
There are very few lesions with simple R➔L shunts eg TOF or single ventricle patients with a Glenn. Most lesions have either a L➔R shunt (ASD, VSD, AVC, PDA) or bidirectional shunting (eg. transposition of the great arteries (TGA), total anomalous pulmonary venous return, truncus arteriosus, and stage 1 of single ventricle physiology)

7. In the following diagrams the arrows represents blood flow.
represents deoxygenated systemic venous blood flow (normal saturation about 65-75%)
represents oxygenated pulmonary venous blood flow (normal saturation about %)
represents an admixture of deoxygenated and oxygenated blood flow (saturation approx between 80-95%)
The size of the arrow represents the relative amount of flow in (Liters/min)

8. Normal Heart
In normal patients all deoxygenated systemic venous blood from the superior vena cava (SVC) and inferior vena cava (IVC) (blue arrows) goes out the pulmonary artery (PA) to the lungs (Qp), and all oxygenated pulmonary venous blood (red arrow) goes out the aorta (Qs). PA blood is the same saturation as the cavae blood and aortic saturation is the same as the pulmonary venous saturation. If the PA saturation is higher then the cavae saturation (by more than 5% ) , there is a left to right shunt. If the aortic saturation is lower than the pulmonary venous saturation there is a right to left shunt. There must be cross over of blood in the heart if the PA and aortic saturation are not what they should be. In addition, both the PA and aortic arrows are the same size, meaning that the flows are equal i.e. Qp=Qs. This is usually not the case in patients with cardiac communications. In most, but not all cases of cardiac shunting the Qp does not equal the Qs.
QsSVC, QsIVC-systemic venous blood flow from the superior and inferior vena cava, Qpv-pulmonary venous blood flow, RA-right atrium, RV-right ventricle, LA- left atrium, LV-left ventricle, Ao-aorta, Qp- pulmonary blood flow, Qs- systemic blood flow

9. L R Shunts (ASD) QsSVC QPV QsIVC RA LA LV RV PA Ao Qp Qs ASD
The communication is in the atrial septum (ASD) allowing an almost exclusively left to right shunt. In this case red blood from the LA returns to the RA.
The amount of shunting through an ASD is complicated and depends on the difference in ventricular compliance which in turn is dependent on a complex variety of factors including ventricular muscle mass, pulmonary outflow resistance versus systemic outflow resistance, preload, heart rate etc. PA Ao Qp Qs

10. Qp>>Qs and Qp is more oxygenated than normal.
L R Shunts
QsSVC
QPV PA RA RV LA LV
QsIVC
ASD Qp Qs
Note the size discrepancies in the arrows, Qp>>Qs (purple arrow or Qp) is the sum of the blue arrow (venous blood) and the flow through the ASD (shunt). Also, PA saturation is purple but it should be blue. Aortic saturation is red and it should be red, so it is a simple left to right shunt.
Qp>>Qs and Qp is more oxygenated than normal.

11. Effective pulmonary blood flow (PBF), recirculated PBF and total PBF
QsSVC
QPV PA Ao RA RV LA
Qp>>>Qs
QsIVC
a+b b Qp Qs
Effective PBF = systemic (blue) blood that goes to the lungs (a)
The length of the arrows are simply added or subtracted together depending on the direction of the shunts.
With few exceptions the amount of flow returning to the heart from the pulmonary veins (Qpv) is the same as the amount entering the lungs (Qp) so the size of the Qpv and Qp arrows are usually equal.
Two new terms are introduced. Effective PBF is that part of the blue blood that goes to the lungs. In this case all the systemic flow (blue blood) goes to the lungs so the effective flow is equal to the systemic blood flow (blue arrow). Recirculated PBF is the shunt i.e. the red blood that goes back to the lungs ( in this case, the flow through the ASD). The total PBF is made up of both arrows.
Recirculated PBF= PV (red) blood that goes back to the lungs i.e. L -> R shunt (b)
Qp total=PBFeffective+PBFrecirc OR
Qp total=PBFeffective+L -> R shunt

12. R L Shunts QsSVC QPV PA Ao RA RV LA LV QsIVC Qp Qs TOF
This is a schematic of a patient with tetralogy of Fallot (TOF) which consists of right ventricular outflow tract obstruction (demonstrated by the small PA orifice), right ventricular hypertrophy, a VSD, and an overriding aorta.
Note that the amount of intracardiac shunting depends on the outflow tract resistance from the obstruction to PA flow compared to the resistance from the Ao (systemic vascular resistance). In this case, the pulmonary vascular resistance is usually low BUT the RV outflow resistance is high because of obstruction from a combination of subpulmonary (RV) muscular hypertrophy, hypoplastic pulmonary annulus and/or valve or small pulmonary arteries. The increased resistance to egress of RV flow results in right to left shunting of blood.

13. Qp<<Qs and Qs is less oxygenated than it should be
R L Shunts
QsSVC
QPV PA Ao RA RV LA LV
QsIVC
TOF Qp Qs
Less blood goes out the PA resulting in a Qp:Qs <1. The PA saturation is blue and it should be blue and the aortic saturation is purple but it should be red. Therefore, this is a simple right to left shunt.
Qp<<Qs and Qs is less oxygenated than it should be

14. Effective systemic blood flow (SBF), recirculated SBF and total SBF
QsSVC
QPV PA Ao RA RV LA LV
QsIVC a b
a-b Qp Qs
Effective SBF= pulmonary venous blood (red) that goes out the aorta (a-b)
Recirculated SBF = blue blood that goes out the aorta i.e.R -> L shunt (b)
The effective systemic blood flow describes that fraction of the pulmonary venous blood (high in oxygen) that goes out the systemic circulation (aorta). Therefore, Qs is made up of effective SBF and recirculated SBF (i.e. shunt). The shunt component of the flow has no more oxygen to unload. In this example, the effective systemic flow is equal to the pulmonary venous flow.
Qs total= SBF effective +SBF recirc OR
Qs total= SBF effective + R -> L shunt

15. Understanding Complex (Bidirectional) Intracardiac Shunts
Due to “crossover” of flow, SaO2 is lower than expected and SpaO2 is higher than expected.
Eg. transposition of the great arteries (TGA), single ventricle, total anomalous pulmonary venous drainage (TAPVD), truncus arteriosus
Single ventricle physiology will be used to explain bidirectional shunting.

16. Bidirectional Shunting (single ventricle)
QPV PA Ao SV
atrium
QsIVC
QsSVC
This diagram represents a heart with a “generic” single ventricle (SV) and single atrium. The most common SV lesions include hypoplastic left heart syndrome, double inlet left ventricle, tricuspid atresia and pulmonary atresia. There are other lesions and there are also patients who may have two normal size ventricles but because of other intracardiac pathology have SV physiology.

17. Bidirectional shunting
QsSVC
QPV PA Ao
Qp is redder than it should be and Qs is bluer than it should be. Therefore this is a complex bidirectional shunt. Also, note Qp>>Qs. SV
atrium
QsIVC Qp
Systemic venous blood mixes with pulmonary venous blood so that by the time blood gets to the ventricle it is usually reasonably well mixed and the saturations in both great vessels are almost equal.
The amount of PA versus Ao flow depends on the outflow resistance of each vessel. Assuming there is unobstructed flow from each vessel, the amount of Qp will progressively increase after the first hours of life as pulmonary vascular resistance (PVR) falls. This explains why it is not ideal to have a high saturation (>85% or so) in this type of physiology because it implies there is too much PA flow compared to Ao flow. The heart is simply pumping red blood back and forth to the lungs markedly increasing the volume work of the heart. This will eventually lead to complications (see consequences of shunts).

18. Consequences of Shunts
Depending on the direction and amount of the shunt:
Pulmonary overcirculation resulting in CHF, pulmonary hypertension, pulmonary vascular disease
Ventricular volume overload and increased myocardial work which can lead to cardiogenic shock
Hypoxia
Basically the heart is working to recirculate blood. Eventually the effects of decreased oxygen delivery and/or pulmonary overcirculation causes systemic effects resulting in multisystemic organ failure in addition to cardiac failure..

19. Anesthesia and Cardiac Shunting
Ventilation and acid/base management can alter PVR and increase or decrease PBF.
In the OR, FiO2, pH (via pCO2), and medication (narcotic, milrinone, nitric oxide) are used most frequently to alter PVR.
SVR can be altered with vasopressors/vasodilators.
It is crucial to de-air lines and be vigilant for emboli in the presence of any shunt.
Medication and ventilation strategies can be used to alter the PVR/SVR and attempt to “balance “ the circulations (i.e. Qp/Qs ♒1).
In the presence of a LR shunt, maneuvers that decrease PVR such as over ventilation or oxygen therapy will usually increase the shunt which is often undesirable. However, in the presence of increases in PVR those same maneuvers can improve PBF.
When trying to decrease PBF in patients with single ventricle physiology the addition of pCO2 or subambient oxygen therapy has been used to help increase PVR and help “balance” the circulations. However, because the “natural” decrease in PVR in the first days of life is so great, pharmacologic and other maneuvers are often ineffective. However, it is CRUCIAL not to decrease PVR any further with poor ventilation techniques. Decreasing SVR has also been used to help balance flow in patients with SV physiology.

20. Ready for some numbers?
Due to intracardiac shunts, systemic and pulmonary flows cannot be calculated with thermodilution techniques.
Systemic and pulmonary cardiac outputs are calculated with the Fick equation:
Qs=VO2/CaO2-CvO2
Qp=VO2/CpvO2-CpaO2

21. VO2 is oxygen consumption.
CaO2 is the oxygen content of arterial blood: Hg(gm) X 1.36(mlO2/gm Hg) X arterial saturation+dissolved oxygen
Use the same formula to calculate mixed venous, pulmonary venous and pulmonary artery oxygen contents, by substituting their saturation value.
VO2 is oxygen consumption. In anesthesia we simplify the number to about 2-6 ml of O2/kg/min. In the cath lab a nomogram table is usually used to estimate VO2.
The dissolved oxygen is ignored if the patient is on room air during measurement.

22. Qp/Qs=SaO2-SvO2 / SpvO2 -SpaO2
Qs=VO2 / Hg X 1.36 X {(SaO2-SvO2)} X10 (factor to correct for units)
Qp=VO2 / Hg X 1.36 X {(SpvO2 -SpaO2)} X10
With some simple algebra the above equation can be re-written
Qp/Qs=SaO2-SvO2 / SpvO2 -SpaO2
The SpvO2 is usually estimated to be about 97% in the absence of lung disease. A mixed venous, PA and Ao saturation are necessary.
The “simplified” Qp/Qs formula is extremely helpful in the cardiac OR when you want to see if there is a residual shunt. The surgeon can draw a gas from the SVC and PA . The arterial saturation is known and the pulmonary venous saturations is estimated to be around 97%.

23. Example #1 - Simple shunt
6 month old male with a VSD, VO2 75 ml/min, SVC 70%, IVC 70%, PA 87%, Ao 99%, PV 99%, Hg 11 g/dl
Qs=1.7 L/min; Qp=4.2 L/min
Qs={75÷11 x 1.36 x (99-70) }x10
Qp={75÷11 x 1.36 x (99-87) }x10

24. L R Shunts QsSVC QPV PA Ao RA RV LA LV QsIVC Qp Qs Qp:Qs= 2.5:1 VSD
4.2 L/min
1.7 L/min
The data for Qp and Qs are noted and arrows are placed to indicate the shunt and the direction of flows. In reality it would be rare to perform a cardiac cath on a patient with a simple VSD.
The simplified formula yields:
SaO2-SvO2 / SpvO2 -SpaO2
99-70/99-87 or 29/12 =2.4:1.
A significant shunt is usually considered when the Qp:Qs is >2:1
Qp:Qs= 2.5:1

25. L R Shunts QsSVC QPV PA Ao RA RV LA LV QsIVC Qp Qs Qp/Qs=4.2/1.7=2.5:1
VSD Qp Qs
4.2 L/min
1.7 L/min
2.5 L/min
Qp/Qs=4.2/1.7=2.5:1
Qp total=PBFeffective+L R shunt
L -> R shunt= Qp-PBFeffective
The LV is pumping 4.2L/min but only 1.7L/min is going to the body. The rest of the volume work is being performed to pump red blood back to the lungs. (Wasted work).
The effective systemic blood flow is 1.7 L/min (all the blue blood goes to the lungs) and the effective pulmonary blood flow is also 1.7 L/min. Note they are equal.
The recirculated PBF (L to R shunt) is 2.5 L/min. The total PBF (Qp) and the total systemic blood flow (Qs) are NOT equal!!
L -> R shunt=
L -> R shunt = 2.5L/min

26. Example #2 - Bidirectional shunt
3 day old baby with hypoplastic left heart syndrome. VO2 50 ml/min, SVC 65%, IVC 65%, PA 87%, Ao 87%, PV 100%, Hg 13 g/dl
Qs=1.3 L/min; Qp=2.2 L/min
Qs={50÷13 x 1.36 x (87-65)} x 10
Qp={50÷13 x 1.36 x (100-87)} x 10

27. Bidirectional shunting - calculating effective PBF and R L shunt
QsSVC
QPV PA Ao SV
atrium
1.3L/min
Q R L = Qs-QPBF effective
Q R L =
Q R L =0.5L/min
QsIVC
Q PBF effective
0.8 L/min
Q PBF effective = {50÷13 x 1.36 x (100-65)} x 10 = 0.8 L/min. This is the amount of blue blood going to the lungs. The rest of the systemic venous blood ( ) 0.5 L/min is shunted R➔L.
The data for Qp, Qs and Q PBF effective are noted and arrows are placed to indicate the R➔L shunt and the direction of systemic flows
2.2 L/min
1.3L/min
A new equation is used to calculate effective PBF in patients with bidirectional shunting.
Q PBF effective = VO2/CpvO2-CvO2

28. Bidirectional shunting-calculating effective SBF and L R shunt
QsSVC
QPV PA Ao SV
atrium
QsIVC
1.3L/min
2.2 L/min
2.2L/min
Q SBF effective
0.8 L/min
Next, mark the arrows for PV flow. A little less than half of red PV blood goes out the aorta as effective SBF (0.8L/min), the rest is shunted back to the lungs (L➔R-1.4L/min)).
Note the effective pulmonary and systemic flows are equal.
Q L -> R = Qp-QPBF effective
Q L -> R=
Q L -> R =1.4L/min

29. Bidirectional shunting
QsSVC
QPV PA Ao SV
atrium
QsIVC
1.3L/min
2.2 L/min
2.2L/min
Q PBF effective
0.8 L/min
Q R L = Qs-QPBF effective
Q R L =
Q R L =0.5L/min
Q L -> R = Qp-QPBF effective
Q L -> R=
Q L -> R =1.4L/min
Q SBF effective
Combine the pictures to get a complete idea of flows.
Note that Q PBF effective always equals Q SBF effective . The SV has a cardiac output 3.5L/min (Qp+Qs) but only 1.6L/min is going to the correct place (effective blood flows)

30. Objectives To explain intracardiac shunting using illustrations
To describe the hemodynamic and physiologic consequences of intracardiac shunting