What are the mechanisms of cyanosis in cyanotic heart disease ?
Most of my fellows have difficulty in answering this question. (It is not the lack of knowledge though !) In my view ,cyanosis can occur , by six different modes
- Reduced pulmonary blood flow with some form of anatomical obstruction in RVOT with a communication between ventricles (TOF physiology ) , atria or both
- Reduced pulmonary flow with obstructive pulmonary vasculature (Eisenmenger physiology )
- Wrong way origin ( RV to Aorta/LV to Pulmonary artery ) : Transposition physiology
- Simple mixing of arterial and venous blood channels within the atria ,ventricle or great vessel without RVOT obstruction .This, in fact can causes increased pulmonary blood flow (Technically left to right shunt ) and still there is cyanosis (These are called as Admixture lesions ) It is also to be noted some of the admixture lesions (Truncus, DORV,etc ) the mixing takes place only during systole , while TAPVC,Common atrium, Tricuspid atresia* admixture is more complete as it happens during entire cardiac cycle.
- Isolated Right to left shunt are very rare ( Pulmonary AV fistula , SVC to LA )
- Complex combination of first 4 (Like bi-directional shunting , TGA combines , AV canal defect , with varying degree of pulmonary obstructive disease) Note : TOF and Eisenmenger are physiologically mimic each other , the only difference is site of resistance to pulmonary flow. RVOT vs Lung vasculature )
* Essentially Atrial admixture is more complete than when it happens at ventricular or great vessel level
For advanced readers only
Now, is it possible for “Net” left to right shunt to result in cyanosis ?
Yes*.Very much possible. The bulk of this group is referred to as admixture lesions with certain caveats.There should be an obligatory mixing without contribution from RVOT obstruction or raised PVR( *Please note theoretically admixture can either be right to left or left to right shunt )
All pure admixture lesions are in fact net left to right shunts. (TAPVC, Single ventricle , Common atrium , Common AV canal ,Truncus, ) This is the group we have been traditionally calling cyanosis with increased pulmonary flow.
Its may also to be noted with surprise some admixture lesions often has less intense cyanosis than other forms as long as pulmonary blood flow is normal and the lung does its job perfectly .
*Please note Isolated classical left to right shunts , ASD, VSD, PDA can never cause significant cyanosis unless there is reversal of flow .However ,many Eisenmenger physiology show net Left to right shunting only ( 1.2-1.5 : 1 or so ) but with a definite right to left component .What we call as typically bi-directional shunt .
How can cyanosis be minimal even in some cases of single ventricle ?
- Even though there is single ventricle , there can be preferential (favorable) streaming of right heart blood flow without gross mixing .
- As discussed before good uninterrupted pulmonary blood flow will make the cyanosis less intense .
Is single ventricle with PS admixture lesion or TOF physiology ?
Though single ventricle in isolation is an admixture lesion, when it has associated RVOT obstruction it ceases to be admixture by definition as mixing is augmented by the obstruction rather than by simple mixing.The complexity could be understood in certain situations where admixture lesions like common AV canal go for raised PVR .Here the various quantum of contribution to cyanosis is mind boggling. (Original admixture, augmented by RVOT resistance, differential mixing at atrial and ventricular level , hypoxia at lung level due micro pulmonary AV fistulas in grade 4 heath Edwards etc )
Role of streaming in Admixture lesions
Streaming is selective flow of venous blood into PA and arterial blood into Aorta even in the presence of large septal defects. Favorable streaming implies good systemic saturation. Unfavorable streaming would mean PA saturation more than aorta.(It should be noted streaming and good admixture don’t go together. If good admixture has happened there can’t be any streaming and vice versa)
Streaming is common in which situations?
Inspite of absence of IVS, streaming has been noted in some cases of single ventricle with minimal cyanosis with good saturation in Aorta.
Streaming in TAPVC has some unique features.
Fetal circulation has certain preformed pathways. IVC blood deflects to LA through ASD/PFO .SVC blood preferentially enter RV-PA. In Infradiaphragmatic TAPVC where it drains into IVC highly saturated PV blood may stream into LA thorough ASD and reach LV nd result in higher Aortic saturation.(This is in contrast the classical type of TAPVC draining into RA with little favorable streaming and hence O2 saturation equilibrates between PA/Aorta.)
In Supra cardiac TAPVC that drains into SVC or coronary sinus the streaming is unfavorable as it may preferentially cross tricuspid valve and enter PA making the saturation higher than Aorta.
Streaming is less common in which lesions ?
In common atrium and TAPVC draining into RA streaming is less common.In tricuspid atresia streaming is almost impossible as TV is non existent and this ensures complete mixing in the atria and hence cyanosis is likely to be severe.
Can TOF behave like an admixture lesions ?
Technically yes.If the RVOT obstruction is minimal ,(What was called then as pink Fallot ) We haven’t understood this entity properly for so long. Atleast I was baffled to read when J.K Perloff mentioned in his book during my DM fellowship days, that TOF can manifest with predominant left to right shunt with little or absent cyanosis.
The aortic override in TOF facilitated by large malaligned VSD make it a sort of admixture situation as RVOT resistance is too little to offer any resistance, (rather it welcomes more blood from left side ! ) So , should we call it simple VSD physiology , admixture physiology or just acyanotic forms of TOF ?)
Key points
Though admixure lesions are discussed separately , bulk of them actually represent cyanosis with increased pulmonary blood flow situations.
The net pulmonary blood flow is much more important than the quantum of admixture in determining the degree cyanosis
Finally , one should appreciate there can be combination admixture lesions with obstructive RVOT components . (Tricupid atresia+Pulmonary stenois )
Further reading
An excellent review article on this rare topic of admixture physiology
Dr.S.Venkatesan MD 