All left to right shunts are acyanotic heart disease to begin with. Cyanosis appears if there is progressive PHT and reversal of shunt .We know this happens late in ASD.(third decade)
It is important to remember some of the patients with large ASD can show significant desaturation without severe pulmonary HT. This should not be mistaken for Eisenmenger reaction.
In any large ASD ,
- IVC blood can stream into LA by hitting preferentially the lower part of IAS.( It is the old fetal route that heart does not forget and indulges whenever the local hemo-dynamics permits !)
- During straining , (Valsalva and equivalents) right atrial pressure can exceed LA and small amount of shunts occur across RA.
- ASD is often (15%) associated with systemic venous anomaly. The common one is persistent LSVC. LSVC is usually connected to coronary sinus . If it has a communication with LA (Un-roofed CS) , there can be significant cyanosis .
- Further , a large ASD can act as a single atrium and considerable mixing happens and cyanosis results.
Finally ,two conditions should always be considered
- ASD if associated with VPS auguments R-L shunt .
- TAPVC can be mistaken for Eisenmengerisation of ASD in bedside which presents as clinical signs of ASD + Cyanosis
* It is useful to recall ,even PFOs can shunt right to left at times of extreme RA pressures like during PEEP ventilation and orthostatic deoxia in sick ICU patients are reported (If PFO can shunt R-L , why not huge ASD ?)
Cyanosis in ASD is not always an ominous sign .There are few important causes other than Eisenmenger. Though it occurs intermittently , persistent mild desaturation is also possible.