Insidious (weeks-months)
Progressive heart failure over weeks is more likely to be due to an initial L-R shunt, increasing pulmonary blood pressure
E.g. ASD, VSD, PDA
This may improve naturally as vascular resistance improves
If untreated, pulmonary HTN will occur, causing increased RH pressure and reversal of the shunt to R-L; dilution of ox blood with deox, and child becomes cyanotic (Eisenmenger syndrome)