Dropping Oxygen Saturation

Hi,
  My son is 1 years old and has hypoplastic left heart syndrome.  He is post Glenn with a BT shunt and a pulmonary artery band between glenn shunt and BT shunt.  He has previously been on ECMO and has had 4 heart surgeries as well as the Nissen with G-tube placement.  Currently the problem is that when we feed him through his g-tube, his oxygen saturations drop between 50 and 70 (his normal oxygen saturations are 75-85).  He has done this in the past when we first went to bolus or bulk feeds from continuous feeds.  The only thing that has changed recently is that we have increased the amount of feeds he gets from 4 oz to 6 oz.  Is this normal?  What is a normal drop range while feeding?  What could be causing his drops...is it the increase in feed amount?  Any input you can give on this would be greatly appreciated!
Thanks,
Tammy

[National Jewish Health]

This is not a medical situation with which we are familiar.  However a possible explanation for the fall in saturation would be recurrent aspiration, despite the Nissen procedure.  The chance of this would be increased with higher volume feedings.  The degree of risk for this can be assessed by a two-step swallowing provocation test (S-SPT).  We cannot comment on the normal "drop range."  You should pose this question to those prescribing and supervising your son’s care.

[tcsears]

The problem is that his cardiologist in Chattanooga told me that it was a problem with him refluxing and then aspirating due to his Nissen coming undone and that we needed to get our pediatrician to set up testing.  So we had a chest x-ray with nothing abnormal showing up as well as a barium test where the barium was put through his g-tube and we watched as none of it went up his esophogus.  His tummy empties well and the food moves through his intestines well.  So it's not that.  It's a puzzle and even when he was still in the hosptial at Vandy his cardiologists there weren't sure why it was happening.  So I just thought I'd put it out there to see if anyone else had heard of this happening.  Thanks for taking the time to respond!