A by-product of increased effort -- esp. respiratory effort -- is fatigue.
Feeding is work for babies.  If the baby has a problem (heart defect,
respiratory or swallowing difficulties, low birth weight, etc) then feeding
may be quite challenging.  Often the baby will be able to sustain only short
bursts of effort and then need time to recover oxygen saturation, etc.
Lifting the breast is a good idea as it lessens the work, but it may not be
the only issue.  Observing baby's breathing pattern is useful:  does the
baby need to pause after only a few sucks to do a lot of "catch up"
breathing?  Does baby feed with closed eyes the whole time?  Are there other
signs of stress occuring?

Mothers of such infants are often going to need post-feed pumping to insure
that the breasts are thoroughly drained in order to maintain a good milk
supply.  This pumped hind milk can be used as supplement as short-feed
babies may not be able to reliably access this milk on their own until their
status improves.

Kangaroo care/sling carrying is lovely for such infants as it reduces stress
by helping baby feel very embraced in mother's protective care.

Barbara

Barbara Wilson-Clay, BS, IBCLC
Private Practice, Austin, Texas
Owner, Lactnews On-Line Conference Page
http://moontower.com/bwc/lactnews.html