Chris -
According to Tom Hale's "Medications and Mother's Milk - 1997":
"Nitroprusside is a rapid acting hypotensive agent of short duration (1-10
minutes).  Besides rapid hypotension, nitroprusside us converted
metabolically to cyanogen (cyanide radical) which is potentially toxic.
 Although rare, significant thiocyanate toxicity can occur at higher doses
(>2ug/kg/min.) and longer durations of exposure (>1-2 days).  When
administered orally, nitroprusside is reported to not be active, although one
report suggests a modest hypotensive effect.  No data available on the
transfer of nitroprusside or thiocyanate into human milk.  The half-life of
the thiocyanate metabolite is approximately three days.  Because the
thiocyanate metabolite is orally bioavailable, some caution is advised if the
mother has received nitroprusside for more than 24 hours.
Pediatric concerns: None is reported but caution advised due to thiocyanate
metabolite"
Good luck!
Cynthia D. Payne
LLL of Berkshire County, MA