Questions re: heart problems -- long

[RhndLev@cs.com]

This isn't exactly endurance related, but I hope Steph will cut me some 
slack.  I have an older mare who just hasn't been herself lately.  She 
colicked last November, and had a slight impaction, an elevated CPK and no 
parasite load we could detect.  The impaction resolved, but Wapiti's recovery 
was very slow.  Lately, she's been seeming under the weather and irritable.  
I had the vet out last Friday, and after walking the mare a short distance, 
the vet noticed a heart murmur.  That's something new.  The mare is 22 and 
has never had a murmur.  The vet drew blood and then we did a modified CRI.  
Trotted the mare only about four truck lengths.  The vet immediately checked 
the mare's heart.  She said the heartrate was "incredibly" rapid and 
irregular.  As I stroked my mare's neck, the vet said she could hear the 
heartrate stabilizing slowing.  After one minute, Wapiti's heartrate was 
"down" to 82.  

The vet left a message for me tonight with some of the blood work results.  
She did not mention the CPK, but stated Wapiti is slightly anemic and her 
phosphorus level was low.  The message from the vet says that I should get a 
fecal sample, worm with a double dose of ivermectin, and go ahead with the 
EKG that we already scheduled for Friday.  My vet is unavailable until 
tomorrow morning, so I'm hoping someone out there can answer my questions.  
So, my first question is:  given the sub-Arctic conditions we've had in 
Kansas, is it possible that my mare acquired a significant worm load between 
November and now?  My last vet, who I admittedly have stopped using for a 
variety of reasons, told me I did not need to worm during the winter if I did 
not stall my horse.  Second, what causes a low phosphorus level and what does 
a low phosphorus level do to my horse?  

Rhonda