So glad you've been feeling better!  Still, I think you should be able to get all those pesky symptoms under control using more or different meds.  At first I also had low bp and high heart rate, in the same range as yours, too.  Getting on the right meds has solved that problem.  I still have a little of both sometimes, but it's now usually in the normal range.  Are you staying on the low histamine diet?  Some of the forbidden foods could be causing symptoms otherwise.

  I'm assuming you had the high heart rate before you started on the antihistamines.  If not, then you might want to switch to a different one.  Avoid those that contain any type of decongestant, i.e., zyrtec-D, because decongestants are known to increase heart rate in some people.  Also avoid any nasal decongestant drops or sprays.  If you notice that your heart rate increases after eating a specific, take it out of your diet.  I found that I was getting increased heart rate after eating anything in the morning, so I started the day with mild ginger tea or just a cup or two of warm water and my morning meds.  If I waited 30 min-1 hr., I can eat without side effects.  Now that is only a problem sometimes.

  If you're able to take Allegra in the morning, instead of the Zyrtec, it might help without making you sleepy and could possibly last until evening.  The 60 mg. Allegra works for up to 12 hours, and the 180 mg. is a 24-hr. med.  If not, try taking the entire 10 mg. of zyrtec in the a.m. and add another H1 antihistamine in the afternoon if you need it, for example, Benadryl (diphenhydramine) or Vistaril (hydroxyzine).  The maximum dosage on Zyrtec is much, much higher than you're taking, so you could also ask the doctor if you can take 10 mg., 3 times/day instead of the 5 mg.  My doctors have me taking Allegra in the a.m. and Zyrtec at night, and that works well for me.  If those help, then the mast cell disorder is probably causing the symptoms.  

  Of course, ask your doctor about all additions or changes you want to make.  I don't know if they'll interact with the midodrine, as I'm not familiar with it.  Doxepin was a great med for me.  It can cause some sleepiness at first, though, so I took it at bedtime.  You can ask your doctor if it's possible to switch to doxepin, for example, starting on a low dose of doxepin and working your way up as you decrease on the midodrine.

  Let us know how you're doing and what changes work for you!

As long as your blood pressure is so low, you are going to have some degree of tachycardia. You have to have some way of getting the blood around to all your tissues. If the blood pressure is low, the heart rate goes up to compensate.

One thing that could help is increasing your blood volume. This will increase your blood pressure, and maybe slow down your heart. Drinking extra fluids WITH extra salt is one way to do this. I used to drink very slightly salted water all day to get my blood volume up. If you just drink plain water, you will probably urinate it all back out. It is due to an imbalance in certain hormones that manage blood fluid levels. You can use drugs to correct it, but it is harder on your body than using the extra fluids and salt.

Check with your doctor to be sure this is OK, as I don't know your whole medical history, but many people have used this, including me.

My HR was at 100 or over for several years, and it has not damaged my heart. I know it is hard to tolerate, and makes you feel anxious as your heart beats so fast, but hopefully you will find something that helps soon, and there will not have been any damage from this period of time.

Hi Sweety!
I have to agree with Joan on this--and I know how easily you trigger to meds,, but Im thinking you might do well with w childrens dose Allegra.. ( 60 mg) try it only once a day to begin with and start in the AM... that is the only med that made my tachycardia go away over 80%. I hope you feel better soon
hugs
me

PS as long as your meds (Singulair and Zyrtec) are helping you, dont worry about any small lactose in them==your body is obviously handing it well, If it aint broke dont fix it lol.