Here's my thoughts, for what it's worth. I'd go with the OB on this one. The existing litertature on the subject, (and our own experts here) concur that hypertension before 20 weeks is and should be considered chronic hypertension. I have essential hypertension that complicates my pregnancies, but for me, the hypertension doesn't necessarily 42 days after my kids are born.
On one level, I wouldn't say that it matters what the origin is of the hypertension, but there are preconceived ideas about what those things mean to the people who will treat you. One doc will say it's just chronic hypertension, and not worry about your pressures, another will say it's chronic htn, we'll watch closely. One doc will say 'PIH", no danger in that, others will say "PIH" lets see her twice a week before it turns into preeclampsia.
YOUR job is to make sure that regardless of what label they throw at you, that they take it seriously, ykwim? The boundaries do sort of blur- regardless, you're going to want them to ACT in the same way- frequent monitoring, frequent level II Ultrasounds, frequent visits.
Just for fancy situations like this, I bought Chesley's Hypertensive Disorders in Pregnancy- the pregnancy hypertension 'Bible'. You can mention to your cardio that Chesley's (which he or she should know or own) says on page 546 in the second edition, that hypertension before 20 weeks is considered chronic hypertension, and ask why your situation is different than that. (after they rule out cardiomyopathy, which is a whole different kettle of fish)
My other thought is that yes, medicines can make your pressure go too low, by reducing placental perfusion and causing growth restriction. That's not good. But they can watch for that through ultrasounds checking your baby's growth and adjust your dose accordingly.
I'm absolutely not a doctor- I just had a second pregnancy strikingly like yours- hyperemesis, pulse so fast that I couldn't walk by 26 weeks, hypertension, the whole works. A lot of it was cleared up with meds.
Whew! Was that a mouthful, or what? Take care, and let us know what happens!
Allie 5-13-98 (35 weeks-pre-e)
Baby Camille 4-17-03 (36 weeks- htn and oligo)