I was diagnosed with heart failure the day before I had surgery (my doctors in Mexico pretty much saved my life, as I had no idea something was wrong with my heart.) I’m 17 days post op today. Yesterday and the day before, I started almost blacking out every time I stood up, and then I’d get a headache. When I took my bp yesterday it was 71/48. I emailed my cardiologist and then consulted a pharmacist who told me that with the nearly blacking out symptoms, that was simply too low, and I needed to stop all my meds until I spoke with my doc. My bp slowly rose thru last night and today, and when I spoke to my doc, he took me off a med completely and reduced the dosage of the other 2. He said that at this point in recovering from surgery, I simply wasn’t able to intake enough water to combat the water i was losing because of the meds. Please call your doctor and tell him what’s happening. You very likely need your meds adjusted until your water intake is closer to normal.