Hi Rae, I also experienced heart problems with Herceptin when I was treated during 2012/13. My baseline was much higher than yours (in the 70's I believe) and dropped to around 43% in the final 6 months of the treatment. After a short delay of a few weeks it stabilised and I was able to complete the treatment once on heart medication (think it was Bicor) and was more closely monitored with extra ECGs etc. The good news is that my heart function returned to more normal levels after I finished treatment (though not to the original level) and I no longer need heart medication or see a cardiologist.
Best wishes to you, hopefully things will improve for you soon. Jane xx