What Causes Episodes Of Racing Heartbeat And Confusion In Spite Of Taking Lexapro?

my 15 yr old granddaughter previously checked out by cardiologist, put on a portable monitor for 15 days and was ultimately undiagnosed and put on anxiety meds...lexapro, however is still having sporadic episodes of sudden racing heartbeat weakness and confusion. today there were additional symptoms. her hands and feet started tingling, her knees turned blue and then she hit the floor. this was witnessed by her health teacher and rotc instructor. these episodes happen at random and stress times are rarely a factor as it happens frequently when she is relaxed and at rest.