Cardiac is the amount of blood pumped out by each ventricle in one minute. Cardiac output can increase markedly to meet the demands placed on your body.
Cardiac output normally increases in response to increased demand for oxygen by muscles during exercise, in reaction to danger, in response to certain classes of drugs and in some altered health conditions.
You are the sole provider of nourishment for the embryo and later, the fetus, and so your plasma and blood volume slowly increase by 40-50% over the course of the pregnancy to accommodate the changes.
Cardiac output increases by about 50%, mostly during the first trimester. Reference NIH
Your Dr. has put you on medicine and I would encourage you to talk to him to ask what he feels is the problem and also ask him to explain the use of the atenolol. Be sure to let him know if your pulse is fast - for your sake calling the clinic and talking to the nurse would help you and may decrease your worries.
Now is the time to be your own best advocate.
I will be thinking of you,
and Heart/Cardiovascular Disease. "She Stood in the Storm & When the Wind Did Not Blow Her Away, She Adjusted Her Sails."