Patients who first develop atrial fibrillation during
pregnancy do so as a rule after the fourth month at a
time when the blood volume is already increased
almost to its maximum. It is therefore possible that
the increase in circulating blood volume in a patient
with severe rheumatic heart disease may be an
important factor in determining the onset of atrial
fibrillation. Another possible factor might be
rheumatic activity with carditis, a raised antistreptolysin
titre having been found not uncommonly,
but the evidence for this is conflicting and
in the individual patient the diagnosis of rheumatic
activity may be difficult or impossible to make.