Hello and hope you are doing well.
A ‘stiff heart’ is when the heart fails to relax completely. It is sometimes referred to as diastolic dysfunction. As a result of inadequate relaxation, it does not get filled adequately leading to a decreased cardiac output. This can in turn cause hypoxia and fainting episodes. The commonest cause for this is high blood pressure. If you have high blood pressure, you should try to control this as much as possible with medication and lifestyle changes. This includes avoiding cigarettes, minimizing your salt and alcohol intake, getting regular aerobic exercise (based on your cardiac status), and losing any excess body weight. And follow up with your doctor.
Hope this helped and do keep us posted.