EFFECTS OF PREGNANCY ON HEART (CARDIO-VASCULAR PHYSIOLOGY)

During pregnancy, due to the increased blood volume and increased cardiac output, weight carrying for the foetus, compression of the bases of lungs and the axial rotation of the heart to the left (during late pregnancy), the “reserve force” of the heart is called on but heart lives well within the limits of the myocardial reserve. The increased work of heart in the third month of pregnancy, remains so throughout the pregnancy. The maximum work may be as high as 30 to 40 percent above the non-pregnant level as cardiac output increases in the same proportion. Heart’s increased work continues upto 36 weeks. Thereafter load on heart lessens as term approaches. A few suggestions are given for the rise and fall in cardiac output during pregnancy. The placenta behaves as modified arteriovenous fistula with decreased total peripheral resistance during second trimester with a progressive normal return towards the term; hence there is a variation in the cardiac output. Another explanation is that, increased blood volume during early last trimester of pregnancy causes increased cardiac output. During last month of the pregnancy, there happens pooling of blood in the lower limbs and vena caval compression by the gravid uterus in supine posture of the woman; hence there is fall in the cardiac output due to decreased venous return to the heart.

During labor contractions, heart has to work more as cardiac output increases by about 40 percent. Strain of second stage of labor becomes highest. Immediately after the delivery, cardiac output increases abruptly by about 30 per cent as some additional amount of blood is thrown into circulation from the contracted uterus and also the pressure of gravid uterus on great veins has been released. Cardiac output returns to pre-pregnancy level towards the end of the first week after delivery.

Cardiac changes in a normal pregnancy can simulate the manifestations of a cardiac disease. In a normal pregnancy, dyspnoea on exertion appears in about 60 percent; palpitation of apex of the heart is a common occurrence; splitting of loud first sound, appearance of a third sound at the base of the heart mimic manifestations for a cardiac disease. Moreover on skiagram, cardiac shadow appears enlarged.