Book of Medical Disorders in Pregnancy - Tintash

The Stroke Volume - This increases
considerably during pregnancy. The
change becomes maximum between 19th
to 20th weeks. From 20 weeks until term
the stroke volume increase remains
constant. The difference in results reported
by various authors is probably
due to the position of the patient when
stroke volume and cardiac output was
measured. Some authors in the past had
reported a decrease in stroke volume
near term, but this is not now accepted
as correct by most authorities.
Cardiac Output - There is 40 per cent
increase in cardiac output above normal
by the 28th week. The increase starts
early in the first trimester, and continues
gradually during the second trimester, it
reaches its peak around 28 weeks, and
then remains at this level till term. Most
authorities in the past reported a decline
in cardiac output in the last trimester.
This difference is due to the position of
the patient. When the patient is lying in
supine position, pressure on the inferior
vena cava produces a decrease in the
venous return to the right heart and thus
lowers the cardiac output. These workers
made their measurements while the
patient was lying in the lateral position.
An additional increase in cardiac output
occurs during labour. It is due to an
increase in stroke volume caused by
uterine contractions which can squeeze
an extra 400ml of blood into the general
circulation.
Blood vessels -
There is generalized vasodilatation due
to decrease in tone of the smooth muscles
during pregnancy. This helps in
peripheral vasodilatation and thus produces
a great increase in the peripheral
blood flow. The veins are easily dis-
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tended and therefore there is an increase
in the incidence of varicosities. The
incidence of hemorrhoids is also considerably
increased.
Fig 5.3: Shows inside heart structures
Blood volume - This is markedly
increased, reaching its peak around the
end of the second trimester where it can
be as much as 48 per cent above the
baseline. There is only a slight increase
during the third trimester. This increase
in volume is quite disproportionate to the
red cell mass increase. In the first half of
pregnancy the increase is more in the
volume while in the second half red cell
mass increases more than the plasma.
The net result is a fall in the hematocrit
value, from 41 per cent in the nonpregnant
stage to 37 per cent, earlier in