During pregnancy cardiac output rises gradually, reaching 150% of the pre-pregnant value by term. Labor results in a further increase in cardiac output, from an increase in both stroke volume and heart rate. With each contraction, auto-transfusion results in an additional 1020% increase in cardiac output. Analgesia, with the resulting attenuation of the heart rate increase will alleviate but not eliminate the increase in cardiac output associated with labor.
Further increase in cardiac output occurs immediately postpartum and is a result of sustained uterine contraction and auto-transfusion. This is attenuated by regional analgesia only when it is associated with sympathetic vasodilatation.
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