Cardiac output peaks during pregnancy at what gestational age?
Correct Answer: 32 weeks
Description: Ans. c (32 weeks). (Ref. Textbook of Obstetrics by Dutta, 4th ed., 53)Cardiac output starts to increase from 10th week of pregnancy, reaches its peak of 40% at about 24-30 weeks.PHYSIOLOGICAL CHANGES ASSOCIATED WITH PREGNANCYCardiovascular System1Blood Volume# Increases progressively from 6-8 weeks gestation (pregnancy)# Reaches a maximum at approximately 32-34 weeks.The increase in plasma volume (40-50%) is relatively greater than that of red cell mass (20-30%) resulting in hemodilution and a decrease in haemoglobin concentration.Intake of supplemental iron and folic acid is necessary to restore hemoglobin levels to normal (12 g/dl).The increased blood volume serves two purposes.First, it facilitates maternal and fetal exchanges of respiratory gases, nutrients and metabolites.Second, it reduces the impact of maternal blood loss at delivery.Typical losses of 300-500 ml for vaginal births and 750-1000 ml for Caesarean sections are thus compensated with the so-called "autotransfusion" of blood from the contracting uterus.2Blood ConstituentsAs mentioned above, red cell mass is increased by 20-30%. Leukocyte counts are variable during gestation, but usually remain within the upper limits of normal.Marked elevations, however, develop during and after parturition (delivery).3Fibrinogenas well as total body and plasma levels of factors VII, X and XII increase markedly.The number of platelets also rises, yet not above the upper limits of normal.Combined with a decrease in fibrinolytic activity, these changes tend to prevent excessive bleeding at delivery. Thus, pregnancy is a relatively hypercoagulable state, but during pregnancy neither clotting or bleeding times are abnormal.4Cardiac OutputIncreases to a similar degree as the blood volume.During the first trimester cardiac output is 30-40% higher than in the non-pregnant state.Steady rises are shown on Doppler echocardiography, from an average of 6.7 litres/ minute at 8-11 weeks to about 8.7 litres/minute flow at 36-39 weeks. They are due, primarily, to an increase in stroke volume (35%) and, to lesser extent, to a more rapid heart rate (15%).There is a steady reduction in systemic vascular resistance (SVR) which contributes towards the hyperdynamic circulation observed in pregnancy.5Cardiac Size/ Position/ECGThere are both size and position changes which can lead to changes in ECG appearance. The heart is enlarged by both chamber dilation and hypertrophy.
Category:
Gynaecology & Obstetrics
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