Plasma volume increases 45% at term, RBC volume increases 20%, thus while pregnant patients have increased RBC mass, they appear anemic. Normal hemoglobin is 12 g/dL. During labor, contractions squeeze blood into the systemic circulation, and after delivery, uterine involution autotransfuses 500 cc/blood. Note that clotting factors (I, VII, VIII, IX, X, XII) are elevated, protein s (anticoagulant) is decreased, and there is acquired resistance to protein C. Pregnancy is thus a hypercoagulable state. Also note that both platelet generation and destruction increase, with variable effects. Importantly, 7.6% of term parturients have platelets < 150,000, and 1% have < 100,000 platelets at term.
Both stroke volume and heart rate increase, the end result being a 40-50% increase in cardiac output by the third trimester (maximal at 24 weeks) – immediately after delivery it can be as high as 80% above normal (150% above pre-pregnancy levels). Increased CO may be detrimental to women with valvular lesions (ex. stenotic lesions). Pregnant women may also develop systolic regurgitation murmurs (usually mitral or tricuspid in origin).
Note that the heart is displaced cephalad and laterally, and the EKG changes of pregnancy include 1) sinus tachycardia 2) other dysrhythmias 3) ST depression 4) T wave flattening 5) LVH and 6) LAD
Decreases in SVR as result in reduced SBP (avg 8%, as much as 15%) and DBP (avg 20%), likely due to changes in estradiol, progesterone, nitric oxide, and prostacyclin. Increases in venous capacitance and myocardial remodeling attenuate the increased blood volume (i.e. CVP remains constant)
Aortocaval compression can occur in as many as 20% of pregnant women, and can lead to several problems. First, it can lead to maternal hypotension and subsequent fetal acidosis (usually if SBP is 90 – 100 mm Hg for 10 or more minutes), and second, it can dilate the epidural veins (Batson’s plexus), leading to intravascular injection during epidural anesthesia. Treat with lateral positioning or a right-sided hip wedge
Maternal Hemodynamic Effects
- Hematologic: plasma volume increases 45% at term, RBC volume increases 20%
- Cardiac Output: stroke volume and heart rate increase
- Vascular resistance: decreases in SVR as result in reduced SBP
- Venous Return: aortocaval compression can occur in as many as 20% of pregnant women
Note that approximate blood loss is 300-500 cc for a vaginal delivery and 800-1000 cc for a Cesarean section.
Burrows RF, Kelton JG. Thrombocytopenia at delivery: a prospective survey of 6715 deliveries. Am J Obstet Gynecol. 1990 Mar;162(3):731-4.