Physiologic Changes of Pregnancy: Time Course
Last updated: 05/28/2020
- Cardiovascular: Cardiac output(CO) increases by 50% by 12wks gestational age(GA)(HR and SV each increase by 25%). At term, CO increases an additional 25%(up to 40-50% increase from pre-pregnancy baseline). CO is highest immediately after delivery and can increase by an additional 80% which is 150% above pre-pregnancy baseline1. CO returns to pre-pregnancy baseline around 6 months post-partum2. Systemic vascular resistance(SVR) decreases resulting in decreased SBP (average 8% decrease) and DBP(average 20% decrease). While some degree of hypotension is expected, profound hypotension (e.g., from aortocaval compression) can result in fetal acidosis and hypoxemia.
Venous capacitance is increased in pregnancy and myocardial remodeling occurs. The heart is displaced laterally and cephalad which can result in left axis deviation on ECG. Other common ECG findings include sinus tachycardia, LVH, ST depressions and T-wave flattening. It is not uncommon for parturients to develop an S3 gallop or systolic flow murmur. Regurgitant lesions may also develop during pregnancy3.
- Respiratory: During pregnancy, minute ventilation must increase by ~50% to supply fetus with adequate oxygen delivery. This increase is largely achieved by increasing TV as RR stays relatively stable. Increasing TV is facilitated by expansion of the thoracic rib cage. There is a relative respiratory alkalosis of pregnancy and normal PaCO2 values are ~32mmHg.
FRC(20% upright, 30% when supine)
- Hematologic: Blood volume increases by 50% by 34 wks gestation, there is a relative anemia of pregnancy given a greater increase in plasma volume(55% increase) compared to a 35% increase in RBC volume. This results in an average hemoglobin of 12.0g/dL. Blood volume decreases to 125% pre-pregnancy values by 1 week post-partum; drops to 110% of pre-pregnancy values by 6-9 wks post-partum. Albumin also decreases (3.9 g/dL in 1st trimester, 3.3g/dL by 3rd trimester) which results in decreased oncotic pressure and tissue edema.
Pregnancy is also a hypercoagulable state. There is an increase in production of factors VII, VIII, IX, X, XII and fibrinogen. Production of factors XI and XIII is decreased while factors II and V are unchanged.
- Gastrointestinal: The stomach is displaced left and upward increasing the incidence of GERD. Gastric volume is increased and pH is decreased which increases risk of aspiration.
Gastric emptying is not delayed until during labor but progesterone does slow esophageal peristalsis and intestinal transit time. There is also increased risk of biliary disease as biliary stasis is common. LFTs may appear falsely elevated despite normal liver size and morphology because the placenta produces some of these enzymes as well.
- Renal: there is a 50% increase in GFR and RBF. This leads to increased creatinine clearance; thus, serum creatinine level should be lower than pre-pregnancy baseline.
- Endocrine: Thyroid: thyroid function is increased by ~50-70%; estrogen causes an increase in globulin binding proteins which leads to an increase in total T3 and T4 but levels of free T3 and T4 are unchanged.
Glucose Metabolism: Glucose demands are increased by the fetus but average maternal blood glucose levels are grossly unchanged. The increased glucose delivery to the fetus is facilitated by a relative insulin resistance from placental lactogen.
- Nervous System: Progesterone is thought to lead to significantly reduced MAC requirements in pregnancy (up to 40% decrease). Additionally, plasma and CSF levels of endorphins and enkephalins are increased in pregnancy. Neuraxial medication requirement is also decreased as increased adipose tissue surrounding the epidural and spinal canal spaces results in higher spread of medications than in non-parturients4.
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