Background Maternal and Fetal Physiology Anderson Lo, DO Fellow, Maternal-Fetal Medicine Wayne State University School of Medicine SEMCME Fetal Assessment Course July 20, 2018 Oxygen pathway Mother Placenta Fetus Interruption of the oxygen pathway may cause fetal injury What changes in maternal physiology and fetal developmental characteristics are protective against disruption of oxygen pathway? Objective Maternal physiology during pregnancy Fetal circulation Oxygen transfer between mother and fetus Fetal response to hypoxia Plasma volume Increase of 50% above nonpregnant values Begins to increase at 6-8 weeks Maximal volume ~5 L at 32 weeks Red blood cell mass Increase of 20-30% above nonpregnant values Increase by 250-450 ml by term Due to increased production of red blood cells Increased maternal demand for iron (total requirement 1,000 mg) Physiologic hemodilution Increase in plasma volume > increase in RBC cell mass Decrease in hematocrit Decreased blood viscosity 1
Anatomic Changes Ventricular mass increases in 1 st trimester End-diastolic volume increases in the 2 nd and early 3 rd trimesters Increased cardiac compliance (physiologically dilated heart) Ejection fraction maintained due to increased contractility Cardiac output Cardiac output a measure of the functional capacity of the heart Cardiac output = HR x SV Cardiac output is increased in pregnancy Increases by 30-50% From 4.5 L/min to 6 L/min Earliest rise in CO is due to increase in SV Gradual increase in maternal HR Netter Cardiac output is affected by maternal position Supine hypotension syndrome Compression of IVC decreased venous return 25-30% fall in CO Manifested by: Sudden drop in BP Bradycardia Syncope Distribution of Cardiac Output Percentage of cardiac output to the uterus Non-pregnant state: 2% Pregnancy: 20% by term (500-800 ml/min) Increased blood flow to Kidneys (increases by 50%) Skin Breasts www.whatwhenhow.com Gray, Henry. Anatomy of the Human Body. 1918 Blood Pressure Decrease in peripheral vascular resistance from increased cardiac output Arterial blood pressure decreases in pregnancy as early as 7 weeks Nadir occurs from 24-32 weeks Followed by rise toward nonpregnant values at term Hemodynamic Changes in Labor 1 st Stage 2 nd Stage Postpartum ~10-30% increase in CO ~50% increase in CO 80% increase in CO within 15 minutes after Due to increase in SV Some related to pain vaginal delivery Uterine contractions transfer blood from uterus to general circulation Release of venacaval obstruction Autotransfusion of uteroplacental blood Rapid mobilization of extravascular fluid 2
Review Maternal Lungs Change in pregnancy Plasma volume Red blood cell mass Cardiac output Heart rate Stroke volume Peripheral vascular resistance Ejection fraction Increase in subcostal angle and chest circumference Upward displacement of diaphragm Hegewald, M.J. Clin Chest Med. 2011 Pulmonary Function Minute ventilation is increased in pregnancy Increased TV Decreased ERV RV No change IRV Minute ventilation (MV) volume of air that can be inhaled or exhaled during 1 minute MV = TV x RR Increase MV (30-50%) attributed to increase in TV alone Mild respiratory alkalosis is normal in pregnancy PaCO2 Serum bicarbonate Pre-pregnancy ~39 mmhg 23-30 meq/l Pregnancy ~30 mmhg (at term) 18-22 meq/l 3
Fetal Circulation 3 shunts Preferential streaming Distribution of cardiac output 65% of cardiac output from RV 35% of cardiac output from LV O2 transport from mother to the fetus Facilitated by Higher affinity of fetal hemoglobin for oxygen Difference in oxygen concentration gradient between maternal and fetal blood Bohr effect Fetal hemoglobin has higher oxygen affinity than adult hemoglobin Both CO2 and O2 move down their concentration gradient www.cambridge.org Bohr Effect Fetus responds to hypoxia by increasing blood flow to CNS and heart Limit of circulatory defense against hypoxia is reached when perfusion rate of CNS and heart has reached its maximum Respiratory acidosis = acute compromise (reflected in PCO2 in umbilical vessels) Metabolic acidosis = prolonged compromise (reflected in base deficit in umbilical vessels) www.slideshare.net 4
Definitions Hypoxemia decreased O2 in blood Hypoxia decreased O2 in tissue Acidemia increased hydrogen ions in blood Acidosis increased hydrogen ions in tissue Asphyxia hypoxia with metabolic acidosis Perinatal asphyxia Implies hypoxia to a degree leading to metabolic acidosis Apgar score is not a good tool to measure asphyxia Umbilical artery blood ph < 7.0 Base deficit > 12 meq/l Thank You 5