MATERNAL FETAL PHYSIOLOGY

1. MATERNAL & FETAL PHYSIOLOGY Vicente Gonzalez CRNA, MS, ARNP 2011 FLORIDA INTERNATIONAL UNIVERSITY PRINCIPLES ANESTHESIOLOGY NURSING II NGR 6422

2. OBJECTIVES Discuss physiologic changes in the parturient that occur during pregnancy. Explain physiology of placental circulation including the course of fetal circulation. Compare progress of labor and pain during various stages of labor and delivery. Describe various techniques of anesthesia for labor and vaginal delivery and cesarean section. Discuss diagnosis and management of fetal distress. Describe neonatal resuscitation techniques and methods of evaluation for the neonate.

3. REFERENCES Chestnut, D. H., Polley, L. S., Tsen, L. C., Wong, C. A. (4th Ed.)(2009). Chestnut�s Obstetric Anesthesia, Principles and Practice. Mosby Elsevier ISBN 978-0-323-05541-3. Nagelhout, J. J., Plaus, K. L. (4th Ed.)(2010). Nurse Anesthesia. Saunders Elsevier. ISBN 978-1-4160-5025-4.

4. INTRODUCTION GENERAL CONSIDERATIONS: Anesthetic management requires an understanding of the physiologic changes in the parturient during pregnancy and labor The full term parturient is rarely in optimal condition at the time anesthetic care is administered and is always to be considered a �full stomach� During labor, emergencies demand immediate obstetrical intervention that is likely to require administration of appropriate anesthesia management

5. PHYSIOLOGIC CHANGES CARDIOVASCULAR SYSTEM: Intravascular fluid volume: Results in an average expansion of 1500ml at term Plasma volume increases ~ 45% and erythrocyte volume increases ~ 20% Cardiac output: Increases ~ 10% by the tenth week of pregnancy Increases to 40% above normal by the third trimester Largest increase in CO occurs immediately after delivery

6. PHYSIOLOGIC CHANGES CARDIOVASCULAR SYSTEM: Cardiac changes Ventricular enlargement (ventricular mass)by 23% EDV increases but LVESV is unchanged Systolic murmurs may appear. S3 Benign in nature HR increases by 15-25% by end of 1st trimester Peripheral circulation: Systemic vascular resistance decreases by 20% No change in central venous pressure Renin activity increases 12 fold by third trimester but vascular sensitivity to Angiotensin II is reduced. Changes begin as early as 4 weeks and continue into post-partum. CO increases at 5th week. Can be as high as 180% 24hrs after delivery and returns to normal 10 days afterChanges begin as early as 4 weeks and continue into post-partum. CO increases at 5th week. Can be as high as 180% 24hrs after delivery and returns to normal 10 days after

7. Physiologic changes Supine hypotension syndrome: Due to compression of the inferior vena cava by the gravid uterus in the supine position Results in a decreased CO and decline in systemic blood pressure Associated with a decrease in uterine and placental blood flow Treatment: lateral position

8. Stroke volume and HR changes

9. PHYSIOLOGIC CHANGES PULMONARY SYSTEM: Upper airway: Capillary engorgement of the mucosal lining of the upper respiratory tract is present Select a smaller cuffed ETT (6.0-7.0) Weight gain, short neck, and large breasts can result in difficulty inserting the laryngoscope Minute ventilation: Increased ~ 50% during the first trimester Due to increased tidal volume Results in a decreased resting maternal PaCO2

10. PHYSIOLOGIC CHANGES Airway changes Study in 2008 61 parturients examined at onset and at the end of labor In 20 the exam went one grade higher In 3 it went two grades higher There was no correlation with fluid or duration of labor

11. PHYSIOLOGIC CHANGES PULMONARY SYSTEM: Lung volumes: Begin to change about the third month Enlarged uterus results in a 20% decrease in FRC at term Vital capacity not significantly changed Arterial oxygenation: Early in gestation PaO2 above 100 mmHg Later PaO2 normal or slightly decreased Induction associated with marked decreases in PaO2 Treatment: preoxygenation

12. Summary of Pulmonary changes Parameter Change* Lung volumes: Inspiratory reserve volume +5% Tidal volume +45% Expiratory reserve volume -25% Residual volume -15% Lung capacities: Inspiratory capacity +15% Functional residual capacity -20% Vital capacity No change Total lung capacity -5% Dead space +45% Respiratory rate No change Ventilation: Minute ventilation +45% Alveolar ventilation +45%

14. ABG changes Blood Gas Measurements during Pregnancy Trimester Parameter Nonpregnant First Second Third Paco2 (mm Hg) 40 30 30 30 Pao2 (mm Hg) 100 107 105 103 pH 7.40 7.44 7.44 7.44 [HCO3-] (mEq/L) 24 21 20 20 Blood volume changes +45% Plasma +55% RBC +30%

15. Increased Factor Concentrations � Factor I (fibrinogen) � Factor VII (proconvertin) � Factor VIII (antihemophilic factor) � Factor IX (Christmas factor) � Factor X (Stuart-Prower factor) � Factor XII (Hageman factor) Unchanged Factor Concentrations � Factor II (prothrombin) � Factor V (proaccelerin) Decreased Factor Concentrations � Factor XI (thromboplastin antecedent) � Factor XIII (fibrin-stabilizing factor)

16. Other Parameters � Prothrombin time: shortened 20% � Partial thromboplastin time: shortened 20% � Thromboelastography: hypercoagulable � Fibrinopeptide A: increased � Antithrombin III: decreased � Platelet count: no change or decreased � Bleeding time: no change � Fibrin degradation products: increased � Plasminogen: increased

17. PHYSIOLOGIC CHANGES NERVOUS SYSTEM: MAC requirements for volatile anesthetics decreased Decreased epidural space Decreased volume of CSF RENAL SYSTEM: Renal blood flow and glomerular filtration rate Blood urea nitrogen and creatinine concentrations

18. PHYSIOLOGIC CHANGES HEPATIC SYSTEM: Plasma cholinesterase activity is decreased Plasma concentrations of coagulation factors increased GASTROINTESTINAL SYSTEM: Gastric fluid volume and gastric fluid pH Transit time increased in 3rd trimester (a study of 11 women) Laboring patient always considered �full stomach� Sodium citrate and metoclopramide

19. Summary of Changes VARIABLE CHANGE AMOUNT Total blood volume ? 25�40% Plasma volume ? 40�50% Fibrinogen ? 100% Serum cholinesterase activity ? 20�30% Cardiac output ? 30�50% Minute ventilation ? 50% Alveolar ventilation ? 70% Functional residual capacity ? 20% Oxygen consumption ? 20% Arterial carbon dioxide tension ? 10 mm Hg Arterial oxygen tension ? 10 mm Hg Minimum alveolar concentration ? 32�40% From Clinical Anesthesia, Barash, 6th edition

20. PLACENTAL CIRCULATION UTERINE BLOOD FLOW: Not auto-regulated Directly proportional to mean perfusion pressure across the uterus and inversely proportional to uterine vascular resistance Margin of safety, blood flow exceeds fetal O2 requirements HYPOTENSION: Decreases uterine blood flow Epidural or spinal anesthesia does not alter uterine blood flow if maternal hypotension avoided

21. PLACENTAL CIRCULATION UTERINE VASCULAR RESISTANCE: Increased uterine vascular resistance decreases uterine blood flow Ephedrine not associated with significant decreases in uterine blood flow* Uterine contractions decrease uterine blood flow PLACENTAL EXCHANGE: Occurs primarily by diffusion Very low pressure system Diffusion depends on maternal-to-fetal concentration gradients, maternal protein binding, molecular weight, lipid solubility, and degree of ionization of a specific substance

22. Ephedrine vs. Phenylephrine Original studies in the 70�s Uterine blood flow increased by ephedrine. Conclusion: cardiac stimulation is important Follow up studies 2002 7 randomized control trials (n=292) No difference in APGAR scores Lower HR in phenylephrine group 2009; 104 C/S patients. No difference. Ephedrine crosses placenta more readily which increases lactate and catecholamine levels