Physiologic Changes of Pregnancy

John G. Gianopoulos, M.D.

INTRODUCTION

''During pregnancy, multiple physiologic adjustments are made to maintain maternal hemostasis. In a non-pregnant patient, many of these alterations would be considered pathologic rather than physiologic. This lecture will present the adjustments and alterations in maternal physiology.''

MATERNAL PHYSIOLOGY

CARDIOVASCULAR

1. Cardiac Output - is  ­ during pregnancy » 1.5l/min.

   1. Cardiac Output = heart rate x stroke volume.
      CO = HR x SV

   2. Output can be measured using direct Fick method, i.e.:
      Output of L. ventricle = 0₂ consumption (ml/min)
                                                A₀₂ - V₀₂

   3. Pulse rate ­ from 70 ® 85

      1. Heart rate ­ by catecholamines
         (1) Chronotropic
         (2) Inotropic

   4. Stroke volume increases » 10%. Regulated by 2 mechanisms.

      1. Heterometric
      2. Homometric
         
         
         

   5. ­ CO, by ­ SV = ­ heart size

2. Mean arterial pressure - average pressure throughout cardiac cycle - decreased.

3. Total peripheral resistance - dependent upon arteriole diameter - ie., small changes in caliber = large changes in TRP.

R = 8hL
     p r⁴

During pregnancy TPR is reduced.

4. Increased blood flow - distribution.

   1. Uterus*
   2. Kidney
   3. Skin
   4. Breasts

5. Pulmonary Pressure - same as non-pregnant levels, due to:

1. ¯ resistance to flow - vascular dilation
2. ­ volume - capacitance
3. *RADIOGRAPHIC APPEARANCE - increased vascularity, enlarged pulmonary vessels.

6. Venous Pressure - ­ in femoral pressure:

1. Weight of uterus of illac veins, inf. vena cava.
2. Hydrodynamic obstruction - due to ­ uterine outflow.
3. *SUPINE HYPOTENSIVE SYNDROME

7. ECG Changes - elevation of diaphragm heart moved upwards and rotated forward, + enlarged. Therefore, one would predict change in the electrical axis of the heart. Deviation to the left in the electrical axis (15-28°).

8. Volume and Composition of Blood.

1. ­ Plasma volume 40-50%
2. ­ RBC - 25-30%
3. ¯Hematocrit
4. ­ WBC
5. ­ Fibrinogen
6. ­ Sedimentation rate
7. ­ Clotting Factors (VII, VIII, IX, X)
8. ­ Serum alkaline phosphatase

RESPIRATION

1. Definitions

   1. Tidal volume (TV) - amount of air moving into lungs with each inspiration.
   2. Inspiratory Reserve Volume (IRV) - air inspired with maximal inspiratory effort in excess of tidal volume.
   3. Expiratory Reserve Volume (ERV) - volume expelled by active expiratory effort after passive expiration.
   4. Residual Volume (RV) - air left in lungs after maximal expiratory effort.
   5. Vital Capacity - greatest amount of air that can be expired after maximal inspiratory effort. (timed vital capacity)
   6. Respiratory Minute Volume (RMV) - amount of air inspired/minute.
   7. Maximal Voluntary Ventilation - maximal amount of air that can be moved into and out of the lungs in one minute by voluntary effort.
      
      

2. During pregnancy the following occurs:

1. ­ TV
2. ­ RMV - 40%
   1. hyperventilation
   2. respiratory alkalosis - compensated
   3. ¯ alveolar CO₂ 40mm Hg ® 30mm Hg
   4. ­ sensitivity of brain stem respiratory centers to pCO₂ - progesterone.

RENAL

1. Physiologic Changes:

   1. Renal Plasma Flow (RPF) - equals the amount of a substance excreted/unit of time divided by renal arteriovenous difference. (PAH, DIODRAST)

      RPF = U [x] V
                   P [x]
      

      Renal blood flow = RPF x 1                    
                                                    1-hematocrit

      RPF is raised throughout pregnancy by 200-250 ml/1-min.

    

   2. Glomerular Filtration Rate (GRF) - of the plasma perfusing the glomeruli - about 20% reaches the tubular system of the kidney as an ultrafiltrate - i.e. , GFR.
      
      GFR = U [x] V = Clearance
                    P [x]

      1. ­ creatinine clearance
      2. ­ urea clearance
      3. ­ uric acid clearance
         
         

   3. Tubular Function:

      1. Na+ Excretion - progesterone inhibits reabsorption, however, increasing aldosterone levels counteract this effect.
      2. H₂0 = although plasma osmolality is ¯ and ECF ­ , urine volumes are similar to non-pregnant volumes. Hypothalamic resetting of osmoreceptors.
      3. Glycosuria - normal in pregnancy.
      4. Aminoaciduria - due to high circulating levels of cortisol?
         
         

   4. ­ activity of renin - angiotensin - dissociation between pressor and renal effects.

   5. ­aldosterone

   6. ­erythropoietin

GASTROINTESTINAL - Decreased motility, delayed absorption ¯ gastric secretion, ¯ tone of cardiac sphincter (reflex esophagitis).

ENDOCRINE

1. Steroids:

   1. Progesterone - corpus luteum, fetal-placental unit.

      1. hyperpolarization of smooth muscle membrane potential - ¯ tone.
      2. ­ temperature
      3. ­ respiratory rate
      4. combined actions

      5. breast development

   2. Estrogens - ovarian, fetal-lacental unit.

      1. combined actions with progesterone
      2. connective tissue effects
      3. liver - binding globulins ­, serum enzymes ­.
         
         

2. Protein Hormones:

   1. HCG - human chorionic gonadotrophin - syncytial trophoblast

1. corpus luteum
2. diagnosis of pregnancy
3. fetal adrenal

2. hPL - human placental lactogen - syncytium

1. GH like activity like activity
2. diabetogenic

3. HCT - chorionic thyrotropin TSH like activity

3. Pituitary Hormones:

1. Anterior Lobe
2. ADH, Oxytocin

4. Adrenal Hormones:

1. Cortisol - increases
2. Aldosterone - increases

5. Pancreas - ­/s cell function, placental insulinase

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  © 2001 Loyola University Chicago Stritch School of Medicine. All rights reserved.
Please send questions or comments to: Scott Graziano, M.D.
Updated: 07/05/00 ... Created: 05/02/00