Physiologic Changes of Pregnancy

Question 1

Change in MAC

Answer 1

Decrease of 30% because of:

Increased Plasma Endorphins (beta endorphins and kappa opioid receptors) increases pain tolerance

Increased Progesterone (2nd/3rd Trimester) produces CNS depression.

Increased RATE OF RISE of alveolar anesthetic concentration.

i.e. 50% N2O may produce unconsciousness and increases sensitivity to IV agents.

Question 2

Neuraxial Requirements

Answer 2

Decreased by 40% at term because of:

The uterus compresses the vena cava which produces epidural venous plexus distention which decreases CSF volume.

Also, 2nd and 3rd trimester, increased progesterone increases sensitivity to local anesthetics.

Question 3

Oxygen Consumption

Answer 3

Increased 50% vs pre-pregnancy because the growing uterus, placenta, and fetus increase oxygen demand.

Question 4

Rapid Oxygen Desaturation

Answer 4

With proper preoxygenation and denitrogenization, it takes 4 minutes of apnea for SpO2 to drop to 90%.

After, it takes 35 seconds to drop to 40%.

Rapid Sequence Induction with Cricoid Pressure.

Question 5

Change in Response to CO2

Answer 5

Increases sensitivity to CO2 because increased progesterone.

Increased Tidal Volume and Respiratory Rate leads to pregnancy induced hyperventilation which drops CO2.

Question 6

Blood Gas (pH)

Answer 6

Non-pregnant: 7.40

Pregnant:
1st Trimester- 7.41 to 7.44 (alkalotic)
2nd Trimester- 7.41 to 7.44 (alkalotic)
3rd Trimester- 7.41 to 7.44 (alkalotic)

Question 7

Blood Gas (PO2)

Answer 7

Non-pregnant: 100 mmHg

Pregnant:
1st Trimester- 107
2nd Trimester- 105
3rd Trimester- 103

Question 8

Blood Gas (PCO2)

Answer 8

Non-pregnant: 40 mmHg

Pregnant:
1st Trimester- 30 to 32
2nd Trimester- 30 to 32
3rd Trimester- 30 to 32

Question 9

Blood Gas [HCO3-]

Answer 9

Non-pregnant: 24 mmHg

Pregnant:
1st Trimester- 21
2nd Trimester- 20
3rd Trimester- 20

Question 10

Risk of Hypoxemia

Answer 10

Increased because uterus pushes diaphragm up and decreases FRC.

Supine position further decreases FRC where closing capacity is greater than FRC and causes small airway closure.

Decreased cardiac output decreases mixed venous saturation and arterial saturation.

Question 11

Changes in Tidal Volume and Minute Ventilation

Answer 11

Increased Tidal Volume by 45% increases Minute Ventilation by 45%.

Question 12

FRC changes

Answer 12

Decreased Expiratory Reserve Volume by 20% and decreased Residual Volume by 20% decreases FRC by 20%.

Question 13

Changes in Pulmonary Resistance

Answer 13

Decreased by 50%.

Question 14

Changes in Dead Space

Answer 14

Increased by 50% to 3 ml/kg

Question 15

Why do contractions cause hypoxemia?

Answer 15

Contractions lead to hyperventilation which decreases PaCO2 (10-15 mmHg) and increases O2 consumption.

Question 16

Changes in O2 Delivery

Answer 16

Increased O2 Delivery

Hyperventilation leads to increased dissolved O2.

Maternal Right Shift (P50 from 27mmHg to 30mmHg) of the Oxy-Hgb Dissociation Curve because of an increase in 2,3-DPG promotes easier unloading of O2 at placenta which crosses over to fetal Hgb which is left shifted (P50 of 19mmHg) and rapidly takes on O2.

Question 17

Airway Changes

Answer 17

Increased vascularity leads to mucosal friability. This increases the risk of injury during DVL.

Pre-eclampsia further increases vascularity.

Intubate with 6.0 to 6.5 tube.

NASAL INTUBATION and AIRWAYS CONTRAINDICATED.

Question 18

Causes of Anemia of Pregnancy

Answer 18

Blood volume increases by 45% while plasma volume increases by 55% mostly during the 2nd trimester. This cause “Anemia of Pregnancy”

Hgb 9 to 10 (Hct 35) is normal. 13+ is considered hemoconcentrated and may indicate pre-eclampsia.

Question 19

Estimated Blood Volume

Answer 19

90 ml/kg in single pregnancy and 105 ml/kg in twins vs 65 ml/kg non-pregnant female.

15 to 20% is allowable blood loss.

Question 20

Changes in Cardiac Output

Answer 20

Increased Stroke Volume by 25% throughout pregnancy and an increase in Heart Rate by 15% the 2nd half of pregnancy produces an increased in Cardiac Output of 50%.

Question 21

Changes in Ejection Fraction

Answer 21

Increased Left Ventricular End DIASTOLIC Volume and an unchanged Left Ventricular End SYSTOLIC Volume results in an increased Ejection Fraction.

Question 22

Change in Systemic Vascular Resistance

Answer 22

Decreased by 20% because:

Increase in Prostacyclin which is a potent vasodilator and increase in Progesterone which promotes vasodilation of vascular smooth muscles.

Placental circulation runs in parallel to systemic circulation which decreases afterload.

Decreased blood viscosity from anemia of pregnancy decreases afterload.

Question 23

EKG Changes

Answer 23

Leftward shift of heart by uterus produces:

QRS shift in any direction
QT Inversion in lead III
Transient ST-T Wave Changes.

Question 24

Most common dysrhythmias in pregnancy:

Answer 24

PAC, PVC, and Sinus tachycardia from cardiac ion channel changes and hormones.

Question 25

Aortocaval Compression

Answer 25

Decreased Venous Return after 20 weeks when supine from the uterus compressing the vena cava against the spine. This decreases Preload which decreases Stroke Volume which then decreases Cardiac Output. Aortic Compression leads to an increased BP in upper extremities but decreased blood flow distally. The uterine artery which is a branch off the hypogastric artery has decreased blood flow which causes uteroplacental insufficiency. LEFT UTERINE DISPLACEMENT or FULL LEFT LATERAL POSITION especially after neuraxial anesthesia.

Question 26

Platelets and Coagulation

Answer 26

Normal platelet count is down to 90 to 100. They function normally. Increased coagulation increases risk of thromboembolism (600%). It is not detectable with PT/PTT but is with CAT Test.

Question 27

Leukocytes

Answer 27

Increased to 9k to 11k from 6k.

Question 28

Changes in Gastric Emptying

Answer 28

Overall, gastric emptying is UNCHANGED in pregnancy. There is a decrease in gastric emptying from pain (contractions) and neuraxial opioids. Clear liquids (OK by ASA) promotes gastric emptying. Returns to normal 18 hours postpartum.

Question 29

Risk of Regurgitation and Aspiration

Answer 29

Increases because: Decreased Lower Esophageal Sphincter tone from increased progesterone and estrogen. Uterus rotates stomach which eliminates the "pinch point". Note: Decreased Gastrin leads to decreased Gastric Acid Secretions (20-30 weeks pregnancy). However, there is no difference between pregnant and non-pregnant females regarding pH less than 2.5 and gastric volume greater than 25mL.

Question 30

Hepatic Changes

Answer 30

Cardiac output to liver drops 35%. Non-pregnant is 25%, pregnant is 18%. This leads to a decrease in clearance of drugs dependent on hepatic blood flow. Increase splanchnic, portal, and esophageal venous pressures leads to an increased risk of esophageal varices by 60%. Albumin concentration decreases by 60% because of increased plasma volume.

Question 31

Renal Changes

Answer 31

Kidney grows because of a 75% increase in renal blood flow and returns to normal after 6 months. GFR increases from 100 ml/min to 150 ml/min. This decreases BUN and Creatinine. A normal BUN (12 to 20) or Creatinine (0.6 to 1.2) could indicate renal insufficiency. Increased proteinuria is normal.

Question 32

Pancreatic Changes

Answer 32

Decreased insulin sensitivity because of placental lactogen increases blood glucose on a heavy carb diet. Decreased fasting blood glucose during the 3rd trimester from increased glucose consumption by fetoplacental unit.

Question 33

Effects of Lumbar Lordosis

Answer 33

Meralgia Paresthetica- sensory loss over anterolateral thigh due to stretching of lateral femoral cutaneous nerve. Brachial Plexus Neuropathy from anterior flexion of neck and slumping shoulders.

Question 34

Major changes caused by Progesterone

Answer 34

CNS depression (decreased MAC) Increased sensitivity to local anesthetics. Increased response to CO2. Vasodilation of vasc. smooth muscle (decreased SVR) Decreased lower esophageal sphincter tone (GERD)

Question 35

Goals of Ventilation

Answer 35

Maintain PaCO2 around 30mmHg which is an ETCO2 of 27mmHg by INCREASING minute ventilation and DECREASING tidal volume.

Question 36

Effects of Pregnancy on Neuromuscular Blockers

Answer 36

25% decrease in plasma cholinesterase requires slightly less rocuronium, titrate to Train of Four. Use normal dose of succinylcholine to optimize intubation.

Question 37

Risk of Morbidity and Mortality

Answer 37

Emergence from general anesthesia. Induction of neuraxial anesthesia can lead to CV collapse or high level block.