Physiologic Changes of Pregnancy Flashcards
Change in MAC
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.
Neuraxial Requirements
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.
Oxygen Consumption
Increased 50% vs pre-pregnancy because the growing uterus, placenta, and fetus increase oxygen demand.
Rapid Oxygen Desaturation
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.
Change in Response to CO2
Increases sensitivity to CO2 because increased progesterone.
Increased Tidal Volume and Respiratory Rate leads to pregnancy induced hyperventilation which drops CO2.
Blood Gas (pH)
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)
Blood Gas (PO2)
Non-pregnant: 100 mmHg
Pregnant:
1st Trimester- 107
2nd Trimester- 105
3rd Trimester- 103
Blood Gas (PCO2)
Non-pregnant: 40 mmHg
Pregnant:
1st Trimester- 30 to 32
2nd Trimester- 30 to 32
3rd Trimester- 30 to 32
Blood Gas [HCO3-]
Non-pregnant: 24 mmHg
Pregnant:
1st Trimester- 21
2nd Trimester- 20
3rd Trimester- 20
Risk of Hypoxemia
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.
Changes in Tidal Volume and Minute Ventilation
Increased Tidal Volume by 45% increases Minute Ventilation by 45%.
FRC changes
Decreased Expiratory Reserve Volume by 20% and decreased Residual Volume by 20% decreases FRC by 20%.
Changes in Pulmonary Resistance
Decreased by 50%.
Changes in Dead Space
Increased by 50% to 3 ml/kg
Why do contractions cause hypoxemia?
Contractions lead to hyperventilation which decreases PaCO2 (10-15 mmHg) and increases O2 consumption.
Changes in O2 Delivery
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.
Airway Changes
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.
Causes of Anemia of Pregnancy
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.
Estimated Blood Volume
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.
Changes in Cardiac Output
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%.
Changes in Ejection Fraction
Increased Left Ventricular End DIASTOLIC Volume and an unchanged Left Ventricular End SYSTOLIC Volume results in an increased Ejection Fraction.
Change in Systemic Vascular Resistance
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.
EKG Changes
Leftward shift of heart by uterus produces:
QRS shift in any direction
QT Inversion in lead III
Transient ST-T Wave Changes.
Most common dysrhythmias in pregnancy:
PAC, PVC, and Sinus tachycardia from cardiac ion channel changes and hormones.
Aortocaval Compression
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.
Platelets and Coagulation
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.
Leukocytes
Increased to 9k to 11k from 6k.
Changes in Gastric Emptying
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.
Risk of Regurgitation and Aspiration
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.
Hepatic Changes
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.
Renal Changes
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.
Pancreatic Changes
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.
Effects of Lumbar Lordosis
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.
Major changes caused by Progesterone
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)
Goals of Ventilation
Maintain PaCO2 around 30mmHg which is an ETCO2 of 27mmHg by INCREASING minute ventilation and DECREASING tidal volume.
Effects of Pregnancy on Neuromuscular Blockers
25% decrease in plasma cholinesterase requires slightly less rocuronium, titrate to Train of Four.
Use normal dose of succinylcholine to optimize intubation.
Risk of Morbidity and Mortality
Emergence from general anesthesia.
Induction of neuraxial anesthesia can lead to CV collapse or high level block.
