Week 12 - OB Physiology and Anesthetic Considerations Flashcards
What maternal cardiovascular changes are seen in pregnancy?
- Increased intravascular fluid volume
- Increased CO
- Decreased SVR
- Supine aortocaval compression (decreases uteroplacental circulation)
- Systolic BP drop 0-5 mmHg, Diastolic BP drops 15%, MAP remains normal
- Unchanged CVP
- Distension of peripheral vessels (sluggish flow – increases risk of clots)
- Delayed absorption of IM, SubQ drugs
How does maternal cardiac output change during pregnancy?
- Increases 40% by 10 weeks and stays thru delivery
- Increases from 4.5 lpm to 6,5 lpm
- Stroke volume increases 30%
- Heart rate increases 15-30%
- Total peripheral resistance decreases 15% due to estrogen
*at delivery CO increases another 40-60% – rapidly declines within 1 hr of delivery and returns WNL about 2 weeks postpartum
What is supine hypotension syndrome in a pregnant woman?
- Associated w/ pallor, sweating, N&V (symptoms are worse when combined with HoTN due to GA or RA)
- Secondary to compression of the vena cava
- Turn pt left side down or tilt (mandatory when pt is supine)
*uterine contractions relieve vena caval compression but worsen aortic compression
What causes the physiologic anemia of pregnancy?
Plasma volume increases by a greater amount than RBC volume
- Plasma volume increases 30-40%
- Red cell volume increases 20-30%
- Blood viscosity decreases 12-20%
What changes to maternal blood volume occur during pregnancy?
Blood volume increases 1000-1500 mL
-allows for delivery without transfusion (vaginal EBL = 400-500, c-section EBL = 600-1000) – can lose ~20% w/o symptoms
- increased volume is accommodated by uterine vasculature, maternal mammaries, renal, and muscular system, and cutaneous vascular system
- increase is stimulated from fetal adrenals
How does maternal blood protein levels change during pregnancy?
Total protein decreases by 18%
Serum Albumin decreases by 15%
What cardiovascular effects does progesterone have?
Increases RAAS Activity: leading to increased blood volume and CO
Vascular Muscle Relaxation: leading to decreased SVR and PVR, increasing blood flow
Increases Minute Ventilation: leading to decreased PaCO2 and the kidneys eliminate HCO2 to preserve pH
What maternal respiratory changes are seen during pregnancy?
- O2 consumption and minute ventilation increases progressively (TV 40%, RR 10%)
- Oxyhgb dissociation curve shifts to the right (greater tissue extraction of O2)
- PaCO2 decreases to about 28-31 mmHg (decreased HCO3 from increased renal excretion prevents resp acidosis)
How does pregnancy affect the maternal FRC?
FRC decreases up to 20% at term – returns to normal 48 hr postpartum
- decreased FRC and increased O2 consumption leads to rapid desaturation (good pre oxygenation is mandatory)
- closing volume likely to exceed FRC when supine (atelectasis and hypoxemia, usually wear O2 when supine)
What maternal airway changes are seen during pregnancy?
- Engorged airway mucosa – gentle DL and intubation, use smaller ETT
- Exacerbated signs and symptoms of URIs
- Swollen nasal mucosa leads to mouth breathing – NO nasal intubations
*capillary engorgement of larynx, oral pharyngeal, and nasal mucosa begins in first trimester and increases throughout pregnancy
What hematologic changes are seen in pregnancy?
Pregnancy is hypercoagulable – most clotting factors increase
Cell mediated immunity is depressed – may be more susceptible to viral infections
What maternal renal changes are seen during pregnancy?
- Renal blood flow and GFR increase 50% at 3 months until 3 months post partum
- Sodium retention due to increased renin and aldosterone
- Serum BUN and Cr decrease (increased clearance and increased protein excretion)
- Mild glycosuria or proteinuria is common due to increased glucose
What maternal hepatic changes are seen during pregnancy?
- Liver function and blood flow unchanged
- Plasma cholinesterase levels decrease (not associated w/ clinically significant increase in duration of SUX)
- Pregnancy resembles DM – insulin levels rise throughout
- B cell hyperplasia due to increased insulin demand
What maternal GI changes are seen during pregnancy?
- Decreased lower esophageal sphincter tone
- pH decreases – increased acid and volume
- Decreased gastric motility
- Increased intra-gastric pressure
- Increased gastric emptying time – during labor (intrathecal opioid delay gastric emptying)
- Increased gastric residual volume
*ALL lead to increased risk of aspiration
How can you prevent aspiration in a pregnant woman?
- Regional is best method
- NPO
- Non-particulate antacids (BiCitra)
- H2 receptor antagonists (60-90 min)
- Reglan (15-30 min)
- Head up position
- RSI
- Cricoid pressure
- ETT
- Awake extubation
How is MAC affected during pregnancy?
MAC is decreased
- decreases 40% at term
- increased pain tolerance
- related to high progesterone and endorphin levels
- returns to normal about 3 days postpartum
Is sensitivity to local anesthesia increased or decreased in pregnancy?
Increased sensitivity
- may reduce local dose by 30%
- possibly resp acidosis which increases unionized drug
- due to hormones, engorged epidural veins
How does pregnancy effect an epidural?
Obstruction of the IVC engorges epidural veins and venous plexus:
- increases the chances for venous puncture, venous injection, and venous “epidural” catheter
- decreases volume of CSF in subarachnoid space
- decreases epidural space which increases cephalad spread of injections for SAB and epidural
- increases pressure in epidural space which increases the potential for dural puncture
What is uterine blood flow directly proportional to?
Maternal SBP and Difference between uterine artery and venous BP
-uterine artery vasoconstriction decreases UBF, uterine contractions decrease UBF
*NO autoregulation – mom’s BP determine fetal blood flow
How do you maintain uterine blood flow?
By maintaining MAP
UBF = uterine arterial BP - venous BP / uterine vascular resistance
How can a fetus survive 10+ minutes without O2?
- Redistribution to vital organs
- Decreased O2 consumption
- Anaerobic metabolism
- fetal O2 consumption is 21 mL/min
- fetal O2 stores about 42 mL
How do the different anesthetics affect uteroplacental circulation?
- Volatiles: decrease maternal BP and UBF (minimal at <1 MAC)
- Propofol and Barbs: dose dependent decrease in maternal BP, beware low dose and maternal sympathetic response
- Ketamine: <1.5 mg/kg has little effect on BP
- Etomidate: assumed minimal effect
- Local Anesthetics: high levels cause uterine artery vasoconstriction, appropriate levels can improve circulation in preeclampsia
What does diffusion of substances across the placenta depend on?
- Maternal fetal concentration gradient
- Maternal protein binding
- Molecular weight of the substance
- Lipid solubility of the substance
- Degree of ionization of the substance
What principle describes the transfer of anesthetics across the placenta?
Fick’s Principle of Diffusion
What anesthetic drugs transfer across the placenta?
ALL inhaled agents and MOST IV agents
-opioids, BZDs, propofol, barbs all transfer (low molecular weights, lipophilicity, neutral charges)
What anesthetic drugs do not transfer across the placenta?
Paralytics (due to ionization and molecular weight)
Glycopyrrolate
Heparin/Anticoagulants
What three things affect placental transfer of local anesthetics?
pKa of drug
Maternal and fetal pH
Degree of protein binding of drug
*fetal acidosis produces higher fetal/maternal drug ratios which leads to LA toxicity after birth (happens less with Bupivacaine and Ropivacaine – greater protein binding decreases placental transfer)
How does maternal minute ventilation change during labor?
MV can increase 300% during contractions
-PaCO2 can drop below 20 mmHg – hypoventilation and maternal and fetal hypoxemia, hypocarbia reduces UBF and promotes fetal acidosis
What are the stages of Labor?
First Stage (8-12+ hrs in primips, 5-10 in multips):
- latent phase - slow, 2-4cm dilation
- active phase - increased frequency of contraction, progressive cervical dilation to 10cm
- need T10-11 dermatomal level for pain relief
Second Stage (15 min - 2 hrs):
- complete dilation and effacement and delivery of fetus
- need S2-4 dermatomal level for pain relief
Third Stage (15-30 min): -delivery of the placenta
What dermatome levels are needed for pain relief for each stage of labor?
Stage 1 = T10-11
Stage 2 = S2-4
Stage 3 = S2-4
What are the neuraxial anesthesia options for labor?
- Epidural
- Combined spinal-epidural
- Intrathecal narcotics
- Continuous spinal anesthesia
- Spinal anesthesia for delivery
- Blocks: paracervical, pudendal, caudal, saddle
What are the advantages of epidural anesthesia in L&D?
- Ease of technique
- Rapid onset of analgesia
- Decreased dose of local anesthetic necessary
- Ability to provide segmental analgesia
- Avoidance of airway complications
- Ability to titrate the level of analgesia depending on the level of maternal pain
- An awake, pain free mother that can actively participate in the labor and birth process and bond with her newborn
What are the disadvantages of epidural anesthesia in L&D?
- Effect on labor and maternal hemodynamics (can slow 2nd stage if too dense – watch for HoTN)
- Placental transfer and fetal absorption of the local anesthetic
- Accidental dural puncture and PDPH
- Accidental intravascular injection – seizures, fetal distress, or CV collapse
- Backache
What are the general anesthetic effects on labor?
- Rapid induction of anesthesia (increased CO, decreased FRC, increased MV)
- Inhaled agents cause dose-dependent uterine relaxation
- Opioids decrease labor progression minimally
- Regional anesthesia does NOT prolong labor
What does APGAR score assess?
Appearance Pulse Grimace Activity Respiration
What are the different fetal heart tones?
Early Decels - follow with contractions, may be due to head compression-vagal response
Late Decels - some time after contractions and thought to be due to uteroplacental insufficiency
Variable Decels - inconsistent, possible cord compression
What are the characteristics of Pre-eclampsia? (3)
HTN
Edema
Proteinuria
*after 20-24 weeks
What medications are used for uterotonic therapy to treat uterine atony?
- Oxytocin 20-40 units/L infucion
- Ergot Alkaloids (Methergine) 0.2 mg IM
- Prostaglandin (Carboprost) 0.25 mg IM - contraindicated in pt with asthma
- Misoprostol 800-1000 mcg PR/PV/PO
- Dinoprostone 20mg PO
How does magnesium sulfate affect muscle relaxants?
- Increased sensitivity to ALL muscle relaxants
- No fasciculations with SUX
- No defasciculating dose of NDMR with SUX
What are some known teratogens?
- Thalidomide
- Diethylstibesterol (vaginal ca)
- Synthetic Progestagens (masculinization)
- Folic Acid Antagonists (growth retardation)
- Tetracycline (dental staining/bone deform)
- Warfarin (bone malform)
- Iodides (thyroid dysfunction)
- Alcohol (craiofacial abnormalities)