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
