Uterine Blood Flow & Placental Transfer Flashcards
Main blood supply to uterus
Uterine arteries
Secondary source of blood supply to uterus (lesser, variable contribution)
Ovarian arteries
what supplies oxygenated blood to the intervillous space?
spiral arteries
where does maternal and fetal gas, nutrient, and waste exchange occur?
the intervillous space
How does deoxygenated blood return to maternal circulation?
Through uterine and ovarian veins
What mechanism allows increased uterine blood flow to meet needs of the growing uterus and fetus?
Uterine arteries are maximally dilated allowing more flow
What supplies blood to the placenta?
maternal arcuate, radial and spiral arteries?
or maybe the intervillous space?
who fcking knows but it aint me and it aint MBG
What happens to pressure as it goes towards the intervillous space?
Decreases
Which do you expect to have the highest pressure?
Spiral arteries?
Intervillous space?
Uterine arteries?
Radial arteries?
Lowest?
Uterine arteries- highest
Intervillous space- lowest
Uterine blood flow at term
800 ml/min
Blood flow to myometrium
150 ml/min
Blood flow to decidua (placenta)
100 ml/min
What percent of cardiac output is directed towards the uterus?
10-20%
When do vascular changes occur in pregnancy?
FIrst 3 months
What happens to vessel and diameter lenghth pregnancy?
Both increase, but diameter more so than length
What can decrease uterine blood flow?
- Contractions
- Hypertension
- Hypotension
- Exogenous vasoconstrictors
- Local anesthetic toxicity
- Increased uterine tone associated with sympathetic block
What are the two mechanisms by which uterine blood flow is decreased?
Decreased perfusion pressure
Increased uterine vascular resistance
What are the two mechanisms by which perfusion pressure is decreased to uterus?
Decreased uterine arterial pressure
Increased uterine venous pressure
Causes of decreased uterine arterial pressure
- Supine position (aortocaval compression)
- Hemorrhage/hypovolemia
- Drug induced hypotension
- Hypotension during sympathetic block
Causes of increased uterine venous pressure
- Venacaval compression
- Uterine contraction
- Drug induced tachysystole
- Skeletal muscle hypertonus (seizures, valsalva)
Which endogenous vasoconstrictors cause increased uterine vascular resistance?
What causes these to be released?
- Catecholamines (in response to stress)
- Vasopressin (in response to hypovolemia)
Which exogenous vasoconstrictors cause increased uterine vascular resistance?
- Epi
- Vasopressors (phenylephrine > ephedrine)
- Local anesthetics in high concentrations
How to determine uterine blood flow?
Uterine perfusion pressure / Uterine vascular resistance
How does neuraxial anesthesia cause increased uterine blood flow?
- Pain relief
- Decreased sympathetic activity
- Decreased maternal hyperventilation
How does neuraxial anesthesia cause decreased uterine blood flow?
- Hypotension
- Unintentional IV injection of local anesthetic or epi
- Absorbed local anesthetic (little effect)
What effect do commonly used induction agents have on uteroplacental blood flow?
Little to no effect
What effect do inhalational agents have on uterine blood flow?
Little to no effect
Indirect effects of GA on uteroplacental blood flow
- Blood pressure changes
- Sympathetic response (laryngoscopy/intubation
What changes in hemodynamics are associated with deeper planes of anesthesia that may effect uterine blood flow?
Negatives: reduction in CO, maternal BP, and uterine artery blood flow
Positives: vasodilation
What occurs as a result of dose dependent reduction in uterine tone associated with inhalational agents?
Increased blood flow
Which inhalational agent is BEST for a lil mama based on blood flow?
Trick question…
No reason to choose one over another
What carries oxygen poor blood from the fetus to the placenta?
Umbilical arteries (2)
How does blood enter fetal circulation?
Through the ductus venosus
What carries oxygen rich blood from the placenta to the fetus?
Umbilical vein (1)
At what maternal SBP does inadequate fetal oxygenation occur in awake healthy patients during labor epidural analgesia?
<100 mm Hg
Label 1-5
- Umbillical arteries (2)
- Umbilical vein (1)
- Fetal capillaries
- Intervillous space
- Spiral arteries
Flow may differ between the R and L uterine arteries, whcih USUALLY gets more?
The side the placenta is implanted on
What four things affect placental transfer of drugs?
- Concentration
- Molecular weight
- Lipid solubility
- Ionization
Which drug will cross placenta more readily?
Smaller molecular weight (<500 Da) or Larger MW (>100 Da)?
More lipid or water soluble?
Ionized or unionized?
Smaller
More lipid soluble
More unionized
How are drugs diluted as they get closer to the baby?
Diluted in maternal blood
Diluted in intervillous blood
Diluted in Placental blood
Circulate to fetus then diluted in fetal blood
Then first pass effect through liver
How does blood bypass the fetal liver?
Ductus venosus
What percent of fetal CO returns to the placenta without circulating?
20 percent
Does all umbilical venous blood bypass the liver via the ductus venosus?
No
Can the fetus and neonate metabolize drugs at all?
Yes, but at a much slower rate than adults
When the fetus excretes drugs into the urine what happens to them?
How are they actually eliminated?
Excreted and later swallowed as amniotic fluid
Actual elimination is mostly dependent on placental transfer
How does maternal FRC and MV differ from norm?
Effect on uptake and absorption of inhaled agents?
Reduced FRC
Increased MV
Increased (more rapid) uptake of inhaled agents
What effect does increased CO have on distribution to tissues?
Increased distribution to all tissues
How much does total body water increase on average?
What is the significance of this?
Increases on average 8L
Increased Vd for hydrophilic drugs
How much is hepatic blood flow increased?
Ha… gotcha…. It is not significantly increased
Maternal changes in CYP enzymes
Some have increased activity
some have decreased activity
………….
Maternal effect on renal blood flow and GFR
Significance?
Increased
Drgus that are excreted by kidneys unchanged (like cephalosporins) are eliminated faster
How can drugs be transferred across the placenta?
- Simple diffusion
- Simple diffusion via channels
- Facilitated diffusion
- Active transport
- Endocytosis
Which anticholinergics readily cross the placenta?
Atropine, Scopolamine, Glycopyrrolate
Atropine and scopolamine
Which commonly given drugs in anesthesia make for a floppy baby if given before cord is cut because they readily cross placenta?
Opioids and benzos (Diazepam and midazolam)
Which vasopressor readily crosses placenta?
Phenylephrine or ephedrine?
ephedrine
Do induction agents and inhalation agents readily cross the placenta?
Yes
Propofol, ketamine, etomidate, thiopental
All the volatiles and nitrous
Do local anesthetics readily cross the placenta?
Yes
Except chlorprocaine
Do muscle relaxants readily cross the placenta?
Nope
Does heparin readily cross placenta?
Nope
Do antihypertensives readily cross placenta?
Yes
Beta blockers
Nipride
Nitroglycerine
what is primarily responsible for supplying oxygenated maternal blood to the uterus?
(quiz question)
uterine arteries
