Viva G4(ii)h

This viva tests Section G4(ii) of the 2017 CICM Primary Syllabus, which expects the exam candidate to "describe the distribution of blood volume and flow in the various regional circulations and explain the factors that influence them, including autoregulation. These include, but not limited to, the cerebral and spinal cord, hepatic and splanchnic, coronary, renal and uteroplacental circulations."

Specifically, this viva is all about uteroplacental blood flow.

Describe the blood supply of the uterus and placenta
  • The uterus is supplied by an anastomotic loop of the uterine and ovarian arteries.
  • Arcuate branches wrap around the organ and radial branches penetrate the myometrium
  • Spiral arteries then suppy the intervillous space (a 150-500ml blood reservoir).
  • Two circulatory systems exist in parallel (foetoplacental and maternoplacental)
  • The maternoplacental circulatory system has no arterioles.

uteroplacental circulation

What is the intervillous space?
  • A vast blood-filled space which contains about 500ml of maternal blood at term.
  • Into this space, branching processes of the syncytiotrophoblast (the tertiary chorionic villi) are extended
  • These processes are highly vascular, allowing for gas exchange between the foetal blood and the pool of maternal blood in the intervillous space
  • The intervillous space is therefore the gas exchange surface for the foetus, with an area of approximately 12m2
Describe the changes in uterine blood flow during pregnancy
  • The blood supply of the uterus is normally about 200ml/min, or about 3-4% of the total cardiac output.
  • This figure quadruples at term.
  • The blood flow rate increases to about 700-800ml/min, or about 14-15% of the normal cardiac output.
  • About 90% of this blood flow nourishes the placenta, and the rest of it supplies the hypertrophied myometrium
  • The vascular resistance is quite low, and the pressure gradient across the intervillous space is only about 8-14 mmHg
  • The mean pressure in the spiral arteries of the uterus is about 12mmHg
How is the uterine circulation regulated?

It's not, really.

  • There are no arterioles
  • The spiral arteries are devoid of any autonomic innervation
  • Radial uterine arteries are under sympathetic control, and are highly sensitive to noradrenaline, but this does not seem to play a role under normal physiological conditions
  • Thus, uterine blood flow increases (or decreases) linearly, in proportion to pressure. There is no "plateau" of stable flow, as there would be with other organs (eg. kidney and brain)
  • Because of this, the uterus compensates for decreasing flow by increasing its oxygen extraction ratio.
Which vasoactive substances can influence uteroplacental blood flow?
Vasoactive Influences Affecting Uteroplacental Vessels
Vasoconstriction Vasodilation
  • α-1 receptor stimulants
  • Hypoxia (severe)
  • Nicotine
  • Vasopressin
  • Angiotensin
  • Thromboxane A2
  • β-2 receptor stimulants
  • Hypoxia (mild)
  • Adenosine
  • Bradykinin
  • Oestrogens
  • Ischaemia
  • Nitrates
  • Prostaglandins E1, E2, and I
  • Vasoactive intestinal polypeptide

References

Osol, George, and Maurizio Mandala. "Maternal uterine vascular remodeling during pregnancy." Physiology 24.1 (2009): 58-71.

Thaler, Israel, et al. "Changes in uterine blood flow during human pregnancy." American journal of obstetrics and gynecology 162.1 (1990): 121-125.

Moll, Waldemar, et al. "The blood pressure in the decidual part of the uteroplacental arteries (spiral arteries) of the rhesus monkey." Pflügers Archiv 346.4 (1974): 291-297.

Cunningham, F. Gary, et al. "Implantation, embryogenesis, and placental development." Williams obstetrics 22 (2005): 39-90.

Carter, Anthony M. "Regulation of maternal placental blood flow: A review.Placenta 20 (1999): 271-291.

Greiss, Frank C., and Frank C. Greiss Jr. "Uterine and placental blood flow." The Global Library of Women's Medicine 2008 (p1756).

Khong, Yee, Janine H-C. Tee, and Andrew J. Kelly. "Absence of innervation of the uteroplacental arteries in normal and abnormal human pregnancies." Gynecologic and obstetric investigation 43.2 (1997): 89-93.

Steele, Simon C., Averil Y. Warren, and Ian R. Johnson. "Effect of the vascular endothelium on norepinephrine-induced contractions in uterine radial arteries from the nonpregnant and pregnant human uterus." American Journal of Obstetrics & Gynecology 168.5 (1993): 1623-1628.

Carter, Anthony M., and Jan Göthlin. "Effects of Angiotensin, Oxytocin and Vasopressin on Placental Circulation: An Angiographic Study in Rabbits." Investigative Radiology 5.2 (1970): 86-91.

Huckabee, William E., et al. "Blood flow and oxygen consumption of the pregnant uterus." American Journal of Physiology-Legacy Content 200.2 (1961): 274-278.

Greiss Jr, Frank C. "Pressure-flow relationship in the gravid uterine vascular bed." American journal of obstetrics and gynecology 96.1 (1966): 41-47.

Clapp III, James F. "The relationship between blood flow and oxygen uptake in the uterine and umbilical circulations." American Journal of Obstetrics and Gynecology 132.4 (1978): 410-413.