Anesthesia Student Survival Guide.pdf - Index of

PhysiOlOGy And AnesthesiA FOr OBstetriCs ● 307
helps to offset the undesirable effects of lordosis. Ligaments tend to become
more lax near term as the body prepares for vaginal delivery. Many operators
have noted that the ligamentum flavum (see Chap. 13, Regional Anesthesia)
has a more spongy texture at term when compared to the non-pregnant state.
Uteroplacental Blood Flow
By the end of the third trimester, uterine blood flow may represent up to 12%
of cardiac output. Perfusion of the uterus is adversely affected by decreased
uterine arterial pressure (hypovolemia, aortic compression), increased uterine
venous pressure (vena cava compression), and increased uterine vascular
resistance (uterine contractions, severe preeclampsia). Derangement of these
variables may adversely affect fetal oxygen delivery.
Exogenous vasoconstrictors can also adversely affect uterine perfusion.
Animal data from several decades ago led many to avoid the use of a-agonists
(phenylephrine) because of supposed increases in uterine vascular resistance.
However, more recent human studies have confirmed the safety of low dose
a-agonists, and many providers use either ephedrine or phenylephrine for
treatment of maternal hypotension.
Maternal Fetal Exchange
Blood from the maternal uterine spiral arteries bathes fetal villi capillaries within
the maternal intervillous spaces of the placenta. Since placental exchange
occurs across a membrane, it is dependent on diffusion, bulk flow, and active
mechanisms. Oxygen and carbon dioxide diffuse readily across the placenta.
Unloading of maternal oxygen is facilitated by a rightward shift in the
oxyhemoglobin dissociation curve. Fetal oxygen transfer is further bolstered
by fetal hemoglobin’s high affinity for oxygen (leftward shift of the oxyhemoglobin
dissociation curve compared to adult hemoglobin).
The maternal-to-fetal transfer of drugs is a complex topic that is beyond
the scope of this text. In general, molecules that are small and lipophilic (e.g.,
most anesthetics) cross the placenta easily, while large, hydrophilic molecules
that are protein-bound diffuse poorly (e.g., neuromuscular blocking drugs,
insulin). Unfortunately, the situation is often more complicated. For example,
local anesthetics may accumulate in the fetus through so-called ion­trapping.
This occurs when local anesthetics (which are non-ionized weak bases) cross
into the relatively acidotic fetus and become ionized and “trapped”.