Anesthesia for cesarean section. Tom Archer, MD, MBA UCSD Anesthesia. A unique psychosocial surgery. Outline. C-section – a unique psychosocial surgery How the OB anesthetist should behave. Evolution of techniques Neuraxial block physiology and management GA physiology and management.
Anesthesia for cesarean section
Tom Archer, MD, MBA
A unique psychosocial surgery
38 y.o. female, repeat c/s, 420 #, gestational hypertension, continuous spinal: fall in SVR, rise in CO with onset of block. Increased SVR with phenylephrine.
LV dilation / hypertrophy
Aortic stenosis at rest
Cardiac output not sufficient to cause critically high LV intracavitary pressure / LV failure.
Pulmonary capillaries (edema)
LV failure / ischemia
Aortic stenosis with SAB: increased cardiac output / arteriolar vasodilation:
Decreased SVR Fall in systemic BP and / or increase in LV intracavitary pressure ischemia or LV failure.
Resistance arterioles– decreased SVR
Decompensated patient with REAL RL shunt.
Decreased SVR desaturation
Increased pulmonary vascular resistance desaturation
Decompensated patient with ASD, VSD or PDA-- Decreased SVR or increased pulmonary vascular resistance increased RL shunt and increased arterial desaturation.
Compensated patient with POTENTIAL RL shunt.
Minimal RL shunt
Low pulmonary vascular resistance
Normal, compensated patient with ASD, VSD or PDA-- high SVR and low pulmonary vascular resistance minimal RL shunt.
Virtually all patients immediately develop warm, dry hands and leave the hospital the same day as surgery.
Left Uterine Displacement(LUD)
Colman-Brochu S 2004
Chestnut chap. 2
www.siumed.edu/~dking2/erg/images/placenta.jpgfrom Google images
Normal placental function: fetal and maternal circulations separated by thin membrane (syncytiotrophoblast).
Umbilical vein (UV)
“Lakes” of maternal blood
Fetal capillaries in chorionic villi
Precariously oxygenated environment
Archer TL 2006 unpublished
Ohm’s Law of the placenta: O2 delivery = Placental blood flow = (P1 – P2) / R
Aorto-caval compression decreases P1 (“aorto”) and increases P2 (“caval”)
Therefore, aorto-caval compression decreases O2 delivery to fetus.
R = placental resistance (fixed in short term)
P1 = uterine artery pressure
Placenta blood flow (O2 delivery) =
(P1 – P2) / R
P2 = uterine vein pressure
Archer TL 2006
Abdominal compression in non-pregnant subjects compresses the IVC. Which leads to epidural vein engorgement.
Hyperventilation and abdominal pressure shrink the volume of the lumbar thecal sac via engorgement of epidural veins and vasoconstriction of cerebral vessels (CSF shift).
Abdominal compression and hyperventilation
Clinical bottom line of decreased lumbar CSF volume in pregnancy:
Decrease local anesthetic dose by 30%, compared to non-pregnant dose.
Corrolary: Loss of resistance may not be felt if patient is holding her breath (crying or straining).
You can be fooled into thinking you have not reached the epidural space, when you have!
Thecal sac at L4-5: more “triangular” than at L3-4?
Naji M, Anaesthesia, 2009, 64, pages 39–42
Triangular thecal sac at L4-5 may explain dry CSF tap after epidural localization for CSE.
Functional residual capacity (FRC) is our “air tank” for apnea.
www.picture-newsletter.com/scuba-diving/scuba... from Google images
Pregnant Mom has a smaller “air tank”.
Managing common problems