Obstetric Anesthesia

▪Obstetric Patient and Anesthesia

Decr in overall anesthetic requirements; MAC decreased by 40%.
Decr gastric motility w/incompetent gastroesophageal junction leads to regurgitation of gastric contents. All OB pts need RSI (rapid sequence induction).
Serum creatinine and BUN normal is lower than normal b/c RBF incr 40%-90%, and GFR incr 50%. Glycosuria during pregnancy is not necessarily abnormal as there is an incr in GFR with impaired tubular reabsorption for filtered glucose; may be reason for incr incidence of UTI in pregnancy.
Increased plasma volume from 40 to 70 mL/kg is greater than RBC mass increase leading to dilutional anemia. Normal HCT in obstetric patient will be 34%.
Cardiac output doubles during active labor; immediately after birth there is an autotransfusion of 500-750 mL.
Aortocaval compression (s/s similar to shock): HOTN, tachy, pallor, sweating, N/V, changes in cerebration. Tx by left uterine displacement (LUD) with wedge.
Respiratory tract mucosa engorged and friable, needs one size smaller ETT than normal.
Progesterone increases minute ventilation, relaxes bronchial muscle, sensitizes respiratory center to CO₂ and stimuli and vasodilates vessels, decreasing SVR.

OB pts have decreased FRC and expired volumes and can develop hypoxia and hypercarbia rapidly. Normal PCO₂ is 27-32 d/t increased minute ventilation and tidal volume (with incr AP diameter); this all creates a respiratory alkalosis. If pt hyperventilates during labor, resp alkalosis may worsen and shift oxyhgb curve to left, decreasing UBF and amount of O₂ delivered to fetus.

Factors with uteroplacental perfusion: Uterine vasculature is not autoregulated; it is maximally dilated but can constrict.

Aortocaval compression
HOTN: SBP < 100 mm Hg or fall of normal BP 25% cause decreased uterine blood flow.
Increased uterine vascular resistance caused by: Contractions, IV ketamine, Pitocin, abruptio placenta, maternal hypoxia, hypercarbia, hypocarbia, catecholamines (ephedrine affects less).

Non-Obstetric Surgery in Pregnant Patient

Surgery increases perinatal mortality. Avoid surgery if possible to protect developing fetus; 2nd trimester is safest.

If surgery must be done, OK drugs to use:

Propofol, thiopental, ketamine 0.5-1 mg/kg, sevoflurane, isoflurane, vecuronium, neuromuscular blockers, and opiates.

DO NOT USE: Nitrous oxide, diazepam, morphine, ketorolac, benzodiazepines.

GETA: Nonparticulate antacid (Bicitra), preoxygenation, RSI with cricoid. If elective, consider a proton inhibitor: Aciphex, Nexium, Prevacid, Prilosec, and Protonix; or H2 blockers: Famotidine, ranitidine, and metoclopramide (a dopamine antagonist).

▪Anesthesia for Labor and Delivery

Normal Blood Loss

Vaginal: 400-600 cc

Twins: 1000 cc

C/S: 1000 cc

Labor Pain

1st stage: Stage of cervical dilation; lasts 8-12 hrs. Labor pain arises from T11-T12: Autonomic C-fibers from cervical dilation/effacement in uterus; block to level T10-L1.

2nd stage: Stage of expulsion; lasts 20-50 min. Labor pain arises from S2-S4: A-delta fibers from vagina, vulva, and perineum; block to level S2-S4.

▪Obstetric Labor and Delivery Medications

AMPLE history: Allergy/Meds/Past Illnesses/Last meal/Events

Labor

Spinal:

Bupivacaine 1.25-2.5 mg + fentanyl 25 mcg. Lasts 1-1.5 hr with very little motor loss; low placental transfer to fetus.

Epidural:

Bupivacaine 0.0625%-0.125% (high quality analgesia with minimal motor blockade) or ropivacaine 0.125%-0.2% with fentanyl 1-2 mcg/mL. Load 8-12 mL, maintain 8-12 mL/hr infusion.

Lidocaine < 1%-2%

Ropivacaine 0.1%-0.2%

Vaginal Delivery

Spinal (in sitting position):