Cesarean section under regional anesthesia
The following case will emphasize regional anesthesia and obstetric issues.
· reflux: risks, prevention
· physiology: fluid dynamics
· neuraxial anesthesia: technique, epidural hematoma risk, hypotension risk
· vasopressors: ephedrine vs. phenylephrine
· neuraxial opioids: pros/cons, risks.
A 28-year-old primiparous (first baby), pre-eclamptic woman requires a cesarean section for breech presentation of a 38-week fetus.
History. She had a normal prenatal course. This morning she complained of headache, blurred vision and swelling in her face, feet and hands. She is hypertensive, has proteinuria and generalized edema.
Her constellation of symptoms and findings suggest severe preeclampsia.1 We need to control her blood pressure, start a magnesium infusion to reduce the risk of eclamptic seizure, and deliver the baby as soon as possible.
Review of systems: Reflux and low back pain with pregnancy.
These are normal findings in pregnancy. Progesterone-induced relaxation of the lower esophageal sphincter, increased acid secretion, and elevated intra-abdominal pressure increase the risk of aspiration of acidic gastric contents, a dangerous and potentially fatal complication during general anesthesia for delivery.
Physical examination: Anxious Caucasian woman in no distress; weight 100 kg; height 5’6” (165 cm)
BP 160/110 mmHg; HR 100 beats/min; respiratory rate 18 breaths/min
Generalized edema including face
Airway: Mallampati I, 4 fb mouth opening, 4 fb thyromental distance, full neck extension
CV: S1, S2 no murmur
Respiratory: Lungs clear to auscultation, no ralesˆ
Neurologic: Reflexes 4+
To interpret these findings, we must recognize the normal physiologic changes of pregnancy:
· increased blood volume – plasma volume increases more than red blood cell mass leading to a dilutional anemia
· vasodilation to accommodate the increased volume
· increased respiratory drive – a central progesterone effect → a decrease in baseline PaCO2 to 30 mmHg → increased minute ventilation. Surprisingly this is achieved mostly by increasing the tidal volume.
Pre-eclampsia reverses the gestational vasodilation leading to hypertension, and increases capillary permeability resulting in proteinuria, reduced intravascular volume, and cerebral, peripheral and potentially pulmonary edema.
Pre-operative studies: Urine protein dipstick 4+; hemoglobin 12 g/dL; hematocrit 36%; platelets 120 000/µL.
Pre-eclampsia can progress to Hemolysis, E levated Liver enzymes, Low P latelets (HELLP) syndrome. Therefore, before placing an epidural or spinal anesthetic we obtain a platelet count. A low platelet count would raise the specter of an epidural or subdural hematoma, a dreaded complication of neuraxial anesthesia. Proteinuria is routinely tested in pregnancy, and is one of the diagnostic criteria of pre-eclampsia.
Anesthetic preparation: We discuss the risks/benefits of regional vs. general anesthesia, she chooses regional. She drinks 15 mL of a non-particulate antacid (sodium citrate) just before moving back to the OR, where we keep her nerves in check with engaging conversation.
Anesthetic options in this ASA III patient include neuraxial anesthesia (spinal or epidural) or general anesthesia. We prefer regional anesthesia for several reasons:
all anesthetics that reach the brain also cross the placenta, therefore if the mother is asleep, we will deliver a drowsy baby, increasing the risk of neonatal depression; (ii) the mother (and often a companion) can witness the birth; and
in most cases we have no need to manage her airway. The latter is particularly important in this population where the usual risks associated with general anes-thesia are increased (inability to ventilate and/or intubate, aspiration of gastric contents). We explain the risks to the mother.
With an epidural, we avoid an all too rapid development of a sympathetic block and, with it, of hypotension in this hypovolemic patient. Because we do not rou-tinely sedate these patients, reassuring conversation throughout the procedure is essential.
Establishment of regional anesthesia: We have the patient sit on the bed, attach standard monitors, and increase the flow rate of crystalloid (normal saline or Ringer’s lactate) into her i.v. We gently place a lumbar epidural catheter and confirm that it is neither in a vein nor the intrathecal space (no change in hemodynamics or sensation following 3 mL of 2% lidocaine with 1:200 000 epinephrine). We then help the patient lie down with left uterine displacement, and dose the catheter with 5 mL aliquots of the same solution until a T4 level is attained or the maximum dose (7 mg/kg) is reached. Maternal blood pressure may require support as the epidural takes effect.
The fluid bolus should help offset the hypotension from sudden vasodilation, but must be administered with caution as this preeclamptic patient is prone to pulmonary edema. Pregnancy increases the risk of epidural vessel cannulation as these vessels dilate to provide collateral circulation around the compressed intra-abdominal vessels. We minimize this compression by either tilting the table to the left or placing a wedge under the right hip, which moves the uterus off the vena cava. If the patient is in labor, we test the catheter position between contractions to avoid pain-induced tachycardia masking a positive test dose. For blood pressure support, we prefer ephedrine in pregnancy. Phenylephrine may constrict the uterine vasculature and, in large doses, reduce perfusion. If instead she remains hypertensive, we choose labetalol or hydralazine to control her blood pressure.
Maintenance of anesthesia: Following delivery of the child, we start a pitocin infusion, administer prophylactic antibiotics, and inject morphine into the epidural catheter for post-operative pain management.
During uterine closure the patient complains of upper abdominal pain. We apply a face mask with 50% nitrous oxide in oxygen.
Neuraxial duramorph (long-acting morphine) has many side effects – particularly nausea, itching and (rarely) respiratory depression – but these are far outweighed by its benefits in reduced post-operative pain. Pitocin is routinely administered to increase uterine tone and reduce blood loss. It should be given as a rapid infusion as bolus-dosing can cause hypotension. Should the uterus remain atonic, the standard second-line agent, methergine (an ergot alkaloid), is avoided in the pre-eclamptic patient because it may exacerbate vasoconstriction and hypertension.
Nitrous oxide is a fair analgesic with minimal cardiovascular effects. Because MAC (minimum alveolar concentration) is reduced 40% in pregnancy, nitrous oxide gains in effectiveness.
Emergence from anesthesia: Following conclusion of the operation we try to remove the epidural catheter, but in our zest the catheter breaks.
Epidural catheters have great tensile strength, but even the toughest plastic is no match for the over-aggressive. If the catheter does not come out easily, we place the patient in the position in which the catheter was originally placed. Should a catheter break, we inform the patient that a small amount of the plastic tubing remains in her back, but should cause no problem in the future. The tip is radio-opaque and may show up on subsequent imaging.
Post-anesthesia care: The epidural level recedes over 2–3 hours. We continue intravenous fluids, pitocin and magnesium.
Common PACU problems include epidural-induced shivering (Rx: meperidine or tramadol), morphine-induced pruritus (Rx: nalbuphine (a mixed agonist-antagonist opioid)), nausea/vomiting (possibly morphine-induced, Rx: anti-emetics), pain (Rx: ketorolac (safe even with nursing), opioids).
Discharge: After we document that the block is receding, we discharge the patient to the ward for 2–3 days recuperation before letting her go home.
The neuraxial duramorph provides analgesia for 12–18 hours. Additional sedative or opioid analgesics during this time risk respiratory depression, but pain should be treated! Continuous monitoring by pulse oximetry enables early detection of respiratory depression – provided the patient does not receive supplemental oxygen.
N O T E
Severe pre-eclampsia is defined by, among other things, presence of headache and visual disturbance