What anesthetic options are available and what are the concerns regarding each option?
Approximately 30% of preterm neonates have inguinal hernias and 20% of those have them bilaterally. These neonates are at increased risk for incarceration of the hernia and a decision on the timing of the surgical repair needs to be made. This decision may be affected by the experience of the surgeon and the anesthesiologist. The anesthetic options include general anesthesia and regional anesthesia. Since many ex-premature infants may have chronic medical problems, each case must be individually reviewed prior to determining the risks and benefits of each type of anesthesia.
A detailed history of the neonatal course is critical in preparing this patient for anesthesia and avoiding compli-cations. The preoperative evaluation should include a dis-cussion with the neonatologists and nurses who have been caring for the patient as well as a chart review. Specific areas of focus include severity of lung disease and current therapy, presence or absence of tracheal intubation, history of tracheal intubation (including size of ETT and duration of intubation), current oxygen requirements, frequency of apnea episodes, and use of respiratory stimulants. A history of intraventricular hemorrhage (IVH) should be sought. IVH is graded 1–4 with grades 3 and 4 being associated with hydrocephalus. Other medical problems include cerebral palsy, retinopathy of prematurity, and gastroesophageal reflux. Any history of prior anesthe-sia should be obtained and reviewed for any adverse events. Current medications, allergies, family history of anesthetic-related problems, current weight, and NPO status should be obtained in addition to performing a physical and airway examination.
There are several concerns if general anesthesia is cho-sen. First, maintaining normal body temperature is more difficult in the neonate due to the larger body surface area/ volume ratio, which results in the loss of large amounts of heat through the skin. A warmed operating room and the use of a forced-air warming blanket help maintain normal body temperature. A lowered body temperature could con-tribute to postoperative apnea.
Secondly, the infant may require postoperative ventilation and may be at risk for postoperative apnea. There appears to be a direct correlation between the incidence of postoperative apnea and general anesthesia in specific ex-premature infants. Post-conceptual age, gestational age, and the pres-ence of anemia are factors that compound this matter. Most institutions will require apnea monitoring in the postoperative period if the post-conceptual age is less than 60 weeks. Post-conceptual age is calculated in the following manner:
Gestational age + Chronological age = Post-conceptual age
Another concern associated with general anesthesia is retinopathy of prematurity (ROP). ROP is most likely a result of several factors, of which oxygen is a major one. This risk probably exists until the neonate is 44 weeks post-conceptual age. For this reason, it is important to limit the amount of oxygen delivered. A common goal is to maintain the oxygen saturation by pulse oximetry (SpO2) in the mid-nineties. Preductal oxygen saturations should be measured if the ductus arteriosus is still patent.
The two regional anesthetics most commonly used are a spinal and continuous caudal. One major advantage of these techniques is the reduced incidence of postoperative apnea when compared with general anesthesia. This only appears to be true when no additional depressant drugs such as fentanyl, morphine, midazolam, or ketamine are administered at the same time. However, this does not preclude the need for postoperative apnea monitoring in infants identified to be at risk.
Spinal anesthesia has some limitations, most notably the shorter duration of action, even when 1 mg/kg of hyperbaric tetracaine is used. The maximum duration of surgical anesthesia is 60–90 minutes before supplementa-tion would be necessary. Since there is no hemodynamic instability following spinal blockade in neonates (second-ary to the immature sympathetic system), intravenous (IV) access may be placed after establishment of the spinal anes-thetic in an area where there is sensory loss.
Continuous caudal anesthesia is accomplished by plac-ing a catheter in the caudal space. Since caudal anesthesia requires higher doses of local anesthetics compared with spinal anesthesia, toxicity becomes a concern, especially if repeat dosing is necessary. In this scenario, 2% 2-chloro-procaine is preferable because it has a short serum half-life and a low systemic toxicity.