Describe the intraoperative anesthetic considerations for posterior spinal fusion surgery

Describe the intraoperative anesthetic considerations for posterior spinal fusion surgery.

An inhalation or intravenous induction is acceptable. Any of the nondepolarizing muscle relaxants can be used for this procedure. A balanced technique of nitrous oxide, oxygen, volatile agent, neuromuscular blocker, and an opioid should be considered for the maintenance anesthetic. If somatosensory and/or motor evoked potential monitoring are to be used, then it is recommended that the concentration of the volatile agent be 0.2% or less. An opioid infusion rather than bolus injections is the preferred anesthetic technique, particularly in the presence of evoked potential monitoring.

In addition to the standard intraoperative monitors, an arterial catheter, central venous catheter, and a urinary catheter should be placed. The arterial catheter allows for monitoring of arterial blood pressure on a beat-to-beat basis and also facilitates blood sampling. Most anesthesiol-ogists will opt for placement of a central venous catheter to monitor central filling pressures and volume status. Depending on the severity of cardiopulmonary disease, a pulmonary artery catheter may be warranted.

Turning and positioning the patient prone requires extreme care. It is important to avoid pressure on the eyes, which can result in retinal artery occlusion and blindness. It is also necessary to avoid pressure necrosis of the ears, nose, and forehead. The head should be in proper align-ment and positioned in such a way that allows for easy inspection of the face. The chest, abdomen, and pelvic areas should rest on properly positioned parallel rolls or other devices that avoid pressure on the axilla, breasts and genitalia. The arms should rest at the sides with the elbows flexed and the shoulders abducted no greater than 90° to avoid stretching the brachial plexus. Appropriate padding should also be present.

A considerable decrease in body temperature can occur during spinal surgery on account of the large body surface area exposed. Precautions should be taken to avoid intra-operative hypothermia. These include using a forced-air warming blanket, an intravenous fluid warming system, and adjustment of the operating room temperature.

Significant blood loss is not uncommon during this surgery. Predonation of autologous blood is efficacious and recommended for spinal fusion surgery. There are several techniques designed to minimize blood loss and the need for homologous blood transfusion (Table 52.3). More specifically, the intraoperative techniques are acute normo-volemic hemodilution, cell salvage, hypotensive anesthesia, surgical technique, and local infiltration with an epineph-rine-containing solution. The latter technique helps to reduce bleeding at the site of infiltration, but the overall reduction in surgical blood loss is minimal.

A review of each hypotensive anesthetic technique (Table 52.4) is beyond the scope of this chapter. There are a number of excellent resources available for review of these techniques. However, there are several key points to keep in mind when using hypotensive anesthesia. The mean arterial pressure (MAP) should be maintained above 50 mmHg to ensure adequate spinal cord perfusion and cerebral blood flow. In general, a MAP of 50–60 mmHg is ideal. In addition, the arterial blood gases should be monitored during the procedure.

Contraindications to the use of controlled hypotension include pre-existing major end-organ dysfunction, hemo-globinopathies, polycythemia, and elevated intracranial pressure. The clinician must have a thorough understanding of the technique and also be competent with the use of the technique chosen.