Posterior spinal fusion

Posterior spinal fusion

Anesthesia type	General
Airway	ETT
Lines and access	Large bore IV (bolus/resuscitation), 2nd PIV (infusions), A-line
Monitors	Standard, neuromuscular monitoring (e.g. SSEV)
Primary anesthetic considerations
Preoperative	Starting Hb, pulmonary function (restrictive physiology)
Intraoperative	Nerve injury, significant blood loss, controlled hypotension
Postoperative	Nerve injury, pain control
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Posterior spinal fusion is an orthopedic procedure performed to correct idiopathic scoliosis. It is the most common treatment for idiopathic scoliosis. The surgery involves placement of implants (an array of hooks, screws, and wires) which are attached to disc segments and tightened to straighten the spine. Bone grafts are placed between vertebrae and encourage spine fusion.

Overview

Indications

Generally indicated for severe scoliosis (Cobb angle >50 degrees). Even after skeletal maturity, such severe Cobb angles can progress to extreme curvature of up to 80 degrees.

Such severe deviation of spine curvature can lead to chronic back pain and decreased pulmonary function (akin to restrictive lung physiology).

Procedure

Patients are initially supine for intubation, line placement, and monitors. Once complete, they are flipping to the prone position. A large midline incision is made cutting through the back muscles to expose the spine. The surgeon will clear the tissue from the spine in order to create a surface for hardware placement and graft bone. Bone grafts are used between vertebrae to stimulate growth and ultimately spinal fusion. Controlled hypotension (MAPs no greater than the 70s, sometimes lower) limits bleeding during this part of the procedure.

Tightening of the wire implants stretches/distracts the spine into midline position. It is important to maintain normotension once this begins in order to perfuse the spinal cord during distraction (which inevitably causes stretching of the nerves/nerve damage). Close neuromuscular monitoring by a technician allows surgeons to detect this early and stop manipulation. Steroids may be given if concern for nerve injury.

If the spine remains off center from the pelvis, a pelvic fixation may also be performed.

Other Interventions

Harrington rods were the original method but are no longer current as segmental implants allow surgeons more control and early mobilization without the need for bracing.

Anterior spinal fusion is another surgical method that comparatively has less blood loss and risk of neurologic injury. Advantages of posterior spinal fusion over anterior spinal fusion include avoidance of entering the thoracic cavity and potentially impairing pulmonary function.

Preoperative management

Patient evaluation

System	Considerations
Airway	ETT. Prone positioning.
Neurologic	Intra-op neuromonitoring. Avoid NMB. Steroids for protection against nerve injury.
Cardiovascular	Controlled hypotension
Pulmonary	Changes in compliance during surgical manipulation of spine
Gastrointestinal
Hematologic	Anywhere between 300 to 3000 mL of blood loss from the scraping of the epidural veins of the spine
Renal	AKI from hypovolemia or prolonged hypotension
Endocrine
Other

Labs and studies

A pre-op Hb as well as Type and Screen should be drawn pre-procedure. ABG monitoring can be performed if significant blood loss is observed. ABGs should generally include lytes and iCa.

Operating room setup

1. A-line
2. 2 PIV, one large bore (16 gauge) for resuscitation and bolus of meds, one for infusions
3. Standard monitors/equipment including temperature probe/bear hugger (important given prolonged exposure
4. Ancillary equipment: Cell-saver, neuromuscular monitors
5. Tranexamic acid infusion for bleeding, blood pressure agents (norepinephrine infusion for hypotension, nitroglycerin infusion for hypertension)

Patient preparation and premedication

Generally patients receive muscle relaxers (i.e. Valium) to help with muscle spasm that inevitably occurs with such a large surgery.

Intraoperative management

Monitoring and access

1. Evoked potentials (SSEP) are followed by a technologist.
2. Continuous arterial line blood pressure is monitored to ensure precise blood pressure control.
3. ABGs prn

Keeping track of the patient's hourly fluid goal is important to maintain intra-op euvolemia. Consider setting up the following table (example for 52 kg patient):

	Deficit	Maintenance	Insensible losses	EBL	Hourly total	Cumulative total
Hour 1	500	x (can skip while replacing fluid deficit)	x	x	500 mL	500 mL
Hour 2	500	x	375 mL	200 mL (multiply by 2 to get necessary volume to replace, in this case 400 mL)	1275 mL	1775 mL
Hour 3	x	92 mL	375 mL	x	467 mL	2242 mL

Calculating expected blood loss will help guide when to check ABG and consider transfusing blood (for a healthy patient, generally at a Hb of 7 or 8):

Example: 52 kg patient with starting Hb of 12.6

Estimated blood volume: 52 kg x 70 mL/kg = 3500 mL

Estimated cc per gram of Hb: 3500 mL divided by 12.6 g/dL = 277 mL per g Hb

To lose blood to go from Hb of 12.6 to 8.0: 12.6 - 8.0 = 4.6 g/dL Hb

Volume of blood to drop to reach transfusion threshold: 4.6 g/dL x 277 mL = 1274 mL

At an estimated blood loss of 1274 mL, the clinician can expect enough of a drop in Hb to transfuse blood.

Induction and airway management

Standard induction with the addition of large doses of opiate (in preparation for significant pain of the procedure) followed by placement of ETT. Avoid paralysis.

Positioning

Patient is prone during hardware placement and flipped to supine after skin closure is complete.

Maintenance and surgical considerations

Bleeding

Generally, a bolus of tranexamic acid followed by a continuous tranexamic infusion is started to limit excessive blood loss.

Maintenance of anesthesia

A MAC of 0.5 for inhalational agents is used to prevent interference with intra-op neuro monitoring. Iso-nitrous is often used with these procedures but sevo and iso at low MAC is still appropriate. A mix of gas and an IV Propofol infusion can lower the MAC needed to maintain general anesthesia.

Administering ketamine may be considered to improve SSEV signals.

Emergence

Consider slowly starting to wean the Propofol infusion when the surgeons begin with deep dermal suturing. This will help with faster emergence. Wean the gas when skin closure is finished and the patient is flipped back to supine positioning.

Consider extubating in the OR to perform a neuro exam prior to leaving for the PACU.

Postoperative management

Disposition

Admit to inpatient.

Pain management

Ketamine gtt and opiate PCA. Valium prn.

Potential complications

Nerve injury from spine distraction. Significant blood loss leading to hypovolemic shock and increasing risk for spinal cord ischemia/damage.

References

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