14.3 Orthopedic, Spine & Major Vascular Surgery

14.3 Orthopedic, Spine & Major Vascular Surgery

Major orthopedic, complex spine, and open vascular procedures represent some of the most physiologically demanding and technically intricate cases encountered by the anesthesia team. From sudden catastrophic right heart failure during joint arthroplasty cementation, to profound neurovascular injuries during prone spinal positioning, to massive hemodynamic swings during aortic cross-clamping, the Certified Anesthesia Technologist (Cer.A.T.T.) must anticipate critical transitions, operate advanced monitoring equipment, and prepare specialized pharmacotherapy for rapid resuscitation.

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Orthopedic Anesthesia: Bone Cement Implantation Syndrome (BCIS)

In total joint arthroplasty (especially total hip arthroplasty [THA], hemiarthroplasty for femoral neck fracture, and cemented total knee arthroplasty [TKA]), surgeons use polymethylmethacrylate (PMMA) bone cement to anchor prosthetic implants to the host bone.

The Pathogenesis of BCIS

PMMA is formed intraoperatively by mixing liquid methylmethacrylate (MMA) monomer with powdered polymer. As the cement polymerizes through an intensely exothermic reaction (which can generate high local temperatures), the surgeon uses a pressurization gun to drive cement deep into the cancellous bone of the femoral medullary canal:

1. Intramedullary Pressurization & Microembolization: Pressurization can raise intramedullary pressure to several hundred mmHg, far exceeding femoral venous pressure. This massive pressure forces marrow contents—including fat globules, bone marrow debris, air, dissolved methylmethacrylate monomer, and micro-thrombi—directly into torn femoral venous sinusoids.
2. Pulmonary Vascular Occlusion: The embolic shower travels to the inferior vena cava, traverses the right heart, and impacts the pulmonary arterial microcirculation, causing acute pulmonary mechanical obstruction and intense microvascular vasospasm.
3. Monomer Chemical Toxicity: Concurrently, absorbed liquid MMA monomer circulates systemically, stimulating direct myocardial depression, peripheral vasodilation, complement cascade activation, and systemic histamine release.

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|         BONE CEMENT IMPLANTATION SYNDROME (BCIS) SEVERITY CLASSIFICATION    |
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| Grade    | Systolic Blood Pressure Drop | Arterial Oxygen Desaturation      |
+----------+------------------------------+-----------------------------------+
| Grade 1  | Moderate: Fall > 20%         | or SpO2 < 94%                     |
| Grade 2  | Severe: Fall > 40%           | or SpO2 < 88% / sudden LOC        |
| Grade 3  | Cardiovascular Collapse      | Requires CPR / PEA Arrest         |
+----------+------------------------------+-----------------------------------+

Clinical Manifestations & Management

This grading follows Donaldson and colleagues (2009); meeting either the blood pressure or the oxygen criterion is enough for a grade, and LOC means unexpected loss of consciousness. BCIS typically occurs within minutes of cementation, prosthesis insertion, joint reduction, or limb tourniquet deflation. The patient exhibits sudden, severe systemic hypotension, marked tachycardia, acute arterial desaturation, a precipitous plunge or transient surge in end-tidal CO₂, elevated pulmonary artery pressures, and acute right ventricular dilatation progressing to pulseless electrical activity (PEA) arrest.

• Pre-Cementing Preparation: When the surgeon announces "cement going in," the anesthesia provider increases inspired oxygen to 100% (FiO₂ 1.0) and optimizes intravascular volume to augment right ventricular preload.
• Pharmacologic Resuscitation: Immediate treatment requires aggressive intravenous volume expansion and pure alpha-1 adrenergic agonists (phenylephrine boluses or norepinephrine infusion) to counteract systemic vasodilation and maintain coronary perfusion pressure to the struggling right ventricle. Technologists must ensure invasive arterial line transducers are level, zeroed, and responsive.

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Fat Embolism Syndrome (FES)

Fat Embolism Syndrome (FES) is a multisystem complication arising from traumatic orthopedic injuries, particularly closed long-bone fractures (femur, tibia) and unstable pelvic fractures.

Pathophysiological Cascade

Unlike BCIS, which occurs hyperacutely during cement pressurization, FES classically manifests after a delay, typically 12 to 72 hours (often 24 to 72 hours) post-injury. The pathogenesis involves a dual mechanism:

1. Mechanical Obstruction: High intramedullary pressure at the fracture site forces marrow fat droplets into ruptured venous channels, showering the pulmonary capillary bed.
2. Biochemical Endothelial Injury: Circulating neutral fat triglycerides are hydrolyzed by pulmonary lipases into free fatty acids (FFAs). Toxic FFAs cause severe chemical pneumonitis, destroying pulmonary capillary endothelial cells and alveolar type II pneumocytes, triggering non-cardiogenic pulmonary edema, microvascular hemorrhage, and acute respiratory distress syndrome (ARDS).

The Classic Diagnostic Triad

FES is diagnosed clinically based on its classic presentation:

• 1. Acute Hypoxemia & Respiratory Distress: Dyspnea, tachypnea, arterial hypoxemia (PaO₂ < 60 mmHg), and diffuse bilateral pulmonary infiltrates on chest radiography ("snowstorm" appearance).
• 2. Neurological Impairment: Ranging from acute restlessness, confusion, and agitation to delirium, stupor, and coma. Caused by cerebral micro-infarctions as fat droplets squeeze through pulmonary capillaries or cross a patent foramen ovale (PFO).
• 3. Petechial Rash: A characteristic finding (often called pathognomonic), but seen in only a portion of cases (reported in roughly 20% to 50%). Non-blanching, reddish-brown petechiae erupt in a vest-like distribution across the anterior chest, neck, axillae, oral mucosa, and conjunctivae due to microvascular fat embolization and platelet aggregation.
• Treatment: Primarily supportive therapy—aggressive supplemental oxygenation, early lung-protective mechanical ventilation with PEEP, invasive hemodynamic support, and prompt surgical stabilization of fractures to prevent further fat showers.

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Spine Surgery: The Prone Position & Perioperative Visual Loss

Complex spine procedures—such as multilevel posterior lumbar interbody fusions (PLIF), thoracic deformity corrections for scoliosis, and cervical laminoplasties—are performed with the patient positioned prone on specialized surgical frames (e.g., Jackson spinal table, Wilson frame, or Allen table).

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|                      HAZARDS OF THE PRONE SPINAL POSITION                   |
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                                      |
         +----------------------------+----------------------------+
         |                                                         |
         v                                                         v
  ABDOMINAL COMPRESSION                                      PERIOPERATIVE VISUAL LOSS (POVL)
  - Compresses Inferior Vena Cava (IVC).                     - Ischemic Optic Neuropathy (ION: PION > AION).
  - Shunts blood to Batson's valveless                       - Prolonged surgery (> 6 hours).
    epidural venous plexus.                                  - Estimated blood loss > 1,000 mL / anemia.
  - Massive intraoperative epidural bleeding                 - Relative hypotension / decreased perfusion.
    and high spinal cord pressures.                          - Venous congestion (head dependent).
  - MUST allow abdomen to hang free!                         - Central Retinal Artery Occlusion (CRAO)
                                                               from direct globe compression!

Abdominal Compression & Batson's Plexus

A paramount technical rule of prone positioning is that the abdomen must hang completely free between longitudinal chest bolsters and pelvic pads. If the anterior abdominal wall is compressed against the frame:

• Intra-abdominal pressure rises dramatically, compressing the thin-walled inferior vena cava.
• Venous blood is diverted away from the obstructed IVC into the Batson venous plexus—an extensive network of valveless epidural veins surrounding the spinal cord.
• The engorged epidural veins bleed uncontrollably upon surgical exposure, resulting in massive blood loss, obscured visualization, and elevated spinal cord venous pressures.

Perioperative Visual Loss (POVL) & Ischemic Optic Neuropathy (ION)

Perioperative visual loss is a catastrophic complication reported in roughly 0.01% to 0.1% of spine surgeries, and the vision loss is often permanent.

1. Ischemic Optic Neuropathy (ION): The most common etiology of POVL in spine surgery (predominantly Posterior Ischemic Optic Neuropathy [PION]). PION results from hypoperfusion and ischemia of the optic nerve posterior to the lamina cribrosa. Established risk factors include:
   • Prolonged procedures (the ASA practice advisory cites anesthesia times averaging about 6.5 hours).
   • Substantial blood loss (the advisory cites losses averaging about 45% of estimated blood volume).
   • Patient factors from case-control data: male sex and obesity.
   • Use of the Wilson frame and a lower percentage of colloid in non-blood fluid replacement.
   • Venous congestion and tissue edema; anemia and hypotension are also discussed as contributors, although they have not been proven as independent risk factors.
   • Head positioned below the heart. The ASA advisory recommends positioning high-risk patients with the head at or above heart level, in a neutral forward position, when possible.
2. Central Retinal Artery Occlusion (CRAO): Caused by direct external pressure on the ocular globe. If a patient's head slips on a foam cradle or horseshoe headrest, resting the orbit directly on the support, intraocular pressure rises above systolic retinal artery pressure, completely arresting retinal blood flow. Prevented by avoiding any pressure on the eyes, for example with pin fixation (such as a Mayfield skull clamp) or a mirror-equipped prone head support (such as ProneView) that keeps the orbits free, and by checking the eyes periodically during the case.

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Intraoperative Neurophysiological Monitoring (IONM)

During high-risk spinal cord instrumentation, scoliosis corrections, and intracranial neurovascular surgeries, Intraoperative Neurophysiological Monitoring (IONM) is employed to detect evolving neural injury in real time.

Anesthetic Management for IONM: The TIVA Imperative

The anesthesia regimen directly dictates the feasibility and reliability of neurophysiological signal acquisition:

• Volatile Inhalational Anesthetics: Halogenated agents (sevoflurane, desflurane, isoflurane) produce a profound, dose-dependent suppression of cerebral cortical and anterior spinal horn neuronal transmission, decreasing signal amplitude and increasing signal latency. MEPs are especially sensitive, so many teams avoid volatile agents or keep them at low concentrations (often about 0.5 MAC or less) when MEPs are monitored.
• Neuromuscular Blocking Agents (NMBAs): While SSEPs are sensory potentials that are unaffected (or even improved) by paralysis, MEPs rely on transcranial electrical stimulation (TES) conducting down the corticospinal tracts to trigger compound muscle action potentials (CMAPs) recorded from peripheral limb muscles (e.g., abductor pollicis brevis, tibialis anterior, abductor hallucis). Neuromuscular blockers paralyze the neuromuscular junction, completely obliterating all MEP responses!
• Commonly Preferred Regimen: Total Intravenous Anesthesia (TIVA): TIVA utilizing continuous infusions of Propofol (100 to 150 mcg/kg/min) combined with an ultra-short-acting opioid such as Remifentanil (0.1 to 0.5 mcg/kg/min) or Sufentanil best preserves neurophysiological signal amplitudes while providing stable surgical depth.
• Bite Block Safety: Transcranial electrical stimulation drives direct excitation of the motor cortex, causing violent, involuntary contractions of the temporalis and masseter muscles. To prevent catastrophic tongue laceration, tooth avulsion, or mandibular fracture, the anesthesia technologist must place soft, dense bite blocks bilaterally between the upper and lower molars prior to prone positioning.

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Major Vascular Anesthesia: Abdominal Aortic Aneurysm (AAA) Repair

Open abdominal aortic aneurysm (AAA) repair requires cross-clamping the aorta to resect the aneurysmal sac and sew in an interposition Dacron or PTFE prosthetic graft. The application and subsequent release of the aortic cross-clamp induce the most extreme hemodynamic fluctuations seen in non-cardiac surgery.

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|               HEMODYNAMIC CRISIS OF AORTIC CLAMPING VS UN-CLAMPING          |
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                                      |
        +-----------------------------+-----------------------------+
        |                                                           |
        v (Application of Clamp)                                    v (Release of Clamp)
  AORTIC CROSS-CLAMPING CRISIS                                AORTIC UN-CLAMPING SHOCK
  - Massive increase in SVR & Afterload.                      - Precipitous plunge in SVR (Afterload drop).
  - Severe proximal arterial hypertension.                   - Massive venous pooling in revascularized limbs.
  - Acute Left Ventricular wall stress & strain.              - Severe reduction in preload & Cardiac Output.
  - Coronary ischemia risk in CAD patients.                   - Washout of anaerobic metabolites:
  - Renal perfusion drops by 30% - 40%                          * Lactic acid (severe acidosis)
    (even with infrarenal clamping via RAAS).                   * Potassium (K+ arrhythmias)
  - Management: Vasodilators (NTG, Clevidipine).                * Myocardial Depressant Factors
                                                              - Management: Fluid loading, slow release,
                                                                vasopressors, sodium bicarbonate.

The Cross-Clamping Crisis

When the vascular surgeon applies the aortic cross-clamp, blood flow to all distal tissue beds ceases:

• Afterload Explosion: Systemic vascular resistance surges immediately. Mean arterial pressure and left ventricular afterload climb steeply.
• Myocardial Strain: In response to the afterload spike, left ventricular end-diastolic pressure (LVEDP) and pulmonary capillary wedge pressure (PCWP) rise. In patients with coronary artery disease, the resulting increase in myocardial wall tension drives up myocardial oxygen consumption (MVO₂), precipitating acute myocardial ischemia or acute left ventricular failure.
• Renal Hypoperfusion: Renal blood flow decreases by 30% to 40%, even when the clamp is placed strictly infrarenal (below the renal artery orifices). This reduction is mediated by intense reflex renal arteriolar vasoconstriction driven by the renin-angiotensin system and sympathetic activation.
• Management: Anesthesia providers prepare intravenous vasodilators (nitroglycerin, nitroprusside, or clevidipine) to mitigate afterload stress and control proximal hypertension.

The Un-Clamping Shock

Releasing the cross-clamp after graft insertion is the most perilous transition of the procedure:

• Precipitous Hypotension: Removing the clamp opens a vast, previously ischemic vascular tree. Systemic vascular resistance plunges, and blood rushes into the flaccid, vasoparalyzed distal vasculature. Massive venous pooling deprives the central circulation of venous return, causing an acute collapse in preload and cardiac output.
• Metabolic Washout: Ischemia in the distal limbs generates profound anaerobic cellular metabolism. Upon reperfusion, a toxic flood of lactic acid, free hydrogen ions, hyperkalemic blood, adenosine, prostaglandins, and myocardial depressant factors surges into the inferior vena cava and central circulation. This washout triggers acute metabolic acidosis, negative myocardial inotropy, systemic vasodilation, and potentially lethal ventricular dysrhythmias.
• Preventive Protocols: Prior to unclamping, the anesthesia team aggressively volume-loads the patient with crystalloids, albumin, and blood products from the rapid infuser (Belmont/Level 1), guided by filling pressures, dynamic indices, or echocardiography. The surgeon releases the clamp slowly and incrementally (e.g., unclamping one leg first). Vasopressors (phenylephrine, norepinephrine, calcium chloride) and intravenous sodium bicarbonate (guided by blood gas results) may be given to buffer incoming lactic acidosis and restore systemic vascular tone.