How can we proceed with total intravenous anesthesia (TIVA) in a 62-year-old female patient with an incomplete spinal cord injury and hypertension, given that our hospital's equipment only allows for 100% fraction of inspired oxygen (FiO₂)?

Proceeding with TIVA When Only 100% FiO₂ is Available

You can safely proceed with the anesthetic procedure using 100% oxygen initially, but must immediately reduce the FiO₂ to 0.4-0.5 (40-50%) as soon as the patient is intubated and stable, then titrate to maintain SpO₂ 94-98%. 1

Immediate Intraoperative Management

Initial Phase (Induction and Intubation)

• Use 100% oxygen during induction and intubation - this is standard practice and explicitly recommended during initial resuscitation and airway management 2
• Preoxygenate with 100% FiO₂ for 3-5 minutes before induction to maximize oxygen reserves 3
• Continue 100% oxygen through the intubation process 2

Critical Post-Intubation Protocol (Within First 5 Minutes)

Immediately after confirming endotracheal tube placement, you must aggressively reduce FiO₂ because:

• Hyperoxia (PaO₂ >350 mm Hg) causes direct tissue injury through oxygen-derived free radicals, increased brain lipid peroxidation, metabolic dysfunction, and neurodegeneration 1
• When breathing 100% oxygen, PaO₂ can reach 350-500 mm Hg or higher, levels associated with worse neurological outcomes 1
• A pulse oximetry reading of 100% saturation cannot distinguish between a safe PaO₂ of 80 mm Hg and a potentially harmful PaO₂ of 500 mm Hg 1

Step-by-Step FiO₂ Reduction Algorithm

1. Reduce FiO₂ from 1.0 to 0.4-0.5 immediately once the patient is intubated and ventilation is confirmed 1

2. Target SpO₂ of 94-98% (corresponding to PaO₂ 75-100 mm Hg) 2, 1

3. Obtain arterial blood gas within 15-30 minutes to confirm actual PaO₂, as pulse oximetry alone is unreliable when saturation is 100% 1

4. Further titrate FiO₂ downward by 0.1 increments if PaO₂ remains >100 mm Hg 1

5. Use pulse oximetry for continuous monitoring once SpO₂ is <100% 1

Ventilator Settings for This Patient

Initial Settings Post-Intubation

• Tidal volume: 6 mL/kg predicted body weight (approximately 350-400 mL for average 62-year-old female) 3
• Respiratory rate: 10-12 breaths/minute initially 2
• PEEP: 5-10 cm H₂O to maintain alveolar inflation 3
• Target plateau pressure <30 cm H₂O (ideally <28 cm H₂O) 3

Special Considerations for Spinal Cord Injury Patient

This patient with incomplete C4 spinal cord injury requires additional vigilance:

• Monitor for autonomic dysreflexia (paroxysmal hypertension, bradycardia, vasomotor instability) which can be triggered by surgical stimulation below the level of injury 4
• Maintain adequate depth of anesthesia to prevent sympathetic overactivity 4
• The hypertension history may be related to chronic autonomic dysreflexia rather than essential hypertension 4

Monitoring Strategy

Continuous Monitoring Required

• Pulse oximetry - but recognize its limitations when SpO₂ is 100% 1
• End-tidal CO₂ - target PETCO₂ 35-40 mm Hg 2
• Arterial blood pressure - especially important given spinal cord injury and hypertension 4

Intermittent Monitoring

• Arterial blood gas at 15-30 minutes post-intubation to verify PaO₂ is not dangerously elevated 1
• Repeat ABG if FiO₂ adjustments are needed or if clinical status changes 2

Critical Pitfalls to Avoid

Do Not Continue 100% Oxygen Beyond Initial Stabilization

• The American Heart Association gives a Class I recommendation to titrate FiO₂ to minimum concentration needed to achieve SpO₂ ≥94%, avoiding routine use of 100% oxygen beyond initial resuscitation 1
• Prolonged hyperoxia increases mortality and worsens neurological outcomes 1, 3

Do Not Rely on Pulse Oximetry Alone

• When SpO₂ reads 100%, you cannot determine if PaO₂ is 80 mm Hg (safe) or 500 mm Hg (harmful) 1
• Arterial blood gas measurement is essential when saturation is 100% 1

Do Not Use Excessive Tidal Volumes

• Never exceed 8 mL/kg predicted body weight, even if hypercapnia develops, as this increases mortality 3
• Lung-protective ventilation is critical even in patients without pre-existing lung disease 3

Timeline for Safe FiO₂ Reduction

The time required to reduce circuit oxygen concentration depends on fresh gas flow rate:

• With 5 L/min fresh gas flow: median time to reach <30% inspired oxygen is 35 seconds, but expired oxygen may remain >30% for 104 seconds 5
• With 2 L/min fresh gas flow: median time increases to 303 seconds for inspired and 255 seconds for expired oxygen 5
• Use higher fresh gas flows (5 L/min) initially to accelerate FiO₂ reduction after switching from 100% to lower oxygen concentrations 5

Practical Equipment Workaround

If your anesthesia machine truly cannot blend air with oxygen:

• Confirm with biomedical engineering whether a portable air compressor or air cylinder can be connected to the machine's air inlet 6
• Some machines have a manual air inlet that can accept compressed air from a cylinder 6
• If absolutely no air source is available, proceed as outlined above with aggressive early FiO₂ reduction using the oxygen blender on the ventilator itself (most modern ventilators can reduce FiO₂ even with 100% source gas by entraining room air through the inspiratory limb)