College Answer

The principles of weaning are no different in this patient. He now has a reasonable PaO2 on 60%, and has an appropriate PaCO2 for his degree of metabolic alkalosis. General factors preventing weaning need to be excluded (cardiovascular function, metabolic state, nutrition, endocrine function (eg. thyroid), adequate sleep). Carbon dioxide production could be minimised by the use of lower CHO feeds if this is a clinical problem. Metabolic alkalosis could be reduced by the use of acetazolamide (with the risk of increasing ventilatory work required to maintain a given pH). Ventilatory strategies should be similar to (b) above, with more detailed plan including: attention to posture (sitting, lying on side), day/night cycling, minimising work of breathing during rest (triggering work, work to overcome resistance and compliance of lung and that imposed by chest wall and abdomen), and some periods of increased work (to assess ability to breath with no or 
minimal ventilatory support). Rest periods may require high levels of PEEP and pressure support (or equivalent) to counter imposed work. Pressures delivered may overestimate actual trans-pulmonary pressures. Depending on the difficulty of intubation, the patient will need to be kept for a period of time in the Intensive Care Unit after demonstrating ability to breath overnight for himself ( eg. 48 hours) or may stay until the team are happy for him to have the tracheostomy tube removed. Ongoing supportive care is required throughout to prevent Intensive Care Unit related problems (eg. pressure care, DVT prophylaxis, supportive psychological care, prevention of device related infections etc.).

Discussion

If one needed a good locally sourced article to discuss the best trategy for ventilator weaning in a patient with spinal cord injury, one could do much worse than this 2012 pearl scattered by the likes of  Oliver Flower and Sumesh Arora.

To summarise the strategy for weaning a high C-spine quad:

Mechanical ventilation strategy:

• Large tidal volumes, over 10ml/kg (up to 15ml/kg!) - turns out that weaning is faster in high V_T patients (Petersen et al, 1999 - the mean V_T was 1.7L ). Intercostal muscles are initially atonic, but become spastic as time goes on, fixing the chest wall into a rigid bell. A high tidal volume may actually help fix the chest wall into a more productive shape. Arora and Flower also suggest the adding of extra dead space in the circuit, so the high-V_T patient does not become hypocarbic and lose their respiratory drive.
• Low or zero PEEP: provided the V_T is high enough, and there is little atelectasis, you will not need a high PEEP, and it is to be avoided.
• Resistance and endurance protocol (REP): A fond mention is made of the 2003 Gutierrez study, which used a clever weaning protocol to retrain the respiratory systems of seven high quad patients. A simplified version of the protocol would consist of three major training styles:
  • Inspiratory resistance training: patients breathe through a fixed inspiratory resistor applied at the mouth for 10 seconds, four times a day. Resistance is then progressively increased as tolerated.
  • Expiratory resistance training: breathing through an expiratory resistor.
  • Endurance training: gradually reducing ventilator support; then, gradually lengthening periods of off-ventilator breathing time.

Adjunctive measures:

• Minimise sedation
• Maximise postural assistance to respiration: sit in a chair if possible
• Adequate sleep and nutrition
• Respiratory physiotherapy
• Optimise cardiac function; aim for dry fluid balance

Management of a difficult or failed wean:

• Consider tracheostomy
• Consider NAVA
• Extubate on to NIV