A 44 year old man, with morbid obesity (175 cm tall and 210 kg) presents to the Emergency Department with respiratory failure. He is obtunded with an arterial blood gas (ABG) showing pH 7.25, Pa C02 82 mmHg and PaO2 53 mm Hg.
CXR reveals cardiomegaly and clear lung fields.
It is day 7 and he has had a tracheostomy performed. He is on SIMV 12 breaths by 500 ml tidalvolume, with an FIO2 of 0.6 and his ABG is now pH 7.49, PaCO2 49 mm.Hg and Pa02 159.
(c) Describe your strategy for weaning.
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.).
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 VT patients (Petersen et al, 1999 - the mean VT 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-VT patient does not become hypocarbic and lose their respiratory drive.
- Low or zero PEEP: provided the VT 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.
- 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
El Solh, A. Aquilina, et al. "Noninvasive ventilation for prevention of post-extubation respiratory failure in obese patients." European Respiratory Journal28.3 (2006): 588-595.
Arora, Sumesh, et al. "Respiratory care of patients with cervical spinal cord injury: a review." Crit Care Resusc (2012): 14: 64-73
Peterson, W. P., et al. "The effect of tidal volumes on the time to wean persons with high tetraplegia from ventilators." Spinal Cord 37.4 (1999): 284-288.
Boles, Jean-Michel, et al. "Weaning from mechanical ventilation." European Respiratory Journal 29.5 (2007): 1033-1056.
Gutierrez, Charles J., Jeffrey Harrow, and Fred Haines. "Using an evidence-based protocol to guide rehabilitation and weaning of ventilator-dependent cervical spinal cord injury patients." Journal of rehabilitation research and development 40.5; SUPP/2 (2003): 99-110.