Mechanical ventilation in neurosurgical patients is a relatively uncomplicated affair. Beyond isolated traumatic brain injury and various cerebrovascular catastrophes, this group can also be extended to the isolated neurotoxicology case, the intubated drunk, the patient with refractory seizures, meningitis, encephalitis, cerebral lupus and pretty much anything else affecting the patient above the neckline. These pathologies can be grouped together as "indications for ventilation with a clear chest Xray". Nothing is wrong with the lungs, and therefore mechanical ventilation in these patients is a purely supportive process, even more so than usual.
- Goals of mechanical ventilation in neurological/neurosurgical patients:
- Replace or support an ineffective respiratory drive
- Prevent secondary brain injury
- Prevent respiratory complications (eg. atelectasis and VAP)
- Known variables affected by neurosurgical or neurological causes for ICU admission
- Longer ventilation, delayed extubation
- Tracheostomy is more frequent
- NIV and HFNP are less likely to be of benefit
- Specific limitations to ventilation strategies in severe traumatic brain injury:
- Permissive hypercapnia is not appropriate
- Prone ventilation is not appropriate
- High PEEP is counterproductive to ICP control
- Expert recommendations for ventilator settings
- Lung protective ventilation, provided
- PEEP of 5-8, and up to 15 may be safe
- Extubation readiness is signalled by a recovery og GCS to 8-10, and by recovery of upper airway reflexes.
In terms of peer-reviewed reading, the best (free, recent) article on the subject is the comprehensive review by Asehoune et al (2018).
Characteristics of this patient group
Neurosurgical and specifically neurotrauma patients typically end up being ventilated for longer. Lepelletier et al (2010) performed a retrospective analysis of VAP in neurotrauma patients, finding that their average of 14 days of mechanical ventilation tends to put them at greater risk. Passive strategies aimed at protection against VAP (such as head position) become important, given that in this population one cannot exactly expedite extubation (they'll wake up when they feel like waking up). These patients tend to meet standard extubation criteria later, and require tracheostomy more often (Pelosi et al, 2011)
Invasive mechanical ventilation in neurosurgical ICU patients
- Lung protective strategies may be used wherever possible
- A normal PEEP (5-8) is appropriate and high PEEP to maintain normoxia is not contraindicated
- A lower GCS (anything above 8-10) might be a better criterion for extubation readiness
- Recovery of upper airway reflexes is a good guide to extubation readiness
From fear of hypercapnia, these patients generally end up being ventilated with higher tidal volumes than is otherwise accepted for other patient groups. This seems to stem from a reluctance to use lung-protective ventilation early in their treatment course. Pelosi et al (2011) found that this strategy was used in a slightly smaller proportion of neuro patients than in any other group.
For fear of increasing their ICP, these patients are typically ventilated with a lower PEEP. This is in spite of the fact that the evidence for the effect of PEEP on ICP is fairly limited. A group of SAH survivors ended up having a slightly more linear relationship between PEEP and ICP on day 7 (Muench et al, 2005). There is probably no reason to fear a high PEEP in this population, except for where a very high PEEP (15ish) decreases cardiac output and cerebral perfusion in hypovolemic patients. Recommendations from Haddad et al (2012) say as much, recommending that PEEP up to 15 cm H2O "may be used in cases of refractory hypoxemia" because hypoxemia is also bad for the traumatised brain. Normal PEEP (5-8) seems to be quite safe for TBI patients with no lung pathology (Asehnoune et al, 2017)
Delay of extubation in this group seems to be affected by the mistaken application of standard extubation criteria to a group of patients who potentially won't be neurologically perfect for months. Obviously some adjustments are necessary. Namen et al (2001) determined that a GCS better than 8 were the best predictors of extubation success, whereas a strong cough was not. In contrast, Navalesi et al (2008) found it was useful to have a cough in addition to a GCS above 8. Asehnoune et al (2017) felt it would be better to go with a GCS of 10. Overall, it seems the studies all have somewhat different outcomes mainly because nobody can agree on how to score the verbal component of the GCS. Many of the recent trials have agreed that upper airway reflexes should be preserved in order for the extubation attempt to be successful.