UMEM Educational Pearls

Intubation and mechanical ventilation of brain injured patients, which is extremely common in the Emergency Department, can be very challenging and subject to significant practice variation.  It is often said that brain injured patients “can't take a joke”, meaning that they are less tolerant to hemodynamic and metabolic perturbations, and these perturbations tend to be associated with very large swings in their clinical outcomes.  For example, hypo/hyperglycemia, hypo/hypernatremia, hypo/hypertension, hypo/hyperoxia, hypo/hypercapnea, etc are all extremely important to avoid.  This is probably the one patient population where “euboxia” (the notion that we obsess too much about making all the numbers pretty in the EMR) is probably not as applicable.  As such, there is at least good physiologic rationale, and now increasing empirical evidence, that ventilating these patients very thoughtfully is extremely important and likely to have meaningful impact on patient-oriented outcomes (mortality, neurologic outcome, etc).

The VENTIBRAIN study was a prospective observation trial of 2,095 intubated patients in 26 countries who had TBI, ICH (including SAH), or acute ischemic stroke.  Interestingly, they found that patients with lower tidal volume (TV) per predicted body weight had higher mortality (although the majority of their TVs were well controlled and in a fairly tight range), which is contrary to conventional thinking in pulmonary pathologies like ARDS.  They also found that higher driving pressure (DP) was associated with higher mortality, which agrees with data from other conditions.  PEEP and FiO2 had U-shaped curves, but FiO2 in particular tended to favor lower FIO2, also similar to current thinking for ICU patients in general.  

Take Home Points:

1. Although most brain injury patients have relatively normal pulmonary function, lung compliance, ventilator waveforms, etc, their ventilatory parameters (TV, PEEP, DP, pCO2/pH, oxygenation, etc) should be carefully monitored and a deliberate strategy to manage these parameters is essential.  Haphazard ventilatory strategies in these patients are clearly associated with poorer patient-oriented outcomes.
2. It's possible (although not definitively proven) that aggressively low TVs in these patients may lead to hypercapnea - which we know is poorly tolerated in brain injured patients - and worse outcomes.  The role of classic “permissive hypercapnea” (ala ARDS management, goal pH > 7.2) in these patients is unclear, and one should probably be more judicious in letting these patients get overly acidotic or hypercapneic, as opposed to other pathologies like ARDS where this is probably more allowable.  
3. Despite the paradoxical finding with low TVs, high driving pressure remains an important predictor of mortality in essentially all critical patient populations.   Care should be taken to minimize DP (guidelines say < 15 cm H2O, but goal should be minimum achievable value while meeting pCO2/pH targets).  DP/PEEP titrations should be carried out regularly when feasible (not all providers are comfortable with this practice, but it is safe and easy to learn, see references below).
4. Hypoxia and hyperoxia are both extremely dangerous for this population.  The minimum FiO2 needed to achieve a pulse oximetry reading of around 90-96% (exact numbers vary slightly by guideline and any underlying pulmonary pathology) should be used.  Be very wary of the pulse ox sitting constantly at 100% in these patients.

References

VENTIBRAIN: Ventilation practices in acute brain injured patients and association with outcomes: the VENTIBRAIN multicenter observational study | Intensive Care Medicine

TTM2 Reanalysis: Ventilatory settings in the initial 72 h and their association with outcome in out-of-hospital cardiac arrest patients: a preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after out-of-hospital cardiac arrest (TTM2) trial | Intensive Care Medicine

Learning Driving Pressure/PEEP Titration:

UMEM Educational Pearls - University of Maryland School of Medicine, Department of Emergency Medicine

Driving Pressure & PEEP Titration – CriticalCareNow

Driving pressure • LITFL • CCC Ventilation