Respiratory Mechanics Following Brain Injury: The Role of Inhaled Nitric Oxide

Intubation and mechanical ventilation are common treatments in the care of patients with traumatic brain injury (TBI). Intubation allows for airway control and facilitates removal of respiratory secretions. Mechanical ventilation allows control of arterial carbon dioxide to aid in control of intracranial pressure. Recent evidence suggests that lung protective ventilation (tidal volumes of 6 ml/kg of predicted body weight and moderate positive end expiratory pressure) improves outcomes following brain injury and reduces brain-lung cross talk.

The treatment of respiratory failure in TBI must balance the need to improve lung function with the negative consequences of increased intrathoracic pressure on mean arterial pressure, intracranial pressure and venous return. Traditional treatment of increasing positive end expiratory (PEEP) and mean airway pressure then, represent competing interests. Methods for improving arterial oxygenation while avoiding negative hemodynamic effects are needed.

The impact of head injury on respiratory mechanics has been studied in just a few clinical investigations. (1-3) Of note, the earliest of these noted that the ventilation perfusion (V/Q) matching following TBI was not the result of lung collapse or parenchymal lung disease but secondary to alterations in perfusion. There are three possibilities for this finding:

1. redistribution in regional perfusion, which is partially mediated by the hypothalamus
2. pulmonary microembolism, leading to increased dead space
3. lung surfactant depletion due to excessive sympathetic stimulation and hyperventilation.

The introduction of inhaled pulmonary vasodilators such as inhaled nitric oxide or aerosolized epoprostenol offer an opportunity to improve oxygenation in patients with TBI without increasing airway pressures in the face of V/Q inequalities.