icu

Neurosurgical Patients

　　William Coplin, MD,[10] of Wayne State University, Detroit, Michigan, discussed the special case of weaning neurosurgical patients from mechanical ventilation. He described the goals of caring for these patients while on the ventilator as correcting any hypovolemia, preventing aspiration, treating acidosis (particularly respiratory acidosis), and inducing a variable degree of therapeutic hypocapnia. These requirements are traditionally achieved using subtly different approaches from those used to ventilate patients without intracranial compromise; namely, minimal intrathoracic pressure, minimal PEEP (although there is probably little effect on intracranial pressure [ICP] at PEEP values of less than 15 cm H2O), and presedation for tracheal suctioning (which certainly does increase ICP). He stated that the mode of ventilation is probably not important in terms of outcome and has little effect on ICP in the short term.
　　Therapeutic hyperventilation has been used in many centers to reduce ICP. The rationale is that reduced ICP improves cerebral perfusion pressure and that this is beneficial for injured brain tissue, which is unable to autoregulate its blood flow in the usual manner. This approach has definitely been shown to reduce ICP, by reducing the caliber of cerebral blood vessels, but has not been shown to improve outcome. Critics say that the effect is short lived, and that there is always a rebound effect when the PaCO2 is subsequently allowed to rise. One recently published set of guidelines recommends an absolute minimum value of 26 mm Hg for arterial PCO2. Dr. Coplin stated that, in his view, this approach may be appropriate as a temporizing measure (eg, en route to surgery to evacuate a hematoma or other acutely enlarging mass lesion).
　　The decision to remove a tracheal tube in a patient with cerebral compromise can be fraught with difficulty because of the problem of assessing the underlying conscious level as well as the degree of residual respiratory failure. One approach to this problem is to consider why the tube was placed originally (ie, for ventilatory failure or for airway protection). Delays in extubation of these patients leads to a doubling of the ventilator-associated pneumonia rate (40% vs 20%) as well as greatly increased hospital costs. Classical weaning parameters (Table) may be unhelpful in those patients in whom the tube was placed for airway compromise, but remain useful in those with ventilatory failure. Dr. Coplin concluded that neurologic impairment alone is insufficient reason to delay extubation of patients if their ventilatory function is otherwise adequate.