Intrathoracic pressure regulation for intracranial pressure management in normovolemic and hypovolemic pigs.

OBJECTIVE: To evaluate the potential to use subatmospheric intrathoracic pressure to regulate intracranial pressure (ICP) in normovolemic and hypovolemic animals, we tested the hypothesis that mechanical devices designed to reduce intrathoracic pressure will decrease ICP in a dose-related manner. An inspiratory impedance threshold device was used in spontaneously breathing animals and an intrathoracic pressure regulator was attached to a positive pressure ventilator and used in apneic animals: both devices lower intrathoracic pressure.
DESIGN: Prospective, randomized animal study.
SETTING: Animal laboratory facilities.
SUBJECTS: A total of 36 female farm pigs in four different protocols (n = 12, 6, 12, and 6, respectively). INTERVENTIONS, MEASUREMENTS, AND MAIN
RESULTS: In all protocols, endotracheal, right atrial, central aortic, and ICP were measured continuously. In protocol 1, spontaneously breathing animals were randomized to breath for 15 mins through an impedance threshold device with a cracking pressure of -10 or -15 mm Hg. In protocol 2, after untreated ventricular fibrillation for 4 mins and successful defibrillation to a normal rhythm, spontaneously breathing pigs were used to evaluate the effect of two different impedance threshold device cracking pressures (-10 and -15 mm Hg) on increased ICP. In protocol 3, the acute effects of an intrathoracic pressure regulator on ICP were evaluated in combination with a positive pressure mechanical ventilator in apneic hypovolemic hypotensive pigs after 35% or 50% blood loss. In protocol 4, after 40% blood loss, an intrathoracic pressure regulator was applied for 120 mins and ICP was recorded to determine whether the intrathoracic pressure regulator effects were sustained over time. Inspiratory impedance successfully decreased ICP in spontaneously breathing pigs in a dose-dependent manner and decreased elevated ICP immediately after cardiac arrest and successful resuscitation. The same effect was seen in apneic animals with the use of the intrathoracic pressure regulator. The effect was more pronounced in hypovolemia, and it was sustained for >/=2 hrs.
CONCLUSIONS: Reduction of intrathoracic pressure to subatmospheric levels resulted in an instantaneous and sustained reduction in ICP in spontaneously breathing and apneic animals. The effect was most pronounced in the hypovolemic animals.