Intracranial pressure responsiveness to positive end-expiratory pressure in different respiratory mechanics: a preliminary experimental study in pigs

BACKGROUND: Respiratory mechanics affects the effect of positive end-expiratory pressure (PEEP) on intracranial pressure (ICP). Respiratory mechanics of the lung and the chest wall was not differentiated in previous studies. In the present study, we investigated the influence of the following possible determinants of ICP responsiveness to PEEP: chest wall elastance (E(CW)), lung elastance (E(L)), and baseline ICP. METHODS: Eight healthy Bama miniature pigs were studied. The increase of E(L) was induced by instillation of hydrochloride, and the increase of E(CW) was induced by strapping the animals’ chest wall and abdomen. A balloon-tipped catheter was placed intracranially for inducing intracranial hypertension. Six experimental conditions were investigated in sequence: 1) Normal; 2) Stiff Chest Wall; 3) Lung Injury; 4) Lung Injury + Stiff Chest Wall; 5) Lung Injury + Stiff Chest Wall + Intracranial Hypertension and 6) Lung Injury + Intracranial Hypertension. PEEP was gradually increased in a 5 cm H(2)O interval from 5 to 25 cm H(2)O in each condition. Blood pressure, central venous pressure, ICP, airway pressure and esophageal pressure were measured. RESULTS: Hydrochloride instillation significantly increased E(L) in conditions with lung injury. E(CW) significantly increased in the conditions with chest wall and abdomen strapping (all p < 0.05). ICP significantly increased with increments of PEEP in all non-intracranial hypertension conditions (p < 0.001). The greatest cumulative increase in ICP was observed in the Stiff Chest Wall condition (6 [5.3, 6.8] mm Hg), while the lowest cumulative increase in ICP was observed in the Lung Injury condition (2 [1.3, 3.8] mm Hg). ICP significantly decreased when PEEP was increased in the intracranial hypertension conditions (p < 0.001). There was no significant difference in cumulative ICP change between the two intracranial hypertension conditions (p = 0.924). CONCLUSIONS: Different respiratory mechanics models can be established via hydrochloride induced lung injury and chest wall and abdominal strapping. The effect of PEEP on ICP is determined by respiratory mechanics in pigs with normal ICP. However, the responsiveness of ICP to PEEP is independent of respiratory mechanics when there is intracranial hypertension.