Abstract
BackgroundRespiratory 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 (ECW), lung elastance (EL), and baseline ICP.MethodsEight healthy Bama miniature pigs were studied. The increase of EL was induced by instillation of hydrochloride, and the increase of ECW 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 H2O interval from 5 to 25 cm H2O in each condition. Blood pressure, central venous pressure, ICP, airway pressure and esophageal pressure were measured.ResultsHydrochloride instillation significantly increased EL in conditions with lung injury. ECW 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).ConclusionsDifferent 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.
Highlights
Respiratory mechanics affects the effect of positive end-expiratory pressure (PEEP) on intracranial pressure (ICP)
Compared to the Normal condition, hydrochloride instillation significantly increased lung driving pressure and EL in conditions with lung injury (L, L + CW, L + CW + intracranial hypertension (IH) and L + IH conditions); while End-expiratory esophageal pressure (PES-EE), chest wall driving pressure and Chest wall elastance (ECW) significantly increased in conditions with chest wall and abdomen strapping (CW, L + CW and L + CW + IH conditions, Table 1)
The highest ECW/Respiratory system elastance (ERS) ratio was observed in the CW condition which was significantly higher than the Normal condition (p = 0.033, Table 1); while the lowest ECW/ERS ratio was observed in the L condition which was significantly lower than the Normal condition
Summary
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. We investigated the influence of the following possible determinants of ICP responsiveness to PEEP: chest wall elastance (ECW), lung elastance (EL), and baseline ICP. Several possible determinants for the effect of PEEP on ICP have been proposed, including baseline ICP [11], intracranial compliance [12, 13] and respiratory mechanics [9, 10]. Chapin and colleges reported that increased lung elastance (EL) and decreased chest wall elastance (ECW) can minimize the effect of PEEP on pleural pressure [20].
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