Some concern may arise when CPAP is delivered to patients with increased intracranial pressure. Positive intrathoracic pressure may indeed decrease venous return and act as a resistive component to venous return from the head, thus increasing intracranial venous pressure and eventually parenchymal intracranial pressure. Decreased systemic venous return may also decrease cardiac output and blood pressure which will decrease oxygen delivery and perfusion to the already jeopardized cerebral tissues. A CPAP level of 10 to 15 cmH2O increases cerebrospinal fluid pressure by 3 to 5 cmH2O. The use of CPAP greater than 5 cmH2O in this type of patient must be accompanied by frequent monitoring of the patient's neurological status. McGuire et,§( (126.96.36.199) demonstrated that, compared with T-piece breathing, CPAP reduces the total inspiratory work without negative effects on cerebral perfusion pressure. The reduction in the work of breathing appears to be related to the minimization of the variation in airway pressure between inspiration and expiration. Thus the equipment used to deliver CPAP should be able to generate an almost constant airway pressure independently of the inspiratory flow of the patient. Demand-valve CPAP systems require more external inspiratory work than high-flow valve-free systems. However, the success or failure of weaning from mechanical ventilation with CPAP depends not only on the characteristics of CPAP systems, but also on patient characteristics such as muscle strength and compliance of the respiratory system (Fig 2).
Fig. 2 Pressure-volume diagram of elastic work (curves BCD and HJK) and resistive (non-elastic) work (curves BIC and HI'J) done by the lung during breathing at (a) ambient airway pressure and (b) with CPAP. CPAP decreased both elastic and non-elastic work loads as expressed by the area under the curves HJK and HI'J compared with BCD and BIC respectively. (Reproduced with permission from Katzjnd,Maik§..,il9§5.)..)
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