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Objective: To test the hypothesis that in response to increased ventilatory demand, dynamic inspiratory pressure assistance better compensates for increased workload compared with static pressure support ventilation (PSV).
Design: Randomized clinical crossover study.
Setting: General intensive care u nits of a university hospital.
Patients: Twelve patients with acute respiratory failure.
Interventions: Patients received PSV, proportional assist ventilation (PAV), and PAV+ automatic tube compensation (ATC) in random order while maintaining mean inspiratory airway pressure constant. During each setting, ventilatory demand was increased by adding deadspace without ventilator readjustment.
Measurements and Main Results: Cardiorespiratory, ventilatory, and work of breathing variables were assessed by routine monitoring plus pneumotachography; airway, esophageal, and abdominal pressure measurements; and nitrogen washout. After deadspace addition, tidal volume and end-expiratory lung volume increased similarly in all ventilatory modalities. Ventilator work, peak inspiratory flow, and maximum airway pressure increased significantly during PAV+ATC when compared with PSV after deadspace addition. However, increase in ventilator work did not result in a smaller increase in patients' work of breathing with elevated ventilatory demand during PAV+ATC (PSV 807 ± 204 mJ/L, PAV 802 ± 193 mJ/L, and PAV+ATC 715 ± 202 mJ/L, p = .11). Increase in patients' work of breathing was mainly caused by a significantly higher resistive workload during PAV and PAV+ATC.
Conclusion: In patients with acute respiratory failure, dynamic inspiratory pressure assistance modalities are not superior to PSV with respect to cardiorespiratory function and inspiratory muscles unloading after increasing ventilatory demand. The latter might be explained by higher peak flows resulting in nonlinearly increased resistive workload that was incompletely compensated by PAV+ATC.
Pressure support ventilation (PSV) is commonly used to decrease work of breathing (WOB) in patients requiring ventilatory assistance. During PSV, the ventilator applies constant pressure for every detected patient's inspiratory effort. In contrast, proportional assist ventilation (PAV) provides dynamic inspiratory pressure assistance in linear proportion to patient-generated volume and flow. In theory, by adjusting the proportionality between applied pressure and both actual volume and flow, the ventilator should selectively unload the patient's inspiratory muscles for increased elastic and resistive WOB. Endotracheal tube resistance (Ret) imposes an undesirable inspiratory muscle load that is nonlinearly dependent on flow and should, therefore, not be entirely compensated by linearly flow-dependent support with PAV or constant PSV. Automatic tube compensation (ATC) provides ventilatory assistance of each spontaneous breath by increasing airway pressure (Paw) during inspiration and lowering Paw during expiration aiming at compensating nonlinearly flow-dependent Ret. Based on these considerations, PAV and the combination of PAV and ATC should better adapt the dynamic inspiratory pressure assistance to variations in ventilatory demand than PSV.
We hypothesized, that, in response to an increase in ventilatory demand, patients' WOB increases less during PAV+ATC compared with PAV alone or PSV, Therefore, we examined WOB and cardiopulmonary function in patients with acute respirator failure (ARF) and unrestricted breathing with equivalent levels of pressure support during PSV, PAV, and PAV+ATC during normal breathing and after a provoked increase in ventilatory demand.
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