Respiratory care clinics of North America
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During conventional mechanical ventilation, fixed set pressure, flow, and tidal volume result in a mismatch between patient and ventilator inspiratory time and in a patient's inability to adapt to changing ventilatory demand. Synchrony between the patient and ventilator improves neuromuscular coupling and the ability to adapt to increased ventilatory demand or loading. The sensation of dyspnea prevents ineffective inspiratory efforts and attenuates periodic breathing during sleep.
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During assisted mechanical ventilation, the total pressure applied to respiratory system is the sum of ventilator and muscle pressure. As a result, the respiratory system is under the influence of two pumps, the ventilator pump (ie, Paw), which is controlled by the physician's brain and the capabilities of the ventilator, and the patient's own respiratory muscle pump (Pmus), which is controlled by the patient's brain. ⋯ The achievement of this harmony depends exclusively on the physician, who should be aware that during assisted mechanical ventilation the respiratory system is not a passive structure but reacts to pressure delivered by the ventilator via various feedback systems and, depending on several factors both to the ventilator and patient, may modify the function of the ventilator. Finally, the physician should know that the ventilator imposes significant constraints to the respiratory system, the magnitude of which depends heavily on the triggering variable, the variable that controls the gas delivery and the cycling off criterion.
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Respir Care Clin N Am · Jun 2005
ReviewModes of pressure delivery and patient-ventilator interaction.
Differences between assist-control pressure and volume ventilation are minimal provided that peak inspiratory flow delivered at early inspiration matches patient demand Ventilation at constant flow and controlled tidal volume allow instantaneous analysis of the patient-ventilator interactions and the mechanical properties of the respiratory system. The cycle-per-cycle variability of peak inspiratory flow, which is permitted in pressure-limited modes, may allow better patient comfort. We do not know if one mode (pressure controlled or volume controlled) is superior to the other in terms of outcomes.
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Partial patient-controlled mechanical support mode ventilators provide positive pressure assistance whenever a patient's inspiratory effort decreases pressure or flow in the ventilator circuit below the sensitivity set by clinicians; these modes minimize disuse atrophy of the respiratory muscles, can facilitate the weaning process, and usually require lower ventilator pressures. The capability of restoring gas exchange, unloading respiratory muscles, and relieving the patient's dyspnea with partial patient-controlled mechanical support modes depends on matching between the ventilator setting and the patient's ventilatory demand (ie, patient-ventilator interactions).
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Expiratory asynchrony is a universal phenomenon, and expiratory synchrony occurs only by chance. Expiratory asynchrony exists in all breath modes and has a significant impact on the patient's work of breathing and the weaning process. Advancements in ventilator designs and basic physiologic science could lead to the improvement of the expiratory asynchrony.