Resp Care
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Closed-loop mechanical ventilation encompasses a plethora of techniques, ranging from the very simple to the relatively complex. In the simplest form, closed-loop ventilation is the control of one output variable of the mechanical ventilator based on the measurement of an input variable. An example would be pressure support ventilation, in which flow (output) is constantly changing to maintain pressure (input) constant throughout inspiration. ⋯ This review discusses both currently available closed-loop ventilation techniques and those only available outside the United States, along with some cutting-edge techniques that have only limited use. The operation, theoretical advantages, and limitations of each technique are reviewed. When available, the literature supporting or refuting each technique will be reviewed, but, unfortunately, little has been published on certain techniques.
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As many as 20% of extubated patients require reintubation (ie, extubation failure) within 72 hours of extubation, with the exact prevalence depending on numerous factors. The pathophysiologic basis of extubation failure is often different from the cause of weaning failure. Extubation failure substantially prolongs the duration of mechanical ventilation, intensive care unit stay, and hospital stay, and substantially increases hospital mortality. ⋯ New semi-objective measurements of cough strength and secretion volume can help identify patients at increased risk for extubation failure. It is important to note that mortality increases with reintubation delay, which indicates that clinical worsening may take place during the period without ventilatory support. Therefore, improved outcome may derive from rapid identification of patients at increased risk, followed by expeditious reinstitution of ventilatory support when extubation failure occurs.
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Invasive mechanical ventilation can be lifesaving for patients with acute respiratory failure, but numerous complications have been identified. Therefore, once clinical improvement has occurred, emphasis is placed on quickly weaning (ie, liberating) the patient from mechanical ventilation. Weaning can be subdivided into 2 components: readiness testing and progressive withdrawal. ⋯ Recent insights into the pathophysiology of weaning failure have provided a framework for identifying potentially correctable limiting factors. Randomized controlled trials suggest that several approaches to progressive withdrawal may be acceptable, though only a minority of patients require progressive withdrawal. Emerging evidence indicates that protocol-directed weaning, driven by respiratory therapists and intensive care nurses, can improve outcome.
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A common clinical observation is that patients wean more rapidly from mechanical ventilation following tracheotomy. Expected changes in tube resistance and dead space are not adequate to explain this observation in adult patients. Theoretical considerations are too complicated to allow evaluation of expected changes in work of breathing following tracheotomy. ⋯ There may be less late ventilator-associated pneumonia following early tracheotomy. The assumed better safety of tracheostomy has been questioned. That patients appear to wean more rapidly is probably accounted for by the variety of factors mentioned above.
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Volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) are not different ventilatory modes, but are different control variables within a mode. Just as the debate over the optimal ventilatory mode continues, so too does the debate over the optimal control variable. VCV offers the safety of a pre-set tidal volume and minute ventilation but requires the clinician to appropriately set the inspiratory flow, flow waveform, and inspiratory time. ⋯ Further, the beneficial characteristics of both VCV and PCV may be combined in so-called dual-control modes, which are volume-targeted, pressure-limited, and time-cycled. PCV offers no advantage over VCV in patients who are not breathing spontaneously, especially when decelerating flow is available during VCV. PCV may offer lower work of breathing and improved comfort for patients with increased and variable respiratory demand.