22–24There are several other innovations in mechanical ventilation that have not yet been widely implemented into clinical care, but space limitations preclude discussion in this concise review. This support is terminated when the inspiratory flow decreases to a level predetermined by the manufacturer. In this mode, if the patient makes an inspiratory effort that is detected by the machine, the breath is augmented with a preselected level of pressure above positive end-expiratory pressure (PEEP) with a high initial inspiratory flow followed by a decelerating flow pattern to maintain the pressure ( fig. The introduction of inspiratory pressure support allows the spontaneous efforts by the patient to be supported, similar to how the anesthesiologist assists efforts by squeezing the reservoir bag when the patient makes an inspiratory effort. Modern refinements include more sensitive pressure and flow sensors and rapid response times that minimize the work of breathing. Synchronized intermittent mandatory ventilation was the first mode to allow for patient-determined, spontaneous, unassisted breaths between the mandatory breaths. The first refinement of these basic modes of ventilation was the addition of a sensor for patient effort, which triggered the clinician-designed breath. If patients made spontaneous ventilatory effort while undergoing this mode of ventilation, they were working against a closed system (no sensor to detect or respond to patient effort). In pressure-controlled ventilation, the inspiratory pressure was selected by the clinician along with the inspiratory time ( fig. In volume-controlled ventilation, the machine generated a constant flow, and the tidal volume was determined by the inspiratory time. In these modes of ventilation, all breaths were determined by the clinician. The only options were controlled ventilation with time-triggered, volume- or pressure-controlled, and time- or volume-cycled ventilation. Within this classification scheme, the earliest ventilators were very simple. Specificity was high for both clinicians and the automated system (greater than 98%), but the automated system was twice as sensitive as clinicians (83.2% vs. Both the clinicians and the automated system utilized pressure–time and flow–time data but were blinded to esophageal/transdiaphragmatic pressure waveform (a very sensitive monitor of patient effort), which were used to establish the definitive assessment of asynchrony. 13 compared the sensitivity of a group of experienced respiratory therapists to an automated system. More recently there have been reports of using automated frameworks for the analysis of waveforms including frameworks based on machine learning. 8 demonstrated a number of years ago that when esophageal pressure monitoring (a surrogate for pleural pressure and sensitive monitor of patient effort) was added to gas flow/airway pressure monitoring, the incidence of asynchrony was significantly higher. However, the sensitivity of real-time visual detection of asynchrony is low, so one could reasonably expect that the actual incidence of asynchrony is higher than reported. Many of the reports of the incidence of asynchrony have relied on manual inspection of pressure–time or flow–time waveforms.
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