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AutoFlow

Activation of AutoFlow changes the ventilator mode to volume targeted pressure control ventilation. It is activated in the Extra Settings Menu. Once activated, the settings for <Insp. Flow> and <Pmax> are no longer displayed. With AutoFlow activated, inspiratory pressure is automatically adjusted in increments of 3 cm H2O pressure between breaths to maintain the set tidal volume during any given change in the patient's lung compliance and resistance. The inspiratory pressure will automatically increase up to within 5 cm H2O of the the alarm setting for high Paw. AutoFlow ON also provides volume targeted pressure support during spontaneous inspirations. All tidal volumes can be limited by the inspiratory tidal volume (VTi) alarm limit.

The graphic on the right shows an upper flow/time waveform and a lower pressure/time waveform during simulated coughing events in AutoFlow. The airway pressure does not exceed the pressure limit and flow is always available to patient demand.

Pressure Rise Time

Activated through the Extra Settings Menu, pressure rise time is adjustable from 64 ms to 2 seconds.

Airway Pressure Release Ventilation

APRV allows free spontaneous breathing under continuous positive airway pressure with brief periods of pressure release. This ventilation mode is suitable for patients with poor gas exchange and CO2 retention in conventional CPAP. The patient breathes spontaneously at a high-pressure level (P high) for an adjustable length of time (T high). For very short expiration times (T low), the ventilator switches to a low-pressure level (P low) allowing a greater exhalation. The normal lung areas are emptied, but the ‘slow’ lung areas only change volume to a lesser extent. The ventilation/perfusion ratio can be improved this way for patients with poor gas exchange.

Apnea ventilation

The machine will automatic switch to volume controlled mandatory ventilation if the patient stops breathing spontaneously. Apnea ventilation can be switched on in the following ventilation modes: SIMV, PCV+, CPAP, and APRV. If breathing stops, the ventilator will trigger an alarm after the set alarm time (Tapnea), and will start volume controlled ventilation with the set ventilation parameters: breath rate (fapnea), and tidal volume (VTapnea). PEEP and O2 will correspond to the settings effective at the time, and inspiratory time is determined from the start-up value.

Independent Lung Ventilation (ILV)

This mode allows for separate, differential, synchronized ventilation with one Evita 4 ventilator for each lung. This can be accomplished by connecting the two ventilators together via the ILV port on the back of the ventilator.

Intrinsic PEEP

Intrinsic PEEP (Pi) is the actual positive pressure left in the patient’s lungs at the end of exhalation. The Pi measuring maneuver can be performed in all ventilating modes. Pi is measured at the end of a 3 second time period (1.5 sec. peds) in which both the inspiratory and expiratory valves are closed. Three seconds is generally long enough for pressure to equalize between the lungs and the ventilation system. At the end of the 3 sec. interval, the ventilator opens the expiratory valve and measures the expiratory flow generated by intrinsic PEEP until expiratory flow returns to zero, or for up to 7 seconds (3.5 peds). If expiratory flow returns to zero, the actual volume of trapped gas has been measured. During this period, lung pressure is allowed to decrease to the PEEP level.

Occlusion Pressure (P100)

Breathing drive can be measured at the start of an inspiration by measuring the mouth pressure during a short-term occlusion. Occlusion pressure is a direct measurement of neuromuscular breathing drive. To measure occlusion pressure the inspiratory valve is closed after one expiration. The airway pressure produced by the patient’s inspiratory effort is measured at the beginning and at the end of the first 100 ms of patient inspiratory effort. The Occlusion pressure is the difference between these two pressures. This value can be beneficial when weaning COPD patients off the ventilator. P100 values greater than 6 cm H20 correlate with excessive drive and impending respiratory muscle fatigue in COPD patients.

Sighs

In the Evita 4 sighs are accomplished by providing intermittent PEEP. Intermittent PEEP is indicated in patients with problems with atelactasis. Intermittent PEEP will increase end-expiratory pressure for two ventilator breaths every 3 minutes. In order to avoid lung overinflation, pressure peaks during sigh phases can be limited using pressure limit Pmax by the operator, and will not affect sigh function.