Airway pressure release ventilation (APRV)
Airway pressure release ventilation (APRV)
- elevated baseline pressure facilitates oxygenation
- pressure release increases minute ventilation
Advantages of APRV include:
- Lower peak airway pressures
- Lower minute ventilation
- Decr adverse effects upon circulatory fn
- Spont ventilation the entire ventilatory cycle
- Decreased need for sedation
- Near elimination of neuromuscular blockade.
- Decrease respiration work
- Unlike PEEP (involving use of an expiratory flow resistor, which decreases expiratory flow), peak expiratory flow rates are increased during the release phase of APRV, improving expiratory flow limitation.
- exhalation is not limited to the release phase, as it is permitted throughout the respiratory cycle.
- does not compromise circulatory function and tissue oxygenation
- In severe acute respiratory failure,
- significantly lower peak airway pressure, lower minute ventilation cf to CPPV
- lower peak airway pressures than volume assist-control ventilation and SIMV
- lower airway pressure, reduced dead space ventilation, and improved oxygenation and ventilation, cf to IPPV.
Disadvantages
- Because APRV is time-cycled, synchrony with the patient’s spontaneous ventilatory efforts does not occur.
Different from conventional ventilation because
- starts with elevated baseline pressure and followed with a measured pressure release.
- spontaneous breathing may occur at either the plateau pressure or deflation pressure levels.
- it is the high CPAP level [referred to as PEEP high or P high] which enhances oxygenation, while the timed releases to the low CPAP level [referred to as PEEP low or P low] aid in CO2 clearanceOur goals with APRV include the following:
Avoiding extension of lung injury
Minimizing O2 toxicity with reduced Mean airway pressure
Recruiting alveoli & preventing derecruitment
Minimizing peak airway pressure
Preventing atelectasis
Using sedation and paralysis conservatively
Similar modes:
Intermittent mandatory pressure release ventilation (IMPRV):
- synchronizes the release event with the patient’s spontaneous effort. This only occur on pressure release- all spontaneous breaths are pressure supported to overcome the resistance associated with breathing through the endotracheal tube and ventilator tubing.
Intermittent CPAP: - is based on the principles of APRV but is intended for patients undergoing general anesthesia.- CPAP is applied at a level that will provide an adequate tidal volume, then removed for one second to produce tidal ventilation, then reapplied.
- not intended to restore normal functional residual capacity or improve oxygenation
BiLevel ventilation:- Essentially APRV with the option of pressure support. Defined as augmented pressure ventilation that allows for unrestricted, albeit pressure-supported, spontaneous breathing throughout the ventilatory cycle.
Indications:
- acute lung injury
- low compliance lung disease
Constant airway pressure facilitates
- alveolar recruitment
- enhances diffusion of gases
- allows alveolar units with slow time constants to fill, preventing over-distension of alveoli
- augments collateral ventilation.
Settings:
- Ideally, the end-inspiratory pressure, which equates to P High, should be kept beneath 35 cm of water pressure
- the preset pressure limit prevents, or limits, over-distension of alveoli and high-volume lung injury.
- Decreasing lung volume for ventilation further limits air space over-distension and the potential for high-volume lung injury.
- maintaining airway pressure optimizes recruitment and prevents or limits low-volume lung injury by avoiding the repetitious opening of alveoli.
- The clinician converts the plateau pressure of the conventional mode to P High and seeks an expired minute ventilation of 2 to 3 L/minute, less than when on conventional ventilation. This is accomplished by setting P High at the plateau pressure, with a ceiling level for the P High normally at 35 cm of water pressure.
- P Low is often initially set at 0 cm of water pressure. A P Low of zero produces minimal expiratory resistance, thus accelerating expiratory flow rates, facilitating rapid pressure drops.
- T High is set at a minimum of 4.0 seconds. A T High of less than 4.0 seconds begins to impact mean airway pressure negatively.
- T Low is set between 0.5 and 1.0 seconds (often at 0.8 seconds).
- With these settings (P High = 35 cm of water pressure, P Low = 0 cm of water pressure, T High = 4.0 seconds, T Low = 0.8 seconds), the mean airway pressure will equal 29.2 cm of water pressure.
- Primarily, the method to reduce support is through manipulation of P High and T High. P High will be lowered 2 to 3 cm of water pressure at a time, and T High will be lengthened in 0.5- to 2.0-second increments, depending on patient tolerance
- The goal is to arrive at straight CPAP - usually in the range of 10 to 14 cm H2O - and then the clinician weans CPAP [and pressure support if in place]
- Before switching to CPAP, P High often is approximately 14 to 16 cm of water pressure and T High is at 12 to 15 seconds.
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