Time-cycled alternant between two levels of positive airway pressure, with the main time on the high level to maintain adequate lung volume and alveolar recruitment and a brief expiratory release where most of ventilation and CO2 removal occurs
The patient can breathe spontaneously at any time throughout the respiratory cycle
When the trap trends downward (goes from 50% to 40% to 25%), the patient’s lungs are worsening and vice versa
Advantages
The benefit of alveolar recruitment
Less need for neuromuscular blockade and sedation
The preservation of spontaneous breathing (due to floating exhalation valve)
Benefit of Spontaneous Breathing
It may promote:
A redistribution of aeration to the dependent lung regions because during spontaneous breathing, the greatest displacement of the diaphragm is in dependent regions. These regions are the best ventilated. Compared with spontaneously breathing patients, mechanically ventilated patients have a smaller inspiratory displacement of the dependent part of the lung
Pleural pressure decreases, leading to a decrease in the intra-thoracic and right atrial pressure – thus increasing venous return and improving the pre-load and consequently increasing the cardiac output
Better ventilation/perfusion (V/Q) matching
Concerns with Bi-Level (APRV)
Tidal volume is variable, so you must be constantly aware of the patient's minute ventilation to prevent severe hypercapnia or hypocapnia
Overstretching the lung parenchyma. Choosing a P high is not the only determinant of the tidal volume. Spontaneous breathing causes the pleural pressure to become less positive, so there is an increase in the transpulmonary pressure (pressure in alveoli minus pressure in the pleura). This can result in a higher tidal volume and the risk of overdistention and volume-induced lung injury
Atelectrauma/shearing: Damage may occur when airways open and close with each tidal cycle if end-expiratory pressure is below the lower inflection point, as some diseased alveolar units may collapse. Even though the intentional auto-PEEP might maintain a certain end-expiratory pressure, this parameter is truly uncontrolled when the airway pressure is released to zero
If the patient cannot breathe spontaneously. Unfortunately, patients who need heavy sedation or neuromuscular paralysis with lack of spontaneous breathing efforts may lose the physiologic advantages of this mode
Starting or initial setting Before Adjusting to Patient’s Needs:
P high: Set PHIGH to conventional MAP and adjust for 6-8 ml/kg.
P low: 0
T high: Set for a dump rate to fit the patient’s size.
Infant: 25-30 dumps per minute
Toddler: 20-25 dumps per minute
Adolescent: 12-16 dumps per minute
T low: Start at 0.2 seconds and increase to target the TPEF (% trap) over the first sequence of breaths.
Ways to Increase Oxygenation:
Increase FiO2
Increase P-high
Increase T-high
Decrease T-low to be closer to 75% PEFR
Increase P-Low if the target % trap is not obtainable. For example if P-Low is 0 and T-Low is 0.2 seconds, but trap is 10%, you would then think about increasing P-Low.
Ways to Increase Ventilation:
Increase P-high (not to exceed 8ml/kg)
Increase T-low to allow more time for alveolar emptying, but only if trapping doesn’t drop below 25%
Decrease T-high to increase the rate of releases, but this will decrease MAP and possibly oxygenation
Increase T-high if spontaneously breathing, but if PaCO2 increases after this change it may reflect inadequate lung volumes and may need higher P-high to re-establish adequate FRC
Drop and Stretch Weaning Method
Once FiO2 is titrated below 50%, recruitment is maximized, and the patient is spontaneously breathing there is a gradual wean where P-high is dropped and T-high is increased, or ”stretched”