Using Airway Pressure Release Ventilation

The modern variant of this mode of ventilation is featured several modern ventilators. The presence of a dynamic expirtatory valve in these ventilators allows spontaneous breathing at high lung volumes: the ventilator cycles from high CPAP to low CPAP (high lung volume to lower lung volume), and the patient can breath spontaneously at either level. These breaths can be unsupported, pressure supported, or supported by automatic tube compensation. Any patient in critical care can be ventilated on this mode. In general, it is reserved for patients with severe ARDS.
APRV should be seen as full tidal volume ventilation - the patient is ventilated on the expiratory limb of the volume pressure curve.
Several "rules" are involved in APRV:
1,The expiratory time is the key variable - it should be short enough to prevent derecruitment and long enough to obtain a suitable tidal volume. The expiratory time is set between 0.4 to 0.6 seconds - the tidal volume is your target (between 4 and 6ml/kg). If the tidal volume is inadequate, the expiratory time is lengthened; if it is too high (>6ml/kg) the the expiratory time is shortened.
2. The high CPAP (PEEP) level is set at the mean airway pressure level from the previous mode (pressure control, volume control etc). If you are starting off with APRV then start high (28cmH2O of less) and work your way down. Higher transalveolar pressures recruit the lungs.
3. Low PEEP is set at 0cmH2O. The large pressure ramp allows for tidal ventilation in very short expiratory times.
4. The inspiratory time is set at 4-6 seconds (the respiratory rate should be 8 to 12 breaths per minute - never more).
5. Neuromuscular blockade should be avoided: the patient should be allowed to breath spontaneously (this is beneficial). The breaths can be supported with pressure support - but the plateau pressure should not exceed 30cmH2O.
6. There are two different ways to wean patients from APRV. If lung mechanics rapidly return to normal, the patient should be weaned to pressure support. If ARDs is prolonged, then the high CPAP level is gradually weaned down to 10cmH2O, and then the patient is converted to a standard vent wean.
 

 

 

High CPAP level at the previous mean airway pressure or to 28cmH2O or less

Set the Low CPAP level to 0

Set the expiratory time at 0.4 to 0.6 seconds

If tidal volume is > 6ml/kg, reduce expiratory time

If tidal volume is <4ml/kg, increase expiratory time

The expiratory time is now fixed

Set the inspiratory time at 4- 6 seconds

(the respiratory rate is an independent variable)

If High CPAP <30cmH2O, the spontaneous breaths can be supported with
pressure support, but plateau pressure should not exceed 30cmH2O

Wait 4 - 6 hours for a response
If the patient remains severely hypoxemic, lengthen the inspiratory time further
 

If the hypoxemia improves significantly,
reduce the inspiratory time to increase the  respiratory rate and reduce hypercapnia

Allow permissive hypercapnia
If pH <7.2, correct metabolic component (base deficit due to renal or hyperchloremic
acidosis ) with sodium acetate (Na 140mmol/L) drip and, if PCO2 > 60mmHg
start THAM
If hypoxemia worsens further consider:
  • neuromuscular blockade (to reduce chest wall compliance)

  • prone positioning

  • tracheal gas insufflation

  • bilateral chest drains (pleural effusions are inevitable)

Is the patient ready to wean?
If the patient is ventilating but not oxygenating, gradually reduce the high CPAP level.

If the patient is oxygenating well,
Wean the inspiratory time initially

Then increase the  Lower CPAP level to 8 - 10 cmH2O and increase the expiratory time

Switching to PSV:
set the pressure support to the high CPAP- low CPAP
Look at pressure waveform, adjust inspiratory
flow (slope) to optimal setting.
Watch lung compliance: increase PS if
compliance reduces (decreased tidal volumes)
decrease PS if compliance improves
(tidal volumes >6ml/kg).
Address patient sedation (tachypnea in PSV
is often from inadequate sedation)
Beware of tachypnea with small tidal volumes (click here)
       
               
     

Copyright Patrick Neligan 2005