Airway Pressure Release Ventilation (APRV)

Indications

Acute Respiratory Distress Syndrome (ARDS) with Refractory Hypoxemia (see Acute Respiratory Distress Syndrome, [[Acute Respiratory Distress Syndrome]])

Clinical Efficacy

  • Large Randomized Controlled Trial of APRV (Acta Anaesthesiol Scand, 2004) [MEDLINE]: RCT (n = 58) comparing APRV with SIMV with PS (study was terminated early for futility)
    • No Mortality Benefit at 28 Days and 1 Year
    • No Difference in Ventilator-Free Days at 28 Days
    • However, Proning was Used in Both Arms and its Effects May Have Overshadowed the Potential Effects of APRV in this Study
  • Randomized Trial of APRV in Adult Trauma Patients with Respiratory Failure (J Trauma, 2010) [MEDLINE]: n= 63
    • For Adult Trauma Patients Requiring Mechanical Ventilation >72 hrs, APRV Had a Similar Safety Profile as Low Tidal Volume Ventilation
    • Trends for APRV Patients to Have Increased Ventilator Days, ICU Length of Stay, and Ventilator-Associated Pneumonia May Be Explained by Initial Higher Acute Physiology and Chronic Health Evaluation II Scores

Contraindications

Physiology

General Comments

  • Rationale: based on open lung approach
  • Concept: inverse ratio, pressure controlled, intermittent mandatory ventilation with unrestricted spontaneous breathing -> allows patient to breathe spontaneously while receiving high airway pressure with an intermittent pressure release
    • Historically, APRV Has Been Viewed as “Alternating Levels of CPAP”: this gave rise to the P high, P low, etc terminology for settings

Similarities/Differences Between Airway Pressure Release Ventilation (APRV) and Bi-Level Ventilation

  • Confusion Exists in the Literature Regarding Distinction Between APRV and Bi-Level Ventilation
    • Review of 50 published studies noted that 78% of them described APRV, whle 22% described Bi-Level Ventilation (Intensive Care Med, 2008) [MEDLINE]
    • Proprietary Modes Vary by Manufacturer
      • Airway Pressure Release Ventilation (APRV): Drager Evita ventilators
      • BiLevel: Puritan-Bennett 840 ventilator (by Covidien)
      • BiPhasic: CareFusion ventilators
      • Bi-Vent: Maquet Servo-i ventilators
      • DuoPAP: Hamilton C-1 ventilator
  • Similarities
    • Both modes allow unrestricted spontaneous breathing during and between mandatory breaths
  • Differences
    • APRV uses extreme I:E ratios (>2:1), whle Bi-Level Ventilation usually does not
      • APRV usually keeps the duration of T low at < or = to 1.5 sec, while Bi-Level Ventilation has no restriction on T low
    • APRV supplies higher mean airway pressure, but lower minute ventilation (VE) than Bi-Level Ventilation

Advantages of Airway Pressure Release Ventilation

  • Alveolar Recruitment: due to high airway pressure and diaphragmatic contraction during spontaneous breathing
  • Improved Oxygenation: spontaneous breaths allow more even distribution of ventilation (decreasing shunt)
  • Preservation of Spontaneous Breathing: with spontaneous breathing, APRV is better tolerated than inverse ratio ventilation (without the need for deep sedation/paralysis)
    • However, in the Absence of Spontaneous Breathing (i.e. During Paralysis), APRV is Functionally Equivalent to Inverse Ratio Ventilation (Due to the Relatively Long Times Spent at High Pressure)
  • Improved Hemodynamics: spontaneous breaths augment cardiac filling
  • Potential Lung-Protective Effects

Disadvantages of Airway Pressure Release Ventilation

  • Risk of Volutrauma: due to spontaneous breathing during high pressure (with concomitant generation of large tidal volumes and large negative pleural pressure swings)
  • Increased Work of Breathing
  • Increased Energy Expenditure Related to Spontaneous Breathing

Determinants of Ventilator-Provided Tidal Volume During Airway Pressure Release Ventilation

  • Lung Compliance
  • Airway Resistance
  • Timing and Duration of Pressure Release

Time Ratio in Airway Pressure Release Ventilation

  • Time Ratios Reported in Literature: 1:1 to 9:1
  • Significance of Time Ratio
    • The greater the percentage of the total time spent at high pressure (80-95%), the greater the alveolar recruitment
    • The lesser the percentage of the total time spent at low pressure (usually 0.2-0.8 sec in adults), the less alveolar de-recruitment occurs
    • If the Time Spent at Low Pressure is Too Short, Expiration Will Be Incomplete and Auto-PEEP Will Develop
      • However, Some APRV Regimens Use P Low of 0 cm H2O with the Required Development of Auto-PEEP
      • There is a Theoretical Concern About Developing Auto-PEEP, Since (Unlike Applied PEEP Which Distributes Evenly) Auto-PEEP Distributes Predominantly to Lung Units with the Highest Airway Resistance and Lowest Compliance (Chest, 1995) [MEDLINE]
        • Lung Units with Partially Obstructed Airways and Atelectatic Lung Units Will Consequently Have Higher PEEP than the Set P Low

Airway Pressure Release Ventilation Settings and Their Relationship to Gas Exchange

  • Clinical Determinants of Oxygenation (pO2)
    • FIO2
    • Amount of P high
    • Time Spent at T high
  • Clinical Determinants of Ventilation (pCO2)
    • Delta P (P high – P low): Larger Delta P = More Volume Per Release = More CO2 Excretion Per Release
    • Patient’s Spontaneous Breathing: spontaneous breathing typically only occurs during the P high (due to the short duration of time spent at P low)

Technique

Ventilator Settings/Terminology

  • P high: upper pressure level
  • P low or PEEP: lower pressure level
  • T high: time spent at P high
  • T low: time spent at T low
  • Release Rate: number of cycles (or releases) per min
    • General Comments
      • Increasing the release rate will decrease the pCO2
    • T high + T low Combinations with Their Corresponding Release Rates
      • T high 4.0 sec + T low 0.5 sec (cycle length = 4.5 sec) -> release rate = 13.3/min
      • T high 4.5 sec + T low 0.5 sec (cycle length = 5.0 sec) -> release rate = 12.0/min
      • T high 5.0 sec + T low 0.5 sec (cycle length = 5.5 sec) -> release rate = 10.9/min
      • T high 6.0 sec + T low 0.5 sec (cycle length = 6.5 sec) -> release rate = 9.23/min

APRV-REL RATE

General Approach

  • Ventilate the Lung on the Steep Portion of the Pressure-Volume Curve: where mean lung volume and pressures are adequate for oxygenation and ventilation and the tidal volume lies between the lower and upper inflection points of the curves
    • This Strategy Improves Lung Compliance, Venous Admixture, and pO2 in ARDS
    • This Strategy Also Protects the Lung in ARDS by Avoiding Collapse During Expiration (Atelectrauma) and Stretch-Related Lung Injury During Inspiration (Volutrauma, Barotrauma)
  • Minimize Sedation
    • Although, Some Sedation is Usually Required
  • Avoid Use of Paralytics
    • Using Paralytics Will Eliminate the Spontaneous Breaths, One of the Purported Benefits of APRV

Initial Settings

  • No Consensus Exists with How to Set the Initial APRV Parameters: both of the following approaches may be grossly equivalent
    • Approach #1: use short T low + P low of 0 cm H2O -> prolongs I:E ratio and creates auto-PEEP
    • Approach #2: use longer T low (to eliminate auto-PEEP) + higher P low (to avoid alveolar collapse)
  • P high: set using the plateau pressure of the current volume-controlled mode (preferably 20-30 cm H2O)
    • Target tidal volume should be approximately 6 ml/kg PBW
    • Avoid using P high >35 cm H2O, unless the patient has obesity/ascites/etc
  • P low : start at 0 cm H2O
  • T high: start at 4.5 sec
  • T low: start at 0.5 sec

Subsequent Changes

  • General Comments
    • Wait 4-6 hrs for clinical response after a change in ventilator settings
  • To Increase pO2
    • Increase FIO2
    • Increase P high (adjust in 2 cm H2O increments)
      • Usual Range: 20-30 cm H2O
    • Increase T high (adjust in 0.5 sec increments): this will decrease the release rate
      • Range: 4.0-6.0 sec
    • Decrease T low: note that as the T high:T low ratio increases, auto-PEEP can develop (which will decrease effective delta P and VT)
    • Lung Recruitment Maneuvers
  • To Decrease pO2
    • Decrease FIO2
    • Decrease T high: this will increase the release rate
    • Increase T low (adjust in 0.1 sec increments): this will increase the time spent at release
      • Range: 0.5-0.8 sec
  • To Decrease pCO2
    • Increase P high (adjust in 2 cm H2O increments): this will increase the delta P (P high – P low)
      • Usual Range: 20-30 cm H2O
    • Decrease T high (adjust in 0.5 sec increments): this will increase the release rate
    • Increase T low (adjust in 0.1 sec increments): this will increase the time spent at release
      • Range: 0.5-0.8 sec
    • Optimize Spontaneous Breathing
  • To Increase pCO2
    • Increase T high (adjust in 0.5 sec increments): this will decrease the release rate
      • Range: 4.0-6.0 sec
    • Decrease P high (adjust in 2 cm H2O increments): this will decrease the delta P (P high – P low)
      • Usual Range: 20-30 cm H2O
      • Note: this may undesirably decrease the pO2

APRV3

References

  • The effects of applied vs auto-PEEP on local lung unit pressure and volume in a four-unit lung model. Chest. 1995 Oct;108(4):1073-9 [MEDLINE]
  • Airway pressure release ventilation as a primary ventilatory mode in acute respiratory distress syndrome. Acta Anaesthesiol Scand. 2004 Jul;48(6):722-31 [
  • Other approaches to open-lung ventilation: Airway pressure release ventilation. Crit Care Med. 2005 Mar;33(3 Suppl):S228-40 [MEDLINE]
  • Does airway pressure release ventilation offer important new advantages in mechanical ventilatory support? Resp Care. 2007;52:452-460
  • Airway pressure release ventilation and biphasic positive airway pressure: a systemic review of definitional criteria. Intensive Care Med 2008;34(10):1766-1773 [MEDLINE]
  • A randomized prospective trial of airway pressure release ventilation and low tidal volume ventilation in adult trauma patients with acute respiratory failure. J Trauma. 2010;69(3):501-510 [MEDLINE]
  • Comparison of APRV and BIPAP in a mechanical model of ARDS (abstract). Respir Care 2010;55(11): 1516
  • Airway pressure release ventilation: what do we know? Respir Care. 2012 Feb;57(2):282-92 [MEDLINE]