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
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]
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)
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
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
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]
