APRV Flashcards
What is APRV
Inverse ratio, pressure controlled, and time cycled ventilation mode
Applies a continuous airway pressure (Phigh) that is similar to CPAP but has a breif time cycled released phase at a low pressure (Plow) to allow for the removal of CO2
Purpose of APRV
Maintain lung volumes as well as promote alveolor recruitment
What Diseases is APRV used for
This mode is most commonly used for ARDS where it is difficult to oxygenate and recruit the lung
New reserach has soon that to be the most effective APRV should be started at the initiation of ventilation rather than after ARDS has been diagnosed
Has been used for patient with COPD as they will be able to maintain spontaneous breathing
Advantages of APRV
- Lung protective strategy
- Easy to manipulate MAP as well as I:E
- Can help both oxygenation and ventilation
- Is argued to be a more comfortable more mode as the patient is able to breath spontaneously on this mode
- However, in the ICU there are few patients who are breathing spontaneously and any of the positive effect that are derived from APRV are due to the patient spontaneously breathing
- Patient will not need to be paralysed on this mode but made need to be sedated for oxygenation/WOB issues
Disadvantages of APRV
May result in respiratory muscle atrophy if the patient is on this mode for prolonged periods
Theory Behind APRV
Can be thought of as an elevated CPAP pressure (which aids in oxygenation) with a brief intermittent release in airway pressure (which aids in ventilation)
What Kind of Ventilatory Support will APRV give
Can provide full ventilatory support in an apneic patient, but spontaneous breath is desired
Why is Spontaneous Breathing Preferred When Pt is in APRV
*This is the same for spontanesou breathing in all modes
- Contributes to the overall minute ventilation
- Improved V/Q mismatching and results in better gas exchange for a given volume
- Better recruitment and surface area for gas exchange
- Allows for a high MAP at a safe plateu pressure
- Will help with the retention of the thoracic pump mechanism
- Can reduce sedation levels
- Instead of a RASS of -4 to -5 which is done to reduce metabolic rate (VO2 and VCO2) patient can be more alert at a RASS of -2
- Some sources say titrate sedation to a goal MVspontto at least 10% of total MV
Elevated CPAP Pressure Does What
Results in a higher mean airway pressure and improve oxygenation at the same (safe) plateau pressure
This is what is responsible for recruiting the difficult to recruit alveoli
Link Between Thigh and Expiratory Flow Rate
Tlowwill commences once Phighis released, and at this point the expiratory flow rate is the highest (100%) which is equal to PEFR
Tlow should terminate at the time taken for PEFR to reduce to between 25-50% of its peak
Expiratory flow is not finished before inhalation occur, we are doing this in order to maintain PEEP so that even though our PEEP is set at zero we still have some auto PEEP.
What are the Settings that you set
- There will be two set pressures
- Plow which corresponds to PEEP (release phase)
- Phigh which corresponds to the high “CPAP” level
- There is also two times that you will be setting
- Thigh which is the time spent at Phigh
- Tlow which is the time spent at Plow (time at the release phase)
- There will be no RR set as that will be determined through TCT
- On some machines you will have the ability to set a pressure support breath
What Will be Total PEEP in APRV
Total PEEP will be determined by adding Plow plus auto PEEP
What Will be Frequency in APRV
Frequency is 60 seconds divided by the sum of Tlow plus Thigh
APRV is most successful with a limited number of releases. Thus, ventilator frequency should remain around the 10- 12 range. Increases outwith this range promotes derecruitment, and risks a return to refractory hypoxaemia.
Settings When Transferring from Conventional Ventilation
Phigh: Match Pplat on current mode (max 30 cmH2O)
Plow: Set to 0 cmH2O
Thigh: 4.0 sec
Tlow: 0.5-1.0 sec (often 0.8 sec)
Phigh
Transition from conventional ventilation – set Phigh as the plateau pressure in volume-cycled mode or peak airway pressure in pressure-cycled mode
Phigh >30 cmH2O may be necessary in patients with decreased thoracic/abdominal compliance (e.g. tense ascites) or morbid obesity.
Plow
We can leave this at zero because the time at this setting is so short that due to air trapping, we will not be derecruiting the lung
Tlow
Want to adjust to ensure that end expiratory flow is 25-50% of PEFR
I:E in APRV
4:1 or greater
We want to spend 90-95% of the time in Phigh
Settings When Initiating Ventilation on APRV
Phigh: Set at 30 cmH2O
Plow: Set to 0 cmH2O
Thigh: 4.0sec
Tlow: 0.5-1.0 sec (often 0.8 sec)
Setting APRV on G5
Called-APRV
Attempts to synchronize transitions
PS set as a delta pressure, from Plow
Setting APRV on PB 840
Called-BiLevel
Attempts to synchronize transitions
PS set as a delta pressure, from Plow
Setting APRV on Servo-i
Called-Bi-Vent
Attempts to synchronize transitions
PS set separately for each pressure level
Setting APRV on Evita XL
Called-APRV
Time-triggered, time-cycled only
No pressure support options
Increasing Ventilation in APRV
Decreasing PaCO2
- Weaning sedation/increasing spontaneous ventilation
- Increase Phigh
- This is increasing your tidal volume which will increase you minute ventilation and decrease your CO2
- Decrease Thigh
- Optimize Tlow to between the 25-50%
- Increase Tlow
Increasing Oxygenation in APRV
Increase FiO2
Increase Phigh
Increase Thigh
Decrease Tlow
Weaning of APRV
- Weaning is achieved through decreasing Phigh and increasing Thigh
- The ultimately goal is to wean the patient to a low CPAP
- Because patient can breath spontaneously on this mode we will be trying to wean sedation and then wean APRV to a CPAP mode (bring pressure down and then hold them for long) with periodic releases
- Typically you decrease to Phigh to 15 and Thigh to 15 seconds
- The minute volume generated by release volumes decreases and is gradually supplemented by increased spontaneous minute volume, until the patient has essentially been weaned to pure CPAP.
- **Lack of clinician knowledge/comfort
- Concerns about over-distension/over-stretching
- This risk actually increases with spontaneous breathing
- No RCT showing improved outcome in humans (yet)
- It is very hard to enrol people in this type of study as we do not know if will get ARDS
- May be contraindicated in air leak syndromes and conditions of TBI/high ICP
- This is because we are often using permissive hypercapnia with this mode
- We are hold 30 cmH2O in the patient lungs which has shown to be a safe level so that will help to limit over stretching
Auto PEEP Determination in APRV
We are determining auto PEEP via flow instead of just looking at auto PEEP and then adding on PEEP because we want the max delta PEEP in order to get tidal volume
When Should We Assess for Optimial Tlow
As the lung is progressively recruited release volumes will increase.
Tlow should be re-evaluated at least every 1-2 hours in the first six hours after initiation of APRV to ensure release volumes do not exceed 6-8 ml/Kg IBW
Tlow settings should also be re-evaluated after a change in pressure settings.
If appropriate shortening of the Tlow still results in excessive release volumes (> 8ml/Kg IBW), then the patient does not have poor compliance and the rationale for the use of APRV should be reviewed and alternative ventilation options considered.
What About the pathology of ARDS make APRV favorable
For patient with ARDS the FRC and compliance is reduced, and WOB is elevated, through applying Phigh FRC will be restored and inspiration will start from a more favorable pressure-volume relationship, which will help facilitate spontaneous ventilation and improve oxygenation
APRV is What Type of Lung Protective Stratgy
APRV is an open lung strategy as by applying Phigh for 80-90% of the TCT the mean airway pressure will be increased in an almost constant lung recruitment
Mean Airway Pressure in APRV= (Phighx Thigh) + (Plow x Tlow)
(Thigh+ Tlow)
What is the Advantage of a Pt Breathing Spontanesouly
Spontaneous breathing helps drive the inspired gas to the nondependent lung regions by using the patient’s own respiratory muscles and through pleural pressure changes without raising the applied airway pressure to a rather dangerous level, as in conventional mechanical ventilation, producing more physiological gas distribution to the nondependent lung regions and improving ventilation/ perfusion (V/ Q) matching
