APRV

Question 1

What is APRV

Answer 1

Inverse ratio, pressure controlled, and time cycled ventilation mode

Applies a continuous airway pressure (P_high) that is similar to CPAP but has a breif time cycled released phase at a low pressure (P_low) to allow for the removal of CO₂

Question 2

Purpose of APRV

Answer 2

Maintain lung volumes as well as promote alveolor recruitment

Question 3

What Diseases is APRV used for

Answer 3

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

Question 4

Advantages of APRV

Answer 4

• 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

Question 5

Disadvantages of APRV

Answer 5

May result in respiratory muscle atrophy if the patient is on this mode for prolonged periods

Question 6

Theory Behind APRV

Answer 6

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)

Question 7

What Kind of Ventilatory Support will APRV give

Answer 7

Can provide full ventilatory support in an apneic patient, but spontaneous breath is desired

Question 8

Why is Spontaneous Breathing Preferred When Pt is in APRV

Answer 8

*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

Question 9

Elevated CPAP Pressure Does What

Answer 9

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

Question 10

Link Between Thigh and Expiratory Flow Rate

Answer 10

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.

Question 11

What are the Settings that you set

Answer 11

• 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

Question 12

What Will be Total PEEP in APRV

Answer 12

Total PEEP will be determined by adding Plow plus auto PEEP

Question 13

What Will be Frequency in APRV

Answer 13

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.

Question 14

Settings When Transferring from Conventional Ventilation

Answer 14

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)

Question 15

Phigh

Answer 15

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.

Question 16

Plow

Answer 16

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

Question 17

Tlow

Answer 17

Want to adjust to ensure that end expiratory flow is 25-50% of PEFR

Question 18

I:E in APRV

Answer 18

4:1 or greater

We want to spend 90-95% of the time in Phigh

Question 19

Settings When Initiating Ventilation on APRV

Answer 19

Phigh: Set at 30 cmH2O

Plow: Set to 0 cmH2O

Thigh: 4.0sec

Tlow: 0.5-1.0 sec (often 0.8 sec)

Question 20

Setting APRV on G5

Answer 20

Called-APRV

Attempts to synchronize transitions

PS set as a delta pressure, from Plow

Question 21

Setting APRV on PB 840

Answer 21

Called-BiLevel

Attempts to synchronize transitions

PS set as a delta pressure, from Plow

Question 22

Setting APRV on Servo-i

Answer 22

Called-Bi-Vent

Attempts to synchronize transitions

PS set separately for each pressure level

Question 23

Setting APRV on Evita XL

Answer 23

Called-APRV

Time-triggered, time-cycled only

No pressure support options

Question 24

Increasing Ventilation in APRV

Answer 24

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

Question 25

Increasing Oxygenation in APRV

Answer 25

Increase FiO2

Increase Phigh

Increase Thigh

Decrease Tlow

Question 26

Weaning of APRV

Answer 26

• 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.

Answer 27

• **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

Question 28

Auto PEEP Determination in APRV

Answer 28

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

Question 29

When Should We Assess for Optimial Tlow

Answer 29

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.

Question 30

What About the pathology of ARDS make APRV favorable

Answer 30

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

Question 31

APRV is What Type of Lung Protective Stratgy

Answer 31

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

Answer 32

Mean Airway Pressure in APRV= (Phighx Thigh) + (Plow x Tlow)

(Thigh+ Tlow)

Question 33

What is the Advantage of a Pt Breathing Spontanesouly

Answer 33

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