(Previous posts: work of breathing, mode of ventilation, ventilator alarms)
NOTE: this is NOT a peer-reviewed post (pending) and is continuously under construction on the sinaiem.org website!
(Last update: 4/18/20)
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-4.46.06-PM-1024x482.png)
Airway pressure release ventilation (APRV) was used historically as a rescue mode for ARDS, which is classically characterized by heterogeneous lung injury resulting in uneven distribution of ventilation.
The two main theoretical advantages of APRV for intubated COVID patients are:
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-18-at-10.42.52-AM.png)
The benefits of CPAP (covered in work of breathing) include shifting to a more compliant region (steeper slope) of the lung-chest wall compliance curve (which helps work of breathing) and for PEEP to reduce shunt.
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.06.04-PM-1024x436.png)
Allowing the moderately-sedated patient to maintain spontaneous breathing is also advantageous for better V/Q matching:
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.12.11-PM-1024x357.png)
Suppose the COVID patient has progressed to the low-compliance (e.g. C = 20 mL / cm H2O) inflammatory state.
CPAP no longer shifts much along the compliance curve (corresponding to a plateau pressure of 20 cm H2O, assuming VT = 400 mL), and there is increased WOB.
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.25.59-PM-1024x538.png)
Define Phigh to be the high pressure setting of APRV.
This is empirically set equal to plateau pressure, Pplat = Phigh.
(note that Phigh is not known a priori, but is the theoretical sweet spot of compliance between derecruitment / overdistention. It needs to be titrated while observing oxygenation / ventilation)
Spontaneous breaths are still allowed at this high pressure: a transpulmonary pressure ∆P = 3 cm H2O can generate a VT = (3 cm)(40 mL/cm H2O) = 120 mL.
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.31.09-PM.png)
Suppose Phigh is held for time Thigh and is then “released” briefly (i.e. airway pressure release) before returning back to Phigh.
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.36.57-PM-1024x633.png)
Let “briefly” be defined as time Tlow.
This variable is important to:
- allow the volume of air held during Thigh to be ventilated
- intentionally leave safe levels of auto-PEEP to prevent derecruitment
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.43.03-PM-1024x647.png)
Plow = 0 in order to maximize the pressure gradient during the brief release period Tlow for maximal ventilation.
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.51.01-PM.png)
Attention needs to be paid to the end-inspiratory volumes (overdistention) generated by the patient as these can lead to microvascular lung injury (volutrauma).
In summary, near-continuous CPAP at Phigh (i.e. high mean airway pressures) is achieved while allowing spontaneous breaths and mandatory ventilation at the release frequency.
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.52.54-PM-1024x589.png)
Because the clinician sets the the time trigger and cycle, one breath cycle is equal to a total time, T = Thigh + Tlow , and the inverse of this is the mandatory respiratory rate (RR).
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-18-at-10.57.30-AM-1024x273.png)
RR is also known as the pressure release frequency, which can be titrated for hyper/hypo-carbia if the patient is not breathing spontaneously.
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.53.43-PM-1024x604.png)
Inverse-ratio ventilation was also used as rescue mode of ventilation to moderate-severe ARDS, but failed to demonstrate significant improvement in mortality when compared to conventional modes.
Implementation
- Set starting pressure parameters: Phigh = Pplat (initial guess of sweet spot of compliance curve), Plow = 0.
- Set starting time parameters: if the patient is not breathing spontaneously, Thigh = 1.7 and Tlow = 0.3 would yield a total T = 2 seconds/breath and a mandatory RR = 1/T = 30 breaths/min. Increase Thigh if inspiratory efforts present.
- Examine Flow-time curve. Tweak Tlow such that expiratory phase stops 50-75% of peak expiratory flow rate (intentional auto-PEEP).
- Set FiO2 initially to 100%, titrate downward to SpO2 target 90-96%. May need to change Phigh as needed.
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-6.12.21-PM-1024x691.png)
Recall the phase variables characterizing a mode of ventilation; for example, assist/control flow-targeted, volume-cycled (a.k.a. volume-controlled):
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-4.57.53-PM-1024x162.png)
In APRV, spontaneous demand breaths are triggered at the two alternating baseline pressure levels and cycled by time:
![](http://sinaiem.org/wp-content/uploads/2020/04/Screen-Shot-2020-04-16-at-5.56.41-PM-1024x228.png)
APRV in action, titrated to SpO2 90%. This patient was previously on A/C-VC 24 | 530 | 100% | 15 with a Pplat = 30 cm H2O.
Note: (optional) A pressure support (PSV) of 8 cm H2O was added to overcome work from resistive circuit during spontaneous breaths.