The following is a tutorial on basic airway pressure release ventilation (APRV) setup and troubleshooting.
There are multiple theoretical advantages of APRV over conventional ventilator strategies (see review articles below) however some of the benefits specific to the COVID-19 patient population is the prevention of derecruitment and encouragement of spontaneous breathing with consequent decreased need for deep analgosedation. While APRV can be set up directly in the newly intubated patient, it is perhaps easier to transition from more a more familiar conventional modality such as AC/VC.
- After RSI, place pt on ACVC utilizing low tidal ventilation strategy per ARDSnet protocol. Initiate analgesia and sedation strategy.
- Once PEEP has been titrated per ARDSnet PEEP/O2 tables, paralysis has worn off and the patient is spontaneously breathing, consider transition to APRV
- Set Pressure high (Phigh). While still on ACVC, perform inspiratory hold and measure Plateau Pressure. The plateau pressure will be your starting Phigh in APRV. Values are typically between 20-35 cmH20
- Set Time high (Thigh)* in post-op patients and those with normal lungs, Thighs are typically between 4-6s. Longer Thigh in general will increase oxygenation. However, in the setting of COVID-19 an alternative rescue strategy utilizing shortened breath cycles may be preferable. See below for setting Thigh as a rescue strategy, APRV time controlled adaptive ventilation (TCAV), in which Thighs will be much shorter, between 1-3s.
- Set Pressure low (Plow) to 0** (see discussion below)
- Set Time low (Tlow). APRV relies on autopeep(iPEEP) to prevent derecruitment of alveoli, therefore your release time or Tlow is critical. The longer the Tlow, the lower the iPEEP and the greater the likelihood of alveolar collapse. In order to adjust Tlow we need to observe the patients expiratory flow waveform on the ventilator, targeting >50% to 75% T-PEFR (see below). This will typically be between 0.2-0.8s in restrictive lung disease and 0.8-1.5s in obstructive lung disease. The shorter the Tlow, the greater the mean airway pressure which will generally increase oxygenation.
- Spontaneous Breathing in APRV. While traditional long Thigh (4-6s) APRV requires spontaneous breathing to maintain normal minute ventilation, rapid cycling TCAV with shortened Thigh (1-3s) will provide adequate ventilation without additional patient efforts even in the setting of paralysis. In COVID-19, there is concern that abnormal respiratory drive and large Vt spontaneous breathing may be injurious. TCAV with shortened respiratory cycles can be helpful in minimizing patient efforts without the need for exceptional doses of analgesics/sedatives. During weening trials, cautious reintroduction of spontaneous breathing may be considered. On the draeger ventilator, automatic tube compensation (ATC) can be added to provide assistance overcoming the resistance of the endotracheal tube. On other vent models (PB 840, vela, servo-i) leave off pressure support outside of spontaneous breathing trials during weening.
- Troubleshooting Hypoxemia in traditional APRV (for TCAV rescue see below) Overall, to improve oxygenation we need to increase mean airway pressure and/or recruit atelectatic alveoli. Assuming your FiO2 is already 100%, consider the following steps:
- shorten Tlow up to T-PEFR 75%
- increase Phigh and Thigh simultaneously. Phigh >35 may be required in the morbidly obese
- Troubleshooting Hypercapnia in traditional APRV (for TCAV rescue see below) Mild hypercapnia without severe acidemia can be tolerated in these patients. Optimizing ventilation should be performed cautiously in a way that does not compromise oxygenation.
- Lighten sedation to encourage spontaneous ventilation.
- Increase Phigh and Thigh simultaneously.
- Lengthen Tlow by 0.05-1s increments up to 50% T-PEFR. (while this will increase tidal volumes during release, this will also decrease mean airway pressure and likely worsen oxygenation)
- Increase Phigh while decreasing Thigh (not recommended). while this will increase minute ventilation, it will also decrease mean airway pressure and worsen oxygenation.
Below is the table from the Habashi review article which details setup as well as troubleshooting. I highly encourage everyone to read the review prior to your first attempts using APRV.
T-PEFR – ventilator flow waveform (resusreview.com)
Great Review article by Nader Habashi on APRV
APRV by Habashi
Thigh – APRV time controlled adaptive ventilation (TCAV) as RESCUE* While Thighs are routinely set at 4-6s in the postop or trauma population, in patients with COVID-19 Dr. Habashi recommends shortening the respiratory rate considerably to improve bulk ventilation. Transitioning from conventional modalities use the following formula to calculate Thigh. As discussed above, short Thighs may also suppress spontaneous breathing in COVID-19 patients with abnormal respiratory drive and may potentially protect patients from self injury.
(60/current rate) – TLow (if RR 20, then 60/20=3 , if Tlow 0.5s then rescue Thigh is 2.5s)
The full APRV Rescue protocol is below from http://www.APRVnetwork.org
APRV TCAV Rescue Strategy Strategy Guidelines 2020
Alternative approach to Tlow/Plow settings**. (skip this part until comfortable with Habashi method) An alternative strategy for APRV has been proposed by Zhou et al. The Zhou method is notably different in their approach to determining Tlow which is determined by starting with a Tlow of ~1s, multiplied by the time constant (resistance*compliance) and then targeting T-PEFR >50%. In addition, Zhou also utilizes a starting Plow of 5 cmH20, while Habashi recommends Plow of 0. Many pulmonary physiologists criticize APRV because it relies on autoPEEP(iPEEP) to prevent derecruitment of alveoli. iPEEP does not uniformly affect the lung. iPEEP will recruit healthy alveoli with increased compliance preferentially over diseased low compliant alveoli which could potentially worsen atelectatrauma. With that in mind, per the starling resistor model extrinsic PEEP and iPEEP are not additive unless extrinsic PEEP exceeds iPEEP. Adding additional extrinsic PEEP with the ventilator (eg. Plow 5 cmH20) would likely not affect healthy alveoli already stented by iPEEP but may prevent the full derecruitment of diseased/low compliance/stiff alveoli. Adding Plow may prolong the release time necessary to reach T-PEFR 50-75%.
Below is Zhou’s alternative APRV initiation and titration strategy taken from their study protocol. Of note, Zhou utilized Puritan Bennett ventilators (like our 840s) which likely explains some of the differences in initiation/setup. While 840s can be used to provide APRV, the setup is less straight forward than on the VELAs and Draegers.
More recent review article summarizing trials as well as reviewing alternative strategy by Zhou.
EMCRIT post on APRV
another site with tutorial based on Habashi APRV strategy
Any questions about setting this up? Grab me while I’m working clinically and I’ll walk you through it or call me anytime with questions 917-749-1004.