Example of common I:E ratio setting:
APRV 4:1 vs. AC 1:2

      •   It will increase mean airway pressure (Pmean) without increasing peak pressures.

      •   It allows for superimposed spontaneous ventilation by the patient.

      •   There is no respiratory rate on APRV; the cycles are measured as “releases” per minute.

When to use this mode of ventilation

      •   Patients with non-obstructive pulmonary physiology (ARDS) who are difficult to oxygenate using conventional modes. (Reference 1, 2)

                  Example, a patient with ARDS on AC on FiO₂ 100%, PEEP 15–20

Parameters and what they mean

      •   P-High: it is the upper pressure. The airway stays at this level for most of the cycle.

                  The higher the P-High, the higher the risk of barotrauma

                   P-High will V_T better oxygenation

                   P-High will V_T worse oxygenation

     •   P-Low: it is the setting to which the airway pressure drops intermittently, also known as “airway release”.

                  P-low will V_T

                  P-Low will V_T

     •   T-High: this is the time spent during P-High.

                  The longer the T-High the longer the alveoli stay open.

                  The longer the T-High the better oxygenation.

                   T-High will frequency of “releases” per minute PCO₂

                   T-High will frequency of “releases” per minute PCO₂

                  It should be set 8–12 times longer than the T-Low.

     •   T-Low: is the time spent on P-Low.

                  Alveoli collapse occurs during this time.

                  It should be kept short to avoid de-recruitment.

                  If this time is too long, it will result in worsening oxygenation.

                  If it is too short, it will cause hypercarbia.

     •   Termination Peak Expiratory Flow Rate (TPEFR): It is an expiratory flow rate termination point.

                  Used to determine the optimal duration of T-Low

                  Expiration is terminated early in order to create an auto-PEEP and keep the alveoli open.

                  Experts have recommended allowing an expiration of 50% – 75% of normal.

                  T- low will bring TPERF closer to 75% and will improve hypoxemia.

                  T-low will bring TPEFR closer to 50% and will improve hypercarbia.

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