Yongfang Zhou, Xiaodong Jin published a study in Intensive care medicine journal about using APRV early in ARDS.
WHAT THEY DID
Randomized adult patients with ARDS (Berlin definition, P:F ratio <250) who were on mechanical ventilation to APRV(Airway pressure release ventilation) or LTV(low tidal volume) mode of mechanical ventilation. Patients were enrolled within 48 hours.
Exclusion criteria were pregnancy; anticipated duration of invasive mechanical ventilation for <48 h; intracranial hypertension (suspected or confirmed); neuromuscular disorders that are known to prolong the need for mechanical ventilation; severe chronic obstructive pulmonary disease; preexisting conditions with an expected 6-month mortality exceeding 50%; presence of
documented barotrauma; treatment with extracorporeal support (ECMO) at enrollment; refractory shock; lack of commitment to life support; age of <18 years or >85 years.
INTERVENTION
In APRV group, following initial settings: high airway pressure ( Phigh) was set at the Pplat
measured at the previous VCV settings, not to exceed 30 cmH2O; low airway pressure (
Plow) was set at 5 cmH2O. Duration of release phase ( Tlow) was initially set at one- to 1.5-fold the expiratory time constant, and then adjusted to achieve a termination of peak expiratory flow rate (PEFR) of ≥50% of PEFR; release frequency was 10–14 frequency/min; duration of Phigh
( Thigh) was indirectly calculated based on the Tlow and release frequency.
In LTV group, tidal volume target ( VT) was 6 mL/ kg predicted body weight (PBW), with allowances for 4–8 mL/kg PBW to minimize asynchrony between the patient and ventilator; PEEP levels were adjusted, guided by the PEEP-FiO2 table from ARDSnet Protocol.
In both groups, the mechanical ventilation goals were to maintain plateau airway pressure
( Pplat) at no more than 30 cmH2O, PaO2 at between 55 and 100 mm Hg (or pulse oximeter between 88 and 98%), and arterial pH at ≥7.30.
Weaning was slightly different in APRV group then the traditional APRV weaning. The weaning
process consisted of simultaneously decreasing both Phigh by 2 cmH2O and the release rate by two frequencies/min, twice daily unless the patient’s cardiopulmonary function deteriorated. In the second stage, when patients achieved the criteria with a Phigh of 20 cmH2O on 40% FiO2,
respiratory therapists also performed the weaning protocol with the SBT trial as in the LTV group. This was different than current primary APRV weaning process which is based on gradual withdrawal, using an alternate decrease in Phigh by 2 cmH2O, followed by an increase in Thigh
of 0.5–1.0 s , and extubation is assessed until the criteria of a Phigh of 12 cmH2O on 40% FiO2
is achieved.
RESULTS
Total of 138 patients with ARDS were enrolled in this intention to- treat analysis: 71 patients in the APRV group and 67 patients in the LTV group. Baseline characteristics were similar in both groups(Equal numbers for severe ARDS and APACHE Scores).
There was a shorter duration of mechanical ventilation in the APRV group. They also had shorter length of stay in the ICU, higher rate of successful extubation, and lower tracheostomy rate.
There was no difference in hospital length of stay, ICU mortality, hospital mortality, or incidence of pneumothorax between the two groups.
Compared with the LTV group, the sedation depth in the APRV group was lighter (P < 0.001) ,
and the average doses of fentanyl and midazolam were significantly lower (P < 0.01) (Fig. 2i, j), and the average doses of propofol were similar (P = 0.112).
DISCUSSION
APRV is a mode of mechanical ventilation that was described for the first time by Stock and Downs in 1987. In their initial description, the pressure was alternated from a high level to a low level with a switch while the patient breathed spontaneously between the pressure changes. They termed it as 2 levels of CPAP (Phigh and Plow) because they built a prototype using CPAP valves.
The evolution of APRV mostly revolves around modifying the CPAP and release time durations (time at expiration—TLow)
Habashi originally described the modern P-APRV (personalized APRV)breath with the following settings: (1) the inspiratory pressure (PHigh) is set to the desired plateau pressure,; (2) the THigh is typically set to occupy 90 % of the total cycle time of each breath ; (3) the TLow set based on changes in lung mechanics by analyzing the slope of the expiratory flow curve ; and (4) the expiratory pressure (PLow) is set at 0 cmH2O to minimize resistance to convective expiratory gas flow and maximize ventilation.
Theoretical advantages
- Higher Mean Airway Pressures for a given minute ventilation and peak airway pressure. APRV, due to the inverse I-E ratio, will have higher mean airway pressure (P_ aw) than conventional lung-protective ventilation (both pressure or volume control) with a normal I-E ratio of 1:2–3. In APRV, the time spent at the higher pressure is generally 80–90% of the respiratory cycle.
- Ventilation occurs with potentially less risk of ventilator- induced lung injury
- The rate of mandatory breaths will be lower than in conventional pressure or volume control ventilation.
- Work of breathing from the patient may be decreased.
APRV may be named in various ways based on the ventilator in use such as (1) APRV (Drager Evita, Savina and V series, Hamilton G5), (2) Bi-Vent (Maquet Servo-i), (3) BiLevel (Engström Carestation, Puritan Bennett 840 & 980), and (4) APRV/Biphasic (Viasys Avea). However, even more problematic than the different names is the wide variation in their implementation of APRV.
In particular, the ability to control key APRV parameters such as the TLow varies, and fine control of this parameter is critical to properly set P-APRV.
LIMITATIONS
Prior to this trial, there is only one randomized trial in trauma patients which showed both groups had similar safety profile though they used pressures of 40 cm water. There are more retrospective studies which suggest improvement in mortality.
This study was not blinded. There were more patients with coexisting chronic diseases in the LTV group than in the APRV group (P = 0.029). Furthermore, this study did not measure the
patient–ventilator interaction.
CONCLUSIONS
This study demonstrates that APRV in patients with ARDS was associated with better oxygenation and respiratory system compliance, lower plateau airway pressure, less sedation requirement, more ventilator-free days at day 28, and a shorter duration of ICU stay.
Further large blinded randomized trial is warranted to replicate and validate the results.