Disadvantages Of APRV

Airway Pressure Released Ventilation was first introduced in the late 1980s, by Dr. Christine Stock and Dr. John Browns. APRV is a time triggered, pressure limited, and time cycled ventilation that provides two levels of continues positive airway pressure (CPAP). It allows the patient to breathe spontaneously without pressure support, throughout the periods of inspiratory and expiratory phases and characterized by higher mean airway pressure. This modality of mechanical ventilation was originally used as a rescue therapy to manage critically ill patients who have difficulty in oxygenation.1 APRV reduces the risk of lung injury and provides better ventilation-perfusion matching, patient synchrony and cardiac preload than other modes that do

not provide spontaneous ventilation.
Indeed, APRV has received national attention by critical care physicians, even though there is limited evidence its effectiveness. Liu et al assessed the advantages and adverse effects of APRV compared with conventional mode of ventilation in fifty eight patients with severe ARDS2. APRV showed improvement in oxygenation and significant decrease in mortality rate with no untoward effects. This study had several limitations, the reduction in mortality rate requires further randomized controlled studies to validate these findings. Other limitations included uncontrolled variables, small patient population, and new strategies have been developed since the study was published. 2
APRV may be used as an alternative and safe mode of ventilation to manage patients with ARDS and ALI. APRV provides a potential lung protective effect. According to Acute Respiratory Distress Syndrome Network (ARNSnet), the standard ventilation strategy for patients with ARDS and ALI is a low tidal volume strategy. Clinicians are often looking for alternative methods of treatment when current ventilator strategies failed. Dr. Maxwell and colleagues discussed a study, where APRV was compared to the low tidal volume (LOVT) strategy in sixty three trauma patients at high risk for ARDS.3 The study was carried out by the use of different scoring methods to monitor the severity of illness (GCS, ISS and APACHE II). APRV showed improvement in mean airway pressure with minimum decrease in peak airway pressure. As well as, APRV group demonstrated an increase in the number of ventilator days, intensive care unit stay and ventilator-associated pneumonia. The authors explained this finding might be due to a significant higher baseline of APACHE II scores for APRV patients. Other limitations included the differences between the two groups in stabilization and weaning periods. However, there was no differences between both groups in mortality rate, PaO2/FiO2 ratio and sedation need. 3
There is a recent study, where Dr. Sareach and colleagues investigated the differences between APRV and the low tidal strategy in animals models with peritoneal sepsis at high risk for ARDS.4 In this experiment, APRV was examined to be a preventive strategy rather than supportive therapy. APRV showed to prevent lung injury by providing higher PaO2/FiO2 ratio, improving lung compliance, and higher mean airway pressure. Also, APRV maintained alveolar stability by preventing lung edema and surfactant deterioration. Base on these findings, the authors were recommended to use airway pressure release ventilation as prophylactic strategy to prevent exacerbation of ARDS. 4
Dr. Emr and colleagues presented a study to support the use of APRV in prevention of lung injury on postoperative patients.5 The clinicians attempted to demonstrate that mechanical ventilation with control mandatory ventilation settings used routinely on surgery patients causes acute lung injury, while the early application of APRV prevent lung injury. In this study, lungs with CMV presented with significant lungs changes; which include lung flooding, infiltrate, decrease lung permeability and decrease surfactant proteins. However, animal's lungs with APRV showed normal lung histology as APRV prevented alveolar recruitment. According to the authors, ARDS is not a disease to be treated, it should to be prevented. 5
Anther conceptual advantage of APRV is allowing patients unrestricted spontaneous breathing during the application of two levels of CPAP.

Facilitating spontaneous ventilation during APRV aids in alveolar recruitment, and improves distribution of lung volume to collapsed lung units. In one year retrospective study, APRV was compeered with pressure support ventilation(PSV) in eighteen patients with ALI and ARDS. Pressure support ventilation is a patient triggered, pressure limited, and flow cycled ventilation, it allows the patient to control the rate and depth of each breath. The effectiveness of spontaneous ventilation was investigated by the use of both computed tomography scan and volumetry for a period of three days.6 This study showed superiority of APRV in providing better gas distribution, pulmonary oxygenation, and decreasing lungs atelectasis faster than PSV. The clinicians recorded the main reason for this finding was derived from alveolar recruitment without overdistention during APRV. Airway Pressure Released Ventilation allows spontaneous ventilation while providing an open lung protective strategy. 6 Dr. Varpula and colleagues also compared APRV with other forms of partial mechanical ventilation, SIMV with PS, to study the effect of spontaneous ventilation in improving gas distribution. They observed no differences in clinical outcome between APRV and SIMV in gas distribution. Authors interpreted the finding due to the long study period and the differences

between the two groups in weaning periods. 7
APRV is similar to inverse ratio ventilation (IRV) with a prolong inspiratory phase (T_high) ( 4 to 6 seconds), which is associated with a tendency to increase intrinsic PEEP due to short expiratory times (auto-PEEP). This application helps to maintain an open lung and prevents repeated collapse and re-expansion of alveoli.8 However, due to improvement in the exhalation valve performance, APRV differs from IRV by allowing spontaneous breathing at any time regardless of the respiratory cycle. This explains why APRV required less sedation. For the initial setup of the higher CPAP level (P_high), it is approximately set as the patient's inspiratory pressure, plateau pressure or his ideal mean airway pressure during conventional mechanical ventilation. It is usually range from 20 to35 mmHg. P_high level controls mean airway pressure to improve oxygenation, which is interrupted intermittently to allow pressure drop to a lower level (P_low). The interval during which the pressure briefly drop to lower CPAP level the patient's functional residual capacity (FRC) reduced and allows patient exhaled carbon dioxide. The low CPAP level is initially set to zero and lasts about (T_low) .6 to .8 seconds, depending on whether or not air trapping is desired. As soon as expiratory period is completed, pressure return to higher CPAP level. 8
The duration of T_low is very critical and needs close observation to prevent alveolar derecrutiment.

Generally, T_low set closer to terminate at 50% - 70% of the peak expiratory flow rate (T-PEFR). 8 Many studies have advocated setting of T_low according to the peak expiratory flow termination.9 Inappropriate mechanical ventilation setting can lead to ventilation-induce lung injury (VILI). Dr. Kollisch-Singule and colleagues discussed a study where they hypothesized that lung injury can be reduced by modifying specific components of ventilation waveforms.9 In their study, control mandatory ventilation (CMV) was compare to APRV in analyzing the effect of mechanical breath on the lung units for both ventilation modes. During APRV when expiratory time was adjusted to regulate peak expiratory flow rate termination point to 75% , the gas distribution to the terminal airways was almost similar to that of the normal lung. Whereas, in CMV and APRV smaller percentage of peak expiratory flow termination, the gas distribution to the terminal airways was lower and increase "conduction airway micro-strain". The finding indicated that APRV with 75% of peak expiratory flow termination is the optimal setting to achieve lung protective goals.

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Generally, APRV approach is frequently used to manage patients with stiff lungs; patients with acute respiratory disease (ARDS) or acute lung injury (ALI). APRV is considered to be more effective in improving oxygenation than in improving alveolar ventilation. It is designed to maximize ventilation by recruiting collapsed alveolar units with minimizing ventilation-induced baratrouma in patients with ARDS and/or ALI. All studies which were reviewed have not shown any improvement on mortality rate with APRV. APRV shares some similar feature with other partial mechanical ventilation in improving gas distribution. Many advantages of APRV are likely attributable to the preservation of spontaneous breathing. Most of APRV studies are weakened by using animals as paradigm or used a small number of patients. Taken together, these small trills and animal studies presented advantages of APRV over conventional ventilation. However, they still cannot proved the superiority of APRV over the small volume strategy. APRV shows superior promise in some study, and further studies are needed to answer many questions. Some research papers show superiority of APRV to the conventional ventilator methods, but none of them showed any mortality differences. So, Can APRV makes mortality differences? Can APRV replaces ARDSnet? What is the best time to initiate an APRV?