The physiologic rationale
ARDS is a heterogeneous disease process, characterized by a mix of relatively normal, collapsed, fluid-filled, and consolidated alveoli. The functional lung tissue is relatively small, and has been described as the “baby lung” (Fig 1). Cyclical opening and closure of collapsed alveoli leads to shear stress at alveolar interphases and leads to ventilator-induced lung injury (VILI). Ventilation with low tidal volumes may prevent overdistension; however, it cannot prevent cyclical opening and closure of collapsed alveoli. There may be collapsed, potentially aeratable units in patients with ARDS that may require higher opening pressures compared to the airway pressures attained during tidal ventilation. Recruitment maneuvers (RMs) aim to open collapsed alveoli with a sustained increase in airway pressure. Following this, an appropriate level of PEEP is applied to keep the lungs open. Through the use of this strategy, we aim to open up alveoli that are potentially “recruitable”. Keeping them open will hopefully, prevent damage resulting from cyclical opening and closure. Furthermore, as collapsed alveoli open, oxygenation may improve.
When to perform RMs?
There is scant evidence to support appropriate indications to perform RMs. RMs are generally resorted to in patients with moderate to severe hypoxemia with a P/F ratio of less than 150 as a rescue intervention. RMs may be indicated only in the early stage of ARDS (within the first 5 days). RMs may also be effective in enhancing recruitment of dorsal areas of the lung during prone ventilation.
Performing an RM involves a transient increase in intrathoracic pressure. A rise in the intrathoracic pressure may lead to reduced venous return and preload, with a fall in cardiac output and cause hypotension. Hence, it is important to assess volume responsiveness prior to an RM and infuse intravenous fluids if appropriate. A rise in intrathoracic pressure also increases pulmonary artery pressures and the afterload to the right ventricle (RV). Excessive afterload may precipitate RV dysfunction and hemodynamic instability. RV dysfunction may be monitored in real time by bedside transthoracic echocardiography. Application of excessive pressure may be harmful in patients with chronic obstructive pulmonary disease and emphysema; the risk of inducing a pneumothorax is increased in these patients. Clearly, in a patient with a pre-existing pneumothorax or bronchopleural fistula, RMs are contraindicated. Any significant rise in the intrathoracic pressure may be transmitted to the intracranial compartment; hence, RMs may be inadvisable in patients with raised intracranial pressure, including traumatic brain injury. Importantly, it is inadvisable to attempt RM in patients in the late phase of ARDS (more than 5–7 days). At this stage of ARDS, the potential for recruitment is low due to the onset fibrotic changes.
How to perform RMs
There are different methods of performing RMs. The most commonly used method until recently was the sustained inflation method, wherein a pressure of 30–40 cm of H2O is applied for 30–40 seconds in the CPAP mode.1 However, this method may not be the most optimal method of applying an RM. In contrast to sustained pressure, multiple cycles of tidal ventilation may be required after an increase of PEEP before the functional residual capacity increases and stabilizes at a particular level.2 Hence, the time over which pressure is sustained may be a less important factor in recruitment compared to the level of applied pressure. The efficacy of recruitment fades over the duration for which it is applied; an arbitrary duration of 30–40 seconds may be too lengthy and lead to a higher incidence of complications, particularly, increased right ventricular afterload and hemodynamic instability leading to hypotension.2
A more optimal method may be the “staircase” method of performing an RM. On the pressure controlled ventilation mode, the PEEP level is increased in a stepwise manner. The inspiratory pressure is set at 15 cm of H2O above the PEEP level. The PEEP level is increased to 20, 30, and 40 cm H2O. At each step, the PEEP level is sustained for 2 min, while the change in oxygen saturation is observed and blood pressure is continuously monitored. After a maximum inspiratory pressure around 55 cm H2O is reached, the PEEP level is titrated backwards at 3 min intervals in a stepwise manner, by 2.5 cm of H2O at each step to a minimum of 15 cm H2O or until a decrease in oxygen saturation by 1–2% is observed. This is defined as the decruitment point. A “re-recruitment” maneuver is then performed, followed by setting the PEEP at 2.5 cmH2O above the de-recruitment point.3
What is the evidence?
There have been about seven RCTs that have addressed the use of RMs in patients with ARDS. Most of these have used a higher PEEP level as a co-intervention; hence, it is difficult to evaluate the effect of RMs alone. RMs have generally shown to improve the P/F ratio; however, the effect appears to be transient. One of the studies that used RMs without co-interventions included 110 patients in which all subjects received a low tidal volume strategy of 6–8 ml/kg of the predicted body weight.4 RM was performed by a sustained inflation maneuver of 40 cm H2O maintained for 40 seconds, conducted every eight hours for the first five days. This study showed significantly improved ICU mortality and ventilation-free days; however, there was no difference in hospital or 28-day mortality. No significant hemodynamic instability or barotrauma were noted.
The latest RCT that addressed the effect of RMs was the Alveolar Recruitment for Acute Respiratory Distress Syndrome Trial (ART).5 The control group received PEEP and FiO2 titrated according to ARDS-net table. The intervention group received the following interventions in a stepwise manner:
- Pressure controlled ventilation with a driving pressure (delta p) of 15 cm H2O
- PEEP of 25 cm H2O for 1 min, 35 cm H2O for 1 min, and 45 cm H2O for 2 min (reduced to 25-30-35 for 1 min each, starting with the 556thof 1010 patients, as three patients suffered cardiac arrest with the higher pressures)
- Volume controlled ventilation with PEEP of 23
- Decrease PEEP by 3 cm H2O at each step; 4 min at each decremental level of PEEP (changed to 3 min at each level); down to a minimum of 11 cm H2O)
- Measure compliance at each step
- PEEP with the best compliance + 2 cm H2O was set (optimal PEEP)
- Switch to PCV again, another one-step, 2 min recruitment: PEEP 45 cm H2O, (changed to 35) delta p 15 cm H2O.
- Following the RM and PEEP titration, the tidal volume was maintained between 4–6 ml/kg predicted body weight to keep plateau pressure ≤ 30 cm H2O.
Outcomes were significantly worse in the recruitment group. The primary outcome, 28-day mortality, was significantly higher in the recruitment group. Six-month mortality was higher with RMs; ventilator-free days at 28 days was more in the control arm. The incidence of barotrauma, including pneumothorax, was also significantly higher with RMs.
We do not perform RMs routinely for patients with moderate to severe ARDS in our practice. In patients with a low P/F ratio, we perform upward titration of PEEP. In the pressure-controlled mode, we increase the PEEP in a step-wise manner, with increments of about 5 cm of H2O at a time, to a maximum level of 15–20 cm of H2O. The driving pressure is held constant. We evaluate the patient at each incremental level of PEEP for at least 10–15 minutes. The compliance and oxygen saturation are monitored closely at each level of PEEP; we select a final PEEP level that we feel offers the best compliance. An improvement in oxygen saturation with incremental PEEP would also suggest “recruitability”.
The bottom line
- The use of high plateau pressures of up to 60 cm of H2O, even for brief periods, may lead to adverse outcomes, if used as a routine intervention in patients with moderate to severe ARDS. The poor outcomes including three instances of cardiac arrest and significantly higher barotrauma in the intervention group in the ART trial clearly demonstrated this.
- The efficacy of RMs may depend on the extent of “recruitability”. If RMs are performed on lungs that are non-recruitable, it may lead to harm.
- Improvements in compliance, oxygen saturation, and driving pressures are commonly employed to evaluate the efficacy of RMs; however, it is unclear whether these parameters are sufficiently predictive of recruitability.
- Future research is necessary to evaluate more optimal tools to assess recruitability. Lung ultrasound needs to be investigated as a possible option to evaluate the efficacy of RMs.
1. Passos AMB, Valente BCS, Machado MD, et al. Effect of a Protective-Ventilation Strategy on Mortality in the Acute Respiratory Distress Syndrome. N Engl J Med. 1998:8.
2. Marini JJ. Recruitment by sustained inflation: time for a change. Intensive Care Med. 2011;37(10):1572-1574. doi:10.1007/s00134-011-2329-7
3. Hodgson C, Cooper DJ, Arabi Y, et al. Permissive Hypercapnia, Alveolar Recruitment and Low Airway Pressure (PHARLAP): a protocol for a phase 2 trial in patients with acute respiratory distress syndrome. Crit Care Resusc J Australas Acad Crit Care Med. 2018;20(2):139-149.
4. Xi X-M, Jiang L, Zhu B, RM group. Clinical efficacy and safety of recruitment maneuver in patients with acute respiratory distress syndrome using low tidal volume ventilation: a multicenter randomized controlled clinical trial. Chin Med J (Engl). 2010;123(21):3100-3105.
5. Writing Group for the Alveolar Recruitment for Acute Respiratory Distress Syndrome Trial (ART) Investigators, Cavalcanti AB, Suzumura ÉA, et al. Effect of Lung Recruitment and Titrated Positive End-Expiratory Pressure (PEEP) vs Low PEEP on Mortality in Patients With Acute Respiratory Distress Syndrome: A Randomized Clinical Trial. JAMA. 2017;318(14):1335. doi:10.1001/jama.2017.14171