Do you recall our 42-year-old patient admitted to the hospital with acute pancreatitis who subsequently developed severe ARDS? (See ARDS: Part 1.)
Several days into his illness, our patient developed respiratory distress that rapidly progressed to ventilator-dependent respiratory failure meeting the clinical criteria for diagnosis of severe acute respiratory distress syndrome (ARDS) (Ranieri et al., 2012):
- Acute onset
- Bilateral opacities
- Refractory hypoxemia
- FiO2/PaO2 ratio ≤ 100
Upon recognition of ARDS and continued decline in respiratory status, ventilator settings were adjusted; tidal volume was decreased to 6 mL/kg of his ideal body weight, PEEP was titrated up to 10 cm H2
O which allowed us to decrease his FiO2 to 60%. Several hours later, he had a precipitous drop in SpO2
to 78% and no improvement with upwards titration of FiO2
. He began over breathing the ventilator (rate set at 18) and a dyssynchronous breathing pattern was observed. A decision was made to attempt a recruitment maneuver (RM) during which 30 cm H2
O of positive pressure was applied for 30 seconds. The response was not clear, a second RM was attempted, this time applying 40 cm H2
O for 40 seconds. The patient remained hemodynamically stable and SpO2
improved, PEEP was increased 16 and SpO2
was persistently at 92% which was considered acceptable within the goals of ARDS. Several hours later, his FiO2 was titrated down to 55%.
The cornerstone of treatment for ARDS is mechanical ventilation. The challenge of mechanical ventilation is the risk of lung injury and barotrauma. Treatment strategies for ARDS are aimed at minimizing ventilator-induced lung injury (VILI) while maximizing ventilation and oxygenation, all aimed at reducing morbidity and mortality associated with ARDS. The most recent clinical practice guidelines in the management of ARDS endorse the following strategies (Fan et al., 2017):
- Low tidal volume ventilation (all patients with ARDS)
- 4-8 mL/kg predicted body weight with lower inspiratory pressure to target lower plateau pressures < 30cm H2O
- Adjust tidal volume to maintain goal plateau pressures.
- Higher positive end-expiratory pressure (PEEP) as opposed to lower PEEP (in those with moderate to severe ARDS)
- Recruitment maneuvers (in those with moderate to severe ARDS)
- A transient, sustained increase in airway pressure with goal to open collapsed alveoli
- Involves applying high PEEP for a specified time and evaluating improvements in oxygenation (example: 30-40 cm H2O PEEP for 30-40 seconds)
- Prone positioning (severe ARDS) for > 12 hours/day (involves placing patient in prone position while on ventilator, shifts weight of heart to ventral wall)
Remember that the pathophysiology of ARDS involves diffuse alveolar damage due to accumulation of protein-rich inflammatory mediators in the alveoli causing non-cardiac pulmonary edema. The overall effect is stiffness and non-compliance in the alveoli, dead space, and impaired gas exchange leading to hypoxia and ineffective ventilation.
Recommendation: Low tidal volume ventilation
Let’s start with the primary recommendation for ARDS, low tidal volume ventilation. Although over-simplified in description, the changes in the lung as a result of ARDS lead to a significant decrease in the area of the lung (or alveoli) available for gas exchange. If one attempts to ventilate the ARDS lung at volumes and pressures required for a non-ARDS lung, this would lead to over-ventilation, barotrauma and ventilator-associated lung injury (VALI). Since a goal of ventilation in ARDS is to avoid VALI, providing tidal volumes to match the presumed available lung can provide the same amount of oxygen exchange with less risk for over ventilation and less risk of alveolar overdistention. Attempts to ventilate the stiff lung can lead to high airway pressures. Low tidal volume ventilation is considered “lung protective ventilation.”
Other strategies for moderate to severe ARDS include higher PEEP and recruitment maneuvers which can decrease atelectasis and improve end-expiratory lung volumes. PEEP is intended to keep alveoli open. PEEP is pressure applied to alveoli by the ventilator at the end of expiration. When alveoli are open, air exchange can take place. Have you heard the balloon analogy to understand PEEP? Inflating a new balloon (collapsed alveoli) can be difficult, once some air (pressure) is applied, it becomes easier to inflate. The goal of PEEP is to keep the alveoli open (preventing collapse) during expiration so air exchange continues to take place during expiration and less pressure is necessary to inflate the alveoli with the next inhalation. The pressure to overcome the initial resistance in a collapsed alveolus (deflated balloon) will not be required if the alveoli remains open. In the ARDS lung, the alveoli are stiff and difficult to inflate, adequate PEEP can improve air exchange and is often titrated up to decrease FiO2
Recommendation: Recruitment maneuvers
Recruitment maneuvers (RM) involve applying increased airway pressure to transiently increase transpulmonary pressures (difference between airway and pleural pressure) with a goal of opening up collapsed lung and increasing the number of alveoli participating in air exchange.
There are several methods of applying RMs involving varying pressures and duration and may include prolonged high continuous positive airway pressures (30-40 cm H2
O) or incremental increases in PEEP until attaining the desired physiologic improvements. The technique used will typically be that which is preferred or most familiar to the ordering provider, or the technique accepted and utilized by the facility at which you practice.
The application of both PEEP and RM place a patient at risk of overdistention and barotrauma from the application of higher pressures and should not be used without experienced clinical judgement and understanding of hemodynamics and potential physiologic change associated with the maneuvers. Furthermore, the RM may require the use of increased sedation and/or paralytics for the patient to tolerate. The use of high level of sedation and paralytics may cause further hemodynamic compromise and do not come without risks.
Now back to our patient. When we last discussed him, he was on AC mode of ventilation with tidal volume at 6 mL/hr, RR 18, FiO2
60% and PEEP of 16. Again, he began to decline from a respiratory standpoint. A decision was made to attempt prone positioning.
Recommendation: Prone Positioning
Prone positioning involves placing a ventilated patient on their abdomen. Theoretically, by placing the patient in the prone position, you can achieve expansion of dependent alveoli and reduce pressure on the lungs from cardiac structures and abdominal organs. The most recent guidelines for ventilation in the patients with ARDS recommend this strategy for 12 hours per day in those with refractory hypoxemia and ARDS. The use of this strategy requires a policy and procedure endorsed by your institution with strict protocols to reduce patient injury.
In order to utilize prone positioning, the practice must be supported by the hospital or facility that you work for as its success is dependent on the coordination of many staff members from multiple disciplines including nursing, medicine, respiratory, and wound care. It often requires 1:1 ICU nursing care as well. Each facility will have a specific list of contraindications but in general, the procedure is contraindicated in those with unstable fractures to the face, spine, cervical spine, pelvis, ribs or femur, or any spinal instability, and those with intracranial hypertension. Other common relative contraindications include pregnancy, life-threatening arrhythmias, burns or open wounds to the ventral surface, acute bleeding, shock, multiple trauma, increased ICP greater than 30 mm Hg, and cerebral perfusion pressure less than 60 mm Hg. Furthermore, each patient should be evaluated individually for potential contraindications to prone positioning. Each institution may have specialty beds or devices to assist in prone positioning and it always requires a highly coordinated effort from multiple disciplines. Successful prone positioning can decrease morbidity in severe ARDS.
Our patient did eventually recover from ARDS but required tracheostomy due to his long-term ventilator dependence and associated deconditioning due to a prolonged course of acute illness. He was discharged to a skilled nursing facility for rehabilitation. Following a patient with ARDS does not come without its challenges. It can be daunting to watch a patient deteriorating under your care while knowing the high mortality associated with ARDS and the limited treatment options. It can be equally rewarding to feel confident knowing that you and your team provide the most up-to-date, appropriate care available in the treatment of this condition.
Please share you experience with ARDS or placing a patient in prone positioning!
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Fan, E., Del Sorbo, L., Goligher, E.C., Hodgson, C.L., Munshi, L., Walkey, A.J.,…Brochard, L.J. (2017). An Official American Thoracic Society/European Society of Intensive Care Medicine/Society of Critical Care Medicine Clinical Practice Guideline: Mechanical Ventilation in Adult Patients with Acute Respiratory Distress Syndrome. American Journal of Respiratory and Critical Care Medicine, 195(9), 1253-1263. doi: 10.1164/rccm.201703-0548ST
Han, S.H. & Mallampalli, R.K. (2015). The Acute Respiratory Distress Syndrome: From Mechanism to Translation. The Journal of Immunology, 194(3), 855-860. doi: https://doi.org/10.4049/jimmunol.1402513
Howell, M.D. & Davis, A.M. (2018). Management of ARDs in Adults. JAMA, 319(7), 711-712. doi: 10.1001/jama.2018.0307
Ranieri, V.M., Rubenfeld, V.M., Thompson, B.T., Ferguson, N.D., Caldwell, E., Fan, E., Camporota, L. & Slutsky, A.S.; ARDS Definition Task Force. (2012). Acute respiratory distress syndrome: The Berlin Definition. JAMA, 307(23), 2526-2533. doi: 10.1001/jama.2012.5669
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