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Mechanical Ventilation Strategy for Acute Respiratory Distress (ARDS)

By Currents Editor posted 06-15-2020 09:32

  

Tom_Kallstrom.jpgBy Peter J. Papadakos, MD, FCCM, FAARC, FCCP, professor of Anesthesiology, Surgery, Neurology and Neurosurgery at the University of Rochester; and Thomas J. Kallstrom, MBA, RRT, FAARC, executive director of the American Association of Respiratory Care

The COVID-19 worldwide pandemic has brought to light the importance of proper mechanical ventilation strategy in that the majority of patients succumb with acute respiratory distress syndrome (ARDS).  Even prior to this viral pandemic, ARDS had a high mortality of approximately between 36-52% per 100,000 depending on the agency or study you reviewed.  With such a wide range spread of mortality, it is paramount that providers on the treatment team have an understanding of the principals of ARDS management. This is especially true now that patients are being treated by providers who may not regularly treat large numbers of ARDS patients.

Several evidence-based guidelines have been generated by several critical care organizations during the pandemic — one of the major ones was the Society of Critical Care Medicine (Albazzani, 2020). The majority of these guidelines and protocols have physiologically based parameters of ventilator management at their core. A major theme across the literature is that non-physiologically based ventilator settings can increase mortality.  This is because improper settings can trigger or magnify the systemic inflammatory response (SIRS) and cause an increased cytokine storm that cannot only affect the lung but also other end organs. This fact is especially important in critically ill COVID patients who have elevated levels of cytokines.

The most important primary parameter to set is tidal volume (TV) and is the first setting that should be calculated for your patient with ARDS. The importance of TV was reinforced in the ARDS-NET trials (ARDS Network, 2000), which clearly gave us the best physiological-based range. This is 5-7cc/kg per lean body weight. There are a wide number of apps available to rapidly calculate the TV based on height and other body habitus-based parameters, such as forearm length.

Another keystone of ARDS management is lung recruitment. This strategy of lung recruitment, or open lung concept (OLC) coined by Lachmann, refers to the dynamic process of opening previously collapsed alveolar lung units by increasing trans-pulmonary pressure (Papadakos, 2007). The concept is not new but highly important as massive numbers of patients present with COVID-19.  These patients may have large areas of atelectasis prior to intubation due to progression of respiratory distress and work of breathing. This OLC may also be important in decreasing shear forces within the lung by decreasing the cyclic opening and closing of collapsed lung units. This use of OLC can also preserve and affect surfactant by stopping the cycle of surfactant depletion by ventilator-induced trauma. Surfactant is also a key in pulmonary immunity and is at the forefront of the battle against this virus. It also affects cytokine modulation and may decrease circulating levels of cytokines.

The application of positive end-expiratory pressure is also a basic principal of the management of mechanical ventilation of ARDS. In patients receiving mechanical ventilation, the term positive end-expiratory pressure (PEEP) refers to the pressure in the airway at the end of passive expiration (Acosta, 2007). In patients with COVID -19 hypoxic respiratory failure, one of the methods widely used to improve oxygenation is through the use of physiologically adjusted PEEP. This acts as a splint to keep alveolar units recruited. The majority of mechanical ventilators in use today have graphic displays to help titrate PEEP to the individual compliance of each patient to adjust to the best PEEP for that specific lung. This is done via the pressure volume curve.  It can also be calculated in non-graphic enabled vents by calculating the inflection point. The general range in this COVID sub group of ARDS in many international protocols is 10-15.

Prone positioning is another way to recruit alveolar lung units in ARDS from COVID-19. Prone was first proposed in ARDS management in the 1970s. A subsequent observation of oxygenation improvement with simple rotation of the patient to the prone position has demonstrated a significant mortality benefit (Schulten, 2017). The use of the prone position is especially true in COVID-19 pandemic where it has been used by centers in Italy, Spain and Korea. We are currently using it actively in my center. Nurses should develop a protocol on how to safely prone a patient and maintain them sedated without causing complications, such as pressure sores and damage to eyes.

The key to maintain proper mechanical ventilation also hinges on proper sedation and chemical paralysis. It is important to realize that ARDS ventilation especially in the prone position will need to have ongoing titration of sedation and paralysis to best deliver mechanical ventilation. Protocols and guidelines should be developed and used on the local level based on supplies of medications and institutional experience.

You will note that I have not suggested specific modes of ventilation. This is in light of national and international variability in ventilators used during this COVID-19 pandemic. Facilities may have to face using ventilators not normally used. In this crisis, anesthesia machines have also been placed in service as ICU ventilators — a role that they were not usually intended. The general guidelines in ARDS management that I have outlined are basic and can be used across all machines and modes. It is also important to educate colleagues and therapists in these basics of ARDS management, as some clinicians who are not normally involved in ARDS management or ICU care may be pressed into this service more and more throughout the pandemic.

References

  1. Alhazzani , W Moller MH, Arabi Y, et al. The Surviving Sepsis Campaign Guidelines on the management of Critically ill Adults with Coronavirus Disease 2019 (COVID-19). Crit Care Med supplement March 2020.
  2. ARDS Network Ventilation with lower tidal volumes compared with traditional tidal volumes for acute lung injury and acute respiratory syndrome. N England J Med 2000, 342: 1301-8
  3. Papadakos PJ, Lachman B. The open lung concept of mechanical ventilation: The Role of Recruitment and stabilization Crit Care Clin 2007 23: 241-250
  4. Acosta P, Santisbon E, Varon The use of Positive End-Expiratory Pressure in Mechanical Ventilation. Crit Care Clin 2007 23: 251-261

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