April 03, 2020
Mechanical ventilation has proved to be the defining intervention of intensive care medicine. As an inherently dynamic and complex yet essential life-sustaining measure, mechanical ventilatory support also has the potential to cause discomfort and injury. Low tidal volume strategy and open-lung strategy have had temporary positive impacts among patients with acute respiratory distress syndrome (ARDS). However, as mortality improvements in ARDS have stagnated and the concept of safe ventilation continues to evolve, the search for better and more-integrative therapeutic targets has ensued.
Although an individual's respiratory system mechanics can be estimated through direct measurements at the bedside, assessing the elastic properties of the lung is more invasive and requires specialized equipment and expertise to provide reliable data. Recent interest has arisen in airway driving pressure — the quotient of tidal volume and respiratory system compliance — which could serve as a direct and easily measured marker for ventilator-induced lung injury (VILI) risk. Additionally, as esophageal pressure estimates pleural pressure, esophageal manometry enables the calculation of the average force actually distending the lung. This force is the transpulmonary pressure — the difference between alveolar pressure and pleural pressure.
"According to the influential observations of Daniel S. Talmor, M.D., M.P.H., and others published in The New England Journal of Medicine in 2008 and Jeremy R. Beitler, M.D., M.P.H., and others published in JAMA in 2019, adjusting positive end-expiratory pressure based on transpulmonary pressure significantly improves oxygenation and respiratory system compliance in patients with ARDS," says Gustavo A. Cortes Puentes, M.D., Pulmonary and Critical Care Medicine at Mayo Clinic in Rochester, Minnesota. "At Mayo Clinic, our current clinical practice concentrates on the static inflation features of a single breath cycle, which include transpulmonary pressure, plateau pressure, positive end-expiratory pressure and airway driving pressure. All of these elements have long served as the primary variables guiding prevention of VILI."
Despite their prominence in current practice, such nondynamic pressures cannot act alone to inflict damage, as pressure must be paired with a volume change, thereby expending energy. Additionally, exclusively monitoring these parameters does not consider other key factors shown to influence VILI. Experimental evidence and supportive data from randomized clinical trials have demonstrated that changes in the tidal inflation pattern — influenced by flow amplitude, inspiratory time fraction, and inspiratory inflation contour — and frequency of ventilation also may increase the risk of VILI.
At a basic level, energy must be involved in VILI generation. Mechanical power, which is the energy load applied to the respiratory system per unit of time, has increasingly become recognized as the variable unifying all potential factors contributing to VILI. Mechanical power integrates all the machine-derived variables including tidal volume, pressures, flow and respiratory rate. "We have previously reported that higher mechanical power correlated with higher mortality among patients with moderate ARDS," says Dr. Cortes Puentes. "However, as appealing as this mechanical power hypothesis for integrating the determinants of VILI may be for critical care providers, it must be acknowledged that the concept of mechanical power is still too bluntly defined to precisely describe the link between ventilation strategy and tissue damage.
"Many important conceptual and research questions persist regarding how energy is channeled to cause tissue disruption and signal inflammation. Can we confidently assign a direct causal role to VILI regarding mortality? If so, what is the attributable risk of the different factors contributing to VILI? Although the answers to these questions remain unclear, at Mayo Clinic, we strive to explore the concept of integration of pulmonary mechanics in the path to personalization of mechanical ventilation."
For more information
Talmor D, et al. Mechanical ventilation guided by esophageal pressure in acute lung injury.The New England Journal of Medicine. 2008;359:2095.
Beitler JR, et al. Effect of titrating positive end-expiratory pressure (PEEP) with an esophageal pressure-guided strategy vs an empirical high PEEP-fio2 strategy on death and days free from mechanical ventilation among patients with acute respiratory distress syndrome: A randomized clinical trial. JAMA. 2019;321:846.