Static and Dynamic Contributors to Ventilator-induced Lung Injury in Clinical Practice. Pressure, Energy, and Power

Abstract

Ventilation is inherently a dynamic process. The present-day clinical practice of concentrating on the static inflation characteristics of the individual tidal cycle (plateau pressure, positive end-expiratory pressure, and their difference [driving pressure, the ratio of Vt to compliance]) does not take into account key factors shown experimentally to influence ventilator-induced lung injury (VILI). These include rate of airway pressure change (influenced by flow amplitude, inspiratory time fraction, and inspiratory inflation contour) and cycling frequency. Energy must be expended to cause injury, and the product of applied stress and resulting strain determines the energy delivered to the lungs per breathing cycle. Understanding the principles of VILI energetics may provide valuable insights and guidance to intensivists for safer clinical practice. In this interpretive review, we highlight that the injuring potential of the inflation pattern depends upon tissue vulnerability, the number of intolerable high-energy cycles applied in unit time (mechanical power), and the duration of that exposure. Yet, as attractive as this energy/power hypothesis for encapsulating the drivers of VILI may be for clinical applications, we acknowledge that even these all-inclusive and measurable ergonomic parameters (energy per cycle and power) are still too bluntly defined to pinpoint the precise biophysical link between ventilation strategy and tissue injury.

Keywords: acute respiratory distress syndrome; energy; lung protective ventilation; power; ventilator-induced lung injury.

Figure 1.

Left panel: airway pressure profile during inflation with constant flow. Under these conditions, time and inspired volume are linearly scaled. Total positive end-expiratory pressure (PEEP) is comprised of the set PEEP and auto-PEEP. Areas A, B, and C correspond to the flow-resistive, tidal-elastic, and PEEP-related energy components. Right panel: the shaded area is the pressure–volume area that defines the mechanical work performed by the ventilator during passive inflation, equivalent to the energy it delivers to the respiratory system. P_peak = peak dynamic pressure; P_plat = static (“plateau”) pressure.

Figure 2.

Potential importance of compliance to consequences of driving pressure on injuring strain. Damage from a given driving pressure depends jointly on associated lung unit compliance and delivered energy. In this example, the same driving pressure of 20 cm H₂O overstretches the compliant alveolus (top), whereas the less-compliant alveolus (bottom) undergoes less volume change and tolerates the associated strain. Open units of varying specific compliance are embedded in different zones within the same injured “baby lung.”

Figure 3.

Influence of positive end-expiratory pressure (PEEP) on lung tissue strain for the same driving pressure (bidirectional arrow). Dashed red line represents the threshold pressure at which ventilator-induced lung injury begins. The width of each rectangle indicates the number of high-risk junctional interfaces between open and closed lung units. PEEP may reduce number of junctional interfaces but increases strain on those remaining unrecruited, as indicated by the deepening hues.

Figure 4.

Catastrophic breakdown of the alveolar–capillary barrier. In this isolated, ventilated, and perfused rabbit lung, weight gain rapidly accelerates (arrow) after the first 10 minutes of ventilation at the higher ventilating frequency (f = 20 vs. 5 tidal cycles/min). Stress amplitude intensifies at the higher frequency with the alveolar dropout that occurs as alveoli progressively flood.

Figure 5.

Tidal energy during constant flow. The rectangular crosshatched areas represent positive end-expiratory pressure (PEEP)-related inflation energy at two different levels of PEEP (PEEP₁ and PEEP₂). The triangular crosshatched areas indicate the corresponding “driving energy” components. The solid area corresponds to flow-resistive energy. The sigmoidal line indicates the pressure–volume curve of inflation for the respiratory system. Note that the total elastic energy increases at the higher PEEP level, despite the same driving pressure as at the lower PEEP.

Figure 6.

Proposed key contributors to ventilator-induced lung injury risk based on ergonomic principles. PEEP = positive end-expiratory pressure; VILI = ventilator-induced lung injury.