Ratchet recruitment in the acute respiratory distress syndrome: lessons from the newborn cry

Abstract

Patients with acute respiratory distress syndrome (ARDS) have few treatment options other than supportive mechanical ventilation. The mortality associated with ARDS remains unacceptably high, and mechanical ventilation itself has the potential to increase mortality further by unintended ventilator-induced lung injury (VILI). Thus, there is motivation to improve management of ventilation in patients with ARDS. The immediate goal of mechanical ventilation in ARDS should be to prevent atelectrauma resulting from repetitive alveolar collapse and reopening. However, a long-term goal should be to re-open collapsed and edematous regions of the lung and reduce regions of high mechanical stress that lead to regional volutrauma. In this paper, we consider the proposed strategy used by the full-term newborn to open the fluid-filled lung during the initial breaths of life, by ratcheting tissues opened over a series of initial breaths with brief expirations. The newborn's cry after birth shares key similarities with the Airway Pressure Release Ventilation (APRV) modality, in which the expiratory duration is sufficiently short to minimize end-expiratory derecruitment. Using a simple computational model of the injured lung, we demonstrate that APRV can slowly open even the most recalcitrant alveoli with extended periods of high inspiratory pressure, while reducing alveolar re-collapse with brief expirations. These processes together comprise a ratchet mechanism by which the lung is progressively recruited, similar to the manner in which the newborn lung is aerated during a series of cries, albeit over longer time scales.

Keywords: ARDS; airway pressure release ventilation; lung recruitment; time-controlled adaptive ventilation; ventilator-induced lung injury.

FIGURE 1

Comparisons between newborn cry and TCAV lung volume vs. time and respiratory flow vs. time. The newborn lung volume waveform shows data from a surrogate measurement (i.e., relative volume change since birth assessed by electrical impedance tomography) published by Tingay et al. (2021), then smoothed and time-differentiated to obtain flow. The TCAV waveforms were simulated in a single-compartment resistance-compliance-inertance model of respiratory mechanics. TCAV applies a prolonged high airway pressure, maintaining high lung volume. Both waveforms exhibit a rapid initial exhalation (red) with large negative flow rates for a brief time before an expiratory “brake” abruptly halts volume change. However, the newborn waveform continues with a second, slower exhalation phase, whereas TCAV immediately reinflates. Note the increase in the newborn lung volume between the beginning and end of the first breath. Note that the newborn breath duration is approximately 0.7 s, whereas each simulated TCAV cycle is 5 s.

FIGURE 2

(A) Single-compartment model of the respiratory system in which the alveolar compartment expands vertically to represent tissue distension, but horizontally to represent recruitment. Variables for the model parameters are defined in the text. (B) Open fractions of the lung $F\left(t\right)$ , simulated by the computational model depicted in (A). The model was mechanically ventilated with APRV using a breath period of 6 s, starting with 25% (black), 50% (red), and 75% (green) derecruitment. Simulations were performed with three different expiratory durations (T _low ) as indicated.