Consequences to the Lungs When Gas Swings Between Lung Units During Patient Triggered Mechanical Ventilation

In this issue of the Journal, Enokidani et al studied the impact of ventilator settings on the extent of pendelluft during mechanical ventilation using a lung simulator model. Pendelluft was first described in patients with chronic lung disease, in whom it can occur during inspiration or expiration, in spontaneous breathing as well as in controlled ventilation. Initially, pendelluft was defined as the movement of gas from one lung unit to another (ie, air movement within the lungs) during either inspiration or expiration. More recently, pendelluft has been detected in patients with ARDS before initiation of the breath, in the absence of tidal volume changes. In the presence of acute lung injury, vigorous spontaneous breathing effort causes a pendelluft phenomenon, with redistribution of air from nondependent to dependent lung regions, following different regional compliance properties of the lungs. This movement results in overdistention of dependent lung and has the potential to cause patient-induced lung injury. Pendelluft can occur during patient triggered mechanical ventilation or spontaneous breathing, but typically does not occur during controlled ventilation, except in patients with chronic lung disease. Pendelluft is also considered one of the mechanisms that explain gas transport during highfrequency jet ventilation or high-frequency oscillation. Pendeluft may be marked when mechanical ventilation is used in a patient with heterogeneous impedances. The presence of pendelluft can be identified with auscultation of the chest or, more practically and definitively, with the use of electrical impedance tomography (EIT). Figure 1 illustrates an EIT image of a swine model of severe ARDS during controlled and patient triggered mechanical ventilation. During controlled ventilation, the impedance changes are synchronous in the ventral and dorsal regions of the lungs during a respiratory cycle. Once the ventilator is switched to assisted mechanical ventilation, the pendelluft phenomenon appears. In the early phases of inspiration, gas moves within the lungs from the nondependent regions of the lungs to the most dependent. With vigorous inspiration, the pleural pressure changes in the dependent area of the lung are greater than the pleural pressure changes of the nondepend-