A Novel Negative Pressure-Flow Waveform to Ventilate Lungs for Normothermic Ex Vivo Lung Perfusion

Abstract

Ex vivo lung perfusion (EVLP) is increasingly used to treat and assess lungs before transplant. Minimizing ventilator induced lung injury (VILI) during EVLP is an important clinical need, and negative pressure ventilation (NPV) may reduce VILI compared with conventional positive pressure ventilation (PPV). However, it is not clear if NPV is intrinsically lung protective or if differences in respiratory pressure-flow waveforms are responsible for reduced VILI during NPV. In this study, we quantified lung injury using novel pressure-flow waveforms during normothermic EVLP. Rat lungs were ventilated-perfused ex vivo for 2 hours using tidal volume, positive end-expiratory pressure (PEEP), and respiratory rate matched PPV or NPV protocols. Airway pressures and flow rates were measured in real time and lungs were assessed for changes in compliance, pulmonary vascular resistance, oxygenation, edema, and cytokine secretion. Negative pressure ventilation lungs demonstrated reduced proinflammatory cytokine secretion, reduced weight gain, and reduced pulmonary vascular resistance (p < 0.05). Compliance was higher in NPV lungs (p < 0.05), and there was no difference in oxygenation between the two groups. Respiratory pressure-flow waveforms during NPV and PPV were significantly different (p < 0.05), especially during the inspiratory phase, where the NPV group exhibited rapid time-dependent changes in pressure and airflow whereas the PPV group exhibited slower changes in airflow/pressures. Lungs ventilated with PPV also had a greater transpulmonary pressure (p < 0.05). Greater improvement in lung function during NPV EVLP may be caused by favorable airflow patterns and/or pressure dynamics, which may better mimic human respiratory patterns.

Figure 1.

Schematic showing experimental setup for positive pressure and negative pressure Ex vivo lung perfusion. NPV, negative pressure ventilation; PPV, positive pressure ventilation.

Figure 2.

Negative pressure ventilation (NPV) lungs demonstrate decreased mechanical injury but no difference in oxygenation after 2 hour Ex vivo lung perfusion (EVLP). A: Dynamic compliance during positive pressure ventilation (PPV) and NPV ventilation. B: Pulmonary vascular resistance (PVR) during PPV and NPV. C: Change in lung weight change after starting EVLP during PPV and NPV. D: Perfusate oxygenation during PPV and NPV, calculated as perfusate oxygen distal to lung (Pa)/fraction inspired oxygen (FIO2). * indicates a statistically significantly effect (p < 0.05) of ventilation type using two-way ANOVA. n = 8 for NPV and PPV.

Figure 3.

Negative pressure ventilation (NPV) lungs exhibit decreased perfusate concentrations of some proinflammatory cytokines after 2 hour Ex vivo lung perfusion (EVLP). A: IL-6 concentrations after 2 hours of positive pressure ventilation (PPV) and NPV EVLP. B: TNF-α concentrations 2 hours of PPV and NPV EVLP. C: IL-1β concentrations 2 hours of PPV and NPV EVLP. D: IL-8 concentrations 2 hours of PPV and NPV EVLP. * indicates p < 0.05 via t-test. n = 8 for PPV at both time points, n = 6 for NPV at 0 hours, n = 5 for NPV at 2 hours.

Figure 4.

Ventilated lungs have distinctly different respiratory cycle waveforms depending on ventilation mode. Average air flowrate (A), pressure (B), and volume (C) over time for one respiratory cycle in positive pressure ventilation (PPV) and negative pressure ventilation (NPV) lungs. Data represent average values after 30 minute ventilation. * indicates p < 0.05 between ventilation type using two-way ANOVA. n = 8 for NPV and PPV.

Figure 5.

Pressure dynamics are significantly different between positive and negative pressure ventilation modes. A: Peak airway pressure during 2 hours of positive pressure ventilation (PPV) and negative pressure ventilation (NPV) Ex vivo lung perfusion (EVLP). B: Maximum peak airway pressure measured throughout 2 hour EVLP. C: Rate of pressure change throughout the inspiratory phase, calculated for each point as change in pressure/change in time. * indicates p < 0.05 between ventilation types using two-way ANOVA for (A) and (C), and t-test for (B). n = 8 for NPV and PPV.

Figure 6.

Positive pressure ventilation results in alveolar collapse and inflammatory cell infiltration compared with negative pressure ventilation and unventilated controls. A: Unventilated lung, 100× magnification. B: Unventilated lung, 200× magnification. C: Lung ventilated with negative pressure, 100× magnification. D: Lung ventilated with negative pressure, 200× magnification. Black arrow highlights areas of alveolar wall thickening, red arrow highlights inflammatory cell infiltration. E: Lung ventilated with positive pressure, 100× magnification. Blue arrow highlights regions alveolar collapse. F: Lung ventilated with positive pressure, 200× magnification. Black arrow highlights areas of alveolar wall thickening, red arrow highlights inflammatory cell infiltration.