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. Author manuscript; available in PMC: 2022 Jun 23.
Published in final edited form as: ASAIO J. 2021 Jan 1;67(1):96–103. doi: 10.1097/MAT.0000000000001168

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

CHRISTOPHER M BOBBA *,†, KEVIN NELSON *,†, CURTIS DUMOND ‡,§, EMRE EREN ‡,§, SYLVESTER M BLACK ‡,§, JOSHUA A ENGLERT *,†, SAMIR N GHADIALI *,†, BRYAN A WHITSON ‡,§
PMCID: PMC9218878  NIHMSID: NIHMS1809206  PMID: 32404613

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.

Keywords: ex vivo lung perfusion, negative pressure ventilation, ventilator mechanics, ventilator, transplantation

Lung transplantation is the only viable therapy for many patients with end-stage lung disease and improves survival.1 Unfortunately, there is a dramatic shortage of suitable donor lungs, and many patients die waiting for transplant.2 Current consensus criteria for accepting a lung for transplant has led to low donor lung utilization. Only about 20% of lungs are deemed viable for transplant and the remainder are rejected as unsuitable.3–5 Expansion of the lung donor pool is of paramount importance to the field of lung transplantation.

Ex vivo lung perfusion (EVLP) has emerged as a technique for assessment and treatment of donor lungs before lung transplant6 and serves as a novel platform for transplantation research.7 During EVLP, the donor lung is explanted, placed in a sterile chamber, mechanically ventilated, and perfused with Steen solution (an acellular buffered solution with physiologic oncotic pressure). Ex vivo lung perfusion has the potential to increase donor lung utilization by coupling the ability to observe lung physiology in real time with a means of treating the lung to improve its function. This includes selective treatment of the pulmonary microcirculation via the perfusate8 or instillation of therapeutics to the airway.9,10 Ex vivo lung perfusion is clinically available in the United States, Europe, and Canada, with evidence suggesting it could be useful as standard practice for all donor lungs.11

Effectively utilizing EVLP requires adequate ventilation of the lung. Typically, the airway is intubated and subjected to standard positive pressure ventilation using an ICU ventilator with nomogram-driven volume control settings that do not take into account atelectrauma, barotrauma, or volutrauma. Theoretically, any ventilation whether in vivo or ex vivo, may expose the lung to supraphysiologic mechanical forces resulting in barrier disruption or biotrauma (i.e., inflammation), known as ventilator induced lung injury (VILI). Recent studies suggest that VILI may be occurring during EVLP,12,13 and further refinement of EVLP for clinical use requires optimization of ventilation protocols.

Several in vivo studies have investigated whether negative pressure ventilation may decrease VILI.14–16 However, it is not known if the application of negative pressures are intrinsically lung protective or if differences in respiratory pressure-flow dynamics used during that ventilation mode is responsible for the reduced lung injury observed during NPV. In this study, we sought to evaluate the impact of positive and negative pressure ventilation and respiratory pressure-flow dynamics on lung injury during normothermic EVLP.

Methods

This investigation utilized a well characterized rodent model of ex vivo lung perfusion18 to evaluate the impact of positive and negative pressure ventilation on pulmonary mechanics, inflammation, and other injury parameters.

Animals

All procedures were performed according to the guidelines of the Institutional Animal Care and the National Research Council’s Guide for the Humane Care and Use of Laboratory Animals (IACUC) and has undergone approval by The Ohio State University IACUC committee (2012A00000126). Male Sprague Dawley rats (200–300 g) were purchased from Harlan Laboratories (Indiana), maintained in laminar flow cages in a pathogen-free animal facility at Ohio State University and fed a standard diet and water ad libitum. N = 8 animals per group were used for all mechanical measurements (Figure 1D).

Figure 1.

Figure 1.

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

Heart-Lung Bloc Extraction, EVLP, and Data Monitoring

Detailed protocols to extract the heart-lung bloc and integrate into the EVLP circuit have been described previously.18 The EVLP circuit is shown in Figure 1 and Figure 1, Supplemental Digital Content, http://links.lww.com/ASAIO/A498. N = 16 animals were ventilated in either NPV or PPV mode. Ventilations were performed in randomized order. Animals were anesthetized with ketamine and xylazine and a tracheostomy tube was placed. The thoracic cavity was opened, and the rat was exsanguinated via incision of the inferior vena cava. The pulmonary artery and left atrium were cannulated via a transapical approach. The surrounding tissue was bluntly dissected, and the heart-lung bloc removed from the chest cavity. The heart-lung bloc was then connected to the ex vivo perfusion system (IL-2 Isolated Lung System, Harvard). The system consists of an ISMATEC peristaltic pump, 500 mL perfusate reservoir, GD100 circulating thermostatic water bath (Grant Scientific), D150 hollow fiber oxygenator (Medica), heat transfer module, and outlet pressure regulator. The D150 hollow fiber oxygenator was used to deoxygenate the perfusate using a 6% O2, 8% CO2, and 86% N2 gas mixture. The circulating water jacket was controlled to 37°C and jacketed the reservoir, the artificial thorax, and heat transfer module. The pulmonary arterial pressure, respiratory flow, and airway opening pressure were measured continuously and were recorded and analyzed using the Power Lab software (Ad Instruments). Steen solution was used for all perfusions. To measure oxygenation, perfusate samples were taken every 15–30 minutes starting at time t = 0 from two designated sample ports in the perfusate line. The sample ports were located directly before the perfusate entered the heart-lung bloc and directly after exiting. Samples were immediately read using a RAPIDpoint 405 blood gas analyzer (Siemens). The RAPIDpoint 405 measured partial pressure of oxygen and CO2, glucose levels, and pH. Additional perfusate samples were taken at t = 0 hours and t = 2 hours for cytokine measurement. Following each experiment, perfusate samples were flash-frozen in liquid nitrogen and saved for cytokine analysis. Lungs were immediately fixed at 20 cmH2O in formalin and paraffin embedded. Embedded lung slices were cut to 5 μm and placed on microscope slides and stained with hematoxylin and eosin for histological analysis.

Positive and Negative Pressure Ventilation

Lungs were ventilated using either positive pressure or negative pressure and enclosed within a glass chamber. Figure 1 diagrams the differences in our experimental setup between the two ventilation modes. Positive pressure ventilation was administered using the Model 683 Small Animal Ventilator (Harvard) and the glass chamber was open to atmosphere. During negative pressure ventilation, the glass chamber was connected to the PLUGSYS Ventilation Control Module Type 681 (Harvard), and the trachea was open to atmosphere. For both ventilation modes lungs were matched for tidal volume (TV), positive end-expiratory pressure (PEEP), and breaths per minute (bpm). Lungs were ventilated for 2 hours with ambient air at 80 bpm and a TV of 4 mL/kg with a PEEP of 2 cmH2O. These settings were chosen to mimic a “preservation” ventilation mode. Pressure and flow rate were independently measured within the tracheal line during both PPV and NPV. In both ventilation modes, a positive transpulmonary pressure is measured (Figure 1C). Positive end-expiratory pressure was controlled by monitoring the pressure-time waveform until the pressure following the expiratory phase was 2 cmH2O. During the first 15 minutes of the experiment, the perfusate flow rate was increased from 5% to 10% predicted cardiac output (CO) to 20% predicted CO.

Pulmonary Mechanics

All lung mechanics measurements were derived from direct measurements of airway pressure, airway flowrate, and pulmonary artery pressure. Tidal volume was calculated as the integral of airway flowrate for one respiratory cycle. Transpulmonary pressure was calculated by subtracting airway pressure from chamber pressure (Figure 1C). Peak airway pressure (PAP) and PEEP were calculated as the maximum and minimum transpulmonary pressures throughout one respiratory cycle, respectively. Dynamic compliance was calculated as TV/(PAP−PEEP). Pulmonary vascular resistance (PVR) was calculated as (pulmonary artery pressure−left atrial pressure)/perfusate flow rate.

Cytokine Analysis

Perfusate samples were taken after exiting the heart-lung block at t = 0 hours and t = 2 hours. These samples were then analyzed using enzyme-linked immunosorbent assay (ELISA, RD Biosystems). Enzyme-linked immunosorbent assay was performed for IL-1β, IL-6, IL-8, TNF-α, IL-4, IL-10, TRAIL, and TGF-β in the perfusate. Storage and procedures were followed according to manufacturer’s directions. 96-well plates were read using an ElmerPerkins Enspire multimode plate reader. Enzyme-linked immunosorbent assay was optimized according to manufacturer’s protocol. Because of perfusate losses during ELISA optimization, N = 8 for PPV groups at 0 and 2 hours, N = 6 for NPV at 0 hours and N = 5 for NPV at 2 hours. Perfusate samples were read in triplicate and averaged for each sample at each time point (Figure 1D).

Statistical Analysis

Results are expressed as mean ± standard deviation. Data were analyzed using GraphPad Prism 7. Data were first tested for normal distribution and equal variances, then by t-test. The D’Agostino and Pearson or Shapiro-Wilk tests were used to test for normal distribution of residuals, depending on group size. Sample sets with unequal variances were tested with Welch’s t-test, otherwise data were analyzed with Student’s t-test. Mechanics measurements were analyzed via ANOVA. If ANOVA indicated a significant difference between groups, a post hoc Tukey test performed for multiple comparisons. A p value <0.05 was considered statistically significant.

Results

Lung Injury Parameters

As shown in Figure 2, A–C, lungs subjected to PPV or NPV demonstrated time-dependent changes in lung compliance, PVR and lung weight over the 2 hours of ventilation. For dynamic compliance, two-way ANOVA indicates that ventilation type (PPV versus NPV) is a statistically significant factor (F = 53.02, p < 0.0001) where PPV leads to a significantly lower compliance as compared with NPV. For PVR, two-way ANOVA indicates ventilation type (PPV versus NPV) is statistically significant factor (F = 85.49, p < 0.0001) where PPV leads to a significantly higher PVR as compared with NPV. For lung weight, two-way ANOVA indicates that both ventilation type and time are statistically significant factors (F = 20.06, p < 0.0001 for ventilation mode, F = 4.124, p < 0.0001 for time) where PPV increases lung weight compared with NPV. Importantly, as shown in Figure 2D, we did not observe any statistically significant difference in perfusate oxygenation between ventilation modes during the 2 hours EVLP, as measured by PaO2/FiO2 ratio (two-way ANOVA, F = 0.474), though a time effect was observed (two-way ANOVA, F = 10.49, p < 0.0001).

Figure 2.

Figure 2.

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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.

Perfusate Cytokine Levels

As shown in Figure 3, 2 hours of either NPV or PPV induced increased protein expression of several proinflammatory cytokines including IL-6, TNF-α, IL-1β, and IL-8 over baseline conditions. Importantly, at the 2 hour time point, the protein expression level of IL-6 in the perfusate from PPV lungs was significantly higher than the IL-6 levels in NPV lungs (p < 0.004). Similarly, there was a trend toward higher TNF-α levels in the perfusate from PPV lungs as compared with the TNF-α levels from NPV lungs (p = 0.07). No statistically significant differences in the levels of IL-1β and IL-8 were detected between PPV and NPV lungs. There was also no statistically significant difference in the levels of other mediators (TRAIL, TGF-β, or IL-4) between PPV and NPV lungs (Figure 2, Supplemental Digital Content, http://links.lww.com/ASAIO/A498).

Figure 3.

Figure 3.

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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.

Comparison of Positive and Negative Pressure Ventilation Waveforms

To assess potential differences in pressure-flow waveforms between positively and negatively ventilated lungs, airflow and airway pressure were recorded continuously (0.0025 s increments, 400 Hz) during PPV or NPV ventilation. Figure 4 shows compiled data from all lungs for a single respiratory cycle at 30 minutes. Pressure is displayed as transpulmonary pressure and is therefore positive for both NPV and NPV modes. For flowrate versus time data (Figure 4A), ventilation type (F = 72.23, p < 0.001) and time (F = 41.11, p < 0.0001) were statistically significant factors. For pressure versus time (Figure 4B), ventilation time (F = 54.76, p < 0.0001) and ventilation type (F = 24.5, p < 0.0001) were significant factors. Volume versus time (Figure 4C) data was also significantly different for both ventilation type (F = 901.3, p < 0.0001) and time (F = 216.6, p < 0.0001). Positive pressure ventilation had a low inspiratory flow rate followed by rapid exhalation, whereas NPV lungs had a higher inspiratory flow and slower exhalation (Figure 4A). Because of the lower flow rate during PPV, flow was present for a longer time during the inspiratory phase of the respiratory cycle. Consequently, PPV lungs demonstrated a slow increase to a peak airway pressure just before exhalation while NPV lungs exhibited a rapid increased in pressure to a peak airway pressure early in the respiratory cycle (Figure 4B). Note, this peak airway pressure was lower in NPV lungs as compared with PPV lungs (Figure 5B).

Figure 4.

Figure 4.

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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.

Figure 5.

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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.

Comparison of Positive and Negative Pressure Ventilation Pressure Dynamics

Peak airway pressure is an important determinate of lung injury and we therefore quantified how the peak airway pressure varied as a function of time on the ventilator. As shown in Figure 5A, lungs subjected to PPV exhibited higher peak airway pressure (measured as maximum recorded pressure over one respiratory cycle) for the duration of perfusion (Figure 5A) and two-way ANOVA indicated that ventilation type (F = 8.116, p < 0.016) and time (F = 3.521, p < 0.002) were significantly different factors. The rate of airway reopening has also been shown to be an important determinate of lung injury,20 and we therefore also calculated the rate of pressure change (dP/dt) using simple forward difference derivative calculation (P2−P1)/(t2−t1). As shown in Figure 5B, PPV demonstrated a lower dP/dt magnitude that occupied a longer proportion of the inspiratory phase of the respiratory cycle (Figure 5) while NPV exhibited a large but transient increase in dP/dt early in the inspiratory phase. Two-way ANOVA indicated that ventilation type (F = 48.94, p < 0.0001) and time (F = 12.09, p < 0.0001) were signifi different factors in the dP/dt versus time curves.

Histologic Analysis

Changes to alveolar architecture following ventilation was assessed using hematoxylin and eosin (H&E) stained lung slices following PPV, NPV, or unventilated control. Representative images for each ventilation condition are shown in Figure 6. Unventilated lungs (Figure 6, A and B) have intact, thin alveoli, with very few inflammatory cells present. Lungs subjected to NPV (Figure 6, C and D) have some basement membrane thickening (Figure 6D, black arrow) and inflammatory cell recruitment (Figure 6D, red arrow). Following PPV (Figure 6, E and F), there was even greater membrane thickening (Figure 6F, black arrow), and inflammatory cell infiltration (Figure 6F, red arrow). Additionally, areas of alveolar collapse or wall destruction were observed in mice subjected to PPV (Figure 6E, blue arrows).

Figure 6.

Figure 6.

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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.

Discussion

Our study used a rat EVLP model to compare the effects of positive and negative pressure ventilation on pulmonary mechanics and inflammation. By applying identical TV, PEEP, and respiratory rates, we found that NPV decreased various indicators of lung injury compared with PPV over a 2 hour period of EVLP. We applied a novel ventilation strategy and found that NPV had reduced lung injury and a more favorable cytokine profile compared with PPV during normothermic EVLP.

In lungs subjected to NPV, there was reduced proinflammatory cytokine concentration (Figure 3), reduced pulmonary vascular resistance (Figure 2), reduced edema (Figure 2), reduced peak airway pressure (Figure 5), and reduced histologic evidence of lung damage (Figure 6) compared with PPV. Additionally, NPV lungs demonstrated higher compliance following EVLP (Figure 3). It should be noted that although we did not detect a difference in oxygenation, likely because of the relatively short perfusion time (2 hours), the mechanical, histologic, and inflammatory differences observed between the two ventilation groups suggest that difference in oxygenation would likely occur with a longer time of EVLP. In addition to demonstrating that NPV EVLP leads to less lung injury, this study also demonstrates that these changes occur in the setting of markedly different respiratory waveforms (Figures 4 and 5). Specifically, faster inspiratory flow rates during inspiration were observed during NPV and this rapid lung inflation may be less injurious than the slower, more sustained rate observed in PPV. The clinical implication of this finding is that a rapid inspiratory phase may be a more important determinant of lung injury as opposed to any intrinsic benefit derived from the application of negative pressures.

Developing ventilation protocols that minimize VILI is critical to improving EVLP for lung transplant applications that seek to maximally rehabilitate lungs before transplant. In addition to establishing optimal TV and PEEP, investigating different modes of ventilation, (i.e., positive versus negative pressure) may lead to novel ways to prevent VILI. With regard to pulmonary mechanics, the terms negative and positive pressure are expressed relative to atmospheric pressure. In NPV EVLP, a vacuum pressure (relative to atmospheric pressure) is applied to the external surface of an isolated lung in a chamber. This negative pressure expands the lungs and draws air into the pulmonary system and is similar to the mechanics that occur during normal breathing in vivo. In PPV EVLP, the lung is inflated by applying positive pressure (relative to atmospheric pressure) directly to the airway via an endotracheal tube and thus inflating the lung. Basic pulmonary mechanical theory contends that the transpulmonary pressure (in EVLP, this is the difference between pressure at the external lung surface and pressure at the airway opening) drives alveolar inflation. Therefore, lungs subjected to identical transpulmonary pressures (whether by positive or negative ventilation) should have identical tidal volumes, as well as pulmonary physiology and biologic outcomes. This concept has been debated extensively in the literature with some studies showing a beneficial effect of NPV compared with PPV in reducing VILI in vivo,14,21 while other studies show no difference between the two modes.16,22 Small scale clinical studies and case reports have shown a potential benefit of NPV as well,23,24 though limited by an extremely small sample size. In vivo, NPV may favorably alter hemodynamics compared with PPV by increasing right ventricular preload, decreasing left ventricular afterload and increasing stroke volume.25 One mechanism that has not been fully explored in EVLP is that the dynamics of airflow and pressure may vary between NPV and PPV even when VT, PEEP, and respiratory rates are matched. Therefore, in this study, we specifically investigated how NPV and PPV alter the dynamic pressure-flow rate data recorded during ventilation.

Our data suggest that differences in pressure waveforms and/or peak airway pressures may be responsible for the differences in lung injury/inflammation during positive and negative pressure EVLP. In a recent study using a porcine model of EVLP that compared these modes, NPV demonstrated a reduction in proinflammatory cytokine secretion and edema; however, pressure waveforms for these ventilation modes were not reported.17 This study also showed a reduction in peak airway pressures in NPV ventilated lungs compared with PPV. Assuming identical TV was delivered in both ventilation modes, a difference in peak airway pressure suggests a difference in applied pressure waveforms, even though both ventilators operate by volume-controlled ventilation. In our study, there are two possible explanations for the favorable function of NPV lungs compared with PPV. The first is an intrinsic difference between positive and negative pressure ventilation. The second is that our negative pressure ventilator utilizes a more favorable pressure waveform. In our model, PPV lungs needed higher peak airway pressure to achieve the same tidal volume. Increased peak airway pressure is a well-documented driver of lung mechanical injury26 and may be causing greater VILI in this setting. Alternatively, PPV lungs received lower airflow magnitude for a longer duration during the inspiratory portion of the respiratory cycle while NPV lungs received a more rapid and transient increase in airflow during inspiratory phase. As a result, the time-rate of pressure application (dP/dt) was faster in NPV lungs as compared with PPV lungs. Previous in vitro and in silico studies indicate that rapid reopening of pulmonary airways leads to less cellular and tissue damage.20 Therefore, the apparent benefit of NPV that we observed may be because of the larger rate of pressure application (dP/dt) and the rapid rate of lung inflation during inspiration.

The current study has several limitations. It utilizes an isolated organ perfusion system to study lung function during EVLP, not a full animal transplant model. An important future study would utilize the negative pressure ventilatory protocols outlined here to analyze graft performance after transplant. Second, to reduce costs, a rodent model was used instead of a larger animal model. Given the importance of pressure-flow waveforms, future studies should investigate the benefit of using a positive pressure ventilation protocol that matches the negative pressure-flow waveforms documented in this study. It is possible that more rapid inspiration during NPV may be a reflection of improved efficiency present during this ventilation mode. This could be the result of lung “auto-PEEPing” during PPV or some other yet undescribed mechanism. Unfortunately, given the physical limitations of our current ex vivo ventilation systems, we were not able to apply identical pressure waveforms (Figure 4), and improved efficiency of NPV during inhalation. We do note that previous investigators found no differences in oxygenation or peak airway pressures between PPV and NPV in vivo when the pressure-time and volume history were properly matched.16

Current commercial EVLP systems employ clinical ventilators that operate by standard PPV delivered via an endotracheal tube. These ventilation protocols may lead to significant VILI, limiting the regenerative capacity of these systems. This study suggests that NPV may reduce injury during ventilation, and that the benefit of NPV may be because of differences in pressure-flow waveforms or transpulmonary pressure magnitude (i.e. peak airway pressure) as opposed to an intrinsic benefit of negative pressure ventilation. It is of paramount importance to consider differences in flow and pressure delivery when comparing NPV and PPV during EVLP. In this study, we applied a unique ventilation strategy and found that NPV delivered a more favorable pressure-flow waveform. This novel NPV strategy improved lung function during normothermic EVLP.

Supplementary Material

Supplemental Figures 1 & 2

Acknowledgments

This work was supported by a Thoracic Surgery Foundation for Research and Education (TSFRE) award (B.A.W), K08 GM102695 (J.A.E), The Ohio State University Presidential Fellowship (C.M.B.), and The Ohio State University Medical Scientist Training Program (C.M.B.).

Footnotes

Disclosure: The authors have no conflicts of interest to report.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text, and links to the digital files are provided in the HTML and PDF versions of this article on the journal’s Web site (www.asaiojournal.com).

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Supplementary Materials

Supplemental Figures 1 & 2
NIHMS1809206-supplement-Supplemental_Figures_1___2.pdf (274.9KB, pdf)

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