Rationale: In the original 1974 in vivo study of ventilator-induced lung injury, Webb and Tierney reported that high Vt with zero positive end-expiratory pressure caused overwhelming lung injury, subsequently shown by others to be due to lung shear stress.
Objectives: To reproduce the lung injury and edema examined in the Webb and Tierney study and to investigate the underlying mechanism thereof.
Methods: Sprague-Dawley rats weighing approximately 400 g received mechanical ventilation for 60 minutes according to the protocol of Webb and Tierney (airway pressures of 14/0, 30/0, 45/10, 45/0 cm H2O). Additional series of experiments (20 min in duration to ensure all animals survived) were studied to assess permeability (n = 4 per group), echocardiography (n = 4 per group), and right and left ventricular pressure (n = 5 and n = 4 per group, respectively).
Measurements and Main Results: The original Webb and Tierney results were replicated in terms of lung/body weight ratio (45/0 > 45/10 ≈ 30/0 ≈ 14/0; P < 0.05) and histology. In 45/0, pulmonary edema was overt and rapid, with survival less than 30 minutes. In 45/0 (but not 45/10), there was an increase in microvascular permeability, cyclical abolition of preload, and progressive dilation of the right ventricle. Although left ventricular end-diastolic pressure decreased in 45/10, it increased in 45/0.
Conclusions: In a classic model of ventilator-induced lung injury, high peak pressure (and zero positive end-expiratory pressure) causes respiratory swings (obliteration during inspiration) in right ventricular filling and pulmonary perfusion, ultimately resulting in right ventricular failure and dilation. Pulmonary edema was due to increased permeability, which was augmented by a modest (approximately 40%) increase in hydrostatic pressure. The lung injury and acute cor pulmonale is likely due to pulmonary microvascular injury, the mechanism of which is uncertain, but which may be due to cyclic interruption and exaggeration of pulmonary blood flow.
In their classic 1974 study of ventilation with high Vt and zero positive end-expiratory pressure (PEEP), Webb and Tierney concluded that the resulting severe lung injury was due to vascular interdependence and surfactant depletion. Later studies confirmed these findings and demonstrated major permeability alterations and severe ultrastructural damage likely due to shear stress. These experimental and highly cited works on ventilator-induced lung injury prompted key clinical studies that have changed practice in treating acute respiratory distress syndrome.
We reproduced the key findings of a classic model of ventilator-induced lung injury with high peak pressure and zero PEEP, and our results show respiratory swings in right ventricular (RV) filling and pulmonary perfusion, with both obliterated during peak inspiration. RV systolic pressure, initially maintained, progressively decreases and is ultimately associated with marked RV dilation and a modest (approximately 40%) increase in transmural left ventricular end-diastolic pressure. All of these effects were prevented by addition of PEEP. The mechanism of the acute lung injury and cor pulmonale is uncertain but might be due to pulmonary microvascular injury resulting from cyclic interruption/exaggeration of flow.
There is overwhelming evidence derived from randomized trials that mechanical ventilation (MV) contributes to mortality in patients with acute respiratory distress syndrome (1, 2). These trials were based on earlier clinical studies (3, 4) that in turn were based on laboratory data published in the 1960s to 1980s indicating that MV could directly injure lungs (5–9).
MV was first reported to cause surfactant depletion and atelectasis by Greenfield and colleagues in 1964 (10) and by Faridy and coworkers in 1966 (11). Webb and Tierney, in their highly cited “classic” paper published in 1974 (12), used the in vivo rat to demonstrate fulminant pulmonary edema following high Vt (and protection by positive end-expiratory pressure [PEEP]). Subsequently, Egan and colleagues and Parker and coworkers reported increased permeability (7, 13–15). Dreyfuss and colleagues reported increased microvascular permeability and endothelial lesions (8), and later they (16) and Hernandez and coworkers (17) prevented injury by restricting chest wall expansion. Ventilator-induced lung injury (VILI) was associated with edema due to elevated pulmonary microvascular pressure (Pmv), a phenomenon attributed to unrestricted right ventricular (RV) filling in the open-chest model (18); however, PEEP prevented edema formation in the setting of high peak airway pressure, and because dopamine administration increased edema, this suggested a role for pulmonary microvascular pressure (Pmv) (19).
In pilot experiments, we reproduced the classic study of Webb and Tierney (12) and confirmed the previously reported precipitous pulmonary edema (airway pressure, 45/0 cm H2O). This prompted us to wonder about a cardiac contribution. We therefore reproduced the experiments and determined the effect and contribution of RV function in this model of VILI.
Healthy adult male Sprague-Dawley rats (Charles River Laboratories, Montreal, QC, Canada) weighing 350–500 g were used in all experiments. Institutional ethical approval (conforming to the guidelines of the Canadian Council on Animal Care) was obtained. After anesthesia was induced, venous access, tracheostomy, and carotid artery cannulation were performed. Airway and arterial pressures were continuously monitored, and anesthesia was maintained with continuous infusion of ketamine and xylazine as previously described (20).
Animals were ventilated with baseline settings (Vt, 6 ml·kg−1; respiratory rate, 60 min−1; FiO2, 0.21; PEEP, 3 cm H2O), and baseline parameters were recorded. After stabilization, animals (n = 5 per group) were randomized to one of the following groups, defined by airway pressure (Paw) as 14/0, 30/0, 45/0, or 45/10 cm H2O. The peak inspiratory Paw/PEEP (and corresponding delta pressures) were 14/0 (14), 30/0 (30), 45/0 (45), and 45/10 (35) cm H2O. Whereas the researchers in the original study (12) randomized animals to six groups (control, 14/0, 30/0, 30/10, 45/0, or 45/10 cm H2O), in the present study, we omitted the control (i.e., nonventilated) and 30/10 groups because they were not a focus. Ventilation was maintained for 1 hour (or until death, whichever occurred earlier). Vt was adjusted to maintain assigned Paw as in the original study (12), and respiratory rate was adjusted to maintain normal PaCO2. Dead space (3 ml) was added in the 45/0 and 30/0 groups. Arterial blood gases (ABGs) were measured at baseline (before randomization) and (with dynamic pulmonary compliance [CDyn]) at the start and end of the experiment. The duration of the experiments was 1 hour for groups 14/0, 30/0, and 45/10; for the 45/0 group, the experiment was terminated when the mean arterial pressure (MAP) was 20 mm Hg. After the experiment was complete, lungs were removed and weighed, and the lung/body weight ratio was determined. In selected animals (n = 2 per group), the right lung was pressure fixed in formalin (25 cm H2O) and stained with hematoxylin and eosin for blinded histologic scoring (20).
Animals were randomized to either the 45/0 (n = 4) or 45/10 (n = 4) group to assess microvascular permeability to protein, and an additional comparison group (10/3, n = 2; control) was included. As in series 1, the animals were ventilated at baseline settings before randomization (Vt, 6 ml·kg−1; respiratory rate, 60 min−1; FiO2, 0.21; PEEP, 3 cm H2O). Evans blue dye was given intravenously (30 mg·kg−1). After randomization, animals were ventilated (FiO2, 0.21) for 20 minutes (to ensure survival in the 45/0 group). Ventilation management and ABG (and CDyn) measurement were carried out as in series 1. At the end of the experiment, the left lung was removed, and the ratio of wet-to-dry weight (W/D ratio) was calculated. Right lung lavage was performed with saline (5 ml, repeated three times), and the bronchoalveolar lavage fluid collected was analyzed for Evans blue dye absorbance using photospectrometry (21).
Animals were randomized to either the 45/0 (n = 4) or 45/10 (n = 4) group to assess echocardiographic results (10-MHz probe, Vivid 7 ultrasound system; GE Healthcare, Chicago, IL). Echocardiography was performed at baseline, the start of the experiment, and at 20 minutes. Ventilation management and measurement were carried out as in series 2. The image acquisition plane was standardized (22), and image landmarks remained stable to ensure that a consistent view remained in focus. Inclusion was limited to images where the field of view and Doppler planes were within the standard range. RV output was calculated using the formula CO = ([3.14 × VTI × HR × πr2]/W), where CO is the cardiac output, VTI is the velocity time integral, HR is heart rate, r is the radius of the pulmonary artery, and W is the animal’s weight. VTI was measured using pulsed-wave Doppler at the level of the hinge points of the pulmonary artery annulus in a parasternal short-axis plane at an angle of insonation less than 10 degrees (23). The ratio of RV area to left ventricular (LV) area was measured at midcavity during peak diastole in a parasternal short-axis plane at the level of the tips of the mitral valve leaflets. Right ventricular ejection time (RVET) and pulmonary artery acceleration time (PAAT) were measured using pulsed-wave Doppler, and their ratio was determined (a surrogate of pulmonary vascular resistance [PVR]). The eccentricity index of the left ventricle was calculated in systole and diastole, and fractional shortening was determined by M-mode in a parasternal short-axis plane (24). Tricuspid annular plane systolic excursion (TAPSE, a parameter of global RV function) was determined using M-mode in the four-chamber view (25).
Animals were randomized to either the 45/0 (n = 4) or 45/10 (n = 4) group to assess RV pressures, and an additional comparison group (10/3, n = 2; control) was included. Ventilation management and measurement were carried out as in series 2. A catheter (SPR-407 Mikro-Tip; Millar, Houston, TX) was inserted via the internal jugular vein into the right ventricle to measure right ventricular pressure (RVP). Esophageal manometry (Pes) was performed using a water-filled catheter placed in the lower esophagus as previously described (26). Right ventricular systolic pressure (RVSP) and right ventricular end-diastolic pressure (RVEDP) were noted in inspiration and in expiration.
Transmural (TM) RV pressure was calculated in systole as RVSP TM = RVSP − Pes and at end diastole as RVEDP TM = RVEDP − Pes during inspiration and during expiration. Right coronary perfusion pressure (RCPP) was calculated as RCPP = MAP − mean RVP (27). Transpulmonary pressure (Pl) was calculated as Pl = Paw − Pes in inspiration and in expiration, and ΔPl was calculated as ΔPl = inspiratory Pl − expiratory Pl.
Animals were randomized to either the 45/0 (n = 4) or the 45/10 (n = 4) group to assess LV pressures, and an additional comparison group (10/3, n = 2; control) was included. Ventilation management and measurement were carried out as in series 2. A Millar catheter was placed in the left ventricle via a retrograde right carotid artery approach. Pes was measured as in series 4. LV pressure was measured as an average of 10 beats over a respiratory cycle. For each beat, TM LV pressure was calculated in systole (LVSP) as LVSP TM = LVSP − Pes and at end diastole (LVEDP) as LVEDP TM = LVEDP − Pes. Left coronary perfusion pressure (LCPP) was calculated as LCPP = diastolic SAP − LVEDP, where SAP is systemic arterial pressure (27).
Data were recorded in real time (PowerLab and LabChart software; ADInstruments, Colorado Springs, CO), were expressed as mean ± SD, and were compared using one-way or two-way analysis of variance. Statistical significance was set at P < 0.05.
Five series of experiments were completed.
Four groups were randomized to Paw of 14/0, 30/0, 45/10, or 45/0 cm H2O, respectively, to reproduce the original experiments of Webb and Tierney (12). Twenty animals (n = 5 per group) were studied. Baseline characteristics (e.g., ABGs, Paw) were within normal range in all groups (see Table E1 in the online supplement).
The target Paw was reached in each group. The Vt was highest in the 45/0 group, and in this group, CDyn fell between baseline (0 minutes) and death (Table E1). By the end of the experiment, PaCO2 was comparable in all groups, and the rank order of PaO2 was 14/0 ≈ 30/0 ≈ 45/10 > 45/0 (Table E1). Survival was 100% in all groups except 45/0, in which the duration of survival was 15–30 minutes (mean survival time in 45/0, 26 ± 6 min; P < 0.001 vs. other groups).
Pulmonary edema, expressed as the ratio of wet lung weight/body weight (12), was ranked as 14/0 ≈ 30/0 ≈ 45/10 < 45/0, similar to the results of the original study by Webb and Tierney (Figure 1). Histology revealed greater alveolar damage, edema, and hemorrhage in the 45/0 group than in other groups (Figure E1 and Table E2).
Animals were randomized to ventilation with 45/0 or 45/10, and ventilation was continued for 20 minutes. A small, nonrandomized group of 10/3 was included for illustration but was excluded from statistical comparisons. Permeability (leak of Evans blue dye into the bronchoalveolar lavage fluid) (Figure 2A) and lung W/D ratio (Figure 2B) were significantly higher in the 45/0 group. As in series 1, Vt in the 45/0 group was higher, as was the reduction in CDyn between 0 and 20 minutes. Arterial pH and PaO2 were significantly lower in the 45/0 group (Table E3). In this series, two animals in the 45/0 group survived for 20 minutes, whereas two experiments were terminated early (at 15 and 18 min).
Animals were assigned to ventilation groups as in series 2 above. In the 45/0 group, echocardiography at time 0 minutes revealed that the right ventricle was markedly underfilled during inspiration but not during expiration (Figures 3C and 3D and Video 1). In contrast, in the 45/10 group at 0 minutes, RV filling was constant through inspiration and expiration (Figures 3A and 3B and Video 2). The cyclic inspiratory nonfilling of the right ventricle in the 45/0 group was accompanied by cyclic abolition of RV stroke volume (SV) (Table 1) and pulmonary artery flow. Although the pulmonary artery flow was also reduced in the 45/10 group during inspiration, the magnitude of this effect was less than in the 45/0 group. RV and LV area (a surrogate for volume) was less during inspiration than expiration in the 45/0 group, but it was constant throughout the respiratory cycle in the 45/10 group (Table 1). Over the course of the experiment, the right ventricle became progressively dilated in the 45/0 group (shown at 20 minutes in Video 3), increasing the RV/LV ratio; this did not occur in the 45/10 group (Video 4, Figure 4A, and Table 1). The RVET/PAAT ratio was high in the 45/0 group at 0 minutes during expiration (Table 1). LV eccentricity index progressively increased in the 45/0 group, especially during systole (Table 1). TAPSE declined to a greater extent in the 45/0 group than in the 45/10 group (Figure E2).
|45/0 (n = 4)||45/10 (n = 4)|
|Baseline||0 min||20 min||Baseline||0 min||20 min|
|RV area inspiration||0.07 ± 0.02||0.0*||0.13 ± 0.26*||0.08 ± 0.03||0.16 ± 0.06||0.16 ± 0.06|
|RV area expiration||0.07 ± 0.02||0.20 ± 0.06||0.55 ± 0.12†‡||0.08 ± 0.03||0.16 ± 0.11||0.21 ± 0.10|
|LV area inspiration||0.48 ± 0.07||0.24 ± 0.02‡||0.21 ± 0.11*‡||0.42 ± 0.07||0.34 ± 0.02||0.36 ± 0.12|
|LV area expiration||0.48 ± 0.07||0.37 ± 0.11||0.30 ± 0.06||0.42 ± 0.07||0.36 ± 0.06||0.40 ± 0.11|
|RV/LV ratio inspiration||0.15 ± 0.07||0.00 ± 0.0||1.63 ± 3.3||0.18 ± 0.04||0.43 ± 0.2||0.45 ± 0.09|
|RV/LV ratio expiration||0.15 ± 0.07||0.61 ± 0.36||1.9 ± 0.6†‡||0.18 ± 0.04||0.43 ± 0.23||0.52 ± 0.12|
|RV SV inspiration||0.48 ± 0.15||0.07 ± 0.13*||0.0 ± 0.0*‡||0.45 ± 0.1||0.16 ± 0.03*||0.15 ± 0.07*|
|RV SV expiration||0.61 ± 0.1||0.8 ± 0.17‡||0.44 ± 0.15†||0.57 ± 0.04||0.55 ± 0.08||0.51 ± 0.07|
|LV SV inspiration||0.47 ± 0.1||0.23 ± 0.07*||0.11 ± 0.13*†‡||0.42 ± 0.07||0.26 ± 0.05*||0.27 ± 0.06*|
|LV SV expiration||0.59 ± 0.15||0.51 ± 0.16||0.36 ± 0.17||0.55 ± 0.1||0.47 ± 0.1||0.52 ± 0.05|
|RVET/PAAT inspiration||2.67 ± 0.23||—||—||2.75 ± 0.49||2.68 ± 0.39||3.12 ± 1.1|
|RVET/PAAT expiration||2.67 ± 0.23||3.9 ± 0.63||3.28 ± 0.68||2.75 ± 0.49||2.8 ± 0.48||2.92 ± 0.47|
|Eccentricity diastole||1.03 ± 0.04||1.2 ± 0.27||1.55 ± 0.27||0.96 ± 0.16||1.27 ± 0.13||1.35 ± 0.16|
|Eccentricity systole||1.09 ± 0.15||1.27 ± 0.4||2.98 ± 0.75†‡||1.04 ± 0.41||1.48 ± 0.35||1.69 ± 0.30|
|Fractional shortening||52.8 ± 3.9||45.5 ± 3.8||41.6 ± 4.3||49.4 ± 2.1||44.0 ± 1.4||41.3 ± 4.7|
Ventilation with airway pressure 45/0 at 0 min (series 3). Inspiration is associated with obliteration of the right ventricular cavity.
Ventilation with airway pressure 45/10 at 0 min (series 3). Inspiration is associated with minimal change in right ventricular size.
Video 1) the right ventricular size in expiration is enlarged.Ventilation with airway pressure 45/0 at 20 min (series 3). Inspiration is associated with obliteration of the right ventricular cavity, but (in contrast to 0 min,
Animals were assigned to ventilation groups as in series 2. The RV peak systolic pressure was less during inspiration than during expiration at 0 and 20 minutes in the 45/0 group, but it was not altered by respiration in the 45/10 group (Table 2; specimen traces are provided in Figure E3). The (expiratory) RVSP and RVSP-TM progressively decreased in the 45/0 group during the experiment, but they decreased in the 45/10 group with initial application of PEEP, and they stabilized thereafter (Table 2, Figure 4B). RVEDP was lower in the 45/0 group than in the 45/10 group due to the addition of PEEP (10 cm H2O) (Table 2). The RCPP progressively decreased in the 45/0 group but was not different from that in the 45/10 group at 20 minutes (Table 2). The ΔPl was significantly greater in the 45/0 group than in the 45/10 group (Table E4).
|45/0 (n = 5)||45/10 (n = 5)||10/3 (n = 2)|
|Baseline||0 min||20 min||Baseline||0 min||20 min||Baseline||0 min||20 min|
|RVSP inspiration||41.0 ± 20.4||13.2 ± 7.8*||13.8 ± 7.7*†||28.0 ± 11.9||27.9 ± 12.2||27.4 ± 14.6||28.1 ± 5.2||29.2 ± 3.3||34.2 ± 1.8|
|RVSP expiration||42.0 ± 18.9||37.4 ± 18.1||24.1 ± 12.0||26.4 ± 11.4||25.3 ± 6||24.8 ± 11.6||28.2 ± 2.5||28.7 ± 1.3||34.8 ± 2.5|
|RVSP-TM inspiration||36.5 ± 22.4||8.2 ± 8.5*||9.7 ± 9.5*||27.6 ± 7.5||22 ± 10.6||21.8 ± 13.8||24.5 ± 4.9||25.4 ± 4.0||30.3 ± 1.8|
|RVSP-TM expiration||38.0 ± 21.3||34.5 ± 19.4||21.0 ± 13.1||26.5 ± 7.7||21.1 ± 5.1||21.1 ± 10.7||25.3 ± 3.0||25.8 ± 1.6||32.2 ± 2.4|
|RVEDP inspiration||3.2 ± 3.7||2.7 ± 4.0||3.4 ± 4.8||5.0 ± 3.0||7.6 ± 2.6||8.4 ± 5.1||3.2 ± 0.1||3.8 ± 0.14||3.3 ± 1.3|
|RVEDP expiration||3.4 ± 3.2||2.9 ± 3.8||2.6 ± 4.9†||4.1 ± 3.0||6.1 ± 3.0||7.7 ± 3.7||2.4 ± 0.5||2.8 ± 1.3||2.7 ± 0.8|
|RVEDP-TM inspiration||−0.94 ± 5.4||−1.5 ± 5.4||−0.84 ± 6.7||0.88 ± 1.7||1.6 ± 3.1||3.1 ± 4.7||−0.85 ± 0.5||−0.15 ± 0.6||−0.4 ± 0.7|
|RVEDP-TM expiration||−0.36 ± 5.2||0.16 ± 5.4||−0.4 ± 6.5||0.53 ± 1.3||2.1 ± 2.8||3.9 ± 3.2||−0.5 ± 0.0||−0.05 ± 0.5||0.4 ± 0.2|
|MAP||104 ± 17.2||89 ± 22.8||45.5 ± 14.2||74.4 ± 26.1||55.5 ± 9.2||48 ± 8.2||102 ± 9||100 ± 8||60 ± 2.1|
|Mean RVP||16.2 ± 8.4||12.2 ± 5.1||9.3 ± 4.5†||12.2 ± 4.7||12.6 ± 3.6||13.7 ± 6.1||11.0 ± 0.5||11.3 ± 1||12.7 ± 1.4|
|Right CPP||87.6 ± 21.8||77 ± 20.4||33 ± 12.2||62.2 ± 28.7||42.8 ± 11.3||34 ± 11.8||91.3 ± 9.4||88 ± 7.1||47 ± 0.7|
Animals were assigned to ventilation groups as in series 2. The LVSP and LVSP-TM progressively decreased in the 45/0 group, but they remained stable in the 45/10 group (following initial application of PEEP) (Table E5). In the 45/0 group, the LVEDP and mean LVEDP-TM increased progressively from 9 to 14 mm Hg (Figure 5, Table E5), whereas in the 45/10, group the LVEDP-TM was lower than in the 45/0 group throughout the experiment (Table E5; representative traces are provided in Figure E4).
In the present study, we reproduced the data reported in the classic study by Webb and Tierney (12). The pulmonary edema is due to a large increase in microvascular permeability, as previously shown (7, 8, 16), in addition to a modest (approximately 40%) increase in hydrostatic pressure, which may augment edema formation (18, 28). Ultimately, with high Vt and the absence of PEEP, acute cor pulmonale develops and is lethal. These effects are in addition to lung inflammation that occurs because of repetitive pulmonary strain.
Several points are important in drawing these conclusions. The original study (12) showed that ventilation with either moderate Paw (30 cm H2O) or high Paw with PEEP (45/10 cm H2O) caused mild injury, but that ventilation with high Paw and zero PEEP (45/0 cm H2O) caused rapid, severe (and lethal) injury (12). These important results were reproduced in the present study (series 1) in terms of lung/body weight ratio (pulmonary edema), histology, mortality, and comparable values for ABG and CDyn at the beginning and end of the experiment in each group. As in previous studies (7, 8, 16), the present study also demonstrates that with 45/0 cm H2O, microvascular permeability is increased (series 2, Figure 2), and the high W/D ratio indicates development of substantial pulmonary edema (Figure 2B).
Additional insight in the present study derives from hemodynamic studies (series 3–5). Here, events can be described as those influenced by the respiratory cycle (i.e., inspiration vs. expiration) and those that evolved over the course of the experiment (20 min). In the 45/0 group, echocardiography demonstrated that the end-diastolic RV area (surrogate for end-diastolic RV volume) was reduced during inspiration (Figure 3, Table 1, and Video 1); that is, during inspiration, preload was reduced with concomitant absence of pulmonary artery flow (Table 1). During expiration, filling and SV were greater (Figure 3, Table 1, and Video 1).
The RVET/PAAT ratio, a surrogate for PVR, was high during expiration in the 45/0 group (Table 1); however, during inspiration, the RVET/PAAT ratio is not reportable, owing to absent flow. The high inspiratory pressure can compress alveolar capillaries, creating zones I and II conditions (29); this could impose a high afterload on the right heart (30). Thus, the RV output and afterload undergo large cyclic swings at 45/0 cm H2O. Lesser degrees of oscillation also occurred at 45/10 cm H2O (Video 2). Similar effects of differential lung volume on PVR have been shown in canine lobes (31).
The LVSV also undergoes cyclic changes with respiration, and the magnitude is similar at 45/10 and 45/0 cm H2O (Table 1). It is important to note that in the 45/0 group, during expiration at time 0 minutes, the RVSV was transiently greater than the LVSV, suggesting that phasic pooling of blood occurred in the pulmonary circuit. In contrast, in the 45/10 group, the RVSV and LVSV were similar. High-fidelity pressure recording (Millar catheter; series 4) confirmed that in the 45/0 group, the RVSP was reduced significantly during inspiration, concomitant with interruption of flow (i.e., absent preload) (Table 2, Figure E3), whereas in the 45/10 group, the RVSP was unchanged.
During MV, changes in pleural pressure (Ppl) and Pl (in addition to pulmonary artery and venous pressures, as well as flow) contribute to Pmv (32). We demonstrated high inspiratory Pl in the 45/0 group (Table E4), and this could potentially obliterate postcapillary venous flow, increasing the local Pmv (33). Increases in Pl and Pmv can each cause capillary stretch (34, 35) and could be exacerbated by the expiratory increase in RVSV, especially in zone II or III conditions. Finally, where expiratory Paw is zero, the TM Pmv is high. Taken together, these effects could potentiate capillary wall stress and microvascular damage, which in small animals has been described at relatively modest pressure elevations (36) and would result in increased permeability and lung edema. This is consistent with the sequence reported in this model of VILI by Dreyfuss and colleagues (8), where high Vt injury results in endothelial lesions before epithelial lesions.
The effects of inspiratory swings on Ppl and Pl have been modeled by computer simulation (32): Increases in Ppl primarily reduce RV preload (as we observed; Figure 3, Table 1), and changes in Pl primarily increase RV afterload, by development of West’s zone 1 or 2 conditions (i.e., absent or interrupted pulmonary perfusion). These hemodynamic changes were reduced by addition of PEEP (45/10) reflecting the reduced swings in Ppl and Pl (Table E4). These marked swings in RV volume and pulmonary perfusion resulted in marked reversed pulsus paradoxus in the 45/0 group compared with the 45/10 group (Figure E3).
In the 45/10 group, with each respiratory cycle, the RV area (therefore volume) underwent no change; however, the RV systolic pressure increased, and the output fell. These changes may indicate an effect of PEEP on the peripheral circulation (reviewed in ).
In the 45/0 group, the RV systolic (and the TM systolic) pressure fell over the course of the experiment, the RV area increased (Tables 1 and 2, Figure 4, and Video 3), and TAPSE decreased (Figure E2), all consistent with acute RV failure. With regard to inspiratory (vs. expiratory) RV volume, 45/0 reflects RV preload (a function of swings in Ppl), whereas dilation over the course of the experiment reflects increasing afterload (i.e., cor pulmonale) (Table 1). This was accompanied by decreased LV end-diastolic area, slightly increased LVEDP (i.e., LVEDP-TM, the “effective” pressure) (Figure 5 and Figure E4), higher systolic eccentricity index (Table 1), and reduced LVSV (Table 1). It is important to note that LVEDP-TM (and not LVEDP) is the effective hydrostatic pressure that best reflects the pulmonary microvascular pressure. It is also important to note that the measures of PVR (i.e., RVET/PAAT) are flow dependent; hence, the values are underestimated at 20 minutes due to RV failure (during expiration, due to insufficient flow) and are not reportable during inspiration (due to absent flow).
The LVEDP-TM was increased in the 45/0 group, and the contributors include compression by the dilated right ventricle (cor pulmonale, Figure 4 and Video 3), which parallels increased LV afterload and wall stress (Figure E5B). Although LVEDP-TM is an underestimation of Pmv (35, 38), the modest increase in observed Pmv would not cause edema, but in the presence of increased permeability, it would exacerbate edema (18, 28).
An important insight may be observed in the hemodynamic changes at the beginning of the experiment. The cardiac output decreased to the same extent in the 45/0 and 45/10 groups, but the decrease in MAP was greater in the 45/10 group (Table E1). Therefore, the systemic (and hence LV) afterload must have been higher in the 45/0 group; this is reflected in the increased LV systolic TM pressure (Figure E5), and it would be associated with an increased propensity to developing pulmonary edema.
The pathogenesis of pulmonary edema involves factors affecting fluid filtration and the integrity of the microvascular membrane (39). In the context of VILI, a major contributor to edema is microvascular membrane damage resulting from stretch (8); moreover, reducing this stress (Vt) reduces edema formation (16, 19). Increased permeability to protein resulting from such microvascular damage has previously been demonstrated in experimental VILI (7, 8), and it is recapitulated in the present study (Figure 2A). Pulmonary edema predominates in regions of increased pulmonary perfusion (38) because increased microvascular pressure augments the rate of edema development in the setting of increased permeability (Figure 2A), as shown previously (18, 28), and can augment permeability (35, 40, 41). Also, higher vascular flows (as seen during expiration in this model), together with increasing microvascular pressure, can further enhance edema formation (42). A contribution of microvascular pressure has been reported in isolated perfused (43–46) or open-chested (18) models of VILI and is consistent with the demonstration in the present model of increased permeability (Figure 2A), increased microvascular pressure (Figure 5 and Figure E4), and increased overall edema formation (Figures 1 and 2B).
The edema in the 45/10 group was less than in the 45/0 group (Figure 1). Application of PEEP (higher mean Paw) may further compress collapsible microvasculature and reduce cardiac output, which may in turn derecruit the capillary bed; both higher Paw and lower perfusion may reduce the surface area of perfused capillaries. In addition, with PEEP 10 cm H2O, the higher Paw and the reduced Pmv (Figure 5) together decrease TM Pmv. Together, these factors may have contributed to less edema formation in the 45/10 group. Of note, dopamine infusion in this model (45/10) increased edema formation in another study (19), presumably by increasing pulmonary perfusion and/or microvascular pressure.
Of note, the RVSV and LVSV were less out of phase (Table 1) than would be expected with positive pressure ventilation because the ratio of heart rate to respiratory rate was high (300/30 = 10 times as high). Thus, compression and emptying of the right ventricle in this rodent experiment would result in forward emptying of the left ventricle within the same inspiration.
The acute RV dilation (45/0) noted in this study (Figure 4A) represents acute cor pulmonale, which in turn results from increased PVR. The reasons for increased PVR may include high inspiratory Paw causing phasic zones of nonperfusion and increased RV afterload (see above) (30). However, due to simultaneous volume unloading of the right ventricle, pressure loading could not be accessed contemporaneously. In addition, the absence of PEEP would result in low expiratory lung volume and could independently increase PVR (47) (Table 1, RVET/PAAT ratio). Finally, the development of pulmonary edema and hypoxemia can also increase PVR (although PaCO2 was not elevated). Pulmonary vascular injury may also have contributed to the increased PVR. There is clear evidence of such injury, such as increased permeability in the present (Figure 2A) and previous (7, 8) studies, as well as in previously reported endothelial lesions (8). It is possible, though not proved, that such injury was caused by the cyclic interruption of pulmonary vascular flow. For example, phasic changes in pulmonary blood flow contribute to lung edema (43), and reperfusion of ischemic lungs can lead to pulmonary microvascular injury (48, 49). Also, repetitive “stress” is often more injurious than single stressors (50), and this has been conceptually proposed in VILI (38). In addition, changes in flow can increase endothelial permeability in vitro (51). Finally, the presence of neutrophil infiltration (Figure E1) indicates acute inflammation, but the rapid mortality suggests lethal circulatory failure.
There are limitations to the present data. The original study (12) was not entirely replicated: The 30/10 and control (spontaneously breathing) groups were omitted because they were not relevant to studying the outcome of interest (Table E1). Although the original study had more groups (six vs. four), the present study was randomized, whereas the original one was not. The precise processing details of lung histology were uncertain, and we performed standardized (20) histologic processing and interpretation that reflect well the original patterns of injury (12) (Figure 2 and Table E3). Additional differences in the original study include the use of HLA-W rats (200–250 g; Hilltop Lab Animals, Scottdale, PA) and halothane/pentobarbital anesthesia (12); in contrast, in our present experiments, we employed heavier (300–500 g) Sprague-Dawley rats and ketamine/xylazine anesthesia. Although increased RV afterload is inferred, we did not record increased RV systolic pressure. Finally, although LV pressures and shape were monitored, LV function was not. The classic work by Webb and Tierney (12), one of the foremost in the field of VILI, involved swings in RV filling and SV, acute cor pulmonale, and a modest increase in LVEDP.
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Supported by research funds from the Canadian Institutes of Health Research (B.P.K.). B.P.K. holds the Dr. Geoffrey Barker Chair in Critical Care Medicine. K.A.C. received a new investigator award from the Canadian Institutes of Health Research and an early researcher award from the Ontario Ministry of Research, Innovation and Science.
Author Contributions: Conception and design: B.H.K., R.E.G., D.E., D.Z., A.K., G.O., T.Y., W.M.K., P.J.M., K.A.C., and B.P.K.; analysis and interpretation: B.H.K., R.E.G., D.E., D.Z., A.K., G.O., T.Y., W.M.K., P.J.M., K.A.C., and B.P.K.; and drafting of the manuscript for important intellectual content: B.H.K., R.E.G., D.E., D.Z., A.K., G.O., T.Y., W.M.K., P.J.M., K.A.C., and B.P.K.
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Originally Published in Press as DOI: 10.1164/rccm.201611-2268OC on August 10, 2017