Combining Partial Liquid Ventilation and Prone Position in Experimental Acute Lung Injury 

The experimental protocol was approved by the appropriate governmental institution, and the study was performed according to the Helsinki convention for the use and care of animals.

Twenty female pigs (Deutsches Hausschwein) with body weight of 29.2 ± 2.5 kg (PLV group 29.9 ± 3.2 kg body weight, gaseous ventilation [GV] group 28.5 ± 1.5 kg body weight) were included in the study after confirming the absence of any clinical sign of infection by the responsible veterinary surgeon.

Premedication was performed with azaperon (5 mg/kg intramuscularly) followed by ketamine (10 mg/kg intramuscularly) and atropine (0.01 mg/kg intramuscularly). After venous access via  an auricular vein, anesthesia was induced with a bolus of thiopental (3 mg/kg intravenously) and maintained with a continuous infusion of thiopental (6–10 mg · kg⁻¹· h⁻¹) and fentanyl (0.1 μg · kg⁻¹· min⁻¹). Muscle relaxation was achieved with pancuronium bromide (3 μg · kg⁻¹· min⁻¹). All animals were orotracheally intubated with a 7.5–8.5-mm internal diameter endotracheal tube and submitted to pressure-controlled mechanical ventilation (Servo 300 NO-A; Siemens Elema, Lund, Sweden). Peak inspiratory pressure was limited to 40 cm H²O at a respiratory rate of 20 per minute, an inspiration:expiration time ratio of 1:2, and a positive end-expiratory pressure of 5 cm H²O. The inspiratory pressure was set to achieve tidal volumes of 8–10 ml/kg; the fraction of inspiratory oxygen (FI^O2) was maintained at 1.0 throughout the experiment. Prior to the induction of lung injury, the inspiratory pressure was adjusted to keep the animals normocapnic, as determined by two sequential blood gas analyses, with no further changes of the respiratory setting thereafter.

A continuous infusion of 3–5 ml · kg⁻¹· h⁻¹of balanced electrolyte solution was administered for adequate hydration.

Arterial access was achieved by introducing an 18-G catheter (Vygon, Ecouen, France) into a femoral artery. A pulmonary artery catheter (model 93A-431–7.5 F; Baxter Healthcare Corporation, Irvine, CA) was advanced into a pulmonary artery under transduced pressure guidance through an 8.5-French sheath (Arrow Deutschland GmbH, Erding, Germany) positioned in a femoral vein. The blood temperature, measured by means of the pulmonary artery catheter, was maintained at 37.8 ± 0.9°C during the experiment using an infrared warming lamp.

Measurements were taken in prone and supine positions according to the experimental protocol with zero reference level at the midchest. Mean arterial pressure, mean pulmonary arterial pressure, central venous pressure, and pulmonary artery occlusion pressure were transduced (PVP, Kirschseeon/Eglharting, Germany) and recorded (model CS/3; Datex, Achim, Germany). Cardiac output was determined with standard thermodilution techniques (Baxter Deutschland GmbH, Unterschleiβheim, Germany) and expressed as the mean of three measurements at random phases of different respiratory cycles. Heart rate was traced by the arterial blood pressure curve.

Arterial and mixed venous blood samples were collected anaerobically and immediately analyzed for P^O2, P^CO2, and p  H using standard blood gas electrodes (ABL 520; Radiometer, Copenhagen, Denmark). Species-specific spectrophotometry was performed to obtain arterial and mixed venous oxygen saturation and total hemoglobin concentration (OSM 3 Hemoximeter; Radiometer). The oxygen contents (ml/dl) of arterial (Ca^O2), mixed venous (Cv^O2), and pulmonary capillary (Cc^O2) samples were calculated using the formula: Content of oxygen = hemoglobin concentration × 1.34 ×(% O²saturation/100)+ P^O2× 0.0031. To calculate Cc^O2, the pulmonary capillary oxygen tension at a FI^O2of 1.0 was assumed to be equivalent to the alveolar P^O2, which was estimated as: barometric pressure − water vapor pressure − Pa^CO2/respiratory quotient (assuming that the respiratory quotient = 0.8)− perfluorocarbon vapor pressure when PLV was performed (61 mmHg for the perfluorocarbon used in our study). 15,17The venous admixture (Q^S/Q^T) was derived from the standard shunt equation:(Cc^O2− Ca^O2)/(Cc^O2− Cv^O2).

The anesthetized animals were positioned supine and lung injury was induced by repeated lung lavage as previously described and evaluated by Lachmann et al . 18After the lungs were filled with 30 ml/kg prewarmed saline (0.15 M; 38°C) through the endotracheal tube, the liquid was drained by tilting the animals head down at an angle of about 45 degrees using the gravitational gradient. A Pa^O2consistently less than 100 mmHg for 1 h at a FI^O2of 1.0 and a positive end-expiratory pressure of 5 cm H²O after the last lung lavage was chosen as the only endpoint to define acute lung injury. To achieve this value, 10 ± 1 lung lavages were performed in the PLV group, and 11 ± 1 lavages were necessary to induce acute lung injury in the animals submitted to GV.

Partial liquid ventilation was initiated in the supine animal with the administration of two sequential doses of 15 ml/kg perfluorocarbon (FC 3280; 3M Chemical Products, Neuss, Germany) through the endotracheal tube during inspiration, using a swivel connector (Portex, Kent, UK). Each dose was administered into the airway over 9–10 min resulting in a volume of approximately 2–2.5 ml of perfluorocarbon per inspiration. The total volume of 30 ml/kg perfluorocarbon is approximately equivalent to the normal functional residual capacity of pigs. 1The perfluorocarbon used in our study (C⁸F¹⁸, FC 3280; 3M Chemical Products) is a highly purified industrial perfluorocarbon with a density of 1.75 g/cm³, a vapor pressure of 61 mmHg, and a surface tension of 12 mN/m at 25°C. Up to 40 ml O²and 192 ml CO²can be dissolved in 100 ml of the liquid (data from 3M Chemical Products).

Gas-exchange, hemodynamic, and airway pressure data were assessed in supine position in all animals before (T^Baseline) and after induction of lung injury (T^ALI). Two groups of animals were studied. After investigating 10 animals assigned to PLV, the possibility remained that the results were related not to the perfluorocarbon instillation but to independent changes related to time and position. Therefore, we studied a second group of 10 animals submitted to GV to exclude such influences. Measurements of blood gases, hemodynamics, and airway pressure were performed in both groups at the following time points and experimental settings:(1) T³⁰: 30 min after the induction of acute lung injury, prone;(2) T⁶⁰: 60 min after the induction of acute lung injury, supine;(3) T⁹⁰: 90 min after the induction of acute lung injury and 30 min after initiation of PLV with 15 ml/kg perfluorocarbon in the PLV group, supine;(4) T¹²⁰: 120 min after the induction of acute lung injury and 60 min after the initiation of PLV with 15 ml/kg perfluorocarbon in the PLV group, prone;(5) T¹⁵⁰: 150 min after the induction of acute lung injury and 30 min after the initiation of PLV with 30 ml/kg perfluorocarbon in the PLV group, supine;(6) T¹⁸⁰: 180 min after the induction of acute lung injury and 60 min after the initiation of PLV with 30 ml/kg perfluorocarbon in the PLV group, prone. A 30-min time period was given for equilibration prior to the measurements following each change in position or perfluorocarbon dose.

At the end of the study the anesthetized animals were killed with an intravenous administration of potassium chloride.

All data are expressed as mean ± SD. Statistical analyses were performed using a suitable software package (NCSS 6.0.7.; NCSS, Kaysville, UT). The data were analyzed by analysis of variance (ANOVA) for repeated measurements. The statistical design for the ANOVA procedure was based on three within factors, that is, PLV having two levels (15 ml/kg, 30 ml/kg), positioning having two levels (prone, supine) and group having two levels (GV group, PLV group). For significant ANOVA results Duncan's post hoc  test was used for comparisons between and within the groups. P < 0.05 was considered as significantly different.

Repeated lung lavages (PLV group: 10 ± 1, GV group: 11 ± 1) were performed to induce acute lung injury in all animals as indicated by a decrease of Pa^O2from 510 ± 35 mmHg to 54 ± 11 mmHg and an increase in Q^S/Q^Tfrom 16 ± 7% to 61 ± 10%(table 1). Pa^CO2increased from 38 ± 7 mmHg to 54 ± 9 mmHg after lung lavage. Detailed gas-exchange, hemodynamic, and airway-pressure data at T^Baselineand T^ALIfor each group are presented in tables 1 and 2. There were no significant differences between the two groups at these time points for any parameter tested. All animals survived the entire study period.

Positioning the animals prone during GV (T³⁰) resulted in a significant increase of Pa^O2and Pv^O2and a significant decrease in Q^S/Q^Tin both groups compared with subsequent supine positioning (T⁶⁰) and to T^ALI(P < 0.001;fig. 1, table 1). Decreases in Pa^CO2resulting from prone position were observed in both groups but did not reach statistical significance; however, p  H increased significantly in the PLV group during prone position compared with supine at T⁶⁰(table 1).

Comparison between the GV and the PLV group of gas-exchange and metabolic data at corresponding time points (T³⁰and T⁶⁰) revealed a significant difference only for p  H at T³⁰(table 1).

Analyses of the hemodynamic data within each group showed a significant increase of MAP during prone position at T³⁰for the PLV group compared with T^ALIand T⁶⁰. In the GV group CO decreased significantly if the animals were positioned prone at T³⁰compared with T^ALIbut not compared with subsequent supine position. Mean pulmonary arterial pressure in the same group increased from 28 ± 3 mmHg (T³⁰) to 35 ± 6 mmHg (T⁶⁰) after turning the animals supine. Comparison of hemodynamic parameters at T^ALI, T³⁰, and T⁶⁰revealed no differences for these time points between the GV and the PLV groups (table 2).

The peak inspiratory pressure increased in both groups after the induction of lung injury. Turning the animals prone resulted in a decrease of peak inspiratory pressure compared with T^ALI(P < 0.05), which was reversed when the animals were returned to supine position (T⁶⁰)(table 2).

Partial liquid ventilation in supine position resulted in an increase of Pa^O2in the PLV group, which reached statistical significance compared with T^ALIonly for the higher of the two doses of perfluorocarbon used (P < 0.001, fig. 1). However, a simultaneous increase in mixed venous carbon dioxide tension (Pv^O2) and a decrease of Q^S/Q^Twere significant at T⁹⁰and T¹⁵⁰compared with T^ALI(P < 0.001;table 1). Pa^CO2demonstrated no changes following PLV with either dose of perfluorocarbon;p  H, standard base excess, and HCO³⁻levels increased persistently compared with T^ALIafter perfluorocarbon was instilled.

Cardiac output decreased significantly after the instillation of perfluorocarbon together with a decrease in heart rate from 103 ± 13 (T^ALI) to 88 ± 17 (T⁹⁰) and 80 ± 15 beats/min (T¹⁵⁰) respectively (P < 0.05). All other hemodynamic parameters and the peak inspiratory airway pressure remained unchanged (table 2).

Turning the animals from supine to prone position during PLV resulted in a further significant increase of Pa^O2from 109 ± 42 (T⁹⁰) to 176 ± 83 mmHg (T¹²⁰) and from 192 ± 92 (T¹⁵⁰) to 265 ± 111 mmHg (T¹⁸⁰) when time points with similar doses of perfluorocarbon were compared in the PLV group, although changes in Pa^O2during PLV with 15 ml/kg of perfluorocarbon while positioned prone did not exceed the effect that was achieved with prone position alone in the GV group. However, a significant, additive effect of PLV and prone position on arterial oxygenation was observed at T¹⁸⁰compared with PLV with the same dose of perfluorocarbon in supine position (T¹⁵⁰) and to the corresponding time point in the GV group (T¹⁸⁰) as depicted in figure 1.

Comparison of supine and prone position during PLV with equal doses of perfluorocarbon in the PLV group revealed no changes for Pv^O2, Pa^CO2, Q^S/Q^T, or any hemodynamic or metabolic parameter.

In the GV group, arterial and venous oxygenation and the venous admixture increased reproducibly if the animals were positioned prone compared with supine position and T^ALI, whereas turning them supine again reversed this improvement to lung-injury values. Pa^CO2increased slowly but continuously over the study period, with a concomitant decrease in standard base excess and p  H that was significantly different from the PLV group for all time points from T⁹⁰through T¹⁸⁰. No changes in hemodynamics caused by changes in positioning were observed in the GV group, except a significant increase in mean pulmonary arterial pressure when the animals were supine. Similarly, a decrease in peak inspiratory pressure was observed when the animals were turned prone (table 2).

Comparison of gas-exchange, airway-pressure, and hemodynamic data at T²¹⁰and at T^ALIin the GV group revealed statistical differences only for peak inspiratory pressure, mean pulmonary arterial pressure, and pulmonary artery occlusion pressure. All other hemodynamic and gas-exchange parameters remained stable over the entire study period and were not influenced by intermittent changes in positioning or time.

The objective of our study was to evaluate combined effects of prone position and PLV on pulmonary gas exchange in a saline lung lavage model of acute lung injury in medium-sized pigs. We demonstrated that a combination of prone position and PLV can improve arterial oxygenation depending on the dose of perfluorocarbon. There was a tendency for the prone position to decrease Q^S/Q^Tin the PLV group, although it did not reach statistical significance. The maximum effect of the combined application of these techniques on Pa^O2exceeded the effects that were obtained with prone position or PLV in supine position alone.

Two mechanisms are proposed to explain the increase of Pa^O2during PLV. First, the intratracheal instillation of perfluorocarbon can cause a recruitment of atelectatic lung segments, thereby improving the impaired ventilation/perfusion ratio. Atelectasis, especially of the dependent lung segments is a typical feature generated by the well-established and extensively used lung lavage model employed in this study. 18Second, perfluorocarbon may compress pulmonary capillaries in the dependent lung regions because of the high density of the liquid. 3,19The redistribution of pulmonary blood flow from dependent poorly or nonventilated lung segments to well-ventilated, nondependent lung regions may result in a reduction of pulmonary shunt and an improvement of arterial oxygenation. 4 

In our investigation, PLV in supine position caused a significant increase in Pa^O2only after the larger of the two doses of perfluorocarbon (equivalent to the functional residual capacity of the pigs) was given. A dose-dependent increase of arterial oxygenation during PLV has been described by other authors using the same model of acute lung injury in different species. 5,20However, although improvements in small-animal models and human newborns have been achieved already with small doses of perfluorocarbons, 20,21medium-sized animals and adult patients seem to require larger doses for significant improvements in Pa^O2, as observed in our study. 22It is suggested that the larger anteroposterior thoracic diameter results in a more inhomogenous distribution of perfluorocarbon and gas in the lung, with a pooling of the liquid in the dependent lung regions. 22–24Because of the increase of the preexisting gravitational pleura pressure gradient in supine position during PLV 25, ventilation of the dependent, liquid-filled lung areas and concomitantly the oxygenation of the perfluorocarbon may be poor. 26Therefore, the dorsal lung segments will remain hypoxic until the perfluorocarbon dose is increased and there is a reflux of the liquid into central airways, where it can be better reoxygenated and subsequently redistributed to the peripheral airways by the inspiratory gas flow.

Several investigators report that the prone position increases Pa^O2in patients with ARDS. 10,11,27A number of animal studies have shown that this improvement is associated with more homogenous ventilation, resulting from an increase of the vertical pleura pressure gradient when the animals were turned from supine to prone position. 15,26,28,29An additional increase of the pleura pressure gradient during PLV in supine position has been shown, 25and it can be assumed that a decrease of this gradient in prone position as observed during GV also occurs if PLV is performed. A more uniform dispersal of the gaseous tidal volume and a reduction of the airway pressures necessary to apply sufficient tidal volumes and to move the liquid column in the dependent lung segments could cause a more efficient recruitment and ventilation of alveoli in all lung regions and a facilitated reoyxgenation of the perfluorocarbon as indicated by the increase of Pa^O2and the marked but not significant decrease in Pa^CO2during PLV in prone position in our investigation.

The decrease in cardiac output during PLV has already been observed by other investigators and is likely to be the result of a compression of the pulmonary vascular bed with a subsequent decrease of right ventricular preload and an increase of right ventricular afterload. 22However, the changes in cardiac output during PLV did not result in a decrease of Pv^O2or mixed venous oxygen saturation in our study, indicating that oxygen delivery and cardiac function were still sufficient to avoid an increase in the arterial/venous P^O2difference.

Time may be a crucial factor if changes in gas exchange are evaluated over a prolonged study period. However, in our investigation we did not see any difference in gas-exchange parameters when values of the GV group at the end of the study were compared with those after the induction of lung injury, indicating a high stability of the lung injury model applied.

Despite the efficacy of the perfluorocarbon used in our study, the type of perfluorocarbon may influence the response to PLV, although chemical and physical properties of the different perfluorocarbons presently tested for PLV are very similar. Previous investigations of our group revealed no differences in the efficacy of C⁸F¹⁸, the perfluorocarbon used in our study, compared with the perfluorocarbon recently under investigation in human trials (Perflubron, Liquivent; Alliance, San Diego, CA). 8,21Regarding the physical properties, C⁸F¹⁸reveals a higher vapor pressure than Perflubron. This can result in a faster evaporation of perfluorocarbon from the lungs, which is more likely to account for a slight, continuous decrease of Pa^O2and an increase of Q^S/Q^Tduring long study periods. Other investigators compensated this effect by a continuous substitution for losses resulting from evaporation. 7,21However, because of the short time period between two measurements with the same dose of perfluorocarbon (30 min) we did not administer subsequent doses of perfluorocarbon in the present investigation.

Our data suggest that the combined application of PLV and prone position can be an efficient technique to further improve the beneficial effect on pulmonary gas exchange that has been observed during PLV in supine position. Changing the position of the pigs during the experiment did not cause any incident compromising the animals. The mechanisms by which PLV in supine position improves pulmonary gas exchange and lung mechanics are still not completely clarified. There are controversial data concerning the hypothesized redistribution of pulmonary blood flow in animal models 3and the effect on arterial oxygenation in clinical trials on patients with ARDS. 8Therefore, it is difficult to predict the mechanisms that may account for the observed improvement during PLV in prone position in our study, and explanations have to remain hypothetical. To clarify these questions, further studies investigating the mechanism of PLV when combined with other therapeutic strategies or alone are required.

The authors thank Gabriele Wettmann, Helmut Wiese, and Rolf Dembinski for their excellent technical assistance.