High-frequency oscillatory ventilation with partial liquid ventilation in a model of acute respiratory failure

Conventional vs High-Frequency Ventilation for Weaning from Total Liquid Ventilation in Lambs

OBJECTIVE

To compare conventional gas ventilation (GV) and high-frequency oscillatory ventilation (HFOV) for weaning from total liquid ventilation (TLV).

METHODS

Sixteen lambs were anesthetized. After 1 h of TLV with perflubron (PFOB), they were assigned to either GV or HFOV for 2 h. Oxygen requirements, electrical impedance tomography and videofluoroscopic sequences, and respiratory system compliance were recorded.

RESULTS

The lambs under GV needed less oxygen at 20 min following TLV (40 [25, 45] and 83 [63, 98]%, p = 0.001 under GV and HFOV, respectively). During weaning, tidal volume distribution was increased in the nondependent regions in the GV group compared to baseline (p = 0.046). Furthermore, residual PFOB was observed in the most dependent region. No air was detected by fluoroscopy in that region at the end of expiration in the GV group.

CONCLUSION

GV offers a transient advantage over HFOV with regards to oxygenation for TLV weaning.

Conventional Gas Ventilation, Liquid-Assisted High-Frequency Oscillatory Ventilation, and Tidal Liquid Ventilation in Surfactant-Treated Preterm Lambs

This study was designed to compare the efficacy and potential protective or injurious effects of tidal liquid ventilation (TLV), liquid-assisted high-frequency oscillatory ventilation (LA-HFOV), and high PEEP conventional mechanical ventilation (CMV) in neonatal respiratory distress syndrome. Preterm lambs (124–126 days gestation), prophylactically treated with natural surfactant, were allocated to one of the treatment modalities or to an untreated fetal control group (F), euthanised after tracheal ligation. LA-HFOV animals received an intratracheal loading dose of 5 mL.kg-1 followed by a continuous intrapulmonary instillation of 12 mL.kg-1;h-1 FC-75 perfluorocarbon liquid. The ventilation strategies aimed at keeping clinically appropriate arterial blood gases for a study period of 5 hours. A histological lung injury score was calculated and semiquantitative morphometry was performed on lung tissue fixed by vascular perfusion.

The alveolar-arterial pressure difference for O2 was significantly lower throughout the study in TLV compared to CMV lambs; at 1, 2, and 5 hours, oxygenation was better in TLV when compared to LA-HFOV. Total lung injury scores in TLV lambs were significantly lower than in either CMV or LA-HFOV animals, but higher when compared to F. CMV and LA-HFOV induced an excess of collapsed and overdistended alveoli, whereas in TLV alveolar expansion was normally distributed around predominantly normal alveoli. CMV and LA-HFOV, but not TLV, were associated with an excess of dilated airways.

Thus, in the ovine neonatal RDS model, TLV compared favourably to either gas ventilation strategy by its more uniform ventilation, reduced lung injury, and improved gas exchange.

Gas exchange and lung mechanics during high frequency ventilation in the perflubron-treated lung

OBJECTIVE

To identify the effect of perflubron on gas exchange and lung mechanics during high frequency oscillatory ventilation in an animal model.

DESIGN

Prospective randomized animal trial.

SUBJECTS

Eighteen Yorkshire swine.

INTERVENTIONS

Three groups of six animals each were investigated: control (high frequency oscillatory ventilation alone), low dose perflubron (high frequency oscillatory ventilation plus perfluoro-octyl bromide [PFOB]-Lo, 1.5 mL/kg), and high dose perflubron (high frequency oscillatory ventilation plus PFOB-Hi, 3 mL/kg). Lung injury was induced with repeated saline lavage and amplified for 1 hr using large tidal volumes. Perflubron (Alliance, CA) or a sham dose (room air) was administered with bronchoscopic guidance. The animals were transitioned to high frequency oscillatory ventilation starting at a mean airway pressure of 15 cm H2O. Mean airway pressure was increased (inflation phase) by 5 cm H2O every 15 mins to a maximum mean airway pressure of 40 cm H2O. During the deflation phase, mean airway pressure was reduced by 5 cm H2O every 15 mins to a mean airway pressure of 15 cm H2O.

MEASUREMENTS AND MAIN RESULTS

Oxygenation was improved and pulmonary shunt fraction was reduced for PFOB-Hi compared with the control group only at a mean airway pressure of 15 and 20 cm H2O. At a maximal mean airway pressure of 40 cm H2O, oxygenation was not different between the groups, but pulmonary artery pressures were elevated in both PFOB-groups compared with the control group. During the deflation phase, oxygenation, pulmonary shunt fraction, and pulmonary artery pressures were adversely affected by PFOB-Hi and PFOB-Lo.

CONCLUSIONS

Although PFOB-Hi compared with the control group improved oxygenation and reduced pulmonary shunt fraction only during the first pressure steps of a formal stepwise recruitment maneuver during high frequency oscillatory ventilation, this effect was not sustained during maximal recruitment. During the deflation phase, both PFOB groups were associated with worse gas exchange compared with the control group. PFOB also produced significant pulmonary hypertension in comparison with the control group.

Unsteady propagation of a liquid plug in a liquid-lined straight tube

This paper considers the propagation of a liquid plug driven by a constant pressure within a rigid axisymmetric tube whose inner surface is coated by a thin liquid film. The Navier-Stokes equations are solved using the finite-volume method and the SIMPLEST algorithm. The effects of precursor film thickness, initial plug length, pressure drop across the plug, and constant surface tension on the plug behavior and tube wall mechanical stresses are investigated. As a plug propagates through a liquid-lined tube, the plug gains liquid from the leading front film, and it deposits liquid into the trailing film. If the trailing film is thicker (thinner) than the precursor film, the plug volume decreases (increases) as it propagates. For a decreasing volume, eventually the plug ruptures. Under a specific set of conditions, the trailing film thickness equals the precursor film thickness, which leads to steady state results. The plug speed decreases as the precursor film thins because the resistance to the moving front meniscus increases. As the pressure drop across the plug decreases, the plug speed decreases resulting in thinning of the trailing film. As the plug length becomes longer, the viscous resistance in the plug core region increases, which slows the plug and causes the trailing film to become even thinner. The magnitude of the pressure and shear stress at the tube inner wall is maximum in the front meniscus region, and it increases with a thinner precursor film. As the surface tension increases, the plug propagation speed decreases, the strength of the wall pressure in the front meniscus region increases, and the pressure gradient around the peak pressure becomes steeper.

Combining lung-protective strategies in experimental acute lung injury: The impact of high-frequency partial liquid ventilation

OBJECTIVE

To evaluate the independent and combined effects of high-frequency oscillatory ventilation (HFOV) and partial liquid ventilation (PLV) on gas exchange, pulmonary histopathology, inflammation, and oxidative tissue damage in an animal model of acute lung injury.

DESIGN

Prospective, randomized animal study.

SETTING

Research laboratory of a health sciences university.

SUBJECTS

Fifty New Zealand White rabbits.

INTERVENTIONS

Juvenile rabbits injured by lipopolysaccharide infusion and saline lung lavage were assigned to conventional ventilation (CMV), PLV, HFOV, or high-frequency partial liquid ventilation (HF-PLV) with a full or half dose (HF-PLV1/2) of perfluorochemical (PFC). Uninjured ventilated animals served as controls. Arterial blood gases were obtained every 30 mins during the 4-hr study. Histopathologic evaluation was performed using a lung injury scoring system. Oxidative lung injury was assessed by measuring malondialdehyde and 4-hydroxynonenal in lung homogenates.

MEASUREMENTS AND MAIN RESULTS

HFOV, PLV, or a combination of both methods (HF-PLV) resulted in significantly improved oxygenation, more favorable lung histopathology, reduced neutrophil infiltration, and attenuated oxidative damage compared with CMV. HF-PLV with a full PFC dose did not provide any additional benefit compared with HFOV alone. HF-PLV1/2 was associated with decreased pulmonary leukostasis compared with HF-PLV.

CONCLUSIONS

The combination of HFOV and PLV (HF-PLV) does not provide any additional benefit compared with HFOV or PLV alone in a combined model of lung injury when lung recruitment and volume optimization can be achieved. The use of a lower PFC dose (HF-PLV1/2) is associated with decreased pulmonary leukostasis compared with HF-PLV and deserves further study.

Protective strategies of high-frequency oscillatory ventilation in a rabbit model

High-frequency oscillatory ventilation (HFOV) causes less severe lung injury than conventional mechanical ventilation (CMV) but the optimal frequency for HFOV has not been determined. We hypothesized that 15 Hz HFOV would be more protective than 5 Hz HFOV in a rabbit model of acute lung injury. Surfactant-depleted rabbits were ventilated at 15 Hz or 5 Hz HFOV for 4 h, or not ventilated, to characterize the extent of lung injury before HFOV. PaO(2) and PaCO(2) were measured throughout the experiment, and lung myeloperoxidase (MPO) activity, neutrophil infiltration, and histopathological changes were determined. There were no statistically significant differences in PaO(2) and PaCO(2) between groups (p > 0.05). Neutrophil counts (p = 0.013), airway injury scores (p = 0.007), airspace injury scores (p = 0.029), and total lung injury scores (p = 0.014) differed between non-HFO-ventilated and HFOV animals. Comparing the 2 HFOV regimens, 15 Hz ventilation yielded a lower tissue neutrophil score (p = 0.005). MPO activity, neutrophil count, airway injury score, airspace injury score, and total lung injury score parameters did not differ significantly between the HFOV groups (p > 0.150). We concluded that both frequencies of HFOV efficiently restored O(2) and CO(2) exchange in a rabbit model of severe lung injury, and that 5 Hz HFOV increased neutrophil infiltration relative to 15 Hz HFOV.

Partial liquid ventilation with low-dose perfluorochemical and high-frequency oscillation improves oxygenation and lung compliance in a rabbit model of surfactant depletion

BACKGROUND

Partial liquid ventilation (PLV) with perfluorochemical (PFC) has been advocated as a new therapy for acute respiratory distress syndrome in both clinical and animal studies, meconium aspiration syndrome, and RDS. PFC is referred to as liquid PEEP because it gets distributed to the most gravity-dependent regions of the lung due to its density. High-frequency oscillation (HFO) has been shown to prevent both acute and chronic lung injury in the management of very low birth weight infants with RDS, with gentle ventilation approach. Specifically, HFO with aggressive and adequate lung volume recruitment has been shown to reduce the incidence of chronic lung disease in very low birth weight infants. We hypothesized that PLV along with HFO might be effective in ARDS in an adult rabbit model.

OBJECTIVES

To examine the efficiency of low-dose PLV with with HFO on pulmonary gas exchange and lung compliance in a surfactant-depleted rabbit model.

METHODS

After induction of severe lung injury by repeated saline lung lavage, 19 adult white Japanese rabbits were randomized into two groups that received PLV with HFO (n=9) or HFO gas ventilation (n=10). Physiological and blood gas data were compared between the two groups by analysis of variance.

RESULTS

The HFO-PLV group showed improved total lung compliance with maintenance of significantly lower mean airway pressure as compared with the HFO-GAS group so as to keep SpO2>90%.

CONCLUSIONS

The addition of a low dose of PFC with HFO was effective in achieving adequate oxygenation, with a reduction in further lung injury in neonates.

Differential impact of perfluorochemical physical properties on the physiologic, histologic, and inflammatory profile in acute lung injury

OBJECTIVE

To evaluate the differential effects of physical properties of combinational perfluorochemical liquids (PFC) during partial liquid ventilation (PLV) on inflammatory indexes in the injured lung.

DESIGN

: Interventional laboratory study.

SETTING

Academic medical research laboratory.

SUBJECTS

Seventeen saline lavage-injured juvenile rabbits.

INTERVENTIONS

Rabbits were anesthetized, ventilated, saline lavage-injured, and randomized into groups: group 1 (conventional mechanical ventilation alone-no PFC), group 2 (PLV: lowest viscosity, highest vapor pressure), group 3 (PLV: mid-viscosity, mid-vapor pressure), group 4 (PLV: highest viscosity, lowest vapor pressure).

MEASUREMENTS AND MAIN RESULTS

Arterial blood chemistry and pulmonary mechanics were monitored throughout the protocol. Following 4 hrs, lung tissue was harvested for interleukin-8, myeloperoxidase, and histologic analyses. Oxygenation (Pao2), ventilation (ventilation efficiency index), and respiratory compliance were not significantly different between groups before or following injury. Pao2 increased significantly following treatment in groups 3 and 4. Oxygenation index was significantly lower and respiratory compliance and ventilation efficiency index were significantly higher for group 4 following 4 hrs than all other groups. Total lung tissue interleukin-8 was significantly lower in groups 3 and 4 than groups 1 and 2, and lung myeloperoxidase was significantly lower in all PLV-treated groups than CMV alone. Histologic examination showed increased recruitment of the dependent lung in groups 3 and 4, with significantly greater lung expansion index, than groups 1 and 2.

CONCLUSIONS

PLV, with a single dose of higher viscosity and lower vapor pressure PFC, resulted in significantly improved gas exchange and lung mechanics with significant reduction in lung inflammation compared with conventional mechanical ventilation alone and PLV with lower viscosity and higher vapor pressure liquid. Since PFC evaporative loss and redistribution are minimized by lower VP and higher viscosity, these data suggest that greater mechanoprotection and cytoprotection of the lung are conferred during PLV with PFC liquids that remain distributed throughout the entire lung for a longer duration.

High-frequency oscillatory ventilation: what large-animal studies have taught us!

BACKGROUND

Much of the information on the physiologic effects, mechanisms of gas exchange, and potential utility of high-frequency oscillation (HFO) has been acquired in animal studies. Specifically, large animal data have been useful in assessing adult application because large animals present many of the same concerns and challenges as adults.

OBJECTIVE

To review the literature on HFO testing in large animal models, identifying contributions to the understanding of mechanisms of action and the physiology of HFO.

RESULTS

Large animal studies have clarified the mechanisms of gas exchange during HFO, identified approaches to setting mean airway pressure based on lung mechanics, and identified a potentially better approach to applying partial liquid ventilation.

CONCLUSION

The study of HFO in large animal models has been essential to our understanding of the optimal approach to applying HFO in human studies.

Steady propagation of a liquid plug in a two-dimensional channel

In this study, we investigate the steady propagation of a liquid plug within a two-dimensional channel lined by a uniform, thin liquid film. The Navier-Stokes equations with free-surface boundary conditions are solved using the finite volume numerical scheme. We examine the effect of varying plug propagation speed and plug length in both the Stokes flow limit and for finite Reynolds number (Re). For a fixed plug length, the trailing film thickness increases with plug propagation speed. If the plug length is greater than the channel width, the trailing film thickness agrees with previous theories for semi-infinite bubble propagation. As the plug length decreases below the channel width, the trailing film thickness decreases, and for finite Re there is significant interaction between the leading and trailing menisci and their local flow effects. A recirculation flow forms inside the plug core and is skewed towards the rear meniscus as Re increases. The recirculation velocity between both tips decreases with the plug length. The macroscopic pressure gradient, which is the pressure drop between the leading and trailing gas phases divided by the plug length, is a function of U and U2, where U is the plug propagation speed, when the fluid property and the channel geometry are fixed. The U2 term becomes dominant at small values of the plug length. A capillary wave develops at the front meniscus, with an amplitude that increases with Re, and this causes large local changes in wall shear stresses and pressures.

A Mathematical Model of Alveolar Gas Exchange in Partial Liquid Ventilation

In partial liquid ventilation (PLV), perfluorocarbon (PFC) acts as a diffusion barrier to gas transport in the alveolar space since the diffusivities of oxygen and carbon dioxide in this medium are four orders of magnitude lower than in air. Therefore convection in the PFC layer resulting from the oscillatory motions of the alveolar sac during ventilation can significantly affect gas transport. For example, a typical value of the Pe´clet number in air ventilation is Pe∼0.01, whereas in PLV it is Pe∼20. To study the importance of convection, a single terminal alveolar sac is modeled as an oscillating spherical shell with gas, PFC, tissue and capillary blood compartments. Differential equations describing mass conservation within each compartment are derived and solved to obtain time periodic partial pressures. Significant partial pressure gradients in the PFC layer and partial pressure differences between the capillary and gas compartments PC-Pg are found to exist. Because Pe≫1, temporal phase differences are found to exist between PC-Pg and the ventilatory cycle that cannot be adequately described by existing non-convective models of gas exchange in PLV. The mass transfer rate is nearly constant throughout the breath when Pe≫1, but when Pe≪1 nearly 100% of the transport occurs during inspiration. A range of respiratory rates (RR), including those relevant to high frequency oscillation (HFO)+PLV, tidal volumes VT and perfusion rates are studied to determine the effect of heterogeneous distributions of ventilation and perfusion on gas exchange. The largest changes in PCO2 and PCCO2 occur at normal and low perfusion rates respectively as RR and VT are varied. At a given ventilation rate, a low RR-high VT combination results in higher PCO2, lower PCCO2 and lower PC-Pg than a high RR-low VT one.

Dose response to aerosolized perflubron in a neonatal swine model of lung injury

Aerosolized perfluorocarbon (PFC) improves gas exchange, lung mechanics, and pulmonary artery pressure. The objective of this intervention was to study the dose-response effect to aerosolized perfluorooctylbromide (PFOB; perflubron, LiquiVent, Alliance Pharmaceutical Corp.) in surfactant-depleted piglets. After induction of lung injury by saline lavage, 25 newborn piglets were randomly assigned to receive 0, 1.25, 2.5, 5.0, or 7.5 mL/kg aerosolized PFOB per hour. A 2-h therapy period was followed by a 3-h observation period. In all animals, respiratory support was performed with intermittent mandatory ventilation. After aerosol treatment and 3 h of observation, arterial oxygen pressure was similarly improved in the 2.5-, 5.0-, and 7.5-mL. kg(-1). h(-1) aerosol-PFOB groups and higher compared with the 1.25-mL. kg(-1). h(-1) aerosol-PFOB (P < 0.01) and the control groups (P < 0.001). Compared with the control group, arterial carbon dioxide pressure was significantly reduced with 2.5-, 5.0-, and 7.5-mL. kg(-1). h(-1) aerosol-PFOB (P < 0.001). Treatment with 1.25 mL. kg(-1). h(-1) aerosol-PFOB did not significantly affect arterial carbon dioxide pressure. The 20% terminal dynamic compliance/dynamic compliance was significantly improved in the groups that received 2.5, 5.0, and 7.5 mL. kg(-1). h(-1) aerosol-PFOB compared with control animals. Mean pulmonary artery pressure was lower after therapy with 5.0 and 7.5 mL. kg(-1). h(-1) aerosol-PFOB (P < 0.01) than in the control group. IL-1beta gene expression in lung tissue was significantly reduced with PFOB 1.25 mL. kg(-1). h(-1). In summary, aerosolized PFOB improved terminal dynamic compliance, pulmonary gas exchange, and pulmonary artery pressure in a dose-dependent manner. In terms of oxygenation and lung mechanics, the optimum dose was between 2.5 and 5 mL. kg(-1). h(-1).

Fluorocarbons facilitate lung recruitment

OBJECTIVE

"Open the lung and keep it open" is increasingly accepted as a fundamental principle for mechanical ventilation. However, it is sometimes very difficult, or impossible, to recruit the diseased lung. We questioned whether one could facilitate recruitment by using a low dose of fluorocarbon in a model previously shown to be non-recruitable by conventional sustained inflation maneuvers.

DESIGN AND SETTING

Experimental prospective study in a university laboratory.

ANIMALS AND INTERVENTIONS

Nine saline-lavaged rabbits subjected to prolonged large tidal volume mechanical ventilation to establish significant lung injury were randomly allocated to two groups: control [High Frequency Oscillation (HFO) alone: n=4] or 1 ml/kg fluorocarbon (FC) treated (HFO/FC: n=5) for 2+1 h (experiment 1). An additional four similarly prepared animals were treated by single-lung instillation of 0.5 ml/kg dose of fluorocarbon and underwent serial computerized tomography scans at a series of predetermined step-wise pressure increase in both lungs (experiment 2).

MEASUREMENTS AND RESULTS

In experiment 1 there was a very significant improvement in oxygenation in HFO/FC group (PaO(2) increased from 108 mmHg to 424+/-81 mmHg; P<0.05) while there was no significant change in the control group. In experiment 2 lung volumes were determined using three-dimensional reconstruction. The lung having fluorocarbon showed a 2.4-fold increase in lung volume at inflation pressure of 15 cmH(2)O compared to the lung without fluorocarbon.

CONCLUSIONS

We propose that the low equilibrium surface tension and positive spreading coefficient of fluorocarbon facilitates lung recruitment by ungluing adherent surfaces in this model of lung injury.

Effects of exogenous surfactant supplementation and partial liquid ventilation on acute lung injury induced by wood smoke inhalation in newborn piglets

OBJECTIVE

To investigate the beneficial effects of exogenous surfactant supplementation (ESS) and partial liquid ventilation (PLV) in treating acute lung injury induced by wood smoke inhalation.

DESIGN

A prospective, randomized, controlled, multigroup study.

SETTING

An animal research laboratory at a medical center.

SUBJECTS

Newborn piglets (n = 29; 1.80 +/- 0.06 kg) of either sex.

INTERVENTIONS

Animals were ventilated with a tidal volume of 15 mL/kg, a rate of 30 breaths/min, a positive end-expiratory pressure of 5 cm H(2)O, and an Fio(2) of 1.0. After the induction of acute lung injury by wood smoke inhalation, animals were randomly assigned to receive either conventional mechanical ventilation (CMV) or PLV with or without ESS pretreatment. Animals were grouped as CMV, ESS-CMV, PLV, and ESS-PLV.

MEASUREMENTS AND MAIN RESULTS

Arterial blood gases, cardiovascular hemodynamics, dynamic lung compliance, and total lung injury scores were measured. After smoke inhalation, all four groups displayed similar high arterial carboxyhemoglobin levels, low Pao(2) (<150 mm Hg), and low dynamic lung compliance (<66% of its baseline). In the CMV group, these deleterious conditions remained during the 4-hr observation period, and severe lung injury was noted histologically. All treatment groups demonstrated a significant increase in Pao(2) compared with the CMV group. In addition, both the PLV and ESS-PLV groups displayed significant improvements in dynamic lung compliance and in their histologic outcomes. Nevertheless, none of the variables measured in the PLV group differed from those measured in the ESS-PLV group.

CONCLUSIONS

In a newborn piglet model of smoke inhalation injury, PLV or ESS improved oxygenation. PLV compared favorably with ESS in its greater improvements in lung compliance and lung pathology. However, the combined therapy of ESS and PLV was not clearly superior to PLV alone during the observation period.

Changes in pulmonary function and oxygenation during application of perfluorocarbon vapor in healthy and oleic acid-injured animals

OBJECTIVES

To investigate the changes in pulmonary function and gas exchange during application of 18% perfluorohexane vapor in healthy and in oleic acid-injured animals and compare it with an injured control group.

DESIGN

Prospective randomized controlled study.

SETTING

Experimental research laboratory at a university medical center.

SUBJECTS

Nineteen sheep weighing 31.4 +/- 4.1 kg.

INTERVENTIONS

Lung injury was induced in 14 sheep by the intravenous injection of 0.1 mL/kg oleic acid. After establishment of lung injury (PaO(2)/F(IO(2)) ratio, <200; pulmonary artery occlusion pressure, <19 torr), seven animals were treated with 18% perfluorohexane vapor for 30 mins whereas seven animals served as controls. After the start of perfluorohexane treatment, blood gases and respiratory and hemodynamic data were collected in 10-min intervals throughout the study period of 1 hr. In addition, five healthy animals received perfluorohexane vapor for 30 mins and were followed up for 2 hrs to exclude delayed negative effects.

MEASUREMENTS AND MAIN RESULTS

Treatment of healthy animals with 18% perfluorohexane vapor was not accompanied by any significant adverse effects. It was associated with a significant decrease of alveolar-arterial oxygen difference during perfluorohexane application (p <.05). In injured animals, 18% perfluorohexane led to a sustained improvement of peak inspiratory pressures within 10 mins of treatment (p <.001). The concomitant increase in compliance was equally significant (p <.001). Significant improvements in PaO(2) occurred despite a decrease in F(IO(2)) to 0.81 at the end of vaporization.

CONCLUSION

Healthy animals tolerated perfluorohexane vapor well without significant changes in oxygenation and mechanical lung function for 2 hrs. In injured animals, application of perfluorohexane vapor primarily improved peak inspiratory pressure and compliance. The increase of oxygenation therefore could be secondary to an improvement in compliance.

Respiratory fluid mechanics and transport processes

The field of respiratory flow and transport has experienced significant research activity over the past several years. Important contributions to the knowledge base come from pulmonary and critical care medicine, surgery, physiology, environmental health sciences, biophysics, and engineering. Several disciplines within engineering have strong and historical ties to respiration including mechanical, chemical, civil/environmental, aerospace and, of course, biomedical engineering. This review draws from a wide variety of scientific literature that reflects the diverse constituency and audience that respiratory science has developed. The subject areas covered include nasal flow and transport, airway gas flow, alternative modes of ventilation, nonrespiratory gas transport, aerosol transport, airway stability, mucus transport, pulmonary acoustics, surfactant dynamics and delivery, and pleural liquid flow. Within each area are a number of subtopics whose exploration can provide the opportunity of both depth and breadth for the interested reader.

Liquid plug flow in straight and bifurcating tubes

A finite-length liquid plug may be present in an airway due to disease, airway closure, or by direct instillation for medical therapy. Air forced by ventilation propagates the plug through the airways, where it deposits fluid onto the airway walls. The plug may encounter single or bifurcating airways, an airway surface liquid, and other liquid plugs in nearby airways. In order to understand how these flow situations influence plug transport, benchtop experiments are performed for liquid plug flow in: Case (i) straight dry tubes, Case (ii) straight pre-wetted tubes, Case (iii) bifurcating dry tubes, and Case (iv) bifurcating tubes with a liquid blockage in one daughter. Data are obtainedfor the trailing film thickness and plug splitting ratio as a function of capillary number and plug volumes. For Case (i), the finite length plug in a dry tube has similar behavior to a semi-infinite plug. For Case (ii), the trailing film thickness is dependent upon the plug capillary number (Ca) and not the precursor film thickness, although the shortening or lengthening of the liquid plug is influenced by the precursor film. For Case (iii), the plug splits evenly between the two daughters and the deposited film thickness depends on the local plug Ca, except for a small discrepancy that may be due to an entrance effect or from curvature of the tubes. For Case (iv), a plug passing from the parent to daughters will deliver more liquid to the unblocked daughter (nearly double, consistently) and then the plug will then travel at greater Ca in the unblocked daughter as the blocked. The flow asymmetry is enhanced for a larger blockage volume and diminished for a larger parent plug volume and parent-Ca.

Hemodynamic effects of positive end-expiratory pressure during partial liquid ventilation in newborn lambs

BACKGROUND/PURPOSE

The aim of this study was to compare the effect of positive end-expiratory pressure (PEEP) application on hemodynamics, lung mechanics, and oxygenation in the intact newborn lung during conventional ventilation (CV) and partial liquid ventilation (PLV) at functional residual capacity (FRC). CV or PLV modes of ventilation do not affect hemodynamics nor the optimum PEEP for oxygenation.

METHODS

Seven newborn lambs (1 to 3 days old) were instrumented to measure pulmonary hemodynamics and airway mechanics. Each lamb was used as their own control to compare different modes of ventilation (CV followed by PLV) under graded variations of PEEP (4, 8, 12, and 16 cm H(2)O) on the influence on pulmonary blood flow and pulmonary vascular resistance.

RESULTS

There was a significant drop in pulmonary blood flow (PBF) from baseline (PEEP of 4 cm H(2)O on CV, 1,229 +/- 377 mL/min) in both modes of ventilation on a PEEP of 16 cm H(2)O (CV, 750 +/- 318 mL/min v PLV, 926 +/- 396 mL/min, respectively; P <.05). Peak inspiratory pressure (PIP) was higher on PLV at PEEP states of 4 cm H(2)O (16.5 +/- 1.3 cm H(2)O to 10.6 +/- 2.1 cm H(2)O; P <.05) and 8 cm H(2)O (18.8 +/- 2.2 cm H(2)O to 15.1 +/- 2.6 cm H(2)O; P <.05) when compared with CV. Conversely, PIP required to maintain the pCO(2) was lower on PLV at PEEP states of 12 (22.5 +/- 3.6 cm H(2)O to 24.2 +/- 3.8 cm H(2)O; P <.05) and 16 cm H(2)O (27.0 +/- 1.6 cm H(2)O to 34.0 +/- 5.9 cm H(2)O; P <.05).

CONCLUSIONS

Hemodynamically, CO is impaired at a PEEP above 12 cm H(2)O in intact lungs. PFC at FRC does provide an advantage in lung mechanics more than 10 to 12 cm H(2)O of PEEP by decreasing the amount PIP needed to achieve the similar levels of gas exchange and minute ventilation, implying a reduced risk for barotrauma with chronic ventilation. Thus, selection of the appropriate level of PEEP appears to be important if PLV is to be utilized at FRC. The best strategy for PLV, including the selection of PEEP, remains to be determined.

Persistent improvement of gas exchange and lung mechanics by aerosolized perfluorocarbon

The effect of aerosolized perfluorocarbon (PFC) (FC77) on pulmonary gas exchange and lung mechanics was studied in a surfactant depleted piglet model. Sixty minutes after induction of lung injury by bronchoalveolar lavage, 20 piglets were randomized to receive aerosolized PFC (Aerosol-PFC, 10 ml/kg/h, n = 5), partial liquid ventilation (PLV) at FRC capacity volume (FRC-PLV, 30 ml/kg, n = 5) or low volume (LV-PLV, 10 ml/kg/h, n = 5), or intermittent mandatory ventilation (IMV) (Control, n = 5). After 2 h, perfluorocarbon application was stopped and IMV was continued for 6 h. Sixty minutes after the onset of therapy, PaO2 was significantly higher and PaCO2 was significantly lower in the Aerosol-PFC and the FRC-PLV groups than in the LV-PLV and the Control groups; p < 0.001. Six hours after treatment, maximum PaO2 was found in the Aerosol-PFC group: 406.4 +/- 26.9 mm Hg, FRC-PLV: 217.3 +/- 50.5 mm Hg, LV-PLV: 96.3 +/- 18.9 mm Hg, Control: 67.6 +/- 8.4 mm Hg; p < 0.001. PaCO2 was lowest in the Aerosol-PFC group: 24.2 +/- 1.7 mm Hg, FRC-PLV: 35.9 +/- 2.8 mm Hg, LV-PLV: 56.7 +/- 12.4 mm Hg, Control: 60.6 +/- 5.1 mm Hg; p < 0.01. Dynamic compliance (C20/c) was highest in the Aerosol-PFC group; p < 0.01. Aerosolized perfluorocarbon improved pulmonary gas exchange and lung mechanics as effectively as PLV did in surfactant-depleted piglets, and the improvement was sustained longer.

A rat lung model of instilled liquid transport in the pulmonary airways

When a liquid is instilled in the pulmonary airways during medical therapy, the method of instillation affects the liquid distribution throughout the lung. To investigate the fluid transport dynamics, exogenous surfactant (Survanta) mixed with a radiopaque tracer is instilled into tracheae of vertical, excised rat lungs (ventilation 40 breaths/min, 4 ml tidal volume). Two methods are compared: For case A, the liquid drains by gravity into the upper airways followed by inspiration; for case B, the liquid initially forms a plug in the trachea, followed by inspiration. Experiments are continuously recorded using a microfocal X-ray source and an image-intensifier, charge-coupled device image train. Video images recorded at 30 images/s are digitized and analyzed. Transport dynamics during the first few breaths are quantified statistically and follow trends for liquid plug propagation theory. A plug of liquid driven by forced air can reach alveolar regions within the first few breaths. Homogeneity of distribution measured at end inspiration for several breaths demonstrates that case B is twice as homogeneous as case A. The formation of a liquid plug in the trachea, before inspiration, is important in creating a more uniform liquid distribution throughout the lungs.

High-frequency oscillatory ventilation of the perfluorocarbon-filled lung: Dose-response relationships in an animal model of acute lung injury

OBJECTIVE

To examine dose-response relationships regarding the efficiency of gas exchange and hemodynamic function during high-frequency oscillation and partial liquid ventilation (HFO-PLV) of the perfluorocarbon (PFC)-treated lung in a model of acute lung injury.

SETTING

An animal research laboratory in a university medical center.

DESIGN

A prospective, randomized study comparing animals receiving varying doses (0, 5, 15, and 20 mL/kg) of perflubron during high-frequency oscillatory ventilation (HFOV) with mean airway pressure (Paw) optimized to achieve a minimal percutaneous oxygen saturation (Spo2).

SUBJECTS

Nineteen healthy swine (mean weight 28.9 kg) with saline lavage-induced acute lung injury.

METHODS

Animals were treated with repetitive saline lavage to achieve a uniform degree of acute lung injury (Spo2 < or =90% on an Fio2 of 1.0). After lung injury, subjects were converted to HFOV, and lung volume was optimized. HFO-PLV was initiated by instillation of perflubron at a rate of 0.5 mL.kg-1.min-1 to achieve total doses of 5, 15, and 20 mL/kg. After PFC dosing, the only experimental manipulation consisted of adjustment of Paw to achieve an Spo2 of 90% +/- 2% with Fio2 of 0.6. Gas exchange, hemodynamic variables, and pulmonary mechanics data were collected over a 1-hr period. Five control animals were not dosed with perflubron and remained on HFOV for the 1-hr period of data collection.

MEASUREMENTS AND MAIN RESULTS

After lung volume recruitment with HFOV, the initiation of HFO-PLV was best tolerated with the two lower doses in our protocol. There were essentially no changes in Paco2 or pH between groups over the dosing interval. After dosing, analysis of variance demonstrated a PFC dose-dependent effect for oxygenation index (p =.01) only; the lowest oxygenation index was found in the 15 mL/kg group (p =.01). In the 15 mL/kg group, the Paw decreased steadily from 20.6 +/- 3.4 cm H2O at the end of dosing to 18.0 +/- 4.9 cm H2O at 60 mins. The Pao2 increased from 113 +/- 51 torr (15.06 +/- 6.79 kPa) to 134 +/- 49 torr (17.86 +/- 6.53 kPa) during this period and was associated with a decreasing oxygenation index (from 11.4 +/- 2.0 to 9.3 +/- 1.5). The cardiac index and pulmonary vascular resistance did not change significantly during the dosing period and were relatively stable after the completion of dosing.

CONCLUSIONS

The combination of HFOV and perflubron administration was well tolerated hemodynamically and was not associated with deterioration of gas exchange during dosing. Our data suggest that the optimal dose of perflubron to achieve the lowest oxygenation index during HFO-PLV is between 5 and 15 mL/kg. The combination of HFOV and perflubron administration is a novel strategy in the treatment of acute lung injury that shows some promise and merits additional investigation. We hope in future studies to address the histopathologic effects of varying perflubron doses during HFOV in a long-term study of the lung-protective effects of HFO-PLV.

Optimizing intrapulmonary perfluorocarbon distribution: Fluoroscopic comparison of mode of ventilation and body position

OBJECTIVE

Partial liquid ventilation with the perfluorochemical, perflubron, has been shown to improve lung mechanics and enhance gas exchange in the treatment of severe acute lung injury. However, the most effective strategy to provide optimal intrapulmonary distribution of perflubron has not been fully accessed. The objective of this study was to examine the effect of body position (supine vs. rotational) and mode of ventilation (conventional mechanical ventilation [CMV] vs. high-frequency oscillatory ventilation [HFOV]) on perflubron distribution and oxygenation improvement.

DESIGN

Prospective, randomized, animal trial.

SETTING

Research laboratory at a university medical center.

SUBJECTS

Twenty healthy piglets (4.5-6.6 kg).

INTERVENTIONS

Subjects underwent repetitive saline lavage to achieve a uniform degree of lung injury and then were randomized to either CMV or were converted to HFOV. Within each ventilator group, animals were randomized to supine positioning (S) or rotational positioning with alternation between supine and prone position (R) during incremental dosing of three 5-mL/kg doses of perflubron.

MEASUREMENTS AND MAIN RESULTS

Arterial blood gas tensions, hemodynamic variables, and the oxygenation index were recorded after each dose of 5 mL/kg. Lateral cinefluoroscopic images after each dose were digitized for computer analysis of density. A density index was calculated for a 2-cm2 window in three dorsal and three ventral lung regions. Uniformity of distribution was calculated by comparing the mean density among the six regions. Oxygenation improvements were compared between groups. There were no significant differences in hemodynamic variables or gas exchange after lung injury in the four groups. Rotational positioning produced significantly more uniform perflubron distribution during both CMV and HFOV. This effect was independent of the mode of ventilation. The mean ventral density index was affected by rotating position and HFOV mode of ventilation after 10 mL/kg of perflubron, and rotating position was affected only after 15 mL/kg of perflubron. There was a significant reduction in the oxygenation index from baseline to end lavage in both CMV groups, as well as all of the animals that were rotated.

CONCLUSION

Perflubron is more uniformly dispersed when dosed in a rotational fashion with alternation between supine and prone position during incremental dosing. This effect is independent of mode of ventilation. There was no relationship between oxygenation improvements and nondependent perflubron distribution. CMV and rotating dosing both led to a significant decrease in the oxygenation index after a 15 mL/kg dose of perflubron. This information has important impact on the future development of dosing strategies and clinical trial design.

Liquid ventilation

Partial liquid ventilation (PLV) developed considerably in the clinical and experimental fields during the past few years. In addition to improved oxygenation and lung mechanics by perfluorocarbon (PFC) administration, recent animal studies have tried to optimize PLV by evaluating the most appropriate ventilatory mode to use during PLV and by adjusting the best level of positive end-expiratory pressure (PEEP). Other pathophysiological aspects of acute lung injury that may be positively affected by liquid ventilation have been studied, including regional blood flow redistribution, reduction in ventilator-induced lung injury, and antiinflammatory properties of PFC. Although the precise dosing of PFC is debated, evidence from several experimental studies supports the use of smaller doses of PFC because larger doses increase the occurrence of baro- and volutrauma. In the clinical field, after promising data from preliminary studies, an international randomized controlled trial is on the verge of completion.

Partial liquid ventilation for acute respiratory distress syndrome

PLV represents an intriguing alternative paradigm in the approach to the patient with ALI. Within the past decade, substantial information has become available regarding this technique. Clearly, PLV is feasible in patients with ALI and ARDS, and it appears to be safe with respect to short-term effects on hemodynamics and lung physiology, as well as long-term toxicity (although further research in this area is warranted). Although PLV has not yet been proven to be superior to traditional mechanical ventilation for patients with ALI or ARDS, PLV possesses an intriguing combination of physical, physiologic, and biologic effects: "Liquid PEEP" effect--e.g., more effective recruitment of dependent lung zones than achieved by gas ventilation Anti-inflammatory effects Lavage of alveolar debris Mitigation of ventilator-induced lung injury Direct anti-inflammatory effects--e.g., decreased macrophage release of proinflammatory cytokines, etc. Prevention of nosocomial pneumonia Combination with other modalities--e.g., exogenous surfactant replacement, inhaled NO, prone position Enhanced delivery of drugs or gene vectors into the lung. The results of ongoing and future clinical trials will be necessary to establish whether PLV improves clinical outcomes in patients with ALI or ARDS, or specific subgroups of such patients. Significant work also remains to be done to define the optimum dose level of PLV and the most appropriate ventilatory strategies.

High-frequency oscillatory ventilation and partial liquid ventilation after acute lung injury in premature lambs with respiratory distress syndrome

OBJECTIVE

Conventional mechanical ventilatory support (CV) contributes to lung injury in premature lambs with respiratory distress syndrome, a disease that is characterized by progressive deterioration of gas exchange and increased lung inflammation. Lung recruitment strategies, such as high-frequency oscillatory ventilation (HFOV) and partial liquid ventilation (PLV), improve gas exchange and attenuate lung inflammation when instituted immediately after birth. However, whether these recruitment strategies are effective as rescue treatment after established lung injury is unknown. To determine the separate and combined effects of HFOV and PLV when initiated after the establishment of acute lung injury in severe respiratory distress syndrome, we studied the effects of these strategies on gas exchange and histologic signs of acute lung injury in premature lambs.

DESIGN

Animals were intubated, treated with surfactant and ventilated with 1.00 FIO2 for 4 hrs. After 2 hrs, animals were either continued on CV (controls) or treated with one of three strategies: HFOV; CV + PLV; or HFOV + PLV. The response to low-dose inhaled nitric oxide (5 ppm) was measured in each group at the end of the study.

SETTING

An animal laboratory affiliated with University of Colorado School of Medicine.

SUBJECTS

A total of 20 premature lambs at 115-118 days of gestation (term = 147 days).

MEASUREMENTS AND MAIN RESULTS

In comparison with control animals, each of the rescue therapies improved PaO2 after 1 hr of treatment. The HFOV and HFOV + PLV groups had higher PaO2 than CV + PLV or CV alone (p < .05). Mean airway pressure (Paw) was lower in the PLV groups during CV or HFOV compared with their controls (p < .05). Inhaled NO improved PaO2 in all groups; however, the increase in PaO2 was greatest in the HFOV + PLV group (p < .05). Histologic examination and myeloperoxidase assay were not different between groups.

CONCLUSION

We conclude that each lung recruitment strategy improved oxygenation in premature lambs with established lung injury.

Surfactant and partial liquid ventilation via conventional and high-frequency techniques in an animal model of respiratory distress syndrome

OBJECTIVE: To compare the physiologic and pathologic effects of conventional ventilation (CV) and high-frequency ventilation (HFV) during partial liquid ventilation (PLV) with perflubron after surfactant treatment with the results of HFV plus surfactant in an animal lung-injury model created by saline lavage. We also studied the dose effects of perflubron during HFV. DESIGN: Randomized experimental study. SETTING: Research animal laboratory. SUBJECTS: A total of 32 newborn piglets. INTERVENTIONS: After lung injury was induced, the animals were randomized to one of four groups: a) CV + surfactant + perflubron to functional residual capacity (FRC); b) HFV + surfactant + perflubron to FRC; c) HFV + surfactant + 10 mL/kg perflubron; and d) HFV + surfactant. All then received intratracheal surfactant. After 30 mins, perflubron was administered to the PLV groups. The animals underwent ventilation for 20 hrs. MEASUREMENTS AND MAIN RESULTS: Arterial blood gases and hemodynamic variables were continuously monitored. Pulmonary histologic and morphometric analyses were performed after death or euthanasia at 20 hrs. All animals had sustained improvements in arterial/alveolar oxygen ratios, and no differences were observed among groups. All HFV groups required higher mean airway pressures to maintain oxygenation (p <.05). Hemodynamics did not differ among groups. Pathologic analysis demonstrated decreased lung injury in both cranial-dorsal (nondependent) and caudal-ventral (dependent) lobes of all animals treated with PLV when compared with those treated with HFV + surfactant (p <.05). CONCLUSIONS: After surfactant treatment, physiologic support over 20 hrs was similar during HFV with or without perflubron and CV with perflubron. All PLV modalities improved lung pathologic factors uniformly to a greater degree than did HFV + surfactant. A lower treatment volume of perflubron during HFV produced physiologic and pathologic results similar to those produced by perflubron with respect to FRC during either CV or HFV.

Liquid ventilation: a future modality?

Liquid ventilation, an idea currently being trialled in the United States, is increasingly being discussed as a possible future trend in ventilation. A review of the available literature indicates that this treatment provides effective gas exchange and has a number of potential advantages. These include lower airway pressures, decreased alveolar surface tension, alveolar recruitment and removal of pulmonary exudate. While yet to be seen in this country, liquid ventilation may be introduced in the future. If it is, those caring for patients treated in that way will require knowledge of the mechanics and physiological changes involved, as well as the potential hazards of this modality.

Current status of liquid ventilation

Perfluorochemical liquid has been used experimentally to enhance mechanical ventilation for the past 30 years. Liquid ventilation is one of the most extensively studied revolutionary medical therapies being considered for use in practice. Since 1989, when the first human neonates were treated with perfluorochemical liquid, more than 500 human patients--neonate, pediatric, and adult--have been treated with liquid ventilation as part of clinical trials. However, most of the clinically relevant information known to the medical field about liquid ventilation still comes from the laboratory. This paper seeks to briefly present current information available from studies involving liquid ventilation, both laboratory-based and clinical trials, as well as to inform the reader on patient management. In addition, we attempt to elucidate future directions.

Independent and combined effects of inhaled nitric oxide, liquid perfluorochemical, and high-frequency oscillatory ventilation in premature lambs with respiratory distress syndrome

Acute lung injury caused by tidal volume ventilation in the premature lamb with respiratory distress syndrome (RDS) is characterized by progessive deterioration in gas exchange and lung inflammation. Inhaled nitric oxide (iNO) improves gas exchange and decreases lung neutrophil accumulation in premature lambs with RDS. Mechanical lung recruitment techniques such as high-frequency oscillatory ventilation (HFOV) and partial liquid ventilation (PLV) also decrease lung injury and improve gas exchange in experimental models of neonatal respiratory failure. We hypothesized that two lung recruitment strategies (HFOV and PLV) would have similar effects on gas exchange and lung inflammation, and would augment the response to iNO. We studied the individual and combined effects of iNO, HFOV, and PLV (perflubron) in 31 extremely premature lambs (115 d, 0.78 term) using seven mechanical ventilation protocols. Four groups were treated with conventional ventilation (control CV, CV + iNO, CV + PLV, and CV + PLV + iNO). Three groups were treated with HFOV (control HFOV, HFOV + iNO, HFOV + PLV). Control CV animals had progressive deterioration in gas exchange over the 4-h study period (a/AO2 at 4 h = 0.06 +/- 0.01). In contrast, both HFOV and CV + PLV caused sustained improvements in oxygenation at 4 h (HFOV a/AO2 = 0. 27 +/- 0.06, CV + PLV a/AO2 = 0.25 +/- 0.04; p < 0.01 versus CV). Both lung recruitment strategies improved oxygenation when combined with iNO (5 ppm). Lung neutrophil accumulation was reduced by HFOV, PLV, and iNO compared to CV. We conclude that HFOV and PLV with perflubron cause similar improvements in gas exchange and lung inflammation in the premature lamb with severe RDS, and both strategies augment the oxygenation response to iNO.

Non-conventional respiratory support modalities applicable in the older child. High frequency ventilation and liquid ventilation

HFV, LV, and several other novel therapies offer promise to adults and children that the mortality associated with respiratory failure may be affected. Although there are several forms of HFV, HFOV is presently gaining favor in the treatment of severe respiratory failure and has generally supplanted HFJV in pediatric critical care. HFOV has the advantage of having an active expiratory phase, which helps to minimize air trapping and better modulate mean lung volume. Ventilators with sufficient power to perform HFOV in adults are currently under investigation, although there is a growing experience in using current ventilators in larger patients. To date, however, demonstration of lowered mortality with HFOV is lacking although intermediate outcome indicators are improved. PLV also offers promise in the treatment of ARF through its drastic ability to improve oxygenation, ventilation, and compliance in many lung injury models. Human trials are presently underway, but the optimal delivery of this novel therapy still necessitates extensive investigation. TLV is likely even more removed from general clinical application given the necessity of developing a new generation of ventilators for the delivery of liquid tidal volumes. How these and other modalities may piece together to improve the condition of our patients who have respiratory failure remains to be seen, but certainly, present and future investigation will be intriguing for years to come.

High-frequency oscillation versus conventional ventilation following surfactant administration and partial liquid ventilation

Surfactant followed by partial liquid ventilation (PLV) with perfluorocarbon (PFC; LiquiVent) improves oxygenation, lung compliance, and lung pathology in lung-injured animals receiving conventional ventilation (CV). In this study, we hypothesize that high-frequency oscillation (HFO) and CV will provide equivalent oxygenation in lung-injured animals following surfactant repletion and PLV, once lung volume is optimized. After saline-lavage lung injury during CV, newborn piglets were randomized to either HFO (n = 10) or CV (n = 9). HFO animals were stabilized over 15 min without optimization of lung volume; CV animals continued treatment with time-cycled, pressure-limited, volume-targeted ventilation. All animals then received 100 mg/kg of surfactant (Survanta). Thirty minutes later, all received intratracheal PFC to approximate functional residual capacity. Thirty minutes after PLV began, mean airway pressure (MAP) in both groups was increased to improve oxygenation. MAP was directly adjusted during HFO; PEEP and PIP were adjusted during IMV, maintaining a pressure sufficient to deliver 15 mL/kg tidal volume. Animals were treated for 4 h. The CV group showed improved oxygenation following surfactant administration (OI: 26.79 +/- 1.98 vs. 8.59 +/- 6.29, P < 0.0004), with little further improvement following PFC administration or adjustments in MAP. Oxygenation in HFO-treated animals did not improve following surfactant, but did improve following PFC (0I: 27.78 +/- 6.84 vs. 15.86 +/- 5.53, P < 0.005) and adjustments in MAP (OI: 15.86 +/- 5.53 vs. 8.96 +/- 2.18, P < 0.03). After MAP adjustments, there were no significant intergroup differences in oxygenation. Animals in the CV group required lower MAP than animals in the HFO group to maintain similar oxygenation. We conclude that surfactant repletion followed by PLV improves oxygenation during both CV and HFO. The initial response to administration of surfactant and PFC was different for the conventional and high-frequency oscillation groups, likely reflecting the ventilation strategy used; animals in the CV group responded most to surfactant, whereas animals in the HFO group responded most after PFC instillation. The ultimately similar oxygenation of the two groups once lung volume had been optimized suggests that HFO may be used effectively during administration of, and treatment with, surfactant and perfluorocarbon.

Liquid-assisted ventilation: physiology and clinical application

Liquid-assisted ventilation, as an alternative ventilation strategy for respiratory distress, is progressing from theory and basic science research to clinical application. Biochemically inert perfluorochemical liquids have low surface tension and high solubility for respiratory gases. From early immersion experiments, two primary techniques for liquid-assisted ventilation have emerged: total liquid ventilation and partial liquid ventilation. While computer-controlled, time-cycled, pressure/volume-limited total liquid ventilators can take maximum advantage of these liquids by completely eliminating the gas phase in the distressed lung, partial liquid ventilation takes advantage of having these liquids in the lung while maintaining gas ventilation. The benefits of both partial and total techniques have been demonstrated in animal models of neonatal and adult respiratory distress syndrome, aspiration syndromes and congenital diaphragmatic hernia and also in combination with other therapeutic modalities including extracorporeal membrane oxygenation, high-frequency ventilation and nitric oxide. Additionally, nonrespiratory applications have expanding potential including pulmonary drug delivery and radiographic imaging. Since its use in neonates in 1989, liquid-assisted ventilation in humans has progressed to a variety of clinical experiences with different aetiologies of respiratory distress. The future holds the opportunity to clarify and optimize the potential of multiple clinical applications for liquid-assisted ventilation.