Liquid ventilation: an alternative ventilation strategy for management of neonatal respiratory distress

Effects of surfactant on propagation and rupture of a liquid plug in a tube

Surfactant-laden liquid plug propagation and rupture occurring in lower lung airways are studied computationally using a front-tracking method. The plug is driven by an applied constant pressure in a rigid axisymmetric tube whose inner surface is coated by a thin liquid film. The evolution equations of the interfacial and bulk surfactant concentrations coupled with the incompressible Navier-Stokes equations are solved in the front-tracking framework. The numerical method is first validated for a surfactant-free case and the results are found to be in good agreement with the earlier simulations of Fujioka et al. (2008) and Hassan et al. (2011). Then extensive simulations are performed to investigate the effects of surfactant on the mechanical stresses that could be injurious to epithelial cells such as pressure and shear stress. It is found that the liquid plug ruptures violently to induce large pressure and shear stress on airway walls and even a tiny amount of surfactant significantly reduces the pressure and shear stress and thus improves cell survivability. However, addition of surfactant also delays the plug rupture and thus airway reopening.

Steady Displacement of Long Gas Bubbles in Channels and Tubes Filled by a Bingham Fluid

Bingham fluids behave like solids below a von Mises stress threshold, the yield stress, while above it they behave like Newtonian fluids. They are characterized by a dimensionless parameter, Bingham number (Bn), which is the ratio of the yield stress to a characteristic viscous stress. In this study, the non-inertial steady motion of a finite size gas bubble in both a plane 2D channel and an axi-symmetric tube filled by a Bingham fluid has been studied numerically. The Bingham number, Bn, is in the range 0 ≤ Bn ≤ 3, where Bn=0 is the Newtonian case, while the Capillary number which is the ratio of a characteristic viscous force to the surface tension has values Ca=0.05, 0.10, and 0.25. The volume of all axi-symmetric and 2D bubbles has been chosen to be identical for all parameter choices and large enough for the bubbles to be long compared to the channel/tube width/diameter. The Bingham fluid constitutive equation is approximated by a regularized equation. During the motion, the bubble interface is separated from the wall by a static liquid film. The film thickness scaled by the tube radius (axi-symmetric)/half of the channel height (2D) is the dimensionless film thickness, h. The results show that increasing Bn initially leads to an increase in h, however, the profile h versus Bn can be monotonic or non-monotonic depending on Ca values and 2D/axi-symmetric configurations. The yield stress also alters the shape of the front and rear of the bubble and suppresses the capillary waves at the rear of the bubble. The yield stress increases the magnitude of the wall shear stress and its gradient and therefore increases the potential for epithelial cell injuries in applications to lung airway mucus plugs. The topology of the yield surfaces as well the flow pattern in the bubble frame of reference varies significantly by Ca and Bn.

Long-circulating perfluorooctyl bromide nanocapsules for tumor imaging by 19FMRI

PLGA-PEG nanocapsules containing a liquid core of perfluorooctyl bromide were synthesized by an emulsion-evaporation process and designed as contrast agents for (19)F MRI. Physico-chemical properties of plain and PEGylated nanocapsules were compared. The encapsulation efficiency of PFOB, estimated by (19)F NMR spectroscopy, is enhanced when using PLGA-PEG instead of PLGA. PLGA-PEG nanocapsule diameter, measured by Dynamic Light Scattering is around 120 nm, in agreement with Transmission Electron microscopy (TEM) observations. TEM and Scanning Electron Microscopy (SEM) reveal that spherical core-shell morphology is preserved. PEGylation is further confirmed by Zeta potential measurements and X-ray Photoelectron Spectroscopy. In vitro, stealthiness of the PEGylated nanocapsules is evidenced by weak complement activation. Accumulation kinetics in the liver and the spleen was performed by (19)F MRI in mice, during the first 90 min after intravenous injection. In the liver, plain nanocapsules accumulate faster than their PEGylated counterparts. We observe PEGylated nanocapsule accumulation in CT26 xenograft tumor 7 h after administration to mice, whereas plain nanocapsules remain undetectable, using (19)F MRI. Our results validate the use of diblock copolymers for PEGylation to increase the residence time of nanocapsules in the blood stream and to reach tumors by the Enhanced Permeation and Retention (EPR) effect.

Effects of respiratory rate and tidal volume on gas exchange in total liquid ventilation

Using a rabbit model of total liquid ventilation (TLV), and in a corresponding theoretical model, we compared nine tidal volume-respiratory rate combinations to identify a ventilator strategy to maximize gas exchange, while avoiding choked flow, during TLV. Nine different ventilation strategies were tested in each animal (n = 12): low [LR = 2.5 breath/min (bpm)], medium (MR = 5 bpm), or high (HR = 7.5 bpm) respiratory rates were combined with a low (LV = 10 ml/kg), medium (MV = 15 ml/kg), or high (HV = 20 ml/kg) tidal volumes. Blood gases and partial pressures, perfluorocarbon gas content, and airway pressures were measured for each combination. Choked flow occurred in all high respiratory rate-high volume animals, 71% of high respiratory rate-medium volume (HRMV) animals, and 50% of medium respiratory rate-high volume (MRHV) animals but in no other combinations. Medium respiratory rate-medium volume (MRMV) resulted in the highest gas exchange of the combinations that did not induce choke. The HRMV and MRHV animals that did not choke had similar or higher gas exchange than MRMV. The theory predicted this behavior, along with spatial and temporal variations in alveolar gas partial pressures. Of the combinations that did not induce choked flow, MRMV provided the highest gas exchange. Alveolar gas transport is diffusion dominated and rapid during gas ventilation but is convection dominated and slow during TLV. Consequently, the usual alveolar gas equation is not applicable for TLV.

Congenital diaphragmatic hernia: current status and review of the literature

Treatment of congenital diaphragmatic hernia (CDH) challenges obstetricians, pediatric surgeons, and neonatologists. Persistent pulmonary hypertension (PPHT) associated with lung hypoplasia in CDH leads to a high mortality rate at birth. PPHT is principally due to an increased muscularization of the arterioles. Management of CDH has been greatly improved by the introduction of prenatal surgical intervention with tracheal obstruction (TO) and by more appropriate postnatal care. TO appears to accelerate fetal lung growth and to increase the number of capillary vessels and alveoli. Improvement of postnatal care over the last years is mainly due to the avoidance of lung injury by applying low peak inflation pressure during ventilation. The benefits of other drugs or technical improvements such as the use of inhaled nitric oxide or extracorporeal membrane oxygenation (ECMO) are still being debated and no single strategy is accepted worldwide. Despite intensive clinical and experimental research, the treatment of newborn with CDH remains difficult.

Alterations of alveolar type II cells and intraalveolar surfactant after bronchoalveolar lavage and perfluorocarbon ventilation. An electron microscopical and stereological study in the rat lung

Background

Repeated bronchoalveolar lavage (BAL) has been used in animals to induce surfactant depletion and to study therapeutical interventions of subsequent respiratory insufficiency. Intratracheal administration of surface active agents such as perfluorocarbons (PFC) can prevent the alveolar collapse in surfactant depleted lungs. However, it is not known how BAL or subsequent PFC administration affect the intracellular and intraalveolar surfactant pool.

Methods

Male wistar rats were surfactant depleted by BAL and treated for 1 hour by conventional mechanical ventilation (Lavaged-Gas, n = 5) or partial liquid ventilation with PF 5080 (Lavaged-PF5080, n = 5). For control, 10 healthy animals with gas (Healthy-Gas, n = 5) or PF5080 filled lungs (Healthy-PF5080, n = 5) were studied. A design-based stereological approach was used for quantification of lung parenchyma and the intracellular and intraalveolar surfactant pool at the light and electron microscopic level.

Results

Compared to Healthy-lungs, Lavaged-animals had more type II cells with lamellar bodies in the process of secretion and freshly secreted lamellar body-like surfactant forms in the alveoli. The fraction of alveolar epithelial surface area covered with surfactant and total intraalveolar surfactant content were significantly smaller in Lavaged-animals. Compared with Gas-filled lungs, both PF5080-groups had a significantly higher total lung volume, but no other differences.

Conclusion

After BAL-induced alveolar surfactant depletion the amount of intracellularly stored surfactant is about half as high as in healthy animals. In lavaged animals short time liquid ventilation with PF5080 did not alter intra- or extracellular surfactant content or subtype composition.

Effects of Inertia and Gravity on Liquid Plug Splitting at a Bifurcation

Liquid plugs may form in pulmonary airways during the process of liquid instillation or removal in many clinical treatments. During inspiration the plug may split at airway bifurcations and lead to a nonuniform final liquid distribution, which can adversely affect treatment outcomes. In this paper, a combination of bench top experimental and theoretical studies is presented to study the effects of inertia and gravity on plug splitting in an airway bifurcation model to simulate the liquid distributions in large airways. The splitting ratio, Rs, is defined as the ratio of the plug volume entering the upper (gravitationally opposed) daughter tube to the lower (gravitationally favored) one. Rs is measured as a function of parent tube Reynolds number, Rep; gravitational orientations for roll angle, ϕ, and pitch angle, γ; parent plug length LP; and the presence of pre-existing plug blockages in downstream daughter tubes. Results show that increasing Rep causes more homogeneous splitting. A critical Reynolds number Rec is found to exist so that when Rep⩽Rec, Rs=0, i.e., no liquid enters the upper daughter tube. Rec increases while Rs decreases with increasing the gravitational effect, i.e., increasing ϕ and γ. When a blockage exists in the lower daughter, Rec is only found at ϕ=60deg in the range of Rep studied, and the resulting total mass ratio can be as high as 6, which also asymptotes to a finite value for different ϕ as Rep increases. Inertia is further demonstrated to cause more homogeneous plug splitting from a comparison study of Rs versus Cap (another characteristic speed) for three liquids: water, glycerin, and LB-400X. A theoretical model based on entrance flow for the plug in the daughters is developed and predicts Rs versus Rep. The frictional pressure drop, as a part of the total pressure drop, is estimated by two fitting parameters and shows a linear relationship with Rep. The theory provides a good prediction on liquid plug splitting and well simulates the liquid distributions in the large airways of human lungs.

Effect of gravity on liquid plug transport through an airway bifurcation model

Many medical therapies require liquid plugs to be instilled into and delivered throughout the pulmonary airways. Improving these treatments requires a better understanding of how liquid distributes throughout these airways. In this study, gravitational and surface mechanisms determining the distribution of instilled liquids are examined experimentally using a bench-top model of a symmetrically bifurcating airway. A liquid plug was instilled into the parent tube and driven through the bifurcation by a syringe pump. The effect of gravity was adjusted by changing the roll angle (phi) and pitch angle (gamma) of the bifurcation (phi = gamma =0 deg was isogravitational). Phi determines the relative gravitational orientation of the two daughter tubes: when phi not equal to 0 deg, one daughter tube was lower (gravitationally favored) compared to the other. Gamma determines the component of gravity acting along the axial direction of the parent tube: when gamma not equal to 0 deg, a nonzero component of gravity acts along the axial direction of the parent tube. A splitting ratio Rs, is defined as the ratio of the liquid volume in the upper daughter to the lower just after plug splitting. We measured the splitting ratio, Rs, as a function of: the parent-tube capillary number (Cap); the Bond number (Bo); phi; gamma; and the presence of pre-existing plugs initially blocking either daughter tube. A critical capillary number (Cac) was found to exist below which no liquid entered the upper daughter (Rs = 0), and above which Rs increased and leveled off with Cap. Cac increased while Rs decreased with increasing phi, gamma, and Bo for blocked and unblocked cases at a given Cap > Ca,. Compared to the nonblockage cases, Rs decreased (increased) at a given Cap while Cac increased (decreased) with an upper (lower) liquid blockage. More liquid entered the unblocked daughter with a blockage in one daughter tube, and this effect was larger with larger gravity effect. A simple theoretical model that predicts Rs and Cac is in qualitative agreement with the experiments over a wide range of parameters.

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.

Chemical stability of esters of nicotinic acid intended for pulmonary administration by liquid ventilation

PURPOSE

It has been suggested that fluorocarbon liquid may be a unique vehicle for the delivery of drugs directly to the acutely injured lung. A prodrug approach was used as a means of enhancing the solubility of a model drug (nicotinic acid) in the fluorocarbon. The solubility, the chemical stability of the putative prodrugs, and the sensitivity to enzymatic hydrolysis was investigated.

METHODS

The solubility of each nicotinic acid ester was determined in buffer as a function of pH and in perflubron. The octanol/buffer partition coefficient was determined at pH 7.4. The chemical stability of the putative prodrugs was determined as a function of pH, temperature, buffer content, and ionic strength. In addition, sensitivity of the esters to enzymatic degradation was evaluated.

RESULTS

Compared with nicotinic acid, the solubility in perflubron of the esters was significantly enhanced. In aqueous buffers, the esters exhibited pseudo-first order degradation kinetics, with both acid and base catalyzed loss. Studies of the fluorobutyl ester indicate quantitative loss of the putative prodrug and release of the parent nicotinic acid. Porcine esterase accelerated the loss of fluorobutyl ester by a factor of over 200 compared with chemical hydrolysis at pH 7.4.

CONCLUSIONS

The properties of the fluorinated esters suggest that they may be suitable candidates for further testing as possible prodrugs of nicotinic acid based upon higher solubility in perflubron, rapid release of the parent drug after simple hydrolysis, and sensitivity to the presence of a model esterase enzyme.

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.

Effects of partial liquid ventilation with FC-77 on acute lung injury in newborn piglets

Partial liquid ventilation (PLV) with various types of perfluorochemicals (PFC) has been shown to be beneficial in treating acute lung injury. FC-77 is a type of PFC with relatively high vapor pressure and evaporative losses during PLV. This study tested the hypothesis that using FC-77 for PLV with hourly replacement is effective in treating acute lung injury. Fifteen neonatal piglets were randomly and evenly divided into 3 study groups: 1) lavage-induced lung injury followed by conventional mechanical ventilation (Lavage-CMV); 2) lavage-induced lung injury followed by PLV using FC-77 with hourly replacement (11.2 +/- 1.5 mL/kg/hr) (Lavage-PLV); and 3) sham lavage injury followed by conventional mechanical ventilation (Control). Immediately after induction, repeated saline lavages induced acute lung injury characterized by decreases in dynamic lung compliance, arterial oxygen tension, and arterial pH, and increases in arterial CO(2) tension and oxygenation index, whereas the sham lavage procedure failed to do so. During the 3-hr period of CMV, these pulmonary and cardiovascular parameters remained stable in the Control group, but deteriorated in the Lavage-CMV group. In contrast, after acute lung injury, low lung compliance, abnormal gas exchange, acidosis, and inadequate oxygenation significantly improved in the Lavage-PLV group. Histological analysis of these 3 study groups revealed that the Lavage-CMV group had the highest lung injury score and the Control group had the lowest. These results suggest that, in comparison to CMV, PLV with FC-77 and hourly replacement of FC-77 promotes more favorable pulmonary mechanics, gas exchange, oxygenation, and lung histology in a piglet model of acute lung injury.

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.

Liquid ventilation in an infant with persistent interstitial pulmonary emphysema

We present the case of a full term infant affected by diffuse persistent interstitial pulmonary emphysema (PIPE), who was treated with partial liquid ventilation (PLV) after the failure of conventional management. PIPE is a lethal chronic lung disease of unclear pathogenesis. Clinical history, radiological and histological findings confirmed the diagnosis in our patient. PLV applied for 48 hours resulted in a significant improvement in the infant's respiratory function and was not associated with adverse effects. We concluded that PLV could be effective in prolonging the survival of infants with PIPE; its application represents an effective form of respiratory support in infants with chronic lung disease.

Effects of partial liquid ventilation with perfluorodecalin in the juvenile rabbit lung after saline injury

OBJECTIVE

To evaluate the feasibility of using the perfluorochemical, perfluorodecalin, for partial liquid ventilation (PLV) with respect to gas exchange and lung mechanics in normal and saline-injured lungs of juvenile rabbits.

DESIGN

Experimental, prospective, randomized, controlled study.

SETTING

Physiology laboratory at a university medical school.

SUBJECTS

Seventeen juvenile rabbits assigned to three groups.

INTERVENTIONS

The conventional mechanical ventilation (CMV)-injury group (n = 5) was treated with CMV after establishing a lung injury; the PLV-injury group (n = 6) was treated with PLV after lung injury; and the PLV-healthy group (n = 6) was supported with PLV without lung injury. Lung injury was created by repeated saline lung lavages. PLV-treated animals received a single dose of intratracheal perfluorodecalin at a volume equal to the measured preinjury gas functional residual capacity (functional residual capacity = 18.6+/-1.5 [SEM] mL/kg).

MEASUREMENTS AND MAIN RESULTS

Sequential measurements of total respiratory compliance and arterial blood chemistries were performed in all groups. Oxygenation index (OI) and ventilation efficiency index were calculated. After lung injury, there was a significant (p < .05) decrease in PaO2, total respiratory compliance, and ventilation efficiency index and an increase in OI and PaCO2. In the PLV-injury group, PLV significantly (p < .05) improved PaO2 (+60%) and OI (-33%) over time. Compliance was significantly (p < .05) higher (90%) than in the CMV-injury group over time.

CONCLUSIONS

These results demonstrate that PLV with perfluorodecalin improved oxygenation and increased respiratory compliance in the saline-injured rabbit lung. In addition, similar to the effects of several other perfluorochemical liquids on normal lungs, pulmonary administration of perfluorodecalin was associated with a small impairment in gas exchange and a significant decrease in lung compliance in the juvenile rabbit model.

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.

New ways to ventilate newborns in acute respiratory failure

Out treatment options for acute neonatal failure have expanded greatly in the last 20 to 30 years. This article reviews patient-triggered ventilation, high frequency ventilation, negative extrathoracic pressure ventilation, nitric oxide therapy, liquid ventilation, extracorporeal membrane oxygenation, and advances in pulmonary function monitoring. The authors present background theories, describe equipment, review clinical strategies, and the results of recent trials.

Perfluorochemical rescue after surfactant treatment: effect of perflubron dose and ventilatory frequency

To test the hypotheses that perfluorochemical (PFC) liquid rescue after natural surfactant (SF) treatment would improve pulmonary function and histology and that this profile would be influenced by PFC dose or ventilator strategy, anesthetized preterm lambs (n = 31) with respiratory distress were studied using nonpreoxygenated perflubron. All animals received SF at 1 h and were randomized at 2 h as follows and studied to 4 h postnatal age: 1) conventional mechanical gas ventilation (n = 8), 2) 30 ml/kg perflubron with gas ventilation [partial liquid ventilation (PLV)] at 60 breaths/min (n = 8), 3) 10 ml/kg perflubron with PLV at 60 breaths/min (n = 7), and 4) 10 ml/kg perflubron with PLV at 30 breaths/min (n = 8). All animals tolerated instillation without additional cardiopulmonary instability. All perflubron-rescued groups demonstrated sustained improvement in gas exchange, respiratory compliance, and reduction in pressure requirements relative to animals receiving SF alone. Improvement was directly related to perflubron dose and breathing frequency; peak inspiratory pressure required to achieve physiological gas exchange was lower in the higher-dose and -frequency groups, and mean airway pressure was lower in the lower-frequency group. Lung expansion was greater and evidence of barotrauma was less in the higher-dose and -frequency group; regional differences in expansion were not different as a function of dose but were greater in the lower-frequency group. Regional differences in lung perflubron content were reduced in the higher-dose and -frequency groups and greatest in the lower-dose and -frequency group. The results suggest that, whereas PLV of the SF-treated lung improves gas exchange and lung mechanics, the protective benefits of perflubron in the lung may depend on dose and ventilator strategy to optimize PFC distribution and minimize exposure of the alveolar-capillary membrane to a gas-liquid interface.

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.

Comparison of perfluorochemical fluids used for liquid ventilation: effect of endotracheal tube flow resistance

Neonatal endotracheal tubes with small inner diameters are associated with increased resistance regardless of the medium used for assisted ventilation. During liquid ventilation (LV) reduced interfacial tension and pressure drop along the airways result in lower alveolar inflation pressure compared with gas ventilation (GV). This is possible by optimizing liquid ventilation strategies to overcome the resistive forces associated with liquid density (rho) and viscosity (mu) of these fluids. Knowledge of the effect of rho, mu, and endotracheal tube (ETT) size on resistance is essential to optimize LV strategies. To evaluate these physical properties, three perfluorochemical (PFC) fluids with a range of kinematic viscosities (FC-75 = 0.82, LiquiVent = 1.10, APF-140 = 2.90) and four different neonatal ETT tubes (Mallincrokdt Hi-Lo Jet ID 2.5, 3.0, 3.5, and 4.0 mm) were studied. Under steady-state flow, flow and pressure drop across the ETTs were measured simultaneously. Resistance was calculated by dividing pressure drop by flow, and both pressure-flow and resistance-flow relationships were plotted. Also, pressure drop and resistance were each plotted as a function of kinematic viscosity at flows of 0.01 L.s-1 for all four ETT sizes. Data demonstrated a quadratic relationship with respect to pressure drop versus flow, and a linear relationship with resistance versus flow: both were significantly correlated (R = 0.92; P < 0.01) and were inversely related to ETT size. Additionally, there was a significant correlation between pressure drop or resistance and kinematic viscosity (R = 0.99; P < 0.01). For LV in neonates these data can be used to select the optimum ETT size and PFC liquid depending OR the chosen ventilation strategy.