High-frequency oscillatory ventilation of the perfluorocarbon-filled lung: preliminary results in an animal model of acute lung injury

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.

High-frequency oscillatory ventilation in infants and children

The goal of mechanical ventilation in patients with acute lung injury is to support gas exchange and mitigate ventilator-associated lung injury. High-frequency oscillatory ventilation relies on the generation of a constant distending pressure, small tidal volumes and rapid respiratory rates with the intent to recruit atelectatic lung, reduce peak inflating pressures and limit volutrauma. The utilization of high-frequency oscillatory ventilation has dramatically increased in neonatal and pediatric intensive care units. As there is an overlap between the intensive care unit and the operating room, anesthesiologists must be familiar with recent advances in the care of infants and children with acute respiratory failure. High-frequency oscillatory ventilation has been used successfully to manage patients with severe respiratory failure who have failed conventional mechanical ventilation. When initiated early, high-frequency oscillatory ventilation has been shown to improve oxygenation and reduce acute and chronic lung injury in neonates, infants and children. Further trials are necessary to better delineate the benefits and risks of this therapy in various patient populations.

Combination of Arteriovenous Extracorporeal Lung Assist and High-Frequency Oscillatory Ventilation in a Porcine Model of Lavage-Induced Acute Lung Injury: A Randomized Controlled Trial

BACKGROUND

To compare the combined effects of arteriovenous extracorporeal lung assist (AV-ECLA) and high-frequency oscillatory ventilation (HFOV) on pulmonary gas exchange, hemodynamics, and respiratory parameters in a lavage-induced porcine lung injury model.

METHODS

A prospective, randomized animal study. Saline lung lavage was performed in 33 healthy female pigs, weighing 52 +/- 4.1 kg (mean +/- SD), until the Pao2 decreased to 53 +/- 8 mm Hg. After a stabilization period of 60 minutes, the animals were randomly assigned to four groups: group 1, pressure-controlled ventilation (PCV) with a tidal volume of 6 mL/kg; group 2, PCV with a tidal volume of 6 mL/kg and AV-ECLA; group 3, HFOV; group 4, HFOV and AV-ECLA. In groups 2 and 4, the femoral artery and vein were cannulated and a low-resistance membrane lung was interposed. After isolated evaluation of AV-ECLA, the mean airway pressure was increased by 3 cm H2O from 16 to 34 cm H2O every 20 minutes, accompanied by blood gas analyses and measurements of respiratory and hemodynamic variables.

RESULTS

Only in AV-ECLA-treated animals was normocapnia achieved. No significant increase of Pao2 attributable to AV-ECLA alone was detected. Mean airway pressure augmentation resulted in a significant increase in Pao2 in all groups. Peak inspiratory pressure was significantly lower in HFOV-treated animals.

CONCLUSIONS

The combination of AV-ECLA and HFOV resulted in normocapnia and comparable Pao2, although a smaller ventilator pressure amplitude was applied. Long-term animal studies are needed to assess whether this approach results in further lung protection.

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.

Effect of low-bias flow oscillation with partial liquid ventilation on fluoroscopic image analysis, gas exchange, and lung injury

OBJECTIVE

To evaluate the effect of low-bias flow oscillation (LBFO) with partial liquid ventilation (PLV) on perfluorochemical evaporation, histopathology, 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

Twelve New Zealand White rabbits.

INTERVENTIONS

Juvenile rabbits were anesthetized, paralyzed, and ventilated through a tracheostomy with either high-frequency oscillatory ventilation or LBFO. Lung injury was induced by repeated saline lavage, after which perflubron was instilled through a side port of the endotracheal tube. Lateral fluoroscopic images were performed at baseline and at various postfill intervals of animals in the high-frequency oscillatory ventilation-PLV and LBFO-PLV groups. The images were digitalized for computer analysis of the Lung Lucency Index, a surrogate marker of perflubron evaporation. Histopathologic evaluation was performed using a lung-injury scoring system. Malondialdehyde was measured in lung homogenates to assess oxidative damage.

MEASUREMENTS AND MAIN RESULTS

There were no significant differences in gas exchange and ventilator settings between groups throughout the experiment. At 300 mins, the high-frequency oscillatory ventilation-PLV group had a significantly higher Lung Lucency Index compared with the LBFO-PLV group in both dependent and nondependent lung regions (a high Lung Lucency Index correlates with increased perflubron loss). Malondialdehyde measurements were not different between groups. Animals treated with LBFO-PLV had a lower histopathologic lung-injury score compared with high-frequency oscillatory ventilation-PLV.

CONCLUSION

LBFO-PLV is a viable mode of ventilation in a model of acute lung injury and is associated with significant preservation of perflubron in comparison with high-frequency oscillatory ventilation-PLV. The lower evaporative losses during LBFO-PLV were associated with improved histology scores.

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.

Neutral Endopeptidase Determines the Severity of Pancreatitis-Associated Lung Injury1

BACKGROUND

Neutral endopeptidase (NEP) is a cell-surface metalloprotease that degrades proinflammatory peptides such as substance P, neurokinin A, and bradykinin. Inhibition of NEP exacerbates both experimental pancreatitis and the associated lung injury. It is unclear if worsened lung injury is the indirect result of more severe pancreatitis or if it is a direct effect of NEP inhibition in the lung.

MATERIALS AND METHODS

We used a model of pancreatitis-associated lung injury (PALI) to test the hypothesis that antagonism or genetic deletion of NEP augments PALI inflammation and pulmonary damage irregardless of the degree of pancreatitic inflammation.

RESULTS

In NEP(+/+) mice, intraperitoneal injection of porcine pancreatic elastase (elastase, 0.085 U/g at t = 0 h and t = 1 h) caused a 7-fold increase in lung myeloperoxidase (MPO) activity and marked pulmonary edema, neutrophil infiltration, and hemorrhage at 4 h as compared to control animals. The pattern of lung injury induced by elastase mimicked that observed among a separate group of animals with PALI induced by cerulein but was not associated with pancreatitis. Both NEP(-/-) mice and NEP(+/+) mice pretreated with the NEP antagonist phosphoramidon (10 mg/kg s.c.) had significant elevations of lung MPO and worsened lung histology compared to NEP(+/+) mice given elastase alone. Antagonism of either the vanilloid receptor transient receptor vanilloid 1 or the substance P receptor NK1-R had no effect on elastase-mediated lung injury in NEP-deficient mice.

CONCLUSIONS

NEP is an inhibitor of pancreatic elastase-induced lung injury, presumably via degradation of proinflammatory mediators.

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.

Heliox administration in the pediatric intensive care unit: an evidence-based review

OBJECTIVE

To provide a comprehensive, evidence-based review of helium-oxygen gas mixtures (heliox) in the management of pediatric respiratory diseases.

DATA SOURCE

A thorough, computerized bibliographic search of the preclinical and clinical literature regarding the properties of helium and its application in pediatric respiratory disease states.

DATA SYNTHESIS

After an overview of the potential benefits and technical aspects of helium-oxygen gas mixtures, the role of heliox is addressed for asthma, aerosolized medication delivery, upper airway obstruction, postextubation stridor, croup, bronchiolitis, and high-frequency ventilation. The available data are objectively classified based on the value of the therapy or intervention as determined by the study design from which the data are obtained.

CONCLUSIONS

Heliox administration is most effective during conditions involving density-dependent increases in airway resistance, especially when used early in an acute disease process. Any beneficial effect of heliox should become evident in a relatively short period of time. The medical literature supports the use of heliox to relieve respiratory distress, decrease the work of breathing, and improve gas exchange. No adverse effects of heliox have been reported. However, heliox must be administered with vigilance and continuous monitoring to avoid technical complications.

Positive end-expiratory pressure modulates perfluorochemical evaporation from the lungs

To study the effects of positive end-expiratory pressure (PEEP) level on perfluorochemical (PFC) elimination profiles (E(L)), 6 ml/kg of perflubron were instilled into healthy anesthetized rabbits. The ventilation strategy was to maintain constant minute ventilation (300 ml/kg/min) and mean airway pressure (7-8 cm H(2)O) while randomly changing the PEEP levels from 5 to 0, 1, 3, and 10 cm H(2)O, each for a period of 15 min. The PFC content in the expired gas was measured and the E(L) was calculated. There was a significant reduction in the E(L) when decreasing the PEEP levels from 5 to 0 cm H(2)O, but no differences were seen when the PEEP was increased from 5 to 10 cm H(2)O. The results indicate that PEEP levels influence PFC elimination profiles; therefore, the measurement of the E(L) and PEEP levels should be considered when optimizing supplemental PFCs during partial liquid ventilation.

High-frequency partial liquid ventilation in two infants

Two infants on high-frequency oscillatory ventilation for chronic lung disease and severe respiratory failure, received a bolus of warmed and oxygenated perfluorodecalin up to residual functional capacity, followed by a continuous infusion of 6 ml/kg/hour. Our aim was to improve gas exchange without increasing ventilatory-induced lung injury. Heart rate, oxygen saturation, blood pressure, and TcPO(2)/TcPCO(2) were continuously monitored during treatment. Arterial blood gas was evaluated every 3 hours. Both patients showed improvement of gas exchange with a 13.6 and 12.5% reduction of oxygenation index, respectively. High-frequency partial liquid ventilation is an experimental ventilation technique that could be considered as rescue treatment, to improve oxygenation in subjects with critical respiratory failure. This method could probably produce less damage, than other ventilation modes, to severely injured lungs.

Extended high-frequency partial liquid ventilation in lung injury: gas exchange, injury quantification, and vapor loss

High-frequency oscillatory ventilation with perflubron (PFB) reportedly improves pulmonary mechanics and gas exchange and attenuates lung injury. We explored PFB evaporative loss kinetics, intrapulmonary PFB distribution, and dosing strategies during 15 h of high-frequency oscillation (HFO)-partial liquid ventilation (PLV). After saline lavage lung injury, 15 swine were rescued with high-frequency oscillatory ventilation (n = 5), or in addition received 10 ml/kg PFB delivered to dependent lung [n = 5, PLV-compartmented (PLV(C))] or 10 ml/kg distributed uniformly within the lung [n = 5, PLV(U)]. In the PLV(C) group, PFB vapor loss was replaced. ANOVA revealed an unsustained improvement in oxygenation index in the PLV(U) group (P = 0.04); the reduction in oxygenation index correlated with PFB losses. Although tissue myeloperoxidase activity was reduced globally by HFO-PLV (P < 0.01) and regional lung injury scores (lung injury scores) in dependent lung were improved (P = 0.05), global lung injury scores were improved by HFO-PLV (P < 0.05) only in atelectasis, edema, and alveolar distension but not in cumulative score. In our model, markers of inflammation and lung injury were attenuated by HFO-PLV, and it appears that uniform intrapulmonary PFB distribution optimized gas exchange during HFO-PLV; additionally, monitoring PFB evaporative loss appears necessary to stabilize intrapulmonary PFB volume.

Recombinant human neutral endopeptidase ameliorates pancreatic elastase-induced lung injury

BACKGROUND

Genetic deletion of neutral endopeptidase (NEP), a cell-surface metalloprotease that degrades proinflammatory peptides, exacerbates lung injury induced by pancreatic elastase in a model of pancreatitis-associated lung injury. We tested 3 hypotheses: (1) genetic deletion of NEP prolongs lung recovery after elastase injections; (2) elastase-mediated lung injury is associated with down-regulation of NEP; and (3) pretreatment of NEP (-/-) and (+/+) animals with recombinant human NEP (rhNEP) reduces pulmonary damage in this model.

METHODS

NEP (+/+) or (-/-) mice were injected with pancreatic elastase (0.085 U/g/dose intraperitoneally) or saline carrier at t = 0 hours and t = 1 hour. Some mice were pretreated with rhNEP (3 mg/kg intraperitoneally). Serum elastase, lung histologic score, myeloperoxidase, and NEP activities were measured at 4, 8, or 12 hours.

RESULTS

NEP (-/-) mice had worse pulmonary inflammation at 4 and 8 hours versus (+/+) mice. Lung NEP activity was similar in elastase-treated and control (+/+) animals. Pretreatment with rhNEP reduced myeloperoxidase and improved histology at 4 hours in NEP (-/-) and (+/+) mice.

CONCLUSIONS

Pancreatic elastase induces lung injury that is worse and prolonged in NEP (-/-) mice. Pretreatment with rhNEP ameliorates this injury. Thus, upregulation of NEP is a potential therapeutic approach for pancreatitis-associated lung injury.

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.

Assessment of newly developed perfluorocarbon emulsion: oxygen carrying capacity as the blood substitute in vivo

This study provides the evaluation of oxygen carrying capacity of the novel perfluorocarbon emulsion (Neo-PFC) produced by the new emulsifying technology named High Pressure Process. For the performance comparison of oxygen carrying abilities of Neo-PFC and a representative PFC emulsion, the oxidation states of cerebral tissues in substituted animals were measured by near-infrared spectrometry. After the 70% exchange transfusion of whole blood of rats by Neo-PFC and Fluosol-DA, fractional inspired oxygen (FiO2) was gradually decreased from 100% to 0%. As the control experiments, the blood was substituted by Krebs Ringer bicarbonate buffer containing 3% BSA. When the blood of rats was substituted by Neo-PFC, Cyt. ox., a terminal enzyme in mitochondrial respiratory chain maintained fully oxidized state with FiO2 values between 100 to 40%. By contrast, in the models substituted by Fluosol-DA and BSA-buffer. Cyt. ox. was gradually reduced with FiO2 values below 60% and 80%, respectively. This specific advantage of Neo-PFC was explained by its higher oxygen solubility in arterial blood. The novel PFC emulsion prepared by the new emulsifying technology is a potential basis for blood substitutes.

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.

High-frequency ventilation in the pediatric intensive care unit

OBJECTIVE

To provide a state-of-the-art review of high-frequency oscillatory ventilation in the management of pediatric patients with respiratory failure.

DATA SOURCES

A thorough analysis of the preclinical and clinical literature regarding the pathophysiology of respiratory failure and the efficacy of high-frequency techniques in the neonatal and pediatric populations.

DATA SYNTHESIS

After an overview of the introduction of high-frequency techniques, the following topical areas are addressed: device vs. strategy, indications for use, disease-specific strategies, additional practical considerations, and the future of high-frequency techniques.

CONCLUSIONS

The ideal ventilatory approach in patients with hypoxemic respiratory failure may be early institution of an "open lung" strategy using high-frequency ventilatory techniques. The mechanisms of gas exchange that are most important during high-frequency ventilation are bulk axial flow, interregional gas mixing, and molecular diffusion. Infants with hyaline membrane disease and congenital diaphragmatic hernia have also responded positively to the implementation of high-frequency techniques. The oxygenation index (mean airway pressure x Fio2 x 100/Pao2) provides useful prognostic information in patients being managed with high-frequency oscillatory ventilation and may help to identify those patients with high predicted mortality to offer additional or experimental therapies. In the future, the combination of high-frequency oscillatory ventilation and partial liquid breathing offers the possibility of partitioning the physiologic changes associated with positive pressure ventilation. This approach may prove to be the ultimate lung-protective ventilatory strategy.

Perfluorochemical elimination from the lungs: effect of initial dose

Liquid-assisted ventilation with perfluorochemical (PFC) has been beneficial in a variety of respiratory diseases in animals and humans. Although PFC evaporation from the lungs is in part dependent on ventilation strategy and positioning, guidelines for initial and replacement dosing are unclear. We hypothesized that PFC evaporative loss over time is dependent on the size of the initial dose. Juvenile rabbits (n = 18) were ventilated using constant animal position and ventilator strategy. PFC (perflubron: LiquiVent ) was instilled endotracheally, using four groups with initial doses of 2, 6, 12, and 17 mL/kg. A previously described thermal detector that measures PFC in expired gas was used to calculate loss rate, residual perflubron in the lung, and volume loss as a % of initial fill volume. There was a significant dose, time, and dose-time interaction such that evaporative loss was dependent on initial PFC volume and time after fill (P < 0.05). Evaporative loss rate decreased earlier at lower doses. The percentage of initial volume lost to evaporation over time was inversely related to dose and could not be predicted by decreasing % PFC saturations, independent of dose. Evaporative loss should be considered to optimize both the application of PFC to the lung and replacement dosing during partial liquid ventilation.

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.

Treatment of acute (Adult) respiratory distress syndrome. The holy grail of surfactant therapy

The treatment of neonatal respiratory distress syndrome with surfactant represents a successful culmination of decades of basic and clinical research. In many babies, respiratory distress syndrome is a relatively pure expression of surfactant deficiency. Acute respiratory distress syndrome (ARDS) is a more common disease that is most frequently seen in adults, but the processes are common to lung injuries in newborns and children as well. While some impairment of production and secretion of surfactant constituents may be present in ARDS, surfactant inactivation is probably a more important factor in this disease. Until recently, surfactants available for human use have been easily susceptible to inactivation and this may explain why they have been less successful for treatment of ARDS than for neonatal respiratory distress syndrome. This review outlines recent information on surfactant inactivation and describes initiatives that may result in 'inactivation-proof' surfactants that may be of increased benefit in ARDS.