Liquid-assisted ventilation: physiology and clinical application

Administration of Drugs/Gene Products to the Respiratory System: A Historical Perspective of the Use of Inert Liquids

The present review is a historical perspective of methodology and applications using inert liquids for respiratory support and as a vehicle to deliver biological agents to the respiratory system. As such, the background of using oxygenated inert liquids (considered a drug when used in the lungs) opposed to an oxygen-nitrogen gas mixture for respiratory support is presented. The properties of these inert liquids and the mechanisms of gas exchange and lung function alterations using this technology are described. In addition, published preclinical and clinical trial results are discussed with respect to treatment modalities for respiratory diseases. Finally, this forward-looking review provides a comprehensive overview of potential methods for administration of drugs/gene products to the respiratory system and potential biomedical applications.

In vivo clearance of nanoparticles by transcytosis across alveolar epithelial cells

Nanoparticles in polluted air or aerosolized drug nanoparticles predominantly settle in the alveolar lung. Here, we describe a novel, highly effective pathway for the particles to cross the alveolar epithelium and reach the lymph and bloodstream. Amorphous silica nanoparticles, suspended in perfluorocarbon, were instilled into the lungs of mice for intravital microscopy. Particles formed agglomerates that settled on the alveolar wall, half of which were removed from the lung within 30 minutes. TEM histology showed agglomerates in stages of crossing the alveolar epithelium, in large compartments inside the epithelial cells and crossing the basal membrane into the interstitium. This pathway is consistent with published kinetic studies in rats and mice, using a host of (negatively) charged and polar nanoparticles.

Management of Congenital Diaphragmatic Hernia in Newborn - Paradigm Shift and Ethical Issues

Management of congenital diaphragmatic hernia (CDH) begins soon after it is detected, whether antenatally or postnatally. Assessment of the severity of the condition, associated congenital anomalies, maternal health and related issues, weight of the fetus/baby, mode of delivery, timing of delivery, immediate appropriate management of the baby with CDH at birth, appropriate utilization of available treatment modalities as well as infrastructure of the treating institute have an impact on the outcome of the neonate. Survival without significant long-term/permanent morbidity is considered as good outcome. With advances in antenatal diagnosis, several legal and ethical considerations have cropped up. While on one hand there are proponents of early antenatal diagnosis and medical termination of pregnancy (MTP), on the other hand there are several socio-cultural groups who look upon human life as precious and argue against MTP. There is an ongoing ethical battle between maternal vs. fetal rights; there is no way to put a lid on the controversy whether the mother be allowed to choose in favor of MTP after being aware of the anomalous fetus or, we must attempt to save every fetus irrespective of the antenatal diagnosis of life-threatening anomalies. Notwithstanding, appropriate assessment of the condition, thorough counseling and sound evidence-based decisions could avert ethical dilemma in most cases. This review article provides information about the various choices available in the diagnostic and treatment armamentarium, though it should be kept in mind that the entire spectrum of management strategies may not be universally available.

Liquid ventilation

Human have lungs to breathe air and they have no gills to breath liquids like fish. When the surface tension at the air-liquid interface of the lung increases as in acute lung injury, scientists started to think about filling the lung with fluid instead of air to reduce the surface tension and facilitate ventilation. Liquid ventilation (LV) is a technique of mechanical ventilation in which the lungs are insufflated with an oxygenated perfluorochemical liquid rather than an oxygen-containing gas mixture. The use of perfluorochemicals, rather than nitrogen as the inert carrier of oxygen and carbon dioxide offers a number of advantages for the treatment of acute lung injury. In addition, there are non-respiratory applications with expanding potential including pulmonary drug delivery and radiographic imaging. It is well-known that respiratory diseases are one of the most common causes of morbidity and mortality in intensive care unit. During the past few years several new modalities of treatment have been introduced. One of them and probably the most fascinating, is of LV. Partial LV, on which much of the existing research has concentrated, requires partial filling of lungs with perfluorocarbons (PFC's) and ventilation with gas tidal volumes using conventional mechanical ventilators. Various physico-chemical properties of PFC's make them the ideal media. It results in a dramatic improvement in lung compliance and oxygenation and decline in mean airway pressure and oxygen requirements. No long-term side-effect reported.

Liquid ventilation

Mammals have lungs to breathe air and they have no gills to breath liquids. When the surface tension at the air-liquid interface of the lung increases, as in acute lung injury, scientists started to think about filling the lung with fluid instead of air to reduce the surface tension and facilitate ventilation. Liquid ventilation (LV) is a technique of mechanical ventilation in which the lungs are insufflated with an oxygenated perfluorochemical liquid rather than an oxygen-containing gas mixture. The use of perfluorochemicals, rather than nitrogen, as the inert carrier of oxygen and carbon dioxide offers a number of theoretical advantages for the treatment of acute lung injury. In addition, there are non-respiratory applications with expanding potential including pulmonary drug delivery and radiographic imaging. The potential for multiple clinical applications for liquid-assisted ventilation will be clarified and optimized in future.

Modified haemoglobins and perfluorocarbons

After decades of research activities and product improvements in the field of artificial oxygen carriers based on either haemoglobin modifications or perfluorocarbon emulsions, these products have reached a critical stage of their development. Varieties of haemoglobin-based oxygen carriers and perfluorocarbon emulsions are under current clinical investigation. Although the clinical availability of artificial oxygen carriers may result in profound changes of fluid resuscitation from haemorrhage, the transfusion of human blood components as an integral part of medical trauma management will not be replaced. However, a rapid and effective restoration of tissue oxygenation by the use of artificial oxygen carriers in the treatment of severe haemorrhage may bridge time delays until stored and cross-matched human packed red cells are available. Whether artificial oxygen carriers could provide additional clinical benefits by sustaining tissue oxygenation even under conditions of a disturbed macro- or microcirculation is the subject of current investigations. Therefore, the application of safe and effective artificial oxygen carriers would not only be restricted to the treatment of severe haemorrhage, but additional therapeutic indications of artificial oxygen carriers in emergency medicine, trauma anaesthesia and other medical specialities would emerge.

Prenatal surgery for congenital diaphragmatic hernia

Congenital diaphragmatic hernia (CDH) has a mortality rate of up to 77% despite optimal pre- and postnatal care. Fetuses with liver herniation, a low lung-to-head ratio, and an early diagnosis before 24 weeks have a particularly poor prognosis. In utero open repair of these fetuses does not improve patient survival. The PLUG (Plug the Lung Until it Grows) technique was reported to be able to reverse pulmonary hypoplasia in CDH. A foam plug or a titanium clip is used and the trachea can be unplugged using Ex Utero Intrapartum Tracheoplasty (EXIT) at birth. Since hysterotomy causes premature labour, a video-fetoscopic intrauterine technique of tracheal occlusion called Fetendo-PLUG was developed. Compared to those who receive standard postnatal care or fetal tracheal occlusion via open hysterotomy, patients who undergo Fetendo-PLUG are reported to have a higher survival rate of 75% and fewer fetal and maternal complications. A recent refinement is to use a detachable balloon for intratracheal occlusion through a single 5 mm port under real-time ultrasound guidance. Without the need for neck dissection, injury to the recurrent laryngeal nerves and trachea and vocal cord paresis can be minimized. The result of this form of treatment for CDH is promising, but further refinement of fetal instrumentation and development of effective tocolytic drugs are still required.

Measurements of Alveolar pO2 Using 19F-MRI in Partial Liquid Ventilation

RATIONALE AND OBJECTIVES

Partial liquid ventilation using Perfluorcarbon (PFC) is an innovative treatment of acute respiratory distress syndrome. However, the underlying mechanisms are not totally clear. The aim was to investigate the distribution of oxygen partial pressure within the PFC-filled lung (ppO2).

METHODS

Nine pigs underwent partial liquid ventilation, receiving 20 mL PFC/kg bodyweight (bw). Measurements were obtained by a chemical shift selective TurboFLASH sequence at different axial lung levels. ppO2 was calculated from 19F-MRI by nonlinear curve T1-fitting technique after noise correction.

RESULTS

Quantification and distribution of ppO2 was performed successfully. A narrow relationship of the inspiratory O2 fraction and ppO2, as well as a significant ventral-to-dorsal gradient of ppO2 (ventral:dependent lung = 1.9:1) were detected in all subjects and slice positions.

CONCLUSIONS

In vivo measurement of local ppO2 gains new and clinical important insights into the physiology of PLV. The previously unknown ppO2 gradient within PFC fits to distribution of perfusion. Dependent lung regions appear to have limited access to O2 from central airways.

Effect of ventilatory variables on gas exchange and hemodynamics during total liquid ventilation in a rat model

OBJECTIVES

To investigate the settings necessary to achieve maximum gas exchange and pulmonary function while minimizing effects on cardiovascular hemodynamics during total liquid ventilation with a pressure-limited, time-cycled ventilator in a rat model.

DESIGN

Prospective, randomized controlled animal study.

SETTING

A university research laboratory.

SUBJECTS

Male Sprague-Dawley rats (n = 48).

INTERVENTIONS

All animals had a tracheostomy tube designed for total liquid ventilation placed under anesthesia. The carotid artery was cannulated for blood pressure monitoring and for assessing blood gas data.

MEASUREMENTS AND MAIN RESULTS

Forty 492 +/- 33 g rats were assigned to one of four inspiratory/expiratory ratio groups (inspiratory/expiratory ratio of 1:2, 1:2.5, 1:3, and 1:4). Total liquid ventilation was performed with a pressure-limited, time-cycled total liquid ventilator. Outcome measures were evaluated as a function of respiratory rate and included tidal volume, maximal alveolar ventilation, inspiratory and expiratory mean arterial pressures, the difference of mean arterial pressure between the inspiratory and expiratory phase, static end-inspiratory/expiratory pressures, Paco(2), Pao(2), tidal volume + approximate expiratory reserve volume, and lung volume-induced suppression of mean arterial pressure. Maximal alveolar ventilation increased and decreased in parabolic fashion as a function of respiratory rate and was maximal at rates of 4.3-6.8 breaths/min and high inspiratory/expiratory ratios that corroborated with optimal levels of Pao(2) and Paco(2). Lung overdistention occurred at high respiratory rates and high inspiratory/expiratory ratios. Deleterious effects were observed on the difference of mean arterial pressure between the inspiratory and expiratory phase during total liquid ventilation at low respiratory rates, apparently due to increased tidal volume, and on suppression of mean arterial pressure at high inspiratory/expiratory ratios and high respiratory rate apparently due to "auto-positive end-expiratory pressure." These effects were minimized in this model at respiratory rates >/=5.7 and </=6.8 breaths/min and inspiratory/expiratory ratios </=1:2.5. These settings were successfully tested in eight additional animals.

CONCLUSION

These data demonstrate the feasibility of performing total liquid ventilation in rodents. A balance must be identified where gas exchange is optimal yet hemodynamics are least affected. In the specific system studied, an inspiratory/expiratory ratio of 1:2.5 and respiratory rate of 6.8 breaths/min appeared to provide optimal gas exchange while minimizing the effects on hemodynamics.

Systemic perfluorocarbons suppress the acute lung inflammation after gastric acid aspiration in rats

Perflurocarbons (PFCs) are used during liquid ventilation and as hemoglobin substitutes. PFCs reduce free radical generation and damage to the lung during liquid ventilation. Thus, we examined the effects of parenteral administration of PFCs on lung injury after acid aspiration. Rats were treated with intraperitoneal injection of either FC-77 or IV injection of Fluosol. Controls received intraperitoneal or IV normal saline (NS) before or at the time of injury and then were injured by instillation of NS + HCl (pH = 1.25) into their lungs via a tracheotomy. The animals were exposed to air or 98% oxygen, breathing spontaneously. The rats were injected with 0.05 microCi of (125)I-albumin (bovine serum albumin) before injury. The extent of lung injury was assessed 5 h postinjury by compliance and lung albumin permeability index measurement. Myeloperoxidase (MPO) activity and histologic examination were used to assess neutrophilic infiltration. Both FC-77 and Fluosol decreased the permeability index compared with controls (1.05 +/- 0.08; 1.08 +/- 0. 12, respectively, versus 1.34 +/- 0.21) and improved lung compliance after intratracheal instillation of 1.2 mL/kg of HCl/NS, pH = 1.25 + hyperoxia injury (P < 0.05). Lung MPO activity decreased in the FC-77 group and was associated with a concomitant decrease in neutrophil infiltration. MPO activity of the spleen increased after FC-77 treatment. The administration of FC-77 decreased the severity of lung permeability changes associated with acid in the presence or absence of hyperoxia exposure. These data suggest that attenuation of neutrophilic infiltration by PFCs decreases lung injury.

IMPLICATIONS

Intraperitoneally administered perfluorocarbons in rats attenuate the neutrophilic infiltration in the lung after acid aspiration, thereby decreasing the alveolar protein leakage and improving pulmonary compliance.

Intra-tracheal delivery strategy of gentamicin with partial liquid ventilation

Patients with pulmonary infection often present with ventilation and perfusion abnormalities, which can impair intravenous antibiotic therapy. Intra-tracheal (i.t.) administration has met with obstacles, such as inadequate delivery to affected lung regions and the disruption of gas exchange. We hypothesized that i.t. administration of a gentamicin (G)/perfluorochemical (PFC) suspension (G/PFC) would effectively deliver and distribute gentamicin to the lung, while maintaining gas exchange and non-toxic serum levels. In addition, we sought to compare serum G and lung levels and distribution of G when G/PFC is administered at the initiation of partial liquid ventilation (PLV) vs. during PLV. To test this hypothesis, 17 newborn lambs were ventilated by PLV with perflubron (LiquiVent) for 4 h using three different G (5 mg kg-1) administration techniques: i.t. slow-fill (SF) (n = 6; G/PFC over 15 min at start of PLV), i.t. top-fill (TF) (n = 6; G/PFC 10-65 min after start of PLV), intravenous (i.v.) (n = 5, aqueous injection at start of PLV). Serum levels of gentamicin were obtained 1, 15, 30 and 60 min after administration, and hourly there after for the remainder of the protocol (4 h). Arterial blood gas and pulmonary function measurements were obtained throughout the protocol. At the conclusion of the protocol, representative samples from each lung lobe, the brain and kidney were homogenized and assayed for gentamicin. All results are presented as the mean +/- SEM; P < 0.05. Over time, serum gentamicin levels were greatest (P < 0.05) in i.v. (11.0 +/- 2.3 micrograms ml-1), followed by TF (2.3 +/- 0.1 micrograms ml-1) and SF (0.8 +/- 0.1 microgram ml-1). The percentage of the administered dose remaining in the lungs after 4 h was greater (P < 0.05) following i.t. delivery (SF 23.8 +/- 4.3%, TF 13.7 +/- 2.5%) as compared to i.v. (3.7 +/- 0.5%). These findings suggest that for a given dose of G, both SF and TF delivery methods of G/PFC can enhance pulmonary, relative to systemic, antibiotic coverage.

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.

Recognition and management of pulmonary hypertension

Pulmonary hypertension (mean pulmonary arterial pressure > 20mm Hg at rest or > 30mm Hg during exercise) occurs (i) as primary pulmonary hypertension (no known underlying cause), (ii) as persistent pulmonary hypertension of the newborn or (iii) secondary to a variety of lung and cardiovascular diseases. In the last 10 to 15 years there have been significant advances in the medical management of this debilitating and life-threatening disorder. The main drugs in current use are anticoagulants (warfarin, heparin) and vasodilators, especially oral calcium antagonists, intravenous prostacyclin (prostaglandin I2; epoprostenol) and inhaled nitric oxide. Calcium antagonists, (e.g. nifedipine, diltiazem) are used chiefly in primary pulmonary hypertension. They are effective in patients who give a pulmonary vasodilator response to an acute challenge with a short acting vasodilator (e.g. prostacyclin, nitric oxide or adenosine), and are used in doses greater than are usual in the treatment of other cardiovascular disorders. Prostacyclin, given by continuous intravenous infusion, is effective in patients even if they do not respond to an acute vasodilator challenge. The long term benefit in these patients is thought to reflect the antiproliferative effects of the drug and/or its ability to inhibit platelet aggregation. It is used either as long term therapy or as a bridge to transplantation. Inhaled nitric oxide, which is used mainly in persistent pulmonary hypertension of the newborn, has the particular benefit of being pulmonary selective, due to its route of administration and rapid inactivation. Anticoagulants have a specific role in the treatment of pulmonary thromboembolic pulmonary hypertension and are also used routinely in patients with primary pulmonary hypertension. Nondrug treatments for pulmonary hypertension include (i) supplemental oxygen (> or = 15 h/day), which is the primary therapy in patients with pulmonary hypertension secondary to chronic obstructive pulmonary disease and (ii) heart-lung or lung transplantation, which nowadays is regarded as a last resort. Different types of pulmonary hypertension require different treatment strategies. Future advances in the treatment of pulmonary hypertension may come from the use of drug combinations, the development of new drugs, such as endothelin antagonists, nitric oxide donors and potassium channel openers, or the application of gene therapy.