Maximum expiratory flow and estimated CO 2 elimination in liquid-ventilated dogs' lungs

The buffer capacity of airway epithelial secretions

The pH of airway epithelial secretions influences bacterial killing and mucus properties and is reduced by acidic pollutants, gastric reflux, and respiratory diseases such as cystic fibrosis (CF). The effect of acute acid loads depends on buffer capacity, however the buffering of airway secretions has not been well characterized. In this work we develop a method for titrating micro-scale (30 μl) volumes and use it to study fluid secreted by the human airway epithelial cell line Calu-3, a widely used model for submucosal gland serous cells. Microtitration curves revealed that HCO⁻₃ is the major buffer. Peak buffer capacity (β) increased from 17 to 28 mM/pH during forskolin stimulation, and was reduced by >50% in fluid secreted by cystic fibrosis transmembrane conductance regulator (CFTR)-deficient Calu-3 monolayers, confirming an important role of CFTR in HCO⁻₃ secretion. Back-titration with NaOH revealed non-volatile buffer capacity due to proteins synthesized and released by the epithelial cells. Lysozyme and mucin concentrations were too low to buffer Calu-3 fluid significantly, however model titrations of porcine gastric mucins at concentrations near the sol-gel transition suggest that mucins may contribute to the buffer capacity of ASL in vivo. We conclude that CFTR-dependent HCO⁻₃ secretion and epithelially-derived proteins are the predominant buffers in Calu-3 secretions.

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.

Expiratory flow limitation during gravitational drainage of perfluorocarbons from liquid-filled lungs

Flow limitation during pressure-driven expiration in liquid-filled lungs was examined in intact, euthanized New Zealand white rabbits. The aim of this study was to further characterize expiratory flow limitation during gravitational drainage of perfluorocarbon liquids from the lungs, and to study the effect of perfluorocarbon type and negative mouth pressure on this phenomenon. Four different perfluorocarbons (PP4, perfluorodecalin, perfluoro-octyl-bromide, and FC-77) were used to examine the effects of density and kinematic viscosity on volume recovered and maximum expiratory flow. It was demonstrated that flow limitation occurs during gravitational drainage when the airway pressure is < or = -15 cm H(2)O, and that this critical value of pressure did not depend on mouth pressure or perfluorocarbon type. The perfluorocarbon properties affect the volume recovered, maximum expiratory flow, and the time to drain, with the most viscous perfluorocarbon (perfluorodecalin) taking the longest time to drain and resulting in lowest maximum expiratory flow. Perfluoro-octyl-bromide resulted in the highest recovered volume. The findings of this study are relevant to the selection of perfluorocarbons to reduce the occurrence of flow limitation and provide adequate minute ventilation during total liquid ventilation.

Total liquid ventilation: dynamic airway pressure and the development of expiratory flow limitation

Expiratory flow limitation occurs during total liquid ventilation (TLV), and is characterized by the sudden development of excessively negative intratracheal pressures without increases in flow. The purpose of this study was to identify a dynamic signal for the servoregulation of expiratory flow (Ve), by determining the range of dynamic intratracheal pressures [P(T)], which mark the onset of flow limitation during liquid expiration, where choke occurs at the critical pressure (Pc). The lungs of rabbits were filled with perflurocarbon to an end-inspiratory lung volume (EILV) of 20, 30, or 40cc/kg and connected to a piston driven liquid ventilator, which removed perfluorocarbon at a rate (Vs) of 2.5, 5.0, or 7.5 ml/s. Nine animals per EILV group were used (27 animals total), and within each EILV group each (Vs) was used three times. P(T) and (Ve) (T) were measured at the tracheostomy tube, and dP/dT was calculated from P(T). Pc was determined within each EILV/(Vs) group by examining the average dP/dT curve for the first significant change from baseline. Pc ranged from -6.02 +/- 1.83 to -9.02 +/- 3.2 mm Hg. In general, the higher the EILV, the more negative the Pc. We conclude that Pc during TLV varies within a limited range in rabbits. These data may be used to maximize expired volume during TLV by sequentially tapering flow rates as this critical range of pressures is approached.

Flow Limitation in Liquid-Filled Lungs: Effects of Liquid Properties

Flow limitation in liquid-filled lungs is examined in intact rabbit experiments and a theoretical model. Flow limitation (“choked” flow) occurs when the expiratory flow reaches a maximum value and further increases in driving pressure do not increase the flow. In total liquid ventilation this is characterized by the sudden development of excessively negative airway pressures and airway collapse at the choke point. The occurrence of flow limitation limits the efficacy of total liquid ventilation by reducing the minute ventilation. In this paper we investigate the effects of liquid properties on flow limitation in liquid-filled lungs. It is found that the behavior of liquids with similar densities and viscosities can be quite different. The results of the theoretical model, which incorporates alveolar compliance and airway resistance, agrees qualitatively well with the experimental results. Lung compliance and airway resistance are shown to vary with the perfluorocarbon liquid used to fill the lungs. Surfactant is found to modify the interfacial tension between saline and perfluorocarbon, and surfactant activity at the interface of perfluorocarbon and the native aqueous lining of the lungs appears to induce hysteresis in pressure–volume curves for liquid-filled lungs. Ventilation with a liquid that results in low viscous resistance and high elastic recoil can reduce the amount of liquid remaining in the lungs when choke occurs, and, therefore, may be desirable for liquid ventilation.

A prototype of a liquid ventilator using a novel hollow-fiber oxygenator in a rabbit model

OBJECTIVE

A functional total liquid ventilator should be simple in design to minimize operating errors and have a low priming volume to minimize the amount of perfluorocarbon needed. Closed system circuits using a membrane oxygenator have partially met these requirements but have high resistance to perfluorocarbon flow and high priming volume. To further this goal, a single piston prototype ventilator with a low priming volume and a new high-efficiency hollow-fiber oxygenator in a circuit with a check valve flow control system was developed.

DESIGN

Prospective, controlled animal laboratory study.

SETTING

Research facility at a university medical center.

SUBJECTS

Seven anesthetized, paralyzed, normal New Zealand rabbits

INTERVENTIONS

The prototype oxygenator, consisting of cross-wound silicone hollow fibers with a surface area of 1.5 m2 with a priming volume of 190 mL, was tested in a bench-top model followed by an in vivo rabbit model. Total liquid ventilation was performed for 3 hrs with 20 mL.kg(-1) initial fill volume, 17.5-20 mL.kg(-1) tidal volume, respiratory rate of 5 breaths/min, inspiratory/expiratory ratio 1:2, and countercurrent sweep gas of 100% oxygen.

MEASUREMENTS AND MAIN RESULTS

Bench top experiments demonstrated 66-81% elimination of CO2 and 0.64-0.76 mL.min(-1) loss of perfluorocarbon across the fibers. No significant changes in PaCO2 and PaO2 were observed. Dynamic airway pressures were in a safe range in which ventilator lung injury or airway closure was unlikely (3.6 +/- 0.5 and -7.8 +/- 0.3 cm H2O, respectively, for mean peak inspiratory pressure and mean end expiratory pressure). No leakage of perfluorocarbon was noted in the new silicone fiber gas exchange device. Estimated in vivo perfluorocarbon loss from the device was 1.2 mL.min(-1).

CONCLUSIONS

These data demonstrate the ability of this novel single-piston, nonporous hollow silicone fiber oxygenator to adequately support gas exchange, allowing successful performance of total liquid ventilation.

Assessment of the development of choked flow during total liquid ventilation

OBJECTIVE

The flow rate of a liquid drainage from the lungs is limited because of the elastic nature of the airways. This study was designed to clarify the relationship between intrapulmonary liquid volume and the development of the flow limitation or choked flow phenomenon as a function of expiratory flow rate during total liquid ventilation with perflubron.

DESIGN

Prospective animal study.

SETTING

University research laboratory.

SUBJECTS

Rabbits with a weight of 3.2 +/- 0.3 kg.

INTERVENTIONS

After the rabbits were killed, the lungs were filled to functional residual capacity with perflubron, followed by administration of an additional volume of 30, 45, or 60 mL of perflubron (initial volume = functional residual capacity + additional volume).

MEASUREMENTS AND RESULTS

In one set of five animals, the intratracheal pressure at the occurrence of choked flow was established at -20 mm Hg. In another set of six animals, we demonstrated that the volume remaining in the lung at the point of development of choked flow (Vch) was stable for the first 40 mins after the animals were killed. Flow rates of 1.25, 2.5, 3.75, 5.0, 7.5, 10.0, and 12.5 mL/sec were then applied at an additional volume of 30, 45, or 60 mL to 34 animals. Vch approximately doubled as the flow rate increased from 1.25 mL/sec to 12.5 mL/sec (p <.001). At the same flow, Vch was higher for an additional volume of 60 mL than 30 mL when the flow was > or =2.5 mL/sec.

CONCLUSIONS

From these data, we conclude that choked flow occurs at intratracheal pressure of less than -20 mm Hg, that Vch is stable for the first 40 mins after the animals are killed, and that Vch is a function of flow rate and initial volume.

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.

Development and application of a double-piston configured, total-liquid ventilatory support device

OBJECTIVE

Perfluorocarbon liquid ventilation has been shown to enhance pulmonary mechanics and gas exchange in the setting of respiratory failure. To optimize the total liquid ventilation process, we developed a volume-limited, time-cycled liquid ventilatory support, consisting of an electrically actuated, microprocessor-controlled, double-cylinder, piston pump with two separate limbs for active inspiration and expiration.

DESIGN

Prospective, controlled, animal laboratory study, involving sequential application of conventional gas ventilation, partial ventilation (PLV), and total liquid ventilation (TLV).

SETTING

Research facility at a university medical center.

SUBJECTS

A total of 12 normal adult New Zealand rabbits weighing 3.25+/-0.1 kg.

INTERVENTIONS

Anesthestized rabbits were supported with gas ventilation for 30 mins (respiratory rate, 20 cycles/min; peak inspiratory pressure, 15 cm H2O; end-expiratory pressure, 5 cm H2O), then PLV was established with perflubron (12 mL/kg). After 15 mins, TLV was instituted (tidal volume, 18 mL/kg; respiratory rate, 7 cycles/min; inspiratory/expiratory ratio, 1:2 cycles/min). After 4 hrs of TLV, PLV was re-established.

MEASUREMENTS AND MAIN RESULTS

Of 12 animals, nine survived the 4-hr TLV period. During TLV, mean values +/- SEM were as follows: PaO2, 363+/-30 torr; PaCO2, 39+/-1.5 torr; pH, 7.39+/-0.01; static peak inspiratory pressure, 13.2+/-0.2 cm H2O; static endexpiratory pressure, 5.5+/-0.1 cm H2O. No significant changes were observed. When compared with gas ventilation and PLV, significant increases occurred in mean arterial pressure (62.4+/-3.5 torr vs. 74.0+/-1.2 torr) and central venous pressure (5.6+/-0.7 cm H2O vs. 7.8+/-0.2 cm H2O) (p < .05).

CONCLUSIONS

Total liquid ventilation can be performed successfully utilizing piston pumps with active expiration. Considering the enhanced flow profiles, this device configuration provides advantages over others.

Physiologic, biochemical, and histologic correlates associated with tidal liquid ventilation

Tidal liquid ventilation (TLV) with perfluorochemical fluid (PFC) has been successfully used experimentally for up to 4 h. However, no studies of prolonged TLV have been reported. We hypothesized that full-term newborn lambs can safely and effectively be liquid-ventilated for up to 24 h. To test this hypothesis, 17 lambs were liquid-ventilated; 7 for 4 h, 5 for 12 h, and 5 for 24 h. Arterial blood samples were obtained for PFC uptake, lipid analysis, and blood gas measurements. Tissues were obtained for histologic and biochemical analysis. Arterial blood gas and mean arterial blood pressure were as follows (mean +/- SEM): pH 7.48 +/- 0.04; PaCO2 30.6 +/- 2.8; PaO2 424 +/- 17; mean arterial pressure 76 +/- 16 mm Hg. PFC blood levels increased rapidly to a mean of 5.2 +/- 3.9 microg/mL. PFC tissue levels increased significantly (p < 0.01) from 260 +/- 45 microg/g at 4 h to 400 +/- 140 microg/g at 12 h. There was no further increase in PFC tissue levels by 24 h (456 +/- 181 microg/g). There was a significant difference in PFC concentration as a function of tissue (p < 0.01). Furthermore, there was a significant correlation (r = 0.88; p < 0.01) between the amount of PFC and lipid in blood and tissue. Microscopic examination of the lungs demonstrated no evidence of barotrauma. These data demonstrate that prolonged TLV can be safe and efficacious for up to 24 h in full-term newborn lambs.

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.

Shunt and ventilation-perfusion distribution during partial liquid ventilation in healthy piglets

Replacing gas in the lung with perfluorocarbon fluids (PFC) and periodically ventilating with a gas [partial liquid ventilation (PLV)] has been shown to improve oxygenation in models of respiratory distress syndrome. We hypothesized that the addition of PFC to healthy lungs would result in shunt, diffusion impairment, and increased ventilation-perfusion (VA/Q) heterogeneity. Previously, Mates et al. showed that O2 shunt and arterial-alveolar CO2 difference increased linearly with dose in piglets given graded intratracheal doses of PFC (10, 20, and 30 ml/kg followed by mechanical ventilation with 100% O2) (E.A. Mates, J. C. Jackson, J. Hildebrandt, W. E. Truog, T. A. Standaert, and M. P. Hlastala. In: Oxygen Transport to Tissue XVI, 1994, p. 427-435). Here we report VA/Q distribution in the same animals, showing a 50% increase in VA/Q heterogeneity during PLV independent of PFC dose. Ventilation heterogeneity was the major factor in this increase, and there was no significant change in dead space ventilation. We also report on five animals given a single 20 ml/kg dose of PFC and followed for 3 h. They showed an increase in shunt during PLV but no change in arterial-alveolar CO2 difference.

Gas Exchange during Gas and Liquid Ventilation

Liquid Ventilation with perfluorochemicals (PFC) violates many of our long-held assumptions about how the lung functions. However, the technique has been so successful in animal models of lung disease that it is currently being tested in clinical trials for the treatment of infant and acute (“adult”) respiratory distress syndrome in newborns, children, and adults. A common feature of both infant and acute respiratory distress syndromes is an inability of the lung's surfactant system to adequately lower surface tension, leading to regions of atelectasis. Liquid ventilation with PFC appears to ameliorate the disease process by lowering interfacial tension in the lung, opening regions of atelectasis, and improving gas exchange. To understand how gas exchange is successful during liquid ventilation requires careful re-evaluation of the assumptions underlying our current models of gas exchange physiology during normal gas ventilation. These assumptions must then be examined in light of the alterations in pulmonary physiology during liquid ventilation.

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

Perfluorochemical (PFC) liquids have great potential for biomedical use and the support of respiration. Currently, there are several commercially available PFC fluids which meet the physiochemical property requirements as well as purity specifications necessary to perform many of the discussed biomedical applications. Moreover, state-of-the-art fluorine chemistry should enable production of new PFC liquids uniquely sculptured relative to the proposed specific application (ie. vehicle for pulmonary delivery of drugs, a diluent for pulmonary lavage, a medium for respiratory gas exchange). In addition to PFC fluid requirements, there have been several techniques reported for liquid assisted ventilation. These methods include total liquid ventilation, liquid lavage, and partial liquid ventilation. The efficacy of these various techniques is under extensive investigation with respect to specific types of lung dysfunction. Liquid ventilation (LV) techniques have the potential to treat lung disease with less risk of barotrauma and provide the means for direct and uniform delivery of pulmonary agents to injured or dysfunctional sites in the lung. For LV to assume a role in clinical medicine it must be shown to be safe and effective with respect to other therapies or in combination with current therapies. Although the use of LV in animal and initial clinical studies has been impressive to date, better documentation of efficacy in human disease will be required. Further controlled multi-center clinical trials are warranted and are currently in progress.

Improvement of gas exchange, pulmonary function, and lung injury with partial liquid ventilation. A study model in a setting of severe respiratory failure

STUDY OBJECTIVE

To evaluate gas exchange, pulmonary function, and lung histology during gas ventilation of the perfluorocarbon-filled lung compared with gas ventilation of the gas-filled lung in severe respiratory failure.

STUDY DESIGN

Application of gas (GV) or partial liquid (PLV) ventilation in lung-injured sheep.

SETTING

A research laboratory at a university medical center.

SUBJECTS

Eleven sheep 17.1 +/- 1.8 kg in weight.

INTERVENTIONS

Lung injury was induced by intravenous administration of 0.07 mL/kg oleic acid followed by saline pulmonary lavage. When alveolar-arterial oxygen pressure difference (P[A-a]O2) was 600 mm Hg or more and PaO2 was 50 mm Hg or less with fraction of inspired oxygen of 1.0, bijugular venovenous extracorporeal life support (ECLS) was instituted. For the first 30 min on ECLS, all animals were ventilated with gas. Over the ensuing 2.5 h, ventilation with 15 mL/kg gas was continued without intervention in the control group (GV, n = 6) or with the addition of 35 mL/kg of perflubron (PLV, n = 5).

MEASUREMENTS AND RESULTS

At 3 h after initiation of ECLS, Qps/Qt was significantly reduced in the PLV animals when compared with the GV animals (PLV = 41 +/- 13%; GV = 93 +/- 4%; p < 0.005). At the same time point, pulmonary compliance was increased in the PLV when compared with the GV group (PLV = 0.61 +/- 0.14 mL/cm H2O/kg; GV = 0.41 +/- 0.02 mL/cm H2O/kg; p < 0.005). The ECLS flow rate required to maintain the PaO2 in the 50 to 80 mm Hg range was substantially and significantly lower in the PLV group when compared with that of the GV group (PLV = 25 +/- 20 mL/kg/min; GV = 87 +/- 15 mL/kg/min; p < 0.001). Light microscopy performed on lung biopsy specimens demonstrated a marked reduction in lung injury in the liquid ventilated (LV) when compared with the GV animals.

CONCLUSION

In a model of severe respiratory failure, PLV improves pulmonary gas exchange and pulmonary function and is associated with a reduction in pulmonary pathology.

Diving physiology and pathophysiology

Divers have worked at 500 m depth in the sea and have reached 700 m in simulated chamber dives. A prerequisite for this has been extensive physiological studies of the body's reactions to pressure and pressure changes. This paper reviews such physiological and pathophysiological studies with emphasis on recent developments.