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.

High-Resolution Three-Dimensional Magnetic Resonance Imaging of Mouse Lung In Situ

OBJECTIVES

This study establishes a method for high-resolution isotropic magnetic resonance (MR) imaging of mouse lungs using tracheal liquid-instillation to remove MR susceptibility artifacts.

METHODS

C57BL/6J mice were instilled sequentially with perfluorocarbon and phosphate-buffered saline to an airway pressure of 10, 20, or 30 cm H2O. Imaging was performed in a 7T MR scanner using a 2.5-cm Quadrature volume coil and a 3-dimensional (3D) FLASH imaging sequence.

RESULTS

Liquid-instillation removed magnetic susceptibility artifacts and allowed lung structure to be viewed at an isotropic resolution of 78-90 microm. Instilled liquid and modeled lung volumes were well correlated (R = 0.92; P < 0.05) and differed by a constant tissue volume (220 +/- 92 microL). 3D image renderings allowed differences in structural dimensions (volumes and areas) to be accurately measured at each inflation pressure.

CONCLUSION

These data demonstrate the efficacy of pulmonary liquid instillation for in situ high-resolution MR imaging of mouse lungs for accurate measurement of pulmonary airway, parenchymal, and vascular structures.

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.

MRI detection of tumor in mouse lung using partial liquid ventilation with a perfluorocarbon-in-water emulsion

Transverse relaxation time (T(2*))-weighted (1)H-MRI of mouse lungs has been performed using partial liquid ventilation (PLV) with a perfluorocarbon (PFC)-in-water emulsion as a contrast modality for lung MRI. Significant sensitivity enhancement in MRI of mouse lungs has been demonstrated with the protocol. The results show that the T(2*) value in lung is approximately proportional to the infusion dose up to a dose of 5 ml/kg body weight (BW) (4.5 g PFC/kg BW) and becomes essentially constant beyond this dosage. T(2*) maps of lungs have been calculated and T(2*) in lungs is in the range of 10-35 ms with this technique, which is an order of magnitude greater than the T(2*) value of mouse lungs without using a PFC-in-water emulsion. T(2*)-weighted (1)H-MR images of mouse lungs have been obtained with good quality under our experimental conditions. We have applied this technique to detect tumors in mouse lungs. Our technique can detect small lung tumors of B16 melanoma, about 1 mm in diameter, in mice. With its significant MR sensitivity enhancement and technical simplicity, T(2*)-weighted (1)H-MRI using PLV with PFC-in-water emulsion offers a promising approach to investigate lung cancers using rodent models.

MRI of lungs using partial liquid ventilation with water-in-perfluorocarbon emulsions

A novel (1)H-MRI contrast modality for rat lungs has been developed using water-in-perfluorocarbon (PFC) emulsions for partial liquid ventilation (PLV). The feasibility of the new ventilation protocol for (1)H-MRI studies of lungs has been demonstrated. (1)H-MR images of lungs have been obtained with sensitivity and spatial resolution higher than those of the (19)F-MRI of lungs previously reported. Diffusion-weighted MRI measurements of lungs showed that the results obtained are related to the pulmonary architecture and functional properties of lungs. Although the methodology needs further improvement and evaluation, it appears to have great potential in a wide range of new applications in the field of lung MRI, such as in vivo detection of lung cancer, emphysema, and allograft rejection following lung transplantation. The ability of this technique to achieve high-quality MR images of lungs, together with its technical simplicity, stability, and low cost, makes this method a promising imaging technique for the lungs.

A model of neonatal tidal liquid ventilation mechanics

Tidal liquid ventilation (TLV) with perfluorocarbons (PFC) has been proposed to treat surfactant-deficient lungs of preterm neonates, since it may prevent pulmonary instability by abating saccular surface tension. With a previous model describing gas exchange, we showed that ventilator settings are crucial for CO(2) scavenging during neonatal TLV. The present work is focused on some mechanical aspects of neonatal TLV that were hardly studied, i.e. the distribution of mechanical loads in the lungs, which is expected to differ substantially from gas ventilation. A new computational model is presented, describing pulmonary PFC hydrodynamics, where viscous losses, kinetic energy changes and lung compliance are accounted for. The model was implemented in a software package (LVMech) aimed at calculating pressures (and approximately estimate shear stresses) within the bronchial tree at different ventilator regimes. Simulations were run taking the previous model's outcomes into account. Results show that the pressure decrease due to high saccular compliance may compensate for the increased pressure drops due to PFC viscosity, and keep airway pressure low. Saccules are exposed to pressures remarkably different from those at the airway opening; during expiration negative pressures, which may cause airway collapse, are moderate and appear in the upper airways only. Delivering the fluid with a slightly smoothed square flow wave is convenient with respect to a sine wave. The use of LVMech allows to familiarize with LV treatment management taking the lungs' mechanical load into account, consistently with a proper respiratory support.

Liquid ventilation: a mathematical model of gas diffusion in the premature lung

The liquid ventilation (LV) technique was previously demonstrated to be a valuable alternative to ordinary gas ventilation, particularly for newborn patients with severely distressed lungs. This work describes a mathematical model of gas transfer phenomena occurring within the lungs of a preterm newborn baby ventilated with liquid perfluorocarbon (PFC) RM-101. The model was conceived in order to perform computer simulations of LV treatments. Its input parameters are tidal volume, respiratory frequency, oxygen and carbon dioxide tension in inlet PFC; its output data are the partial pressures of respiratory gases in the alveolar environment. Such values may be evaluated at any instant from the beginning of the treatment, in order to judge whether the therapy is able to meet the necessary conditions to arterialize properly the patient's venous blood. The model also enables optimisation procedures to be defined and performed. Quantitative results and graphs are supplied, with reference to the simulation of LV applied to a preterm newborn of 28 gestational weeks. The main results point out that a relatively short duration of initial transients is attainable (200 to 240 s) and that blood arterialization is possible even with low oxygen tension in inlet PFC (29.7 kPa (223 mmHg)).