Property value estimation for inhaled therapeutic binary gas mixtures: He, Xe, N2O, and N2 with O2

The influence of flowrate and gas density on positive airway pressure for high flow nasal cannula applied to infant airway replicas

High flow nasal cannula (HFNC) therapy has been previously shown to produce positive upper airway pressures in adult and child patients. This work aimed to evaluate and quantify the effects of HFNC flowrate and gas type on airway pressures measured in vitro in infant airway replicas. Ten realistic infant airway replicas, extending from nares to trachea, were connected in turn to a lung simulator and were supplied gas flows through HFNC. Air and heliox were each provided at two weight-indexed flowrates, 1 l/min/kg and 2 l/min/kg. Pressure and lung volume were continuously measured during simulated breathing. For constant simulated patient effort, no statistically significant change in tidal volume was measured between baseline and lower or higher HFNC flowrates, nor was there any significant difference in tidal volume between air and heliox. Tracheal pressure increased with increasing HFNC flow rate, and was highly variable between airway replicas. Higher pressures were measured for air versus heliox. For air supplied at 2 l/min/kg, average airway pressures in excess of 4 cm H₂O were generated, with positive end-expiratory pressure (PEEP) ranging from 2.5 to nearly 12 cm H₂O across the replicas. A predictive correlation for PEEP was proposed based on supplied gas density and flow velocities exiting the cannula and nares, and was able to account for a portion of variability between airway replicas (R² = 0.913). Additionally, PEEP was well correlated with, and predictive of, expiratory peak pressure (R² = 0.939) and average inspiratory pressure (R² = 0.944).

Numerical analysis of mechanical ventilation using high concentration medical gas mixtures in newborns

When administered in relatively high concentrations the mechanical properties of inhaled gas can become significantly different from air. This fact has implications in mechanical ventilation where adequate respiration and injury to the lungs or respiratory muscles can worsen morbidity and mortality. Here we use an engineering pressure loss model to analyze the administration of medical gas mixtures in newborns. The model is used to determine the pressure distribution along the gas flow path. Numerical experiments comparing medical gas mixtures with helium, nitrous oxide, argon, xenon, and medical air as a control, with and without an endotracheal tube obstruction were performed. The engineering pressure loss model was incorporated into a model of mechanical ventilation during pressure control mode, a ventilator mode that is often used for neonates. Results are presented in the form of Rohrer equations relating pressure loss to flow rate for each gas mixture with and without obstruction. These equations were incorporated into a model for mechanical ventilation resulting in pressure, flow rate, and volume curves for the inhalation-exhalation cycle. In terms of accuracy, published values of airway resistance range from 50 to 150 cmH2O/L per second for a normal 3 kg infant. With air, the current results are 55 to 80 cmH2O/L per second for 0.3 to 5 L/min. It is shown that density through inertial pressure losses has a greater influence on airway resistance than viscosity in spite of relatively low flow rates and small airway dimensions of newborns. The results indicate that the high-density xenon mixture can be problematic during mechanical ventilation. On the other hand, low density heliox (a mixture of helium and oxygen) provides a wider margin of safety for mechanical ventilation than the other gas mixtures. The argon or nitrous oxide mixtures considered are only slightly different from air in terms of mechanical ventilation performance.

Experimental evaluation of pressure drop for flows of air and heliox through upper and central conducting airway replicas of 4- to 8-year-old children

Airway resistance describes the ratio between pressure drop and flow rate through the conducting respiratory airways. Analytical models of airway resistance for tracheobronchial airways have previously been developed and assessed without upper airways positioned upstream of the trachea. This work investigated pressure drop as a function of flow rate and gas properties for upper and central airway replicas of 10 child subjects, ages 4-8. Replica geometries were built based on computed tomography scan data and included airways from the nose through 3-5 distal branching airway generations. Pressure drop through the replicas was measured for constant inspiratory flows of air and heliox. For both the nose-throat and branching airways, the relationship between non-dimensional coefficient of friction, C_F, with Reynolds number, Re, was found to resemble the turbulent Blasius equation for pipe flow, where C_F∝Re^-0.25. Additionally, pressure drop ratios between heliox and air were consistent with analytical predictions for turbulent flow. The presence of turbulence in the branching airways likely resulted from convection of turbulence produced upstream in the nose and throat. An airway resistance model based on the Blasius pipe friction correlation for turbulent flow was proposed for prediction of pressure drop through the branching bronchial airways downstream from the upper airway.

Gas transport during in vitro and in vivo preclinical testing of inert gas therapies

New gas therapies using inert gases such as xenon and argon are being studied, which require in vitro and in vivo preclinical experiments. Examples of the kinetics of gas transport during such experiments are analyzed in this paper. Using analytical and numerical models, we analyze an in vitro experiment for gas transport to a 96 cell well plate and an in vivo delivery to a small animal chamber, where the key processes considered are the wash-in of test gas into an apparatus dead volume, the diffusion of test gas through the liquid media in a well of a cell test plate, and the pharmacokinetics in a rat. In the case of small animals in a chamber, the key variable controlling the kinetics is the chamber wash-in time constant that is a function of the chamber volume and the gas flow rate. For cells covered by a liquid media the diffusion of gas through the liquid media is the dominant mechanism, such that liquid depth and the gas diffusion constant are the key parameters. The key message from these analyses is that the transport of gas during preclinical experiments can be important in determining the true dose as experienced at the site of action in an animal or to a cell.

Brain Cooling With Ventilation of Cold Air Over Respiratory Tract in Newborn Piglets: An Experimental and Numerical Study

We investigate thermal effects of pulmonary cooling which was induced by cold air through an endotracheal tube via a ventilator on newborn piglets. A mathematical model was initially employed to compare the thermal impact of two different gas mixtures, O₂-medical air (1:2) and O₂-Xe (1:2), across the respiratory tract and within the brain. Following mathematical simulations, we examined the theoretical predictions with O₂-medical air condition on nine anesthetized piglets which were randomized to two treatment groups: 1) control group (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$n = 4$ \end{document}) and 2) pulmonary cooling group (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$n = 5$ \end{document}). Numerical and experimental results using O₂-medical air mixture show that brain temperature fell from 38.5 °C and 38.3 °C ± 0.3 °C to 35.7 °C ± 0.9 °C and 36.5 °C ± 0.6 °C during 3 h cooling which corresponded to a mean cooling rate of 0.9 °C/h ± 0.2 °C/h and 0.6 °C/h ± 0.1 °C/h, respectively. According to the numerical results, decreasing the metabolic rate and increasing air velocity are helpful to maximize the cooling effect. We demonstrated that pulmonary cooling by cooling of inhalation gases immediately before they enter the trachea can slowly reduce brain and core body temperature of newborn piglets. Numerical simulations show no significant differences between two different inhaled conditions, i.e., O₂-medical air (1:2) and O₂-Xe (1:2) with respect to cooling rate.

Using helium-oxygen to improve regional deposition of inhaled particles: mechanical principles

BACKGROUND

Helium-oxygen has been used for decades as a respiratory therapy conjointly with aerosols. It has also been shown under some conditions to be a means to provide more peripheral, deeper, particle deposition for inhalation therapies. Furthermore, we can also consider deposition along parallel paths that are quite different, especially in a heterogeneous pathological lung. It is in this context that it is hypothesized that helium-oxygen can improve regional deposition, leading to more homogeneous deposition by increasing deposition in ventilation-deficient lung regions.

METHODS

Analytical models of inertial impaction, sedimentation, and diffusion are examined to illustrate the importance of gas property values on deposition distribution through both fluid mechanics- and particle mechanics-based mechanisms. Also considered are in vitro results from a bench model for a heterogeneously obstructed lung. In vivo results from three-dimensional (3D) imaging techniques provide visual examples of changes in particle deposition patterns in asthmatics that are further analyzed using computational fluid dynamics (CFD).

RESULTS AND CONCLUSIONS

Based on analytical modeling, it is shown that deeper particle deposition is expected when breathing helium-oxygen, as compared with breathing air. A bench model has shown that more homogeneous ventilation distribution is possible breathing helium-oxygen in the presence of heterogeneous obstructions representative of central airway obstructions. 3D imaging of asthmatics has confirmed that aerosol delivery with a helium-oxygen carrier gas results in deeper and more homogeneous deposition distributions. CFD results are consistent with the in vivo imaging and suggest that the mechanics of gas particle interaction are the source of the differences seen in deposition patterns. However, intersubject variability in response to breathing helium-oxygen is expected, and an example of a nonresponder is shown where regional deposition is not significantly changed.

A xenon recirculating ventilator for the newborn piglet: developing clinical applications of xenon for neonates

CONTEXT

The clinical applications of xenon for the neonate include both anaesthesia and neuroprotection. However, due to the limited natural availability of xenon, special equipment is required to administer and recapture the gas to develop xenon as a therapeutic agent.

OBJECTIVE

In order to test the xenon recirculating ventilator for the application of neuroprotection in a preclinical trial, our primary objective was to test the efficiency, reliability and safety of administering 50% xenon for 24 h in hypoxic ischaemic piglets.

DESIGN

A prospective observational study.

SETTING

Institute for Women's Health, University College London, January 2008 to March 2008.

ANIMALS

Four anaesthetised male piglets, less than 24 h old, underwent a global hypoxic ischaemic insult for approximately 25 min prior to switching to the xenon recirculating ventilator.

INTERVENTION

Between 2 and 26 h after hypoxic ischaemia, anaesthetised piglets were administered a mixture of 50% xenon, air, oxygen and isoflurane.

MAIN OUTCOME MEASURES

The primary outcome measure was blood gas PaCO2 (kPa) and secondary outcome measure was xenon gas use (l h), over the 24-h duration of xenon administration.

RESULTS

The xenon recirculating ventilator provided effective ventilation, automated control of xenon/air gas mixtures, and stable blood gas PaCO2 (4.5 to 6.3 kPa) for 24 h of ventilation with the xenon recirculating ventilator. Total xenon use was minimal at approximately 0.6 l h at a cost of approximately &OV0556;8 h. Additional features included an isoflurane scavenger and bellows height alarm.

CONCLUSION

Stable gas delivery to a piglet with minimal xenon loss and analogue circuitry made the xenon recirculating ventilator easy to use and it could be modified for other large animals and noble gas mixtures. The technologies, safety and efficiency of xenon delivery in this preclinical system have been taken forward in the development of neonatal ventilators for clinical use in phase II clinical trials for xenon-augmented hypothermia and for xenon anaesthesia.

Extremely low flow tracheal gas insufflation of helium-oxygen mixture improves gas exchange in a rabbit model of piston-type high-frequency oscillatory ventilation

Objective

The purpose of this study was to show the effects of the tracheal gas insufflation (TGI) technique on gas exchange using helium-oxygen mixtures during high-frequency oscillatory ventilation (HFOV). We hypothesized that a helium-oxygen mixture delivered into the trachea using the TGI technique (0.3 L/min) would enhance gas exchange during HFOV.

Methods

Three rabbits were prepared and ventilated by HFOV with carrier 70% helium/oxygen or 70% nitrogen/oxygen gas mixture with TGI in a crossover study. Changing the gas mixture from nitrogen70% to helium70% and back was performed three times per animal with constant ventilation parameters.

Results

Compared with the nitrogen-oxygen mixture, the helium-oxygen mixture of TGI reduced PaCO₂ by 7.6 mmHg (p < 0.01) and improved PaO₂ by 14 mmHg (p < 0.01). Amplitude during TGI was significantly lower with the helium-oxygen mixture than with the nitrogen-oxygen mixture (p < 0.01) and did not significantly affect mean airway pressure.

Conclusions

This study demonstrated that a helium-oxygen mixture delivered into the trachea using the TGI technique would enhance CO₂ elimination and improve oxygenation during HFOV.

Methods for evaluation of helium/oxygen delivery through non-rebreather facemasks

BACKGROUND

Inhalation of low-density helium/oxygen mixtures has been used both to lower the airway resistance and work of breathing of patients with obstructive lung disease and to transport pharmaceutical aerosols to obstructed lung regions. However, recent clinical investigations have highlighted the potential for entrainment of room air to dilute helium/oxygen mixtures delivered through non-rebreather facemasks, thereby increasing the density of the inhaled gas mixture and limiting intended therapeutic effects. This article describes the development of benchtop methods using face models for evaluating delivery of helium/oxygen mixtures through facemasks.

METHODS

Four face models were used: a flat plate, a glass head manikin, and two face manikins normally used in life support training. A mechanical test lung and ventilator were employed to simulate spontaneous breathing during delivery of 78/22 %vol helium/oxygen through non-rebreather facemasks. Based on comparison of inhaled helium concentrations with available clinical data, one face model was selected for measurements made during delivery of 78/22 or 65/35 %vol helium/oxygen through three different masks as tidal volume varied between 500 and 750 ml, respiratory rate between 14 and 30 breaths/min, the inspiratory/expiratory ratio between 1/2 and 1/1, and the supply gas flow rate between 4 and 15 l/min. Inhaled helium concentrations were measured both with a thermal conductivity analyzer and using a novel flow resistance method.

RESULTS

Face models borrowed from life support training provided reasonably good agreement with available clinical data. After normalizing for the concentration of helium in the supply gas, no difference was noted in the extent of room air entrainment when delivering 78/22 versus 65/35 %vol helium/oxygen. For a given mask fitted to the face in a reproducible manner, delivered helium concentrations were primarily determined by the ratio of supply gas flow rate to simulated patient minute ventilation, with the inspiratory/expiratory ratio playing a secondary role. However, the functional dependence of helium concentration on these two ratios depended on the mask design.

CONCLUSIONS

Large differences in mask performance were identified. With continued refinement, the availability of reliable benchtop methods is expected to assist in the development and selection of patient interfaces for delivery of helium/oxygen and other medical gases.

Bench experiments comparing simulated inspiratory effort when breathing helium-oxygen mixtures to that during positive pressure support with air

BACKGROUND

Inhalation of helium-oxygen (He/O2) mixtures has been explored as a means to lower the work of breathing of patients with obstructive lung disease. Non-invasive ventilation (NIV) with positive pressure support is also used for this purpose. The bench experiments presented herein were conducted in order to compare simulated patient inspiratory effort breathing He/O2 with that breathing medical air, with or without pressure support, across a range of adult, obstructive disease patterns.

METHODS

Patient breathing was simulated using a dual-chamber mechanical test lung, with the breathing compartment connected to an ICU ventilator operated in NIV mode with medical air or He/O2 (78/22 or 65/35%). Parabolic or linear resistances were inserted at the inlet to the breathing chamber. Breathing chamber compliance was also varied. The inspiratory effort was assessed for the different gas mixtures, for three breathing patterns, with zero pressure support (simulating unassisted spontaneous breathing), and with varying levels of pressure support.

RESULTS

Inspiratory effort increased with increasing resistance and decreasing compliance. At a fixed resistance and compliance, inspiratory effort increased with increasing minute ventilation, and decreased with increasing pressure support. For parabolic resistors, inspiratory effort was lower for He/O2 mixtures than for air, whereas little difference was measured for nominally linear resistance. Relatively small differences in inspiratory effort were measured between the two He/O2 mixtures. Used in combination, reductions in inspiratory effort provided by He/O2 and pressure support were additive.

CONCLUSIONS

The reduction in inspiratory effort afforded by breathing He/O2 is strongly dependent on the severity and type of airway obstruction. Varying helium concentration between 78% and 65% has small impact on inspiratory effort, while combining He/O2 with pressure support provides an additive reduction in inspiratory effort. In addition, breathing He/O2 alone may provide an alternative to pressure support in circumstances where NIV is not available or poorly tolerated.

Bench and mathematical modeling of the effects of breathing a helium/oxygen mixture on expiratory time constants in the presence of heterogeneous airway obstructions

Background

Expiratory time constants are used to quantify emptying of the lung as a whole, and emptying of individual lung compartments. Breathing low-density helium/oxygen mixtures may modify regional time constants so as to redistribute ventilation, potentially reducing gas trapping and hyperinflation for patients with obstructive lung disease. In the present work, bench and mathematical models of the lung were used to study the influence of heterogeneous patterns of obstruction on compartmental and whole-lung time constants.

Methods

A two-compartment mechanical test lung was used with the resistance in one compartment held constant, and a series of increasing resistances placed in the opposite compartment. Measurements were made over a range of lung compliances during ventilation with air or with a 78/22% mixture of helium/oxygen. The resistance imposed by the breathing circuit was assessed for both gases. Experimental results were compared with predictions of a mathematical model applied to the test lung and breathing circuit. In addition, compartmental and whole-lung time constants were compared with those reported by the ventilator.

Results

Time constants were greater for larger minute ventilation, and were reduced by substituting helium/oxygen in place of air. Notably, where time constants were long due to high lung compliance (i.e. low elasticity), helium/oxygen improved expiratory flow even for a low level of resistance representative of healthy, adult airways. In such circumstances, the resistance imposed by the external breathing circuit was significant. Mathematical predictions were in agreement with experimental results. Time constants reported by the ventilator were well-correlated with those determined for the whole-lung and for the low-resistance compartment, but poorly correlated with time constants determined for the high-resistance compartment.

Conclusions

It was concluded that breathing a low-density gas mixture, such as helium/oxygen, can improve expiratory flow from an obstructed lung compartment, but that such improvements will not necessarily affect time constants measured by the ventilator. Further research is required to determine if alternative measurements made at the ventilator level are predictive of regional changes in ventilation. It is anticipated that such efforts will be aided by continued development of mathematical models to include pertinent physiological and pathophysiological phenomena that are difficult to reproduce in mechanical test systems.