Low-density Gas Mixture Research Articles

Peng: A program for transport properties of low-density binary gas mixtures

The fundamental properties of molecules bridge experiment and theory. Transport properties (diffusion, thermal diffusion, thermal conductivity and viscosity) of binary mixtures are measurable in experiments, and well-defined in theory, but difficult to compute with high accuracy. In addition to high-accuracy inter-molecular potential energy curves (PECs), a reliable and high-order solution program that compute the properties based on the PECs is required. In this work, we present a computer program called Peng that performs the collision integration numerically, and solves the Boltzmann equation in Chapman–Enskog fashion. The program has been devised to perform both parts of the solution procedure to arbitrary order, so that no hard-coded limitation will prevent a user from computing at higher precision, except the amount of RAM and the required computational time. Peng is well-designed in an Object-Oriented Programming (OOP) fashion, which make the program clear and easy to modify. In addition to the end-user oriented program, Peng is also compiled as a dynamic shared library that may readily be extended and embedded in users' programs. Program summaryProgram Title:PengCPC Library link to program files:https://doi.org/10.17632/n5tm9426jx.1Developer's repository link:https://github.com/zhaiyusci/pengCode Ocean capsule:https://codeocean.com/capsule/9951686Licensing provisions: LGPLProgramming language: C++Nature of problem: Nowadays, quantum chemistry provides high-accuracy intermolecular interactions potential energy curves (PECs), and more and more accurate thermophysical properties of dilute gases can be measured experimentally. It is meaningful to build a bridge between thermophysical properties and PECs, so that people can refine the PECs and learn more about the nature of dynamics of gases. For a dilute binary gas mixture, the theory, the Boltzmann equation, and its solution has been available for a long time, but fewer numerical solution packages are available for the public. An easy-to-use and easy-to-extend software package is required.Solution method: To complete the work, we have created a program that computes the collision integrals (Ω(ℓ,s)) for a set of temperatures, following which the thermophysical properties are computed using the Chapman–Enskog solutions of the Boltzmann equation. Both parts of the program are written in C++. The program is easy to use, and a well-designed framework is provided thanks to the Object-oriented design. Users with programming experience can readily extend the present work.Additional comments including restrictions and unusual features: In this version, the program is limited to the computation of the properties of mixtures of structureless atoms. For polyatomic molecules, additional work need to be done.

Electron and nuclear spin polarization in Rb-Xe spin-exchange optical hyperpolarization

Spin-exchange optical hyperpolarization of $^{129}\mathrm{Xe}$ gas enhances the signal-to-noise ratio in nuclear magnetic resonance experiments. The governing parameter of the Rb-Xe spin-exchange process, the so-called enhancement factor, was recently reevaluated experimentally. However, the underlying hyperfine coupling and atomic interaction potential as functions of the internuclear distance of the open-shell Rb-Xe dimer have not been accurately determined to date. We present a piecewise approximation based on first-principles calculations of these parameters contributing to the NMR and EPR frequency shifts in the low-density Rb-Xe gas mixture of relevance to hyperpolarization experiments. Both Rb electron and $^{129}\mathrm{Xe}$ nuclear spin polarizations are estimated based on a combination of electronic-structure calculations, observed frequency shifts, and an estimate of the Rb number density. Finally, an expression for the enhancement factor in terms of modern electronic-structure theory is obtained.

Calculation of the thermal conductivity of low-density CH4-N2 gas mixtures using an improved kinetic theory approach.

The thermal conductivity of low-density CH4-N2 gas mixtures has been calculated by means of the classical trajectory method using state-of-the-art intermolecular potential energy surfaces for the CH4-CH4, N2-N2, and CH4-N2 interactions. Results are reported in the temperature range from 70 K to 1200 K. Since the thermal conductivity is influenced by the vibrational degrees of freedom of the molecules, which are not included in the rigid-rotor classical trajectory computations, a new correction scheme to account for vibrational degrees of freedom in a dilute gas mixture is presented. The calculations show that the vibrational contribution at the highest temperature studied amounts to 46% of the total thermal conductivity of an equimolar mixture compared to 13% for pure nitrogen and 58% for pure methane. The agreement with the available experimental thermal conductivity data at room temperature is good, within ±1.4%, whereas at higher temperatures, larger deviations up to 4.5% are observed, which can be tentatively attributed to deteriorating performance of the measuring technique employed. Results are also reported for the magnitude and temperature dependence of the rotational collision number, Z(rot), for CH4 relaxing in collisions with N2 and for N2 relaxing in collisions with CH4. Both collision numbers increase with temperature, with the former being consistently about twice the value of the latter.

Clinical use of Heliox in Asthma and COPD

Heliox is a low density gas mixture of helium and oxygen commonly used in deep diving (> 6 ATM). This mixture has been also used for clinical purposes, particularly in the critical care setting. Due to of its physical proprieties, Heliox breathing reduces air flow resistances within the bronchial tree; in patients with obstructive lung diseases Heliox may also reduce the work of breathing and improve pulmonary gas exchange efficiency. Beneficial effects have been documented in severe asthma attacks and in patients with chronic obstructive pulmonary disease. A reduction in WOB during mechanical ventilation and an increase in exercise endurance capacity have also been described in COPD. Heliox has been also used in the treatment of upper airways obstruction, bronchiolitis and bronchopulmonary dysplasia. Despite the encouraging results, Heliox use in routine practice remains controversial because of technical implications and high costs.

Numerical and experimental study of horizontal round turbulent forced plume in a static homogeneous environment

This paper investigates a numerical and experimental study about horizontal round turbulent forced plume injected into a static homogeneous environment. Helium–air plume is an example of non-Boussinesq forced plume in which a low-density binary gas mixture is injected into a high density ambient of air. Since the domain temperature is assumed to be constant, the density of the mixture is a function of the concentration only. Numerical model is based on the finite volume method and reports on an application of standard k–e model for steady flow with densimetric Froude numbers of 30–300 and Reynolds numbers of 2000–6000. The basic features of the model are: the conservation of mass, momentum and concentration. The boundaries of plume body, the centerline trajectory, the radius plume and the centerline mixture density are determined. It is found that the plume spreading and behavior depend on the ratio between buoyancy flux and momentum, i.e. Froude number. Laboratory experiments have been conducted for photographic observations of plumes trajectories for different mixture (helium–air) densities with different velocities and injection nozzle diameters. Numerical results are described and compared with the experiments and good agreement has been achieved.

Does impaired O2 delivery during exercise accentuate central and peripheral fatigue in patients with coexistent COPD-CHF?

Impairment in oxygen (O2) delivery to the central nervous system (“brain”) and skeletal locomotor muscle during exercise has been associated with central and peripheral neuromuscular fatigue in healthy humans. From a clinical perspective, impaired tissue O2 transport is a key pathophysiological mechanism shared by cardiopulmonary diseases, such as chronic obstructive pulmonary disease (COPD) and chronic heart failure (CHF). In addition to arterial hypoxemic conditions in COPD, there is growing evidence that cerebral and muscle blood flow and oxygenation can be reduced during exercise in both isolated COPD and CHF. Compromised cardiac output due to impaired cardiopulmonary function/interactions and blood flow redistribution to the overloaded respiratory muscles (i.e., ↑work of breathing) may underpin these abnormalities. Unfortunately, COPD and CHF coexist in almost a third of elderly patients making these mechanisms potentially more relevant to exercise intolerance. In this context, it remains unknown whether decreased O2 delivery accentuates neuromuscular manifestations of central and peripheral fatigue in coexistent COPD-CHF. If this holds true, it is conceivable that delivering a low-density gas mixture (heliox) through non-invasive positive pressure ventilation could ameliorate cardiopulmonary function/interactions and reduce the work of breathing during exercise in these patients. The major consequence would be increased O2 delivery to the brain and active muscles with potential benefits to exercise capacity (i.e., ↓central and peripheral neuromuscular fatigue, respectively). We therefore hypothesize that patients with coexistent COPD-CHF stop exercising prematurely due to impaired central motor drive and muscle contractility as the cardiorespiratory system fails to deliver sufficient O2 to simultaneously attend the metabolic demands of the brain and the active limb muscles.

Heliox breathing equally influences respiratory mechanics and cycling performance in trained males and females.

In this study we tested the hypothesis that inspiring a low-density gas mixture (helium-oxygen; HeO2) would minimize mechanical ventilatory constraints and preferentially increase exercise performance in females relative to males. Trained male (n = 11, 31 yr) and female (n = 10, 26 yr) cyclists performed an incremental cycle test to exhaustion to determine maximal aerobic capacity (V̇o2max; male = 61, female = 56 ml·kg(-1)·min(-1)). A randomized, single-blinded crossover design was used for two experimental days where subjects completed a 5-km cycling time trial breathing humidified compressed room air or HeO2 (21% O2:balance He). Subjects were instrumented with an esophageal balloon for the assessment of respiratory mechanics. During the time trial, we assessed the ability of HeO2 to alleviate mechanical ventilatory constraints in three ways: 1) expiratory flow limitation, 2) utilization of ventilatory capacity, and 3) the work of breathing. We found that HeO2 significantly reduced the work of breathing, increased the size of the maximal flow-volume envelope, and reduced the fractional utilization of the maximal ventilatory capacity equally between men and women. The primary finding of this study was that inspiring HeO2 was associated with a statistically significant performance improvement of 0.7% (3.2 s) for males and 1.5% (8.1 s) for females (P < 0.05); however, there were no sex differences with respect to improvement in time trial performance (P > 0.05). Our results suggest that the extent of sex-based differences in airway anatomy, work of breathing, and expiratory flow limitation is not great enough to differentially affect whole body exercise performance.

High flow nasal cannula (HFNC) with Heliox decreases diaphragmatic injury in a newborn porcine lung injury model

High flow nasal cannula (HFNC) improves ventilation by washing out nasopharyngeal dead space while delivering oxygen. Heliox (helium-oxygen gas mixture), a low-density gas mixture, decreases resistance to airflow, reduces the work of breathing, and facilitates distribution of inspired gas. Excessive lung work and potential injury increases the workload on the immature diaphragm predisposing the muscle to fatigue, and can lead to inflammatory and oxidative stress, thereby contributing to impaired diaphragmatic function. We tested the hypothesis that HFNC with Heliox will decrease the work of breathing thereby unloading the neonatal diaphragm, and potentially reducing diaphragmatic injury. Spontaneously breathing neonatal pigs were randomized to Nitrox (nitrogen-oxygen gas mixture) or Heliox, and studied over 4 hr following oleic acid injury. Gas exchange, pulmonary mechanics indices, and systemic markers of inflammation were measured serially. Diaphragm inflammation biomarkers and histology for muscle injury were assessed at termination. Heliox breathing animals demonstrated decreased respiratory load and work of breathing with lower pressure-rate product, lower labored breathing index, and lower levels of diaphragmatic inflammatory markers, and muscle injury score as compared to Nitrox. These results suggest that HFNC with Heliox is a useful adjunct to attenuate diaphragmatic fatigue in the presence of lung injury by unloading the diaphragm, resulting in a more efficient breathing pattern, and decreased diaphragm injury.

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

BackgroundExpiratory 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.MethodsA 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.ResultsTime 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.ConclusionsIt 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.

Does a Low-Density Gas Mixture or Oxygen Supplementation Improve Exercise Training in COPD?

Low-density gas mixtures and oxygen (O₂) supplementation are known to improve physical performance and ventilatory adaptation during incremental maximal exercise in COPD. We investigated whether their combined use during intense physical training is also effective in ameliorating exercise tolerance in patients affected by moderate to severe COPD. Thirty patients (FEV₁ < 60% of predicted) underwent a 2-month rehabilitation program. Leg-cycle training was conducted thrice weekly at 80% of the initial peak work rate for at least 20 min breathing room air, a 60% helium and 40% O₂ mixture, or supplemental O₂ (40%) inhaled from a Douglas bag. The study was randomized with a double-blind design. Before and at the end of the training period the subjects were tested for spirometry, arterial gas tension, diffusing lung capacity for CO, and incremental and constant work rate exercise test on a cycle ergometer. Physical training resulted in a significant improvement in peak oxygen consumption and power output (P < .01 for both) and in exercise endurance time (P < .05) independently of the kind of inhaled gas during the rehabilitation program (P ≥ .42). No changes were observed in lung function tests or gas exchange. We concluded that the use of a low-density gas mixture or O₂ supplementation does not contribute to improved exercise performance in patients with moderate to severe COPD without a significantly decreased diffusion lung capacity for CO who are able to tolerate intense physical training.

The major limitation to exercise performance in COPD is inadequate energy supply to the respiratory and locomotor muscles

No doubt dynamic hyperinflation and lack of oxidative capacity of skeletal muscles are important causes of exercise limitation in COPD. O'Donnell and Webb and Debigare and Maltais will convince the reader of this by the elegant experiments they have performed. The thesis we will put forward is that

Exercise-induced respiratory muscle fatigue: implications for performance

It is commonly held that the respiratory system has ample capacity relative to the demand for maximal O(2) and CO(2) transport in healthy humans exercising near sea level. However, this situation may not apply during heavy-intensity, sustained exercise where exercise may encroach on the capacity of the respiratory system. Nerve stimulation techniques have provided objective evidence that the diaphragm and abdominal muscles are susceptible to fatigue with heavy, sustained exercise. The fatigue appears to be due to elevated levels of respiratory muscle work combined with an increased competition for blood flow with limb locomotor muscles. When respiratory muscles are prefatigued using voluntary respiratory maneuvers, time to exhaustion during subsequent exercise is decreased. Partially unloading the respiratory muscles during heavy exercise using low-density gas mixtures or mechanical ventilation can prevent exercise-induced diaphragm fatigue and increase exercise time to exhaustion. Collectively, these findings suggest that respiratory muscle fatigue may be involved in limiting exercise tolerance or that other factors, including alterations in the sensation of dyspnea or mechanical load, may be important. The major consequence of respiratory muscle fatigue is an increased sympathetic vasoconstrictor outflow to working skeletal muscle through a respiratory muscle metaboreflex, thereby reducing limb blood flow and increasing the severity of exercise-induced locomotor muscle fatigue. An increase in limb locomotor muscle fatigue may play a pivotal role in determining exercise tolerance through a direct effect on muscle force output and a feedback effect on effort perception, causing reduced motor output to the working limb muscles.

Shock structure in low density gas mixture flows over cylinders and plates

This paper presents the results of direct Monte Carlo simulation of transverse hypersonic flow of gas mixtures around a cylinder and a plate. The main objective of the current analysis was the deceleration of heavy atoms in the shock layer in front of the cylinder and the plate. Data are presented for the structure of the disturbed flow, density, velocity, temperature distribution, and energy distribution function of the heavy atoms colliding with the body. Parametric analysis is provided for a wide range of Knudsen numbers, surface temperatures, adsorption probabilities, and gas mixture compositions.

Nonpharmacological modulation of dynamic hyperinflation

Expiratory flow limitation is the pathophysiological hallmark of chronic obstructive pulmonary disease (COPD), but dyspnoea is the most prominent and distressing symptom. Acute dynamic hyperinflation (DH), which refers to the temporary increase in operating lung volumes above their resting value, is a key mechanistic consequence of expiratory flow limitation and has serious mechanical and sensory consequences. In flow-limited patients, pharmacological interventions using bronchodilating agents act by improving dynamic airway function: enhancing lung emptying and reducing lung hyperinflation. Besides bronchodilators, other factors that reduce the ventilatory demand at a given level of exercise or interventions that aim to improve lung emptying or to reduce resting levels of hyperinflation decrease the rate of development of DH in COPD patients. The different nonpharmacological intervention strategies that aim to reduce lung hyperinflation are summarised in the present article, as well as the different underlying mechanisms contributing to lung deflation. The following interventions are systematically reviewed in the present article: breathing supplemental oxygen or low-density gas mixtures of helium and oxygen, pulmonary rehabilitation and noninvasive positive pressure ventilation. Pulmonary rehabilitation and noninvasive pressure ventilation reduce the DH; the effects of heliox-breathing and hyperoxia are more variable. These nonpharmacological interventions have importantly contributed to the present understanding of the role of hyperinflation in COPD patients.

Heliox Attenuates Lung Inflammation and Structural Alterations in Acute Lung Injury

Low-density gas mixtures, such as heliox, were shown to reduce the work of breathing and facilitate the distribution of inspired gas. Since supplemental ventilatory and oxygen requirements may lead to pulmonary inflammation and structural alterations, we hypothesized that by reducing these requirements, heliox breathing may attenuate the acute inflammatory and structural changes associated with acute lung injury. Spontaneously breathing neonatal pigs were anesthetized, instrumented, supported with continuous positive airway pressure (CPAP), injured with oleic acid, and randomized to nitrox (n = 6) or heliox (n = 5).F(I)O(2) was titrated for pulse oximetry (SpO(2)) 95 +/- 2% for 4 hr. Gas exchange and pulmonary mechanics were measured. Lungs were analyzed for myeloperoxidase (MPO), interleukin-8 (IL-8), and histomorphometery. Relationships between physiologic indices and cumulative lung structure and inflammatory indices were evaluated. With heliox, compliance was significantly greater, while tidal volume, frequency, minute ventilation, F(I)O(2), arterial carbon dioxide tension (PaCO(2)), MPO, and IL-8 were significantly lower compared to nitrox. The expansion index and number of exchange units were significantly greater with heliox, while the exchange unit area (EUA) was smaller. MPO was significantly and positively correlated with F(I)O(2) (r = 0.76) and EUA (r = 0.63), and negatively correlated with number of open exchange units/field (r = -0.73). Compared to breathing nitrox, these data indicate that heliox improved the distribution of inspired gas, thereby recruiting more gas exchange units, improving gas exchange efficiency, reducing ventilatory and oxygen requirements, and attenuating lung inflammation. These data suggest that heliox breathing may have the combined therapeutic benefits of attenuating lung inflammation by reducing mechanical and oxidative stress in the clinical management of acute lung injury.

Comparison of Lorentz–Berthelot and Tang–Toennies Mixing Rules Using an Isotropic Temperature-Dependent Potential Applied to the Thermophysical Properties of Binary Gas Mixtures of CH4, CF4, SF6, and C(CH3)4with Ar, Kr, and Xe

In this paper the isotropic temperature-dependent potential (ITDP) approach and the concepts introduced in our previous papers have been used to calculate equilibrium and transport properties of low-density gas mixtures. The twelve binary mixtures considered here are: Ar–CH4, Ar–CF4, Ar–SF6, Ar–C(CH3)4, Kr–CH4, Kr–CF4, Kr–SF6, Kr–C(CH3)4, Xe–CH4, Xe–CF4, Xe–SF6 and Xe–C(CH3)4. The (n−6) Lennard–Jones potential parameters n (repulsive parameter), R m (equilibrium distance), and ɛ (potential well depth) of the pure noble gases Ar, Kr, and Xe are obtained by a minimization of the sum of squared deviations between experimental and calculated viscosity (η), and second pVT (B) and acoustic (β) virial coefficients normalized to their relative experimental error aexp. The number of included experimental points for B, β, η was N = 305, 210, and 167 for Ar, Kr, and Xe, respectively. For the pure globular gases the potential parameters were taken from previous publications. The calculations of B, η, and ρ D12 of binary mixtures were compared with experimental data by using two different mixing rules (Lorentz–Berthelot and Tang–Toennies). Recommended sets and fitting formulae for the potential parameters that can be used for the calculation of low-pressure thermophysical properties of these mixtures are provided.

Aerosol Deposition in the Human Respiratory Tract Breathing Air and 80:20 Heliox

Aerosol mixing resulting from turbulent flows is thought to be an important mechanism of deposition in the upper respiratory tract (URT). Since turbulence levels are a function of gas density, the use of a low density carrier gas would be expected to reduce deposition in the URT. We measured aerosol deposition in the respiratory tract of 8 healthy subjects using both air and heliox, a low density gas mixture containing 80% helium and 20% oxygen, as the carrier gas. The subjects breathed 0.5, 1, and 2 microm-diameter monodisperse polystyrene latex particles from a reservoir at a constant flow rate (approximately 450 mL/sec) and tidal volume (approximately 900 mL). Aerosol concentration and flow rate were measured at the mouth using a photometer and a pneumotachograph, respectively. Deposition was 17.0%, 20.3%, and 38.9% in air and 16.8%, 18.5%, and 36.9% in heliox for 0.5; 1, and 2 microm-diameter particles, respectively. There was a small but statistically significant decrease in deposition when using heliox compared to air for 1 and 2 microm-diameter particles (p < 0.05). While it could not be directly measured from these data, it is likely that when breathing heliox instead of air, deposition is reduced in the URT and increased in the small airways and alveoli.

Maximal horizontal flight performance of hummingbirds: effects of body mass and molt.

Hovering and fast forward flapping represent two strenuous types of flight that differ in aerodynamic power requirement. Maximal capabilities of ruby-throated hummingbirds (Archilochus colubris) in hovering and forward flight were compared under varying body mass and wing area. The capability to hover in low-density gas mixtures was adversely affected by body mass, whereas the capability to fly in a wind tunnel did not show any adverse mass effect. Molting birds that lost primary flight feathers and reduced wing area also displayed mass loss and loss of aerodynamic power and flight speed. This suggests that maximal flight speed is insensitive to short-term perturbations of body mass but that molting is stressful and reduces the birds' speed and capacity for chase and escape. Hummingbirds' flight behavior in confined space was also investigated. Birds reduced their speeds flying through a narrow tube to approximately one-fifth of that in the wind tunnel and did not display differences under varying body mass and wing area. Hence, performance in the flight tube was submaximal and did not correlate with performance variation in the wind tunnel. For ruby-throated hummingbirds, both maximal mass-specific aerodynamic power derived from hovering performance in low-density air media and maximal flight velocity measured in the wind tunnel were invariant with body mass.

Evaluation of a Pitot type spirometer in helium/oxygen mixtures.

Mixtures of helium and oxygen are regaining a place in the treatment of obstruction of the upper and lower respiratory tract. The parenchymal changes during the course of IRDS or ARDS may also benefit from the reintroduction of helium/oxygen. In order to monitor and document the effect of low-density gas mixtures, we evaluated the Datex AS/3 Side Stream Spirometry module with D-lite (Datex-Engstrom Instrumentarium Corporation, Finland) against two golden standards. Under conditions simulating controlled and spontaneous ventilation with gas mixtures of He (approx. 80, 50, and 20%)/O2 or N2(approx. 21 and 79%)/02, simultaneous measurements using Biotek Ventilator Tester (Bio-Tek Instr., Vermont, USA) or body plethysmograph (SensorMedics System, Anaheim, USA) were correlated with data from the spirometry module. Data were analyzed according to a statistical regression model resulting in a best-fit equation based on density, voltage, and volume measurements. As expected, the D-lite (a modified Pitot tube) showed density-dependent behaviour. Regression equations and percentage deviation of estimated versus measured values were calculated. Measurements with the D-lite using low-density gases are satisfactorily contained in best-fit equations with a standard deviation of less than 5% during all ventilatory modes and mixtures.

Inhaled helium-oxygen revisited: Effect of inhaled helium-oxygen during the treatment of status asthmaticus in children

Objectives: To assess the effects of breathing a low-density gas mixture on dyspnea and the pulsus paradoxus in children with status asthmaticus. Design: In an urban academic tertiary referral center, 18 patients, aged 16 months to 16 years, who were being treated for status asthmaticus with continuously inhaled β-agonist and intravenously administered methylprednisolone and had a pulsus paradoxus of greater than 15 mm Hg received either an 80%:20% helium-oxygen gas mixture (HELIOX patients) or room air (control patients) at 10 L/min by nonrebreathing face mask in a double-blind, randomized, controlled trial. In all patients, baseline data, including pulsus paradoxus (determined by sphygmomanometer or arterial catheter blood pressure readings), respiratory rate, heart rate, investigator-scored dyspnea index, and oxygen saturation, were compared with values obtained 15 minutes during and after intervention. In a subset of patients, peak flows before and after breathing HELIOX or room air were measured. When clinically indicated, arterial blood gases were obtained. Results: The pulsus paradoxus (in millimeters of mercury) fell significantly from an initial mean value of 23.3 ± 6.8 to 10.6 ± 2.8 with HELIOX breathing ( p <0.001) and increased again to 18.5 ± 7.3 after cessation of HELIOX. Peak flow increased 69.4% ± 12.8% during HELIOX breathing ( p <0.05). The dyspnea index decreased from an initial mean value of 5.7 ± 1.3 to 1.9 ± 1.7 with HELIOX breathing ( p <0.0002) and increased again to 4.0 ± 0.5 after cessation of HELIOX breathing. In control patients, there was no significant difference in pulsus paradoxus or dyspnea index at any time during the study period. Mechanical ventilation was averted in three patients in whom dyspnea lessened dramatically during breathing of HELIOX. Conclusion: During acute status asthmaticus, inhaled HELIOX significantly lowered the pulsus paradoxus, increased peak flow, and lessened the dyspnea index. Moreover, HELIOX spared three patients a planned intubation and caused no apparent side effects. Thus HELIOX reduces the work of breathing and may forestall respiratory failure in children with status asthmaticus, thus preventing the need for mechanical ventilation. (J Pediatr 1997;130:217-24)