The effect of heliox on nebulizer function using a beta-agonist bronchodilator

Administration of dry powders during respiratory supports

Inhaled drugs are routinely used for the treatment of respiratory-supported patients. To date, pressurized metered dose inhalers and nebulizers are the two platforms routinely employed in the clinical setting. The scarce utilization of the dry powder inhaler (DPI) platform is partly due to the lack of in vivo data that proves optimal delivery and drug efficacy are achievable. Additionally, fitting a DPI in-line to the respiratory circuit is not as straightforward as with the other aerosol delivery platforms. Importantly, there is a common misconception that the warm and humidified inspiratory air in respiratory supports, even for a short exposure, will deteriorate powder formulation compromising its delivery and efficacy. However, some recent studies have dispelled this myth, showing successful delivery of dry powders through the humidified circuit of respiratory supports. Compared with other aerosol delivery devices, the use of DPIs during respiratory supports possesses unique advantages such as rapid delivery and high dose. In this review, we presented in vitro studies showing various setups employing commercial DPIs and effects of ventilator parameters on the aerosol delivery. Inclusion of novel DPIs was also made to illustrate characteristics of an ideal inhaler that would give high lung dose with low powder deposition loss in tracheal tubes and respiratory circuits. Clinical trials are urgently needed to confirm the benefits of administration of dry powders in ventilated patients, thus enabling translation of powder delivery into practice.

Mechanically Ventilating the Severe Asthmatic

The management of the critically ill patients with asthma can be rather challenging. Potentially devastating complications relating to this presentation include hypoxemia, worsening bronchospasm, pulmonary aspiration, tension pneumothorax, dynamic hyperinflation, hypotension, dysrhythmias, and seizures. In contrast to various other pathologies requiring mechanical ventilation, acute asthma is generally associated with better outcomes. This review serves as a practical guide to the physician managing patients with severe acute asthma requiring mechanical ventilation. In addition to specifics relating to endotracheal intubation, we also discuss the interpretation of ventilator graphics, the recommended mode of ventilation, dynamic hyperinflation, permissive hypercapnia, as well as the role of extracorporeal membrane oxygenation and noninvasive mechanical ventilation.

Fundamentals of aerosol therapy in critical care

Drug dosing in critically ill patients is challenging due to the altered drug pharmacokinetics–pharmacodynamics associated with systemic therapies. For many drug therapies, there is potential to use the respiratory system as an alternative route for drug delivery. Aerosol drug delivery can provide many advantages over conventional therapy. Given that respiratory diseases are the commonest causes of critical illness, use of aerosol therapy to provide high local drug concentrations with minimal systemic side effects makes this route an attractive option. To date, limited evidence has restricted its wider application. The efficacy of aerosol drug therapy depends on drug-related factors (particle size, molecular weight), device factors, patient-related factors (airway anatomy, inhalation patterns) and mechanical ventilation-related factors (humidification, airway). This review identifies the relevant factors which require attention for optimization of aerosol drug delivery that can achieve better drug concentrations at the target sites and potentially improve clinical outcomes.

Contemporary treatment of children with critical and near-fatal asthma

Asthma is the most common chronic illness in childhood. Although the vast majority of children with acute asthma exacerbations do not require critical care, some fail to respond to standard treatment and require escalation of support. Children with critical or near-fatal asthma require close monitoring for deterioration and may require aggressive treatment strategies. This review examines the available evidence supporting therapies for critical and near-fatal asthma and summarizes the contemporary clinical care of these children. Typical treatment includes parenteral corticosteroids and inhaled or intravenous beta-agonist drugs. For children with an inadequate response to standard therapy, inhaled ipratropium bromide, intravenous magnesium sulfate, methylxanthines, helium-oxygen mixtures, and non-invasive mechanical support can be used. Patients with progressive respiratory failure benefit from mechanical ventilation with a strategy that employs large tidal volumes and low ventilator rates to minimize dynamic hyperinflation, barotrauma, and hypotension. Sedatives, analgesics and a neuromuscular blocker are often necessary in the early phase of treatment to facilitate a state of controlled hypoventilation and permissive hypercapnia. Patients who fail to improve with mechanical ventilation may be considered for less common approaches, such as inhaled anesthetics, bronchoscopy, and extracorporeal life support. This contemporary approach has resulted in extremely low mortality rates, even in children requiring mechanical support.

Regional Ventilation and Aerosol Deposition with Helium-Oxygen in Bronchoconstricted Asthmatic Lungs

BACKGROUND

Theoretical models suggest that He-O2 as carrier gas may lead to more homogeneous ventilation and aerosol deposition than air. However, these effects have not been clinically consistent and it is unclear why subjects may or may not respond to the therapy. Here we present 3D-imaging data of aerosol deposition and ventilation distributions from subjects with asthma inhaling He-O2 as carrier gas. The data are compared with those that we previously obtained from a similar group of subjects inhaling air.

METHODS

Subjects with mild-to-moderate asthma were bronchoconstricted with methacholine and imaged with PET-CT while inhaling aerosol carried with He-O2. Mean-normalized-values of lobar specific ventilation sV* and deposition sD* were derived and the factors affecting the distribution of sD* were evaluated along with the effects of breathing frequency (f) and regional expansion (FVOL).

RESULTS

Lobar distributions of sD* and sV* with He-O2 were not statistically different from those previously measured with air. However, with He-O2 there was a larger number of lobes having sV* and sD* closer to unity and, in those subjects with uneven deposition distributions, the correlation of sD* with sV* was on average higher (p < 0.05) in He-O2 (0.84 ± 0.8) compared with air (0.55 ± 0.28). In contrast with air, where the frequency of breathing during nebulization was associated with the degree of sD*-sV* correlation, with He-O2 there was no association. Also, the modulation of f on the correlation between FVOL and sD*/sV* in air, was not observed in He-O2.

CONCLUSION

There were no differences in the inter-lobar heterogeneity of sD* or sV* in this group of mild asthmatic subjects breathing He-O2 compared with patients previously breathing air. Future studies, using these personalized 3D data sets as input to CFD models, are needed to understand if, and for whom, breathing He-O2 during aerosol inhalation may be beneficial.

Inhalation therapy in mechanical ventilation

Patients with obstructive lung disease often require ventilatory support via invasive or noninvasive mechanical ventilation, depending on the severity of the exacerbation. The use of inhaled bronchodilators can significantly reduce airway resistance, contributing to the improvement of respiratory mechanics and patient-ventilator synchrony. Although various studies have been published on this topic, little is known about the effectiveness of the bronchodilators routinely prescribed for patients on mechanical ventilation or about the deposition of those drugs throughout the lungs. The inhaled bronchodilators most commonly used in ICUs are beta adrenergic agonists and anticholinergics. Various factors might influence the effect of bronchodilators, including ventilation mode, position of the spacer in the circuit, tube size, formulation, drug dose, severity of the disease, and patient-ventilator synchrony. Knowledge of the pharmacological properties of bronchodilators and the appropriate techniques for their administration is fundamental to optimizing the treatment of these patients.

Heliox-driven β2-agonists nebulization for children and adults with acute asthma: a systematic review with meta-analysis

BACKGROUND

The effect of heliox as a nebulizer β2-agonist driving gas in acute asthma remains controversial.

OBJECTIVE

To perform a systematic review with a meta-analysis of randomized trials designed to evaluate the efficacy of heliox versus oxygen in driving β2-agonist nebulization in patients with acute asthma.

METHODS

A search was conducted of all randomized controlled trials published before August 2013. Primary outcomes were change in spirometric measurements and severity composite score (pediatric studies); secondary outcomes were hospitalizations and serious adverse effects.

RESULTS

Eleven trials from 10 studies (697 participants) met the inclusion criteria (7 included adults and 3 included children). The mean duration of heliox therapy was 120 minutes and the most common helium-oxygen mixture used was 70:30. Patients receiving heliox presented a statistically significant difference for mean percentage of change in peak expiratory flow (17.2%; 95% confidence interval 5.2-29.2, P = .005). Post hoc subgroup analysis showed that patients with severe and very severe asthma showed a significant improvement in peak expiratory flow compared with those with mild to moderate acute asthma. Heliox-driven nebulization also produced significant decreases in the risk of hospitalizations (odds ratio 0.49, 95% confidence interval 0.31-0.79, P = .003) and severity of exacerbations (pediatric studies; standard mean difference -0.74, 95%% confidence interval -1.45 to -0.03, P = .04). There were no group differences for serious adverse effects.

CONCLUSION

This review suggests that heliox benefits in airflow limitation and hospital admissions could be considered clinically significant. Data support the use of heliox as a nebulizing β2-agonist driving gas in the routine care of patients with acute asthma.

Severe acute asthma exacerbation in children: a stepwise approach for escalating therapy in a pediatric intensive care unit

OBJECTIVES

An increasing prevalence of pediatric asthma has led to increasing burdens of critical illness in children with severe acute asthma exacerbations, often leading to respiratory distress, progressive hypoxia, and respiratory failure. We review the definitions, epidemiology, pathophysiology, and clinical manifestations of severe acute asthma, with a view to developing an evidence-based, stepwise approach for escalating therapy in these patients.

METHODS

Subject headings related to asthma, status asthmaticus, critical asthma, and drug therapy were used in a MEDLINE search (1980-2012), supplemented by a manual search of personal files, references cited in the reviewed articles, and treatment algorithms developed within Le Bonheur Children's Hospital.

RESULTS

Patients with asthma require continuous monitoring of their cardiorespiratory status via noninvasive or invasive devices, with serial clinical examinations, objective scoring of asthma severity (using an objective pediatric asthma score), and appropriate diagnostic tests. All patients are treated with β-agonists, ipratropium, and steroids (intravenous preferable over oral preparations). Patients with worsening clinical status should be progressively treated with continuous β-agonists, intravenous magnesium, helium-oxygen mixtures, intravenous terbutaline and/or aminophylline, coupled with high-flow oxygen and non-invasive ventilation to limit the work of breathing, hypoxemia, and possibly hypercarbia. Sedation with low-dose ketamine (with or without benzodiazepines) infusions may allow better toleration of non-invasive ventilation and may also prepare the patient for tracheal intubation and mechanical ventilation, if indicated by a worsening clinical status.

CONCLUSIONS

Severe asthma can be a devastating illness in children, but most patients can be managed by using serial objective assessments and the stepwise clinical approach outlined herein. Following multidisciplinary education and training, this approach was successfully implemented in a tertiary-care, metropolitan children's hospital.

Evaluation of lung function and deposition of aerosolized bronchodilators carried by heliox associated with positive expiratory pressure in stable asthmatics: a randomized clinical trial

While administration of medical aerosols with heliox and positive airway pressure are both used clinically to improve aerosol delivery, few studies have differentiated their separate roles in treatment of asthmatics. The aim of this randomized, double blinded study is to differentiate the effect of heliox and oxygen with and without positive expiratory pressure (PEP), on delivery of radiotagged inhaled bronchodilators on pulmonary function and deposition in asthmatics. 32 patients between 18 and 65 years of age diagnosed with stable moderate to severe asthma were randomly assigned into four groups: (1) Heliox + PEP (n = 6), (2) Oxygen + PEP (n = 6), (3) Heliox (n = 11) and (4) Oxygen without PEP (n = 9). Each group received 1 mg of fenoterol and 2 mg of ipratropium bromide combined with 25 mCi (955 Mbq) of Technetium-99m and 0.9% saline to a total dose volume of 3 mL placed in a Venticis II nebulizer attached to a closed, valved mask with PEP of 0 or 10 cm H2O. Both gas type and PEP level were blinded to the investigators. Images were acquired with a single-head scintillation camera with the longitudinal and transverse division of the right lung as regions of interest (ROIs). While all groups responded to bronchodilators, only group 1 showed increase in FEV1%predicted and IC compared to the other groups (p < 0.04). When evaluating the ROI in the vertical gradient we observed higher deposition in the middle and lower third in groups 1 (p = 0.02) and 2 (p = 0.01) compared to group 3. In the horizontal gradient, a higher deposition in the central region in groups 1 (p = 0.03) and 2 (p = 0.02) compared to group 3 and intermediate region of group 2 compared to group 3. We conclude that aerosol deposition was higher in groups with PEP independent of gas used, while bronchodilator response with Heliox + PEP improved FEV1 % and IC compared to administration with Oxygen, Oxygen with PEP and Heliox alone. Trial registration NCT01268462.

Inhalation therapy in patients receiving mechanical ventilation: an update

Incremental gains in understanding the influence of various factors on aerosol delivery in concert with technological advancements over the past 2 decades have fueled an ever burgeoning literature on aerosol therapy during mechanical ventilation. In-line use of pressurized metered-dose inhalers (pMDIs) and nebulizers is influenced by a host of factors, some of which are unique to ventilator-supported patients. This article reviews the impact of various factors on aerosol delivery with pMDIs and nebulizers, and elucidates the correlation between in-vitro estimates and in-vivo measurement of aerosol deposition in the lung. Aerosolized bronchodilator therapy with pMDIs and nebulizers is commonly employed in intensive care units (ICUs), and bronchodilators are among the most frequently used therapies in mechanically ventilated patients. The use of inhaled bronchodilators is not restricted to mechanically ventilated patients with chronic obstructive pulmonary disease (COPD) and asthma, as they are routinely employed in other ventilator-dependent patients without confirmed airflow obstruction. The efficacy and safety of bronchodilator therapy has generated a great deal of interest in employing other inhaled therapies, such as surfactant, antibiotics, prostacyclins, diuretics, anticoagulants and mucoactive agents, among others, in attempts to improve outcomes in critically ill ICU patients receiving mechanical ventilation.

Aerosol therapy in patients receiving noninvasive positive pressure ventilation

In selected patients, noninvasive positive pressure ventilation (NIPPV) with a facemask is now commonly employed as the first choice for providing mechanical ventilation in the intensive care unit (ICU). Aerosol therapy for treatment of acute or acute-on-chronic respiratory failure in this setting may be delivered by pressurized metered-dose inhaler (pMDI) with a chamber spacer and facemask or nebulizer and facemask. This article reviews the host of factors influencing aerosol delivery with these devices during NIPPV. These factors include (1) the type of ventilator, (2) mode of ventilation, (3) circuit conditions, (4) type of interface, (5) type of aerosol generator, (6) drug-related factors, (7) breathing parameters, and (8) patient-related factors. Despite the impediments to efficient aerosol delivery because of continuous gas flow, high inspiratory flow rates, air leaks, circuit humidity, and patient-ventilator asynchrony, significant therapeutic effects are achieved after inhaled bronchodilator administration to patients with asthma and chronic obstructive pulmonary disease. Similarly to invasive mechanical ventilation, careful attention to the technique of drug administration is required to optimize therapeutic effects of inhaled therapies during NIPPV. Assessment of the patient's ability to tolerate a facemask, the level of respiratory distress, hemodynamic status, and synchronization of aerosol generation with inspiratory airflow are important factors contributing to the success of aerosol delivery during NIPPV. Further research into novel delivery methods, such as the use of NIPPV with nasal cannulae, could enhance the efficiency, ease of use, and reproducibility of inhalation therapy during noninvasive ventilation.

In vitro comparison of heliox and oxygen in aerosol delivery using pediatric high flow nasal cannula

Drug administration via high flow nasal cannula (HFNC) has been described in pediatrics but the amount of albuterol delivery with an HFNC is not known. The purpose of this study is to quantify aerosol delivery with heliox and oxygen (O(2)) in a model of pediatric ventilation. A vibrating mesh nebulizer (Aeroneb Solo, Aerogen) was placed on the inspiratory inlet of a heated humidifier and heated wire circuit attached to a pediatric nasal cannula (Optiflow, Fisher & Paykel). Breathing parameters were tidal volume (V(t)) 100 ml, respiratory rate (RR) 20/min, and I-time of 1 sec. Albuterol sulfate (2.5 mg/3 ml) was administered through a pediatric HFNC with O(2) (100%) and heliox (80/20% mixture). A total of 12 runs, using O(2) and heliox were conducted at 3 and 6 L/min (n = 3). Drug was collected on an absolute filter, eluted and measured using spectrophotometry. The percent inhaled dose (mean ± SD) was similar with heliox and O(2) at 3 L/min (11.41 ± 1.54 and 10.65 ± 0.51, respectively; P = 0.465). However at 6 L/min drug deposition was ≥ 2-fold greater with heliox (5.42 ± 0.54) than O(2) (1.95 ± 0.50; P = 0.01). Using a pediatric model of HFNC, reducing delivered flow from 6 to 3 L/min increased inhaled albuterol delivery ≥ 2-fold but eliminated the increase in inhaled drug efficiency associated with heliox.

An in vitro assessment of aerosol delivery through patient breathing circuits used with medical air or a helium-oxygen mixture

BACKGROUND

The bench experiments presented herein were conducted in order to investigate the influence of carrier gas, either medical air or a helium-oxygen mixture (78% He, 22% O2), on the droplet size distribution and aerosol mass delivered from a vibrating mesh nebulizer through a patient breathing circuit.

METHODS

Droplet size distributions at the exit of the nebulizer T-piece and at the patient end of the breathing circuit were determined by laser diffraction. Additional experiments were performed to determine the effects on measured size distributions of gas humidity and of the droplet residence time during transport from the nebulizer to the laser diffraction measurement volume. Aerosol deposition in the nebulizer, breathing circuit, and on expiratory and patient filters was determined by photometry following nebulization of sodium fluoride solutions into the breathing circuit during simulated patient breathing.

RESULTS

With no humidification of the carrier gas, droplet volume median diameter (VMD) at the exit of the nebulizer T-piece was 5.5±0.1 μm for medical air, and 4.3±0.1 μm for helium-oxygen. Varying the aerosol residence time between the nebulizer and the measurement volume did not affect the measured size distributions; however, humidification of the carrier gases reduced differences in VMD at the nebulizer exit between medical air and helium-oxygen. At the patient end of the breathing circuit, droplet VMDs were 1.8±0.1 μm for medical air and 2.2±0.1 μm for helium-oxygen. The percentages of sodium fluoride recovered from the nebulizer, breathing circuit, patient filter, and expiratory filter were, respectively, 29.9±8.3, 40.4±5.6, 8.3±1.5, and 21.5±2.1% for air, and 32.6±2.2, 36.3±0.7, 12.0±1.4, and 19.1±1.1% for helium-oxygen.

CONCLUSIONS

Ventilation with helium-oxygen in place of air-oxygen mixtures can influence both the droplet size distribution and mass of nebulized aerosol delivered through patient breathing circuits. Assessment of these effects on aerosol delivery is important when incorporating helium-oxygen into patient ventilation strategies.

Aerosol drug delivery: developments in device design and clinical use

Aerosolised drugs are prescribed for use in a range of inhaler devices and systems. Delivering drugs by inhalation requires a formulation that can be successfully aerosolised and a delivery system that produces a useful aerosol of the drug; the particles or droplets need to be of sufficient size and mass to be carried to the distal lung or deposited on proximal airways to give rise to a therapeutic effect. Patients and caregivers must use and maintain these aerosol drug delivery devices correctly. In recent years, several technical innovations have led to aerosol drug delivery devices with efficient drug delivery and with novel features that take into account factors such as dose tracking, portability, materials of manufacture, breath actuation, the interface with the patient, combination therapies, and systemic delivery. These changes have improved performance in all four categories of devices: metered dose inhalers, spacers and holding chambers, dry powder inhalers, and nebulisers. Additionally, several therapies usually given by injection are now prescribed as aerosols for use in a range of drug delivery devices. In this Review, we discuss recent developments in the design and clinical use of aerosol devices over the past 10-15 years with an emphasis on the treatment of respiratory disorders.

Acute severe asthma: new approaches to assessment and treatment

The precise definition of a severe asthmatic exacerbation is an issue that presents difficulties. The term 'status asthmaticus' relates severity to outcome and has been used to define a severe asthmatic exacerbation that does not respond to and/or perilously delays the repetitive or continuous administration of short-acting inhaled beta(2)-adrenergic receptor agonists (SABA) in the emergency setting. However, a number of limitations exist concerning the quantification of unresponsiveness. Therefore, the term 'acute severe asthma' is widely used, relating severity mostly to a combination of the presenting signs and symptoms and the severity of the cardiorespiratory abnormalities observed, although it is well known that presentation does not foretell outcome. In an acute severe asthma episode, close observation plus aggressive administration of bronchodilators (SABAs plus ipratropium bromide via a nebulizer driven by oxygen) and oral or intravenous corticosteroids are necessary to arrest the progression to severe hypercapnic respiratory failure leading to a decrease in consciousness that requires intensive care unit (ICU) admission and, eventually, ventilatory support. Adjunctive therapies (intravenous magnesium sulfate and/or others) should be considered in order to avoid intubation. Management after admission to the hospital ward because of an incomplete response is similar. The decision to intubate is essentially based on clinical judgement. Although cardiac or respiratory arrest represents an absolute indication for intubation, the usual picture is that of a conscious patient struggling to breathe. Factors associated with the increased likelihood of intubation include exhaustion and fatigue despite maximal therapy, deteriorating mental status, refractory hypoxaemia, increasing hypercapnia, haemodynamic instability and impending coma or apnoea. To intubate, sedation is indicated in order to improve comfort, safety and patient-ventilator synchrony, while at the same time decrease oxygen consumption and carbon dioxide production. Benzodiazepines can be safely used for sedation of the asthmatic patient, but time to awakening after discontinuation is prolonged and difficult to predict. The most common alternative is propofol, which is attractive in patients with sudden-onset (near-fatal) asthma who may be eligible for extubation within a few hours, because it can be titrated rapidly to a deep sedation level and has rapid reversal after discontinuation; in addition, it possesses bronchodilatory properties. The addition of an opioid (fentanyl or remifentanil) administered by continuous infusion to benzodiazepines or propofol is often desirable in order to provide amnesia, sedation, analgesia and respiratory drive suppression. Acute severe asthma is characterized by severe pulmonary hyperinflation due to marked limitation of the expiratory flow. Therefore, the main objective of the initial ventilator management is 2-fold: to ensure adequate gas exchange and to prevent further hyperinflation and ventilator-associated lung injury. This may require hypoventilation of the patient and higher arterial carbon dioxide (PaCO(2)) levels and a more acidic pH. This does not apply to asthmatic patients intubated for cardiac or respiratory arrest. In this setting the post-anoxic brain oedema might demand more careful management of PaCO(2) levels to prevent further elevation of intracranial pressure and subsequent complications. Monitoring lung mechanics is of paramount importance for the safe ventilation of patients with status asthmaticus. The first line of specific pharmacological therapy in ventilated asthmatic patients remains bronchodilation with a SABA, typically salbutamol (albuterol). Administration techniques include nebulizers or metered-dose inhalers with spacers. Systemic corticosteroids are critical components of therapy and should be administered to all ventilated patients, although the dose of systemic corticosteroids in mechanically ventilated asthmatic patients remains controversial. Anticholinergics, inhaled corticosteroids, leukotriene receptor antagonists and methylxanthines offer little benefit, and clinical data favouring their use are lacking. In conclusion, expertise, perseverance, judicious decisions and practice of evidence-based medicine are of paramount importance for successful outcomes for patients with acute severe asthma.

Heliox in airway management

Helium-oxygen ("heliox") mixtures have been used for decades in the treatment of various respiratory problems ranging from acute upper airway obstructions to lower airway derangements, such as asthma and exacerbations of chronic bronchitis. This review presents a brief history of helium and helium-oxygen mixtures and their potential clinical uses, summarizes the results of past research into heliox in respiratory applications, explains the physiology of heliox, and presents more recent literature relating to heliox in the clinical setting.

[Heliox-driven bronchodilator nebulization in the treatment of infants with bronchiolitis]

INTRODUCTION

Heliox is a helium-oxygen mixture which improves laminar flow and decreases airway resistance and the work of breathing. The aim of this study was to assess the effects of salbutamol or epinephrine nebulization driven by heliox in infants with moderate-to-severe bronchiolitis.

MATERIALS AND METHODS

This prospective, observational, interventional, controlled and randomized study included ninety-six children who came to our pediatric emergency department with first episode of moderate-to-severe bronchiolitis. The patients were randomized to receive salbutamol or epinephrine nebulized with either oxygen (control group) or heliox (70% helium and 30% oxygen) as the driving gas. Heart rate, respiratory rate, pulse oximetry oxygen saturation and clinical score were measured before and after the treatment period. We also reported hospitalization rates and the number of patients who returned to the emergency department in the following seventy two hours.

RESULTS

There were no significant differences between both groups. The only statistically significant difference was that, in the heliox group, patients with severe bronchiolitis needed a lower number of nebulizations than infants in the control group.

CONCLUSIONS

According to our study, heliox-driven salbutamol or epinephrine is not an effective therapy in patients with acute bronchiolitis.

How best to deliver aerosol medications to mechanically ventilated patients

Pressurized metered-dose inhalers (pMDIs) and nebulizers are employed routinely for aerosol delivery to ventilator-supported patients, but the ventilator circuit and artificial airway previously were thought to be major barriers to effective delivery of aerosols to patients receiving mechanical ventilation. In the past two decades, several investigators have shown that careful attention to many factors, such as the position of the patient, the type of aerosol generator and its configuration in the ventilator circuit, aerosol particle size, artificial airway, conditions in the ventilator circuit, and ventilatory parameters, is necessary to optimize aerosol delivery during mechanical ventilation. The best techniques for aerosol delivery during noninvasive positive-pressure ventilation are not well established as yet, and the efficiency of aerosol delivery in this setting is lower than that during invasive mechanical ventilation. The most efficient methods of using the newer hydrofluoroalkane-pMDIs and vibrating mesh nebulizers in ventilator-supported patients also require further evaluation. When optimal techniques of administration are employed, the efficiency of aerosolized drug delivery in mechanically ventilated patients is comparable to that achieved in ambulatory patients.

Aerosol delivery during mechanical ventilation: from basic techniques to new devices

Pressurized metered-dose inhalers (pMDIs) and nebulizers are routinely employed for aerosol delivery in mechanically ventilated patients. A significant proportion of the aerosol deposits in the ventilator circuit and artificial airway, thereby reducing the inhaled drug mass. Factors influencing aerosol delivery during mechanical ventilation differ from those in spontaneously breathing patients. The English language literature on aerosol delivery during mechanical ventilation was reviewed. Marked variations in the efficiency of drug delivery with pMDIs and nebulizers occur due to differences in the technique of administration. Careful attention to five factors, viz., the aerosol generator, aerosol particle size, conditions in the ventilator circuit, artificial airway, and ventilator parameters, is necessary to optimize aerosol delivery during mechanical ventilation. Factors influencing drug delivery during NPPV are not well understood, and the efficiency of aerosol delivery in this setting is lower than that during invasive mechanical ventilaiton. With an optimal technique of administration the efficiency of aerosol delivery during mechanical ventilation is similar to that achieved during spontaneous breathing. Further research is needed to optimize aerosol delivery during NPPV.

Inhaled bronchodilator administration during mechanical ventilation: how to optimize it, and for which clinical benefit?

Bronchodilators are frequently used in ICU patients, and are the most common medications administered by inhalation during mechanical ventilation. The amount of bronchodilator that deposits at its site of action depends on the amount of drug, inhaled mass, deposited mass, and particle size distribution. Mechanical ventilation challenges both inhaled mass and lung deposition by specific features, such as a ventilatory circuit, an endotracheal tube, and ventilator settings. Comprehensive in vitro studies have shown that an endotracheal tube is not as significant a barrier for the drug to travel as anticipated. Key variables of drug deposition are attachments of the inhalation device in the inspiratory line 10 to 30 cm to the endotracheal tube, use of chamber with metered-dose inhaler, dry air, high tidal volume, low respiratory frequency, and low inspiratory flow, which can increase the drug deposition. In vivo studies showed that a reduction by roughly 15% of the respiratory resistance was achieved with inhaled bronchodilators during invasive mechanical ventilation. The role of ventilatory settings is not as clear in vivo, and primary factors for optimal delivery and physiologic effects were medication dose and device location. Nebulizers and pressurized metered-dose inhalers can equally achieve physiologic end points. The effects of bronchodilators should be carefully evaluated, which can easily be done with the interrupter technique. With the non-invasive ventilation, the data regarding drug delivery and physiologic effects are still limited. With the bilevel ventilators the inhalation device should be located between the leak port and face mask. Further studies should investigate the effects of inhaled bronchodilators on patient outcome and methods to optimize delivery of inhaled bronchodilators during non-invasive ventilation.

Ventilation strategies and adjunctive therapy in severe lung disease

Respiratory failure caused by severe lung disease is a common reason for admission to the pediatric and neonatal intensive care units. Efforts to decrease morbidity and mortality have fueled investigations into innovative methods of ventilation, kinder gentler ventilation techniques, pharmacotherapeutic adjuncts, and extracorporeal life support modalities. This article discusses the rationale for and experience with some of these techniques.

The uses of helium and xenon in current clinical practice

The noble gases have always been an enigma. Discovered late in the history of chemistry and in seemingly small quantities in our atmosphere, they are some of the most unreactive elements known. However, despite being extremely inert, the noble gases (helium, neon, argon, krypton, xenon and radon) have found diverse and ever expanding applications in medicine. Of all of them, the gases that have found the greatest number of uses in the field of anaesthesia and related specialties are helium and xenon. This review focuses on the history of the discovery of both gases, their unique physicochemical properties and describes their uses in clinical practice with particular emphasis on those applicable to anaesthesia.

The effects of Heliox on the output and particle-size distribution of salbutamol using jet and vibrating mesh nebulizers

There are theoretical benefits of delivering drug aerosols to patients with asthma and chronic obstructive pulmonary disease (COPD) using Heliox as a carrier gas. The objective of this study was to develop systems to allow bronchodilators nebulized by a breath enhanced jet nebulizer and a vibrating mesh nebulizer to be delivered to patients in Heliox. This was achieved by attaching a reservoir to the nebulizers to ensure inhaled Heliox was not diluted by entrained air. For the vibrating mesh nebulizer, the total output was significantly higher after 5 min of nebulization when Heliox rather than air was used as the delivery gas (p < 0.001). The proportion of drug in particles <5 microm was 58.1% for Heliox and 50.1% when air was entrained. When the breath enhanced nebulizer was used a much higher driving flow of Heliox, compared to air, was required to deliver a similar dose of drug (p < 0.05). The total amount of drug likely to be inhaled was significantly higher when the vibrating mesh nebulizer (Aerogen) was used compared to the breath enhanced jet nebulizer (Pari LC plus) (p < 0.001). The amount of drug likely to be inhaled was also significantly greater for the adult as opposed to pediatric breathing pattern for all nebulizers and flows tested with the exception of the Aeroneb and Heliox entrainment. In this case, total amounts were similar for both patterns but for the pediatric pattern, the time taken to reach this output was longer. Such information is required to allow appropriate interpretation of clinical trials of drug delivery using Heliox.

Inhalation therapy in invasive and noninvasive mechanical ventilation

PURPOSE OF REVIEW

The aim of this article is to discuss the various factors that influence aerosol delivery in mechanically ventilated patients and clarify optimal techniques for aerosol administration in this patient population. Clinical use of various inhaled therapies in patients receiving invasive and noninvasive mechanical ventilation is also discussed.

RECENT FINDINGS

With optimal techniques for using pressurized metered-dose inhalers and nebulizers in ventilator circuits, the efficiency of inhaled drug delivery in mechanically ventilated patients is comparable to that in ambulatory patients. Techniques for enhancing inhaled drug delivery during noninvasive positive pressure ventilation are also being investigated.

SUMMARY

Pressurized metered-dose inhalers of bronchodilator and corticosteroid aerosols are more efficient and convenient to use than nebulizers for routine therapy in ventilated patients. Nebulizers are, however, more versatile and are employed to generate aerosols of bronchodilators, corticosteroids, antibiotics, prostaglandins, surfactant, and mucolytic agents. Factors influencing drug delivery during noninvasive positive pressure ventilation are not fully understood as yet, and further work is needed to enhance drug delivery in this setting. Improvements in drug formulations and the design and efficiency of aerosol generating devices have led to increasing application of inhaled therapies in mechanically ventilated patients.

A model protocol for emergency medical services management of asthma exacerbations

Emergency medical services (EMS) is an important part of the continuum of asthma management. The magnitude of the EMS responsibility is very large, with millions of patients with asthma treated each year by EMS personnel. In response to inconsistencies between the 1997 National Asthma Education and Prevention Program asthma guidelines and a variety of existing EMS protocols on the management of asthma exacerbations, the Centers for Disease Control and Prevention convened a workgroup in 2004 to discuss the various opportunities and challenges ahead. At the meeting, and over the ensuing year, the workgroup created a model protocol that was derived from the National Asthma Education and Prevention Program guidelines. The model protocol is available in both text and algorithm format and offers guidance for EMS systems to develop and implement treatment protocols in their local areas. The workgroup recommendations emphasize flexibility, simplicity, and low-risk practices. By integrating these recommendations into existing protocols, we believe that EMS systems could improve prehospital care for patients with asthma. Demonstration projects are needed to carefully examine the implementation process and the actual impact of the model protocol on various outcomes. The workgroup also encourages more research on EMS management of asthma exacerbations. In the meantime, improved collaboration between EMS and national asthma organizations is an immediate priority and will continue to advance future discussions on how to improve asthma management in the prehospital setting. The workgroup hopes that state and local EMS systems will see the value of the model protocol and encourage its use.

Room air entrainment during beta-agonist delivery with heliox

Studies of the efficacy of heliox in patients with severe asthma have shown mixed results. Among the factors that are responsible for variable outcomes, the failure of heliox delivery systems to prevent room air entrainment (RAE) during beta-agonist delivery is probably the most critical. While keeping the rotameter flow rate (FR) of heliox mixed 70:30 to a nebulizer at 10 L/min, the FR of heliox from a second gas source to a T-connector (TC) was increased during the delivery of the beta-agonist with a conventional T-nebulizer delivery system (TNDS). A negative peak inspiratory flow (pneumotachometer reading) or a helium concentration of < 70% (quadralizer reading) were indicators of RAE. RAE was tested during spontaneous tidal breathing and acute asthma. A rotameter FR of 10 L/m to the nebulizer with no flow from a second gas source to a TC (conventional TNDS) resulted in a significant drop in helium concentration during tidal breathing (46.2%) and acute asthma (27.5%) due to RAE. This degree of helium dilution can negate the beneficial effects of heliox to lung mechanics almost completely. A rotameter FR of 10 L/m each to a nebulizer and a TC resulted in a helium concentration 69.8% during tidal breathing (no RAE), but 49% (significant RAE) during asthma events. A rotameter FR of 15 L/m (pressure regulator setting, 100 lbs per square inch) to a TC, while maintaining a rotameter FR of 10 L/m to a nebulizer prevented RAE during asthma (helium concentration, 69.9%). Conventional TNDS may be used to deliver the beta-agonist with heliox during asthma without RAE.

Heliox for nonintubated acute asthma patients

BACKGROUND

Helium and oxygen mixtures (heliox), have been used sporadically in respiratory medicine for decades. Their use in acute respiratory emergencies such as asthma has been the subject of considerable debate. Despite the lapse of more than 60 years since it was first proposed, the role of heliox in treating patients with severe acute asthma remains unclear.

OBJECTIVES

To determine the effect of the addition of heliox to standard medical care on the course of acute asthma, as measured by pulmonary function testing and clinical endpoints.

SEARCH STRATEGY

Randomised controlled trials were identified from the Cochrane Airways Group Specialised Register. In addition, we contacted primary authors and experts and searched reference lists of articles. Searches are current to August 2005.

SELECTION CRITERIA

1) randomised, single or double blind, controlled trials; 2) children or adults with a clinical diagnosis of acute asthma seen in emergency departments or equivalent acute care settings; and 3) compared treatment with inhaled heliox to placebo (oxygen or air).

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed the studies for inclusion and quality assessment; disagreement was resolved by a third review author and consensus.

MAIN RESULTS

This review has been updated in 2006 to include four new trials giving atotal of ten trials involving 544 acute asthma patients. Seven studies involved adults and three studies dealt solely with children. Three were assessed as high quality (Jadad score > 3). Pulmonary function tests were recorded during heliox administration (15 to 60 min). Pooling of the eight trials contributing data to this review showed no significant group differences (standardised mean differences -0.28; 95% confidence interval (CI) -0.56 to 0.01). There was significant heterogeneity among the studies. Heliox use did improve pulmonary function only in the subgroup of patients with the most severe baseline pulmonary function impairment; however, this conclusion is based on a small number of studies. There were no significant differences between groups when adults versus children, and high versus low heliox dose studies were compared. Finally, at the end of treatment, participants treated with heliox showed no significant different risk of admission to hospital (RR 0.83 (95%CI 0.66 to 1.08, P = 0.17, I(2) = 0%).

AUTHORS' CONCLUSIONS

The existing evidence does not provide support for the administration of helium-oxygen mixtures to all ED patients with acute asthma. At this time, heliox treatment does not have a role to play in the initial treatment of patients with acute asthma. Nevertheless, new evidence suggests certain beneficial effects in patients with more severe obstruction. Since these conclusions are based upon between-group comparisons and small studies, they should be interpreted with caution.

Effect of Heliox on Albuterol Delivery by Metered-Dose Inhaler in Pediatric In Vitro Models of Mechanical Ventilation

STUDY OBJECTIVE

To determine the effect of varying concentrations of heliox, a mixture of helium and oxygen, on albuterol delivery administered by metered-dose inhaler (MDI) in pediatric mechanically ventilated models.

DESIGN

Prospective in vitro laboratory study.

SETTING

University-affiliated research laboratory.

MODELS

The lungs of a 10-kg infant and 30-kg child receiving humidified pressure-regulated volume-controlled ventilation were simulated. The infant settings were an endotracheal tube (ETT) of 4.0 mm, tidal volume of 150 ml, positive end-expiratory pressure of 2 cm H(2)O, rate of 20 breaths/minute, inspiratory time of 0.7 second; the child settings were an ETT of 6.0 mm, tidal volume of 450 ml, positive end-expiratory pressure of 2 cm H(2)O, rate of 16 breaths/minute, and inspiratory time of 0.8 second.

MEASUREMENTS AND MAIN RESULTS

Ten albuterol MDI canisters with chlorofluorocarbon propellants were each actuated once sequentially (total dose 1000 mug) with a commercially available aerosol holding chamber. Albuterol was collected onto a filter proximal to a lung simulator. The filter was rinsed, and concentrations were determined by high-performance liquid chromatography. In the infant model, heliox mixtures of 70:30, 60:40, and 50:50 were compared with nitrogen:oxygen (N(2):O(2)) mixtures in the same ratios. The effect of the 70:30 mixtures was also explored in a child model. Each gas mixture was tested 5 times. At all three ratios, albuterol delivery to the end of the ETT was improved with heliox compared with N(2):O(2) (approximately 7% vs 3-4%, p<0.0001, one-way analysis of variance [ANOVA] with a Bonferroni correction for multiple comparisons). No significant difference was noted in mean percentage albuterol delivery among the varying ratios of heliox studied. By two-way ANOVA, significantly greater albuterol delivery was noted with 70:30 heliox compared with 70:30 N(2):O(2) (7-8% vs 3%, p<0.0001), with no significant difference between the infant and child model (p=0.21). The gas mixture, model, and interaction of the two explained 88% of the variability in mean percentage albuterol delivery.

CONCLUSION

Heliox increased albuterol delivery administered by MDI to the end of the ETT in these in vitro pediatric models of mechanical ventilation. Further studies are needed to determine if the improved albuterol delivery with heliox enhances clinical response in infants and children needing mechanical ventilation.

Deposition pattern of heliox-driven bronchodilator aerosol in the airways of stable asthmatics

To compare the deposition patterns of heliox-driven and air-driven radiolabeled bronchodilator aerosol, a prospective randomized study was undertaken at the Ben Taub Hospital in Houston, Texas. The working hypothesis was that nebulization with heliox would improve the peripheral deposition of a bronchodilator aerosol. Twelve mild-moderate known asthmatics were recruited for the study. They were asked to withhold medications for 8-24 hours prior to reporting for the study. Each subject was randomized to receive either heliox or oxygen for delivery of albuterol labeled with Tc-99m DTPA. Prior to the nebulization, baseline spirometry was performed. Following nebulization, the subjects were scanned for 1,000,000 counts on the ADAC Genesis Scanner. A postbronchodilator spirometry was then performed. Subjects returned about a week later, this time to have the same process repeated with the other gas. The normalized pixel counts were obtained, and the frequency distribution histograms were constructed for each of the deposition images. Skew and kurtosis were calculated. A lower skew and kurtosis value suggests a more peripheral distribution of the bronchodilator, whereas a higher pixel count corresponds with an increased area and uniformity of deposition. There were no statistically significant differences in baseline PFTs on the 2 days of the study. The pixel count was statistically higher after the heliox-driven nebulization than the air-driven nebulization. The skew and kurtosis values were lower after the heliox-driven nebulization than after the air-driven nebulization of radiolabeled aerosol. All patients had a good bronchodilator response with either driving gas. However, the degree of improvement was more with heliox-driven nebulization than with air-driven nebulization. We conclude that aerosol delivery with heliox results in more uniform and peripheral deposition. Thus, this mode of delivery can be used when uniform, peripheral deposition is desired as with drug delivery during an exacerbation or with aerosolized gene therapy.

A prototype infant incubator for heliox therapy

Heliox (Hx) gas has been shown to improve pulmonary function in infants, but methods for its delivery are invasive and problematic. To this end, we modified an Isolette (Hill-Rom Air-Shields) infant incubator (Hxl) to deliver Hx respiratory gas mixtures noninvasively while providing thermal stability for neonatal care in the Neonatal Intensive Care Unit (NICU). In vitro tests and in vivo animal studies were performed to compare the original design specifications and established baseline performance criteria for the Hxl design. The experimental environments at 50% and 80% relative humidity (RH) consisted of helium (He) with 21% and 50% O2 and control (C) of 21% and 50% O2 with the balance nitrogen (N). Elapsed times to steady state (SS) and recovery time back to SS (OCDss) due to opening and closing the door were recorded for each variable. All rabbits survived and appeared comfortable during all experimental conditions. These data show that the newly designed Isolette provides similar thermal, O2, CO2, and RH responses as the control incubator. Based on these positive safety/efficacy studies, study of the therapeutic impact of Hxl care on neonatal growth and development is in progress.

Heliox administration in the pediatric intensive care unit: an evidence-based review

OBJECTIVE

To provide a comprehensive, evidence-based review of helium-oxygen gas mixtures (heliox) in the management of pediatric respiratory diseases.

DATA SOURCE

A thorough, computerized bibliographic search of the preclinical and clinical literature regarding the properties of helium and its application in pediatric respiratory disease states.

DATA SYNTHESIS

After an overview of the potential benefits and technical aspects of helium-oxygen gas mixtures, the role of heliox is addressed for asthma, aerosolized medication delivery, upper airway obstruction, postextubation stridor, croup, bronchiolitis, and high-frequency ventilation. The available data are objectively classified based on the value of the therapy or intervention as determined by the study design from which the data are obtained.

CONCLUSIONS

Heliox administration is most effective during conditions involving density-dependent increases in airway resistance, especially when used early in an acute disease process. Any beneficial effect of heliox should become evident in a relatively short period of time. The medical literature supports the use of heliox to relieve respiratory distress, decrease the work of breathing, and improve gas exchange. No adverse effects of heliox have been reported. However, heliox must be administered with vigilance and continuous monitoring to avoid technical complications.

Development of Aerosol Drug Delivery with Helium Oxygen Gas Mixtures

Aerosol drug delivery using helium-oxygen gas mixtures (heliox) is considered in terms of flow physics, atomization, and aerosol mechanics. Theoretical considerations are then related to past studies of the physiological effects of the inhalation of heliox and its potential use as a drug delivery medium. Past clinical trials of heliox investigating this use are reviewed and technical recommendations made for its successful development. It is proposed that improved peripheral lung drug delivery with heliox is highly dependent on proper administration, especially the inclusion of proper reservoir system for the gas.

Acute asthma in adults: a review

All patients with asthma are at risk of having exacerbations. Hospitalizations and emergency department (ED) visits account for a large proportion of the health-care cost burden of asthma, and avoidance or proper management of acute asthma (AA) episodes represent an area with the potential for large reductions in health-care costs. The severity of exacerbations may range from mild to life threatening, and mortality is most often associated with failure to appreciate the severity of the exacerbation, resulting in inadequate emergency treatment and delay in referring to hospital. This review describes the epidemiology, costs, pathophysiology, mortality, and management of adult AA in the ED and in the ICU.

Inhaled therapy for acute adult asthma

PURPOSE OF REVIEW

To evaluate recent developments on emergency department inhalotherapy in non-intubated acute adult asthma patients.

RECENT FINDINGS

There is evidence that high-flow oxygen can be associated with hypercarbia, and that full humidification of the inspired gases should be recommended. On the contrary, there is a lack of evidence to support the role of heliox in the initial treatment of acute asthma. Specific short-acting inhaled beta(2)-agonists are the drugs of choice. A more rapid and profound bronchodilatation with fewer side effects and less time of treatment can be achieved when sufficient doses are given using pressurized meter dose inhalers and large-volume valved-spacers, particularly in patients with the most severe obstruction. Findings argue against the routine use of continuous nebulization. High and repetitive doses of ipratropium bromide in combination with beta(2)-agonists are indicated as first line treatment of severe acute asthma. There is insufficient evidence that inhaled corticosteroids alone are as effective as systemic corticosteroids. Finally, the combination of nebulized magnesium and albuterol provides no benefit in addition to that provided by therapy with albuterol in patients with mild-to-moderate asthma exacerbations.

SUMMARY

According to the latest evidence, the goals of treatment may be summarized as follows: maintenance of adequate arterial oxygen saturation with supplemental oxygen, relief of airflow obstruction by administration of inhaled beta-agonists and anticholinergics, and reduction of airway inflammation and prevention of future relapses by using early administration of systemic corticosteroids.

Heliox vs air-oxygen mixtures for the treatment of patients with acute asthma: a systematic overview

OBJECTIVE

To evaluate, by systematic review, the efficacy of heliox on respiratory mechanics and outcomes in patients with acute asthma.

METHODS

The search strategy included searching electronic databases (MEDLINE, EMBASE, and The Cochrane Library) and the references of relevant articles. Study quality was assessed based on allocation concealment. Randomized controlled trials (RCTs) comparing heliox to an air-oxygen mixture (airO(2)) as an adjunct treatment in patients with acute asthmatic attacks were analyzed. For the qualitative portion of the analysis, all reports of the use of heliox in patients with acute asthma were included.

RESULTS

Four RCTs (n = 278) were found to have a common respiratory parameter (peak expiratory flow rate as a percentage of predicted) suitable for meta-analysis. Within the 92% confidence interval (CI), there was a small benefit with the use of heliox compared to airO(2) (weighted mean difference, + 3%; 95% CI, - 2 to + 8%). There was also a slight improvement in the dyspnea index (weighted mean difference, 0.60; 95% CI, 0.04 to 1.16) with the use of heliox over airO(2). Overall, five RCTs, one nonrandomized unblinded parallel trial, one retrospective case-matched control trial, three case series, and one case report had results in favor of heliox; one RCT and one case series showed no improvement with heliox; one RCT showed a possible detrimental effect with heliox; and 1 small RCT was inconclusive. Most investigators did not prevent entrainment of room air during heliox use or compensate for the lower nebulizing efficiency of heliox.

CONCLUSION

Based on surrogate markers, heliox may offer mild-to-moderate benefits in patients with acute asthma within the first hour of use, but its advantages become less apparent beyond 1 h, as most conventionally treated patients improve to similar levels, with or without it. The effect of heliox may be more pronounced in more severe cases. There are insufficient data on whether heliox can avert tracheal intubation, or change intensive care and hospital admission rates and duration, or mortality.

Use of helium-oxygen mixtures in the treatment of acute asthma: a systematic review

STUDY OBJECTIVE

To determine the effect of the addition of heliox to standard medical care on the course of acute asthma.

DESIGN

Systematic review of randomized and nonrandomized prospective, controlled trials of children and adults that compared heliox to placebo when used in conjunction with other standard acute treatments.

MAIN OUTCOME MEASURES

Pulmonary function tests, hospital admissions, physiologic measures, side effects, and clinical outcomes.

RESULTS

Seven trials were selected for inclusion, with a total of 392 patients with acute asthma. Six studies involved adults, and one study dealt solely with children. The main outcome variable was spirometric measurements (peak expiratory flow or FEV(1)) in six trials. Two studies evaluated the effect of heliox on airways resistance. No significant differences were demonstrated between heliox or oxygen/air groups (standardized mean difference [SMD], - 0.20; 95% confidence interval [CI], - 0.91 to 0.51; p = 0.6). However, the four studies that used heliox to deliver nebulized therapy showed a nonsignificant increase in pulmonary function (SMD, - 0.21; 95% CI, - 0.43 to 0.01; p = 0.06). In two studies of the same subgroup, heliox mixtures produced a significantly greater increase of heart rate than oxygen/air (weighted mean difference, 9.0; 95% CI, 1.27 to 16.8; p = 0.02). However, the four studies that used heliox to deliver nebulized therapy reported a nonsignificant difference in hospital admissions (odds ratio, 1.07; 95% CI, 0.46 to 2.48; p = 0.9). Overall, heliox appears to be safe and well tolerated.

CONCLUSIONS

The existing evidence does not provide support for the administration of helium-oxygen mixtures to emergency department patients with moderate-to-severe acute asthma. However, these conclusions are based on between-group comparisons and small studies, and these results should be interpreted with caution.

Usefulness of helium-oxygen mixtures in the treatment of mechanically ventilated patients

The density of helium is markedly lower than that of air or any of its components, leading to a substantial decrease in airway resistance to flow when it is inhaled. In mechanically ventilated patients with obstructive airway disease, replacing the usual air-oxygen mixture with helium-oxygen has been shown to reduce dynamic hyperinflation and intrinsic positive end-expiratory pressure; to decrease lung inflation pressures, respiratory acidosis, and work of breathing; and to improve arterial blood gases. Aerosol delivery to distal airways is enhanced with helium-oxygen. Preliminary data also suggest that the use of helium-oxygen could be a valuable approach to decrease postextubation respiratory distress. However, interference with ventilator function and added costs are two major disadvantages of helium-oxygen. Hence, before its widespread use in mechanically ventilated patients can be recommended, studies are needed to determine whether these favorable short-term effects can influence patient outcome.