Helium oxygen mixtures in the intensive care unit

Heliox, a mixture of helium and oxygen, has a density that is less than that of air. Breathing heliox leads to a reduction in resistance to flow within the airways, and consequently to a decrease in the work of breathing (WOB), particularly in disorders that are characterized by increased airways resistance. Beneficial effects have been observed in patients with asthma, chronic obstructive pulmonary disease (COPD), bronchiolitis, bronchopulmonary dysplasia and upper airways obstruction. Until we have conclusive data that attest to the efficacy of heliox in such conditions, its use will remain controversial. Meanwhile, it appears wise not to incorporate heliox therapy into routine practice because of technical complications and high costs.

Human airways are complex tubes, and airflow in a tube may occur in a laminar, a transitional or a turbulent manner [1]. Reynolds number can approximately predict the nature of flow [2]. Flow characteristics are determined by complex interplays between many different factors, including the branching patterns of trachea and bronchi, the diameter of the conduits, the angles of branching, and the degree of roughness of the walls. The flow characteristics in the airways may also vary depending on the inspiratory and expiratory flow rates [2,3].
During quiet breathing in normal humans, a transition from turbulent to laminar flow occurs through the trachea to around the second generation of bronchi. In patients with airways obstruction, however, turbulent flow occurs very frequently, even in low breathing rates and quiet breathing patterns [4]. This leads to increased work of breathing (WOB).

Breathing helium decreases work of breathing
In turbulent flows, the pressure necessary to generate a given flow rate is dependent on the density of the inspired gases [4,5]. Helium has a density that is significantly lower than that of air [4]. Therefore, a mixture of helium and oxygen, termed heliox, has a lower density than does a mixture of nitrogen and oxygen (air, or airox) [6,7]. Breathing heliox leads to a reduction in resistance to flow within the airways, and consequently to a decrease in the WOB, particularly in disorders that are characterized by increased airways resistance [8]. These beneficial effects have been observed in a few patients with asthma, chronic obstructive pulmonary disease (COPD), bronchiolitis, bronchopulmonary dysplasia and upper airways obstruction [9]. Heliox has not only been used in patients breathing spontaneously, but also in the settings of both invasive mechanical ventilation (ie with tracheal intubation) [10] and noninvasive mechanical ventilation (NIV) [11,12]. Nevertheless, the usefulness of heliox in the intensive care unit remains debatable.

The controversy regarding the use of heliox Upper airways obstruction
In upper airways obstruction, heliox appears to be a reasonable option in severe respiratory distress that results from upper airways obstruction [13]. The gas mixture may allow the intensivist to 'buy time' while awaiting treatment that is aimed at alleviating the obstruction. Of course, helium per se has no curative action [9,14], and Commentary Helium oxygen mixtures in the intensive care unit in a randomized protocol in children with moderate upper airways obstruction [15] there was no difference between heliox and racemic epinephrine (adrenaline) in alleviating symptoms.

Acute asthma crisis
There is controversy regarding the use of heliox in acute asthma crisis. In a meta-analysis of four randomized controlled trials performed in children and adults presenting at the emergency room with moderate to severe asthma [16], there was no evidence favouring the use of heliox. However, there are several studies (randomized protocols) that reported findings in favour of heliox in acute severe asthma [17][18][19], showing a more rapid decrease in pulsus paradoxus and in airflow obstruction with helium than with air oxygen mixtures in nonintubated patients. As in upper airways obstruction, there is no curative effect of heliox in asthma crisis, and in no circumstances should heliox replace aggressive bronchodilator and corticosteroid therapy [9].
Our present policy is that, in acute severe asthma in spontaneously breathing patients, heliox can be given after failure of the initial bronchodilator therapy. In so doing we aim to avoid intubation, which (as reported by many authors [20,21]) is a dangerous procedure, particularly in asthmatic persons. Heliox has also been used in intubated asthma patients, and an improvement in blood gases with a reduction in inflation pressure was observed [22]. From a practical standpoint, we usually restrict the use of heliox to patients who are difficult to ventilate, with high inflation pressures and haemodynamic instability.

Chronic obstructive pulmonary disease
Heliox has been used in both nonintubated and intubated COPD patients. NIV has become the standard initial treatment in the majority of such patients [23]. COPD patients in acute decompensation are at risk for respiratory failure precipitated by an increase in their WOB, leading to inspiratory muscle fatigue. NIV decreases the WOB in these patients [24]. Therefore, it was hypothesized that the WOB associated with NIV could be further reduced by combining that form of ventilation with heliox. Two studies have been reported [11,12] that support this hypothesis. However, the proof that NIV with heliox is superior to NIV with an air-oxygen mixture in avoiding intubation in COPD patients is nevertheless lacking. We are aware of two ongoing multicentre trials in Europe that are attempting to resolve this issue. Until the results of those studies are known, it seems wise to refrain from introducing heliox into routine practice because of the costs of the gas.
Heliox has also been used in COPD patients once intubated and mechanically ventilated, in order to reduce intrinsic positive end-expiratory pressure and hyperinflation, and to lessen the impact of high alveolar pressure on haemody-namic and respiratory mechanics [25]. By reducing the respiratory resistance, heliox could enhance expiratory flow rate, and it could help to reduce intrinsic positive end-expiratory pressure. This was recently demonstrated in intubated and acutely decompensated COPD patients [10]. Thus, heliox may be administered to mechanically ventilated COPD patients with high levels of dynamic hyperinflation and severe consequences on haemodynamics and high respiratory pressures. Use of heliox while weaning the patient from the respirator remains to be investigated.

Other uses
Other more anecdotal uses of heliox have been reported, including use in children with acute bronchiolitis [26] or bronchopulmonary dysplasia [27], during bronchoscopy [28], and even as a treatment for pneumothorax [29] and hyperammonaemia [30].

Technical and cost issues may discourage widespread use … for now
Several words should be added regarding the technical and the financial issues associated with the use of helium. In spontaneously breathing patients, heliox (generally pressurized as 78 : 22 helium:oxygen) should be administered with a nonrebreather face mask, and if necessary oxygen can be added through a nasal canula. The use of heliox with a ventilator raises various technical and safety issues (fractional inspiration of oxygen measurements, volume measurements, valve functioning, etc) that need to be resolved before we can begin to use this mixture of gas in ventilated patients [31,32].
Finally, even if helium is the second most abundant element in the universe, the extraction and processing costs on Earth are high. In Switzerland, a 60-l tank of 78 : 22 heliox pressurized at 200 Bars (10,000 l at atmospheric pressure) costs US$275. However, these high costs should be balanced against the benefits, such as improving patient outcome and reducing the duration of intensive care unit stay. Such issues should be addressed in further studies into heliox therapy.