The Advanced Ventilator Book by Owens William

Author:Owens, William [Owens, William]
Language: eng
Format: epub
Published: 2017-04-01T16:00:00+00:00

Heliox

Heliox refers to a blend of helium and oxygen, usually in a 70:30 ratio. A gas blender can be used to change this ratio to 60:40. When the fraction of oxygen exceeds 40%, the potential benefit of Heliox is lost. Therefore, Heliox should only be used when the patient can be adequately oxygenated with an FiO 2 of 40% or less.

The benefit of Heliox is with the helium having a much lower density than nitrogen gas. This translates into Heliox improving the tendency of inspired gas toward laminar flow, which improves gas flow through narrowed proximal and larger airways. More laminar and less turbulent gas flow in the conducting airways leads to better gas exchange and aerosol delivery of medications like albuterol to the respiratory and terminal bronchioles. This can also help reduce the work of breathing. Heliox can be delivered by facemask, through noninvasive positive pressure breathing, or via the mechanical ventilator.

Two issues must be addressed when using Heliox through the ventilator. The first is the mode of ventilation. Most newer ventilators are not calibrated for a mixture of helium and oxygen. The inspiratory valve may permit a larger volume of gas to be delivered than what is set in volume-control mode, and some ventilators will not register an accurate exhaled tidal volume. For this reason, pressure-control ventilation is preferable. The inspiratory pressure should be set at a level sufficient to attain rise of the chest and provide acceptable (if not normal) alveolar ventilation. In order to avoid barotrauma, keeping the inspiratory pressure less than 30 cm H 2 O is advised, if possible. If volume-control ventilation is desired, then the clinician should either use a conversion table for the particular ventilator being used 21 to estimate the true tidal volume, or else periodically measure the exhaled tidal volume at the level of the endotracheal tube with a density-independent pneumotachograph. 22 Using pressure-control ventilation is easier.

The second issue is the FiO 2 . Most ventilators have two gas inlets—one for 100% oxygen, and one for air (21% oxygen). The gas blender will mix the two to attain the FiO 2 selected on the ventilator. Administration of helium through the air inlet can result in a different FiO 2 actually delivered to the patient. In the case of pure helium going through the air inlet, the delivered FiO 2 may be less than what's selected (normally the air inlet has 21% oxygen, and with pure helium there's 0% oxygen). More commonly, a premixed Heliox tank is connected to the air inlet. If the Heliox is, say, 70% helium and 30% oxygen, and the FiO 2 on the vent is set at 30% oxygen, then the patient will end up receiving a higher FiO 2 than the selected 30% (due to mixture of the gases). This may reduce the efficacy of the Heliox, especially if the true FiO 2 exceeds 40%. It can also increase the volume of gas delivered to the patient and increase the airway pressures. To make this even more complicated, different ventilators use different mixing valves and blenders.

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