Effects of helium and oxygen mixtures on pulmonary mechanics during airway constriction.

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Abstract
Patients with obstructive airway diseases differ with respect to the effects of low-density gases on pulmonary mechanics. Theoretically, this could result from differences in the anatomic locus of the obstruction. The effects of breathing He-O2 mixtures upon pulmonary mechanics were studied in dogs with airway constricted by various methods. When dogs breathed air, histamine injection, which constricts distal airways, was associated with increased nonelastic work of breathing and decreased pulmonary compliance. Similar changes occurred when histamine was injected while the dogs breathed 22% oxygen, 78% helium (helium air). With air breathing, mechanical obstruction of the lower trachea and vagal stimulation were both accompanied by increased nonelastic work. This effect was minimal or absent when the dogs breathed helium air. It is concluded that helium air reverses the effects on airway resistance of proximal airway constriction but has little influence upon that resulting from distal airway constriction. This conclusion is consistent with the laws governing gas flow in branching tubes.

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