The inhalation of helium to relieve obstructive dyspnea is based on the molecular lightness of this gas in comparison to introgen.1 A mixture of 80 per cent nitrogen and 20 per cent oxygen is three times as heavy as a mixture of 80 per cent helium and 20 per cent oxygen. According to Graham's law, the rate of diffusion of a gas is inversely proportional to the square root of its density. This law applies also to effusion, i. e., the passage of a gas through small orifices, which is especially applicable to the problem here discussed. Since the viscosity of helium is slightly greater than that of nitrogen, there is no reduction in frictional resistance, and Poiseuille's law does not become involved in an explanation of the results. The point involved is that the obstruction exists for a space so limited in extent as to obey the the law
JOHN D. KERNAN, ALVAN L. BARACH. RÔLE OF HELIUM IN CASES OF OBSTRUCTIVE LESIONS IN THE TRACHEA AND LARYNX. Arch Otolaryngol. 1937;26(4):419–447. doi:10.1001/archotol.1937.00650020461006
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