Relation of Respiratory Ventilation to Basal Metabolism in Thyroid Disorders

Abstract

An analysis of basal metabolism records made available to one of us (ATB) from the files of the United States Veterans'Facility, Fort Miley, California, shows a relation between respiratory ventilation and metabolism which we believe has not been reported previously. The coefficient of ventilation (CV), or volume percent removed as oxygen from the tidal volume, 1 has been used as a measure of respiratory activity. All data were obtained by one of us (ATB), using a conventional closed-circuit metabolism apparatus, with 16-minute recording periods.

Ventilation, due to the mechanism of control normally depending on the CO₂ content of the blood, makes available, under ordinary circumstances, a considerable surplus of oxygen over the metabolic needs. Thus at rest the average man uses only 4% of the tidal volume (CV = 4) or about one-fifth of the oxygen available. This factor of safety of 5 for oxygen may be increased to 20 by over-ventilation, that is, the CV may fall as low as one. The data contained 76 records from 37 persons diagnosed as suffering from various thyroid disorders. Combining all types the BMR ranged from -54% to 114% (Aub-DuBois standards). The CV for a group of 14 having “normal’ BMR (mean +0.857%) is 4.393, slightly above the 3.996 found for normal men. With either raised or lowered metabolism the value of the CV falls in a degree roughly proportionate to the severity of the metabolic disturbance. The means of the groups are given in Table I. The falling trend in either direction is clear but the variability is so great that the reliabilities must be considered. The probabilities (Fisher's t method), expressed as chances per hundred, that a difference as great or greater than that recorded would arise by random sampling from a population in which no difference existed are given in Table II. The average CV of persons with normal BMR is doubtfully above the mean of the group with lowered metabolism (P = 10 times in 100). The normals are not significantly above the group having the next higher BMR but are significantly higher than the second group (1.5 in 100). It is thus clear that the fall of CV with raised metabolism is statistically significant, and that with lowered BMR, suggestive.