A disturbance in the respiratory function is the outstanding clinical feature of heart disease. Respiration and circulation serve a dual purpose in supplying oxygen to and removing carbon dioxide from the tissues. When the circulation fails, respiration is stimulated as a means of compensation. By measuring the extent of this stimulation, the degree of circulatory failure may be estimated. The ordinary methods of measuring respiration are unsatisfactory; the volume of air breathed must be measured in relation to the oxygen absorbed or the carbon dioxide produced. It has been shown that determination of the concentration of carbon dioxide in the expired air is a simple and highly accurate means of measuring respiration.1 It is remarkably constant in large groups of subjects and shows little variation between the sexes or with wide differences in age.
Dyspnea is a common complaint in hyperthyroidism and in obesity, whether the heart is abnormal
BOYER PK, BAILEY CV. CONCENTRATION OF CARBON DIOXIDE IN EXPIRED AIR IN HEART DISEASE. Arch Intern Med (Chic). 1943;71(4):529-535. doi:10.1001/archinte.1943.00210040088009