Various chemical processes are occurring within the tissues of the body at all times. These processes occur with any muscular activity and with all the vital body functions.
With these chemical reactions heat is produced, and energy and oxygen are used. Metabolism is the term employed to collectively embrace all these chemical processes. The rate at which metabolism goes on depends on many things. In health it varies with the age, size, and sex of the individual, activity, emotional state, diet, climate, drugs, and many other influences. Therefore, if the metabolic rate is to be measured in cases of suspected disease, the conditions must be standardized so that only the metabolic processes occurring without activity or other influences are measured. Metabolic rates are taken under these basal conditions, and thus the term basal metabolic rate, or B.M.R. Under these conditions only the metabolism occurring involuntarily without activity is measured; there is no muscular action at the time of the test except that of the heart, intestines, and breathing muscles. All that is measured is the rate of chemical processes which work constantly, or the basal metabolism. Since oxygen is consumed by all these chemical reactions, the oxygen consumption furnishes the basis for measurement of the B.M.R. The B.M.R. machine is a device which measures the oxygen a patient breathes in, and plots a graph of the amount consumed over a period of time.
If your doctor orders a B.M.R., you will be given specific instructions. If they are not carried out accurately, the results of the test will not be reliable. The test is usually done before breakfast in the morning. You must have had a good night’s sleep. No drugs should be taken for at least 12 hours before the test. You will not be allowed to smoke, eat, or drink after midnight until after the test on the next morning. Other specific instructions in preparation for the test will be given.
The B.M.R. is done with the patient lying down. The oxygen mask is placed over the mouth and nose and the technician sets the machine to record your oxygen consumption. Never fear that you will not be able to breathe with the mask on your face, because you are breathing pure oxygen (air is about 20% oxygen). This is all there is to the B.M.R. determination. The graph the machine records is then interpreted by the technician and the report given to your doctor. The B.M.R. is below normal in suppressed activity of the thyroid and other internal glands, starvation and malnutrition, and after sedatives. It is elevated with excess thyroid gland secretion, any disease with accompanying fever, anxiety states, some diseases of the blood, and occasionally in heart and kidney disease.