Respiratory Quotient

RESPIRATORY QUOTIENT, the volumetric ratio of carbon dioxide eliminated by an organism to the oxygen absorbed by it in the same interval of time, or in other words, the ratio (volumetric or weight) of the oxygen contained in the exhaled CO₂ to the absorbed O₂. Carbon dioxide, as is known (see Dissimilation), is one of the final products of the destruction of organic substances in the body, and this destruction is carried out through a series of chemical reactions in which O₂ is an obligatory component; therefore, a quantitative relationship must exist between the absorbed O₂ and the eliminated CO₂, which is expressed in the respiratory quotient. The very term respiratory coefficient, or respiratory quotient, was introduced by Pflüger; however, Lavoisier had already noticed that the volume of O₂ absorbed during respiration is greater than the volume of exhaled CO₂, and found an explanation for this in the fact that part of the inhaled O₂ is used for the oxidation of H₂ into water and does not participate in the formation of CO₂. Fifty years later, the first precise experiments conducted by Regnault and Reiset yielded numerical values for the respiratory quotient, and it was discovered that variations in the respiratory quotient do not depend on the species of animal, but are related to the nature of the food, with the authors observing fluctuations from 0.64 to 1.02 in the same animal. Subsequent researchers, confirming Regnault's data, established that the normal limits of variation of the respiratory quotient indeed lie within the range of 0.7 to 1. The cause of the variations in the respiratory quotient is related to the nature of the substances broken down in the organism; their varying O₂ content affects the magnitude of the consumption of the latter from the atmospheric air. Thus, carbohydrates upon their conversion to CO₂ and H₂O in the process of dissimilation require external O₂ only for the oxidation of C, since H and O in their molecule are in a ratio that precisely determines the formation of water (i.e., 1 : 8). For every atom of its C, a carbohydrate requires one molecule of O₂, with one molecule of CO₂ being produced (e.g., C₆H₁₂O₆ + 6 O₂ = 6 CO₂ + 6 H₂O). According to Avogadro-Gerhardt's law, equal quantities of molecules occupy equal volumes, from which it follows that the volumetric ratio of CO₂ to O₂ upon the combustion of carbohydrates must be equal to 1. In fats, such a ratio cannot be equal to 1, because there is very little O₂ in their molecule, and external O₂ during the combustion of fats must be distributed between the atoms of C and H, such that the resulting CO₂ will not contain all the O₂ that went into combustion, but only a fraction of it; thus, the ratio must be less than 1. From the formula 2C₅₁H₉₈O₆ + 145 O₂ = 102 CO₂ + 98 H₂O (tripalmitin), it is seen that 145 volumes of O₂ absorbed during the reaction correspond to 102 volumes of formed CO₂, so that the ratio, i.e., the respiratory quotient = 102 / 145 = 0.703. It is more difficult to calculate the respiratory quotient of proteins, since the products of their breakdown in the body are not only CO₂ and H₂O, but also a number of organic substances passing into the urine, and furthermore, the elementary composition of protein bodies is known to us only approximately. As an example, the following calculation can be given: 100 g of fat-free intestinal protein contain C 52.38 g, H 7.27 g, O₂ 22.68 g, N 16.66 g, S 1.02 g. Passing into urine: 9.406 (C), 2.663 (H), 14.099 (O), 16.28 (N), 1.02 (S). Passing into feces: 1.471 (C), 0.212 (H), 0.889 (O), 0.38 (N). Residue: 41.5 (C), 4.4 (H), 7.69 (O). If this residue burns to the end, then 41.5 g of C and 4.4 g of H will require 145.87 g of O₂; subtracting from this 7.69 g of O₂ already present in the composition of the burning protein residue, we obtain a requirement of 138.18 g, or 96.63 liters of O₂; at the same time, 152.17 g, or 77.39 liters of CO₂ will be formed; consequently, for proteins we have the respiratory quotient: 77.39 / 96.63 = 0.801. Under certain conditions, the respiratory quotient can be greater than 1 and less than 0.7. Thus, during excessive carbohydrate nutrition, the organism converts them into fats, and since the latter are considerably poorer in oxygen than carbohydrates, a certain amount of O₂ is liberated during this conversion, passing into the general disposal of the organism, as a result of which relatively less O₂ is taken from the outside than would be necessary to obtain the CO₂ produced at that time. Therefore, in the fraction, the denominator will be reduced, i.e., the fraction CO₂ / O₂ will increase. The indicated metamorphosis of carbohydrates can take place only in the case of their abundance in the organism, when they are also the principal combustion material, as a result of which the respiratory quotient must be close to 1; and the parallel-running conversion of carbohydrates into fat, deposited in the body, and the associated reduction of O₂ absorbed from the air quickly make the fraction CO₂ / O₂ greater than 1. Thus, for example, Bleibtreu obtained a respiratory quotient of up to 1.38 when fattening geese. Conversely, the conversion of fats into carbohydrates with the deposition of the latter in the body requires significant absorption of O₂ without causing changes in CO₂ production. This must be reflected in the ratio CO₂ / O₂ in the sense of its decrease (the denominator increases). If a similar process occurs in the body on a significant scale, the respiratory quotient can reach low values. Such a phenomenon occurs during starvation, and in certain pathological conditions (diabetes), when in addition a number of under-oxidized products are produced (acetone bodies, etc.), for the formation of which nevertheless a considerable amount of O₂ is expended, which in turn increases the denominator of the fraction CO₂ / O₂, causing the respiratory quotient to drop to 0.6, and sometimes even lower. It goes without saying that proteins, fats, and carbohydrates undergo combustion in the organism at the same time in various combinations; therefore, the volumes of respiratory gases obtained in respiration experiments yield a total respiratory quotient, the magnitude of which lies within the indicated limits. In prolonged respiration experiments, when it is easy to determine the amount of protein destroyed in the body from the urine nitrogen (N of urine × 6.25 = protein), one can calculate what amount of O₂ went to protein and how much CO₂ could be formed from it, since it is known that 1 g of protein requires 0.96 liters of O₂ for its oxidation and produces 0.78 liters of CO₂. By subtracting from the volumes of total O₂ and total CO₂ the volumes of these gases associated with the destroyed protein [when gas volumes are compared among themselves, they are always taken under identical conditions of temperature and pressure (0° and 760 mm Hg)], one obtains the volumes of CO₂ and O₂ relating exclusively to nitrogen-free substances destroyed in the body during the experiment. The ratio of such volumes of CO₂ and O₂ is called the "true respiratory quotient" (or protein-free respiratory quotient). From the value of the true respiratory quotient, it is easy to evaluate the caloric value of each absorbed liter of O₂, proceeding from the fact that at a true respiratory quotient = 0.707, when the combustible nitrogen-free material is represented solely by fat, 1 liter of O₂ accounts for 0.495 g of it, while 4.686 calories (= 0.495 × 9.461) are released (heat of combustion of fat); whereas when the true respiratory quotient = 1 (combustible nitrogen-free material—carbohydrates), 1 liter of O₂ accounts for 1.207 g of carbohydrate, which is associated with the production of 5.047 calories. By means of interpolation, the following table is compiled: Respiratory quotient | Grams of fat per 1 L O₂ | Grams of carbohydrate per 1 L O₂ | Calories. 0.707: 0.495, 0.000, 4.686. 0.75: 0.425, 0.170, 4.739. 0.80: 0.345, 0.367, 4.801. 0.85: 0.262, 0.569, 4.863. 0.90: 0.176, 0.777, 4.924. 0.95: 0.089, 0.989, 4.985. 1.00: 0.000, 1.207, 5.047. Often, especially in short-term experiments, such a table (or a corresponding diagram) is used, proceeding from the total respiratory quotient and neglecting the inevitable error in this case, which is of course incorrect, but permissible in approximate calculations, especially in comparative experiments. As is evident from all of the above, great importance is attached to the respiratory quotient in characterizing metabolism, but one must always remember that the respiratory quotient is as it were a certain "summary" magnitude resulting from various processes—different degrees of oxidation of organic materials and parallel-running reductions (taking place, for example, during the conversion of carbohydrates into fats). In short-term experiments, the respiratory quotient does not precisely correspond to the processes occurring in the organism, because in these cases pulmonary ventilation acquires a relatively greater influence; its increase (with a voluntary or involuntary increase in the frequency and depth of respiratory movements) causes a decrease in the tension of CO₂ in the lung air, and as a result, CO₂, weakly chemically bound in the blood, tissues, and body fluids, passes in greater quantities into the exhaled air, which leads to an increase in the ratio CO₂ / O₂, since the absorption of O₂ remains at the previous level in this case. After pulmonary ventilation returns to normal, the tension of CO₂ in the alveolar air rises; from this, CO₂ is retained in the blood and tissues in accordance with the new equilibrium of tensions of this gas; then the respiratory quotient decreases due to a decrease in the numerator of the fraction CO₂ / O₂.

V. Lavrov. Clinical study of the respiratory quotient. The fluctuations of the respiratory quotient are of extremely great importance not only from a physiological point of view, but also purely clinically. Reflecting under certain conditions to some extent the state of intermediary metabolism, the respiratory quotient makes it possible to approach more closely the understanding of the metabolic status of both a healthy and a sick organism. While considerable place has been allotted to the study of basal metabolism in all sorts of pathological conditions, sufficient attention has not yet been paid to the study of changes in the respiratory quotient. The changes in the respiratory quotient in diseases of metabolism and endocrine glands have been studied the most. At the same time, the study of the fluctuations in the value of the respiratory quotient has not only made it possible to understand somewhat more deeply the essence of intermediary metabolism in a number of diseases, but has also provided the clinician with data for making a differential diagnosis, for checking the therapy performed, and in some cases for prognosis. In phenomena of chronic exhaustion, the metabolism in patients changes sharply. Their carbohydrate reserves are more and more depleted and replenished by carbohydrates originating from fats. At the same time, the respiratory quotient gradually decreases. When the respiratory quotient drops below 0.76, a moment arrives when, according to Shaffer's data, the balance in the formation of ketogenic and antiketogenic substances is disturbed—a moment when less than one molecule of glucose accounts for the oxidation of each molecule of fatty acids. Such a drop in the respiratory quotient corresponds to the onset of ketonemia, prolonged in diabetics and starving individuals and transient in healthy individuals. In cases where the drop in nutrition is a transient phenomenon, with the beginning of the restoration of weight loss, a sharp rise in the respiratory quotient begins, which reaches a value slightly exceeding 1 (1.02-1.04), which indicates an energetic process of carbohydrate restoration, accompanied by the deposition of fats. The aforesaid makes understandable the high respiratory quotient frequently found in obesity. The study of the increase in the value of the respiratory quotient during insulin fattening (Mast-kur, according to Falta) serves as a good check for this method of treatment. It should be noted that such a pronounced disease of metabolism as gout proceeds with a respiratory quotient corresponding to the average figures of the norm (0.90-0.87). This is completely understandable, because in this disease protein metabolism suffers, while carbohydrate and fat metabolism, which affect the value of the respiratory quotient most of all, do not suffer. Among diseases of the endocrine glands, the most drastic changes in the respiratory quotient occur in diabetes mellitus. This disease, proceeding with a sharp disturbance of carbohydrate and fat metabolism, gives not only the lowest figures of the norm for the respiratory quotient, but the value of the respiratory quotient drops even below these figures (down to 0.58 after food intake), since in these cases vital processes are maintained mainly by the burning of fats; the more severe the case of diabetes, the lower the respiratory quotient. In some cases of far-advanced diabetes accompanied by a sharp drop in nutrition, after several days of starvation and subsequent introduction of carbohydrates, a significant increase in the respiratory quotient is observed, which, as Richardson and Ladd showed, was accompanied by the assimilation of part of the introduced carbohydrates. Of interest is Richardson's observation on changes in the respiratory quotient under the influence of a carbohydrate load, making it possible to differentiate various types of glycosuria. While diabetics lower their respiratory quotient under a sugar load, patients with Basedow's disease and renal glycosuria, although responding to the load with glycosuria, nevertheless have their respiratory quotient increase, i.e., their ability to oxidize carbohydrates is not impaired. In thyroid diseases, the respiratory quotient stays within normal limits, thereby indicating that the burning of food substances in these cases proceeds in almost the same quantitative ratios as in healthy ones. But while in hypothyroidism the respiratory quotient approaches the upper limits of the norm, in hyperthyroidism, Basedow's disease, and upon the administration of thyroxine, the respiratory quotient either reaches the lower limits or drops even below normal. The decrease in the respiratory quotient should be attributed to a decrease in carbohydrate reserves, rather than anomalies of their oxidation. With a deterioration of the condition, the respiratory quotient in Basedow's disease sometimes drops even below normal (burning of fats). It should be noted that in response to a protein load in all types of thyroid diseases, a normal rise in the respiratory quotient is also obtained. Improvement in the course of both Basedow's disease and myxedema is accompanied by the approach of the respiratory quotient value to the average figures of the norm (0.90-0.87). Unfortunately, the respiratory quotient in cases of adrenal diseases has not been investigated. On the other hand, there are numerous observations on changes in the respiratory quotient under the influence of the administration of adrenaline. As might be expected, the respiratory quotient initially increases and then drops below the initial value, which indicates the nature of the substances involved in oxidative processes; namely, carbohydrates burn first, and then fats. It is quite obvious that the magnitude of the increase in the respiratory quotient under the influence of adrenaline depends on the nutritional state of the subject. Pituitary diseases proceed without changes in the value of the respiratory quotient. However, the combustion processes under a protein load in disease of this gland proceed by the type of exothermic reactions, i.e., the respiratory quotient drops below the abscissa. Injections of the anterior lobe hormone of the gland increase the respiratory quotient. Diseases of the sex glands also do not affect the respiratory quotient. To a protein load, cases with congenital insufficiency respond with a drop in the respiratory quotient, and with acquired insufficiency—with its increase. The influence of the remaining endocrine glands on the value of the respiratory quotient is unclear. While blood diseases (leukemia, pernicious anemia, polycythemia) significantly affect basal metabolism, the respiratory quotient in these diseases does not deviate from the average figures of the norm. Its approach to the lower figures of the norm is observed only with the onset of exhaustion. In kidney patients, despite severe disorders of intermediary metabolism, the respiratory quotient does not differ from the average figures of the norm. The extremely low respiratory quotients observed by Kraus and Grafe in cardiac patients turned out to be erroneous, and upon studying metabolism in a calorimeter, the respiratory quotient of cardiac patients is completely normal. In tuberculosis patients, a strong increase in the respiratory quotient was noted under the influence of a carbohydrate load; at the same time, the specific dynamic action is also sharply expressed, exceeding several times the specific dynamic action of proteins, while fats have almost no effect on oxidative processes. Mac Cannon saw in this an experimental confirmation of the necessity of a fat diet in tuberculosis patients. During febrile diseases (typhoid fever, pneumonia), the respiratory quotient sometimes reaches the lowest figures of the norm. During the recovery period after typhoid fever, the respiratory quotient rises above 1.02, which indicates an energetic restoration of the body's fat reserve. Already by the end of the temperature period, by strengthening nutrition, the respiratory quotient can be raised. Those recovering from pneumonia and other infectious diseases do not give such sharp rises. It is clear that fluctuations in the respiratory quotient in infectious diseases depend on the nutritional state of the body and on the degree of intoxication.

S. Zhislina.