 The untoward effects of carbon dioxide accumulation on vital physiologic functions are well known. The following experiments were designed to investigate the effects of an amine buffer of low toxicity on hypercapnic acidosis developing in a dog maintained under conditions of apnic oxygenation. Anesthesia of the animal is designed so that a minimum of barbiturate is required. Atropine, 4-tenths of a milligram, is first administered. Through the intravenous needle, 10 milligrams per kilogram of pentothal are administered to induce anesthesia. During the next 20 minutes, the animal will receive another 10 milligrams per kilogram. After that time, no more pentothal will be given. The dog is immediately placed on his back and the vocal cords are sprayed with 1% xylokane in order to allow intubation of the trachea under light anesthesia. A cuffed endotracheal tube is inserted into the trachea. Attention will now be given to the ventilatory circuit. The oxygen flows from the tank through the adjustable pressure limiting valve to the ventilator. During apnea, the ventilator is stopped, the breathing tubes are clamped. The three-way stopcock is turned so that the circuit bypasses the ventilator. The spirometer becomes the source of oxygen. The low resistance to flow of this system is indicated by the fact that the spirometer bell immediately begins to fall when the endotracheal tube is open. The animal is then placed on the operating table and the endotracheal tube connected with the ventilator. Complete relaxation of the dog is obtained by intravenous administration of divided doses of succinyl choline, one half of a milligram per kilogram as needed. Here we see the deep, regular respirations produced by ventilation of the paralyzed animal. The timer is turned to zero, started, and ventilation with 100% oxygen will continue for one hour. During that time, all unnecessary preparations will have been completed. We shall next consider the process of denitrogenation, which is the preliminary maneuver to the production of ethnic oxygenation. Denitrogenation is the process whereby nitrogen is removed from the lungs and tissues and replaced by oxygen. This process is achieved by using non-rebreathing valves on the inspiratory and expiratory sides of the ventilator, which is fed with 100% oxygen. As a result, with each expiration, some portion of the nitrogen present in the lungs is exhaled and not rebreathed. After one hour, almost all of the nitrogen in the lungs and tissues will have been removed and replaced by oxygen. Now the preparation is nearly completed. A puncture into the cisterna magna has been made and the needle is connected to the strain gauge by a plastic catheter. Just before the onset of apnea, here, after 58 minutes of ventilation, the bladder is catheterized and emptied. This is a schematic presentation of the completed preparation, which will be described to orient the viewer in the laboratory. During the period of denitrogenation, the dog is ventilated with 100% oxygen by the ventilator. During the period of apnea, the dog absorbs oxygen from a spirometer by means of the three-way stopcock connected to the endotracheal tube. A femoral vein is cannulated for administration of an infusion of saline in the controlled dog and of bufferamine in the treated animal. During denitrogenation and apnea, cerebrospinal fluid is measured through the needle placed in the cisterna magna. The electrocardiogram is recorded through leads as indicated and arterial pressure is measured through a catheter placed in the femoral artery. The three-way stopcock in the arterial line permits samples to be taken for pH, PCO2, carbon dioxide content, catecholamines, and oxygen saturation. Introthoracic venous pressure is measured through a plastic catheter inserted through the femoral vein and passed up into the chest. All pressure measurements are made by pressure transducers and recorded with the electrocardiogram on a four-channel recorder. The urethral catheter leads to a graduated flask for measurement of urine excreted. On this control record, the baselines indicate the point of zero pressure. From right to left are recorded mean arterial blood pressure, electrocardiogram, central venous pressure with typical respiratory waves, and cerebrospinal fluid pressure. On the record, taken immediately after the onset of apnea, the influence of thoracic movements is no longer visible. Seven minutes after stopping the ventilator, there are no respiratory efforts or movements on the part of the dog. The source of oxygen for the animal is now the spirometer, and the circuit for this oxygen flow is indicated. From the bell through the stopcock, which has been repositioned and into the endotracheal tube, completely bypassing the ventilator. The animal is now being kept alive through the process of apnic oxygenation. We will now explain this process in the following diagrams. We shall represent total lung volume by a single alveolus containing 2,000 milliliters. At the end of denitrogenation and at the start of apnea, only oxygen, carbon dioxide, and water vapor are present in the lung in the following amounts. Oxygen, 1,770 milliliters. PO2, 673 millimetres. Carbon dioxide, 106 milliliters. PCO2, 40 millimetres. Water vapor, 124 milliliters. And PH2O, 47 millimetres of mercury. We shall assume that blood flows around this alveolus in discrete quantities of 1,000 milliliters every 30 seconds. The first 1,000 milliliters of mixed venous blood entering the lung contain 150 milliliters of oxygen with a partial pressure of 40 milliliters of mercury and 440 milliliters of carbon dioxide with a partial pressure of 46 milliliters of mercury. As mixed venous blood flows through the alveolar capillary, 40 milliliters of carbon dioxide diffuse from the blood into the alveolus and 50 milliliters of oxygen diffuse from the alveolus into the blood indicating a respiratory exchange ratio of 0.8. When this process is complete, the arterial blood leaving the lung will contain 200 milliliters of oxygen at a partial pressure of 673 milliliters and 400 milliliters of carbon dioxide with a partial pressure of 40 milliliters of mercury. The net effect of this gas transfer is to decrease the lung volume by 10 milliliters and lower the total gas pressure from 760 to 756 milliliters of mercury, which is 4 milliliters of mercury less than the pressure of oxygen in the spirometer. Under the effect of this pressure gradient, a mass flow of oxygen will proceed from the spirometer into the lung so that total gas pressure in the lung is restored to 760 milliliters of mercury and total lung volume to 2,000 milliliters. 30 seconds after the onset of apnea, the second 1,000 milliliters of mixed venous blood enters the lung. However, in this case, only oxygen will diffuse into the blood. No transfer of carbon dioxide between mixed venous blood and the alveolus will occur because the partial pressure of carbon dioxide is the same in both. As a result, the blood leaving the lung will be fully oxygenated, but its carbon dioxide content will be 440 milliliters the same as venous blood. The transfer of 50 milliliters of oxygen from the lung into the blood will decrease the lung volume by the same amount and lower total pressure by 19 milliliters from 760 to 741 milliliters of mercury. Under the effect of this pressure gradient, a mass flow of oxygen will proceed from the spirometer into the lung so that the total gas pressure in the lung is restored to 760 milliliters of mercury and the volume to 2,000 milliliters. It is now apparent that apnea following denitrogenation will provide sufficient oxygen for saturation of reduced hemoglobin so long as a source of oxygen is available through a patent airway. However, apnic oxygenation does not permit removal of carbon dioxide. The mass flow of oxygen into the lung prevents the transport of measurable amounts of carbon dioxide from the lung to the spirometer. Furthermore, accumulation of carbon dioxide in the lungs proceeds slowly, PCO2 increasing by 5 to 6 millimeters of mercury per minute while the respiratory exchange ratio tends toward zero. The slow accumulation of carbon dioxide in the lungs is accounted for by its large solubility which permits it to accumulate in body fluids, tissues, and lungs nearly simultaneously. These conditions permit the study of progressive hypercapnic acidosis in the dog. It will be demonstrated in the following scenes that acidosis is the limiting factor in survival of the animal. After 15 minutes of apnea, arterial, venous, and cerebrospinal fluid pressures have all risen. The cyclic variations of the tracings are due to the inherent discharges of the vasomotor and respiratory centers. Radicardia is present. During the whole period of apnea, isotonic saline is infused intravenously at the rate of 1 milliliter per kilogram per minute. After 15 minutes of this infusion, there is a persisting anuria. After 30 minutes of apnic oxygenation, spinal fluid pressure has increased 300 percent so that the pen is off the scale and a change in attenuation is required. Mean arterial blood pressure has risen 70 percent, pulse rate has increased, and central venous pressure has fallen back to its control level. After 45 minutes of apnic oxygenation, arterial pressure has fallen below control level. There is marked tachycardia. Spinal fluid pressure has also decreased, but it is still twice above control. The tongue is hypoxic. At the end of 60 minutes of apnea, despite the volume of fluid infused, anuria prevails. The 60-minute record contrasts the pronounced fall of mean arterial pressure with the still elevated cerebrospinal fluid pressure. Marked arterial oxygen desaturation is reflected in the color of the tongue. Within two more minutes, complete cardiovascular collapse has occurred with only residual electrical activity of the heart being recorded. There is no pulse pressure measurable at this time. At 64 minutes, minimal electrical manifestations of agonal cardiac activity are the only tracings recorded. These blood samples were drawn at 15-minute intervals during apnea. The arterial oxygen desaturation can be accounted for by the Bohr effect, cardiovascular collapse, and pulmonary adolescences, which develops when the lungs are not ventilated. The last half of this presentation shows a second dog undergoing a similar one-hour period of ethnic oxygenation. This dog, however, was treated with an intravenous infusion of 2-amino, 2-hydroxymethyl, 1-3-propane diol, or trishydroxymethylamino-methane, or T-H-A-M. A compound of low toxicity will buffer carbon dioxide as shown in the following reaction. The second animal is presented here after completion of all preparations. The baselines indicate zero pressures and are followed by the control recordings at the end of denitrogenation. These values are similar to those recorded in the first animal. The disappearance of the respiratory waves on the record indicate the transition from mechanical ventilation to apnea. At the onset of apnea, a 3-tenth molar infusion of tham is administered to the animal at the rate of 1 milliliter per kilogram per minute. After 40 minutes of this infusion, 180 milliliters of urine has been excreted. This matches fluid input. Oxygenation of the tongue is fully maintained. The recording made at that time does not differ from the control which is presented for comparison. The rate of infusion has remained constant throughout the 45 minutes of apnea. And during that time, 200 milliliters of urine have been produced. There are no significant changes in the records. After 57 minutes of apnea, as contrasted with the untreated animal, the tongue is well oxygenated. The animal has now excreted 400 milliliters of urine. This record was taken after 60 minutes of apnea. Here, 20 minutes after the cessation of apnea, the animal is attempting to resume spontaneous respirations and limb movements. The record taken at this time shows two spontaneous breaths and also indicates that arterial blood pressure, electrocardiogram, central venous and spinal fluid pressures are the same as control values. Comparison with the control record clearly indicates this. Comparison of blood samples, drawn every 15 minutes, demonstrates maintenance of normal arterial oxygenation in spite of some degree of long adolescence which develops during prolonged apnea. Here, the dog is shown in full recovery. There have been no apparent sequelae of the experience. She is alert and healthy. The following charts contrast humoral changes occurring in these two experiments. In the untreated animal, arterial blood pH fell during one hour from 7.41 to 6.42. In the treated animal, it did not change significantly. Concomitant with the increase in hydrogen ion concentration observed in the untreated animal, there was a considerable rise in plasma epinephrine which increased from 1 to 38 micrograms per liter while norepinephrine rose from 0.5 to 22 micrograms per liter. This rise in plasma catecholamines did not prevent a cardiovascular collapse from occurring when pH fell below 6.80. In the treated animal, plasma epinephrine and norepinephrine levels did not change significantly. After 60 minutes of apnea, total plasma CO2 increased from 21 to 41 millimoles per liter in the untreated animal. The ratio of bicarbonate to carbonic acid falling from 20 to 3. In the treated dog, there was after 60 minutes of apnea a still greater increase in plasma CO2 which rose to 49 millimoles per liter. However, in this case, the ratio of bicarbonate to carbonic acid remained normal in spite of the higher CO2 accumulation. The lesser CO2 production by the untreated animal may be accounted for by the depression of metabolic function caused by acidosis. The partial pressure of carbon dioxide rose from 38 to 346 millimetres of mercury in the untreated animal. In the treated one, it rose from 32 to 98 millimetres of mercury, which is still a considerable rise, two and a half times above normal, but it took place in the presence of a normal pH. The results shown on the next chart indicate that FAM will neutralize the untoured effects of carbon dioxide retention in two ways. One, the compound's buffering capacity maintains pH within the normal range. Two, 25% of the total amount of carbon dioxide produced during apnea will be excreted in the urine. This presentation has demonstrated that carbon dioxide accumulation during apneic oxygenation will be well tolerated when the two fractions, bicarbonate and carbonic acid, are kept in suitable proportions to maintain the acid-based balance of the blood within normal limits and preserve the neutrality of the internal environment.