H. H. Bendixen

Ventilation and Oxygen Requirements during Prolonged Artificial Ventilation in Patients with Respiratory Failure

IN a person with normal lungs the amount of ventilation that is sufficient to eliminate the carbon dioxide produced (and thus to maintain carbon dioxide homeostasis) is predictable with considerabl...

ATELECTASIS AND SHUNTING DURING SPONTANEOUS VENTILATION IN ANESTHETIZED PATIENTS.

The hypothesis is tested that progressive atelectasis. with shunting of venous blood into the arterial bloodstream, may occur with ventilation which is normal by the usual criteria, but lacking in periodic deep breaths capable of reinflating collapsed airspaces. A previous study found such shunting to occur in anesthetized patients, ventilated by mechanical respirators; in the present study 25 patients were anesthetized with ether and oxygen and breathed spontaneously for an average period of 130 minutes. At the end of this period of spontaneous ventilation the average arterial oxygen tension was 402 mm. of mercury. Following a period of three to five minutes of controlled ventilation, using large tidal volumes, the average arterial oxygen tension rose to 553 mm. of mercury. The greatest fall in arterial oxygen tension occurred in the patients whose spontaneous tidal volumes were the most shallow. It is concluded that spontaneous ventilation in anesthetized patients, even when adequate in terms of carbon dioxide elimination, should be supplemented with periodic passive hyperinflations.

Influence of Respiratory Acidosis on Circulatory Effect of Epinephrine in Dogs

The circulatory depressant effects of respiratory acidosis and the relation between acidosis and the circulatory effects of epinephrine were studied in anesthetized intact dogs after sympathetic and parasympathetic block. Respiratory acidosis invariably depressed myocardial contractile force, cardiac rate, and mean arterial pressure, but only the fall, in contractile force showed a relation with the degree of acidosis. The effect of epinephrine on myocardial contractile force decreased with decreasing arterial pH, but no correlation was found between pH and the effect of epinephrine on arterial blood pressure and on cardiac rate. During continuous infusion of epinephrine, arterial pressure continued to rise after the contractile force had reached its maximum. With repeated periods of epinephrine infusion in the same animal, the response of the myocardial contractile force declined, while the response of the arterial blood pressure was unchanged. Such “differential tachyphylaxis” may occur under clinical conditions with prolonged infusions of vasopressor agents.

Arterial Oxygen Tension Measurements During Nitrous Oxide-Oxygen Anesthesia

Arterial oxygen tensions were determined while commonly used mixtures of nitrous oxide and oxygen were administered. These mixtures were administered in random order to 60 patients without clinical evidence of cardiac or pulmonary disease. The patients were divided into 3 equal groups according to whether they were undergoing upper abdominal, lower abdominal, or peripheral operations. Intermittent positive pressure ventilation was provided by means of a respirator. Each ratio of N2O:O2 was administered for 10 minutes, and at the end of this period an arterial blood sample was withdrawn. Arterial oxygen tension, arterial carbon dioxide tension and arterial PH were determined. All patients had arterial carbon dioxide tensions of less than 40 mm. Hg. Arterial oxygen tensions were found lower than expected in almost all patients. Because suboptimal oxygenation occurred so frequently when N2O in excess of 66 per cent was administered to patients undergoing intra-abdominal operations, it is strongly recommended that during surgery of this type anesthetic mixtures contain at least 33 per cent oxygen.

Arterial Oxygenation During Hypothermia

The aims of this stady were: to investigate a possible significant relation between the size of the tidal volume during constant volume ventilation and the rate of change of the alveolar-arterial oxygen tension gradient (AaDO2): to determine whether hypothermia significantly affects this relationship; and to test the accuracy of correction factors previously proposed for the effect of temperature on the oxygen tension of fully saturated blood. Ten patients were surface cooled to 30° C. for over 24 hours. Anesthesia consisted of pentobarbital sodium and d-tubocurarine. Constant volume pulmonary ventilation with 100 per cent oxygen was provided by a piston pump. A significant relationship (P < 0.01) was found between the size of the constant tidal volume and the rate of increase or decrease of the AaDO2. This relation showed little evidence of being affected by hypothermia per se. The accuracy of previously determined temperature correction factors for oxygen tension was confirmed.

Differential Effects of Catecholamines on the Distribution of Aortic Blood Flow

During continuous intravenous infusion of isoproterenol, epinephrine, and norepinephrine, in the intact dog under barbiturate anesthesia, mean aortic blood flow was measured at the aortic root (RAF) and in the descending (thoracic) aorta (TAF). With graded doses of isoproterenol, RAF increased markedly while TAF increased more moderately and then decreased at higher doses. With infusion of epinephrine the dose—effect curves of RAF and TAF were more nearly parallel, rising with lower doses and declining at higher infusion rates. With infusion of norepinephrine there was little change in either flow at any dose. During infusion of isoproterenol and norepinephrine there were definite changes in the distribution of flow: with isoproterenol the greater portion of flow was shifted toward the head and forelimbs, with norepinephrine toward the viscera and lower limbs (despite the small variation in RAF and TAF with norepinephrine). Epinephrine infusion induced a slight redistribution in the same direction as that produced by isoproterenol. Partial or complete autonomic block prior to catecholamine infusion had no important effect on the responses elicited by any of the three amines.