THE "indicator-dilution" method of Stewart [1921] has provided the theoretical basis for a number of different techniques for measurement of cardiac output. In the experiments to be described here the same principle is employed; but instead of measuring the "time-concentration curve" of an injected substance, such as a dyestuff, a similar kind of curve is obtained for the changes in temperature which occur at selected sites in the circulatory system after experimental reduction of the blood temperature in the right auricle.Should the method prove to be sufficiently accurate, its advantages over other existing methods are of two kinds. First, an indefinite number of measurements can be made since they are not limited by the accumulation of a foreign substance in the circulation; and, secondly, it permits the simultaneous measurement of the outputs of both the right and the left ventricles.A preliminary account of the method has already been published [Fegler, 1953]. The purpose of the present paper is to give a fuller description of the technique, and to describe some of its applications. METHODS.In the model circulation water was made to flow through a closed system consisting of a Dale Schuster pump connected through its inflow valve to a mixing chamber (c. 120 ml. in capacity) which contained a quantity of glass beads and was enclosed in a glass or Perspex jacket.The whole apparatus was immersed in water thermostatically controlled at any required temperature between 21°and 380 C.Test injections were made into the mixing chamber, and the temperature of the fluid flowing through the tube beyond the mixing chamber was recorded by means of a constantan-copper thermocouple similar to those used for recording blood temperatures (see subsequent text). The space between the mixing chamber and its outer jacket 153

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RESPIRATORY responses to the intratracheal inhalation of lung irritants have been described by many workers since the time of Paul Bert. Laqueur and Magnus [1921], and more recently Newton [1941], Fegler [1942], Smythe [1943], and Whitteridge [1948], have described some of the more immediate effects of phosgene inhalation seen in the cat. In the experiments described in the first part of this paper we have studied the action of phosgene on anwesthetised cats and dogs and compared it with that of ammonia, which is a typical respiratory irritant capable of initiating prompt and characteristic changes in the breathing. After analysis of these experiments it seemed desirable to determine whether there was any direct action on the lungs by these gases which, apart from the stimulation of afferent systems of the animal, might influence the total response. Experiments specially designed for this purpose were carried out on isolated blood-perfused dog lungs and are described in the second part of this paper. METHODS.AnCesthetics and Operative Procedure8.Mixtures of chloralose 005 g. and urethane 05 g./kg. given subcutaneously were used for ancesthesia in the phosgene experiments. In the ammonia series dogs were given either the chloralose and urethane mixture, or nembutal (0032 g./kg.) or sodium barbital; cats were given nembutal 0-032-0-06 g./kg.; and rabbits 0032 g./kg. The barbiturate anesthetics were given either intraperitoneally (dogs and cats) or intravenously (rabbits). So far as we could judge the type of aneesthetic was not a factor which decisively modified the results.All animals were tracheotomised. One group of five cats had their lungs chronically sympathectomised (bilateral removal of the stellate and T 1-4 ganglia) in a two-stage operation four and three weeks before