322 PHYSIOLOGICAL CHEMISTRY 



observer. The subject breathes back and forth from the bag 4 times in 20 

 seconds, emptying the bag at each inspiration. The observer should indicate 

 when to breathe in and out. Breathing more frequently will not greatly alter the 

 results. At the end 20 seconds, the tube is clamped off and the air analyzed. 

 The analysis should be carried out within 3 minutes' time, as carbon dioxide 

 rapidly escapes through rubber. 



The foregoing precedure applies to patients who are capable of cooperating 

 to some extent. In the case of comatose pateints, the initial amount of air in the 

 rubber bag must be greater (1000 c.c. at least), and the period of rebreathing 

 prolonged to 30 seconds. 1 This is necessary, as it is not feasible that the bag be 

 completely emptied of air at each inspiration; and therefore a longer time is required 

 for the carbon dioxide tension in the bag and in the lungs to become equal. The 

 initial amount of air in the bag should be such that it is at least one-half and prefer- 

 ably as much as two-thirds emptied at each inspiration. Since comatose patients 

 cannot hold the mouthpiece, some form of mask is necessary. This may be a gas 

 anesthetic mask 2 or such a device as described below for use with infants. 



A special mask has been devised for the collection of alveolar air from infants. 3 

 It is made from the nipple of a wide-mouth (Hygeia) nursing bottle and a piece 

 of thin rubber tissue (dental dam). A sheet of the tissue (8 by 10 inches) is per- 

 forated in the center by a piece of hot metal or glass tubing of large bore. The 

 hole is stretched and pulled over the nipple and slipped down to the lower rim. 

 A small amount of rubber cement is applied to hold the tisssue and nipple together. 

 A strip of adhesive plaster % inch wide is applied around the rim of the nipple 

 so as to overlap the rubber tissue and hold it firmly in place. The extreme tip of 

 the nipple is cut off and a short glass tube, % inch in diameter, inserted. 



"In making a collection of alveolar air from infants a rubber bag of 500 c.c 

 capacity is connected with the mask and partially filled with air by means of an 

 aspirator bulb. The neck of the bag is closed off by a pinchcock or with the fingers, 

 the mask placed over the nose and mouth of the infant and the rubber tissue closely 

 drawn around the face so as to prevent the escape of air. The mask should, if 

 possible, be placed over the face just at the end of expiration. Respirations are 

 allowed to continue for from 28 to 32 seconds, and at the end of an expiration 

 the neck of the bag is closed off and the mask removed from the face. We 

 have found that it is necessary that the infant should be breathing quietly 

 for i minute previous to the collection of the air sample, as vigorous crying, 

 just before the mask is put on, leads to a lowering of the carbon dioxide 

 tension, as determined, by several millimeters. Crying during the collection of 

 the sample almost invariably occurs and facilitates mixing of the gases. The 

 effect is to raise the tension somewhat if crying is very vigorous, but not to such an 

 extent as to be significant. The initial amount of air in the bag must be such that 

 during inspiration the bag is from one-half to two-thirds empty, but never com- 

 pletely collapsed. The amount of air required for infants under i year of age varies 

 from 250 c.c. to 400 c.c." 



1 The bag is clamped at the end of that expiration occurring nearest to 30 seconds, 

 as no great error is introduced by prolonging the time of rebreathing by 2 or 3 seconds. 



2 Some advantage may be gained by inserting a large three-way stopcock of metal 

 or glass between the mask and the bag. When such a stopcock is used, the mask is first 

 put in place on the patient's face with the cock so turned that he breathes the outside air 

 for a few respirations. At the end of an expiration the cock is quickly turned so as to 

 bring the bag and the mask into connection. 



3 Howland and Marriott: Am. Jour. Dis. Child., May, 1916. 



