The cause of the respiratory distress syndrome (RDS) is unknown, hence management of the condition must at present rest on (i) recognition of the physiological disturbances present in the individual case; and (ii) correction of these disturbances, so far as this may be possible, in order that the baby can be given optimal conditions for survival during what is, in a proportion of cases at least, a self-limited illness.The function of the lungs being to take in 02 and to excrete CO2, the course of the respiratory failure seen in infants with RDS can best be followed by observing the changes in the amounts of these gases in arterial blood. A secondary function of the lungs is in the maintenance of normal pH by regulating the amount of carbonic acid in the body. In studying respiratory function in individual cases of RDS we have, therefore, largely depended on serial measurements of four variables-the 02-saturation, Pco2, pH and bicarbonate of arterial blood.Material and Methods The infants studied were those who, because of the presence of respiratory symptoms during the first few hours of life, were considered likely to develop RDS. These symptoms were rapid or laboured breathing, apnoeic spells, indrawing of ribs or sternum, or expiratory moan. A majority of the infants were premature, but some of the most severe cases of RDS were born at term.The infants were nursed in an incubator, the temperature of which was adjusted to maintain the baby's rectal temperature near to 370 C. As it was desired to observe the effect of widely different concentrations of inspired 02, and at times to use high concentrations, it was sometimes convenient to enclose the baby's head in a small 'perspex' hood which easily fitted within the incubator (see Fig. 6). High 02 levels could thereby be maintained with low flow rates of 02 to the hood. This 02 supply could be independently warmed and humidified. Ambient 02 levels were measured with a Beckmann D2 analyser.Arterial samples from an iliac artery were obtained by means of a plastic catheter (O.D. 2 mm.) inserted into an umbilical artery. With experience it was found possible to introduce the catheter in almost every infant, including those of less than 1 kg. After taking a blood sample the catheter was filled with heparin saline (50 units/ml.) which prevented clotting for a period of up to eight hours. The end of the catheter was then closed, and the cord stump and surrounding skin powdered with chlorhexidine and covered with a sterile dressing. The catheter has been left in situ for up to two and a half days. After this time femoral artery puncture has been used when required.Blood samples of about 1-5 ml. were taken under sterile and anaerobic conditions with a syringe, the dead space of which had been filled with heparin saline. A metal washer incorporated in the syringe provided a means of stirring. The syringe was capped and kept on ice until analysis which was done within one hour. 02-content was measured in duplicate or triplicate with the Roughton-Scholander syringe techniqu...

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