Lung Function and Acid‐base Balance in the Newborn Infant

Summary During gestation the placenta subserves the external respiration of the fetus as well as its metabolism. With the initiation of extrauterine life the placental circulation is eliminated and the fetus emerges from a watery environment into air. This implies the establishment of a new autonomic function, that of pulmonary respiration, which process is intimately linked to a profound readjustment of the circulation. The present series of investigations has been performed to elucidate certain of the processes involved in pulmonary gas exchange and to evaluate its efficiency compared to adult standards during the period of adjustment to extrauterine life. Though the most important and rapid changes in this adaptative process occur during the first seconds and minutes of extrauterine life they continue far beyond this period. Observations have therefore been extended until the end of the neonatal period defined as the first week of life; this has been considered particularly important as information as to the later period of this adjustment is scarce or lacking. Emphasis is laid on development of function. Because of the significant morbidity and mortality of the newborn infant in disorders of respiration all investigations throwing more light on the normal respiratory mechanisms and their alterations in disease have important practical implications by improving diagnostic possibilities and facilitating objective evaluation of therapeutic measures. A total of 172 healthy fullterm infants born after an uneventful pregnancy of normal length and uncomplicated vertex delivery are the subjects of the different studies. The size of the subjects to be studied and the lack of cooperation posed special methodological problems. Suitable procedures and methods permitting reproducible and valid measurements have been developed or adapted for the study of mechanics of breathing, functional residual capacity, pulmonary ventilation, diffusing capacity and gas exchange. Since capillary PO 2 does not accurately reflect arterial PO 2 a safe procedure permitting arterial blood sampling without disturbing steady state conditions had to be developed. The onset of respiration and initial aeration of the lungs is characterized by the generation of very high resistive forces, probably mainly due to high surface tension. Subsequently, lung compliance increases while lung resistance and inspiratory work decrease, the speed of these changes being highest in the beginning and slowing down subsequently. Within one hour after delivery, the functional residual capacity amounts to 40—80 ml and shows a relation to lung compliance similar to that later in life. Changes after 24 hours of life are insignificant. There is hypoxemia, CO 2 retention and accumulation of fixed acids, i. e. respiratory and metabolic acidosis at birth. The respiratory component of acidosis is on the average abolished at 30 minutes and metabolic acidosis at 24 hours of life. P CO2 attains a minimum of about 33 mm Hg at 24 hours of life and increases slightly to 36 mm Hg at 7 days. This hyperventilation appears to be counterbalanced by a corresponding hypobasemia which recedes during the subsequent days. following closely the increase of P CO2 . This regulation shows a striking similarity with the changes occurring during and after adaptation to altitude. Arterial oxygen tension during air breathing increases within 5 hours to, on the average, about 73 mmHg, after which time no further significant changes occur. Arterial oxygen tension is thus significantly lower than in normal adults. Arterial oxygen tension during oxygen breathing averages 309 mmHg at 55 minutes of life and between 527 and 589 mmHg during the period between 5 hours and 7 days of life, corresponding to an anatomic R—L shunt of about 24 and between 7.6 and 10.9 per cent of cardic output respectively. Among the different variables studied in connection with the determination of diffusing capacity during steady state conditions at the age of 24 hours and 7 days, only the following variables show a statistically significant change during this period: heart rate, respiratory rate, minute ventilation (V̇ E ), the alveolar ventilation/carbon dioxide elimination ratio (V̇ A /V̇ CO2 ), the respiratory exchange ratio (R), the dead space/tidal volume ratio (V D /V T ), the pulmonary diffusing capacity for CO, the total venous admixture, the anatomic shunt/total venous admixture ratio. Alveolar ventilation in relation to oxygen consumption, physiological dead space in relation to tidal volume, the pulmonary diffusing capacity for CO in relation to oxygen consumption, alveolar ventilation and body surface area, and the alveolar ‐ mean pulmonary end capillary oxygen tension difference are of the same order as in healthy resting adults. The alveolar‐arterial oxygen tension difference is in the order of 30 mm Hg and mainly unchanged during the age of 24 hours and 7 days. It is thus significantly higher than in healthy adults. Both anatomic shunt and ventilation/perfusion inequality, i. e. alveoli with low ventilation/perfusion ratio account for this difference. After the initial adaptation, the contribution from ventilation/perfusion inequality decreases during the first week of life. In accordance with these findings, the overall ventilation/perfusion ratio was estimated to be on the average about 0.65 with a slight tendency to increase between 24 hours and 7 days. The changes of most of the variables followed continuously from birth to 7 days can be expressed as a logarithmic function of age showing that the adaptative changes proceed most rapidly in the initial stage. Even at the age of 7 days the lung of the newly born infant has not yet attained the same efficiency with respect to oxygen exchange as that of the healthy adult. Résumé Durant la gestation, le placenta subvient à la respiration du foetus aussi bien qu'à son métabolisme. C'est avec le début de la vie extra‐utérine que la circulation placentaire est supprimée et que le foetus surgit à l'air quittant un mileu aqueux. Ceci implique, d'urgence, l'établissement d'une nouvelle fonction autonome, celle de la respiration pulmonaire. Ce processus est intimement liéà une profonde réadaptation de la circulation. Les présentes séries de recherches ont eu pour but d'éclaircir quelques‐uns des processus mis en jeu dans l'échange gazeux pulmonaire et d'évaluer leur efficacité compareée à la norme chez l'adulte, durant la période de l'adaptation à la vie extra‐uté. Bien que les changements les plus rapides et les plus importants de cette adaptation se réalisent pendant les premières secondes et minutes de la vie extra‐utérine, ils se prolongent au‐delà de cette période. On a done continué les observations jusqu'à la fin de la période néo‐natale, la première semaine de vie par définition. On a considéré cela comme d'autant plus important que les informations sur la période tardive de l'adaptation sont rares ou inexistantes. Le point a été mis sur le déve‐loppement de la fonction respiratoire. Le taux tojours considérable de la mortalité néo‐natale est en grande partie attribuable aux difficultés de l'adaptation de l'hématose à la naissance. En améliorant les possibilités de diagnostic et d'évaluation desmoyens thérapeutiques employés au cours des troubles respiratoires, les recherches destinées àéclaircir les mécanismes respiratoires physiologiques et leurs déviations patho‐physiologiques au cours de »détresse' respiratoire ont une importance pratique considerable. Un total de 172 enfants venus à terme et en bonne santé, après une grossesse sans histoire, d'une longueur habituelle et un accouchement à présentation normale sans complications, a servi de sujets aux diverses études. La taille des sujets àétudier et leur manque de coopération ont posé des problèmes de méthodes particuliers. Des procédés et des méthodes appropriés permettant des mesures reproductibles et exactes ont été développés ou adaptés à l'étude de la mécanique respiratoire, de la capacité fonctionnelle résiduelle, de la ventilation pulmonaire, de la capacité de diffusion et de l'échange gazeux pulmonaire. Enfin, on a důétudier la validité du sang capillarie pour remplacer le sang artériel. Le résultat étant négatif en ce qui concerne P O2 , on a dů développer un procédé sans danger permettant de prélever du sang artériel sans troubler les conditions stables. Le début de la respiration et l'aération initiale des poumons sont caracterisés par l'apparition de forces de très haute résistance, dues surtout probablement, à la haute tension de surface. Ensuite, l'accomodation (compliance) pulmonaire augmente tandis que la résistance pulmonaire et le travail inspiratoirc diminuent, la vitesse de ces changements étant très élevée au début et diminuant ensuite. Une heure après la naissance, la capacité fonctionnelle résiduelle atteint de 40 à 80 ml et présente un rapport avec la compliance pulmonaire semblable à ce qu'il sera plus tard dans la vie. Après 24 heures, les changements sont insignifiants. A la naissance, il y a hypoxémic, rétention de C O2 et accumulation d'acides fixes, c'està‐dire acidose respiratoire et métabolique. La composante respiratoire d'acidose est abolie à 30 minutes, en moyenne, et l'acidose métabolique à 24