EFFECT OF NEGATIVE-PRESSURE BREATHING ON LUNG MECHANICS AND VENOUS ADMIXTURE

Anesthetized dogs in the supine position show a spontaneous decrease in total respiratory compliance and an increase in venous admixture to the pulmonary circulation. Both these changes can be increased by negative-pressure breathing and reversed by positive-pressure breathing. If the changes in total respiratory compliance are due only to changes in lung compliance and these in turn result directly from the closure of alveoli, the relationship between compliance and inspiratory and expiratory pressure allows one to determine the scatter of opening and closing pressures in the alveoli. The venous admixture measures blood flow through collapsed alveoli, and its relationship to the negative pressure applied indicates the perfusion of the alveoli collapsing with each increment in negative pressure. By studying simultaneously changes in compliance and venous admixture, and using two basic assumptions, the dog's lungs can be described as a system composed of some elements receiving nearly 50% of the ventilation and 20% of the perfusion, relatively unstable mechanically, and having a very high Va/ Q ratio, while the remaining air spaces receive the same ventilation, but 80% of the perfusion.

lung compliance; atelectasis; ventilation-perfusion ratio

Submitted on October 28, 1963

MECHANISM OF DECREASE IN CARDIAC OUTPUT CAUSED BY OPENING THE CHEST

The cardiac output in ten dogs fell an average of 18.8% (±1.1 sem) when the chest was opened. Opening the chest, which increased the extracardiac pressure, shifted the cardiopulmonary-function curve (relating right atrial pressure to cardiac output) toward higher atrial pressure levels. This caused the cardiopulmonary curve to equate with the animal's systemic-function curve at a lower cardiac output; the greater the extracardiac pressure the lower the output. Thus, changes in extracardiac pressure can alter cardiac output even though the pumping ability of the heart is not altered.

PULMONARY ALVEOLAR-VASCULAR REFLEX

Lung mechanics and pulmonary circulation were studied in lightly anesthetized, spontaneously breathing, supine sheep. Aerosol atropine restored lung compliance (Cl) in animals after fluid inhalation. Ventilation-perfusion relationship was studied by comparing Cl and effective pulmonary capillary blood flow (Qe = 100 minus venous admixture in per cent of cardiac output). The relation between Cl and Qe in unpremedicated sheep before and after fluid inhalation suggested homeostatic redistribution of blood flow away from nonventilated areas. Compensation for nonventilated areas was enhanced by severe pulmonary hypertension and abolished by atropine or isoproterenol; vagotomy had no effect. Homeostasis therefore appears to depend on a local reflex response of the parasympathetic nervous system— “alveolar-vascular reflex.” Pulmonary hypertension of generalized alveolar hypoxia was a different reaction not blocked by atropine. Consistent with this analysis, an infusion of acetylcholine increased pulmonary arterial resistance. The increase in Cl which followed atropine or isoproterenol indicated that in addition to surface forces, smooth muscle contraction may result in closure of lung units.

lung mechanics and pulmonary circulation; fluid aspiration; atropine aerosol effect on lung compliance (Cl) after fluid inhalation; isoproterenol restored Cl; local homeostatic reflex response to pulmonary hypoxia; parasympathetic control of pulmonary ventilation; lung compliance (Cl) and effective pulmonary capillary flow (Qe) correlation

Submitted on March 25, 1963

THE CURRENT STATUS OF THE RESPIRATORY DISTRESS SYNDROME OF THE NEWLY BORN

The respiratory distress syndrome (RDS) occurs in 14% of premature infants and is twice as common and twice as lethal in males as in females. Recent work suggests that, during the intrauterine period a disturbance in nutrition of the lung resulting from fetal pulmonary vascular constriction results in alveolar-cell damage and a decrease in pulmonary surface activity with resultant atelectasis. Data on respiratory work levels, oxygen consumption and arterial oxygen tension suggest that there is an oxygen debt in the acute stage of the disease. Such data have further clarified the pathogenesis of the metabolic and respiratory components of the acidosis and the secondary effects thereof. In prevention, prophylaxis of prematurity is of major importance. A program of treatment designed to combat the various aspects of the pathophysiological disturbances is described in the form of a case profile. Modern methods of observation, biochemical control and treatment, as well as the necessity for critical evaluation, suggest that infants with RDS are best cared for in special centres.