The assessment of the severity of respiratory disease in infants relies on clinical observation because of the difficulties in measuring ventilation in the infant. There are at least two clinical methods of assessing the severity, both of which include an assessment of chest wall distortion (CWD) during inspiration as an element of the severity score. Because this is a nonspecific respiratory system abnormality, CWD is thus interpreted as reflecting increased pleural pressure swings because of a change in the resistance to airflow within the respiratory system.
Chest wall distortion occurs during inspiration as the abnormal paradoxic inward movement of the lower chest wall, demarcated by the insertion of the diaphragm into the ribcage, representing a collapse of the lower chest wall at a time when the thoracic cavity is increasing in volume (ie, “out of phase” or paradoxic). This uncoupling of the ribcage-abdominal motion may result in a decrease in tidal volume (Vt), a decrease in alveolar minute ventilation (Ve), and thus ventilatory failure.
Chest wall distortion is frequently seen in the newborn, although it may be present at any time during the preschool years. Two recent publications have suggested that objective assessment of the severity of respiratory disease in infants is possible by using measurements of the timing of motion of the chest wall and abdominal excursions, to avoid calibrating an inductance plethysmograph for volume. Viral laryngotracheobronchitis (LTB) is representative of a group of diseases in infants and young children associated with acute partial upper airway obstruction, increased resistance to inspiratory airflow, and severe CWD. In the most severe form, elevation of arterial carbon dioxide tension occurs because of alveolar hypoventilation. Therefore, we studied infants with acute severe LTB to determine the physical displacement of the chest wall and abdominal compartments during the progression of the disease. The study determined during the clinical illness the magnitude of the CWD, the changes in Vt, and the effects on gas exchange.
Six previously healthy infants, admitted to the Pediatric ICU of the Alberta Childrens Hospital because of a clinical diagnosis of severe LTB and ventilatory failure, were included in this study. Infants with preexisting pulmonary or cardiac disease or with acute epiglottitis were excluded from the study. The study was approved by the Ethics Review Committee of the University of Calgary.
At the time of hospital admission, all had severe partial upper airway obstruction and a clinical diagnosis of LTB. All required a fractional inspired oxygen (Flo,) greater than 0.35 to maintain a transcutaneous oxygen tension (tcPoJ >55 mm Hg and each infant had a transcutaneous carbon dioxide tension (tcPcOj) greater than 45 mm Hg to meet the requirements of ventilatory failure. Infants who required immediate endotracheal intubation were excluded from this study. The initial treatment of the ventilatory failure was determined by the attending physician.