The mechanics of breathing in children with acute severe croup

The assessment of the severity of croup and response to therapy has remained a clinical one. Despite recognition of the importance of a reproducible and easily applicable method for objectively measuring severity, currently, no such technique exists.

We postulated that measurements of air flow and intrathoracic pressure changes in patients with severe croup would provide detailed information about the mechanics of breathing and the potential for the development of continuous bedside methods for objective monitoring of upper airway obstruction.

Twenty out of 21 eligible infants and children with severe upper airway obstruction from croup, and 5 control participants, were studied under light sedation utilizing face masks and nasogastric feeding tubes for flow and esophageal pressure measurements.

Children with croup had lower tidal volumes, but breathed faster, thus maintaining similar minute volumes to the controls. During inspiration, all but 2 croup patients (but no controls) displayed flow limitation. Area within the flow-volume curve was significantly decreased and minute ventilation for effort expended was nearly 4.5 times higher in croup patients than in controls. Peak-to-trough pleural pressure swings, pressure-rate product and pressure-time integral were also significantly higher than in controls (p<0.001) and returned to the normal range in the 9 patients who were subsequently intubated (p<0.001).

Patients with severe croup maintain minute ventilation by means of huge increases in intrathoracic pressure changes. Inspiratory flow limitation is present. In future outcome studies, measurements of respiratory function that do not include intrathoracic pressure changes are unlikely to be effective measures of the severity of croup.