Intensive Care Med (2008) 34:324–332 DOI 10.1007/s00134-007-0910-x PEDIATRIC ORIGINAL Andrew C. Argent Christopher J. L. Newth Max Klein The mechanics of breathing in children with acute severe croup Received: 20 December 2006 Accepted: 26 September 2007 Published online: 16 November 2007 © Springer-Verlag 2007 Electronic supplementary material The online version of this article (doi:10.1007/s00134-007-0910-x) contains supplementary material, which is available to authorized users. Declaration: The authors are neither supported by, nor maintain any financial interest in, any commercial activity associated with the topic of this article. A. C. Argent (✉) University of Cape Town, Division of Paediatric Critical Care and Children’s Heart Disease, School of Child and Adolescent Health, Cape Town, South Africa e-mail: aargent@ich.uct.ac.za Fax: +27-21-6891287 A. C. Argent · M. Klein Red Cross War Memorial Children’s Hospital, Cape Town, South Africa C. J. L. Newth University of Southern California, Division of Critical Care Medicine, Children’s Hospital Los Angeles, Keck School of Medicine, Los Angeles CA, USA M. Klein University of Cape Town, Paediatric Pulmonology Services, School of Child and Adolescent Health, Cape Town, South Africa Abstract Rationale: 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 applica- ble method for objectively measuring severity, currently, no such technique exists. Objectives: We postulated that measurements of air flow and intratho- racic 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. Methods: 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. Measurements and main results: 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 con- trols) 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 pa- tients 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). Conclusions: Patients with severe croup maintain minute ventilation by means of huge increases in intrathoracic pressure changes. In- spiratory 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. Keywords Infants · Children · Flow–volume relationship · Pressure flow relationship · Pulmonary func- tion tests · Face mask · Pneumotachy- graph · Esophageal pressure measure- ment Introduction Viral croup remains a cause of potentially lethal upper air- way obstruction in early childhood [1, 2] despite the intro- duction of epinephrine inhalations and steroid therapy. As- sessment of the severity of croup and response to therapy has been clinical, although over the years many attempts have been made to assess the severity objectively.