Pediatric Pulmonology 50:1304–1312 (2015) Tonic Diaphragmatic Activity in Critically Ill Children With and Without Ventilatory Support Alexandrine Larouche, MD, 1 Erika Massicotte, MD, 1 Gabrielle Constantin, MD, 1 Laurence Ducharme-Crevier, MD, 1 Sandrine Essouri, MD, PhD, 1,2 Christer Sinderby, PhD, 3,4 Jennifer Beck, PhD, 3,5 and Guillaume Emeriaud, MD, PhD 1 * Summary. Background: Infants have to actively maintain their end expiratory lung volume (EELV). In mechanically ventilated infants, the diaphragm stays activated until the end of expiration (tonic activity), contributing to EELV maintenance. It is unclear whether tonic activity compensates for the lack of laryngeal braking due to intubation or if it is normally present. Objective: To determine if tonic diaphragm activity remains after extubation in infants, and if it can be observed in older children. Methods: Prospective observational study of pediatric patients ventilated for >24hr. Diaphragm electrical activity (EAdi) was recorded using a specific nasogastric catheter during four periods: (i) the acute phase, (ii) pre-extubation, (iii) post- extubation, and (iv) at PICU discharge. Tonic EAdi was defined as the EAdi in the last quartile of expiration. Results: Fifty-five patients, median age 10 months (Interquartile range: 1–48) were studied. In infants (<1 year, n ¼ 28), tonic EAdi was always present, and represented 33% (22–43) of inspiratory EAdi at PICU discharge. No significant change was observed between pre- and post- extubation periods. In older patients (n ¼ 27), tonic activity was negligible as a whole, but 10 patients exhibited significant tonic EAdi at one time-point during PICU stay. Bronchiolitis was the only independent factor associated with tonic EAdi. Conclusions: In infants, tonic EAdi remains involved in ventilatory control after extubation and restoration of laryngeal braking. Tonic EAdi may play a pathophysiological role in bronchiolitis and it can be reactivated in older patients. The interest of tonic EAdi as a tool to titrate mechanical ventilation warrants further evaluation. Pediatr Pulmonol. 2015;50:1304–1312. ß 2015 Wiley Periodicals, Inc. Key words: mechanical ventilation; pediatric intensive care unit; diaphragm function; expiration; end expiratory lung volume; positive end expiratory pressure. Funding source: Respiratory Health Network of the Fonds de la Recherche du Qu ebec Sant e; Number: 24470, CHU Sainte-Justine and Sainte-Justine Research Center. 1 Pediatric Intensive Care Unit, CHU Sainte-Justine, Universit e de Montr eal, Montreal, Quebec, Canada. 2 Pediatric Intensive Care Unit, CHU Kremlin Bic^ etre, Universit e Paris Sud, Le Kremlin Bic^ etre, France. 3 Keenan Research Centre for Biomedical Science and Li Ka Shing Knowledge Institute of St. Michael’s Hospital, Toronto, Canada. 4 Department of Medicine and Interdepartmental Division of Critical Care Medicine, University of Toronto, Toronto, Ontario, Canada. 5 Department of Pediatrics, University of Toronto, Toronto, Ontario, Canada. Part of the results of this study have been presented at the Critical Care Congress in San Francisco, in 2014. The study was primarily conducted in CHU Sainte-Justine. Conflict of interest: Neurovent research, Inc. provided a recording device. Maquet Critical Care provided the ventilator and catheters for the study. This company was not involved in the result analysis and reporting. Ã Correspondence to: Guillaume Emeriaud, MD, PhD, CHU Sainte Justine, 3175 Chemin de la c^ ote Sainte-Catherine, Montreal, Quebec, Canada H3T 1C5. E-mail: guillaume.emeriaud@umontreal.ca Received 10 July 2014; Revised 27 January 2015; Accepted 15 March 2015. DOI 10.1002/ppul.23182 Published online 4 May 2015 in Wiley Online Library (wileyonlinelibrary.com). ß 2015 Wiley Periodicals, Inc.