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Abstract

Introduction: In contrast with adults, infants have to actively maintain their end expiratory lung volume because of the high compliance of their rib cage. In mechanically ventilated infants, we previously showed that the diaphragm stays activated until the end of expiration (tonic activity). We hypothesized that this tonic activity was present to compensate for the lack of laryngeal braking secondary to endotracheal intubation. We conducted a clinical study to determine if the tonic activity of the diaphragm remains after extubation in infants, and if it can be observed at all ages in children. Methods: This is a prospective longitudinal observational study. Patients 1 week to 18 year old admitted to the pediatric intensive care unit (PICU), mechanically ventilated for at least 24 hours and with parental consent were eligible. Diaphragm electrical activity was recorded using a specific nasogastric catheter at four different times during PICU stay: in acute phase, before and after extubation and at PICU discharge. Diaphragmatic activity was analyzed in a semi-automatic manner to determine inspiratory and expiratory times. Tonic diaphragmatic activity was defined as the activity during the last quartile of expiration. Results: 55 patients with a median age of 10 months (Interquartile range: 1–48) were studied. In infants (< 1 year-old, n=28), tonic activity expressed as a percentage of inspiratory activity was 48% (30–56) in acute phase, 38% (25–44) pre-extubation, 28% (17–42) post-extubation and 33% (22–43) at PICU discharge (p<0.05, ANOVA, with significant difference between recording 1 and recordings 3 and 4). No significant change in tonic activity was observed between before and after extubation. As expected from physiology, the tonic diaphragmatic activity in older patients (> 1 year-old) was negligible during normal breathing (Median: 0.6mcv). However, significant tonic activity (> 1mcv and >10% of inspiratory activity) was observed at one point during the stay of 10 patients (aged 13 months to 118 months) out of 27 patients > 1 year-old. Conclusions: In infants, tonic diaphragmatic activity remains important after extubation despite restoration of laryngeal braking. Older patients breathing normally don’t show significant tonic activity but it can be re-activated in pathological conditions. Tonic activity seems to be an indicator of patient’s efforts to maintain their end expiratory lung volume. Further studies should evaluate if this monitoring could facilitate the detection of situations of inappropriate ventilatory support.