Increased Incidence of Sighs (Augmented Inspiratory Efforts) During Synchronized Intermittent Mandatory Ventilation (SIMV) in Preterm Neonates Helmut Hummler, MD, Tilo Gerhardt, MD,* Alvaro Gonzalez, MD, Nelson Claure, MS, Ruth Everett, RT, RN, and Eduardo Bancalari, MD Summary. A reflex resulting in a deep, sigh-like inspiratory effort (augmented breath) is fre- quently triggered during synchronized mechanical ventilation in preterm infants. We studied the incidence of augmented inspiratory efforts and their effect on ventilation and lung compliance during conventional IMV and synchronized IMV (SIMV) in 15 preterm neonates (GA 26.7 ± 1.5 wks (mean ± SD), BW 925 ± 222 g, age 1–8 days). Augmentation of spontaneous inspiratory effort was defined as an esophageal pressure deflection occurring coincident with a synchro- nized mechanical breath and exceeding the previous unassisted spontaneous effort by more than 50%. The incidence of augmented breaths was higher during SIMV (11.1 ± 7.7%; P < 0.01) than during conventional IMV (5.1 ± 6.1%). However, when the synchronized breaths were triggered late (200–300 msec) after the onset of inspiration, augmented breaths occurred no more fre- quently than during conventional IMV (6.0 ± 4.7%). The incidence of augmented breaths cor- related inversely with dynamic lung compliance (P = 0.014), but was not significantly influenced by a change in PEEP. Although inspiratory effort increased nearly three times during the aug- mented breaths, tidal volume increased only 12%. The change in tidal volume was limited because the augmented effort reached its maximal negativity only approximately 500 ms after the beginning of the synchronized, mechanical breath and at a time when the mechanical breath had already ended. For this reason the augmented effort did not contribute significantly to minute ventilation, but only prolonged inspiration. Dynamic lung compliance did not change significantly after an augmented breath. The results indicate that augmented inspiratory efforts are more common in preterm neonates ventilated with SIMV than with conventional IMV, but do not contribute significantly to ventilation. Pediatr. Pulmonol. 1997; 24:195–203. © 1997 Wiley-Liss, Inc. Key words: newborn infant; mechanical respiration; augmented inspiration; Head’s paradoxical reflex; synchronized ventilation; pulmonary function. INTRODUCTION Lung inflation coinciding with a spontaneous inspira- tory effort generally induces relaxation of inspiratory muscle activity, shortens inspiratory time, and prolongs expiration (Hering-Breuer Reflex). 1,2 However, aug- mented inspiratory efforts (sighs) in response to me- chanical lung inflation have been described in several animal species, 3–6 as well as in human neonates. 2,7,8 Whether this ‘‘sigh’’ or ‘‘gasp reflex’’ is an equivalent of the originally described ‘‘Head’s paradoxical reflex’’ 9 is controversial. 10 During previous studies evaluating SIMV in small preterm infants, 11 we observed that aug- mented inspiratory efforts occurred more frequently dur- ing SIMV than during conventional IMV. Large aug- mented spontaneous inspiratory efforts coinciding with the inflation pressure of mechanical ventilation increase transpulmonary pressure and may increase the risk of barotrauma and subsequent chronic lung disease. We hypothesized that augmented inspiratory efforts are triggered more frequently during synchronized me- chanical ventilation than during conventional IMV. If this is true then timing of the inflation in relation to spontaneous inspiration is important. We therefore hy- University of Miami School of Medicine, Department of Pediatrics, Division of Neonatology, Jackson Memorial Medical Center, Miami, Florida. Presented at the Annual Meeting of the Society of Pediatric Research, May 7–11, 1995, San Diego, CA. Contract grant sponsor: University of Miami: Project New Born. *Correspondence to Dr. Tilo Gerhardt, University of Miami School of Medicine, Department of Pediatrics (R-131), Division of Neonatology, Jackson Memorial Medical Center, P.O. Box 016960, Miami, FL, 33101. Received 8 August 1996; accepted 10 June 1997. Pediatric Pulmonology 24:195–203 (1997) © 1997 Wiley-Liss, Inc.