The respiratory events of birth are even more fascinating than we thought

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Birth is arguably the most unique, and poorly understood, event in human respiratory physiology.1 To create functional residual capacity and support air breathing, the fluid-filled fetal lung must rapidly clear itself of fluid. Only once aerated can the lung engage in tidal ventilation.2 During the initial inflations, lung liquid is absorbed into the interstitium.3 Through this period the infant is at great risk; inadequate lung liquid clearance creates heterogenous aeration, risk of fluid efflux back into the alveoli, and unstable tidal volumes (VT). That most infants achieve this unsupported is equally remarkable. For those who do not, delivering safe and effective respiratory support is still largely a skill that is experience based rather than founded on scientific evidence.4 The study of Gaertner and colleagues published in this issue of American Journal of Respiratory and Critical Care Medicine provides unique insights into how term infants achieve independent breathing at birth.5 In a study of 46 term newborns delivered vaginally, a total of 10 546 breaths were analyzed using electrical impedance tomography (EIT) data collected immediately after birth. A 32-electrode EIT belt was placed around the newborn’s thorax during vaginal delivery. Measurements were then made while the infant remained in the supine position between the mother’s legs for 60 seconds (delayed cord clamping). After the umbilical cord was clamped, the infant was then placed prone on the mother’s abdomen, and EIT data were collected for a further 10 minutes. Changes in end-expiratory lung volume (EELV) were computed with the EELV value at breath 10 as a baseline reference. From the cross-sectional images of dynamic conductivity changes, relative distributions of VT within lung regions were calculated, and lung regions without any tidal ventilation were identified and functional lung size estimated. Finally, commonly used measures of heterogeneity of ventilation were calculated: center of ventilation for the left/right and ventral/dorsal orientation, global inhomogeneity index, and spatial coefficient of variation.6 The measures were computed for each of the first 10 breaths and minute-by-minute up to 10 minutes after birth, and correlated to breath type as recorded by a researcher present. The authors found that EELV increased rapidly during the first 5 breaths at birth and in most infants plateaued by the 10th breath. During the first inflations, VT were 3 times as high as post-transitional VT and associated with crying, a process that likely aids fluid shift and aeration via high intrathoracic pressure and flow gradients mediated by forceful diaphragmatic activity.1,4,7 This also explains the predominance of central and dorsal lung regions during initial aeration. Central regions contain the major airways (and alveoli closest to them), whereas the dorsal lung has more lung tissue and there is greater diaphragmatic motion during spontaneous breathing.8 The rapid attainment of aeration is consistent with other delivery room studies using EIT or pneumotachography.1,4,7 Importantly, most of these studies did not include infants born by vaginal delivery, or did not reliably capture the first few breaths. As none of the infants in the current study required supportive care, it can be inferred that the observed aeration pattern defines the normal physiology, providing a reference for potential clinical translation. A lack of increasing aeration may suggest the need for clinical intervention. This is supported by the previous studies using EIT that involved term infants born via elective cesarean delivery1 or preterm infants, most of whom needed some respiratory support and required longer times to stable aeration.9 The most notable finding of Gaertner and colleagues’ work is the regional ventilation pattens. During the first breath, ventilation preferentially occurred in the right lung, followed by preferential ventilation to the left lung in the second breath. Thereafter, tidal ventilation occurred in both lungs, although inhomogeneity measures were high. As evident in the EELV data, aeration was occurring in both lungs throughout. This has not been described before and begs the question as to why? The answer may be in the breathing pattern. As hypothesized by previous studies,1 crying is clearly helpful for aeration at birth and is more than a shock response to the birth experience. Crying explains the initially better ventilated central and dorsal lung regions during the initial 10 breaths the authors observed, with ventilation to the lung periphery increasing over the subsequent 10 minutes of regular tidal breathing. But it is in expiration that crying is likely most important. Previous EIT studies suggest that partial glottal closure and diaphragmatic hold during crying facilitate expiratory braking, and establishes functional residual capacity better than tidal breathing.1,9,10 Not only does this “auto-PEEP” (positive end-expiratory pressure) maintain intra-alveolar aeration, reducing fluid efflux during falling pressure gradients, but also allows pendelluft gas flow between well and poorly aerated lung regions, possibly serving to aid ventilation during the next inflation. This rationale is illustrated in Figure 1. It is for this reason that PEEP is the current focus of delivery room clinical trials in preterm infants, and has been recently hypothesized as relevant to the mechanical challenges during expiration in severe adult acute respiratory distress syndrome.11 Proposed rationale for the regional VT patterns during vaginal birth in term infants. Representative functional electrical impedance tomography images of VT (light blue: most VT) distribution shown for the first (A), second (B), and subsequent (C) inflations. Initial inflations are mediated by crying, which creates “auto-PEEP” and pendelluft flow via glottic closure and diaphragmatic breaking. Crying progresses to tidal breathing once FRC, aeration, and VT are stable. Functional electrical impedance tomography images reproduced with permission from Gaertner et al.5 Created in BioRender (Tingay, D, 2025; https://BioRender.com/1hxlq8u). Abbreviations: Cdyn, Dynamic compliance; FRC, functional residual capacity; PEEP, positive end-expiratory pressure; RRS, respiratory system resistance; VT, tidal volume. Utilizing EIT is a strength of this study, as it allows for quantitative and visual assessment of dynamic changes in the regional distribution of air throughout the lung. Data were collected from the very first breath in 18 of the 46 infants, from the second breath in 15 infants, and the third in 4 infants. This would not be possible using, for example, a pneumotachograph, since the mouthpiece would interfere with the natural respiratory transition. Currently available commercial EIT systems use soft, nonadhesive belts that permit measurement without interfering with mother–baby interactions, as this study demonstrated.12 Combined with the high temporal resolution, EIT permits near-continuous imaging of regional aeration distribution in the natural birth environment without ionizing radiation or discomfort. While absolute volumes cannot be measured with time-differencing EIT, as is used here, relative measures still capture increases in regional EELV, VT characteristics, and measures of inhomogeneity. Current EIT systems use a single-slice (2D) image to characterize these changes, which requires consistency of belt position. In infants the nipple provides a suitable reference that captures a large cross-section of the lungs and has been shown to approximate total lung volume.6 Ideally a refined analysis could be obtained in a future study using 3D EIT imaging in which 2 rows of electrodes are used, allowing interrogation across caudocranial and cross-sectional planes. This would serve to reduce out-of-plane image artefacts and allow for lobular estimates of ventilatory distribution but needs to be balanced against

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.