Tidal volume monitoring during initial resuscitation of extremely prematurely born infants

Introduction

Tidal volumes of very prematurely born infants may often be very low during initial resuscitation in the delivery suite, particularly before the infant makes an inspiratory effort. We demonstrated that expiratory tidal volumes (VTes) rarely exceeded 4.4 mL/kg, twice the anatomical dead space [1], [2], until the first inspiratory effort coincided with an inflation [3], [4]. It has been suggested that these low tidal volumes during resuscitation via a facemask (FM) may be due to airway obstruction [5], [6]. Obstruction due to FM hold and positioning has been reported [7], but others have suggested that airway obstruction may occur due to laryngeal closure [8]. Laryngeal closure is overcome by the presence of an endotracheal tube (ETT) and, therefore, the occurrence of low VTes would be expected to be less in intubated babies compared to those resuscitated by an FM. The first spontaneous breath coinciding with an inflation, which we have termed active inflation [3], resulted in higher VTe and end tidal carbon dioxide (ETCO₂) levels in infants resuscitated via an FM or an ETT [9]. The aim of this study, therefore, was to compare the VTe and ETCO₂ levels prior to the first active inflation in infants during initial resuscitation via an FM or an ETT. We wished to determine whether there were significant differences between the two groups in the proportion of babies with low median VTes, that is, a median VTe less than 2.2 mL/kg (the anatomical dead space [1], [2]) or in the proportion of inflations with low VTes. Furthermore, we compared the VTes before and after intubation in a group of babies who had not made a spontaneous respiratory effort before intubation, as those results would further inform whether VTe levels remained low after intubation and hence strategies as well as reducing airway obstruction were needed to optimise tidal volume delivery during initial resuscitation.

Materials and methods

This was a retrospective analysis of respiratory function monitoring recordings at delivery of infants born at less than 29 weeks of gestational age who required resuscitation at birth either via an FM or an ETT. For the purposes of this study, recordings between March 2010 and March 2014 at two hospital sites were analysed. Certain results from some of the infants have been previously published [4]. Ethical approval was granted by the Outer North London Research Ethics Committee who required parental written consent only for analysis of the data which was obtained when the mother was on the postnatal ward.

Results

Sixty-three traces were reviewed. Analysis was not undertaken of traces where the infant had breathed before the onset of positive pressure ventilation via an FM or an ETT, the infants had received continuous positive airway pressure rather than positive pressure ventilation or the recording commenced after the onset of positive pressure ventilation. In the intubated group, only infants who were successfully intubated were included.

Traces from 29 babies who had undergone initial resuscitation with an ETT or an FM were analysed. The demographics between those who were resuscitated via an ETT or via an FM were comparable (Table 1). Only one infant died in the delivery room, this was an infant intubated at birth. There were no significant differences in the number of inflations examined in the two groups (Table 2). The median time to the first active inflation was 13.5 s (range 2–76 s) in the ETT group and 24 s (range 2–82 s) in the FM group (P=0.53). There were no significant differences in the PIP, PEEP, inflation pressures, ETCO₂ or VTe levels between those resuscitated via an ETT or via an FM, but the magnitude of leak was greater in the FM group (P=0.007) (Table 2). In the FM group, there was a significant negative correlation between the magnitude of leak and VTe (r=−0.627, P=0.039). The proportion of infants with a median VTe <2.2 mL/kg and the proportion of inflations with a VTe <2.2 mL/kg were similar in the two groups (Table 2). Comparison of infants with a low VTe and those with a VTe greater than 2.2 mL/kg demonstrated that the only significant difference was that the former group had a lower compliance (Table 3).

After 60 s of monitoring, all but two infants in each of the FM and ETT groups had made a spontaneous respiratory effort. The only significant difference in the results of the two groups was the magnitude of leak, being greater in the FM group (P=0.028) (Table 4).

Traces from seven infants were analysed before and after intubation. These seven infants were significantly more immature than those in the FM group (median gestational age 24 compared to 27 weeks, P=0.015), but otherwise there were no significant demographic differences between these infants and those in the ETT and FM groups (data not shown). In the seven infants, there was no significant difference in the median VTe achieved during FM and ETT resuscitation (3 mL/kg vs. 2.6 mL/kg, P=0.401). In addition, the mean VTe during FM ventilation in those infants was not significantly different to that in the FM group overall (P=0.085) and the mean VTe following intubation was not significantly different to the VTe of the infants in the ETT group overall (P=0.883).

Discussion

We have demonstrated that, during the resuscitation at delivery of prematurely born infants, VTe levels before the first active inflation were low in infants resuscitated via an ETT or via an FM. These results suggest that there may be reasons other than airway obstruction for the low tidal volumes during initial FM resuscitation. In a previous study, in 10 preterm infants, VTi and VTe were larger during mask compared to endotracheal ventilation [12]. The authors postulated that this was due to distension of the upper respiratory tract contributing to the tidal volumes during mask ventilation, as they had calculated that in seven lambs the combined trachea and oropharynx contributed to 14% of the total tidal volume measured at the mouth [12]. They, however, assessed the infants for 2 min before and 2 min after intubation [12]. In contrast, we assessed the infants before they took their first breath (active inflation). We demonstrated no significant differences in the VTe or ETCO₂ levels before the first active inflation between infants resuscitated via an ETT or an FM. Indeed, the VTe was as low as 0.59 mL/kg in the FM group, suggesting that there was not significant oropharyngeal distension contributing to the tidal volume results during initial resuscitation. Indeed, the lack of significant difference in the ETCO₂ results between the two groups further suggests that the VTe is from the lung and not the upper airway which is bypassed by the ETT. Furthermore, we did not demonstrate significant differences in the VTe before and after intubation in infants who had not made a respiratory effort. The difference between our results and those of the previous study may be explained by the fact that we examined initial resuscitation, that is, before the first breath, when both of our groups would have had low functional residual capacities.

A study of simulated FM resuscitation using a mannequin showed that, whilst complete airway obstruction was rare (0–2% of cases), partial airway obstruction (defined as achieving a tidal volume less than 60% of the optimum tidal volume) was seen in approximately 40% of inflations [13]. Airway obstruction occurred frequently during efforts to minimise leak [13]. Use of an oropharyngeal airway in one study of initial resuscitation, however, did not abolish apparent complete or partial obstruction [14]. It has been suggested that FM ventilation is ineffective at aerating the lungs if there is laryngeal closure [15]. Indeed, in a recently reported study [16], phase contrast X-ray imaging showed that the glottis and epiglottis were predominately closed immediately after birth in apnoeic, prematurely born rabbits. Our results suggest that laryngeal closure does not always explain the low expiratory volumes before infants made a respiratory effort, as the VTes were similar in those resuscitated via an ETT or an FM. We propose that the low tidal volumes seen prior to the first active inflation may also be due to the currently advised inflation pressures when applied over short periods of time [3], which may be insufficient to overcome the resistance of fluid-filled small airways. In a previous study, we demonstrated that the time to the first active inflation correlated with the magnitude of the inflation pressure and the duration of the inflation [17]. Use of higher pressures and longer inflation durations were associated with higher tidal volumes [17]. The low ETCO₂ levels before the first active inflation likely reflect that the pulmonary circulation was not fully vasodilated. We [18] and others [19], [20], [21] have shown ETCO₂ levels increase when the infant makes inspiratory efforts.

The anatomical dead space in infants has been shown to be approximately 2.2 mL/kg [1], [2]. Tidal volumes below this level may not make significant contributions to gas exchange. In addition, such low tidal volumes may not stimulate Head’s paradoxical reflex which is important in resuscitation [4], [22]. Head’s paradoxical reflex is stimulated when there is air entry into the lungs [23], 63% of inflations in our study resulted in VTes of less than 2.2 mL/kg. We, therefore, compared the proportions of infants who had a median tidal volume below 2.2 mL/kg in the two groups which were found to be similar, as was the proportion of inflations in the two groups which were associated with a VTe of less than 2.2 mL/kg.

There was a higher magnitude of leak in the FM group which may have led to the small tidal volumes. Repositioning of the FM may have reduced the leak. This, however, does not alter our conclusions. If the leak had been reduced by repositioning of the FM leading to larger expiratory volumes, this would confirm that clinically important airway obstruction was not a consistent feature of FM ventilation. An alternative explanation for lower VTes during FM resuscitation is that, in the presence of a large leak, there is lower pressure transmission.

There are strengths and some limitations to our study. We were able to compare VTe and ETCO₂ levels in infants resuscitated either via an ETT or an FM. Whilst the infants included were heterogeneous in terms of gestational age and birth weight, the two groups of infants had similar demographics and compliance results and all were born extremely prematurely. It is possible that spontaneous respiratory efforts prior to the application of the FM or insertion of the ETT were missed. Nevertheless, the VTes were generally low before the first documented respiratory effort. In addition, the low levels of expired CO₂ suggest that significant gas exchange had not occurred. We were also able to assess VTes before and after intubation in another group of infants demonstrating low VTes at both times. As these infants were their own controls, differences in lung function do not explain their results. The anatomical dead space might have been lower in the infants who were intubated as the upper airway was bypassed, but there were no significant differences in the VTe results in those studied before and after intubation. We included 29 infants in this retrospective analysis of previously collected respiratory function monitoring traces; our results, however, would inform a prospective study with a large sample size.

Conclusion

We have demonstrated similarly low VTe and ETCO₂ levels in infants undergoing initial resuscitation via an ETT or an FM. In addition, the proportion of infants with a low VTe and the proportion of inflations with a low VTe were similar in the two groups. These results show that mechanisms other than upper airway obstruction can result in low tidal volumes during resuscitation. The infants with the lowest VTes had the stiffest lungs, further emphasising that initial resuscitation pressures may be inadequate. Respiratory function monitoring should be recommended in the delivery suite so clinicians can objectively identify low tidal volumes rather than rely on the subjective assessment of chest rise as indicating appropriate pressure delivery. Importantly, they must, however, be trained in the appropriate interpretation of the respiratory function monitoring [24].