Pulmonary Aeration and Posterior Collapse Assessed by Electrical Impedance Tomography in Healthy Children: Contribution of Anesthesia and Controlled Mechanical Ventilation

Anesthesia is associated with the development of atelectasis, which may affect lung ventilation. These changes persist into the postoperative period and are associated with postoperative respiratory complications.¹  Electrical impedance tomography has been used in the operating room to monitor changes in aeration and to guide intraoperative ventilatory settings in a strategy to prevent atelectasis.²  The end-expiratory lung impedance tracks changes in lung aeration, and its correlation with functional residual capacity has been previously validated.³ 

The aim of this study was to evaluate the changes in the lung aeration estimated by electrical impedance tomography as the end-expiratory lung impedance after anesthesia induction in pediatric patients. This was a prospective, single-center, observational study including healthy children younger than 5 yr who underwent nonthoracic surgery. Monitoring was performed continuously before and throughout the surgical period. Data analysis was divided into five periods: induction-spontaneous breathing, ventilation-5 min (initial 5 min of controlled ventilation), ventilation-30 min, ventilation-late (final 5 min of controlled ventilation), and recovery-spontaneous breathing (after removal of the artificial airway and restoration of spontaneous breathing). End-expiratory lung impedance was analyzed cycle by cycle in these five periods.

A total of 20 pediatric patients undergoing nonthoracic surgery with general anesthesia were included. Their mean age was 27.5 months and 90% were male. Postectomy was the most frequent surgery (n = 16), followed by hernioplasty (n = 8) and cholecystectomy (n = 1), while five patients underwent two associated procedures. Ventilatory parameters were stable during the three controlled mechanical ventilation moments analyzed, with a mean positive end-expiratory pressure (PEEP) of 4 cm H₂O (fig. 1).

There was an initial increase in the global aeration (measured with the end-expiratory lung impedance; fig. 1) when mechanical ventilation was initiated (difference, 27.52 [95% CI, 26.27 to 28.77]; P < 0.001), and a subsequent progressive decrease during the time that patients were under general anesthesia (difference, –15.88 [95% CI, –17.42 to –14.34]; P < 0.001). After spontaneous breathing resumption, during recovery, the dependent posterior region presented aeration loss in comparison with the induction period, depicted by the lower end-expiratory lung impedance values found by electrical impedance tomography (difference, –7.46 [95% CI, –8.78 to –6.14]; P < 0.001). In opposition to this finding, there was a rise in the anterior nondependent, end-expiratory lung impedance when comparing induction with recovery periods (difference, 4.96 [95% CI, 3.9 to 6.52]; P < 0.001; fig. 2).

Some authors have studied the negative impact of anesthesia on children’s functional residual capacity,^4,5  and this has encouraged a debate around the importance of using enough PEEP to prevent atelectasis also in this age group.^4,6  During general anesthesia in the adult population, PEEP values can vary depending on patients’ characteristics and on the surgical intervention proposed. As a result, the use of a one-size-fits-all PEEP seems inappropriate, even for patients with healthy lungs.^7,8 

We monitored the entire surgical time since the start of mechanical ventilation until the restoration of spontaneous breathing in the recovery period. This made it possible to evaluate the temporal trend of lung aeration during controlled mechanical ventilation. With PEEP values of 3 to 4 cm H₂O, we found a progressive decrease in the end-expiratory lung impedance in the posterior lung regions, which suggests that these pressures were insufficient to maintain the lung aeration over time. This concept was previously demonstrated in children under general anesthesia, where a PEEP of 3 cm H₂O was not enough to prevent aeration loss, but a PEEP of 6 cm H₂O kept aeration stable even under high inspired oxygen fractions.⁹  Other authors found that a PEEP of 5 cm H₂O after a recruitment maneuver avoided lung collapse in children under general anesthesia for laparoscopy. These authors suggested that PEEP choice should be individualized, given the impact of individual and surgical factors over the risk of atelectasis.¹⁰ 

Our study has several limitations. First, it was an observational, descriptive study, and as such we cannot draw any conclusions about the causal influence of the variables studied on the outcome. Second, because of the small sample size, we could not study relevant subgroups such as age categories, use of neuromuscular blockers (in two patients), use of laryngeal masks versus tracheal tubes. Third, this was a single-center study, and the results reflect the local practice.

In conclusion, we demonstrated that aeration decreased progressively especially in the dorsal lung regions during controlled mechanical ventilation. This finding supports the hypothesis of progressive atelectasis formation predominantly in dorsal regions. Future studies should test whether different ventilatory settings, such as higher PEEP values, can prevent this atelectasis formation and even prevent postoperative complications in children.

This study was submitted to the Ethics and Research Committee of Israelita Albert Einstein Hospital, CAAE 89911218.6.0000.0071, and to the Ethics and Research Committee of the School of Medicine of the University of São Paulo, CAAE 89911218.6.3001.0065.

Support was provided solely from institutional and/or departmental sources.

Dr. Costa has the following financial relationships: consulting fees from Magnamed SA (São Paulo, Brazil) and consultant for Timpel S.A. (São Paulo, Brazil). Dr. Rossi is consultant for Timpel S.A and Marcelo B. Dr. Amato is a minority shareholder of Timpel S.A., and his research laboratory has received grants in the past 5 yr from the Covidien/Medtronics (São Paulo, Brazil; mechanical ventilation), Orange Med/Nihon Koden (São Paulo, Brazil; mechanical ventilation) and Timpel S.A (São Paulo, Brazil; electrical impedance tomography). The other authors declare no competing interests.