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On-line version ISSN 1806-907X
Rev. Bras. Anestesiol. vol.58 no.6 Campinas Nov./Dec. 2008
LETTERS TO THE EDITOR
To the Editor,
We welcome the comments of our colleague Fabiano Timbó Barbosa. On his letter, he makes important observations on the diagnosis of intraoperative atelectasis and on the use of intraoperative alveolar recruiting maneuvers. The response to his observations is as follows:
1) A few methods, based on the use of X-rays 1-3 and electrical impedance tomography 4,5, can be used to visualize the presence of atelectasis; however, those methods have not been used intraoperatively. CT scans allow precise visualization of the distribution and magnitude of atelectasis; however, the patient has to be dislocated to the radiology department, impairing its intraoperative use. Although thoracic electrical impedance tomography can detect non-ventilated lung areas 6, the electric scalpel and placement of electrode belts in upper abdominal and thoracic surgeries make it impossible to use this method. Besides, electric impedance tomography equipment for clinical use (non-experimental) is not available in Brazil. Serial measurements of functional residual capacity using dilution of some gases, such as helium, would be another way of evaluating the development of intraoperative atelectasis; however, the routine use of this method in the operating room is not practical 7. Other methods of indirect assessment of the development of atelectasis include the determination of pulmonary shunts by catheterization of the pulmonary artery, measurement of the residual functional capacity by helium thermodilution, and ventilation/perfusion studies 8, whose routine use for detection of intraoperative atelectasis is not justified.
On the other hand, since the first reports on intraoperative atelectasis, physiological parameters that change with the development of lung collapses, such as static complacency and reduction in the arterial partial pressure of oxygen, have been considered the standard for the detection of lung collapse 9. In the absence of other factors, such as pulmonary edema or the presence of copious secretions in the airways that can affect some patients, a reduction in PaCO2 and static complacency reflect the development of atelectasis.
2) In the section "use of alveolar recruitment maneuvers during surgery", it was described that alveolar recruitment maneuvers can be used whenever necessary, i.e., if oxygenation worsens significantly due to the absence or inadequate PEEP, alveolar recruiting maneuver (ARM) can benefit the patient. In fact, studies have demonstrated that ARM is the most effective treatment for reversal of intraoperative hypoxemia related to alveolar collapse and not only after anesthetic induction 10-13. However, it has not been suggested that alveolar recruitment maneuvers should be used indiscriminately as a preventive method of pulmonary collapse because, as it was explained in the same section, those maneuvers can have deleterious effects on hemodynamic parameters, trigger a systemic inflammatory response originated in the lungs, and barotrauma, whose repercussions range from changes in pulmonary microstructure and matrix to pneumothorax.
We agree with what was stated in the consensus. Almost all patients undergoing general anesthesia develop atelectasis in the first minutes after relaxation of the respiratory musculature 14-16. However, without proper prevention in the form of adequate levels of PEEP (the literature does not have a consensus on which level is adequate), use of 100% FiO2, and frequent disconnection of the tracheal tube from the respiratory circuit, the patient will reestablish lung collapse because the factors related to the development of atelectasis are still present and, in this context, repetition of the alveolar recruiting maneuver is justified.
Finally, ARMs are applied to reverse alveolar collapse related to anesthetic induction and not as an attempt to minimize this phenomenon, as described in the Consenso Brasileiro 17. Manual ventilation with adequate PEEP levels after reduction of the tonus of the respiratory musculature and the use of low FiO2 might be alternatives to minimize the development of atelectasis after anesthetic induction. As for the Consenso Brasileiro, we do not agree that specific studies demonstrating the efficacy of intraoperative alveolar recruitment maneuvers in reversing hypoxemia are lacking. The opposite is true, i.e., ventilation-related atelectasis and the use of alveolar recruitment maneuver were initially reported in the intraoperative period1. A large number of studies demonstrating the effects of ARMs on oxygenation10-13 can be found in the literature, but it is still the subject of ongoing debates in the intensive care setting.
3) Our colleague is right. Randomized and controlled multicenter studies showing that the use of intraoperative ARM can change the evolution of respiratory complications, as suggested in our study, are lacking. Only small studies showing a favorable evolution are available. Miranda et al. observed that the use of strategies to prevent pulmonary collapse such as ARM and PEEP at the beginning of the surgery resulted in the maintenance of residual functional capacity and decreased incidence of postoperative hypoxemia for up to 5 days when compared with the group treated with conventional ventilation 11. Despite the lack of evidence on the impact of ARMs on morbidity and mortality, it is clinically plausible that a patient without atelectasis and consequently less hypoxic will have shorter ventilatory support 18, decreased incidence of ventilation-related pneumonias 19, and shorter hospitalization.
Finally, on this last aspect, although we agree with the Grade B recommendation of the Consenso Brasileiro 17, it should be mentioned that the chapter on intraoperative ventilation has few references on the intraoperative application of ARMs, despite several reports on the subject: a review article, two references on the dynamics of reversal of alveolar collapse by ARMs using tomography, and one study on the benefits of ARMs on oxygenation of intensive care patients. Despite the extensive literature on the intraoperative use of alveolar recruitment maneuvers in different settings, such as bariatric surgery 20,21, cardiac surgery 22,23, and in other surgeries, this consensus does not emphasize the intraoperative use of ARMs.
Once more, we appreciate the comments of Fabiano Timbó Barbosa and the opportunity to explain the topics he mentioned.
Luiz Marcelo Sá
Malbouisson PhD, TSA, TE-AMIB
Flavio Humberto de Souza Neves, PhD student at USP
Roseni dos Reis Rodrigues, PhD student at USP, TSA
Maria José Carvalho Carmona PhD, TSA, TE-AMIB
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