What is the Ideal Tidal Volume During One-Lung Ventilation?

Abstract

One-lung ventilation (OLV) in the thoracic surgical patient facilitates surgical exposure with a collapse of the non-dependent lung, while the dependent lung must be ventilated to maintain arterial oxygenation.1 Intraoperative mechanical ventilation with different tidal volume (VT) has traditionally been applied to prevent hypoxia and atelectasis during OLV,2 the ideal VT during OLV remains controversial. Previous reports have shown that lung protective ventilation (LPV) utilizes lower VT, reduces stretch-induced lung injury, and decreases inflammatory mediators in patients with acute respiratory distress syndrome (ARDS), decreasing postoperative morbidity and mortality.3 However, there has been paucity of literature demonstrating a similar robust evidential foundation in patients that do not have ARDS. Lately, there has been interest in exploring this reduction of complications in patients undergoing thoracic surgery for which they need OLV. It does pose significant challenges to patients due to the associated lung collapse, resulting in reduced oxygenation and ventilation. In recent years, researchers have focused on optimizing ventilatory strategies during OLV to minimize complications and improve patient outcomes. In the last decade, an emphasis has been focused on the impact of low VT during OLV to prevent or reduce postoperative pulmonary complications (PPC) and in the meantime provide lower airway pressure while maintaining oxygenation. The definition of low VT is still very inconsistent and ranges from 4-7 mL/kg of predicted body weight in contrast to what is considered a higher VT which is considered 8-15 mL/kg. A study4 had reported data analysis from multiple institutions (retrospective analysis) and it showed that low VT 5-6 mL/kg without adequate positive end expiratory positive pressure (PEEP) does not prevent PPC. Another study5 from the same multicenter perioperative outcomes group analyzed data from 3,232 cases and reported that LPV with low VT and 5 cmH2O of PEEP was not associated with reduced PPC. A recent meta-analysis report published in 2022 by Peel, et al related to the same topic,6 showed that low VT with a mean value of 5.6 ± 0.9 mL/kg was not associated with significant differences in partial pressure of oxygen or the PaO2/FiO2 ratio or compliance when compared to conventional VT ventilation of 8.1 ± 3.1 mL/kg. The only findings in their report were that low VT were associated with significant decrease odds of pulmonary complications. In this volume of Journal of Cardiothoracic and Vascular Anesthesia, El-Tahan, et al7 have reported a meta-analysis of randomized controlled trials (RCT), on the impact of lower VT during OLV where they have evaluated 17 RCT to establish the effect of low VT (4-7 mL/kg) in comparison to higher VT (8-15 mL/kg) during OLV on gas exchange and PPC. The same author had previously published a similar meta-analysis in 2017, 8 based on 14 RCT's indicating that using lower VT, lowers airway pressure and increased carbon dioxide (CO2) tension during OLV, with no demonstrable effect on PPC's and hospital-length-of-stay (HLOS), possibly due to heterogeneity between the patient study groups. Since then, the authors have updated their dataset, including 3 new RCT's to evaluate any differences. This new meta-analysis reports that a lower VT strategy is associated with a better PaO2/FiO2 at the end of surgery, along with a reduction of PPC (OR 0.50, p<0.001). The traditional approach to mechanical ventilation during OLV involved the use of higher VT to maintain adequate oxygenation. However, this strategy has been associated with an increased risk of ventilator-induced lung injury (VILI), including pulmonary barotrauma, volutrauma, and biotrauma.9 As a result, recent research has aimed to explore the benefits of lower VT, balancing lung protection with appropriate oxygenation. The updated meta-analysis by El-Tahan7 does support the lower VT strategy during OLV as a LPV strategy. This information has critical implications for anesthesiologists and thoracic surgeons, guiding clinical decision-making and enhancing patient care. However, several avenues for future research and exploration remain, which the authors have themselves mentioned as pitfalls in their evaluative strategy and would be important to discuss. El-Tahan7 meta-analysis focused primarily on the impact of lower VT. Future studies could explore the comparative effectiveness of different ventilation strategies during OLV, such as the use of higher PEEP levels or recruitment maneuvers, to optimize lung protection and gas exchange.10 It is worth noting that studies evaluating the effect of low VT ventilation on perioperative outcomes often incorporate the use of PEEP, as it is considered an integral part of LPV strategies. Therefore, it can be challenging to assess the independent effect of low VT ventilation on outcomes, without considering the concurrent use of PEEP. It is also essential to recognize that patients may exhibit substantial heterogeneity in their response to lower VT. The VT and the PEEP might need to be individualized per the patient's specific anatomy, demographic, and ongoing comorbidities. Further research should identify patient-specific factors that influence the optimal choice of VT and PEEP, allowing for personalized ventilation strategies during OLV. While the meta-analysis provides insight into the immediate impact of lower VT, it would be valuable to investigate the long-term effects on patient outcomes, including postoperative pulmonary function, respiratory complications, and mortality rates. The authors have tried to improve upon their dataset, by adding 3 more RCT's. These studies each have their own limitations, such as an insufficient inclusion rate as reported by Marrett, et al.11 A different study by Zhang, et al12 evaluated only elderly patients in 4 different modes of ventilation, and the study by Qutub, et al13 evaluated 3 different VT. Hence, even the addition of these 3 new RCT's does not solve the heterogeneity challenge that is posed in these studies, and the authors recognize this themselves. There is also significant variance in the multiple studies included in the meta-analysis regarding the definition of PPC's. Since these definitions are inconsistent in different studies, yet is reported as a significantly different outcome, it makes it harder for the readers to apply these results to the general patient demographic. Also, multiple factors influence the incidence of PPC's including smoking cessation, early ambulation, pain management strategy, pulmonary hygiene, surgical technique, and fluid management; and hence it is challenging to evaluate just the effect of LPV on these outcomes.14 The ongoing debate surrounding PEEP vs. low VT ventilation in OLV ventilation highlights the need for further research and RCT's comparing these techniques head-to-head. It is crucial to identify which patients may benefit from PEEP or low VT ventilation individually or in combination, as well as to explore potential patient-specific factors that influence the efficacy of these approaches. El-Tahan7 meta-analysis of RCT provides an updated understanding of the impact of lower VT during OLV. The findings support the use of LPV strategies, which mitigate the risk of ventilator-induced lung injury, improve gas exchange, and attenuate the systemic inflammatory response. Nonetheless, further research is needed to refine individualized approaches and explore long-term outcomes. As the field continues to evolve, anesthesiologists and thoracic surgeons must remain cognizant of emerging evidence to provide optimal care for their patients, considering factors such as lung compliance, pre-existing lung pathology, and surgical procedure. Table 1 shows 3 different meta-analysis studies on the clinical impact of low VT vs higher VT during OLV. It is essential to perform new studies with adequate selection of patients including patients with healthy lungs, patients with pre-existing pulmonary diseases particularly chronic obstructive pulmonary disease (COPD), obese and morbidly obese patients with acceptable power analysis of subjects studied. Tidal volume settings require special attention in obese patients. Obese patients are more often exposed to greater VT. It is important to highlight, particularly in obese patients, that the desired VT should be calculated based on the predicted body weight and not on the actual body weight because the increase thoracic appearance is due to excessive adipose tissue but not a greater intrathoracic lung volume. This important information is missing in all meta-analysis studies reported currently, these studies must be prospective, RCT rather than retrospective analysis of multiple institutions where there is a high degree of bias among centers. It is possible that these new studies will provide a better understanding on the advantages of the use of low VT during OLV, this must include FiO2 settings, alveolar recruitment maneuvers, use of different levels of PEEP and outcomes to determine the ideal VT in the thoracic surgical patient undergoing OLV. We appreciate El-Tahan and colleagues’7 meta-analysis report on the controversial issue of the benefits of lower VT during OLV.Table 1Meta-Analysis Summary on Impact of Low Tidal Volumes During One-Lung VentilationAuthorPrimary End PointsTrials SelectedOutcomesEvidence GapsEl Tahan MR, et alJ Cardiothorac Vasc Anesth2017;31:1767-1773•Low VT (4-6 mL/kg)•Higher VT (8-12 mL/kg) during OLV•Gas exchange•Airway pressures during OLV•Postoperative pulmonary complications•LOS•14 RCT•820 patients assigned to two groups•n=429 low VT•n=391 higher VT•Lower VT•Lower arterial oxygen tension•Lower airway pressures•Higher arterial PaCO2•Low incidence pulmonary infiltration•Retrospective design studies•Heterogeneity on VT among studies•Different primary objective among studiesPeel JK, et alJ Thorac Cardiovasc Surg2022;163:1573-1585•Low VT (5.6 ± 0.9 mL/kg) vs•Higher VT (8.1 ± 3.1 mL/kg) during OLV•Oxygenation compliance•Clinical outcomes•18 studies•13 of them RCT•3,693 patients overall•No significant differences in oxygenation, compliance, LOS, or mortality in low vs high VT•Retrospective design studies•Included observational studiesEl Tahan MR, et alJ Cardiothorac Vasc Anesth2023 (in press)•Low VT (4-7mL/kg)•Higher VT (8-15 mL/kg)•PaO2/FiO2 ratio at end of surgery•Secondary outcomes postoperative pulmonary complications•17 RCT•1,463 patients•751 patients low VT•712 patients higher VT•Low VT•Increases PaO2/FiO2 ratio•Reduces the incidence of postoperative pulmonary complications•Retrospective design studies•Missing data from previous reports•Multiple secondary outcomesAbbreviationsVT = tidal volumeOLV = one-lung ventilationRCT = randomized controlled trialLOS = length of stay Open table in a new tab Abbreviations VT = tidal volume OLV = one-lung ventilation RCT = randomized controlled trial LOS = length of stay none 1. 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Eur J Cardiothorac Surg 2019;55:91-115 The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Impact of Lower Tidal Volumes During One-Lung Ventilation: A 2022 Update of the Meta-analysis of Randomized Controlled TrialsJournal of Cardiothoracic and Vascular AnesthesiaPreviewTo clarify the influence of lower tidal volume (4-7 mL/kg) compared with higher tidal volume (8-15 mL/kg) during one-lung ventilation (OLV) on gas exchange and postoperative clinical outcome. Full-Text PDF