In both groups, static compliance decreased when bilateral-lung ventilation was switched to one-lung ventilation (Table 2). After an alveolar recruitment maneuver, static compliance increased significantly in the study group (corrected 95% CI of the difference was +3 to +30 mL cm·H2O−1, corrected P < 0.007) and remained increased throughout one-lung ventilation (Table 2). The results obtained for compliance in the control group after alveolar recruitment were inconclusive (Table 2).
Peak inspiratory pressures showed no differences between groups despite the higher PEEP levels in the study group (Table 4). Airway resistance increased in both groups when bilateral-lung ventilation changed to one-lung ventilation (in the study group, corrected 95% CI of the difference was +3 to +14 cm·H2O, corrected P < 0.001 and in the control group, corrected 95% CI of the difference was +2 to +12 cm·H2O, corrected P < 0.001), and remained increased throughout one-lung ventilation (Table 4). No differences were found between groups (Table 4). None of the study patients had auto-PEEP during the study period.
After an alveolar recruitment maneuver and PEEP adjustment, tidal volume was reduced in most patients in the study group to keep plateau pressure below 25 cm·H2O. In these patients, the ventilatory rate was increased to keep PaCO2 within the target range. Hence, tidal volume trended slightly lower, and ventilatory rate tended higher in the control group during one-lung ventilation. PaCO2 was higher in the study group since the initial measurement during bilateral-lung ventilation and the between-group differences did not vary along the study period (Table 3).
Dead-space/tidal volume showed no differences between groups when switching from bilateral-lung ventilation to one-lung ventilation (dead-space volume/tidal volume, P = 0.06 and alveolar dead-space volume/alveolar tidal volume, P = 0.14). In the study group, alveolar dead-space volume/alveolar tidal volume decreased slightly after an alveolar recruitment maneuver during one-lung ventilation (one-lung ventilation 20 minutes after starting PEEP, corrected 95% CI of the difference was +0.01 to +0.04, corrected P < 0.001 and end one-lung ventilation, corrected 95% CI of the difference was +0.007 to +0.04, corrected P = 0.002). The results obtained in the control group for alveolar dead-space volume/alveolar tidal volume were inconclusive (Table 2).
The cardiac index did not differ between groups, and the alveolar recruitment maneuver did not produce a cardiac index decrease in any patient (Table 3).
The results of this clinical study show that oxygenation and lung mechanics improvement secondary to the alveolar recruitment maneuver were better maintained during one-lung ventilation with an individualized PEEP level determined with a PEEP decrement trial than with a standardized PEEP level.
We found that an alveolar recruitment maneuver improved oxygenation during one-lung ventilation in both groups. The improvement in oxygenation should be related to a decrease in intrapulmonary shunt as shown by several studies.26 However, our results showed that only the study group maintained this oxygenation improvement throughout the procedure until the end of one-lung ventilation; the study group also maintained improved static compliance after an alveolar recruitment maneuver, suggesting a constant end-expiratory lung volume. In contrast, in the control group, the improvements in oxygenation and static compliance were not maintained after an alveolar recruitment maneuver, possibly due to a partial loss in the end-expiratory lung volume. These findings suggest that an optimal PEEP level kept the lung open, while an inadequate PEEP level could not prevent alveolar recollapse after an alveolar recruitment maneuver in thoracic surgeries.
Our results are in agreement with other studies, showing that PEEP improves oxygenation during one-lung ventilation. However, the effects of different PEEP levels on oxygenation during one-lung ventilation have been controversial, because heterogeneity in lung pathology produces different responses to PEEP. Michelet et al.13 found that 5 and 10 cm·H2O of PEEP improved oxygenation to the same degree, but 15 cm·H2O worsened oxygenation because over distension can increase shunt by diverting pulmonary blood flow to nonaerated areas. Several studies showed that 4 to 5 cm·H2O of PEEP during one-lung ventilation improved oxygenation, but increasing PEEP level to 8 to 10 cm·H2O did not improve oxygenation and was sometimes counterproductive.16,17 Leong et al.18 compared PEEP levels 0, 5, 8, and 10 cm·H2O during one-lung ventilation and found no differences in oxygenation.
Based on these results, several authors have promoted the use of 5 cm·H2O during one-lung ventilation for all patients.11,21,35 However, some studies suggest that it is unreasonable to apply a standardized PEEP level for all patients during one-lung ventilation and that PEEP levels should be individualized. Valenza et al.19 showed that PEEP was more effective in nonobstructive patients (high forced expiratory volume in 1 second) with lower risk of auto-PEEP than in patients with low high forced expiratory volume in 1 second. Slinger and Scott36 also showed that PEEP effectiveness regarding oxygenation and lung mechanics depends on the interaction between PEEP and auto-PEEP, which in turn depends on patient mechanical characteristics. In another study, Slinger et al.12 showed that PEEP effectively prevented atelectasis, but that the applied PEEP level should be individualized based on the static compliance curve.
The physiological and clinical effects of a particular level of PEEP are different when PEEP is used isolated or in combination with an alveolar recruitment maneuver.37 Therefore, it is difficult to compare the above studies with the ones by using the concept of lung recruitment.
Alveolar recruitment maneuver strategies are not routinely done by anesthesiologists during one-lung ventilation and are usually conducted only when hypoxemia appears.11,35,38 However, previous studies have shown that an alveolar recruitment maneuver during one-lung ventilation improves oxygenation, ventilation efficiency, and lung mechanics due to reopening of atelectatic areas.22–26 Each of these studies fixed PEEP levels between 5 and 10 cm·H2O without determining individual optimized PEEP settings. Therefore, the difference between using a standard PEEP level versus an individualized level after an alveolar recruitment maneuver in one-lung ventilation has not been elucidated.
Optimal PEEP is defined as the postalveolar recruitment maneuver PEEP level that prevents alveolar collapse while minimizing overdistension. Optimal PEEP encourages maximal arterial oxygen tension and compliance and minimal dead-space29 in restrictive,27 healthy,37 and obstructive12 lungs. Our study showed that the improved oxygenation after an alveolar recruitment maneuver was only maintained at the end of one-lung ventilation in the group with an individualized PEEP level. These results suggest that an optimal PEEP level keeps the lung open, while 5 cm·H2O of PEEP level may not prevent alveolar recollapse or derecruitment. Despite the differences of PEEP with and without an alveolar recruitment maneuver, our results are not comparable with previous studies due to methodological differences. First, we recruited both groups; to our knowledge, this is the first study comparing the effects of individualized PEEP and standardized PEEP after applying an alveolar recruitment maneuver in both groups. Second, we performed the alveolar recruitment maneuver during one-lung ventilation. Cinnella et al.24 and Tusman et al.25 recruited during one-lung ventilation, but they established a standardized PEEP level in all patients and did not evaluate oxygenation at the end of one-lung ventilation.
Our hypothesis was reinforced by the lung mechanics results. In the study group, static compliance improved after the alveolar recruitment maneuver, and the improvement was maintained during the whole procedure with one-lung ventilation. In contrast, in the control group, the postalveolar recruitment maneuver static compliance improvement was lost, presumably due to alveolar recollapse. These results are compatible with those obtained by Unzueta et al.26 and Park et al.,23 who found no differences in static compliance between groups with and without an alveolar recruitment maneuver when a fixed PEEP was applied.
Previous studies22–26 showed that an alveolar recruitment maneuver decreases the dead-space effect produced by atelectasis and improves ventilation efficiency as demonstrated by reduced alveolar dead-space volume/alveolar tidal volume in cardiothoracic surgery. We hypothesized that an optimal PEEP level might more effectively maintain the benefits of an alveolar recruitment maneuver in terms of ventilation efficiency, compared with by using a standardized PEEP level; however, our results did not confirm this. We believe that the lack of difference in dead space observed between groups depended on the amount of lung collapse. Because both groups were recruited, it is reasonable to think that the control group kept some recruitment effect by 5 cm·H2O of PEEP and that such an effect minimized the difference in dead space. Alveolar dead-space volume/alveolar tidal volume increased <5% in our study while Unzueta et al.26 showed an increase >35% because an alveolar recruitment maneuver was not performed in their control patients. Increased levels of atelectasis in patients make the changes produced by an alveolar recruitment maneuver more evident.
Despite a lack of statistical differences in pH, PaCO2 was significantly higher in the study group than in the control group. Based on the results of previous studies,39 this may have contributed to a decreased shunt and improved oxygenation in the study group through an improvement in hypoxic pulmonary vasoconstriction. This limitation should be considered in future studies.
In concordance with previous studies, we found no differences in cardiac index between groups with different levels of PEEP.40 No clinically relevant changes to cardiac index occurred during the alveolar recruitment maneuver.
Our study has some limitations. The main limitation of the study is that our discussion is based on the effects of the alveolar recruitment maneuver and PEEP on atelectasis without providing evidence from lung images or shunt measurements. Previous studies showed the effect of an alveolar recruitment maneuver on shunt and/or atelectasis by using a computed tomography scan and magnetic resonance images.27,41 We determined the effect of lung recruitment by using classical indirect measurements such as PaO2, static compliance and alveolar dead space. The second limitation is that the use of 100% FIO2 may have contributed to an increase in the amount of reabsorption atelectasis, thereby reducing PaO2 in the control group. In this way, the use of lower levels of FIO2 may have varied the differences in oxygenation observed between groups. However, the use of 100% FIO2 is the first rescue therapy when hypoxemia appears. In this case, the use of an individualized level of PEEP would prevent reabsorption atelectasis more than a standardized level of PEEP.
In conclusion, the present results showed that during one-lung ventilation, the effects of an alveolar recruitment maneuver on lung function is better preserved with an individualized level of PEEP based on a PEEP decrement trial compared with that of simple arbitrary PEEP levels of 5 cm·H2O.
Name: Carlos Ferrando, MD, PhD.
Contribution: This author helped with study design, acquisition and analysis of data, interpretation of data, and writing the article.
Attestation: Dr. Ferrando approved the final manuscript, attests to the integrity of the original data and the analysis reported in this manuscript, and is the archival author.
Name: Ana Mugarra, MD.
Contribution: This author helped with acquisition and interpretation of data.
Name: Andrea Gutierrez, MD.
Contribution: This author helped with acquisition and analysis of the data.
Name: Jose Antonio Carbonell, MD.
Contribution: This author helped with acquisition and analysis of the data.
Name: Marisa García, MD.
Contribution: This author helped with analysis and interpretation of the data.
Name: Marina Soro, MD, PhD.
Contribution: This author helped with study design.
Name: Gerardo Tusman, MD.
Contribution: This author helped with interpretation of data, drafting, and revising the manuscript.
Attestation: Dr. Tusman approved the final manuscript.
Name: Francisco Javier Belda, MD, PhD.
Contribution: This author helped with interpretation of data, drafting, and revising the article.
Attestation: Dr. Belda approved the final manuscript and attests to the integrity of the original data and the analysis reported.
This manuscript was handled by: Steven L. Shafer, MD.
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