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The main stream of intraabdominal surgery has changed from laparotomy to laparoscopy, but anesthetic care for laparoscopic surgery is challenging for clinicians, because pneumoperitoneum might aggravate respiratory mechanics and arterial oxygenation. The authors reviewed the literature regarding ventilation strategies that reduce deleterious pulmonary physiologic changes during pneumoperitoneum for laparoscopic surgery under general anesthesia and make appropriate recommendations.
As the previous standard laparotomic procedure now alternates with laparoscopy, anesthetic attentions should be reestablished for laparoscopic procedures. Although laparoscopy has the merits of early recovery, less pain, and short hospital stay, pneumoperitoneum for suitable surgical visualization and instrument manipulation, produces negative physiologic changes and modifies respiratory mechanics .
Carbon dioxide (CO2) pneumoperitoneum increases intraabdominal pressure, and thus, may alter respiratory mechanics, by decreasing lung volume and increasing airway pressure and end-tidal CO2 tension. During pneumoperitoneum, peak and plateau airway pressures increase by more than 50% and lead to around 50% decreases in dynamic and static lung compliances in the Trendelenburg position . Furthermore, in this position, diminished respiratory compliance might not recover completely even after CO2 deflation . In an earlier study, pulmonary compliance decreased by up to 50% at an intra-abdominal pressure of 16 mmHg in a head-up tilt position during laparoscopic cholecystectomy .
Regarding gas exchanges, a previous clinical study revealed that alveolar dead space-to-tidal volume ratio was significantly greater during laparoscopic than open surgery . Consequent diaphragm cephalic shift induces basal lung collapse, reducing functional residual capacity and increasing intrapulmonary shunt and ventilation-perfusion mismatch. These changes might also cause hypoxemia or increase the alveolar-arterial oxygen tension gradient (AaDO2) . Here, we propose appropriate ventilator strategies to reduce deleterious pulmonary physiologic changes during laparoscopic surgery under general anesthesia.
Volume-controlled ventilation (VCV) has been widely used for general anesthesia, and has the merit of a guaranteed preset tidal volume. However, it presents the risk of increased airway pressure when pulmonary compliances change. Unlike VCV, pressure-controlled ventilation (PCV) has less risk of barotrauma because peak airway pressure is limited, but it cannot ensure tidal volume.
During pneumoperitoneum, PCV might be advocated because of a significant increase in airway pressure after CO2 insufflation. A previous randomized trial showed that PCV provides lower peak airway pressures and greater dynamic lung compliances at 60 and 120 min during robot-assisted laparoscopic radical prostatectomy in the steep Trendelenburg position than VCV at similar tidal volumes . Other reports have also demonstrated significantly higher compliance with lower peak airway pressure for PCV than VCV during gynecologic laparoscopy [8,9]. In a study of urologic laparoscopy, dynamic lung compliance improved after switching to PCV at the end of a VCV sequence when tidal volume, respiratory rate, and FiO2 were maintained at constant levels . However, despite positive results for respiratory compliance, PCV might not improve arterial oxygenation. The majority of clinical studies on the topic have suggested PaO2 levels are similar for PCV and VCV [7,8,9,10], and that PaO2/FiO2 ratios were comparable after adapting PCV or VCV (446 ± 77 vs. 458 ± 64) . In a prospective comparative gynecologic laparoscopy study, similar PaO2 values were reported for VCV and PCV from anesthesia induction to extubation .
Even in the steep Trendelenburg position, PCV might not improve oxygenation despite improving dynamic lung compliance. Choi et al.  reported that PaO2 significantly decreased after pneumoperitoneum during both PCV and VCV and there was no difference in PaO2, despite PCV had significantly higher dynamic compliance than VCV after 60 or 120 min in the steep Trendelenburg position with pneumoperitoneum. However, in obese patients (body mass index [BMI] > 35 kg/m2), PCV might help improve oxygenation during pneumoperitoneum . In a previous clinical study, it was observed that PCV produced significantly higher PaO2 than VCV during laparoscopic bariatric surgery , and the authors suggested that improved ventilation/perfusion ratio as evidenced by a marked difference in AaDO2, might be responsible for oxygenation differences in these two ventilator modes . It was concluded that greater inspiratory flow for PCV than VCV (52 vs. 41 L/min) at a given tidal volume might better allow alveolar recruitment . These findings suggest PCV might have more beneficial effects on the respiratory mechanics and arterial oxygenation than VCV in the morbidly obese during laparoscopy.
Many clinicians express concern about hemodynamic deterioration when positive end-expiratory pressure (PEEP) is added at an elevated intra-abdominal pressure. Theoretically, pneumoperitoneum increases intra-abdominal pressure, and consequently intra-thoracic pressure, which might inhibit venous return and reduce cardiac output, and the application of PEEP might aggravate these responses. A previous animal study of laparoscopic pelvic lymphadenectomy showed that pneumoperitoneum at 15 mmHg had no negative cardiovascular effect, but adding 8 cmH2O of PEEP markedly aggravated hemodynamic parameters . In addition, in another animal model, the application of 5 or 10 cmH2O of PEEP during pneumoperitoneum significantly perturbed hemodynamic variables. The authors suggested that pneumoperitoneum might increase ventricular afterload, and thus, aggravate the adverse effects of PEEP, especially in critically ill patients .
Optimal PEEP might improve arterial oxygenation without inducing hemodynamic deterioration during pneumoperitoneum. In a comparative study of the PEEP application at 5 cmH2O vs. no PEEP during endoscopic robot-assisted radical prostatectomy in the steep Trendelenburg position, PEEP significantly increased PaO2 from 3 h after pneumoperitoneum as compared with no PEEP, but no intergroup differences were observed between mean arterial pressures, heart rates, or central venous pressures during pneumoperitoneum . In a study undertaken to determine optimal PEEP level during laparoscopic urologic surgery, PaO2 and AaDO2 were found to be higher in patients with any level of PEEP (3, 5, 7, or 10 cmH2O) than in those without . It was suggested that PEEP at 7 cmH2O might be optimal for improving PaO2 without excessively increasing peak airway pressure or aggravating hemodynamic parameters . During laparoscopic cholecystectomy, levels of PEEP from 5 to 20 cmH2O did not reduce abdominal perfusion pressure, which was calculated by subtracting intra-abdominal pressure during pneumoperitoneum at 15 mmHg from mean arterial pressure . Another study also reported that PEEP at 10 cmH2O increased lung compliance and arterial oxygenation without hemodynamic compromise during laparoscopic cholecystectomy .
In the morbidly obese during laparoscopic gastric banding surgery, PEEP of 10 cmH2O alone did not improve PaO2 in the supine position, but a combination of PEEP and the head-up position improved oxygenation versus no PEEP in the supine position . Erlandsson et al.  suggested based on electric impedance tomographic findings, the optimal level of PEEP was 15 ± 1 cmH2O in morbidly obese patients with stable cardiac index during laparoscopic procedures. Taken together, it appears that the application of an optimal level of PEEP might improve arterial oxygenation adversely affecting hemodynamic parameters.
The alveolar recruitment maneuver is used to improve arterial oxygenation in cases of lung injury [20,21]. It opens collapsed alveoli while PEEP prevents alveolar de-recruitment under low tidal volume ventilation [22,23].
During laparoscopic gynecologic surgery, pre-emptive ap plication of alveolar recruitment (10 manual breaths at peak inspiratory pressure of 40 cmH2O with PEEP at 15 cmH2O) before peritoneal gas insufflation significantly improved PaO2 and alveolar-to-arterial oxygen tension gradient at 15 and 30 min after gas insufflation as compared with no recruitment maneuver during constant FiO2 of 0.35 and tidal volume of 10 ml/kg without PEEP . In a comparative study of obese patients (BMI > 35 kg/m2) and normal weight patients (BMI < 25 kg/m2) in the 30° head-up position, the recruitment maneuver combined with PEEP at 10 cmH2O improved end-expiratory lung volume, respiratory mechanics, and arterial oxygenation, whereas PEEP alone caused no improvement in obese or normal weight patients . Another study compared the effects of only PEEP (10 cmH2O), only recruitment maneuver (inspiratory hold at 55 cmH2O for 10 s), and recruitment maneuver followed by PEEP in the morbidly obese (BMI > 40 kg/m2) and showed that the recruitment maneuver and PEEP in combination significantly reduced atelectasis and increased PaO2/FiO2 ratio versus baseline, but PEEP or the recruitment maneuver alone did not improve these parameters .
Although the recruitment maneuver improves oxygenation deficits, its effect might not be persistent after surgery [27,28]. In a robot-assisted laparoscopic prostatectomy study, a single recruitment maneuver of 40 cmH2O for 40 seconds at constant PEEP of 15 cmH2O in the Trendelenburg position improved PaO2 during pneumoperitoneum, but their effects were short-lived after desufflation . Also, in morbidly obese patients, the recruitment maneuver (4 sustained lung inflations at peak inspiratory pressure of 50 cmH2O with PEEP at 12 cmH2O) significantly increased intraoperative PaO2 and dynamic compliance versus PEEP at 4 cmH2O alone, but its beneficial effects disappeared rapidly after tracheal extubation . Furthermore, application of the recruitment maneuver increased vasopressor requirements due to hemodynamic deterioration .
In summary, the recruitment maneuver with optimal PEEP might be beneficial for improving arterial oxygenation during mechanical ventilation, but cautious hemodynamic monitoring is required.
Inverse ratio ventilation means extending the inspiratory time beyond that required to maintain a conventional inspiratory to expiratory (I : E) ratio of 1 : 2. Prolonged I : E ratio ventilation has the possible merit of improving oxygen delivery by preventing an alveolar collapse in acute lung injury or acute respiratory distress syndrome. However, excessive prolongation of inspiratory time poses the risk of reduced cardiac output, especially in patients with poor pulmonary compliance [29,30,31,32]. In addition, theoretically, a prolonged inspiratory time might result in CO2 retention due to a reduced expiratory time.
The effects of prolonged inspiratory time on respiratory mechanics and oxygenation remain controversial during general anesthesia [33,34,35,36,37]. The application of equal I : E ratio (1 : 1) ventilation for 20 min did not improve respiratory mechanics or arterial oxygen saturation during laparoscopic surgery as compared with conventional ratio ventilation (1 : 2) . However, in an analysis of seven trials, including one-lung ventilation or pneumoperitoneum, equal ratio ventilation significantly improved oxygenation at 60 min after intervention, but not at 20 or 30 min after intervention . In the same study, during laparoscopic surgery, equal ratio ventilation improved oxygenation throughout intervention . In a comparative study, the PaO2/FiO2 ratio was significantly higher at I : E ratios of 1 : 1, 2 : 1, and 1 : 2 with PEEP at 5 cmH2O than at an I : E ratio of 1 : 2 without PEEP at 60 min after pneumoperitoneum for gynecologic laparoscopy , and PaCO2 was significantly lower at an I : E ratio of 2 : 1 than at other ratios at 60 min after pneumoperitoneum . The authors that suggested prolonged I : E ratio ventilation provided superior arterial oxygenation and CO2 elimination without compromising lung mechanics as compared with conventional ratio ventilation with PEEP in patients undergoing laparoscopic surgery . A recent clinical study on laparoscopic bariatric surgery demonstrated that sequentially changes in I : E ratio from 1 : 2 to 1 : 1 to 2 : 1 while applying PEEP at 5 cmH2O, significantly reduced peak airway pressure and increased mean airway pressure and dynamic compliance without hemodynamic consequences . Also, PaO2 was significantly higher and AaDO2 was significantly lower at an I : E ratio of 2 : 1 than at the conventional ratio of 1 : 2 . During this previous clinical study, PaCO2 levels were similar regardless of I : E ratio differences . Taken together, increased I : E ratio (1 : 1 or 2 : 1) ventilation might improve respiratory mechanics and arterial gas exchanges, including arterial oxygenation and CO2 elimination, by reducing dead space and increasing alveolar ventilation during pneumoperitoneum [31,32,35,36].
The benefits of deep neuromuscular blocks for laparoscopic procedures are controversial and most of the studies undertaken have only sought to improve surgical conditions [38,39,40]. Theoretically, deep neuromuscular block permits a lower abdominal insufflation pressure, which leads to better respiratory mechanics and gas exchange. In one clinical study, it was noted that the average adjusted intra-abdominal pressure was significantly lower after deep neuromuscular block as compared with moderate block (9.3 vs. 12 mmHg) . However, another study demonstrated no benefit in terms of reducing insufflation pressure . To the best of our knowledge, no published clinical evidence demonstrates that deep neuromuscular block has a beneficial effect on respiratory mechanics during pneumoperitoneum. However, one case report described a successful anesthetic experience with a deep neuromuscular block for laparoscopic appendectomy in a patient with end-stage lung disease .
In conclusion, PCV rather than VCV, the application of PEEP and the recruitment maneuver, and prolonged I : E ratio ventilation might improve respiratory mechanics or oxygenation during laparoscopic surgery, but if employed, hemodynamic compromise should be considered.