pulmonary gas exchange and respiratory mechanics are usually impaired during general anesthesia and muscle paralysis . functional residual capacity ( frc ) decreases approximately 20  % during induction of anesthesia . reduced frc can be restored somewhat through ventilation using positive end - expiratory pressure ( peep ) . nevertheless , few reports in the literature describe studies of the influence of peep level upon frc in ventilated patients during administration of general anesthesia . frc was reduced by induction of anesthesia , but no report in the literature has described a comparison of frc values with those of stepwise increase of peep during upper abdominal surgery . we measured frc for mechanically ventilated general anesthesia with different peep levels during upper abdominal surgery and subsequently compared them with awake levels measured for the subject in the supine position . we hypothesized that frc , compliance , and arterial oxygenation show positive effects of increased peep , and that 5  cmh2o peep or 10  cmh2o peep increased frc nearly to the awake level . the wash - in or wash - out of a tracer gas in a multiple breath maneuver is apparently best applied at bedside . promising techniques for nitrogen or oxygen wash - in / wash - out with reasonable accuracy and repeatability have been presented . this technique , which estimates frc with good precision using a change of fio2 of only 0.1 , has been evaluated in volunteers and in patients with acute respiratory distress syndrome . we studied nine patients with american society of anesthesiologists physical status classification system grade 1 or grade 2 who had been scheduled for elective upper abdominal surgery . the arterial line , inserted under local anesthesia , was used for the study protocol . monitoring included invasive arterial blood pressure , a continuous electrocardiogram , and peripheral oxygen saturation ( spo2 ) . after induction of anesthesia and tracheal intubation , end - tidal carbon dioxide concentration ( etco2 ) , airway pressure , tidal volume , and respiratory rate were monitored and recorded . before induction of anesthesia ( awake ) , a tight - seal facemask was used for spontaneous ventilation ( engstrm carestation ; ge healthcare , madison , wi , usa ) . the facemask was applied with 2  cmh2o pressure support ventilation using an inspired oxygen fraction ( fio2 ) of 1.0 during frc measurement ( frc inview ) . the frc was designated as the facemask frc value minus facemask dead space ( 100  ml ) . arterial blood gas samples were collected from subjects who were in a supine position ( fig .  1 ) . anesthesia was induced as target - controlled infusion of propofol 45  g / ml with a calculated target plasma , fentanyl 12  g / kg i.v . , and rocuronium bromide 0.6  mg / kg i.v.fig . ps pressure support ventilation , frc functional residual capacity , abg arterial blood gases , tv tidal volume , peep positive end - expiratory pressure . predicted body weight : men , 49.9  +  0.91 [ height ( cm ) , 152.4 ] ; women , 45.4  +  0.91 [ height ( cm ) , 152.4 ] schematic protocol used for this study . ps pressure support ventilation , frc functional residual capacity , abg arterial blood gases , tv tidal volume , peep positive end - expiratory pressure . predicted body weight : men , 49.9  +  0.91 [ height ( cm ) , 152.4 ] ; women , 45.4  +  0.91 [ height ( cm ) , 152.4 ] predicted body weight was calculated as 45.4  +  0.91    [ height ( cm )    152.4 ] for women or 49.9  +  0.91    [ height ( cm )    152.4 ] for men . after tracheal intubation , anesthesia was maintained with a propofol target plasma drug concentration of 2.53  g / ml and remifentanil 0.20.4  g / kg / min with thoracic epidural analgesia . lungs were ventilated in bilevel airway pressure with volume guarantee ( pressure - regulated volume control ) . the tidal volume of 7  ml / kg was estimated according to the predicted body weight . frc measurements and arterial blood gas samples were taken from 30  min after intubation ( pre - ope ) . from 1  h after intubation , frc measurements and arterial blood gas samples were obtained during upper abdominal surgery ( opep0 ) . after 1 and 2  h peep 5  cmh2o , frc measurements and arterial blood gas samples were collected ( opep5.1 and opep5.2 ) . frc measurements and arterial blood gas samples were also collected at 1 and 2  h after the stepwise increase of peep 10  cmh2o ( opep10.1 and opep10.2 ) . the quasistatic compliance of the respiratory system was calculated as the tidal volume divided by the inspiratory plateau pressure minus the end - expiratory pressure . comparison among groups was performed using two - way analysis of variance for repeated one - way measurements . correlation between peep and frc or the pao2/fio2 ratio was analyzed using pearson s correlation . no premedication was administered . before induction of anesthesia ( awake ) , a tight - seal facemask was used for spontaneous ventilation ( engstrm carestation ; ge healthcare , madison , wi , usa ) . the facemask was applied with 2  cmh2o pressure support ventilation using an inspired oxygen fraction ( fio2 ) of 1.0 during frc measurement ( frc inview ) . the frc was designated as the facemask frc value minus facemask dead space ( 100  ml ) . arterial blood gas samples were collected from subjects who were in a supine position ( fig .  1 ) . anesthesia was induced as target - controlled infusion of propofol 45  g / ml with a calculated target plasma , fentanyl 12  g / kg i.v . , and rocuronium bromide 0.6  mg / kg i.v.fig . ps pressure support ventilation , frc functional residual capacity , abg arterial blood gases , tv tidal volume , peep positive end - expiratory pressure . predicted body weight : men , 49.9  +  0.91 [ height ( cm ) , 152.4 ] ; women , 45.4  +  0.91 [ height ( cm ) , 152.4 ] schematic protocol used for this study . ps pressure support ventilation , frc functional residual capacity , abg arterial blood gases , tv tidal volume , peep positive end - expiratory pressure . predicted body weight : men , 49.9  +  0.91 [ height ( cm ) , 152.4 ] ; women , 45.4  +  0.91 [ height ( cm ) , 152.4 ] predicted body weight was calculated as 45.4  +  0.91    [ height ( cm )    152.4 ] for women or 49.9  +  0.91    [ height ( cm )    152.4 ] for men . after tracheal intubation , anesthesia was maintained with a propofol target plasma drug concentration of 2.53  g / ml and remifentanil 0.20.4  g / kg / min with thoracic epidural analgesia . lungs were ventilated in bilevel airway pressure with volume guarantee ( pressure - regulated volume control ) . the tidal volume of 7  ml / kg was estimated according to the predicted body weight . frc measurements and arterial blood gas samples were taken from 30  min after intubation ( pre - ope ) . from 1  h after intubation , frc measurements and arterial blood gas samples were obtained during upper abdominal surgery ( opep0 ) . after 1 and 2  h peep 5  cmh2o , frc measurements and arterial blood gas samples were collected ( opep5.1 and opep5.2 ) . frc measurements and arterial blood gas samples were also collected at 1 and 2  h after the stepwise increase of peep 10  cmh2o ( opep10.1 and opep10.2 ) . the quasistatic compliance of the respiratory system was calculated as the tidal volume divided by the inspiratory plateau pressure minus the end - expiratory pressure . comparison among groups was performed using two - way analysis of variance for repeated one - way measurements . correlation between peep and frc or the pao2/fio2 ratio was analyzed using pearson s correlation . the value of frc awake was significantly higher than that for pre - ope or opep0 ( p  <  0.01 ) . the frc ope10.1 and ope10.2 values were significantly higher than that for opep5.1 ( p  <  0.01 ) . the frc for opep10.2 was significantly higher than that for ope5.2 ( p  <  0.01).fig . 2functional residual capacity for peep levels studied . * * p  <  0.01 versus preoperative ( pre - op ) and opep0 , p  <  0.01 versus opep5.1 , p  <  0.01 versus opep5.2 pre - induction ( pre - ind ) . p postoperative , frc measured pre - ind frc  mask dead space ( 100  ml ) functional residual capacity for peep levels studied . * * p  <  0.01 versus preoperative ( pre - op ) and opep0 , p  <  0.01 versus opep5.1 , p  <  0.01 versus opep5.2 pre - induction ( pre - ind ) . p postoperative , frc measured pre - ind frc  mask dead space ( 100  ml ) figure  3 presents oxygenation data . the pao2/fio2 awake was significantly higher than that for pre - ope , opep0 , or ope5.1 ( p  <  0.01 ) . peep 5  cmh2o and peep 10  cmh2o were significantly higher than either pre - ope or opep0 ( p  <  0.01 ) ( table  1).fig .  3pao2/fio2 for peep levels studied . * * p  <  0.01 versus pre - op and opep0 , p  <  0.01 versus opep5.1table  1patient characteristics ( n  =  9)gender ( male : female)5:4age ( years)69    7weight ( kg)64    8height ( cm)158    6predicted body weight ( kg)53    6body mass index ( bmi)26    43pancreaticoduodectomy4pancreatectomy2hepatic lobectomy2reconstructive operation of biliary tract1values are mean    sd pao2/fio2 for peep levels studied . * * p  <  0.01 versus pre - op and opep0 , p  <  0.01 versus opep5.1 patient characteristics ( n  =  9 ) peep levels at opep0 , opep5.2 , and opep10.2 were correlated with frc ( r  =  0.671 , p  <  0.01 ) and pao2/fio2 ( r  =  0.642 , p  <  0.01 ) ( fig 4peep levels at opep0 , opep5.2 , and opep10.2 were correlated with frc ( r  =  0.671 , p  <  0.01 ) and pao2/fio2 ( r  =  0.642 , p  <  0.01 ) peep levels at opep0 , opep5.2 , and opep10.2 were correlated with frc ( r  =  0.671 , p  <  0.01 ) and pao2/fio2 ( r  =  0.642 , p  < 0.01 ) compliance was significantly higher for opep10.2 than for opep5.1 ( p  <  0.01).table  2respiratory mechanics and hemodynamic datapre - opeopep0opep5.1opep5.2opep10.1opep10.2pip ( cmh2o)12    213    217    2 * 16    2 * 21    2 * 20    2*paco2 ( mmhg)44    544    643 642    643    643    6compliance(ml / cmh2o)36    1035    935 937    938    941    10map ( mmhg)98    12114    9 * 117    22 * 109    14110    19113    14heart rate ( bpm)63    1072    1675    16 * 76    15 * 82    17 * 81    17*data are expressed as mean    sdquasistatic compliance  =  tidal volume/(inspiratory plateau pressure    peep)ope operation , p postoperative day , pip peak inspiratory pressure , map maximum arterial pressure * p  <  0.01 versus pre - operative ( pre - ope ) , p  <  0.01 versus opep0 , p  <  0.01 versus opep5.1 ,   p  <  0.01 versus opep5.2 respiratory mechanics and hemodynamic data data are expressed as mean    sd quasistatic compliance  =  tidal volume/(inspiratory plateau pressure    peep ) ope operation , p postoperative day , pip peak inspiratory pressure , map maximum arterial pressure *  p  <  0.01 versus pre - operative ( pre - ope ) , p  <  0.01 versus opep0 , p  <  0.01 versus opep5.1 ,   p  <  0.01 versus opep5.2 maximum arterial pressure and heart rate were lowest at pre - ope . no significant difference was found in hemodynamics among different peep groups . peep levels at opep0 , opep5.2 , and opep10.2 were correlated with frc ( r  =  0.671 , p  <  0.01 ) and pao2/fio2 ( r  =  0.642 , p  <  0.01 ) ( fig . opep10.2 were correlated with frc ( r  =  0.671 , p  <  0.01 ) and pao2/fio2 ( r  =  0.642 , p  <  0.01 ) peep levels at opep0 , opep5.2 , and opep10.2 were correlated with frc ( r  =  0.671 , p  <  0.01 ) and pao2/fio2 ( r  =  0.642 , p  <  0.01 ) compliance was significantly higher for opep10.2 than for opep5.1 ( p  <  0.01).table  2respiratory mechanics and hemodynamic datapre - opeopep0opep5.1opep5.2opep10.1opep10.2pip ( cmh2o)12    213    217    2 * 16    2 * 21    2 * 20    2*paco2 ( mmhg)44    544    643 642    643    643    6compliance(ml / cmh2o)36    1035    935    937    938    941    10map ( mmhg)98 12114    9 * 117    22 * 109    14110    19113    14heart rate ( bpm)63    1072    1675    16 * 76    15 * 82    17 * 81    17*data are expressed as mean    sdquasistatic compliance  =  tidal volume/(inspiratory plateau pressure    peep)ope operation , p postoperative day , pip peak inspiratory pressure , map maximum arterial pressure *  p  <  0.01 versus pre - operative ( pre - ope ) , p  <  0.01 versus opep0 , p  <  0.01 versus opep5.1 ,   p  <  0.01 versus opep5.2 respiratory mechanics and hemodynamic data data are expressed as mean    sd quasistatic compliance  =  tidal volume/(inspiratory plateau pressure    peep ) ope operation , p postoperative day , pip peak inspiratory pressure , map maximum arterial pressure *  p  <  0.01 versus pre - operative ( pre - ope ) , p  <  0.01 versus opep0 , p  <  0.01 versus opep5.1 ,   p  <  0.01 versus opep5.2 maximum arterial pressure and heart rate were lowest at pre - ope . no significant difference was found in hemodynamics among different peep groups . the first main finding of our study was that peep showed positive effects on frc , compliance , and pao2/fio2 ratio , indicating that patients undergoing upper abdominal surgery might benefit from peep 10  cmh2o . the frc examinations were performed using an engstrm carestation ( ge healthcare ) equipped with the frc inview monitoring feature . the frc is determined using the change of lung nitrogen volume after a step change in the inspired oxygen fraction . in the engstrm carestation , when ventilated with fio2  1.0 , where the o2 consumption was calculated from the co2 production with a default rq of 0.85 , very high measurement precision was attained . results show that frc was reduced by 37  % from the awake level after the induction of anesthesia . as previously reported , frc is reduced by approximately 15  % during anesthesia induced with thiopentone and maintained with halothane at fio2 0.35 . our protocol was fio2 1.0 at induction of anesthesia , which engendered absorption atelectasis because of the high concentration of oxygen . the mechanisms underlying reduction of frc include muscle paralysis , decreased chest wall recoil , increased abdominal pressure , atelectasis formation , and gas entrapment behind a closed airway . our results showed no significant change in frc between that after induction of anesthesia and that during operation at zero end - expiratory pressure ( zeep ) . during abdominal operation , however , during the surgery procedure , frc decreased and returned to post - induction levels at the end of the procedure . indeed , pulmonary gas exchange , which is impaired following induction of anesthesia , deteriorates during laparotomy but not during peripheral surgery . the difference is probably explained by the effect of surgical influences ( packs , retractors , etc . ) on frc . at opep0 , results of this study demonstrated that peep 10  cmh2o reached nearly the same frc level as that of awake frc . no data exist to elucidate waking frc for different peep levels , showing intraoperative frc changes . presented measured mean frc data for postoperative patients at different peep levels ( 0 , 5 , 10  cmh2o ) . their study revealed that peep increased frc at levels of peep 5 and 10  cmh2o ; peep decreased frc after reversion to peep of 0  cmh2o . pelosi et al .   reported that peep 10  cmh2o did not improve respiratory function in paralyzed or anesthetized postoperative patients . their mean pao2/fio2 ratio was 436  mmhg , and their tidal volume was 812  ml / kg ideal body weight . our mean pao2/fio2 ratio was 253  mmhg , and tidal volume was 7  ml / kg ideal body weight at peep 0  cmh2o intraoperatively . lower tidal volume might engender atelectasis , especially if peep is low or not used at all . described a randomized controlled nonblinded preventive trial comparing mechanical ventilation with tidal volume of 10 versus 6  ml / kg in critically ill patients without acute lung injury ( ali ) at the onset of mechanical ventilation . mechanical presentation with 10  ml / kg is associated with sustained cytokine production in plasma . those results suggest that mechanical ventilation with conventional tidal volumes contributed to the development of lung injury in patients without ali at the onset of mechanical ventilation . administration of peep alone increased the normally aerated lung fraction , which combined with a reduction of poorly aerated lung tissue while atelectasis remained unchanged . an earlier study of postoperative mechanically ventilated obese patients ( mean body mass index , 51 ) after abdominal surgery showed that peep 10  cmh2o increased pao2 , respiratory compliance , and frc . reportedly , peep higher than 10  cmh2o is associated with marked derangement of hemodynamics . reported that static compliance and the amount of atelectasis estimated using computed tomography ( ct ) did not change in parallel . described that compliance indicates an optimal level of peep after recruitment in anesthetized patients , reporting that optimal peep was 10  cmh2o because , at that pressure level , the highest compliance value in conjunction with the lowest dead space fraction revealed a maximum amount of effectively expanded alveoli . our results demonstrated that the quasistatic compliance was significantly higher for opep 10.2 than for opep 5.1 . peep 10  cmh2o did not produce the pre - operation level of pao2/fio2 , probably because of a ventilation and circulation mismatch by mechanical ventilation or because recruitment maneuvers were not used . the increased intrapleural pressure caused by peep might also increase the risk of barotrauma and cause changes to cardiovascular dynamics . two trials reported postoperative barotrauma in both peep and zeep [ 14 , 15 ] . their comparison revealed rr of 0.3 for the peep group , which was not statistically significant . it remains unclear whether the increases in frc and p / f ratio are attributable to the increase in peep level , or to a time - dependent effect of peep , or both . during a time study of 5  h operation , pao2 showed constant values at much lower tidal volumes of 6  ml / kg and peep 10  cmh2o ( 21 patients were enrolled ; 13 cases were upper abdominal surgeries with abdominal opening ) . following coronary artery bypass grafting , significant reduction of p ( a  a ) o2 during positive pressure ventilation at 10  cmh2o peep was compared with 0  cmh2o peep and 5  cmh2o peep during 6  h . these data show that pulmonary oxygenation might maintain constant values at equal peep levels . as one limitation of our study , peep was applied in a stepwise increasing fashion . we have no data for p / f ratio and frc on time - dependent effect of peep . however , the same peep level was maintained for 2  h. two hours after changing peep , the frc data were slightly higher than at 1  h after changing peep , but the values were not significantly different . the peep levels at opep0 , opep5.2 , and opep10.2 were correlated with frc and with pao2/fio2 . the peep levels might be applied in a random sequence to mitigate this potential bias in the results . data reported herein suggest that if low tidal volume ventilation is used during upper abdominal surgery , then peep 10  cmh2o is necessary to maintain lung function .