Monitoring alveolar derecruitment at bedside using functional residual capacity measurements in cardiac surgery patients

Abstract

Background: Monitoring of functional residual capacity (FRC) may help detecting alveolar derecruitment and guiding countermeasures like recruitment maneuvers. The aim of this study was to detect the possible decrease of FRC after a suctioning procedure indicating alveolar derecruitment and the effect of a successful alveolar recruitment maneuver on pulmonary function. Methods: We studied 20 postoperative mechanically ventilated cardiac surgery patients. FRC was assessed by oxygen washout using a sidestream O2-analyser (LUFU system (Drager Medical AG, Luebeck, Germany)). FRC, respiratory compliance, paO2/FiO2 (PF-ratio) and paCO2 were recorded at baseline, after a standard suctioning procedure with disconnection of the ventilator (20 sec, 14 F catheter, 200 mmHg negative pressure) (post ETS), and after a standard recruitment manoeuvre (PEEP 15 mbar, PIP 35–40 mbar for 30 sec) (post RM). Results: Mean FRC decreased post ETS (3.2 L ± 1.2) compared to baseline (3.4 L ± 1.1, p = 0.046) and increased post RM compared to post ETS (3.5 L ± 1.0, p=0.039). Mean respiratory compliance showed no significant changes. paCO2 decreased and PF-ratio increased post RM compared to post ETS (p = 0.005, p = 0.026, respectively). Relative FRC changes post ETS correlated to changes of PF-ratio post RM (Pearson: -0.896, p<0.001). Conclusion: In post cardiac surgery patients changes of FRC after alveolar deand recruitment can be detected at bedside using the oxygen washout technique. There was a strong association of a FRC-decrease after open ETS with an improvement of oxygenation from a consecutive RM. A FRC decrease may help to identify those patients who profited from a RM in terms of increased oxygenation, even in absence of blood gas data. 28 H. Heinze, B. Sedemund-Adib, U. W. Gosch, W. Eichler this open lung approach. As there is a high inter-individual variability in the tendency for alveolar derecruitment (6), in some patients reconnection to the ventilator with unchanged ventilatory settings may be enough to restore lung volume, but others might need a RM. In clinical routine this is difficult to judge. Many methods currently available for assessment of alveolar deor recruitment are only used in research situations (7,8) or are not suitable at bedside in clinical routine (9). Changes of oxygenation or CO2-removal are probably the most widely used criteria to monitor the effect of RM, but have the disadvantage of being labor intensive, as repeated blood gas analyses are necessary. In addition, oxygenation may change not only by alveolar deor recruitment, but also by pulmonary hemodynamic changes (10). CO2-removal is influenced by CO2 production, which can be extremely variable in critically ill patients. Measures of lung mechanics have also been used for guiding therapy and respiratory compliance has been shown to correlate with changes in lung aeration in an experimental study in pigs (11), but is difficult to assess in patients, especially during spontaneous breathing. Recently, direct measurements of lung volume have gained more interest, because functional residual capacity (FRC) is independently affected by alveolar deor recruitment (12). Routine FRC measurement using the oxygen washout technique is now applicable at bed-side and has been introduced into clinical practice (13,14). A decrease in FRC after disconnection and / or endotracheal suctioning indicates alveolar derecruitment and may identify patients who need a RM to restore lung volume. The aim of this study was to detect the possible decrease of FRC after a suctioning procedure indicating alveolar derecruitment and the effect of a successful alveolar recruitment manoeuvre on pulmonary function.