Images in Pulmonary, Critical Care, Sleep Medicine and the Sciences Regional Distribution of Air Trapping in Chronic Obstructive Pulmonary Disease Tommaso Mauri1,2, Giacomo Bellani1,2, Domenico Salerno1, Francesco Mantegazza1, and Antonio Pesenti1,2 1
Department of Health Sciences, University of Milan–Bicocca, Monza, Italy; and 2Department of Emergency and Intensive Care, San Gerardo Hospital, Monza, Italy
Figure 1. Airway pressure (Paw) waveform of one intubated patient with COPD undergoing constant-flow volume-controlled ventilation: endexpiratory occlusion (arrow) disclosed elevated auto–positive end-expiratory pressure (i.e., z8–9 cm H2O). Figure 2. Patient was connected to electrical impedance tomography (EIT), a relatively novel lung imaging method, while undergoing decreasing external positive end-expiratory pressure (PEEP) trial. As expected, the patient’s peak inspiratory pressure and end-expiratory lung volume stopped decreasing when external PEEP dropped below the patient’s auto-PEEP (top two panels). Interestingly, EIT showed that end-expiratory deflation of dependent lung regions (third panel, black–green–white scale) stopped at higher external PEEP levels in comparison to nondependent regions, thus implying heterogeneous distribution of regional gas trapping. Finally, the bottom panel shows that ventilation was preserved across all lung regions at all PEEP levels. Paw ¼ airway pressure.
A morbidly obese 51-year-old woman with chronic obstructive pulmonary disease (COPD) was intubated and mechanically ventilated for respiratory failure and septic shock. She was sedated and paralyzed and undergoing 10 ml/kg 3 10 breaths/min volume-controlled ventilation with a constant inspiratory flow of z10 ml$kg21 $s21, which resulted in an inspiratory:expiratory time ratio of 1:4.5. In an effort to promote early assisted ventilation, we interrupted paralysis and decreased sedation. Successful switching from controlled to assisted ventilation in patients with severe COPD largely depends on careful selection Author Contributions: Conception and design: T.M., G.B., and A.P.; analysis and interpretation: T.M., G.B., D.S., F.M., and A.P.; drafting the manuscript for important intellectual content: T.M., G.B., and A.P. Am J Respir Crit Care Med Vol 188, Iss. 12, pp 1466–1467, Dec 15, 2013 Copyright ª 2013 by the American Thoracic Society DOI: 10.1164/rccm.201303-0463IM Internet address: www.atsjournals.org
Images in Pulmonary, Critical Care, Sleep Medicine and the Sciences
1467
of the external positive end-expiratory pressure (PEEPext) value (1). Intubated patients with severe COPD usually develop distal airway collapse that leads to air trapping and to increased end-expiratory airway pressure (i.e., auto-PEEP). Therefore, the patient’s inspiratory effort is, in part, dissipated by reopening collapsed airways, and the work of breathing is usually higher than in normal subjects. Application of PEEPext that equals the patient’s auto-PEEP might offset the critical airway closing pressure, thus reducing the work of breathing and the risk of developing respiratory distress (2). However, the gold standard for measuring auto-PEEP in nonparalyzed patients undergoing assisted mechanical ventilation (i.e., the drop in esophageal pressure occurring before flow becomes inspiratory) is not easily feasible in the clinical setting (3). Thus, in this patient, we decided to measure auto-PEEP using the end-expiratory airway occlusion method (4) while she was still paralyzed and on volume-controlled ventilation and to use it for PEEPext titration during assisted mechanical ventilation. As expected, the patient had significant gas trapping and auto-PEEP (z8–9 cm H2O) at PEEPext ¼ 0 cm H2O (Figure 1, arrow), whereas auto-PEEP became negligible at PEEPext ¼ 9 cm H2O (Figure 2, top panel). Gas trapping that disappears by application of PEEPext is a sign of small airway collapse during expiration (i.e., expiratory flow limitation) and regional inhomogeneity of lung mechanics, as suggested also by the long time taken for auto-PEEP to reach a plateau (Figure 1) (4). Thus, we reasoned that an imaging technique sensitive to regional heterogeneity of gas trapping might have provided additional information, as compared with the end-expiratory occlusion method, which measures only the average global value of auto-PEEP. To this end, we applied electrical impedance tomography (EIT), a relatively new, noninvasive, radiation-free, bedside lung imaging method (5). The EIT monitor (PulmoVista 500, Dra¨ger Medical GmbH, Lu¨beck, Germany) tracks changes of regional intrathoracic impedance by the application of small alternate electrical currents rotating on a 16-electrode belt placed around patient’s thorax and recording resulting potentials at 20 Hz. As air offers the highest intrathoracic impedance, EIT allows continuous monitoring of regional distribution of tidal ventilation and of regional changes in end-expiratory lung inflation (6). Thus, EIT should detect regional lung deflation caused by decreasing application of PEEPext, as long as PEEPext does not drop below the patient’s auto-PEEP. In this patient, we performed a 2-min, 3-cm-H 2 O step decreasing PEEP ext trial between 15 and 0 cm H 2 O (Figure 2, top panel), leaving all other ventilator settings unchanged and while the patient was resting in semirecumbent position. When PEEP ext was reduced from 15 to 12 cm H 2 O, from 12 to 9 cm H 2 O, and from 9 to 6 cm H 2 O, peak inspiratory pressure decreased as well (Figure 2, top panel). From the 6-cm-H 2 O step on, instead, peak pressure remained substantially unchanged (Figure 2, top panel), thus confirming that average global auto-PEEP in this patient lay somewhere between 9 and 6 cm H 2 O. Similarly, global end-expiratory lung volume decreased with PEEP ext until the 6-cm-H 2 O step (i.e., until PEEP ext remained above global auto-PEEP measured by occlusion) (Figure 2, second set of traces). Interestingly, EIT showed that regional lung deflation stopped at different PEEP ext levels (Figure 2, third panel). In particular, dependent lung regions stopped deflating and developed air trapping between 12 and 9 cm H 2 O of PEEP ext , whereas the same was true in nondependent regions at PEEP ext values between 9 and 6 cm H 2 O (Figure 2, third panel). Thus, EIT showed that auto-PEEP was 9–12 cm H 2 O in dependent lung regions and 6–9 cm H 2 O in nondependent regions. An alternative explanation, especially for dependent regions, might be alveolar collapse and atelectasis at the lowest PEEP ext levels. However, this hypothesis is contradicted by preserved ventilation (Figure 2, bottom panel) in dependent lung regions at all PEEP ext levels. Clinically, we set PEEPext at 12 cm H2O (i.e., the highest regional auto-PEEP measured by EIT), and the patient was successfully switched to assisted mechanical ventilation. Unfortunately, the patient acquired a multidrug-resistant infection and died 2 months later. In conclusion, we documented in a patient with severe COPD undergoing controlled MV the preferential regional closure of dependent airways, which has already been described in both COPD and obesity (7). However, our data generate the new hypothesis that, in such patients, EIT might be a valid tool to image regional gas trapping and to guide titration of mechanical ventilation settings. In particular, this technique could prove useful if the purpose is to facilitate ventilation of the most disadvantaged dependent regions. Author disclosures are available with the text of this article at www.atsjournals.org.
References 1. Petrof BJ, Legaré M, Goldberg P, Milic-Emili J, Gottfried SB. Continuous positive airway pressure reduces work of breathing and dyspnea during weaning from mechanical ventilation in severe chronic obstructive pulmonary disease. Am Rev Respir Dis 1990;141:281–289. 2. Ranieri VM, Giuliani R, Cinnella G, Pesce C, Brienza N, Ippolito EL, Pomo V, Fiore T, Gottfried SB, Brienza A. Physiologic effects of positive end-expiratory pressure in patients with chronic obstructive pulmonary disease during acute ventilatory failure and controlled mechanical ventilation. Am Rev Respir Dis 1993;147:5–13. 3. Bellani G, Mauri T, Coppadoro A, Grasselli G, Patroniti N, Spadaro S, Sala V, Foti G, Pesenti A. Estimation of patient’s inspiratory effort from the electrical activity of the diaphragm. Crit Care Med 2013;41:1483–1491.
4. Marini JJ. Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years. Am J Respir Crit Care Med 2011;184:756–762. 5. Bellani G, Mauri T, Pesenti A. Imaging in acute lung injury and acute respiratory distress syndrome. Curr Opin Crit Care 2012; 18:29–34. 6. Mauri T, Bellani G, Confalonieri A, Tagliabue P, Turella M, Coppadoro A, Citerio G, Patroniti N, Pesenti A. Topographic distribution of tidal ventilation in acute respiratory distress syndrome: effects of positive end-expiratory pressure and pressure support. Crit Care Med 2013;41: 1664–1673. 7. Salome CM, King GG, Berend N. Physiology of obesity and effects on lung function. J Appl Physiol 2010;108:206–211.
Department of Health Sciences, University of Milan–Bicocca, Monza, Italy; and 2Department of Emergency and Intensive Care, San Gerardo Hospital, Monza, Italy
Figure 1. Airway pressure (Paw) waveform of one intubated patient with COPD undergoing constant-flow volume-controlled ventilation: endexpiratory occlusion (arrow) disclosed elevated auto–positive end-expiratory pressure (i.e., z8–9 cm H2O). Figure 2. Patient was connected to electrical impedance tomography (EIT), a relatively novel lung imaging method, while undergoing decreasing external positive end-expiratory pressure (PEEP) trial. As expected, the patient’s peak inspiratory pressure and end-expiratory lung volume stopped decreasing when external PEEP dropped below the patient’s auto-PEEP (top two panels). Interestingly, EIT showed that end-expiratory deflation of dependent lung regions (third panel, black–green–white scale) stopped at higher external PEEP levels in comparison to nondependent regions, thus implying heterogeneous distribution of regional gas trapping. Finally, the bottom panel shows that ventilation was preserved across all lung regions at all PEEP levels. Paw ¼ airway pressure.
A morbidly obese 51-year-old woman with chronic obstructive pulmonary disease (COPD) was intubated and mechanically ventilated for respiratory failure and septic shock. She was sedated and paralyzed and undergoing 10 ml/kg 3 10 breaths/min volume-controlled ventilation with a constant inspiratory flow of z10 ml$kg21 $s21, which resulted in an inspiratory:expiratory time ratio of 1:4.5. In an effort to promote early assisted ventilation, we interrupted paralysis and decreased sedation. Successful switching from controlled to assisted ventilation in patients with severe COPD largely depends on careful selection Author Contributions: Conception and design: T.M., G.B., and A.P.; analysis and interpretation: T.M., G.B., D.S., F.M., and A.P.; drafting the manuscript for important intellectual content: T.M., G.B., and A.P. Am J Respir Crit Care Med Vol 188, Iss. 12, pp 1466–1467, Dec 15, 2013 Copyright ª 2013 by the American Thoracic Society DOI: 10.1164/rccm.201303-0463IM Internet address: www.atsjournals.org
Images in Pulmonary, Critical Care, Sleep Medicine and the Sciences
1467
of the external positive end-expiratory pressure (PEEPext) value (1). Intubated patients with severe COPD usually develop distal airway collapse that leads to air trapping and to increased end-expiratory airway pressure (i.e., auto-PEEP). Therefore, the patient’s inspiratory effort is, in part, dissipated by reopening collapsed airways, and the work of breathing is usually higher than in normal subjects. Application of PEEPext that equals the patient’s auto-PEEP might offset the critical airway closing pressure, thus reducing the work of breathing and the risk of developing respiratory distress (2). However, the gold standard for measuring auto-PEEP in nonparalyzed patients undergoing assisted mechanical ventilation (i.e., the drop in esophageal pressure occurring before flow becomes inspiratory) is not easily feasible in the clinical setting (3). Thus, in this patient, we decided to measure auto-PEEP using the end-expiratory airway occlusion method (4) while she was still paralyzed and on volume-controlled ventilation and to use it for PEEPext titration during assisted mechanical ventilation. As expected, the patient had significant gas trapping and auto-PEEP (z8–9 cm H2O) at PEEPext ¼ 0 cm H2O (Figure 1, arrow), whereas auto-PEEP became negligible at PEEPext ¼ 9 cm H2O (Figure 2, top panel). Gas trapping that disappears by application of PEEPext is a sign of small airway collapse during expiration (i.e., expiratory flow limitation) and regional inhomogeneity of lung mechanics, as suggested also by the long time taken for auto-PEEP to reach a plateau (Figure 1) (4). Thus, we reasoned that an imaging technique sensitive to regional heterogeneity of gas trapping might have provided additional information, as compared with the end-expiratory occlusion method, which measures only the average global value of auto-PEEP. To this end, we applied electrical impedance tomography (EIT), a relatively new, noninvasive, radiation-free, bedside lung imaging method (5). The EIT monitor (PulmoVista 500, Dra¨ger Medical GmbH, Lu¨beck, Germany) tracks changes of regional intrathoracic impedance by the application of small alternate electrical currents rotating on a 16-electrode belt placed around patient’s thorax and recording resulting potentials at 20 Hz. As air offers the highest intrathoracic impedance, EIT allows continuous monitoring of regional distribution of tidal ventilation and of regional changes in end-expiratory lung inflation (6). Thus, EIT should detect regional lung deflation caused by decreasing application of PEEPext, as long as PEEPext does not drop below the patient’s auto-PEEP. In this patient, we performed a 2-min, 3-cm-H 2 O step decreasing PEEP ext trial between 15 and 0 cm H 2 O (Figure 2, top panel), leaving all other ventilator settings unchanged and while the patient was resting in semirecumbent position. When PEEP ext was reduced from 15 to 12 cm H 2 O, from 12 to 9 cm H 2 O, and from 9 to 6 cm H 2 O, peak inspiratory pressure decreased as well (Figure 2, top panel). From the 6-cm-H 2 O step on, instead, peak pressure remained substantially unchanged (Figure 2, top panel), thus confirming that average global auto-PEEP in this patient lay somewhere between 9 and 6 cm H 2 O. Similarly, global end-expiratory lung volume decreased with PEEP ext until the 6-cm-H 2 O step (i.e., until PEEP ext remained above global auto-PEEP measured by occlusion) (Figure 2, second set of traces). Interestingly, EIT showed that regional lung deflation stopped at different PEEP ext levels (Figure 2, third panel). In particular, dependent lung regions stopped deflating and developed air trapping between 12 and 9 cm H 2 O of PEEP ext , whereas the same was true in nondependent regions at PEEP ext values between 9 and 6 cm H 2 O (Figure 2, third panel). Thus, EIT showed that auto-PEEP was 9–12 cm H 2 O in dependent lung regions and 6–9 cm H 2 O in nondependent regions. An alternative explanation, especially for dependent regions, might be alveolar collapse and atelectasis at the lowest PEEP ext levels. However, this hypothesis is contradicted by preserved ventilation (Figure 2, bottom panel) in dependent lung regions at all PEEP ext levels. Clinically, we set PEEPext at 12 cm H2O (i.e., the highest regional auto-PEEP measured by EIT), and the patient was successfully switched to assisted mechanical ventilation. Unfortunately, the patient acquired a multidrug-resistant infection and died 2 months later. In conclusion, we documented in a patient with severe COPD undergoing controlled MV the preferential regional closure of dependent airways, which has already been described in both COPD and obesity (7). However, our data generate the new hypothesis that, in such patients, EIT might be a valid tool to image regional gas trapping and to guide titration of mechanical ventilation settings. In particular, this technique could prove useful if the purpose is to facilitate ventilation of the most disadvantaged dependent regions. Author disclosures are available with the text of this article at www.atsjournals.org.
References 1. Petrof BJ, Legaré M, Goldberg P, Milic-Emili J, Gottfried SB. Continuous positive airway pressure reduces work of breathing and dyspnea during weaning from mechanical ventilation in severe chronic obstructive pulmonary disease. Am Rev Respir Dis 1990;141:281–289. 2. Ranieri VM, Giuliani R, Cinnella G, Pesce C, Brienza N, Ippolito EL, Pomo V, Fiore T, Gottfried SB, Brienza A. Physiologic effects of positive end-expiratory pressure in patients with chronic obstructive pulmonary disease during acute ventilatory failure and controlled mechanical ventilation. Am Rev Respir Dis 1993;147:5–13. 3. Bellani G, Mauri T, Coppadoro A, Grasselli G, Patroniti N, Spadaro S, Sala V, Foti G, Pesenti A. Estimation of patient’s inspiratory effort from the electrical activity of the diaphragm. Crit Care Med 2013;41:1483–1491.
4. Marini JJ. Dynamic hyperinflation and auto-positive end-expiratory pressure: lessons learned over 30 years. Am J Respir Crit Care Med 2011;184:756–762. 5. Bellani G, Mauri T, Pesenti A. Imaging in acute lung injury and acute respiratory distress syndrome. Curr Opin Crit Care 2012; 18:29–34. 6. Mauri T, Bellani G, Confalonieri A, Tagliabue P, Turella M, Coppadoro A, Citerio G, Patroniti N, Pesenti A. Topographic distribution of tidal ventilation in acute respiratory distress syndrome: effects of positive end-expiratory pressure and pressure support. Crit Care Med 2013;41: 1664–1673. 7. Salome CM, King GG, Berend N. Physiology of obesity and effects on lung function. J Appl Physiol 2010;108:206–211.
