Institute of Physics and Engineering in Medicine Physiol. Meas. 36 (2015) 1137–1146
Physiological Measurement doi:10.1088/0967-3334/36/6/1137
Influence of tidal volume on ventilation inhomogeneity assessed by electrical impedance tomography during controlled mechanical ventilation T Becher, M Kott, D Schädler, B Vogt, T Meinel, N Weiler and I Frerichs Department of Anaesthesiology and Intensive Care Medicine, University Medical Centre Schleswig-Holstein, Campus Kiel, 24118 Kiel, Germany E-mail: [email protected] Received 30 November 2014, revised 10 March 2015 Accepted for publication 23 March 2015 Published 26 May 2015 Abstract
The global inhomogeneity (GI) index is a parameter of ventilation inhomogeneity that can be calculated from images of tidal ventilation distribution obtained by electrical impedance tomography (EIT). It has been suggested that the GI index may be useful for individual adjustment of positive end-expiratory pressure (PEEP) and for guidance of ventilator therapy. The aim of the present work was to assess the influence of tidal volume (VT) on the GI index values. EIT data from 9 patients with acute respiratory distress syndrome ventilated with a low and a high VT of 5 ± 1 (mean ± SD) and 9 ± 1 ml kg−1 predicted body weight at a high and a low level of PEEP (PEEPhigh, PEEPlow) were analyzed. PEEPhigh and PEEPlow were set 2 cmH2O above and 5 cmH2O below the lower inflection point of a quasi-static pressure volume loop, respectively. The lower inflection point was identified at 8.1 ± 1.4 (mean ± SD) cmH2O, resulting in a PEEPhigh of 10.1 ± 1.4 and a PEEPlow of 3.1 ± 1.4 cmH2O. At PEEPhigh, we found no significant trend in GI index with low VT when compared to high VT (0.49 ± 0.15 versus 0.44 ± 0.09, p = 0.13). At PEEPlow, we found a significantly higher GI index with low VT compared to high VT (0.66 ± 0.19 versus 0.59 ± 0.17, p = 0.01). When comparing the PEEP levels, we found a significantly lower GI index at PEEPhigh both for high and low VT. We conclude that high VT may lead to a lower GI index, especially at low PEEP settings. This should be taken into account when using the GI index for individual adjustment of ventilator settings.
0967-3334/15/061137+10$33.00 © 2015 Institute of Physics and Engineering in Medicine Printed in the UK
1137
T Becher et al
Physiol. Meas. 36 (2015) 1137
Keywords: EIT, lung inhomogeneity, global inhomogeneity index, tidal volume, PEEP, ARDS S Online supplementary data available from stacks.iop.org/PM/36/061137/ mmedia (Some figures may appear in colour only in the online journal) Abbreviations ARDS Acute respiratory distress syndrome EIT Electrical impedance tomography ETT Endotracheal tube FiO2 Fraction of inspired oxygen GI index Global inhomogeneity index GREIT Graz consensus reconstruction algorithm for EIT ID Inner diameter LIP Lower inflection point NA Not applicable Arterial partial pressure of carbon dioxide PaCO2 PaO2 Arterial partial pressure of oxygen Pat Patient PBW Predicted body weight PEEP Positive end-expiratory pressure pH Decimal logarithm of the reciprocal of the hydrogen ion activity ROI Region of interest SD Standard deviation TT Tracheostomy tube VT Tidal volume 1. Introduction In the recent years, a growing interest in electrical impedance tomography (EIT) has evolved focusing on the use of EIT as a bedside tool for monitoring of regional lung function and for guidance of ventilator settings (Frerichs et al 2014). It has been suggested that patients suffering from acute respiratory distress syndrome (ARDS) could especially benefit from a standardized protocol using EIT-derived information on ventilation distribution. ARDS is a severe pulmonary disease that, despite improvements in therapy, is still associated with a high mortality rate (Ranieri et al 2012). Ventilator therapy is the most important life-saving intervention for patients suffering from ARDS. Despite its life-saving potential, ventilator therapy carries the risk of ventilator-induced lung injury (VILI) caused by overdistension and by repeated opening and closing of alveoli (‘tidal recruitment’) and the associated inflammation (Slutsky and Ranieri 2014). Inhomogeneity in lung aeration appears to be a major contributor to VILI, mainly because of the additional shear forces that occur at the interfaces between ventilated and non-ventilated alveoli during tidal ventilation (Cressoni et al 2014). The global inhomogeneity (GI) index is a promising measure that quantifies ventilation inhomogeneity, as assessed by EIT, in a single number (Zhao et al 2009). It is calculated 1138
T Becher et al
Physiol. Meas. 36 (2015) 1137
from functional EIT images showing the tidal ventilation distribution by dividing the sum of the absolute differences between the individual pixel values and the median value of the lung region of interest (ROI) by the sum of the impedance values in the ROI. When performing a positive end-expiratory pressure (PEEP) trial, the changes in GI index are closely correlated to the changes in dynamic respiratory system compliance (Zhao et al 2010). During a low-flow pressure-volume maneuver, the GI index is highly correlated with the amount of recruitable lung tissue, as assessed by EIT (Zhao et al 2014). It has been suggested that PEEP titration according to the GI index could lead to a PEEP setting providing a reasonable balance between tidal recruitment and regional overdistension (Zhao et al 2010). When trying to optimize ventilator settings for a patient with ARDS, one must consider the fact that the amount of tidal recruitment and overdistension is influenced not only by PEEP but also, and markedly, by the selected tidal volume (VT) (Bruhn et al 2011, Zick et al 2013). Ventilation with low VT has repeatedly been proven to improve survival in patients with ARDS (Amato et al 1998, ARDS-Network 2000, Villar et al 2006). However, the influence of VT on the GI index value has not been studied so far. Since increasing VT may simultaneously cause regional recruitment and regional overdistension, we hypothesized that changes in VT would have measurable effects on the GI index. We also hypothesized that an increase in VT at low levels of PEEP could be used to predict the effect of an increase in PEEP on the GI index. 2. Methods 2.1. Materials and study procedure
We retrospectively analyzed the data of 9 sedated and mechanically ventilated patients suffering from ARDS (table 1). Some of the data have previously been published (Zick et al 2013). Patients suffering from ARDS according to the Berlin definition (Ranieri et al 2012) with need for invasive mechanical ventilation were included. Exclusion criteria were age
Physiological Measurement doi:10.1088/0967-3334/36/6/1137
Influence of tidal volume on ventilation inhomogeneity assessed by electrical impedance tomography during controlled mechanical ventilation T Becher, M Kott, D Schädler, B Vogt, T Meinel, N Weiler and I Frerichs Department of Anaesthesiology and Intensive Care Medicine, University Medical Centre Schleswig-Holstein, Campus Kiel, 24118 Kiel, Germany E-mail: [email protected] Received 30 November 2014, revised 10 March 2015 Accepted for publication 23 March 2015 Published 26 May 2015 Abstract
The global inhomogeneity (GI) index is a parameter of ventilation inhomogeneity that can be calculated from images of tidal ventilation distribution obtained by electrical impedance tomography (EIT). It has been suggested that the GI index may be useful for individual adjustment of positive end-expiratory pressure (PEEP) and for guidance of ventilator therapy. The aim of the present work was to assess the influence of tidal volume (VT) on the GI index values. EIT data from 9 patients with acute respiratory distress syndrome ventilated with a low and a high VT of 5 ± 1 (mean ± SD) and 9 ± 1 ml kg−1 predicted body weight at a high and a low level of PEEP (PEEPhigh, PEEPlow) were analyzed. PEEPhigh and PEEPlow were set 2 cmH2O above and 5 cmH2O below the lower inflection point of a quasi-static pressure volume loop, respectively. The lower inflection point was identified at 8.1 ± 1.4 (mean ± SD) cmH2O, resulting in a PEEPhigh of 10.1 ± 1.4 and a PEEPlow of 3.1 ± 1.4 cmH2O. At PEEPhigh, we found no significant trend in GI index with low VT when compared to high VT (0.49 ± 0.15 versus 0.44 ± 0.09, p = 0.13). At PEEPlow, we found a significantly higher GI index with low VT compared to high VT (0.66 ± 0.19 versus 0.59 ± 0.17, p = 0.01). When comparing the PEEP levels, we found a significantly lower GI index at PEEPhigh both for high and low VT. We conclude that high VT may lead to a lower GI index, especially at low PEEP settings. This should be taken into account when using the GI index for individual adjustment of ventilator settings.
0967-3334/15/061137+10$33.00 © 2015 Institute of Physics and Engineering in Medicine Printed in the UK
1137
T Becher et al
Physiol. Meas. 36 (2015) 1137
Keywords: EIT, lung inhomogeneity, global inhomogeneity index, tidal volume, PEEP, ARDS S Online supplementary data available from stacks.iop.org/PM/36/061137/ mmedia (Some figures may appear in colour only in the online journal) Abbreviations ARDS Acute respiratory distress syndrome EIT Electrical impedance tomography ETT Endotracheal tube FiO2 Fraction of inspired oxygen GI index Global inhomogeneity index GREIT Graz consensus reconstruction algorithm for EIT ID Inner diameter LIP Lower inflection point NA Not applicable Arterial partial pressure of carbon dioxide PaCO2 PaO2 Arterial partial pressure of oxygen Pat Patient PBW Predicted body weight PEEP Positive end-expiratory pressure pH Decimal logarithm of the reciprocal of the hydrogen ion activity ROI Region of interest SD Standard deviation TT Tracheostomy tube VT Tidal volume 1. Introduction In the recent years, a growing interest in electrical impedance tomography (EIT) has evolved focusing on the use of EIT as a bedside tool for monitoring of regional lung function and for guidance of ventilator settings (Frerichs et al 2014). It has been suggested that patients suffering from acute respiratory distress syndrome (ARDS) could especially benefit from a standardized protocol using EIT-derived information on ventilation distribution. ARDS is a severe pulmonary disease that, despite improvements in therapy, is still associated with a high mortality rate (Ranieri et al 2012). Ventilator therapy is the most important life-saving intervention for patients suffering from ARDS. Despite its life-saving potential, ventilator therapy carries the risk of ventilator-induced lung injury (VILI) caused by overdistension and by repeated opening and closing of alveoli (‘tidal recruitment’) and the associated inflammation (Slutsky and Ranieri 2014). Inhomogeneity in lung aeration appears to be a major contributor to VILI, mainly because of the additional shear forces that occur at the interfaces between ventilated and non-ventilated alveoli during tidal ventilation (Cressoni et al 2014). The global inhomogeneity (GI) index is a promising measure that quantifies ventilation inhomogeneity, as assessed by EIT, in a single number (Zhao et al 2009). It is calculated 1138
T Becher et al
Physiol. Meas. 36 (2015) 1137
from functional EIT images showing the tidal ventilation distribution by dividing the sum of the absolute differences between the individual pixel values and the median value of the lung region of interest (ROI) by the sum of the impedance values in the ROI. When performing a positive end-expiratory pressure (PEEP) trial, the changes in GI index are closely correlated to the changes in dynamic respiratory system compliance (Zhao et al 2010). During a low-flow pressure-volume maneuver, the GI index is highly correlated with the amount of recruitable lung tissue, as assessed by EIT (Zhao et al 2014). It has been suggested that PEEP titration according to the GI index could lead to a PEEP setting providing a reasonable balance between tidal recruitment and regional overdistension (Zhao et al 2010). When trying to optimize ventilator settings for a patient with ARDS, one must consider the fact that the amount of tidal recruitment and overdistension is influenced not only by PEEP but also, and markedly, by the selected tidal volume (VT) (Bruhn et al 2011, Zick et al 2013). Ventilation with low VT has repeatedly been proven to improve survival in patients with ARDS (Amato et al 1998, ARDS-Network 2000, Villar et al 2006). However, the influence of VT on the GI index value has not been studied so far. Since increasing VT may simultaneously cause regional recruitment and regional overdistension, we hypothesized that changes in VT would have measurable effects on the GI index. We also hypothesized that an increase in VT at low levels of PEEP could be used to predict the effect of an increase in PEEP on the GI index. 2. Methods 2.1. Materials and study procedure
We retrospectively analyzed the data of 9 sedated and mechanically ventilated patients suffering from ARDS (table 1). Some of the data have previously been published (Zick et al 2013). Patients suffering from ARDS according to the Berlin definition (Ranieri et al 2012) with need for invasive mechanical ventilation were included. Exclusion criteria were age
