Influence of tidal volume on ventilation inhomogeneity assessed by electrical impedance tomography during controlled mechanical ventilation

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.

Associations between positive end-expiratory pressure and outcome of patients without ARDS at onset of ventilation: a systematic review and meta-analysis of randomized controlled trials

“…Indeed, clinical studies have shown that ventilation with higher levels of PEEP reduces the incidence of atelectasis [6, 41, 42] and improves respiratory system compliance mainly in patients receiving general anesthesia for surgery [43, 44]. Also, higher levels of PEEP could alleviated lung inhomogeneity, decreasing the impact of the stress raisers and the stress concentration due to inflation of well-aerated alveoli adjacent to collapsed or fluid-filled alveoli [45, 46]. …”

Chest electrical impedance tomography examination, data analysis, terminology, clinical use and recommendations: consensus statement of the TRanslational EIT developmeNt stuDy group

“…37 40 41 Identification of pixel end-expiratory minima before and after a change in ventilator settings (eg, in PEEP) creates a functional EIT image of the local changes in end-expiratory lung volume (EELV) 29. If EIT data are acquired during the forced full expiration then the calculation of the electrical impedance change within the first second of the forced exhalation will image the distribution of local FEV 1 42…”

“…One subgroup describes the overall degree of spatial heterogeneity of ventilation. Examples are the global inhomogeneity index36 40 49 or the coefficient of variation,36 42 50 calculated from the maps of tidal impedance variation. Another subgroup describes the orientation of the spatial distribution of functional measures, usually in the anteroposterior direction.…”

Regional lung function in nonsmokers and asymptomatic current and former smokers

“…However, this study is the largest one ever carried out in adults for the determination of regional lung function by EIT worldwide to the best of our knowledge. The majority of the existing EIT studies on ventilation distribution focus on mechanically ventilated intensive care patients [14,[28][29][30]. We believe that EIT chest examinations may be beneficial in spontaneously breathing patients with pulmonary diseases like COPD, [6,31] asthma [12,13,32] or cystic fibrosis [11,33,34] as well and that this new functional method has a potential in pneumology.…”