Regional pulmonary effects of bronchoalveolar lavage procedure determined by electrical impedance tomography

Background

The provision of guidance in ventilator therapy by continuous monitoring of regional lung ventilation, aeration and respiratory system mechanics is the main clinical benefit of electrical impedance tomography (EIT). A new application was recently described in critically ill patients undergoing diagnostic bronchoalveolar lavage (BAL) with the intention of using EIT to identify the region where sampling was performed. Increased electrical bioimpedance was reported after fluid instillation. To verify the accuracy of these findings, contradicting the current EIT knowledge, we have systematically analysed chest EIT data acquired under controlled experimental conditions in animals undergoing a large number of BAL procedures.

Methods

One hundred thirteen BAL procedures were performed in 13 newborn piglets positioned both supine and prone. EIT data was obtained at 13 images before, during and after each BAL. The data was analysed at three time points: (1) after disconnection from the ventilator before the fluid instillation and by the ends of fluid (2) instillation and (3) recovery by suction and compared with the baseline measurements before the procedure. Functional EIT images were generated, and changes in pixel electrical bioimpedance were calculated relative to baseline. The data was examined in the whole image and in three (ventral, middle, dorsal) regions-of-interest per lung.

Results

Compared with the baseline phase, chest electrical bioimpedance fell after the disconnection from the ventilator in all animals in both postures during all procedures. The fluid instillation further decreased electrical bioimpedance. During fluid recovery, electrical bioimpedance increased, but not to baseline values. All effects were highly significant (p < 0.001). The fractional changes in individual regions-of-interest were posture-dependent. The regional fall in electrical bioimpedance was smaller in the ventral and larger in the dorsal regions after the fluid instillation than after the initial disconnection to ambient pressure in supine animals (p < 0.001) whereas these changes were of comparable amplitude in prone position.

Conclusions

The results of this study show a regionally dissimilar initial fall in electrical bioimpedance caused by non-uniform aeration loss at the beginning of the BAL procedure. They also confirm a further pronounced fall in bioimpedance during fluid instillation, incomplete recovery after suction and a posture-dependent distribution pattern of these effects.

Electronic supplementary material

The online version of this article (10.1186/s40635-019-0225-6) contains supplementary material, which is available to authorized users.

Influence of different electrode belt positions on electrical impedance tomography imaging of regional ventilation: a prospective observational study

Background

Electrical impedance tomography (EIT) is a non-invasive bedside tool which allows an individualized ventilator strategy by monitoring tidal ventilation and lung aeration. EIT can be performed at different cranio-caudal thoracic levels, but data are missing about the optimal belt position. The main goal of this prospective observational study was to evaluate the impact of different electrode layers on tidal impedance variation in relation to global volume changes in order to propose a proper belt position for EIT measurements.

Methods

EIT measurements were performed in 15 mechanically ventilated intensive care patients with the electrode belt at different thoracic layers (L1-L7). All respiratory and hemodynamic parameters were recorded. Blood gas analyses were obtained once at the beginning of EIT examination. Off-line tidal impedance variation/tidal volume (TV/VT) ratio was calculated, and specific patterns of impedance distribution due to automatic and user-defined adjustment of the colour scale for EIT images were identified.

Results

TV/VT ratio is the highest at L1. It decreases in caudal direction. At L5, the decrease of TV/VT ratio is significant. We could identify patterns of diaphragmatic interference with ventilation-related impedance changes, which owing to the automatically adjusted colour scales are not obvious in the regularly displayed EIT images.

Conclusions

The clinical usability and plausibility of EIT measurements depend on proper belt position, proper impedance visualisation, correct analysis and data interpretation. When EIT is used to estimate global parameters like VT or changes in end-expiratory lung volume, the best electrode plane is between the 4th and 5th intercostal space. The specific colour coding occasionally suppresses user-relevant information, and manual rescaling of images is necessary to visualise this information.

Lung volume assessment in acute respiratory distress syndrome

PURPOSE OF REVIEW

Measurements of lung volumes allow evaluating the pathophysiogical severity of acute respiratory distress syndrome (ARDS) in terms of the degree of reduction in aerated lung volume, calculating strain, quantifying recruitment and/or hyperinflation, and gas volume distribution. We summarize the current techniques for lung volume assessment selected according to their possible usage in the ICU and discuss the recent findings obtained with implementation of these techniques in patients with ARDS.

RECENT FINDINGS

Computed tomography technique remains irreplaceable in terms of quantitative aeration of different lung regions, but the commonly used cut-offs for classification may be questioned with recent findings on nonpathological lungs. Monitoring end expiratory lung volume using nitrogen washout technique enhanced our understanding on lung volume change during positioning, pleural effusion drainage, intra-abdominal hypertension, and recruitment maneuver. Recent studies supported that tidal volume could not surrogate tidal strain, which needs measurement of functional residual capacity and which is correlated with pro-inflammatory lung response.

SUMMARY

Although lung volume measurements are still limited to research area of ARDS, recent progress in technology provides clinicians more opportunities to evaluate lung volumes noninvasively at the bedside and may facilitate individualization of ventilator settings based on the specific physiological understandings of a given patient.