Lung aeration and ventilation after percutaneous tracheotomy measured by electrical impedance tomography in non-hypoxemic critically ill patients: a prospective observational study

Abstract

Background: Percutaneous dilatational tracheotomy (PDT) may lead to transient impairment of pulmonary function due to suboptimal ventilation, loss of positive end-expiratory pressure (PEEP) and repetitive suction maneuvers during the procedure. Possible changes in regional lung aeration were investigated using electrical impedance tomography (EIT), an increasingly implied instrument for bedside monitoring of pulmonary aeration.

Methods: With local ethics committee approval, after obtaining written informed consent 29 patients scheduled for elective PDT under bronchoscopic control were studied during mechanical ventilation in supine position. Anesthetized patients were monitored with a 16-electrode EIT monitor for 2 min at four time points: (a) before and (b) after initiation of neuromuscular blockade (NMB), (c) after dilatational tracheostomy (PDT) and (d) after a standardized recruitment maneuver (RM) following surgery, respectively. Possible changes in lung aeration were detected by changes in end-expiratory lung impedance (Δ EELI). Global and regional ventilation was characterized by analysis of tidal impedance variation.

Results: While NMB had no detectable effect on EELI, PDT led to significantly reduced EELI in dorsal lung regions as compared to baseline, suggesting reduced regional aeration. This effect could be reversed by a standardized RM. Mean delta EELI from baseline (SE) was: NMB - 47 ± 62; PDT - 490 ± 180; RM - 89 ± 176, values shown as arbitrary units (a.u.). Analysis of regional tidal impedance variation, a robust measure of regional ventilation, did not show significant changes in ventilation distribution.

Conclusion: Though changes of EELI might suggest temporary loss of aeration in dorsal lung regions, PDT does not lead to significant changes in either regional ventilation distribution or oxygenation.

Figures

Fig. 1

Exemplary sequences of four original impedance tracings at the four study time points acquired in two patients. a baseline: before application of muscle relaxant, b NMB: 2 min after application of muscle relaxant, c PDT: after tracheotomy and d RM: after a final standardized recruitment maneuver. The corresponding graphical displays of regional ventilation (“minute images”) are shown above. The top patient (age 57 years, BMI 24 kg/m2; duration of MV 234 h) showed a pronounced decrease in global EELI after PDT which was reversed by application of a RM. The bottom patient (age 54 years; BMI 19 kg/m2, duration of MV 228 h) did not show any marked changes in EELI among the four study time points. This latter result was detected in the majority of the studied patients and represents the overall study findings

Fig. 2

Changes in end-expiratory lung impedance (delta EELI). Data are given in arbitrary units (a.u.) presented as mean (error bars indicate standard error of the mean) compared to baseline measurements (NMB) after neuromuscular blockade, (PDT) after tracheotomy and (RM) after a subsequent recruitment maneuver, n = 29. Mean changes were significantly different from baseline values in dorsal lung parts after PDT (p = 0.01) and in ventral parts after the recruitment maneuver (p = 0.01), RM ANOVA

Fig. 3

Mean regional changes in EELI compared to baseline measurements (NMB) after neuromuscular blockade, (PDT) after tracheotomy and (RM) after a subsequent recruitment maneuver, n = 29

Fig. 4

Mean global tidal variation during each measurement phase: a before neuromuscular blockade (baseline), b after neuromuscular blockade (NMB), c after tracheotomy (PDT) and d after a subsequent recruitment maneuver (RM) (n = 29; RM ANOVA; p > 0.05)

Fig. 5

Regional tidal variation within 32 horizontal rows of the right and left halves of EIT image in each measurement phase a before neuromuscular blockade (baseline), b after neuromuscular blockade (NMB), c after tracheotomy (PDT) and d after a subsequent recruitment maneuver (RM). No significant changes in regional tidal variation were detected among the four time points of our standard PDT procedure (n = 29; RM ANOVA; p > 0.05)

Fig. 6

Center of ventilation (CoV) describes the distribution of ventilation within the scanned thoracic cross section. A value below 50% indicates a ventrally distributed ventilation, while values above 50% indicate a preferential distribution of ventilation to the dorsal aspects of the scanned volume. No significant changes in CoV were detected among the four time points of our standard PDT procedure (n = 29; p > 0.05; RM ANOVA)

Fig. 7

Horovitz index (paO2/FiO2) at each measurement phase: (baseline) before neuromuscular blockade, (NMB) after neuromuscular blockade, (PDT) after tracheotomy and (RM) after a subsequent recruitment maneuver, n = 29 (p > 0.05; repeated measures ANOVA)