Effect of Positive End Expiratory Pressure and level of Pressure Support Ventilation
on ventral-to-dorsal tidal ventilation redistribution and heterogeneity in lung ventilation
evaluated with Electrical Impedance Tomography
1,
2,
2,
1
A,
1,
2,
2,
2,
1,2
Turella M Mauri T Confalonieri A Pradella
Sala V Coppadoro A Bellani G Patroniti N Pesenti A
1 Postgraduate school of Anesthesia and Intensive Care, University of Milan-Bicocca, Monza, Italy;
2 Department of Perioperative Medicine and Intensive Care, San Gerardo Hospital, University of Milan-Bicocca, Monza, Italy
METHODS
We enrolled 10 intubated semi-recumbent ALI patients undergoing
Pressure Support Ventilation (PSV). We applied on each patient’s thorax a
16-electrode belt connected with a bedside EIT continuous recording
monitor (PulmoVista 500®, Dräger Medical GmbH, Lübeck,
Germany);(Figure 1,2).
*
80
Regional Ventilation, %
AIM OF THE STUDY
Acute lung injury (ALI) is a clinical syndrome characterized by bilateral
inflammatory lung edema, mostly involving dependent regions.
Mechanical ventilation (MV) is usually implemented in ALI patients to
correct gas exchange impairment. Application of assisted MV modes and
positive end expiratory pressure (PEEP) promote redistribution of tidal
ventilation towards dependent collapsed lung regions. Electrical
impedance tomography (EIT) is a non-invasive bedside lung imaging
technique. EIT might be useful to investigate the effects of different
assisted MV and PEEP settings on tidal ventilation distribution in ALI
patients.
Ventral ROI
*
70
Dorsal ROI
60
50
40
30
20
10
0
1
PSV High
PSV Low
Figure 3: Ventilation in ventral and Dorsal ROI at different PS levels.* p<0.05
PEEPlow was 7±2 cmH2O and PEEPhigh 12±3 cmH2O. PEEPhigh was
associated with increased dorsal (41±16% vs. 36±15%, p =0.006) and
decreased ventral ventilation (59±16% vs. 65±14%, p =0.006), as
compared to PEEPlow;(Figure 4). During PEEPhigh, ventral heterogeneity
decreased, as compared to PEEPlow (p =0.008), while dorsal heterogeneity
did not differ (p = 0.26).
*
*
Regional Ventilation, %
80
Ventral ROI
70
Dorsal ROI
60
50
40
30
20
10
Figure 1: : Electrode belt with patient cable connected.
0
PEEP Low
Low PS
(n=10)
High PS
(n=10)
P-value
Low PEEP
(n=10)
PS (cmH20)
3±2
12±3
<0,001
8±4,75
8±4,75
p 0,1
3,3±1,2
1,0±0,6
<0,001
2,03±1,73
1,81±1,67
0,61
PEEP (cmH20)
7±2
7±2
7±2
12,3±2,66
<0,001
Crs (mL/cmH20)
62,6±31,3
60,3±25,9
0,68
59,5±28,42
64,9±26,0
0,26
Paw (cmH20)
8,7±2,5
10,1±2,7
0,0128
9,9±2,84
14,5±3,24
<0,01
300
0,38
248,8±51,68
268,3±58,1
0,00103
250
PaCO2 (mmHg)
41,6±5,81
39,12±4,19
0,077
39,9±4,62
40,81±4,86
0,37
pH
7,40±0,066
7,42±0,053
0,046
7,41±0,049
7,44±0,054
0,42
PEEP High
Figure 4: Ventilation in Ventral an Dorsal ROI at different PEEP levels. * p<0.05
PaO2/FiO2 ratio correlated with dorsal tidal ventilation during both PEEP
and PS phases (R2=0.390 and 0.251, respectively p<0.05;(Figure 5,6).
350
PaO2/FiO2
PaOc/FiO2 (mmHg) 262,6±31,7 250,0±53,9
High PEEP
P-value
(n=10)
1
Table 1: Characteristics of the study population.
R2 =0,251
p <0.05
200
150
100
We randomly performed 2 steps of PSV for 15 minutes: one at lower levels
of PSV (PSlow) and one at higher levels (PShigh). In the same population, we
also performed 2 steps at different PEEP levels for 15 minutes: the level
used for clinical purpose (PEEPlow) and a higher level (PEEPhigh: PEEPlow+5
cmH2O).
50
0
0
10
20
30
40
50
60
70
Vt to Dorsal Lung ROI, %
Figure 5: PS study: changes in PaO2/FiO2 and percentage of Tidal Volume (Vt) to Dorsal ROI.
350
Figure 2: Current application and voltage measurements around the thorax.
PaO2/FiO2
300
250
R2 =0,39
p <0.05
200
150
100
EIT imaging field was divided into 2 contiguous equal-size regions of
interest (ROI): ventral and dorsal. From EIT data offline analysis we
calculated, for each phase: percentage of tidal ventilation distribution in
ventral and dorsal ROIs; heterogeneity, calculated as the standard
deviation of the voxel-level impedance differences between endinspiration and end-expiration/global impedance changes. Together, we
collected ventilation parameters and arterial blood gas analysis.
RESULTS
PSlow was set at 3±2 cmH2O and PShigh at 12±3 cmH2O; Dorsal ventilation
increased during PSlow, as compared to PShigh (42±10% vs. 32±11%,
p=0.004), while ventral ventilation decreased (58±9% vs. 68±10%,
p=0.005);(Figure 3). Comparing PShigh with PSlow, heterogeneity increased
in dorsal ROI while it decreased in ventral ROI (p=0.006 and p=0.008,
respectively).
50
0
0
20
40
60
80
Vt to Dorsal Lung ROI, %
Figure 5: PEEP study: changes in PaO2/FiO2 and percentage of Tidal Volume (Vt) to Dorsal ROI.
CONCLUSIONS
EIT was able to detect ventral-to-dorsal tidal ventilation redistribution
associated with higher PEEP and lower PSV levels.
Increased dorsal ventilation was associated with better oxygenation
probably because of improved ventilation-perfusion matching.
Comparable heterogeneity levels in presence of increased dorsal
ventilation may be related to alveolar recruitment induced by PEEP.