Materials and methods

Statistical analysis

Quantitative variable are expressed as mean±sd. In order to overule pseudoreplication due to repeated measurement to the same animal, mixed effect regression models were use built with nlme package. In these cases, the animal was included as a random intercept. Model marginal (related to fixed factors) and conditional coefficients of determination for mixed-effect models were calculated using MuMIn package. A similar approach was used when fitting analysis of variance models.
Statistical analysis was performed with R version 3.3.3 (2017-03-06).

Results

Baseline respiratory parameters were as follows: Vt=295±83.95 ml, PaFiO2=125.68±70.16 mmHg, pH=7.2±0.06, PaCO2=62.09±13.63 mmHg, inspiratory static PAW=31.43±4.15 cmH2O and compliance=14.51±5.48 ml/cmH2O.

Measurements by PEEP

Table 1: Measurements by PEEP level

0 5 10 15 20 25 30 P
Airway mechanics
freqm 23.09±6.09 22.91±5.75 23.09±6.09 22.73±5.46 23.09±6.09 22.73±5.46 23±5.68 0.43
Tvem 303.02±82.56 293.18±85.71 299.66±83.02 300.55±82.66 298.64±83.97 301.32±83.4 297.1±91.46 0.12
autoPEEP 2.25±2.07 1.07±1.01 0.74±1.19 0.86±0.85 0.8±0.65 0.78±0.73 0.64±0.91 0.00034
Pplat 33.32±5.94 34.02±5.58 33.62±4.32 35.49±5.07 40.46±4.68 48.1±7.19 57.59±8.31 <2e-16
DeltaPAirway 31.07±6.63 27.95±5.78 22.88±4.59 19.62±4.84 19.66±4.77 22.32±7.41 26.96±8.66 7.4e-10
CStatic 9.97±2.9 10.74±3.55 13.42±4.03 15.82±5.01 15.75±5.25 14.67±6.25 12.16±6.75 2.7e-09
Raw 38.18±7.93 35.54±5.92 25.38±6.4 23.81±4.85 22.14±5.38 22.06±4.62 19.97±7.26 4.6e-15
Transpulmonary mechanics
Ptp-esp -8.11±7.03 -3.82±6.5 -1.06±5.91 3.18±4.95 6.23±5.27 9.29±5.24 12.54±5.79 <2e-16
Ptp-insp 19.33±4.31 19.87±4.38 17.9±3.88 19.2±3.13 21.76±4.48 25.66±5.54 33.31±4.5 1.1e-10
DeltaPTp 27.44±6.69 23.69±5.68 18.96±4.78 16.02±4.54 15.54±4.75 16.37±4.47 20.77±5.5 8.8e-08
Etorax 1.51±0.48 1.67±0.51 1.34±0.5 1.31±0.5 1.37±0.94 1.62±0.96 1.78±1.05 0.23
Ep/Etot 0.85±0.05 0.82±0.04 0.82±0.06 0.8±0.05 0.79±0.11 0.77±0.1 0.8±0.09 0.33
Ptp-inspElas 29.63±5.73 28.93±5.57 28.2±4.75 29.03±4.74 32.28±5.74 36.53±5.36 45.76±5.77 1e-08
Gas exchange
PH 7.16±0.06 7.16±0.08 7.2±0.06 7.21±0.07 7.2±0.08 7.23±0.09 7.2±0.06 0.00011
PaCO2 72.59±15.88 70.55±12.32 64.7±13.91 60.74±10.74 60.62±12.77 57.55±15.7 59.37±14.34 4.1e-07
HCO3 25.26±3.89 25.55±3.85 25.05±3.74 24.37±3.64 23.27±3.43 23.7±4.61 23.47±5.01 0.0035
Pa/FiO2 50.95±22.01 54.74±19.54 105.72±58.81 215.75±124.33 332.49±169.2 548.55±140.55 571.19±56.03 <2e-16
Dead space
PeCO2 10.87±3.41 13.02±5.48 12.99±4.24 13.72±4.33 12.24±3.31 11.17±3.23 10.65±3.1 0.0018
PECO2 10.13±2.3 11.7±5.18 12.25±3.82 13.47±4.51 11.71±2.99 10.65±2.88 9.54±3.22 0.00075
PACO2 21.17±6.62 24.35±10.03 27.39±9.59 30.97±12.41 31.32±10.87 32.27±12.24 34.02±13.53 3.2e-07
PetCO2 27.99±16.06 33.18±16.47 35.88±12.6 39.93±13.93 39.66±14.11 38.08±12.78 40.29±15.09 0.00018
Pa-ETCO2 44.6±23.92 37.37±21.11 28.82±18.65 20.8±15.43 20.95±15.33 19.47±13.16 19.08±14.1 1.2e-10
Pa-ACO2 51.43±12.88 46.21±10.9 37.31±10.95 29.77±10.21 29.3±8.06 25.29±4.95 25.35±4.26 5.6e-12
VCO2br 4.32±2.13 5.07±3.11 5.37±2.89 5.71±2.95 5.02±2.74 4.59±2.5 4.18±2.7 5.3e-05
Bohr 0.51±0.08 0.52±0.09 0.53±0.06 0.55±0.05 0.61±0.07 0.65±0.08 0.69±0.1 <2e-16
Vdphys 149.33±48.88 147.74±45.8 155.5±42.02 166.8±47.34 178.48±49.81 190.97±56.04 198.6±59.73 <2e-16
alV 55.6±20.07 48.66±16.25 47.41±14.36 48.8±14.12 49.2±15.86 50.53±16.54 48.4±19.13 0.061
VTalV 202.79±64.6 192.65±67.85 190.31±61.41 183.44±53.01 167.5±58.1 158.08±56.54 142.77±63.18 2.7e-14
VdalV/VTalV 0.27±0.05 0.26±0.05 0.25±0.02 0.27±0.03 0.3±0.03 0.33±0.03 0.35±0.06 6.7e-10
VdB-E/Vt 0.86±0.02 0.83±0.06 0.8±0.06 0.77±0.05 0.8±0.03 0.81±0.04 0.84±0.04 0.00034
VdalvB-E 157.95±51.56 139.7±47.12 130.95±41.69 112.07±40.26 105.89±41.04 98.41±38.83 87.81±39.63 4e-15
VdalvB-E/Vtalv 0.79±0.04 0.75±0.11 0.71±0.12 0.61±0.11 0.63±0.07 0.64±0.07 0.65±0.12 1.3e-05
Hemodynamics
TAS 123.55±31.71 126.27±28.92 124.73±18.43 125±16.73 114.55±14.31 104.18±20.4 100.3±16.71 0.0035
TAM 94±30.1 96.45±29.04 94.82±17.57 94.27±18.3 90.73±14.17 81.09±17.41 78.9±14.25 0.058
IC 4.84±1.1 3.68±1.13 4.22±0.32 3.58±0.9 3.42±1.28 3.76±1.5 2.98±1.1 0.11
VVS 6.5±2.56 6.5±1.77 7.25±1.49 9.62±3.89 15.38±6.16 19±9.77 29.57±14.36 5.3e-09

Figure 1: PL and PAW relationship

As expected, PL shows a linear relationship with PAW (marginal and conditional mixed model R2 are 0.83 and 0.98 respectively, P= \(2.693\times 10^{-47}\)).

Figure 2: Comparison VD/VT and PL relationship according to the equation.

Total VD/VT and expiratory PL disclose a curve relationship with differents slopes. Airway VD/VT increases as expiratory PL increases. The relationship between alveolar VD/VT and expiratory PL appears to fit a curved shape function when Bohr method is used, whereas it is a straight line when using BE equation. Lines in figure 2 represent individual and pooled fixed effect (dotted blue line) predicted values from quadratic functions. Table 2 and 3 show summary results from mixed models for both VD/VT equations.

Table 2: Summary from the quadratic linear model of Bohr VD/VT and its components, as a function of expiratory PL.

\(\beta\) Marginal R2 Conditional R2
Total 0.53 0.93
Intercept 0.54 (0.50-0.59)
PL 0.0072 (0.0062-0.0083)
PL2 0.00024 (0.00016-0.00032)
Airway 0.54 0.94
Intercept 0.38 (0.32-0.43)
PL 0.0086 (0.0075-0.0097)
PL2 0.00020 (0.00012-0.00028)
Alveolar 0.26 0.73
Intercept 0.27 (0.24-0.30)
PL 0.0022 (0.0011-0.0033)
PL2 0.000168 (0.000084-0.000252)

Table 3: Summary from the quadratic linear model of BE VD/VT and its components, as a function of expiratory PL.

\(\beta\) Marginal R2 Conditional R2
Total 0.12 0.36
Intercept 0.81 (0.79-0.83)
PL -0.00203 (-0.00370–0.00035)
PL2 0.00013471 (-0.00000043- 0.00026985)
Airway 0.49 0.85
Intercept 0.117 (0.079-0.155)
PL 0.0062 (0.0047-0.0076)
PL2 0.000094 (-0.000016- 0.000203)
Alveolar 0.36 0.68
Intercept 0.69 (0.64-0.75)
PL -0.0081 (-0.0110–0.0053)
PL2 0.000037 (-0.000187- 0.000261)

Comparison among physiological data according to different optimized parameters

Optimized parameter where defined as follows:

  • VdalV/VTalV: lowest value
  • VCO2br: greatest value
  • Ptp-esp: nearest to 0 cmH2O
  • CStatic: greatest value
  • Pplat: nearest to 30 cmH2O
  • Ptp-inspElas (elastance derived inspiratory PTP): nearest to 27 cmH2O

Table 4 shows physiological values recorded with each optimized parameter in the rows.

Table 4: Physiological data depending on optimized parameters

VdalV/VTalV VCO2br Ptp-esp CStatic Pplat Ptp-inspElas P
PEEPtot 8.17±5.07 11.48±6.84 11.06±6.22 19.1±6.33 8.06±5.88 10.64±8.16 0.002024
Pplat 33.22±4.53 35.32±7.19 36.47±7.6 37.24±5.35 32.91±3.98 34.99±7.1 0.073583
DeltaPAirway 25.04±7.6 23.84±7.35 25.4±4.33 18.13±3.62 24.85±6.48 24.35±3.49 0.010644
CStatic 12.73±5.76 13.59±6.02 13.05±4.63 17.1±5.87 12.3±3.36 13.05±3.15 0.0027749
Bohr 0.5±0.06 0.52±0.05 0.53±0.05 0.59±0.08 0.52±0.07 0.54±0.03 3.5862e-05
VdalV/VTalV 0.23±0.03 0.25±0.04 0.26±0.04 0.29±0.03 0.26±0.03 0.28±0.02 4.8528e-05
VCO2br 5.44±3.02 6.07±2.88 5.86±3.25 5.33±2.36 4.97±2.14 4.85±2.4 0.0065323
Pa-ETCO2 30.65±19.41 27.19±18.58 33.54±25.37 18.59±14.12 32.97±22.38 35.86±29.6 0.021485
VdB-E/Vt 0.8±0.06 0.77±0.06 0.81±0.06 0.78±0.04 0.81±0.06 0.84±0.05 0.058531
Ptp-esp -1.81±6.11 0.6±6.99 -0.26±1.58 5.69±8.17 -2.54±6.91 -1.95±5.82 0.032026
Ptp-inspElas 28.2±4.76 29.74±6.14 30.16±7.41 29.33±5.94 27.69±4.37 27.12±2.4 0.5356
DeltaPTp 20.87±6.67 19.85±6.51 21.01±4.29 13.79±3.5 20.33±5.88 19.41±4.52 0.06245
Pa/FiO2 92.31±60.16 176.89±169.55 93.31±67.81 336.64±162.16 119.44±103.05 122.42±126.27 0.00027092
PaCO2 65.35±12.73 63.5±14.44 70.72±15.38 59.24±12.17 66.45±13.25 68.04±20.85 0.11035
HCO3 25.16±3.35 24.63±3.48 25.3±4.14 24.31±3.65 25.45±3.25 24.62±4.41 0.26509

Multiple comparisons plots

Only those with significant ANOVA P above are displayed

## $PEEPtot

## 
## $DeltaPAirway

## 
## $CStatic

## 
## $VCO2br

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## $`Pa-ETCO2`

## 
## $`Ptp-esp`

## 
## $`Pa/FiO2`