airway closure and VP curves – ICM_2005_00895_R4_ESM

ASSESSMENT OF AIRWAY CLOSURE FROM DEFLATION LUNG VOLUME-PRESSURE CURVE: SIGMOIDAL EQUATION REVISITED

Electronic supplemental material

MATERIAL AND METHODS

Tidal volume was 101 ml.kg-1, respiratory frequency of 161 breaths per minute and inspiratory time 33% of total cycle duration. Inspired fraction of oxygen (FiO2) was 0.21 during animal preparation and up to 1 for the whole experiment before/after induction of ALI. Respiratory frequency was adjusted to obtain an arterial pH greater than 7.2. An 8.5 French catheter was inserted into the right carotid artery and an 8.5 French three-lumen venous catheter into the right external jugular vein. Arterial blood samples were analysed with a blood gas device (ABL™5; Radiometer, Copenhagen, Denmark).

Equipment

Pressure at airway opening (Pao) was measured proximal to endotracheal tube using a piezoresistive pressure transducer (Gabarith™ 682002, Becton Dickinson, Sandy, Utah, USA). Pleural pressure was estimated from esophageal pressure (Pes) using a balloon inserted into the lower third of the esophagus and connected to a piezoresistive pressure transducer (Gabarith™ 682002). The balloon was inflated with 1 ml of air and its correct position ascertained using occlusion test 11. V’ was measured by a Fleisch # 2 pneumotachograph (Fleisch, Lausanne, Switzerland) and changes in lung volume obtained from numerical integration of V’ signal. Equipment dead space was 40 ml.

The respiratory system was inflated and deflated by means of a 3-liter syringe filled with air at room temperature whose piston was driven by an electric motor at low constant V’ (0.12 L.s-1). It was attached to expiratory line of ventilator circuit upstream from exhalation valve via a 3-way tap. A 2-way tap was inserted downstream from inspiratory valve. The two taps were operated manually.

Signals of Pao, Pes, PL (= Pao – Pes) and V’ signals were sampled at 200 Hz using a laptop computer (MP 100, Biopac system. Inc.) and Acqknowledge software (version 3.7.1. for Microsoft 98®, Biopac System. Inc.) and continously monitored.

Experimental procedure

ALI

ALI was induced by administering 30 ml.kg-1 saline at 37°C through endotracheal tube from a height of approximately 75 cm 12. Lavages were repeated until absence of foam in lavage liquid and PaO2<100 mmHg with FIO2 of 1, 15 minutes after last saline lavage, was reached.

Measurements

Arterial blood was withdrawn for blood gas determinations. After 10 baseline breaths, airway closure was perfomed as follows. Airways were occluded at end of baseline expiration and the 3-way tap turned to connect respiratory system to the syringe. Simultaneously, the 2-way valve on the inspiratory line was turned in such a way to avoid inflation from the ventilator; 2) the respiratory system was inflated via expiratory line using the syringe until PL reached 30 cmH2O (Figure 1). An investigator closely monitored PL on the computer screen to detect inflation end; 3) deflation was started at the same flow rate up to the point at which Pes did not exhibit further change (Figure 1). Then the taps were turned to return to baseline ventilation.

Measurements were done before and two hours after ALI.

Data analysis

The point at which Pes no longer changed was taken as the minimal airway closing pressure (Pc,min) 3, defined as: 1) the value of Pes at zero V’ whilst the syringe was still suctioning, reflecting that there was no further change in thoracic gas volume and 2) no change in Pes by more than 5% (Figure 1). Pes at EELV was also measured. Pes was read at mid-height of the cardiac artifact.

The volume at Pc,min was Vtrapped. EELV was obtained by subtracting the change in V during inflation from the corresponding deflation to Vtrapped. All volumes were expressed as % of vital capacity (VC), which was the volume exhaled from end-inflation to Vtrapped (electronic supplemental material).

Digital records of PL and volume were transferred into a spreadsheet program (Matlab®6.1; The Mathworks, Inc., Natick, MA, USA) and analysed into two ways.

First, according to Glaister et al. 2 an inflection point (Pc,max) was defined by eye and experimental points above Pc,max were fitted to Eq. 3 2. It was obtained 2 a computation of the zero volume intercept (PD) which is the pressure the lung would reach at low lung volume if functional properties were the same as above Pc,max. Next, data were fitted to Eq. 4 5 where a’ (%VC) is lower asymptote, b’ (%VC), inspiratory capacity, c’ pressure of “true” inflection point (slope changes direction), d pressure from c’ of the zone of high compliance. Pmci and Pmcd are c’-1.317xd and c’+1.317xd, respectively 6.

Table 1. Values of Pc,max, Pc,min, Pes at EELV and Vtrapped in control and ALI pigs

Pig # / Pc,max
(cm H2O) / Pc,min
(cm H2O) / Pes, EELV
(cm H2O) / Vtrapped
(ml)
control / ALI / control / ALI / control / ALI / control / ALI
1 / 1.5 / 1.5 / - 12.0 / 2.0 / 3.0 / 1.3 / 576 / 72
2 / 1.8 / 9.7 / - 5.8 / 2.0 / - 0.8 / 2.4 / 114 / 28
3 / 5.6 / 7.6 / - 5.0 / 0.5 / - 2.5 / 0.7 / 66 / 37
4 / 1.5 / 7.6 / - 7.0 / - 9.5 / 0.0 / - 2.6 / 313 / 95
5 / 1.2 / 6.2 / - 15.0 / 0.4 / 0.4 / 1.8 / 479 / 141
6 / 2.4 / 2.4 / - 6.9 / - 2.5 / 1.4 / 2.5 / 573 / 245
7 / 5.9 / 7.1 / - 6.2 / - 2.1 / - 1.1 / 0.0 / 84 / - 21
8 / 4.1 / 5.9 / - 8.8 / - 1.4 / 2.4 / 0.0 / 351 / - 61
Mean / 3.0 / 6.0 * / - 8.3 † / -1.3 * / 0.3 / 0.8 / 320 / 67 **
SD / 1.9 / 2.8 / 3.5 / 3.7 / 1.9 / 1.7 / 213 / 93

Pc,max: maximal closing pressure; Pc,min: minimal closing pressure; Pes, EELV: esophageal pressure at end-expiratory lung volume; Vtrapped: trapped gas volume

* p < 0.05 ** p < 0.01 vs control

† p < 0.001 vs Pes, EELV

Table 2. Values of parameter PD from Eq. 3, of parameters c’ and d from Eq. 4 and Pmci and Pmcd in control and ALI

PD / c’ / d / Pmci / Pmcd
Pig# / control / ALI / control / ALI / control / ALI / control / ALI / control / ALI
1 / -1.7 / -4.0 / 1.7 / 6.1 / 3.1 / 7.2 / -2.4 / -3.4 / 5.8 / 15.6
2 / -1.1 / 2.8 / 4.2 / 12.4 / 4.3 / 8.0 / -1.4 / 1.9 / 9.8 / 22.9
3 / 0.0 / 1.5 / 5.9 / 9.2 / 5.1 / 7.3 / -0.8 / -0.4 / 12.7 / 18.9
4 / -3.2 / 1.0 / 2.5 / 8.3 / 4.3 / 6.0 / -3.2 / 0.5 / 8.2 / 16.2
5 / -2.7 / 1.4 / 0.8 / 7.9 / 2.3 / 5.7 / -2.2 / 0.4 / 3.7 / 15.4
6 / -3.1 / -2.1 / 1.4 / 3.5 / 2.7 / 4.7 / -2.2 / -2.7 / 5.0 / 9.7
7 / -1.2 / -0.3 / 4.5 / 7.0 / 5.0 / 4.4 / -2.1 / 1.3 / 11.0 / 12.8
8 / 0.7 / 2.1 / 4.2 / 6.8 / 4.1 / 3.4 / -1.3 / 2.2 / 9.6 / 11.3
mean / -1.5 / 0.3 * / 3.2 / 7.7 ** / 3.9 / 5.8 * / -1.9 / -0.03 * / 8.2 / 15.4 *
SD / 1.4 / 2.3 / 1.8 / 2.6 / 1.0 / 1.6 / 0.7 / 2.1 / 3.1 / 4.2

Values are in cm H2O.

* p < 0.05** p < 0.001 vs control

Figure 1. The 3 steps of analysis of Volume (V)-Transpulmonary (PL) pressure curve (pig # 4 in control condition). A: raw experimental data of the inflation-deflation V-PL curve with inflation from end-expiratory lung volume (EELV) up to the end of inflation (1720 ml) then deflation to trapped volume (Vtrapped =- 313 ml). B: the V axis is now refered to Vtrapped and ranges from 0 (Vtrapped) to vital capacity (VC). C: the V axis from Vtrapped to VC is now expressed as % VC.

Figure 2. Relationship of the difference to the mean of Pc,max and c’, PD and Pmci and Pmci and Pc,min in control and acute lung injury (ALI) in 8 pigs. Horizontal continuous lines are mean of the difference and dotted lines mean + (upper bound) or – (lower bound) 2 SD.





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