(S-148) Walder, B., Sunday 9:15

TITLE: A PROTECTIVE MECHANICAL VENTILATION STRATEGY IN RATS WITHOUT LUNG INJURY INDUCES A TIME DEPENDENT LOCAL AND SYSTEMIC INFLAMMATORY RESPONSE

AUTHORS: Bernhard Walder, Research Fellow1, Nadia Elia, Research Resident2, Jerôme Pugin, Associate Prof3, Marie- Anne Bründler, Fellow4, Martin Tötsch, Fellow4, Denis R. Morel, Associate Prof2
AFFILIATION: 1Div. of Anesthesiology, Geneva, Switzerland; 2Div. of Anesthesiological Investigations, Geneva, Switzerland; 3Div. of Medical Intensive Care, Geneva, Switzerland; 4Div. of Clinical Pathology, Geneva, Switzerland.

INTRODUCTION: Whereas high pressure and/or high volume ventilation produces lung injury, the impact of mechanical ventilation with a protective ventilation strategy on lung or systemic alterations over time has not been investigated. Our aim was to evaluate the effects of two different mechanical ventilation durations in rats without preexisting lung injury ventilated with the same protective strategy, on lung and systemic alterations.
METHODS: Rats (300 - 350 g) were anaesthetized with isoflurane, tracheotomized and ventilated with a tidal volume of 7 ml/kg and a PEEP of 2.5 cm H2O. Group S: 15 rats were mechanically ventilated for 1 h in a supine position. Group L: 14 rats were mechanically ventilated for 7 h in the supine position. The femoral artery and vein were cannulated in a sterile manner. Airway and arterial pressures were continuously measured. Arterial blood gas measurements, WBC count, and determination of the percentage of circulating lymphocytes were performed. At the end of the study period a BAL or a histological analysis were performed.
RESULTS:
Table: Ventilation parameter, blood and BAL analysis

Mean ± SD

Group S (n = 15)

Group L (n = 14)

Peak inspiratory pressure

(cm H2O)

11.6 ± 1.6

13.1 ± 1.5*

PaO2 (kPa) (FiO2 = 0.4)

24.8 ± 2.6

24.8 ± 3.6

WBC (103/ml)

5.9 ± 1.1

5.7 ± 1.6

Blood lymphocytes (%)

76.3 ± 6.6

51.0 ± 10.7*

Other white blood cells (in the majority neutrophils) (%)

23.7 ± 6.6

49.0 ± 10.7

BAL - Neutrophils (%) (Group S: n=7; group L: n=6)

4.8 ± 3.8

54.0 ± 16.5*

Average of median histological score of lung oedema (n=8; best score,3; worst score, 12)

7 (R, 6 to 8)

8 (R, 4 to 9)

Average of median histological score of lung inflammation (best score, 6; worst score, 24)

13 (R,10 to14)

12 (R,12 to 14)

* Mann-Whitney test; p<0.01; R, range

CONCLUSION: Low tidal volume ventilation induces early (< 1 h) mild histological lung lesions. Once these lesions are established, they remain unchanged over time, as well as the pulmonary gas exchange, which is most probably related to the low mechanical forces of this kind of ventilation. The increase of neutrophils in blood and in BAL over time may be due to a local and systemic inflammatory reaction despite the applied protective ventilation strategy.