Ventilation strategies do not affect apoptosis in early acute lung injury (ALI)
Introduction: We have shown that the mode of ventilation, e.g. high vs. low PEEP vs. HFOV, etc, affects early histologic and biochemical markers of ALI. If these early markers of lung injury translate into a decreased severity of ALI with an associated better outcome, we speculated that indirect markers of cellular injury, i.e. commitment to cell death, might be apparent. To test this hypothesis, we studied apoptosis under 4 different modes of mechanical ventilation of experimental ALI. Methods: Forty-two New Zealand White rabbits were anesthetized and instrumented in conventional fashion. Lung injury was via repeated saline lavage (PaO2<100 torr). After 15 min of HFOV (Paw: 16 cmH2O, IT: 33%) interposed with 15 sec. periods of static inflation (Paw: 30 cm H2O) to recruit lung and equalize volume history, animals were assigned to one of 4 strategies: (1) Low PEEP: Vt=10 ml/kg, PEEP=2 cmH2O; (2) High PEEP: Vt=10 ml/kg, PEEP=10 cmH2O; (3) Permissive hypercapnia (P-Hy): Vt=6 ml/kg, PEEP>Pflex cmH2O; (4) HFOV: Paw=16 cmH2O, f=10Hz, Ti=33%. Uninjured rabbits (controls) were ventilated with Vt=10ml/kg, PEEP 5 cmH2O. Ventilation was continued for 4 hours with FiO2=1.0 prior to sacrifice. Arterial blood gases were performed every 30 minutes. Tissues was fixed in Carnoy's fixative and embedded in paraffin. Sections were mounted on slides, stained by conventional nick-end labeling with DNA counter stain to identify nuclei and examined by two observers by fluorescent microscopy at 40x magnification. Apoptosis was quantified by counting the number of fluorescent nuclei per field in 4 fields per slide. Results and Conclusions: There was a mean of 8.6 apoptotic cells per field most commonly in intra-alveolar cells appearing to be macrophages by morphology and location. There was no significant difference between modes of ventilation. We conclude that a 4-hour injury model is insufficient to demonstrate differences in programmed cell death in this model of lung injury treated by different modes of ventilation. We speculate that other early markers, i.e. message or stress response proteins, will be more productive in future studies. Partial support by SensorMedics Corporation, Yorba Linda, CA.
Steinhorn, DM; Rotta, AT; Gunnarsson, B; Fuhrman, BP; Hernan, L
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