Session VII - Polytrauma
Sat, 10/9/04 Polytrauma, Paper #37, 10:36 am
The Influence of Hypotension on Cytokine Production following Pulmonary Fat Embolism: A Two-Hit Model of Trauma
Purpose: The objective of this study was to assess the contribution of pulmonary fat embolism caused by intramedullary femoral canal pressurization to the development of acute lung injury in the presence of resuscitated hemorrhagic shock. We hypothesized that sequential insults of hemorrhagic shock, resuscitation, and pulmonary fat embolism lead to pulmonary inflammation that is greater than either injury alone, as indicated by proinflammatory cytokine expression in plasma and bronchoalveolar lavage fluid (BALF).
Methods: All animal procedures were approved and performed in accordance with the local animal care committee guidelines. After anesthetic administration and preparation, New Zealand White rabbits were randomly assigned into one of four groups: resuscitated hemorrhagic shock + fat embolism (HR/FE), resuscitated hemorrhagic shock (HR), fat embolism (FE), and control. In the HR/FE group (N = 6), hypovolemic shock was induced via constant carotid bleeding into a polypropylene syringe (containing 3.8% sodium citrate). Mean arterial pressure (MAP) was maintained at 30 to 40 mm Hg for 1 hour. Animals were resuscitated with the entire volume of shed blood titrated with an additional volume of saline solution necessary to restore baseline MAP values. After a 1-hour stabilization period, we exposed both distal femoral condyles through a medial parapatellar approach to the right knee in preparation for fat embolism induction. After drilling into the intramedullary cavity in a retrograde fashion, we successively reamed the canal by using 3.5-, 4-, and 4.5-mm diameter T-handle reamers. The intramedullary canal was pressurized via a standardized injection of 1 to 1.5 ml of low-viscosity bone cement. The patella was then reduced and the incision was closed. In the HR group (N = 6), after the 1-hour stabilization period following resuscitated shock, a sham knee incision was made but was closed immediately without drilling, reaming, or pressurization. In the FE group (N = 8), prior to fat embolism induction, the animals were ventilated for a 3-hour period. This time corresponds to the time it took to establish shock, resuscitate, and stabilize the animals in the HR group. In the control group (N = 7), after the 3-hour ventilation period, a sham knee incision was made, but was immediately closed without further manipulations. Animals were mechanically ventilated for 4 hours after surgical closure.
After the 4-hour monitoring period, the rabbits were sacrificed by an intravenous overdose of pentobarbital. A postmortem thoracotomy was performed, the pulmonary vessels were ligated, and the lungs were removed en bloc. Postmortem bronchoalveolar lavage was performed through the right mainstem bronchus on the right lung with use of 30 ml saline solution (repeated three times to better sample alveolar space). Heparin (0.5 ml) was added to salvaged fluid (approximately 15 ml). Fluid was then centrifuged at 2000 rpm at 4°C for 10 minutes to remove the cells. The cell-free supernatant was divided into several aliquots and stored at -70°C until assayed. Analyses of plasma and BALF interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1) concentrations were carried out in triplicate and in a blinded fashion with use of the ELISA technique and standardized methods.
Results: Only the animals in the combined resuscitated hemorrhagic shock plus fat embolism group (HR/FE) had bronchoalveolar lavage fluid IL-8 and MCP-1 levels that were significantly higher than controls (0.72 ng/ml vs. 0.26 ng/ml, P = 0.03; 18.3 ng/ml vs. 2.0 ng/ml, P = 0.01, respectively). Animals in the fat embolism alone and resuscitated hemorrhagic shock alone groups had no significant elevations compared with controls. Moreover, no significant plasma cytokine levels were detected in any of the groups.
Conclusion/ Significance: We investigated the effects of fat-induced lung injury in the setting of acute resuscitated hemorrhagic shock. Only the animals that underwent resuscitated shock and fat embolism displayed amplified BALF proinflammatory cytokine expression. We have previously proposed that the "two-hit" combination of shock plus fat embolism leads to greater pulmonary dysfunction. Subsequently, we were able to display neutrophil activation and enhanced alveolar infiltration in the combined hemorrhage/resuscitation plus fat embolism group. The cytokine measurements in the current study support our previous findings. In support of our hypothesis, the combined HR/FE did elicit greater cytokine expression, which was not significantly elevated in the FE or HR groups alone. These selected cytokine markers may be used as early indicators of an augmented inflammatory response. Our findings suggest that the two-hit combination of resuscitated shock with fat embolism initiates an inflammatory response, which may play a role in the development of fat embolism syndrome.