Severe Traumatic Brain Injury.

Abstract

Background: Inadvertent hyperventilation is associated with poor outcomes from traumatic brain injury (TBI). Hypocapnic cerebral vasoconstriction is well described and causes an immediate and profound decrease in cerebral perfusion. The hemodynamic effects of positive-pressure ventilation (PPV) remain incompletely understood but may be equally important, particularly in the hypovolemic patient with TBI. Objective: Preliminary report on the application of a previously described mathematical model of perfusion and ventilation to prehospital data to predict intrathoracic pressure. Methods: Ventilation data from 108 TBI patients (76 ground transported, 32 helicopter transported) were used for this analysis. Ventilation rate (VR) and end-tidal carbon dioxide (PetCO2) values were used to estimate tidal volume (VT). The values for VR and estimated VT were then applied to a previously described mathematical model of perfusion and ventilation. This model allows input of various lung parameters to define a pressure–volume relationship, then derives mean intrathoracic pressure (MITP) for various VT and VR values. For this analysis, normal lung parameters were utilized. Separate analyses were performed assuming either fixed or variable PaCO2–PetCO2 differences. Ground and air medical patients were compared with regard to VR, PetCO2, estimated VT, and predicted MITP. Results: A total of 10,647 measurements were included from the 108 TBI patients, representing Received July 11, 2014 from the Department of Emergency Medicine, University of California at San Diego, La Jolla, California (DPD, SAA, GV, JVD), School of Medicine, University of California San Diego, La Jolla, California (KS), Division of Cardiology, University of California San Diego, La Jolla, California (RDH), Division of Cardiology, The Ohio State University, Columbus, Ohio (RDH), Department of Anesthesiology (AM) and Division of Pulmonary and Critical Care (RS), University of California at San Diego, La Jolla, California, and Rural/Metro Ambulance, San Diego, California, and San Diego Department of Fire–Rescue, San Diego, California (RF, CB, JVD). Accepted for publication August 4, 2014. The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper. This paper was presented orally at the Resuscitation Science Symposium, 2011. Address correspondence to Steve A. Aguilar, MD, UC San Diego Emergency Medicine, 200 West Arbor Drive #8676, San Diego, CA 92103, USA. E-mail: SteveAguilarMD@gmail.com doi: 10.3109/10903127.2014.959228 about 13 minutes of ventilation per patient. Mean VR values were higher for ground patients versus air patients (21.6 vs. 19.7 breaths/min; p < 0.01). Estimated VT values were similar for ground and air patients (399 mL vs. 392 mL; p = NS) in the fixed model but not the variable (636 vs. 688 mL, respectively; p < 0.01). Mean PetCO2 values were lower for ground versus air patients (30.6 vs. 33.8 mmHg; p < 0.01). Predicted MITP values were higher for ground versus air patients, assuming either fixed (9.0 vs. 8.1 mmHg; p < 0.01) or variable (10.9 vs. 9.7 mmHg; p < 0.01) PaCO2–PetCO2 differences. Conclusions: Predicted MITP values increased with ventilation rates. Future studies to externally validate this model are warranted.