An impedance threshold device (essentially a one-way valve) attached to an endotracheal tube prevents air from leaving the chest during compressions, improving venous return, cardiac output and (in animal studies) perfusion.
Auferheide and the ROC investigators report results of a huge randomized trial testing the ITD. 8,718 victims of out-of-hospital cardiac arrest were randomized to receive the ITD or a sham device during CPR. There was no difference in the primary outcome (6% in each group were alive and able to walk and toilet unassisted at discharge), nor in any secondary outcome (survival to admission or to discharge; return of spontaneous circulation at arrival to the ED).
Aufderheide TP et al. A Trial of an Impedance Threshold Device in Out-of-Hospital Cardiac Arrest. N Eng J Med 2011;365:798-806.
This was half of the ROC-PRIMED trial; the other half (reported in the same issue of NEJM) showed no difference in survival according to at what time resuscitators interrupted CPR to check cardiac rhythm.