What Is an Impedance Threshold Device?
An impedance threshold device (ITD) is a valve that attaches to the face mask or advanced airway during CPR. During the decompression phase of chest compressions (when the chest recoils upward), the ITD limits air entry into the lungs, creating greater negative intrathoracic pressure. This enhanced negative pressure improves venous return to the heart, which increases coronary and cerebral perfusion pressure during the next compression.
How ITDs Improve CPR Hemodynamics
During standard CPR, the decompression phase allows air to flow freely into the lungs, which partially reduces the negative intrathoracic pressure generated by chest recoil. By impeding airflow during decompression, the ITD allows greater vacuum effect in the chest, pulling more blood back into the right heart. When the next compression is delivered, more blood is available to be pumped to the coronary arteries and brain.
Think of it like a bellows with a one-way valve: during decompression, the chest expands and draws blood inward rather than air. During compression, the blood is pushed forward through the circulatory system.
Clinical Evidence
Research on ITDs has shown mixed results. Some studies have demonstrated improved survival to hospital discharge and neurological outcomes, particularly when the ITD is used in combination with active compression-decompression CPR. Other studies have not shown a significant benefit with standard CPR alone. The current AHA guidelines consider the ITD a reasonable adjunct when used by trained providers.
Practical Considerations for EMS
- The ITD is attached between the bag-valve mask and the face mask or advanced airway
- It includes a timing light or indicator to guide ventilation delivery
- Ventilations are delivered normally through the device when positive pressure is applied
- The device is only effective when chest compressions produce adequate recoil
- It should be used in conjunction with high-quality CPR, not as a substitute for good technique
- Remove the ITD if the patient has return of spontaneous circulation (ROSC)
For the NREMT exam, understand the concept of how ITDs enhance venous return and the principle of negative intrathoracic pressure during CPR decompression. This connects to the broader understanding of CPR physiology that the exam tests.