Inspiratory resistance as a potential treatment for orthostatic intolerance and hemorrhagic shock.

Loss of consciousness due to central hypovolemia can occur due to sudden cardiovascular decompensation in normal individuals or hypovolemic shock in wounded patients. A variety of devices have been developed to sustain perfusion to the brain including anti-G suits worn by pilots and returning astronauts and applied to patients as "shock trousers." However, all countermeasures developed to date suffer from problems that limit their utility in the field. An "impedance threshold device" (ITD) has recently been developed that acutely increases central blood volume by forcing the thoracic muscles to develop increased negative pressure, thus drawing venous blood from extrathoracic cavities into the heart and lungs. We review here a series of experiments that demonstrate the application of the ITD to a variety of experimental conditions, including its use to: 1) increase heart rate, stroke volume, and arterial BP in normovolemia and hypovolemia; 2) increase cerebral blood flow velocity; 3) reset cardiac baroreflex function to a higher operating range for BP; 4) lower intracranial pressure; and 5) reduce orthostatic symptoms. In this brief review, we present evidence which supports further consideration of using inspiratory resistance as a countermeasure against circulatory collapse associated with orthostatic instability and hemorrhagic shock.