Non-Invasive Measurement of Physiologic Stress
The AIX monitors vascular tone to help paint a picture of patient health.
Vascular tone can be an important marker for whole- body phenomena. A measure of vascular resistance, obtained noninvasively, provides useful information across various populations. In an elderly population, for example, increased vascular resistance is found in patients with diffuse atherosclerosis. Vascular tone in otherwise healthy people is largely controlled by the autonomic nervous system. In cases of hemorrhagic shock and other severe trauma, the body first responds by increasing vascular tone, keeping the available blood to the core. Combined with the heart rate variability, which is also controlled by the autonomic nervous system, a measure of vascular tone can provide an important picture of the current state of health/injury and the short term prognosis of the patient.
Outside of an obvious traumatic injury, increased stress levels also result in increased vascular tone, and so vascular resistance can be used as a marker for stress even for non-cooperative individuals. Vascular tone can also be controlled voluntarily, through considerable effort such as that exerted by pilots in the anti-G straining maneuver (AGSM).
Measuring the Vascular Resistance
The tone of the vascular tree can be summarized in a single nondimensional value: the Augmentation Index (AIx). The AIx indicates the strength by which the pressure wave leaving the heart is reflected at the end of the vascular tree, relative to its original strength. Augmented Pressure (AP) is the amount by which the initiating pulse is strengthened (augmented) by the reflected wave. The AIx is then calculated as the AP divided by the pulse pressure (PP). Since the AIx is nondimensional (being a ratio of two pressures) the absolute pressure does not need to be known in order for the AIx to be measured. The AIx has components that can be drawn out from its time plot, however the key features that identify the AIx components are found with an analysis of the progression of the time signal in its second and higher derivatives.
*Product under development, not yet FDA approved