Prediction of arterial wall failure under acceleration stress in high-performance aircraft.

Experimental results indicate that the passive mechanics (no smooth muscle action) of the larger arteries, such as the aorta and pulmonary, in humans are closely related to the elastin content at low strain and to the collagen content at high strain. In an aerial combat maneuver, the acceleration stress typically rises to a maximum, remains there for several seconds, and then falls off. The arterial wall viscoelastic property relevant here is hysteresis. The strain remaining when the stress is removed is called "permanent set." This so-called permanent set will decay away exponentially with a comparatively long time constant on the order of several hundred seconds. A first approximation to the permanent set, after a hysteresis loop has been executed, has been derived. Collagen can only stretch by 20% before failing. Choosing combat scenarios in which the maneuver is repeated at 1-min intervals so that the decay in permanent sets can be ignored, the number of maneuvers required to damage or rupture the collagen is computed for ranges of acceleration (g) values equivalent to increases in the arterial intraluminal pressures. The calculations are done for the external carotid, abdominal aorta and pulmonary arteries. For example, applying the continual recruitment of collagen fibers theory to the pulmonary artery produces the following result. The permanent set at net acceleration value of 5 g is 0.085 and the number of sequential maneuvers required to damage 2.5% of the collagen is 2.4. Based on discontinuous recruitment of 100% collagen at a strain of 1.7 in the pulmonary artery, the number of maneuvers required to rupture the collagen would be 58 at an acceleration of 5 g. At 10 g, this number falls to about 10 maneuvers. The results emphasize the importance of present protective measures, such as the use of G suits and valsalva maneuvers. But these measures can only reduce the effects predicted here, not eliminate them. The results also highlight the necessity for servo-controlled seat orientation, and for the introduction of integrating accelerometers for all aircrew to record acceleration exposure history.