Water Hammer effect

Q: What is the "water hammer" effect in blunt aortic injury (BAI) during Motor Vehicle Accidents (MVA)?

Answer: The mechanism revolves around the isthmus

To understand "water hammer" in BAI, it helps to understand the role of the isthmus between the ascending and descending thoracic aorta as a "transition zone." The ascending aorta is more mobile, and as it transits around the isthmus, it becomes relatively fixed as the descending thoracic aorta. Because the isthmus is intrinsically weaker than the rest of the aorta, it stretches with rapid deceleration. For this reason, most BAIs occur at the aortic isthmus just distal to the (left) subclavian artery.

"Water hammer" effect: In MVA, compression of the abdomen secondary to sudden impact occludes the aorta and leads to rupture of the proximal, intrinsically weak isthmus. This phenomenon explains the correlation of geographically distant diaphragmatic rupture and BAI. This effect was first described more than six decades ago.

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References:

Nikolic S, Atanasijevic T, Mihailovic Z, et al. Mechanisms of aortic blunt rupture in fatally injured front-seat passengers in frontal car collisions: an autopsy study. Am J Forensic Med Pathol 2006; 27:292.

LUNDERVALL J. THE MECHANISM OF TRAUMATIC RUPTURE OF THE AORTA. Acta Pathol Microbiol Scand 1964; 62:34.

Rizoli SB, Brenneman FD, Boulanger BR, Maggisano R. Blunt diaphragmatic and thoracic aortic rupture: an emerging injury complex. Ann Thorac Surg 1994; 58:1404.

Richens D, Field M, Neale M, Oakley C. The mechanism of injury in blunt traumatic rupture of the aorta. Eur J Cardiothorac Surg 2002; 21:288.