Thanks Dr. D & Lloyd, and as always kudos to Dr. D's unique insight. This is clearly a potential problem for divers, and your references make a very convincing point about cavitation bubble formation in the arterial circulation.
I've never seen a case of a stroke, either trasient or permanent, in patients with prosthetic valves that were not due to low levels of anticoagulant or from bacterial endocarditis.
However, you bring up a vital point concerning to divers and flyers.
Consider gas loads in patients at 1 ATA: such patients tolerate TEE with bubble contrast without AGE symptoms, and that no supersaturated state exists to stimulate the bubbles to grow beyond a critical size ... suggest that cavitation is less an issue for patients breathing ambient air and acclimatized to pressure. However, in rapid decompression it could be an issue in flight or diving.
For patients in flight, the reduced pressure in planes may induce bubble growth to the critical diameter, but again, I've not heard of an sudden in-flight transient or permanent stroke ... and moreso if it symptoms were improved by 02 [which is common protocol by attendants treating emergency inflight issues.] Further, bubbles would be difficult to find, as landing increases ambient pressure and compress the bubble to terrestrial dimensions.
Diving poses interesting issues, while it maybe possible to dive with a prolonged decompression stop or surface 02 .. a constant source of cavitation seeds the diver until inert gas levels drop below a critical pressure, what such a pressure is from ambient is unknown, and worse, there is an undefined hazard for in-water AGE as the diver begins to surface.
Again, a most unique and helpful insight. I have not heard this issue addressed in diving medicine anywhere but here. The common knowledge is this:
http://www.diversalertnetwork.org/medical/faq/faq.asp?faqid=65
... and I do not recall a mention of bubbles induced by mechanical prosthetic valves in either Bove or Bennett's textbook.
Dr Deco:
Hello readers:
Mechanical Valves
As a biophysicist, I am interested in mechanical prosthetic heart valves because they have been suspected of creating cavitation bubbles within the blood. When monitored with transcranial Doppler ultrasound, emboli are detected. Since it is currently not possible to resolve gaseous from solid emboli, they are referred to as high intensity transient events or HITS.
The process by which this might occur has been studied by the manufacturers of these devices. They are considered to be a potential source of neurological problems because the valve-formed bubbles could flow to the brain and embolize it. Of the many devices in use, it is not clear that there are serious problems from gaseous emboli; anticoagulants are used to mitigate the formation of small clots. Cognitive problems have been reported from prosthetic valves [(1) REFERENCES below].
Experiments have purported to demonstrate that these are gas bubbles and are indeed detectable in the circulation of the brain (2,3). Clots produced from, e.g., rheumatic heart disease, are well know to cause health problems. A literature search on PubMed indicates that severe neurological problems have not been demonstrated by gaseous emboli (4).
Diving
Since there are not any reported problems, this must indicate that whatever is produced while diving does not appear to be in sufficient numbers to result in a problem. That is not an entirely satisfying answer, however.
One can always ask, What if there is a first time, and I am it?
Dr Deco :doctor:
References
- (1) Deklunder G, Prat A, et al. Can cerebrovascular microemboli induce cognitive impairment in patients with prosthetic heart valves? Eur J Ultrasound. 1998 ;7(1): 47-51.
- (2) Bumgartner RW, Frick A, et al. Microembolic signal counts increase during hyperbaric exposure in patients with prosthetic heart valves. J Thorac Cardiovasc Surg. 2001 Dec;122(6):1142-6.
- (3) Georgiadis D, Wenzel A, et al. Influence of oxygen ventilation on Doppler microemboli signals in patients with artificial heart valves. Stroke. 1997 ;28(11): 2189-94.
- (4) Georgiadis D, Lindner A, Zierz S.Intracranial microembolic signals in patients with artificial heart valves: drowning in numbers. Eur J Med Res. 1998 Feb 21;3(1-2):99-102