dive computers with Doppler ultrasond bubble monitors

[Dr Deco]

Dear Dr. White:

There certainly is a lot in this question. It is additionally a very contemporary one since much interest lies in detection of gas bubbles. This is not only true for decompression but for determination of the void fraction in industrial processes. Unfortunately, some of the questions that you ask cannot have answers that are readily available since some of this work is being performed for NASA and is proprietary. They are individuals in a similar position to yours in that they are associated with small R and D laboratories and are connected to NASA through Small Business Innovative Research (SBIR) grants.

Generally speaking, veins are not a good place to look.

• First regarding bubble size as a function of time, the following was done using a Coulter Counter. Hills BA, Butler BD 1981. Size distribution of intravascular air emboli produced by decompression. Undersea Biomed Res. Sep; 8 (3): 163-70. (For the benefit of FORUM readers, this measures size on a basis of changes in electrical conductivity.) The years have not demonstrated that this information has any particular predictive capabilities for DCS, however.
  
  Next regarding bubble size below acoustic Doppler. This is more a function of background signals than anything else is. If you are using transducers in the megahertz range, the resonant frequency of bubbles will be microns in diameter. The problem with blood is that there are also many other scatters present, viz, red blood cells. Thus, clusters of microbubbles are detectable but not individual ones.
  
  The contribution of microbubbles in the blood to DCS is not obvious since there is none. You cannot produce DCS by injecting air into the blood stream. You cannot even amplify DCS by injecting bubbles into the blood stream after diving (an animal). This is because DCS (joint pain, the bends) is the result of an extravascular (outside of the capillaries) gas phase that is present in the tissues. This gas phase grows by inward diffusion of gas into micronuclei (seeds) in the tissue until a compartment syndrome results (excess pressure in a tissue that causes pain and the local collapse of the capillaries). Bubbles probably do not coalesce, and they certainly do not embolize (occlude) blood vessels under normal conditions (though they do embolize lung capillaries and are eventually expelled by breathing).
  
  Gas does not “precipitate out of the blood” but rather diffuses from tissue, where it is dissolved, and into gas micronuclei present in capillaries. There they grow and are eventually expelled into the veins by movement of the muscles. In the venous return (venous system), they are essentially harmless. { MR Powell. Leg pain and gas bubbles in the rat following decompression from pressure: monitoring by ultrasound: Aerospace Med., 43, 168-172 (1972): MR Powell. Gas phase separation following decompression in asymptomatic rats: visual and ultrasound monitoring: Aerospace Med., 43, 1240-1244 (1972). see also MR Powell, MP Spencer, and O von Ramm. Ultrasonic Surveillance of Decompression. In: The Physiology and Medicine of Diving, 3rd Edition, [P. Bennett, D. Elliott, eds.] pp. 404-434, Baillière Tindall, London (1982).}
  
  Gas bubbles do not appear throughout the blood stream, and they are not an exclusive indicator of forthcoming DCS. They are actually from capillaries in muscle and fat tissue and arise predominately (though not exclusively) from tissues that have a higher probability of developing joint pain DCS. { Conkin J, MR Powell, PP Foster, JM Waligora. Information about venous gas emboli improves predictions of hypobaric decompression sickness. Aviat. Space Environ. Med., 69, 8 - 16, (1998); Conkin J, PP Foster, MR Powell, JM Waligora. Relationship of the time course of venous gas bubbles to altitude decompression illness. Undersea Hyperbaric Med.; 23: 141- 49. (1996); Conkin J, PP Foster, MR Powell. Evolved gas, pain, the power law, and probability of hypobaric decompression sickness. Aviat. Space Environ. Med. 69, 352 - 359, (1998). } The bubbles are generated predominantly in tissues that are capable of musculoskeletal activity (though this is not exclusively true as the depth increases.)
  
  The bubbles are generally detected in the pulmonary artery, near the heart, as this is the confluence of all veins. Away from the heart (nearer the source of bubbles) gives information about the locus of formation. They give some information about impending DCS when detected, for example, during saturation decompression. Here the depress is halted until the problem is alleviated, and then the ascent to the surface resumes. Bubble detection at depth and in the water has not been attempted. Generally speaking, decompression is not do in the water, except for technical divers, and they do not report DCS until surfacing.
  
  Gas bubble in the circulatory system give only limited information. The real culprits are those gas bubble that grow in tissues and are stationary. These bubbles do not form with decompression but rather grow during the supersaturation. They grow from (presumably) nuclei already present. These nuclei or seeds are generated by stress-assisted nucleation processes (hydrodynamic cavitation) as occurs in all natural waters. Those that are above a certain radius, as determined by the Young-Laplace equation (above the Laplace cutoff), will grow during decompression. If present in sufficient quantities, DCS is the outcome.

Unfortunately, this answer is very sketchy and represents only the “tip of the iceberg.” Most systems for gas phase detection evolve around ultrasound since this modality is able to capitalize of the very large acoustic impedance difference between water and a gas. In addition, at resonance, the scattering cross section is orders of magnitude larger than the geometric cross section.

If you have further questions that are sparked by this response, please resubmit them to the FORUM.

____________
Dr. Deco
"The King" (Neptune, that is,) is currently constructing a Dr Deco web site. This material in more detail will appear in a short time.

 

[scubadoc]

For those interested-here is the web link for the article published in 'Wellcome News Online', Issue 24, Q3, 2000.
http://www.wellcome.ac.uk/en/1/awtpubnwswnoi24res5.html

Robert Forbes apparently has received a 'Showcase Grant' for this study of recreational divers being done in the Orkney Islands for Heriot-Watt University’s International Centre for Island Technology.