Posted by John Walker on June 25, 2001 at 19:43:10:
I would like to share with you guys one of the refered articles found by Mr. Pettey.
The Effects of Pressure on Blood Filterability
In the pursuit of the idea that an increase in environmental pressure can cause a change in the stiffness and therefore the filterability of blood cells. We carried out a series of shallow saturation dives to a maximum depth of 110 meters using both nitrogen (for shallow exposures) and helium as the diluent gas.
The filterability of whole blood is measured by passing a suspension containing the cells through a 5 micron polycarbonate filter and relating the time taken for a cell suspension to pass to the time taken for the suspending buffer alone to pass. The resultant measurement which is termed the Index of Filtration (IF) is corrected for variations in cell numbers and haemotocrit. A 5 micron filter is chosen because this approximates in size to the diameter of the smaller capillaries through which the cells have to pass in vivo.
The Whole Blood results were as follow:
(1) . Oxy-nitrogen Exposure
Depth/p02 Control At exposure % Change
15m/400mb 4.3 (0.9) 7.5 (0.9) 153%
30m/400mb 7.3 (0.9) 152%
(11). Oxy-helium Exposure
15m/400mb 3.37 (0.22) 3.2 (0.17) 94%
30m400mb 3.5 (0.32) 103%
110m/400mb 3.6 (0.70) 106%
These studies support the idea that a change in the filterability of individual blood cells may adversely influence the flow properties of blood in divers breathing nitrogen and may play a part in the physiology of decompression and the pathogenesis of decompression sickness.
Introduction:
Aseptic necrosis of bone (ABN) in divers is the consequence of shut down of the blood supply to the bone marrow. (1) Many hypothesis have been advanced to account for the mechanisms of ABN. We have in previous studies demonstrated that changes in the ability of the cells of the blood to deform occurs under hyperbaric conditions and may provide a mechanism for the causation of damage in the working diver. (2) Using Sickle Cell Disease as a model we have been able to postulate that not only bone damage but also insult to central nervous system and to the inner ear may also be the consequences of altered blood deformability. In pursuit of this idea that an increase in environmental pressure can cause a change in the stiffness and therefore the filterability of blood cells. We carried out a series of shallow saturation dives to a maximum depth of 110 meters using both nitrogen (for shallow depths) and helium as the diluent gas.
Method:
All control measurements on the cellular elements in the blood are taken outside the chamber before pressurization. The blood samples taken from the subjects under pressure are placed on a roller in a plastic tube containing Lithium-Heparin. Normally, the interval from sampling to the first measurement is 20 minutes.
Our technique for the measurement of filterability involves the passage of a suspension of blood cells in phosphate buffer 0.15MP04 at a pH of 7.3 which is isotonic and iso-osmolar with plasma at a haematocrit of 0.07 (7%), through a filter whose pores are 5 microns in diameter at a filtration pressure of 10 cmH20. The resultant measurement known as the Index of Filtration (IF) is obtained by noting the time taken for 0.5ml of cell suspension to pass divided by the time taken for 0.5ml of buffer alone.
The equipment consists of a base upon which is placed the 5 micron filter. On the top of this is clamped the upright apparatus which consists of a long Perspex tube which leads into an expanded portion into which an injection of blood call suspension can be made. In the use the apparatus is assembled with the membrane in place and the suspension rises above the top mark, the observer notes the time taken for the blood cell suspension meniscus to fall from the first mark to the second mark. (3)
Results:
In the dives using oxy-nitrogen, it was shown that there is a progressive rise in the
index of filtration under pressure, and it is attributed to a modest increase in the red
cell IF and a much greater increase in the white cell value. The relative change in the
filtration in both elements is probably sufficiently significant to cause associated
pathology in certain situations.
The next objective was to check the effect using oxy-helium with a partial pressure of 400mb throughout the dive. On the compression measurements were made at 15 and 30 meters to confirm the results obtained in the oxy-nitrogen dives. We can see from the above graph that the index of filtration increased with depth but to a less greater extent than previously seen with the nitrogen diving. These experiments demonstrate that helium is superior to nitrogen as a diluent gas for diving not only on account of its physical characteristics which aid decompression but also because it has less effect on the flow characteristics of blood cells and this in tern may influence the long term effects of hyperbaric gas breathing on the tissues of the body.
The effect of the partial pressure of inert gas and that of oxygen are important in determining the net change in whole blood filterability. Oxygen clearly plays an important role in both helium and nitrogen diving, the effect is more pronounced in helium with the blood cells becoming more flexible, in the nitrogen series the blood cells remain stiff. The effect of oxygen breathing through a mask during decompression for two twenty minute cycles can be clearly seen in the paragraph overleaf.
(Graph not shown)
The filterability in both the nitrox and heliox dive rose from their surface control. There was a considerable increase in the filterability of the blood cells prior to the two oxygen breathing periods. It is interesting to note that in the nitrox series the filterability improved slightly after 100% oxygen was breathed, but in the heliox series the filterability of blood improved considerably and the blood cells became far more flexable. The effect of oxygen in the oxy-helium environment is much greater than that seen in the nitrox environment. Presumably the improved haemorology resulting from the lowered filterability must have some benefit at the level of tissue oxygenation. One might speculate that if at the start of the decompression the filterability of the blood improved as much as possible the deleterious effects of decompression may be offset than abolished.
Conclusion:
The results obtained in both series of dives lead us to believe that the greater concern must be shown to nitrox diving and its implications than with helium. With air diving we should avoid low p02 and maximize the use of oxygen which has been shown to help improve filterability of blood and so the consequent improvement in tissue blood flow. This will assist considerably in the elimination of dissolved gas from the tissue and in turn may influence the long term effects of hyperbaric gas breathing on the tissues of the body.
on Blood Filterability.
Pimlott.J.K; Bovisand.Plymoth. PL9 OAB. England