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This article first appeared in the
September 1992 issue of Underwater USA

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Often at a gathering of divers discussing decompression you’ll hear statements like, "there’s no such thing physiologically as a tissue halftime it’s just a mathematical concept." That is not really so.

Halftimes occur in processes as real as assessing the age of dinosaur bones to knowing how fast you clear drugs from your system. One of the many substances that has been experimentally measured leaving your body in halftime fashion is nitrogen gas after a dive.

RADIOACTIVITY. In a fixed and characteristic period of time, a radioactive substance will lose one-half its mass. After another equal period, half the remaining 50% also exits. A useful way of expressing the unit of time related to half a substance’s life is a ‘half-life’. This is the same as a half-time, just a different name. The time when any one particular nucleus will decay is impossible to predict but the rate of decay, or half-life, of a mass quantity is precise and real.

Decay rates of different radioisotopes vary considerably. Artificially made radioisotopes often have half-lives as short as micro-seconds. Natural radioisotopes have half-lives up to billions of years. Half-lives of natural radioisotopes are useful to determine age of archaeological remains and the geological age of fossils, rocks, and the earth itself. The process is called Radiometric Age Dating.

DRUGS. Drug metabolism also behaves according to half-lives. Your body takes predictable units of time to get rid of one-half of a standard dose. In pharmacology it’s common to call this unit time a halftime. It is also sometimes called a half-life. The halftime varies a bit among individual people. However, a general range is determinable.

Valium, for example, has a halftime of about 24 hours. Twenty-four hours after taking a five milligram dose (5 mg), about 2.5 mg are still in your body. Given a 5 mg daily dose, Valium takes about 2 weeks to build up until the body's ability to clear the drug balances the daily intake. This is called a steady state. The level falls quickly if the person then stops taking the drug, bringing on withdrawal symptoms. Different parts of your body have different affinities for the drug, and have different times to reach steady state and to clear half the dose. As a generality, blood and plasma levels usually rise and fall more rapidly than levels in fat.

If you wanted to visualize the transit of radioactivity or Valium, you could make a dot on graph paper after each fixed and equal time interval showing how much remained. If you connected the dots you would have a curved line that is characteristic of halftimes. The equation describing that line is called exponential. What would happen if you tried the same dot test with nitrogen leaving a diver?

A REAL NITROGEN HALFTIME. You could catch a diver’s exhaled air in a bag, or pipe it directly to an analyzer and measure how much nitrogen comes out over time. That, more or less, is how a measurement called total body nitrogen washout is estimated. If you charted the time for a total body washout of nitrogen it would look like a curve. That curve is described by the exponential equation of halftimes.

INDIVIDUAL TISSUE HALFTIMES. Total washout curves, like most composite descriptions, lose detail from individual contributors. They tell nothing about how much or little nitrogen makes its way in and out of each of the different parts of you. Different structures of your body gain and lose nitrogen at varying rates. Different nitrogen pressures resulting in those different parts appears to matter. Some parts of you may be relatively quiet on the nitrogen front. Too much nitrogen pressure in any of your other parts from diving too deep, too long, or surfacing too fast, may begin the journey of “Tiny bubbles in your veins, going slowly to your brains…”

EXPERIMENTAL AND THEORETICAL EVIDENCE. Short submarine escape tower work and longer experimental decompression from regular compressed air dives show certain tissue washouts really do proceed faster than others, thereby identifying faster and slower nitrogen halftimes for areas of the body. It was also found that adding more halftimes representing those different body areas brings predictions for decompression table safety closer to the actual outcomes. Most decompression models today don’t use one halftime to represent the entire body.

NOT JUST NUMBERS. The US Navy tables conveniently lower the high number of possible halftimes by grouping them into multiples of minutes, for example, 5, 10, 20, 40, 60, 80, 90, 100, and 120 minutes. Other models use other groupings of minutes.

Yes, halftimes are numbers. However, the numbers are descriptive of real things happening in your body, not just concepts. Numbers are an economical way of describing mathematically something that is complicated biologically. And much more convenient than running after divers putting dots on paper.

IT’S INTERESTING. Tissue nitrogen transport may look and behave exponentially, but is it really? Not all systems gain or lose their components according to exponential decay law. And even if nitrogen does come and go exponentially under controlled conditions, practical factors and things you do during dives change the calculations.

The idea of each of your body compartments offgassing separately but at the same time is called offgassing ‘in parallel’. It’s highly probable that not all gas diffuses in parallel from each body compartment separately back into your blood stream for offgasing by exhalation. If a higher nitrogen pressure area is next to a lower pressure area, nitrogen will flow from the higher to the lower producing serial offgasing of one tissue to another. Serial transfer has already been observed in pharmaceuticals. There is also a difference in the time it takes things to get into your body compared to back out again.

A more important monkey wrench in the works is that in practical application divers are often creative with dive rules and guidelines, producing conditions that affect orderly and explainable nitrogen transport. That means they screw up. That has practical significance.

Halftimes describe calculations for dive time limits based on, among other things, eliminating nitrogen that is dissolved in your body, not nitrogen that has become a gas again before you can breathe it out. Sometimes bubbles can help remove nitrogen, but other times, once bubbles form in your body they can sometimes obstruct further nitrogen exit by several mechanical and chemical means.

What can you do to reduce or prevent bubble causing trouble?

1. Slow ascent rates
2. Safety stops
3. Keep your cardiovascular system healthy

Done together, these can make the difference between having time to offgas nitrogen before it evolves into bubbles, or allowing your body to fill with inert gas grenades.

So do you really have a 60 minute compartment, or a 5 or 120 minute compartment? It’s likely you do have body structures that gain or lose half their nitrogen burden in 5, 60, or 120 minutes. Of course those parts are not an entire organ like your heart or your stomach, but would be similar structures scattered all over you.

We may not yet have the complete system to describe nitrogen leaving your body and thereby completely prevent decompression sickness, but halftimes are very real.

The contents of this site are copyright © 1996-2009
 Ernest Campbell, MD, FACS All Rights Reserved.

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