M. Lang: I would like to proceed to the next topic and am asking the nitrox training agencies for assistance. There is a logical flow to this agenda. We started by highlighting and discussing the operational nitrox certification and nitrox dive data. The following exercise is a sort of mitigation for not allotting each representative a 20-minute dose of "death by PowerPoint".
Since the agency heads are all present here, we will proceed with the completion of this comparative table. Please remember to only present values for recreational nitrox diving.
Table 3. Recreational, Scientific and Governmental Nitrox Diving Training Requirements Recreational Nitrox Training Requirements
Max PO2 limit (atm) O2 Content Range O2 Cleaning O2 Limits (atm) OTU/UPTD Mix Analysis Accuracy
EANx Table/DC*
Agency Tables Table Model Encourage DC Prerequisites
IANTD 1.6 22-40%
>40%
NOAA 300/day
± 1 % T/DC
Y
B-PiN2 Y none
ANDI 1.6 22-50%
>21%
NOAA N/A
± 1 % T/DC
Y B-PiN2
Y none
TDI 1.6 22-40%
>40%
NOAA N/A
± 1 % T/DC NOAA USN-EAD**
Y OW
PADI 1.4 22-40%
Mfr.
NOAA N/A
± 1 % T/DC Y Rogers/RDP
Y OW
NAUI 1.4 22-40%
>40%
NOAA 350/day
± 1 % T/DC Y mUSN-EAD
Y none
SSI 1.6 22-40%
>40%
NOAA NOAA
± 1 % T/DC Y USN-EAD
Y OW Government and Scientific Nitrox Training Requirements
Max PO2 Limit (atm) O2 Content Range 0 2 Cleaning 0 2 Limits (atm) OTU/UPTD
Mix Analysis Accuracy EANx Table/DC*
Agency Tables Table Model Encourage DC Use
NOAA 1.6 32% & 36%
>40%
NOAA Repex
± 1 % T NOAA USN99-EAD
N
NASA 1.6 46%
>23%
NOAA 415/day
± 1 % T USN USN-EAD
N/A
AAUS 1.6 22-40%
na NOAA
Repex
± 1 % T/DC NOAA mUSN99-EAD
N/A
UNCW 1.6 28-40%
>40%
NOAA Repex
± 1 % T/DC
USN mUSN99-EAD
N/A
*DC = Dive Computer
** m=modified; USN=United States Navy; EAD=Equivalent Air Depth
USN99=United States Navy 1999 Dive Tables which have not been published at time of this proceeding.
Lang (ed.): DAN Nitrox Workshop, Divers Alert Network, November 2000
M. Lang: Discussion of table entries.
B. Gilliam: We found out a long time ago that if you keep the divers within the CNS limits, the OTU/UPTD count doesn't make any difference. You can't obtain numbers that are worth mentioning.
K. Shreeves: PADI recognizes the track record of the 40 percent guideline, but we defer to the manufacturer's recommendation.
C. Borne: OTU's are 415 for the suited subject. We take the divers in with a recreational diver's certification and then put them through NASA's training program. Most of my population is former commercial or ex-military, except for the scuba instructors.
D. Rutkowski: The mix analysis should read +/-1 percent.
E. Thalmann: The NOAA tables and USN tables are the same. The NOAA tables just continue what the EAD already is. If you look at the NDL's, these are just the USN tables converted to the EAD's. If you're using USN tables, you're using NOAA tables.
D. Kesling: That's not the case. You aren't rounding off the ten-foot increments or going to the actual depth for the maximum allowable bottom time.
E. Thalmann: That's a small detail and you're really very close. One of the problems with that column is there's a lot of non-information there. If you say USN, what do you mean? Are you using U.S. Navy air tables with EAD conversion? Do you mean a table in which the actual oxygen percentage is used to compute the decompression? There's a big difference.
I'm going to guess that most of the people are referring to the 1955 U.S. Navy air tables using an EAD conversion factor. Are they using air tables with an EAD conversion or using the actual inspired oxygen PO2 to compute the tables (PiO2)?
T. Mount: IANTD uses the actual oxygen fraction. They are not rounded off on an EAD. They are actually calculated from the oxygen fraction.
D. Kesling: As far as the U.S. Navy tables, UNCW is using the 1999 edition that has not been released yet. We also have modified our tables to reflect the points that were made in a report from the NEDU in 1993, which found some errors in tabulation when those tables were generated.
B. Wienke: NAUI has mUSN (small m for modified).
P. Denoble: What is the maximum range of mixtures that you are teaching?
J. Hardy: 22 to 40%. There is no 21% in the nitrox system.
D. Kerem: Can anyone report on incidences of oxygen seizures in recreational nitrox diving over and above what Dick Vann reported earlier?
M. Lang: Nothing to report. There is not a single response from the audience, so the answer to that question is zero incidents.
M. Wells: Under table model, PiO2 should really be PiN2.
/. Nitrox Operational Data Discussion
R. Moon: I presume that mix analysis means absolute value rather than percent of target concentration, is that correct? What does one percent mean?
E. Betts: It's one percent by volume.
D. Kesling: If your target mix is 36, is it a range from 35 to 37, or is it 35.5 to 36.5?
B. Gilliam: They're all one percent absolutes.
B. Hamilton: In other words, if it says 36 percent, the range is from 35 percent to 37 percent rather than the fraction of 36 percent.
B. Gilliam: That's the way we all do it.
M. Lang: That's a general agreement by everyone.
B. Bjorkman: For those who teach outside of the United States, do these values hold true for international communities?
B. Gilliam: TDI, yes B. Wienke: NAULyes.
T. Mount: IANTD,yes.
E. Betts: ANDI, yes. I'd like to point out that for international marketplaces worldwide cleaning is required either by local ordinances or law for 21 percent or greater.
K. Shreeves: PADI, if there's a local regulation, obviously then the instructor has to conform to that, but otherwise, yes.
J. Hardy: SSI, yes, because the materials are simply translated directly and converted to the metric system.
NITROX AND CO2 RETENTION Dan H. Kerem Israel Marine Mammal Research and Assistance Center The Leon Recanati Institute for Maritime Studies University of Haifa Mt. Carmel, Haifa, 31905 ISRAEL Ran Arieli Yochanan I. Daskalovic Avi Shupak Mirit Eynan Israeli Naval Medical Institute, IDF Medical Corps, POB 8040 Haifa 31080 ISRAEL
Introduction
Breathing hyperoxic nitrox ("nitrox") entails a tradeoff between the benefits of a reduced nitrogen tension and the hazards of an increased oxygen tension. Of the latter, pulmonary and/or whole body oxygen toxicity need not concern sport divers who should only worry about central nervous system (CNS) toxicity. When the risk of CNS toxicity is compared for nitrox versus pure oxygen, at an equivalent PO2, the former is potentially riskier on account of two factors:
1. The presence of nitrogen (and seemingly of any "inert" gas) on top of toxic O2 levels hastens the toxic manifestations in several experimental systems:
• In vitro free radical formation
• Survival of fruit flies
• Appearance of seizure activity in the EEG of rats.
2. The increased density of the mixture and the increased work of breathing it, favor hypoventilation and CO2 buildup (see below).
It falls to reason that safe PO2 limits would be more conservative with nitrox, as reflected by the NOAA limits, which are universally adopted by the recreational and scientific nitrox diving communities.
The threshold of CNS oxygen toxicity is notorious for its inter- and intra-subject variability and unpredictability and safe PO2 limits are aimed below the threshold of the most sensitive breathers, on their "worst day". This paper reassesses the attempt to identify individuals with an
Lang (ed.): DANNitrox Workshop, Divers Alert Network, November 2000
allegedly inherent and expected increased risk and to tailor-make them individual PO2 limits, for their own safety as well as for possible liberalization of the global limits.
The inherent risk alluded to is that of CO2 retention resulting from exercise hypoventilation, a term that describes a situation where the increase in ventilation does not match that of CO2 production. In this case, CO2 is temporarily "retained" until a new steady state is reached where the product of (the lower) alveolar ventilation and (the higher) alveolar CO2 again equals CO2 production.
Diving hypoventilation can ensue from either one or any combination of the following, all of which pertain to nitrox diving:
1. A depth-related increase in gas density and work of breathing, 2. A PO2 -related blunting of ventilatory drive,
3. A dive-related individual trait, which could be one or both of:
• Inborn - the diving vocation selects natural exercise hypoventilators, who feel more comfortable under water.
• Acquired - being the only aerobic sport limited by the quantity of breathing gas, a gas-saving breathing pattern is adopted with time.
There is individual variability of the ventilatory response to all 3 factors and the actual degree of hypoventilation resulting from their combination is subject-specific. However, compared to the sensitivity to CNS O2 toxicity, it is much more consistent and reproducible in a given subject.
The rationale for CO2 retention exacerbating CNS O2 toxicity is given below:
Hypoventilation
I
Arterial hypercapnia ( t PaCO2 )
I
Cerebral vasodilation
I
tCerebral tissue PO2
I
1 CNS oxygen toxicity threshold
The weakest link in the above chain, where individual traits are concerned, is that the cerebro-vascular response to CO2 has not been measured during exercise in known "normal chronic hypoventilators", alias "CO2 retainers", and not shown to behave in accord with the general population. Frequent complaints of immediate and transient post-dive headaches in established retainers (who usually "learn how to live with it") may hint that vasodilation indeed exists, at least in some subjects.
Kerem etal.: Nitrox and CO2 Retention
Scanning Nitrox Divers For Degree Of CO2 Retention Professional Divers
Screening for CO2 retention during nitrox breathing is more readily justified in select small groups, backed by a hyperbaric physiological research facility. The Israeli Navy opted to test its construction divers, immersed at their current maximal operational depth, using the Ch-richest nitrox mixture, their personal regulators and at a realistic exercise level (1.2-1.4 L O2/min).
Specialized equipment required includes a wet pressure chamber and a fast-response CO2 analyzer or, better still, a mass spectrometer to accurately measure end-tidal CO2 tension in lieu of the more pertinent arterial tensions.
Measurements were also made, at the same exercise level, breathing air at 1 ATA. This attempted to isolate the contribution of the third hypoventilation promoter from that of the first (depth related) two and also to devise a much simpler test that may predict the response at the above-listed conditions. The figure below shows the correlation of exercise end-tidal PCO2 while breathing air at 1 ATA and 40% nitrox at 4 ATA, in 34 professional Navy divers.
The 40 torr mark line in the figure is the mean end-tidal PCO2 for several hundred young novice divers exercising on 1 ATA air. The 50 torr line marks 2 SD above this mean and the defined upper limit for end-tidal PCO2 of "non-retainers". The 55 torr (3 SD above the mean) line brackets the defined range of "moderate CO2 retainers", whilst anything above it is defined as an "extreme CO 2 retainer":
65
60 -
3 55 -
50 -
46-1
35 -
30
I
30
—1—
35 40 45 50 5545
end-tidal Pco2 - Air (torr)
60 65
CORRELATION BETWEEN END-TIDAL Pco2
IN I ATA AIR & IN 4 ATA NITROX
Lang (ed.): DAN Nitrox Workshop, Divers Alert Network, November 2000
It will be seen that as a whole, group values tend to lie above the 40 torr mean, during both 1 ATA air and, especially, during nitrox breathing at depth. The group mean end-tidal PCO2 was not significantly higher with the latter, attesting to the dominance of the depth-unrelated individual trait in determining the degree of hypoventilation. Four non-retainers on air became so on nitrox (3-6) and two went the other way (1-2). For a small select group, this rather high percent of false negatives in the air-predictor test is unacceptable and the simulation of actual conditions is called for.
We had occasion to test 5 of the above subjects again after four years. The comparison between the two time-spaced measurements for both air and nitrox is shown in the figure below.
Numbers 1-5 identify air-Nitrox pairs for each of the 5 subjects. The 1 ATA air values for 4 subjects (2-5) have maintained their definition (3 non-retainers and one moderate retainer) over the years. Subject 2, with time, became a moderate retainer on nitrox and subject 1, a very experienced diver who of late very rarely dives, has moved from being an extreme retainer on both mixtures to being a moderate one on nitrox and a non-retainer on air.
• 1 ATA AIR
• 4 ATA NITROX
6O Q.
T3£
"5.
X
9
•o
1 S
00
o>
5 5 -
5 0 -
45 -
An
35 -
• 3
I 4
•
•4
• 1
5
35 40 45 50 55
1994 mixed-expired Pco2(torr)
4-YEAR REPEAT TEST OF CO2 RETENTION IN 5 PROFESSIONAL DIVERS
Kerem et al: Nitrox and CO2 Retention
Recreational Divers
The first questions that come to mind are "why bother?" and if we do bother, "is it realistic?". Let me address the second question first in saying: "It is". A nitrox diving club may, at a modest cost, maintain a facility, which will provide a simple if not conclusive test for CO2 retention. The required equipment is a simple graded bicycle ergometer, a hose connecting the exhaust of a scuba regulator to the inlet of a mixing box and a simple CO2 analyzer, the probe of which resides in a "flow container" at the outlet of the box. A thermometer (tested gas is assumed to be fully saturated), barometer and a conversion chart from V0CO2 to CO2 tension completes the list.
The relevant physiological measurement is that of exercise mixed-expired CO2 tension, which is generally highly correlated with end-tidal CO2 tension, as seen in the next figure, constructed with available data from 24 of the 34 construction divers in the first figure:
50 -
o
•4->
N
o
oQ_
£ '5.
Xo
1 4540
35
30
25
Y = 0.91X-3.48 r = 0.84
35 40 45 50
end-tidal Pco
2(torr)
55 60