High-Pressure Nervous Syndrome

High-pressure nervous syndrome (HPNS – also known as high-pressure neurological syndrome) is a neurological and physiological diving disorder that results when a commercial diver or scuba diver descends below about 500 feet (150 m) while breathing a helium–oxygen mixture. The effects depend on the rate of descent and the depth

Symptoms of HPNS include :

• tremors
• myoclonic jerking
• somnolence,
• EEG changes,
• visual disturbance
• nausea, dizziness
• decreased mental performance

Oxygen Toxicity Notes

Oxygen toxicity is a condition resulting from the harmful effects of breathing oxygen at elevated partial pressures.

Pulmonary Toxicity

Pulmonary toxicity is caused caused by long term exposure to oxygen with partial pressure in excess of 0.5 bar.

Symptoms of pulmonary oxygen poisoning begin slowly as a substernal irritation that becomes progressively more intense and widespread along with  with increased coughing.  Uncontrollable coughing occurs in severe cases, symptoms originating in the trachea and major bronchi associated with a constant burning sensation, which is worsened by inspiration.  The most severe symptoms are associated with shortness of breath on exertion or even at rest.  The onset of symptoms is variable among individuals but usually occurs after about 12 to 16 hours of exposure at 1.0 ata, 8 to 14 hours at 1.5 ata, and 3 to 6 hours at 2.0 ata. ( ata= 33 ft sea water)

Central Nervous System Toxicity

Central nervous system toxicity is caused by short term exposure to high concentrations of oxygen, typically involving partial pressures exceeding 1.6 bar. The exact mechanism isn't fully understood but it is thought that the high concentration of oxygen increases the formation of free radicals (atoms, molecules, or ions with unpaired electrons) to a point that damages cells and enzymes.

Signs and symptoms of central nervous toxicity:

• Convulsions - frequently without any warning.
• Visual symptoms including dazzle and constriction of visual field
• Ear related symptoms including ringing of the ears and auditory hallucinations.
• Nausea and spasmodic vomiting
• Twitching - this usually first appears in the lips and other facial muscles. This is the most common and well defined symptom.
• Irritability and behavioral changes including anxiety, confusion, and apprehension.
• Dizziness or vertigo.

PPO2	MAX	5	10	15	20	25	30	35	40	45	50	55	60
1.20	210	2%	5
1.25	195
1.30	180
1.35	165
1.40	150
1.45	135
1.50	120
1.55	82
1.60	45

NOAA CNS Oxygen Exposure limits: Table gives percentage of NOAA "allowable" limits for a single dive. At the 1.6 ATM exposure level the "CNS Clock" runs almost 4 times faster as at a PO2 exposure level of 1.4 atm. An exposure to a PO2 1.4 ATM is the maximum "recommended" limit for the working portion of any dive. Exposure levels above a PO2 of 1.4 ATM are shown for contingency planning and for calculations concerning the decompression (resting) portion of stage decompression dives.

Tracking Central Nervous System Toxicity

Multiple dives may be conducted providing that the final CNS exposure in a 24 hour period does not exceed 80%. If the 80% figure is reached, then a 12 hour surface break breathing normoxic air must be taken. Where repeat dives are conducted with a surface interval, a 50% surface reduction in the CNS load (breathing normoxic air) is assumed for every 90 minutes at the surface. This Residual CNS load is added to the next dives CNS load to give the two dive total at the end of the second dive.

Equivalent Narcotic Depth Notes

The Equivalent Narcotic Depth (END) is used in technical diving as a way of estimating the narcotic effect of a breathing gas mixture, such as Heliox and Trimix. The method is, for a given mix and depth, to calculate the depth which would produce the same narcotic effect when breathing air.


There are three different ways you can calculate the END of a mix:

1. Only Nitrogen is narcotic
2. Nitrogen and Oxygen are narcotic - This is the accepted BSAC standard
3. Nitrogen and helium are narcotic
4. Nitrogen, oxygen and helium are narcotic

Q1. What is the Equivalent Narcotic Depth of an 18/35 mix at 60 meters?

This answer assumes that only nitrogen is narcotic

First we need to work out the total narcotic fraction of the mix:

1 - (0.18 + 0.35) = 0.47

Then we need to work out the absolute pressure at 60 meters and adjust it by the relative difference of the narcotic constituents of the mix compared to air.

((60 / 10) + 1) * (0.47 / 0.79) = 4.164 bar

Then we need to convert this pressure back into a depth:

(4.164 -1) * 10 = 31.64 meters

These steps can be expressed in a single formula as:


We can further simplify the calculation by

END = (Depth + 10) × (1 − Fraction of helium) − 10

Q2. You are planning a closed circuit trimix dive. You intend to dive to 75m for 25 minutes using a 15/55 bottom mix using twin 7lt stage cylinders for bailout. One stage cylinder contains 18/55 and the other contains 60% nitrox. What is the equivalent narcotic depth at 75m?

This answer assumes that only nitrogen and oxygen are narcotic

The back gas and bailout both contain 55% helium so the END would be:

END = (Depth + 10) × (1 − Fraction of helium) − 10

END = (75 + 10) × (1 − 0.55) − 10

28 = (75 + 10) × (1 − 0.55) − 10

The Nitrox 60% has MOD of about 15 meters so its not breathable at 75 meters but for the sake of completeness the END would be:

33 = (75 + 10) * (0.4 / 0.79) - 10

Equivalent Air Depth Notes

The equivalent air depth is the depth at which an air mixture has the same nitrogen partial pressure as a Nitrox mix for a given depth or put another way its tells you what depth you would have to dive to on air to receive the same nitrogen exposure. It is used as a way of approximating the decompression requirements of a Nitrox dive using standard air tables.

What is the equivalent air depth of a 27 meter dive using Nitrox 36?

First we need to work out the absolute pressure at 27 meters. Assuming one atmosphere exerts a pressure of 1 bar and 10 meters of water is equivalent to 1 atmosphere. The absolute pressure will be 

(27 / 10) + 1 = 3.7 bar

now we need to adjust the absolute pressure by the relative reduction in nitrogen

3.7 * (0.64 / 0.79) = 2.997 bar

Next we need to convert this pressure back into a depth:

10 * (2.997 - 1) = 19.97 meters

These steps can be expressed in a single formula as:

Equivalent Air Depth = ((( Depth / 10)  + 1) * (Nitrogen Fraction / 0.79)) -1 ) * 10

Maximum Operating Depth Notes

The maximum operating depth of a gas is the depth at which the partial pressure of oxygen exceeds a safe limit. This is generally accepted as being 1.4 bar for the dive phase and 1.6 bar for the decompression phase.

What is the maximum operating depth of a 36% Nitrox mix assuming a maximum partial pressure of 1.4 bar?


We know that the partial pressure of each gas in the mixture is proportional to its concentration and we know that the partial pressure of O2 will be 1.4 bar at our MOD. Therefore we can write

1.4 = 0.36 * ATA

We can then rearrange this to give us the absolute pressure at which a 36% mix gives a PPO2 of 1.4 bar:

ATA = 1.4 / 0.36

Next we need to convert this absolute pressure into a depth. Assuming every 10 meters of water exerts the same pressure as one atmosphere

28  =  (3.8 * 10) - 10


Calculating the best mix

The best mix is the mix with the highest fraction or percentage of oxygen that can be used at the target depth.Calculating the best mix is similiar to the calculation of MOD but in reverse.

What is the best mix for a 30 meter dive using a maximum PPO2 of 1.4 bar?

Step 1: Determine the absolute pressure at the target depth:

(30 / 10) + 1 = 4 bar

Step 2: Determine what fraction will produce the target partial pressure at that absolute pressure:

1.4 / 4 = 0.35 or 35%

The two steps can be combined into a single formula:

Best Mix = (1.4 / ((Depth / 10) + 1)) * 100