Oxygen and Pressure: It's the Law

 

What makes up atmospheric pressure?

Of course, the answer must be: the atmosphere.

 

What is Atmospheric Pressure?

The pressure exerted on a body on the Earth's atmosphere is a direct result of the weight of the gases in the atmosphere above. These gases include nitrogen (N2), oxygen (O2), argon (Ar), carbon dioxide (CO2), carbon monoxide (CO), helium (He), hydrogen (H2) and other trace gases.

At sea level, we define the pressure as one atmosphere absolute (1 atm), or 760 mmHg. Oxygen and nitrogen are the two mayor players in the atmospheric pressure: at sea-level, oxygen has a partial pressure (PO2) of 160 mmHg; nitrogen has a PN2 of 600 mmHg.

This pressure can increase by submerging ourselves in sea water, as we do when we go diving, where the weight of the water is added to the column of gas from the atmosphere.

 

How does oxygen behave in our bodies?

The bigger the pressure, the smaller the volume, the more molecules stack up in our lungs and get dissolved in our bodies.

As the atmospheric pressure around us increases, we breathe more molecules.

Oxygen readily goes in solution in our body water. We have many (5 million/ML) tiny red blood cells which contain a protein, hemoglobin. At sea level, this protein temporarily binds most of the oxygen in the lungs, and transports it into the tissues via tiny blood vessels, the capillaries. The oxygen then is released in the tissues, where the cells use it for fuel, and changes into CO2 and water. Normally, only 3% of oxygen is free, not bound to hemoglobin.

With increase in pressure, more oxygen is dissolved in our body water, 60% of our weight. It is mainly through our body water that oxygen diffuses to all tissues during a hyperbaric session: there are virtually no barriers. This is why oxygen can reach places where it ordinarily cannot, such as scars and wounds, which normally have very few capillaries and little oxygen.

 

In patients with a normal alveolar-arterial gradient, the concentration off oxygen diffused in plasma is directly proportional to alveolar oxygen.

PAO2= (FiO2x (760-47)-(PaCO2/0.8)

Where PAO2= Partial Pressure of Alveolar Oxygen

FiO2= Fraction of Inspired Oxygen

PaCO2= Partial Arterial Pressure of Carbon Dioxide

It follows that:

1.Room-air PAO2=117.73 mmHg

2.100% Oxygen by non-rebreather mask PAO2 (60% in the alveoli)=395.80 mmHg

3.Room-air 1.3 ATA PAO2=162.65 mmHg

4.100% Oxygen by mon-rebreather mask plus room-air 1.3 ATA PAO2(60% in the alveoli)= 524.14 mmHg

Significance:

A mild 1.3 ATA hyperbaric chamber associated with a 10 Liters/min oxygen concentrator deliver over 500 mmHg oxygen pressure to the body water. This concentration is safe and more effective than the chamber alone, or the concentrator at room air. At this oxygen concentration, we have calculated that it would take 36 hours of continuous exposure to develop oxygen toxicity!