## Oxygen window

At atmospheric conditions, the total pressure of gas in the airways (Ptot ) is 101.3 kPa (1atm, 760mm Hg). Arterial pO2 and pCO2 do not exactly equal alveolar partial pressures, because perfusion and ventilation of lungs is never optimal and so, gas exchange between lungs and blood is never complete. Therefore arterial total pressure is slightly inferior to atmospheric pressure (e.g. arterial Ptot 756mm Hg). Venous Ptot is only 706mm Hg, because pO2 drops from 95mm Hg to 40mm Hg whereas pCO2 only rises from 40mm Hg to 45mm Hg5.

PW7S6flimH(| Ptol 706 mmKj in

 inspired \$as\$s alveolar gases arterial gases venous gas es : 1 ■ pN2 □ pH20 ■ pCQZ □ pQ2

Figure 1.3-5. Breathing gas pressures during air breathing at 101.3kPa (1atm, 760mm Hg), (Welslau2; modified from Brian5)

Figure 1.3-5. Breathing gas pressures during air breathing at 101.3kPa (1atm, 760mm Hg), (Welslau2; modified from Brian5)

The difference between barometric (PtotB) and venous total pressure (Ptotv) is called "oxygen window" (PtotB 760mm Hg - Ptotv 706mm Hg = APtot 54mm Hg). This difference is the reason why confined amounts of gas can not persist in the body under normal conditions. Ptot of any amount of gas will equal atmospheric pressure (101.3kPa, 1atm, 760mm Hg) whereas Ptot of surrounding tissue - like venous blood - is ~ 706mm Hg. The gas will diffuse into the tissue as dissolved gas until it is reabsorbed completely.

Figure 1.3-6. Breathing gas pressures during 100 % O2 breathing at 101.3kPa (1atm, 760mm Hg) (Welslau2; modified from Brian5)

When breathing 100% O2 arterial pO2 rises according to Fick's diffusion laws. In the tissue capillaries, the physically dissolved O2 is utilized before haemoglobin will be deoxygenized. Hereby intravascular pO2 drops far more than during air breathing; arterio-venous pO2 difference is enhanced and the oxygen window is: PtotB 760mm Hg - Ptotv 142mm Hg = APtot 618mm Hg. Resolution of free gas will be accelerated5.

When 100% O2 is breathed at increased pressure, ie 162kPa (1.6atm, 1216mm Hg), arterial pO2 and utilization of physically dissolved O2 in tissue capillaries will rise. In consequence arterio-venous pO2 difference is further enhanced and oxygen window is: PtotB 1216mm Hg - Ptotv 150mm Hg = APtot 1066mm Hg. Resolution of gas will be further accelerated.

Figure 1.3-7. Breathing gas pressures during 100 % O2 breathing at 162kPa (1.6atm, 1216mm Hg) (Welslau2; modified from Brian5)
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