Increased Partial Pressure of Oxygen

Breathing O₂ at increased ambient pressure will lead to elevation of alveolar O₂ tension (PAO₂ ), which can be calculated according to the alveolar gas equation for O₂ :





where PIO₂ and FIO₂ are the inspired partial pressure and fractional O₂ concentration, respectively; PACO₂ is alveolar PCO₂ , assumed to equal arterial PCO₂ (PaCO₂ ); and R is the respiratory exchange ratio (usually ≅0.8 at rest). Calculated values are shown in Table 70-4 . Arterial PO₂ (PaO₂ ) has

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Figure 70-2 Ambient pressure as a function of altitude and water depth. Whereas ambient pressure increases linearly with depth, pressure and altitude are not linearly related. As air is inspired and humidified, there is a small and usually insignificant drop from atmospheric PO₂ to inspired PO₂ . At altitude, however, this decrease accounts for a greater proportion of the total ambient pressure. The O₂ partial pressure line in the water is shown for a constant FIO₂ of 21%. At increasing depth, the inspired PO₂ eventually exceeds the pulmonary toxic limit (≅14 m in depth) and the central nervous system toxic limit (≅70 m in depth). The threshold for high-pressure nervous syndrome and pressure reversal of anesthesia (observed in non-narcotic atmospheres) is around 150 to 200 m in depth. The shaded red bars represent the depth or altitude ranges over which risk progresses from low (light shading) to high (dark shading). AMS, acute mountain sickness; HACE, high-altitude cerebral edema; HAPE, high-altitude pulmonary edema.

Increased PaO₂ has at least four pharmacologic effects:

Increased blood O₂ content

Vasoconstriction

Antibacterial action, particularly against anaerobic bacteria

Inhibition of endothelial neutrophil adhesion in injured tissue

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Figure 70-3 Blood O₂ content versus PO₂ . Virtually complete saturation of hemoglobin with O₂ occurs at a PO₂ of 100 mm Hg. Further increases in PO₂ do not alter the quantity of O₂ bound to hemoglobin. However, there is a linear increase in total blood O₂ content with PO₂ because of increasing quantities of O₂ dissolved in plasma.

The increased arterial O₂ content underlies the rationale for administering HBO for the treatment of ischemic conditions, for example, ischemic, nonhealing wounds. The elevation in PaO₂ leads to an increase in tissue PO₂ , which can be estimated by using transcutaneous PO₂ electrodes, even in ischemic tissue.^{[65] [66]} The second effect is an explanation for the effectiveness of HBO in the treatment of traumatic edema (e.g., crush injury). The mechanism of HBO-induced vasoconstriction appears to be inactivation of nitric oxide (NO) as a result of increased production of superoxide^{[67] [68]} and possibly decreased release of NO from circulating S-nitrosohemoglobin. ^{[67] [69] [70]}

These two effects, increased O₂ content and vasoconstriction, lead to hemodynamic changes,^{[70] [71]} which are shown in Table 70-5 . The elevation in PaO₂ that occurs while breathing 100% O₂ at 3 ATA results in a drop in cardiac output and heart rate and an increase in total peripheral resistance. There is also a slight increase in mean arterial pressure.

The other major pharmacologic effect of increased PaO₂ is the inhibition of toxin production and growth of certain anaerobic bacteria. Additionally, increased PaO₂ has been shown to return phagocytic function and the ability of aminoglycosides to kill aerobic bacteria in ischemic tissue to normal.^{[31] [72]}

HBO also has some poorly characterized microcirculatory and cellular effects. Zamboni and colleagues^{[36] [37] [73]} described a reduction in blood flow on reperfusion of ischemic myocutaneous tissue flaps. This decrease in flow appears to be due to leukocyte adherence to the capillary endothelium mediated by leukocyte adhesion glycoprotein CD18,^[74] an effect that is prevented and ameliorated by HBO treatment.^{[36] [37] [73] [75]} In an animal model, timely administration of HBO also appears to decrease the lipid peroxidation in the brain that occurs after treatment of carbon monoxide (CO) poisoning.^[76]