Physiologic Effects of Hyperbaric Oxygen (HBO)
The Primary Effects
The primary effects of administering oxygen at greater than
one atmosphere of pressure are from the increase in partial pressure of oxygen
dissolved in the arterial blood (hyperoxemia) and from the physical effect
of an increased pressure.
Hyperoxemia
- The Alveolar Air Equation is used to calculate the PaO2 at any barometric
pressure. PAO2 = FIO2(Pb - PH20) - (1.25 x PaCO2). It is based on Dalton's
Law of Partial Pressure. Absolute pressure at sea level = 14.7 psi or
760 mmHg. Therefore, 14.7 psi is one atmosphere. The PAO2 at one atmosphere
is .21(760 - 47) - (40 x 1.25) or 100 torr. Arterial PO2 is slightly less
than alveolar PO2.
- Dalton's Law - the total pressure of a gaseous mixture is equal to the
sum of the partial pressures of its components.
- Henry's Law - gas enters into physical solution in direct proportion
to the partial pressure exerted by that gas.
100% oxygen at 2 or 3 atmospheres forces enough oxygen to dissolve in the
plasma to meet metabolic needs. Normally the body needs 5.0 vol% of oxygen.
100% oxygen at 3 atm = 6.5 vol%. 100% oxygen at 2 atm = 4.4 vol%.
- Oxygen Content = O2 combined with hemoglobin + dissolved oxygen.
SaO2(Hgb g% x 1.34) + (PaO2 x .003) = 20.40 vol% breathing air
at sea level. Dissolved oxygen normally contributes 0.30 vol% of the total.
The Physical Effects of Pressure
Compression of gas bubbles in the blood or air pockets in
the body occurs from the application of greater than atmospheric pressure
according to Boyle's Law. As the pressure increases, gas volume decreases.
Hyperbarism relieves vascular occlusion in decompression sickness and traumatic
or iatrogenic air emboli. Decompression sickness is not possible breathing
100% oxygen.
- Boyle's Law - At a constant temperature, the volume of a gas varies
inversely with the pressure to which the gas is subjected.
100% oxygen at 3 atmospheres absolute for 3 hours results in a grand mal seizure.
Therefore, exposure is limited to 90 minutes at 3 atmospheres absolute.
100% oxygen at 2 atmospheres absolute for six hours results
in cough, decreased vital capacity, substernal chest pain, and areas of patchy
atelectasis. At 2 atmospheres exposure is limited to 2 hours
Secondary Effects of HBO
Include: vasoconstriction, neovascularization, and inhibition
of the growth of anaerobic microorganisms.
Vasoconstriction
Peripheral vasoconstriction reduces blood flow to the tissues:
brain, kidney, and eye, and increases peripheral resistance. HBO also reduces
cardiac output due to bradycardia. These combined effects maintain blood pressure.
There is no impairment to the tissues since the elevated PaO2 more than compensates
for the reduction in blood flow.
Vasoconstriction has not been found to occur in hypoxic tissues
such as chronic skin ulcers, and therefore, these tissues may receive proportionately
more blood flow during HBO. In addition, the plasma is able to carry dissolved
oxygen to areas where red blood cells cannot go which may also benefit hypoxic
tissues. Vasoconstriction may also reduce edema.
Neovascularization
When vasoconstriction significantly reduces blood flow, the
tissue becomes hypoxic. All wounds have areas of hypoxia. Anaerobic metabolism
produces lactic acid. Although the mechanism for neovascularization is not
fully understood, hypoxia and high lactate levels stimulate macrophages to
secrete an angiogenesis factor. As long as the exposure to HBO is intermittent,
the regenerating cells receive the oxygen they need, and the macrophages continue
to stimulate the growth of new vessels. Oxygen also improves collagen synthesis
by fibroblasts. HBO has been found to speed the healing of skin grafts/flaps,
wounds, burned, and irradiated tissues.
Inhibition of the Growth of Anaerobic Microorganisms.
The growth of anaerobic, facultative, and aerobic microorganisms
is inhibited when the microbes are subjected to an hyperoxic environment.
Although oxygen is toxic to all organisms, the adverse effects are determined
by the duration of exposure, partial pressure of oxygen, and antioxidant enzyme
defenses of the organism. Hyperoxia blocks production of the exotoxin produced
by the Clostridium species which may be why HBO, in conjunction with antibiotics
and surgery, has been beneficial in treating Clostridial myonecrosis
