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.
- 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.
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%.
- Henry's Law - gas enters into physical solution in direct proportion to the partial pressure exerted by that gas.
- 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.
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.
- 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 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.
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.
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
Last updated 2/6/97