Physics of Hyperbaric Medicine
Adapted from - https://emedicine.medscape.com/article/1464149-overview#a2
- The application of Boyle’s law (p1 v1 = p2 v2) is seen in many aspects of HBOT. This can be useful with embolic phenomena such as decompression sickness (DCS) or arterial gas emboli (AGE). As the pressure is increased, the volume of the concerning bubble decreases. This also becomes important with chamber decompression; if a patient holds her breath, the volume of the gas trapped in the lungs overexpands and causes a pneumothorax.
- Charles’ law ([p1 v1]/T1 = [p2 v2]/T2) explains the temperature increase when the vessel is pressurized and the decrease in temperature with depressurization. This is important to remember when treating children or patients who are very sick or are intubated.
- Henry’s law states that the amount of gas dissolved in a liquid is equal to the partial pressure of the gas exerted on the surface of the liquid. By increasing the atmospheric pressure in the chamber, more oxygen can be dissolved into the plasma than would be seen at surface pressure.
The clinician must be able to calculate how much oxygen a patient is receiving. In order to standardize this amount, atmospheres absolute (ATA) are used. This can be calculated from the percentage of oxygen in the gas mixture (usually 100% in HBOT; 21% if using air) and multiplied by the pressure. The pressure is expressed in feet of seawater (fsw), which is the pressure experienced if one were descending to that depth while in seawater. Depth and pressure can be measured in many ways; some common conversions are 1 atmosphere (atm) = 33 feet of seawater (fsw) = 10 meters of sea water (msw) = 14.7 pounds per square inch (psi) = 1.01 bar.
What is the physiology of hyperbaric oxygen therapy (HBOT)?
Hyperbaric Physiology Table 1 below summarizes the physiologic mechanisms of HBOT. Each of these is discussed in the context of the indications for HBOT.What is the physiology of hyperbaric oxygen therapy (HBOT)?
Decrease gas bubble size
Air or gas embolism
Fibroblast proliferation/collagen synthesis
Leukocyte oxidative killing ‡
Necrotizing soft tissue infections
Reduces intravascular leukocyte adherence
Crush injury/compartment syndrome
Reduces lipid peroxidation
Van Unnik A
Necrotizing soft tissue infections
*Most oxygen carried in the blood is
bound to hemoglobin, which is 97%
saturated at standard pressure. Some
oxygen, however, is carried in solution,
and this portion is increased under
hyperbaric conditions due to Henry's
law. Tissues at rest extract 5-6 mL of
oxygen per deciliter of blood, assuming
normal perfusion. Administering 100%
oxygen at normobaric pressure increases
the amount of oxygen dissolved in the
blood to 1.5 mL/dL; at 3 atmospheres,
the dissolved-oxygen content is
approximately 6 mL/dL, which is more
than enough to meet resting cellular
requirements without any contribution
from hemoglobin. Because the oxygen is
in solution, it can reach areas where
red blood cells may not be able to pass
and can also provide tissue oxygenation
in the setting of impaired hemoglobin
concentration or function.
Additionally, evidence is growing that HBOT alters the levels of proinflammatory mediators and may blunt the inflammatory cascade. More studies are needed to further elucidate this complex interaction.
As HBOT is known to decrease heart rate while maintaining stroke volume, it has the potential to decrease cardiac output. At the same time, through systemic vasoconstriction, HBOT increases afterload. This combined effect can exacerbate congestive heart failure in patients with severe disease; however, clinically significant worsening of congestive heart failure is rare.
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