Delivery, and Physiologic Effects
6.3 Physiologic Effects of Oxygen Breathing
6.3.3 Hyperbaric Oxygen Therapy (HBO)
During hyperbaric oxygen therapy, a patient is placed in a treatment chamber where he or she breathes 100 % oxygen as the pressure of the chamber is increased to greater than 1 atm.
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When the inspired oxygen is increased to 3 atm in a hyperbaric chamber, the amount of dissolved O2 in arterial blood increases from 1.5 mL/dL to 6 mL/dL, and O2 tension in tissues is increased to nearly 400 mmHg. In this altered environment, O2 has various biochemical, cellular, and physio- logic benefits as described below. HBO therapy is indicated for use in carbon monoxide (CO) poi- soning, myonecrosis, necrotizing fasciitis (gas gangrene of soft tissues), arterial air emboli, and decompression sickness.
6.3.3.1 CO Poisoning
Hemoglobin has 240 times the affinity for CO than oxygen. Once bound, carboxyhemoglo- bin (COHb) shifts the oxyhemoglobin curve to the left leading to decreased oxygen-carrying capacity. CO also binds to cytochrome oxi- dase, interfering with electron transport and adenosine triphosphate (ATP) production with resultant cellular dysfunction. In CO poison- ing, the patient’s arterial oxygen content (PaO2) is often normal or elevated, and routine pulse oximetry measurements can be falsely normal since most technologies do not differentiate between oxyhemoglobin and carboxyhemoglo- bin. In general, co-oximetry is required to quan- tify COHb concentration. The most important therapeutic intervention is administration of 100 % oxygen after removing the patient from the carbon monoxide source. Only very high PaO2 levels can compete with CO for Hb bind- ing, thus driving CO out of the blood. At 3 atm of pressure in an HBO chamber, the half- life of COHb is reduced from 5.3 h to just 23 min.
However, the COHb level is not reflective of the severity of illness, and patients should be treated based on presence of symptoms, espe- cially neurologic abnormalities. Referral to a facility with hyperbaric oxygen (HBO) capa- bilities is recommended for concerning clinical signs such as seizures, altered mental status, syncope, chest pain, severe acidosis, preg- nancy. The risk of fatality is high with levels over 70 %. However, it must be noted that no consistent dose response relationship has been demonstrated between carboxyhemoglobin level and clinical effect.
6.3.3.2 Wound Healing
Since HBO increases the capillary-tissue oxy- gen gradient, this likely explains potential ben- efits that have been seen in wound healing. HBO helps to promote angiogenesis and can help restore neutrophil-mediated bacterial killing.
HBO is also helpful in treating gas gangrene as the organisms cannot survive in such a high PO2
environment.
6.3.3.3 Air Emboli
Based on Boyle’s law, gas volume is inversely proportional to pressure. Therefore, the “bubble volume” in cases of air emboli and decompres- sion sickness is reduced within the highly pres- surized HBO chamber, helping to relieve small vessel obstruction and restore blood flow to com- promised tissue beds.
6.3.3.4 Toxicity/Complications
Complications of HBO can include seizures, gas embolization, pneumomediastinum, pneu- mothorax, perforated tympanic membranes, and oxygen toxicity (reversible myopia, decreased pulmonary compliance). Extreme care must be taken to avoid fires and explosions at such high partial pressures of oxygen.
Essentials to Remember
s The most important factor in determin- ing if an oxygen gas delivery system is sufficient to meet a patient’s inspiratory demand is the gas flow rate.
s The three key components of respiratory control are sensors, central control, and effectors (muscles of respiration).
s Pulmonary vasodilators decrease pul- monary artery pressure and can halt or reverse the vascular changes related to elevated pulmonary vascular resistance.
s Decreased oxygen delivery to the organs and tissues of the body results from inadequate cardiac output, low arterial oxygen saturation level, inadequate hemoglobin concentration, or low PaO2.
Pediatric and Neonatal Mechanical Ventilation
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Nasal continuous positive airway pressure (NCPAP) has become extremely popular over the past few years. Though its use is based in strong physiologic principles, more data as to the bene- fi cial short- and long-term effects as well as pos- sible complications are still needed. Personal opinions and biases have played a role in this arena as well. In this chapter, we will describe the major ways to provide NCPAP that are currently employed as of this writing. We will then discuss the studies of NCPAP that have been done to date and classify them according to level of evidence.
Finally, we will draw some conclusions based on the evidence presented and suggest areas for future investigation.