HYPERBARIC TREATMENT SCHEDULES

Ideally, a patient who has a therapeutic indication for HBO would be exposed to HBO for an unlimited time until the condition resolves. Unfortunately, several factors limit the dose and duration of HBO:

O₂ toxicity

Decompression obligation for the nursing staff (or other tenders) accompanying the patient

Difficulty of monitoring

Patient isolation and boredom in a confined environment

Treatment schedules have been developed that are compromises between O₂ partial pressure and exposure time on the one hand and O₂ toxicity and other practical limiting factors on the other. The original schedules (or "tables") were developed by the various navies of the world to treat decompression sickness and gas embolism in divers. Two U.S. Navy tables are shown in Figure 70-8 .

U.S. Navy Table 6 prescribes an initial exposure to 2.8 ATA (60 ft of sea water [fsw], equivalent to 18 m of sea water [msw]), followed by slow decompression to 1.9 ATA (30 fsw). Periods of O₂ breathing are interspersed with 5- or 15-minute periods of air breathing to decrease O₂ toxicity (see later). This schedule remains the mainstay for treatment of decompression sickness in multiplace chambers throughout the world. Patients who remain symptomatic during treatment may have their exposure extended in time at both 2.8 and 1.9 ATA. U.S. Navy Table 6A is occasionally used for the treatment of air embolism, in which case larger amounts of intravascular gas may be present. In Table 6A, the Table 6 schedule is preceded by a period of 30 minutes at 6 ATA while breathing air. More recently, 40% or 50% O₂ has been advocated during this period to hasten the elimination of nitrogen bubbles and increase O₂ delivery to ischemic tissue. In practice, patients are treated once according to either of these schedules and then reassessed. Incomplete relief of signs or symptoms can be treated with repeated application of the protocols of these tables on a daily basis.

2676

Figure 70-8 U.S. Navy treatment tables. A, U.S. Navy Table 6A. This schedule is occasionally used for the treatment of arterial gas embolism. During the 30-minute period at 6 ATA, air or 40% to 50% O₂ can be administered. B, U.S. Navy Table 6. This table was originally designed for the treatment of decompression sickness, but it is now the most commonly used table for gas embolism as well. The red areas represent 100% O₂ breathing; the white areas represent air-breathing periods (see Figure 70-10 ). Further details can be found in the U.S. Navy Diving Manual.^[144]

An alternative method of treating severe gas bubble disease is "saturation" treatment, the principle of which is that the patient is subjected to elevated pressure (e.g., 2.8 ATA) and allowed to remain there for an unspecified period (often 1 to 2 days) until manifestations have stabilized. Periodic O₂ breathing is given according to a recommended schedule as tolerated. Because saturation treatment results in a much larger degree of nitrogen uptake in both the patient and the tender, decompression must occur much more slowly, usually over a period of 24 to 36 hours.^{[147] [148]} Although this therapy avoids the theoretical disadvantage of intermittent treatment—failure of resolution of gas bubbles—it is also considerably more labor intensive. Because the hyperbaric chambers used for saturation treatment require additional hardware (e.g., CO₂ scrubbing capability) and personnel, their application outside the military and commercial diving has been limited.

A schedule used for the treatment of patients with clostridial myonecrosis or other life-threatening anaerobic infections is shown in Figure 70-9 . This schedule consists of 85 minutes at 3.06 ATA followed by a 33-minute decompression stop for the tenders at 1.3 ATA. This stop may be omitted for the patient if a second team is available to lock in and decompress with the patient. The original tenders can then remain at 1.3 ATA for the required decompression time. This treatment schedule was designed to maximize Pao₂ (and hence tissue bactericidal activity as a result of O₂ ) without an undue risk of hyperoxic seizures.

Figure 70-9 Clinical hyperbaric O₂ treatment schedules. A, The patient breathes 100% O₂ for 2 hours at an ambient pressure of 2 ATA. Generally, this schedule is used for repetitive treatment of chronic conditions (e.g., osteoradionecrosis). B, Therapeutic schedule for the treatment of clostridial myonecrosis. The patient spends 85 minutes at an ambient pressure of 3.06 ATA and breathes 100% O₂ , with the exception of two 5-minute air breaks to minimize pulmonary and central nervous system O₂ toxicity. A decompression stop is made at 1.3 ATA, according to the U.S. Navy standard air decompression table. This decompression stop is designed to prevent decompression sickness in the tenders, who breathe air throughout the period at 3.06 ATA.

Administration of HBO to patients with chronic diseases (e.g., radionecrosis) is usually performed by using shorter tables at lower ambient pressure. The most commonly used schedule is 1.5 to 2 hours at 2 to 2.5 ATA (see Fig. 70-9 ). Because of the lower ambient pressure, the risk of O₂ toxicity is minimal, and an exposure of this duration is not particularly onerous for the patient on a repetitive basis one to two times daily.