E SPECIFICACIONES DE C ASOS DE USO

Much of what is known about the physics of bubbles comes from in vitro studies of visible bubbles. Except for a few transparent marine animals, knowledge of bubbles in living systems has relied on a few low-resolution, noninvasive imaging or detection methods.

Table 4–4. Lifetimes of spherical bubbles as a function of diameter, surface tension (γ, dyne/cm), and the oxygen window according to the model of Epstein and Plesset¹⁰¹

O₂W Bubble Lifetime (s, m, h)

O₂W = 0.0 atm O₂W = 0.08 atm O₂W = 0.83 atm

μ mm γ = 0 γ = 72 γ = 0 γ = 72 γ = 0 γ = 72

10 0.01 ∞ 5 s 6 s 1 s 1 s 0 s

50 0.05 ∞ 1 m 2 m 1 m 14 s 13 s

100 0.1 ∞ 23 m 10 m 6 m 1 m 1 m

250 0.25 ∞ 5 h 1 h 1 h 6 m 6 m

500 0.5 ∞ 40 h 4 h 4 h 23 m 23 m

1000 1 ∞ 322 h 16 h 15 h 92 m 91 m

O₂W, oxygen window.

Radiography

Bubbles were first detected in the human body by radiography in 1910 and are de-scribed extensively in the clinical litera-ture.¹⁰²Known today as vacuum phenomena, these bubbles often appear in synovial joints placed under tension and form as a result of viscous adhesion (see section Physics of Bubble Formation and Stability). Vacuum phenomena that are filled with water vapor collapse noisily as in the cracking knuckle joint of Figure 4–21^93,103; those filled with gas remain stable, such as the bubble in the spinal canal of a 52-year-old man with a history of chronic low back pain (Fig. 4–22).¹⁰⁴ Gas collections in the spinal canal can persist for at least 10 weeks¹⁰⁴and appear to be associated with vacuum phenomena in intervertebral discs or apophyseal joints.¹⁰⁵

Although vacuum phenomena are usually not associated with symptoms, this is not necessarily the case for bubbles detected after decompression. Figure 4–23 is a radio-graph of a large bubble behind the knee of an experimental subject at an altitude in excess of 30,000 feet (9144 m; Dr. A. A. Pilmanis, per-sonal communication). U.S. Army Air Force aircrew members were routinely exposed to such altitudes during World War II. Many experienced pain (as illustrated in Fig. 4–23 by the notation “muscle dissection, in-tense pain”) associated with the vacuum phenomenon behind the knee. Thomas and

Williams obtained radiographs of both knees of 35 subjects at altitude when pain occurred in one knee and found that all subjects had gas in the knee joints: 62% with pain had bubbles behind the knee, and 76% with pain had streaking along tendons and facial planes.^106,107These examples do not prove a causal association of vacuum phenomena and DCS, but supersaturation in the vicinity of a vacuum phenomenon would promote bubble growth by diffusion and the conse-quences of decompressing a bubble in the spinal canal can be postulated (see Fig. 4–22).

Doppler Ultrasonography

The most common method for detecting vas-cular bubbles, Doppler ultrasonography, operates on the principle that moving bubbles change the frequency of reflected sound waves. The frequency shift is con-verted electronically to an audible signal that a trained operator can interpret as gas emboli Figure 4–21. Radiograph showing bubble formation

during “knuckle-cracking”¹⁰³.

Figure 4–22. Radiograph showing bubble in the spinal canal of a 52-year-old man with a history of chronic low back pain¹⁰⁴.

(Fig. 4–24). The interpretation is subjective and commonly scored according to the five-point Spencer scale¹⁰⁸ (Table 4–5) or the 16-point Kisman-Masurel scale, which collap-ses into the Spencer scale.¹⁰⁹Doppler bubble signal scales are nonlinear and cannot be averaged unless linearized by one of seve-ral suggested transformations.^109,110 Typical Doppler monitoring sites are the precordium, the pulmonary artery, the subclavian or femoral veins, and the cerebral arteries.

Doppler bubble detection was introduced into a diving world dominated by Haldane decompression theory.¹¹¹Because the theory held that DCS did not occur until bubbles formed, Doppler seemed to hold the promise of bubble detection as an early warning of DCS. When Doppler-detected venous gas emboli (VGE) were found to be common in the absence of DCS and DCS occasionally occurred with no detectable VGE, some workers rejected Doppler as too imprecise to Figure 4–23. Radiograph of the leg of a U.S. Army Air Force volunteer at an altitude in excess of 30,000 ft (9,144 m) (Courtesy of Dr. A. A. Pilmanis). A large bubble is visible behind the knee with the notation, “Muscle Dissection, Intense Pain.”

Figure 4–24. Doppler bubble monitor showing transmitting and receiving probe.

be of value.¹¹² Table 4–6 indicates that Doppler scores and DCS were significantly associated in diving¹¹³and high-altitude popu-lations,¹¹⁴ however, and the Canadians used Doppler extensively in developing the DCIEM decompression tables.^49,115,116

Doppler has demonstrated VGE in humans after decompression to an altitude of only 12,000 ft (3658 m)¹¹⁷ and after ascent from a 12-hour dive to only 12 fsw (3.6 msw).¹¹⁸ These are pressure changes of 0.4 atm or supersaturation ratios of 1.6:1 and 1.4:1, respectively. Doppler-detected VGE are also common during routine recreational diving in the absence of DCS (see Chapter 7, Figs.

7–17, 7–18, 7–24).¹¹⁹VGE are certainly abnor-mal, but further study is needed to deter-mine whether they can be pathologic (see the discussion “Possible Roles of Venous Gas Emboli in Neurologic Decompression Sickness” in Chapter 7).

Echocardiography

The echocardiograph is another ultrasonic instrument used in decompression research, but one used less frequently than Doppler because of the high cost (although less expensive portable systems are now avail-able). Two-dimensional echocardiography is based on the same principles as computed tomography but uses ultrasound instead of x-rays. Bubble images are relatively easy to locate within the four chambers of the heart (see Chapter 25, Fig. 25–9).

The principal use of echocardiography in decompression research has been to inves-tigate the question of whether a patent foramen ovale (PFO) predisposes to neuro-logic DCS. The hypothesis is that the PFO provides an anatomic pathway through the right side of the heart by which VGE may bypass the filtering action of the lungs and

reach the brain or spinal cord through the arterial circulation (see Chapter 8). To test for the presence of a PFO, a mixture of saline and microbubbles is injected into a peripheral vein. The appearance of bubbles in the left side of the heart is evidence of a functional PFO. Several workers have found that PFO was more prevalent in divers who had suffered neurologic DCS than in controls.

The U.S. Air Force routinely uses echo-cardiography during experimental altitude exposures to search for arterial bubbles in the left side of the heart.¹⁰⁸ If any are detected, the exposure is immediately terminated because of the potential risk of cerebral arterial gas embolism. To date, left-ventricular bubbles have been observed in 8 of 2587 subject exposures. All 8 had grade 4 VGE; of these, 7 experienced limb-pain DCS¹²⁰ (Dr. J. Webb, personal communication). Of 4 who were evaluated for PFO by saline bubble contrast injection, 3 had a PFO and 1 had a functionally similar abnormality known as a sinus venosus defect. The Air Force experi-ence suggested that, for altitude exposure at least, arterial bubbles were rare, and those that did occur did not predispose to cere-bral DCS during a short interval before recompression. Neurologic DCS at altitude is unusual compared with diving, however, possibly because oxygen breathing before decompression reduces or eliminates nitro-gen supersaturation of the brain and spinal cord.

CONCLUSIONS

Stage decompression, introduced in 1908 by John Scott Haldane, was the most significant achievement of the 20^th century in reducing severe or fatal decompression sickness and was the first model of decompression to be Table 4–5. Doppler bubble signal

scoring system according to Spencer¹⁰⁸

Bubble Grade Definition 0 No bubble signals 1 Occasional signal

2 Signals in less than half the cardiac cycles

3 Signals in all cardiac cycles 4 Signals override cardiac

signals

Table 4–6. Relationship of Doppler bubble scores and decompression sickness

Bubble Air Diving* 30,000 ft Altitude^†

Grade (% DCS) (% DCS)

0 0 10

1 1 11

2 1 50

3 6 60

4 10 78

* 35 DCS in 1761 dives¹¹³

†64 DCS in 275 flights¹¹⁴

based on a physiologic explanation of DCS.

Later workers refined Haldane’s method empirically and further improved decom-pression safety and efficiency. The effect of bubble formation on retarding inert gas elim-ination was not appreciated until the latter half of the century, however, and has only recently been incorporated into decompres-sion models. To a large extent, this was because of limited techniques for detecting bubbles and measuring inert gas exchange.

Although these techniques have improved, the situation remains less than satisfactory.

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