Subcutaneous Wound Tissue Oximetry

In the 1970s Thomas K. Hunt and others began to develop techniques to measure average tissue oxygen from a single site in humans. Hunt developed the silicone tonometer for this purpose (Fig. 7). Silicone is freely permeable to oxygen. When a length of silicone tubing (1 mm OD) is placed subcutaneously, the PO2 within the tonometer equilibrates with that of a cylinder of surrounding tissue, thus providing a mean wound tissue oxygen tension equivalent to the mean of the microelectrode-generated histograms. Initially, Hunt infused saline slowly through the tonometer and measured the Po2 of the effluent (microdialysis). A much more convenient method, however, is to place an

Temperature Probe __Oxygen Probe

Temperature Probe __Oxygen Probe

Licox Tissue Oxygenation Catheter
Fig. 7. Schematic drawing of silicone sc oxygen tonometer. The oxygen probe is inserted through the catheter hub. The temperature probe is inserted into the protruding end of the tonometer. Adapted from: Musculoskeletal Infection (14).

oxygen probe within the tonometer to get a continuous measure of tissue oxygen. The tonometer is filled with saline to speed equilibration with surrounding tissue. Temperature is measured at the same site concurrently, because Po2 is temperature dependent.

Two main types of oxygen probes are currently available: polarographic (Clark) electrodes and optical electrodes (optodes) (15). The polarographic electrodes are based on the observation that when a negatively charged metal and a reference electrode are placed in an electrolyte solution, current flows in proportion to oxygen concentration, with no current flow in the absence of oxygen. Current technology corrects many of the problems that plagued early polarographic probes, including drift, excessive oxygen consumption at the cathode, and poisoning by substances such as halothane. The electrodes are highly accurate even to oxygen tensions in excess of 800 mmHg, although their accuracy diminishes somewhat as current decreases toward zero (no oxygen). optical electrodes use a fluorescent dye (usually ruthenium) in which the fluorescence is quenched by oxygen. Thus, the fluorescent output is inversely proportional to oxygen tension. Optodes are highly accurate at low oxygen tensions, but lose accuracy at higher oxygen concentrations (around 100-300 mmHg, depending on the design).

our laboratory currently uses a polarographic electrode, and the methods described pertain specifically to the Licox probe that we use. However, these methods can easily be adapted to any probe that can be placed securely in the tonometer. Most currently manufactured tissue oxygen electrodes are intended for direct implantation and thus are embedded in a sheath of silicone to increase the averaging area around the electrode. These probes work in a tonometer as well. We continue to use the tonometer because: (1) if there is a question of probe drift or malfunction, the probe can be removed to check the calibration and then can be replaced without causing pain to the subject, whereas with a direct implant, a new stick with a sterile probe is required; (2) the probe does not directly contact the subject, so the probe need not be sterile, only clean; and (3) multiple measurements can be made over the course of several days without having to leave the probe in place the entire time (the tonometer is easy to secure).

Although the methods described here are for measurement in sc tissue (16-20), the same probes can be used with essentially the same technique in a number of tissues, including gut, heart, liver, brain, muscle, and bone.

3.3.1. Preparation of Tonometer

1. Cut silicone tubing into 15-cm lengths.

2. Gently wedge one end of the tubing onto the iv catheter. Wedge the other end onto the 19-g spinal needle (if using a 12 to 14-g spinal needle, omit this step) (see Notes 25-27).

3. Gas sterilize (do not autoclave).

3.3.2. Placement of Tonometer 3.3.2.1. Method 1

The tonometer should be in place for at least 30 min before actual measurements begin (this can include equilibration time). For about 30 min after the tonometer is placed, readings may reflect arterial PO2 because of minor bleeding around the tube. The tonometer may be implanted under general or local anesthesia. Placement is identical, except that local anesthetic is not required for placement during general anesthesia.

1. Inspect the lateral upper arm for a site without obvious veins, bruises, or broken skin. Mark an entry and exit site measured 7 cm apart (this is to ensure that the probe tip is at least 2 cm from the exit site and there is no possibility of diffusion of room air to the probe tip).

2. Raise a skin wheal with 1% lidocaine (without epinephrine) at the planned entry and exit sites; the wheal should be at least 1.5 cm in diameter. It is not necessary to inject local anesthetic into the sc tissue between the skin wheals.

3. Prep the arm with Betadine paint solution in a circular fashion.

4. After 1 to 2 min, use alcohol to wash off the betadine. Drape the site with sterile towels.

5. Make a small stab wound with a no. 11 blade at the planned entry and exit sites. The stab wound should be large enough to allow easy passage of the catheter, but as small and superficial as possible, in order to minimize bleeding. Make the stab wound in line, with skin lines to minimize or eliminate scarring.

6. Immediately hold point pressure over the sites until the bleeding stops, usually 1 to 2 min.

7. Using the spinal needle, pull the tonometer through just under the skin, so that the catheter is in up to the hub. You want the tonometer to be superficial, but if you see skin puckers, you are too shallow and should pull back and go deeper. The subject should not feel pain; if the subject does, you are probably too shallow (in the dermis).

8. Cut the silicone tubing close to the spinal needle, leaving a fairly long distal segment of silicone tubing. Wipe off any blood with either alcohol or saline. Dry the catheter hub.

9. Cover the tonometer with two small Tegaderm dressings. One should cover the exit site, with about 2 cm covering the silicon tubing. Cut the silicon tubing about 1 cm beyond the Tegaderm edge. The second Tegaderm should cover the entrance site, with about 0.5 cm of the iv catheter hub uncovered. Usually the two Tegaderm dressings will overlap somewhat. If a significant amount of blood accumulates, the dressing should be removed, the site dried, and a fresh dressing applied in the same fashion. Write on the Tegaderm: "Do not remove."

1. The tonometer in this case consists just of silicone tubing wedged onto the iv catheter. Prepare the entrance and exit sites as outlined in Subheading 3.3.2.1.

2. Insert a 12 to 14-g spinal needle (see Note 28).

3. Thread the tonometer through the sharp end of the needle (hubless). Remove the spinal needle, leaving the tonometer in place.

3.3.3. Measurement of Psqo2

Each probe has been calibrated at the factory in both nitrogen and room air. Calibration values are listed on the probe package, including the IcO2 (current during room air calibration), IcN2 (current during nitrogen calibration), and TS (temperature coefficient for the probe). These should be input into the proper places on the monitor. Barometric pressure at your site should be entered (760 mmHg at sea level), along with 209 for oxygen concentration in room air (see Note 29). The calibration temperature and tissue temperature settings should be 00, since you will be measuring actual temperature during both calibration and oxygen measurement. Calibration is performed with both the temperature and oxygen probes in room air by pushing the blue calibration button (see Notes 30 and 31). For quality assurance, record room air Po2 and temperature immediately after calibration, and again after removing the probe from the tonometer (see Notes 32 and 33).

1. Draw up 10 mL of saline into the 10-mL syringe using the 16-g needle or the filter straw.

2. Carefully place the calibrated probe through the Y adapter (see Note 10) and flush with saline to remove all bubbles. Then place the assembly into the hub of the tonometer (see Note 34).

3. Tape the tubing to the patient's arm so that the probe lies flat.

4. Tape the monitor cable or probe/cable connection to the side rail.

5. Flush the assembly with at least 8 mL of normal saline.

Pictures Normal Wound Healing
Fig. 8. Tonometer in a patient's arm. The oxygen probe has been placed through the Y connector, and the temperature probe has been inserted through the distal end of the tonometer until the measurement tip is under the skin.

6. Place the thermocouple probe through the distal end of the silicon tubing. Advance it as far as it will go.

7. Flush the system gently with about 0.5 mL of saline. Figure 8 shows a patient with the entire assembly in place in the arm.

8. Cover the entire arm with a Chux or towel to decrease heat loss.

9. Record the baseline value for Po2 (sc oxygen) and Tsq (sc temperature) only after the value has changed <1 mmHg in 5 min, and it has been at least 25 min since probe insertion (see Notes 36-37). Data may be recorded continuously or at specified intervals.

10. For the oxygen challenge, administer 7-10 L/min of oxygen via simple face mask (40-60% oxygen). Record the final value after oxygen has been on for at least 25 min and Psqo2 has changed <1 mmHg in 5 min (see Note 38).

11. After the measurements and any interventions are complete, gently remove the probes from the tonometer and replace in their covers.

12. Record the room air Po2 after about 5-10 min, when the value is stable (see Note 39).

13. Place a 2 x 2 gauze under the catheter hub and tape a 4 x 4 gauze over the catheter hub and the distal tip of the tonometer if reuse is planned.

3.3.4. Removal of Tonometer

1. Remove the Tegaderm.

2. Pull the catheter hub out slightly until you can grasp both the catheter hub and the attached silicone tubing. Normally, you will be able to see the silicone at the catheter hub.

3. While holding the silicone as it attaches to the catheter hub, cut the other, free end of the silicone tubing at the exit site, and pull the tonometer out by the hub.

4. If you do not see any silicone on the catheter, grasp the tonometer by both ends and pull; if the tubing and catheter are disconnected, the entire tonometer will come out in two pieces. If this protocol is followed, you will never leave silicone under the skin.

5. If necessary, wash the skin with alcohol before dressing with a fresh Tegaderm.

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