Clinical EPR Oximetry program for diabetic foot
The oxygen
tension (pO2) in the tissues at risk in the diabetic foot is the parameter of greatest clinical importance. Currently there is no reliable method available to measure it, however, making it very difficult to evaluate the state of the disease and the response to therapeutic measures.
The ability of EPR oximetry to make repeated non-invasive measurements of oxygen at several sites (after the initial placement of the paramagnetic material, India ink, at the sites of interest) would provide an important new way to improve the management of this disease, making it feasible to characterize the status of the disease, and to determine the effects of therapeutic intervention and progression of disease, based on the pO2 at critical sites. We have begun measurements in the human foot at the site of greatest risk in patients with diabetic peripheral vascular diseases (on the plantar surface under the first metatarsal head,
Figure
1b) and also at the site where transcutaneous oxygen measurements often are made (first interosseous dorsal space) (Figure
1a). A typical EPR spectrum obtained from the injection of 7-10µl of ink in the plantar surface is shown in
Figure
1c. The signal to noise ratio observed was about 20 in single EPR scans with a scan time of 15 seconds.
Feasibility of repeated measurements over time
The capability of making repeated measurements from the same site probably will be the clinically most useful aspect of this technique. Tissue pO2 was measured several times before and after local compression to cause transient hypoxia in the site; a repeated measurement five months later gave the same baseline values, Figure 2. Measurements now have been made for about a year and we continue to obtain good data from the original injection, Figure 3.
Measuring acute perturbations of pO2
Changes in the local pO2 after acute perturbations are likely to be clinically valuable indicators of the functional state of the tissue oxygenation. Figure 4 illustrates the effects of two perturbations that appear to be effective and practical for use in the clinical situation:
(i) Local compression, which provides data on the rate of decrease of pO2 and the rate of recovery.
(ii) Responses to breathing increased amounts of oxygen.
Minimization of the foot motion artifact
Clinical measurements require the development of techniques to position the subject comfortably and accurately, Figure 5 illustrates such a development, in this instance the apparatus for immobilizing the foot during measurements.
Temperature Control of the Foot
The variations in the pO2 values of the baseline measurements shown above led us to recognize the well established effects of local temperature on the delivery of blood to the periphery and perhaps also to its utilization, resulting in quite significant effects on the local pO2. Figure 6 illustrates the effects in an experiment in which repeated measurements with EPR oximetry were made while local perfusion with hot air was varied and also while the TcPO2 apparatus was turned on or off. We therefore have developed an apparatus to provide local heating of the foot to enable us to obtain data under comparable conditions over time, Figure 7.
Figure 1: The sites of injection of concentrated Higgins "black magic" India ink: (a, top left) first interosseous dorsal space (between the first and second toes) (b, top right) under the first metatarsal head. (c, bottom) The EPR signals obtained from an injection of 7-10µl of concentrated ink before (upper tracing) and after (lower tracing) temporary muscle compression.
Figure 2: Repeated measurements of pO2 in the foot from India ink under the plantar first metatarsal head of a healthy volunteer: baseline tissue pO2, pO2 after muscle compression, and pO2 after compression was released, i.e. recovery. The EPR measurements for baseline and recovery were done for a period of ~15 minutes and for ~ 5 minutes during muscle compression (Mean ± SD).
Figure 3: Repeated measurements of pO2 in the foot from India ink in the first interosseous dorsal space of a healthy volunteer injected on March 19, 2003: pO2 values were measured under baseline conditions, after several minutes of local compression, and after the compression was released, Mean ± S.E.
Figure 4: Dynamic changes in the measured pO2 during and after compression and in response to breathing increased oxygen.
Figure 5: Foot holder constructed for EPR measurements on foot.
Figure 6: Effect of local environmental temperature on measured pO2 in the foot
Figure 7: Apparatus for temperature control of the foot.
Research planned for the next five years:
The overall aim of our research plan is to establish a pO2 measurement that is directly applicable for use in the relevant clinical population; then to test the validity of the conceptual hypothesis by measurements in appropriate populations and relate these measurements to clinical outcomes. We will make serial measurements in two groups of diabetic patients (with and without clinically evident peripheral vascular disease). In patients that undergo therapy to improve tissue oxygenation, we will make measurements before and after the therapeutic intervention. We will demonstrate the ability to follow changes and will correlate clinical outcomes with measured pO2.
If the promise of these techniques is fulfilled, then the clinicians will have a new powerful tool for management of peripheral vascular disease, based on the fundamental pathophysiology.