Transcutaneous oxygen and carbon dioxide pressure monitoring to determine severity of limb ischemia and to predict surgical outcome

Transcription

1 Transcutaneous oxygen and carbon dioxide pressure monitoring to determine severity of limb ischemia and to predict surgical outcome Stephen G. Lalka, M.D., James M. Malone, M.D., Gary G. Anderson, B.S., Roberta M. Hagaman, M.S., Kenneth E. McIntyre, M.D., and Victor M. Bernhard, M.D., Tucson, Ariz. Transcutaneous oxygen and carbon dioxide pressure (Po2 and Pco2) foot monitoring was compared with ankle Doppler-derived systolic pressure regarding their respective abilities to discriminate the severity of limb ischemia before vascular reconstruction and to predict surgical outcome early in the postoperative period. Transcutaneous Po~ (tcpo~), footchest tcpo2 index, transcutaneous Pco2 (tcpco2), foot tcpo2/tcpco2 index (tcpo2/tcpco2), ankle Doppler systolic pressure (AP), and ankle-brachial pressure index (ABI) were determined in 89 revascularized limbs, The measurement oftcpo~ and foot-chest tcpo2 was found to be more sensitive to degrees of severity of limb ischemia and more closely associated with the outcome of revascularization than AP and ABI. TcPco2 and tcpo2/tcpco2 were not useful in assessment of the vascular patient undergoing reconstructive surgery. Before operation, tcpo2 less than or equal to 22 torr and foot-chest tcpo2 less than or equal to 0.46 indicate severe limb ischemia requiring urgent revascularization. After operation, tcpo2 less than or equal to 22 torr and foot-chest tcpo2 index less than or equal to 0.53 indicate that revascularization is likely to fail. We conclude that tcpo~ monitoring, as a metabolic test of actual tissue perfusion, is a more reliable indicator of preoperative limb ischemia and postoperative outcome of revasoalarization than hemodynamic, Doppler-derived pressure tests. (J VASC SURG 1988;7: ) The decision for expectant or operative treatment of the patient with peripheral vascular disease requires accurate objective assessment of the degree of arterial insufficiency. In addition, the ultimate outcome in terms of success or failure must be accurately predicted early in the postrevascularization period to avoid further limb ischemia, limb loss, or both. Although clinical judgment is important, it lacks sufficient preoperative discriminative and postoperative predictive accuracy to be the benchmark? In most institutions, noninvasive vascular testing provides objective data on which consistent clinical decisions can be made; however, the comparative accuracy of many of these noninvasive tests is poorly documented. From the University of Arizona/Tucson Veterans Administration Medical Center. Supported by research fiands from the Veterans Administration Rehab Engineering Research and Development. Presented at the Second Annual Meeting of the Western Vascular Society, Tucson, Ariz., Jan , Reprint requests: James M. Malone, M.D., Chairman, Dept. of Surgery, Maricopa Medical Center, P.O. Box 5099, Phoenix, AZ Table I. Patients studied No. of patients 62 Mean age (yr) 65 (48-82) Risk factors Diabetes 20/62 (32.2%) Hypertension 34/62 (54.8%) Coronary disease 31/62 (50.0%) Smoking 52/62 (83.9%) Since its introduction in 1965 by Strandness and Bell, 2 Doppler ankle systolic pressure, expressed as an absolute value or as an ankle-brachial pressure index (ABI), is the most commonly employed noninvasive peripheral vascular test. Newer, less commonly used, but potentially more accurate noninvasive vascular modalities include the measurement of transcutaneous oxygen pressure (tcp02), transcutaneous carbon dioxide pressure (tcpc02), or both. To date, there is only one case report on the use of tcpco2 in vascular surgery? On the other hand, tcpo2 has been reported in the vascular literature for preoperative, intraoperative, and postoperative assess- 507

2 Journal of VASCULAR 508 Lalka et al. SURGERY Table II. Results of revascularization Claudication Limb-threatening ischemia Revasculariza- Nondiabetic Diabetic Nondiabetic Diabetic tion outcome (No. of limbs) (No. of limbs) (No. of limbs) (No. of limbs) Success Failure Total Table III. Preoperative ischemic severity classification tcpc02 tcp02 (torr) F-CtcPo2* (torr) tcpofl tcpco2 AP ABI Claudicants (n = 35) Diabetic (n = 4) 20.8 ± ± ± ± ± ± 0.20 (n = 3) Nondiabetic (n = 31) 19.5 ± ± ± ± ± ~ 0.17 (n = 11) Limb threat (n = 54) Diabetic (n = 22) Nondiabetic (n = 32) 11.6 _ _ ± ± ± 0.i8 (n = 11) (n = 20)? (n = 20)? 8.6 ± ± ± _ ± ± 0.26 (n = 15) tcp02 = transcutaneous oxygen tension; F-CtcP02 = foot-chest tcp02 index; tcpco2 = transcutaneous carbon dioxide tension; tcpo2/tcpco2 = transcutaneous P02 and Pc02 index; AP = ankle Doppler-derived systolic pressure; ABI = anlde-brachial pressure index. NOTE: Data expressed as mean ± SD of preoperative noninvasive measurement. *Foot-chest tcp02 was measured in 40 of 89 limbs.?two patients had unobtainable values because of noncompressible vessels. ment of vascular reconstruction, 3-I1 amputation level selection, 6'1 32 and healing of ischemic ulcers. 633 The purpose of this study was to compare transcutaneous (metabolic) monitoring with Doppler pressure (hemodynamic) measurement with respect to their ability to discriminate the severity of limb ischemia before vascular reconstruction and, by repeat measurement postoperatively, to predict surgical outcome. The following six noninvasive parameters were evaluated: tcp02; foot-chest P02 index (F-CtcPo2)~'14;15; tcpco216; the ratio of tcpo2 to tcpco2 (tcpo2/tcpco2); Doppler-derived anldc systolic pressure (AP); and ABI. MATERIAL AND METHODS Sixty-two patients were treated at the Tucson Veterans Administration Medical Center, Tucson, Arizona, between Feb. 1, 1985 and Oct. 23, The patients underwent 76 vascular reconstructions: two axillofcmoral, nine femorofemoral, 16 aortailiac-femoral, eight femoral endartercctomy-patch angioplasty, 10 femoropopliteal above-knee, eight femoropoplitcal below-knee, and 23 femorotibial. The population demographics arc given in Tables I and II; briefly, 89 limbs from 76 operations were evaluated in 62 patients. Data were obtained in each revascularized limb with preoperative and postoperative assessment by AP (with an ankle cuff) and tcpo2 and tcpc02 foot monitoring. The clinical status of each limb was assessed preoperatively on the basis of the history and physical examination and the limb was assigned to one of two ischemic classes: 35 limbs were only associated with claudication and were thought not to be severely ischemic (four of these were in diabetic patients); and 54 limbs were considered to have severe limb-threatening ischemia (rest pain, dependent rubor, ischemic ulcers, or gangrene) (22 of these were in diabetic patients). Values of tcpo2 and tcpc02 were obtained with a Novametrix System 800 monitor (Novametrix Medical Systems, Wallingford, Conn.) with a combined 02/CO2 sensor (Clark-type polarographic electrode) heated to 44 C. The 02 and CO2 sensors were separately calibrated with the use of standardized gas references. The patient's skin was degreased with alcohol and the sensor was applied to the dotsum of the foot over the third metatarsal with a double-sided adhesive ring. Measurements were made at ambient temperature with the patients in the supine position and breathing room air after they had rested for 15 minutes. Readings were recorded

3 Volume 7 Number 4 April 1988 Transcstaneous Poe and Pco2 monitoring 509 gable IV. Postoperative results tcpo2 tcpc02 AP (torr) F-CtcPo2* (torr) tcp02/ co2 (mm Hg) ABI Success (n = 67) Claudicants (n = 27) Diabetic (n = 4) Nondiabetic (n = 23) Limb threat (n = 40) Diabetic (n = 14) Nondiabetic (n = 26) Failure (n = 22) Claudicants (n = 8) Diabetic (n = 0) Nondiabetic (n = 8) Limb threat (n = 14) Diabetic (n = 8) Nondiabetic (n = 6) 34.0 ± ± ± ± _ ± 0.20 (n = 3) 32.3 ± ± ± ± ± ± 0.20 (n = 8) 30.4 ± ± ± ± ± ± 0.14 (n = 7) 30.6 ± ± ± ± ± ± 0.22 (n = 12) 14.0 ± ± ± ± ± ± 0.23 (n = 3) 12.9 ± ± ± ± ± ± 0.21 (n = 4) 12.5 ± ± ± ± ± ± 0.33 (n = 3) For abbreviations see Table III. NOTE: Data expressed as mean ± SD of postoperative noninvasive measurement. *Foot-chest tcpo2 measured in 40 of 89 limbs. after tcpo2 and tcpco2 values were stable for 15 to 20 minutes. To study the utility of F-CtcPo2, tcpo2 measurements at the foot were normalized to values measured on the chest (5 to 7 cm infraclavicular, midaxillary line). F-CtcPo2 was not obtained initially in the study; therefore only 40 of 89 limbs had this index calculated. The methods of AP determinations have been well described and will not be covered here. 17'18 Bccanse this study focused on the noninvasive evaluation of limbs in the early postoperative period, stress testing 6'14'18,19'2 (treadmill or reactive hyperemia by cuff occlusion) was not feasible because of limb swelling, surgical incisions, and pain. Therefore only resting Doppler and transcutaneous measurements were made before and after operation. The postoperative noninvasive measurements on which this study is based wcrc those recorded when clinical evaluation before discharge clearly showed that the revascularization was a success (significant hnprovcment in symptoms and signs of limb ischemia) or failure (no change or worsening of ischemic signs and symptoms). The values were obtained at a mean of 8 days postoperatively (median 6 days; range 1 to 90 days). The outcome of only seven limbs was not clear until more than 14 days postoperatively: Statistical methods. Six variables were analyzed: tcp02, tcpc02, F-CtcP02, tcpo2/tcpc02, AP, and ABI. The measurements obtained with each of these six noninvasive tests were grouped by ischemic classification (claudication vs. limb threat), diabetic cat- ±gory (diabetic vs. nondiabetic), and result (success vs. failure). The means and standard deviations were calculated for each group (Tables III and IV). For each of the six variables measured preoperatively, the statistical significance of differences of the means was computed by two-way analysis of variance (ANOVA) for class effect and diabetic category. The statistical significance of differences of the mean postoperative values for each of the six variables were computed by three-way ANOVA for result (success or failure), class effect (claudication or limb threat), and diabetic category. The discriminative and predictive accuracy of the six variables was analyzed by computation of the error rates and their standard error (Tables V and VI). These were calculated by ranking the values for each particular variable from smallest to largest and then serially computing the error rate for each successive value, assuming it was the cutoff point determining the "positive" range for that variable. Preoperatively, "positive" represented the "limb threat" range (severe ischemia), and postoperatively, "positive" represented the "failure" range (in which a procedure would be very likely to fail). The "best" cut point value chosen as the limit of the "positive" range for each test was determined by the corresponding error rate, which would yield the minimal false-negative

4 510 Lalka et al. Journal of VASCULAR SURGERY Table V. Discriminative value: Claudication vs. limb threat Limb threat range % False-negative ~ % False-positive~ tcpo2 (torr) -< F-CtcPo2 -< tcpco2 (torr) > tcpo2/tcpco2 -< AP (ram Hg) --< ~ ABI -< _ For abbreviations see Table III. NOTE: Error rate -+ standard error. Percentage predicted to fall in claudication category that in fact were under limb threat. t Percentage predicted to be under limb threat that in fact were not. Table VI. Predictive value: Success vs. failure "Failure" range % False-negative ~ % False-positive~ tcpoz (torr) -< F-CtcPo2 -< tcpco2 (tort) > tcpoz/tcpc% -< _ AP (ram Hg) -< _ ABI -< _ For abbreviations see Table III. NOTE: Error rate + standard error. ~Percentage predicted to be a success that in fact failed. t Percentage predicted to fail that in fact were successful. and false-positive rates. First priority was given to minimizing the number of false negatives to minimize the risk of undertreating a patient on the basis of that noninvasive test. For the ability to discriminate preoperative ischemic severity (clandication vs. limb threat), a false negative was defined as a limb having a value that would predict it to be in the claudication category when in actuality it was under threat of amputation. A false positive was defined as a limb with a value that predicted the limb was under threat of amputation when indeed it was in the claudication category. For the ability to predict surgical outcome (success vs. failure), a false negative was defined as a failed revascularization with a value that predicted success. A false positive was a successful operation with a postoperative value that predicted failure. A measurement was considered to have adequate discriminatory ability if its error rates were less than or equal to 5% (determined by a t test with the normal approximation to the binomial). Variables having satisfactory error rates were compared by means of a McNemar test. RESULTS The preoperative means and standard deviations for the six noninvasive tests, grouped by ischemic class and diabetic category, are displayed in Table III. Doppler-derived pressures could not be obtained in only two limbs from diabetic patients because of noncompressible vessels (7.7%). There were no limbs from which transcutaneous values could not be obtained. The postoperative means and standard deviations, grouped by surgical outcome, ischemic class, and diabetic category, are shown in Table IV. In the failure group, there were no diabetic claudicants. Sixty-seven of 89 revascularizations (,75.3%) were successful, and 22 of 89 (24.7%) failed. Five of the 22 failures were due to occluded grafts, three of which were successfully revised; one required a second revision; and one required below-knee amputation. Four additional patients had amputations for continued ischemia (one above-knee and three below-knee). Thirteen patients had no significant improvement in symptoms or signs of ischemia. By retrospective analysis of postdischarge patien t follow-up (from 21/2 to 24 months; five patients lost to follow-up), no initially successful revascularization, as clinically assigned in this study, went on to fail within 6 weeks of surgery. Of the 62 successful revascularizations for which long-term follow-up is available, one graft occluded at 3 months and resulted in above-knee amputation. Four grafts occluded at 6 weeks and 4, 12, and 15 months; these had surgical revision. One patient had a patent but hemodynam-

5 Volume 7 Number 4 April 1988 Transcutaneous Poe and Pco2 monitoring 511 ically failed graft and recurrence of symptoms requiring reoperation at 4 months. One patient with multilevel disease had recurrence of symptoms 2 months after a successful inflow procedure and required surgery for outflow disease. Results of the two-way ANOVAs of the six variables measured preoperatively (Table III) show significant differences between claudication and limb threat classes for tcpo2 (p = 0.003), F-CtcP02 (p = 0.013), tcpo2/tcpc02 (p = 0.011), AP (p = 0.033), and ABI (p = 0.034). Significant differences between diabetic and nondiabetic patients exist for AP (p = 0.029) and ABI (p = 0.031). There were no significant differences involving tcpc02. The three-way ANOVAs of the postoperative measurements (Table IV) show highly significant differences (p < 0.001) between success and failure categories for all six measurements. No significant differences between claudication and limb threat or between diabetic and nondiabetic categories were found for any of the postoperative measurements. When analyzed by error rates, the preoperative ability of the six noninvasive vascular tests to discriminate patients with less severe ischemia (claudication) from those with limb-threatening ischemia was found to be relatively poor (Table V); no falsenegative rate was less than 15% and only one ofsix variables had a false-negative rate less than 20%. The metabolic tests based on tcpo2 (absolute tcp02, F-CtcP02, and tcpo2/tcpc02) were better than tcpc02 or the hemodynamic tests. The F-CtcP02 had the lowest false-negative rate ( ), when all values less than or equal to 0.46 were considered "positive" (positive meaning limb-threatening ischemia). All preoperative false-negative error rates except that for F-CtcP02 were significantly greater than 5% (at the 0.05 level). However, the large standard error and high false-positive error rate for F-CtcP02 cause this measurement to be an inadequate predictor of severity of ischcmia. By error rate analysis, the postoperative association of the six variables with surgical outcome was considerably better than the discriminative ability of the preoperative tests, with all false-negative rates less than 20% (Table VI). The three metabolic tests based on tcp02 (absolute tcp02, F-CtcP02, and tcpo2/tcpc02) were considerably more accurate than tcpc02 or the hemodynamic tests. Again, F-CtcP02 had the lowest false-negative rate ( ) when all values less than or equal to 0.53 were considered "positive" (positive meaning failure). The falsenegative error rates for tcp02, F-CtcP02, and tcp02/tcpc02 are not significantly different from 5 %. By McNemar's tests, these three error rates are not significantly different from each other. DISCUSSION For almost 20 years, Doppler-derived ankle pressure measurement has been the most commonly employed noninvasive vascular test. Although this technique is simple, inexpensive, and feasible for most patients, its applicability can be limited. It is a hemodynamic test that measures pressure in the tibial vessels. In terms of regional blood flow distribution, it primarily reflects skeletal muscle flow and is only an indirect indicator of the adequacy of skin perfusion. 5'6'8'21 Cuff occlusion tests are often inaccurate in patients with diabetes because of medial calcification of the peripheral vessels, which causes artificially elevated systolic pressures. 12,19,22 In 5% to 10% of diabetic patients, systolic pressures will not be obtainable because of noncompressible vesselsy In addition; because of the predilection for more distal small vessel disease in diabetic patients, foot lesions may develop or progress at higher ankle pressures than in the nondiabetic patient. = Even in the nondiabetic patient, Doppler-derived pressure readings can be unreliable because of calcified noncompressible vessels, particularly in the elderly patient, and because of distal small vessel disease as in a patient with Buerger's disease. 6 Finally, Doppler pressure measurements may not be feasible in the postoperative patient with a fresh surgical incision in the distal extremity. 21 Transcutaneous monitoring was introduced by Huch et al.24 in 1969 as a noninvasive method of measuring arterial oxygen pressure (Pa02). Values of tcp02 differ from Pa02 because of four temperaturedependent factors: rightward shift in the oxyhemoglobin dissociation curve with heating of the capillary blood, skin resistance to oxygen permeation (stratum corneum permeability), metabolic consumption by dermal tissue; and effective rate of cutaneous blood flow. Since the effect of the first factor is the opposite of the third factor, they tend to cancel each other out so that the relationship of tcp02 and Pa02 depends primarily on permeability and flowy Skin permeability to oxygen is optimized by heating the skin sensor to 44 C, which causes a temperaturedependent microstructural change from solid to liquid of the lipid phase of the stratum corneumy The value of tcp02 approximates that of Pa02 because maximal capillary vasodilatation induced by heat in the skin beneath the sensor allows oxygen supply to exceed tissue demand as long as tissue perfusion is normal. However, with limb ischemic states, there is

6 512 Lalka et al. Journal of VASCULAR SURGERY a decrease in perfusion pressure paralleled by a decrease in skin blood flow so that oxygen delivery (function of blood flow and arterial oxygen content) approaches tissue oxygen requirements and tcpo2 decreases relative to Pao Once oxygen delivery equals the metabolic consumption of the skin, no PO2 will be sensed by the transcutaneous electrode (tcp02 = 0), although skin blood flow is present.4,8,n, 2a It is dear that both systemic and local factors affect tcpo> Systemic factors include oxygen content (dependent on ventilation and hemoglobin) and blood flow (dependent on cardiac output and perfusion distribution).8,26 To eliminate all but perfusion distribution as systemic factors, an F-CtcPoz ratio has been employed by others as well as in this study. 15,2 As limb perfusion becomes progressively limited by arterial disease, there is a redistribution in favor of the muscles and away from the skin. 4'11'15'20'23 Although the value of tcp02 is reasonably linear with respect to skin blood flow, with severe limb ischemia and associated arteriovenous shunting tcp02 may decrease disproportionately (and nonlinearly) to skin blood flows Local factors that can limit the reliability oftcp02 include increased skin thickness (obesity and hyperkeratosis), edema, or cellulitis, all of which reduce the diffusion of oxygeny Carbon dioxide has a greater skin transmissibility than oxygen and should be less affected by the presence of such local factors. However, tcpco2 monitoring in the vascular patient as reported by Kram and Shoemaker 3 appears less sensitive an indicator of changes in limb perfusion than tcp02. Therefore, in this study, we compared tcpcoz to tcp02 and investigated the applicability of a tcpojtcpco2 index. On the basis of this comparative study of Doppler-derived pressure and transcutaneous monitoring, Doppler-derived pressure measurement is less reliable in discriminating the severity of limb ischemia. In addition, Doppler-derived pressures were unobtainable in 7.7% of limbs of diabetic patients because of noncompressible vessels whereas transcutaneous measurements were obtained in all limbs. Although ANOVA showed a significant difference between limbs with claudication and limbthreatening ischemia as assessed by AP and ABI, the difference was not so great as with transcutaneous measurements involving PO2. One of the goals of this study was to determine a critical value for each noninvasive variable, which would define the limb-threat range, indicating that a 7" particular limb was in urgent need of revascularization. Values outside such a range would indicate the safety of expectant management. This was done by error rate analysis, with a priority on minimizing the false-negative rate, since underdiagnosis is more critical than overdiagnosis in terms of the risk of limb loss. We found that all six preoperative tests have relatively high false-negative rates (none less than 15%). The reason for this may be more apparent than real because of the inherent difficulty of subjective clinical categorization of the severity of limb ischcmia. A patient's history can bc inaccurate, symptoms can be misleading (i.e., diabetic neuropathy vs. true ischemic rest pain), and skin lesions can have multifactorial origins (especially in the diabetic). Recognizing those limitations, it was found that metaboric tests based on P02, (tcp02 and F-CtcP02) had lower false-negative rates than the hemodynamic test (AP and ABI). This increased reliability and applicabirity of tcp02 and F-CtcP02 make transcutaneous monitoring the superior diagnostic noninvasive vascular modality for the clinical situations defined in this study. ANOVA revealed that there was a highly significant difference between successful and failed revascularizations for all six tests. Each test was examined by error rate analysis, with the goal of finding the "failure" range. A value in this range, even if a revascularization was patent, indicates that the procedure has a high likelihood of failure. This would lead one to closely reassess the limb to determine whether revision or additional surgical intervention (in the case of multilevel disease) was indicated. We found tcpo2 and F-CtcP02 to be significantly better than Doppler-derived pressure measurements in terms of false-negative rates. AP and ABI had lower falsepositive rates but, again, in terms of uncovering pos. sible continued poor tissue perfusion in a newly revascularized limb before it becomes irreversibly ischemic, it is essential to use a more sensitive test (i.e., lower false-negative rate). In theory, F-CtcP02 index is a valuable adjunct to absolute tcpoz (the index compensates for anemia, low cardiac output, ventilatory compromise, and hypotension). In this study, F-CtcP02 had as low a falsenegative error rate and a lower false-positive rate than absolute tcp02, indicating that it is a valuable noninvasive vascular measurement. The use oftcpc02 in vascular Surgery has not been so extensively studied as tcp02. We found tcpc02 to have no ability to discriminate ischemic severity and it was a poor predictor of surgical outcome. The ratio

7 Volume 7 Number 4 April 1988 Transcutaneous Po2 and Pco2 monitoring 513 of tcpo2/tcpco2 had good discriminative and predictive value relative to Doppler-derived pressure measurements but had no better accuracy than tcpo2 or F-CtcPo2. Therefore the use of tcpco2 has no value in the prerevascularization or postrevascularization assessment of the vascular patient. In summary, this study retrospectively evaluated noninvasive vascular test values that characterized limbs with clinically defined ischemic severity and surgical outcome. The demonstration of the better applicability and more sensitive discriminative value of tcpo2 and F-CtcPo2 compared with AP and ABI supports our hypothesis that a metabolic test of actual tissue perfusion should be a more sensitive indicator of limb ischemia than a hemodynamic test. The determination of critical values of tcpo2 and F-CtcPo2 that define a range of limb-threatening ischemia n preoperative analysis (tcpo2 ~<22 mm Hg and F-CtcPo2 ~<0.46) and a range of likely postoperative failure ofa revascularization (tcp02 ~<22 mm Hg and F-CtcPo2 ~<0.53) can now be prospectively applied to the clinical assessment of limbs in which the vascular status is clinically unclear. CONCLUSIONS 1. The limbs of diabetic patients have significantly elevated AP and ABI compared with those of nondiabetic patients, thus risking underdiagnosis of ischemic severity. 2. The measures tcpo2 and F-CtcPo2 are more sensitive to degrees of severity of limb ischemia than AP and ABI. 3. The measures tcpo2 and F-CtcPo2 are more closely associated with the outcome of revascularization than AP and ABI. 4. Ttle values tcp02 ~<22 mm Hg and F-CtcPo2 ~<0.46 indicate severe limb ischemia requiring urgent revascularization. 5. 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