CLN. CHEM. 27/3, 391-396 (1981) Laser DopplerVelocimetryvs Heater Poweras ndicatorsof Skin Perfusion duringtranscutaneous02 Monitoring Louis Enkema, Jr.,1 G. llen Holloway, Jr.,2 Daniel W. Piraino,3 David Harry,1 Gregory L. Zick,3 and Margaret. Kenny 4 Laser Doppler velocimetry (LDV) and heater power (HP) were compared as indicators of skin perfusion during transcutaneous oxygen tension (p2) monitoring in adults. The WV probe was fitted to a Radiometer Ptco2 attachment ring, which was fixed to the forearm. ptco2, electrode temperature, HP, and LDV measurements were recorded continuously and simultaneously. LDV was more specific and sensitive to changes in cutaneous perfusion. Moreover, changes in perfusion were signalled four to 24 times faster by WV than by HP. Changes in p2 corresponded to changes in LDV more closely than to changes in HP in all experiments. HP was more specific and sensitive to changes in electrode and air temperatures. Once hyperemia was established at 44 #{176}C and the electrode heater was turned off (HP = 0), LDV measurements frequently showed persistence of hyperemia (up to 25 mm). Under these circumstances ptco2 decreased linearly with decreasing electrode temperature in the range of 44-32 #{176}C. LDV performance was referenced by monitoring velocity measurements taken on a cylinder revolving at known velocities. We conclude that LDV is a useful adjunct to Ptc.02monitoring and better than HP in assessing skin blood f low. t should enhance quality control of transcutaneous 02 monitoring. ddtonal Keyphrases: blood gases. peripheral perfusion skin blood flow Transcutaneous oxygen-tension measurements (po2) are now a widely accepted alternative to arterial whole-blood oxygen tension determinations (Pa02) in the management of acutely ill neonates because they are continuous and noninvasive (1-3). However, the correlation of ptco2 with Pa02 depends in part upon adequate peripheral cutaneous perfusion at the measurement site (2). For this reason, transcutaneous 02 electrodes incorporate heating elements, which can be set to 43-45 #{176}C and which thus thermally induce a localized hyperemia when the electrode is fixed on the skin. The power required to maintain the heating element at the preset temperature, heater power (HP), has frequently been used as an indicator of changes in perfusion. Peabody et al. (4) have challenged the validity of HP as a measure of cutaneous blood flow. However, until the recent development by Stern et al. (5) and Watkins and Holloway (6) of a device that monitors superficial capillary blood flow by laser Doppler velocimetry (LDV), there was no practical alternative. Clinical Chemistry Division, Department of Laboratory Medicine, University of Washington School of Medicine, Seattle, W 98195. 2 Center for Bioengineering, University of Washington. Department of Electrical Engineering, University of Washington. To whom communications should be addressed. Nonstandard abbreviations used: p,o2, transcutaneous oxygen tension; Pe02, oxygen tension in arterial blood; LDV, laser Doppler velocimetry (or laser Doppler velocimeter); HP, heater power. Received Nov. 6, 1980; accepted Dec. 16, 1980. The LDV instrument operates on the same Doppler principle as that used for diagnostic ultrasound devices, except that light is substituted for sound energy. Light from a 5-mW He-Ne laser (frequency 4.75 X 1014 Hz) is transmitted to the skin through a quartz optical fiber. The light is then backscattered from both stationary skin components and moving objects, primarily erythrocytes in the capillaries, which are encountered as the laser light penetrates to an estimated maximum depth of 1-1.5 mm. The backscattered light, which is transmitted to a photodiode through another optical fiber, consists of a mixture of frequency-shifted and -nonshifted waves. These different frequencies are optically mixed on the surface of the photodiode and produce a beat frequency which reflects the Doppler shift. The photodiode output signal is composed of a spectrum of Doppler-shifted frequencies, generally in the range of 1-8 khz, caused by the different velocities of erythrocytes encountered. From this, a single flow parameter is obtained by taking the normalized root mean square bandwidth of the signal, which has been shown (5) to be proportional (r = 0.89) to peripheral cutaneous perfusion as measured by radioisotope (xenon) clearance studies. n this paper, using the LDV integrated with a commercially available transcutaneous oxygen monitor (Radiometer /S, Copenhagen), we report the comparison of HP and LDV as indicators of cutaneous perfusion during transcutaneous oxygen monitoring in healthy adult subjects, and we demonstrate circumstances in which LDV is a better measure of peripheral perfusion. Methods pparatus and Materials Our transcutaneous oxygen-measurement system is the Radiometer TCM-1 monitor (The London Co., Cleveland, OH 44145), which was calibrated and operated as described by Graham and Kenny (1) except that a 25-tm Teflon (fluorinated ethylene-propylene, FEP type) electrode membrane replaced the polypropylene membrane and that no spacer was used. ll measurements were made with 44 #{176}C electrode temperature except where noted. They are reported in mmhg (1 mmhg = 133 Pa = 133 N/m2). four-channel recorder was used to display the PtcO2, electrode temperature, heater power, and laser Doppler velocimetry signals simultaneously at a chart speed of either 25 mm/mm or 5 mm/mm. The LDV apparatus was that described by Piraino et al. (7), with some modifications: Our laser source (Spectra-Physics nc., Santa Clara, C 95050; Model 120), a 5-mW helium neon laser, is more powerful and our optical fibers have a smaller core diameter (63 nm). n addition, the LDV output voltage is proportional to root mean square bandwidth, our flow parameter, which was electronically calculated according to the algorithm of Stern et al. (5,8). The LDV unit is a preproduction prototype (Nuclear Pacific, nc., Seattle, W 98108). The optical fibers for the LDV are permanently mounted in the Radiometer transcutaneous oxygen electrode retaining ring as illustrated in Figure 1. The two glass optical fibers, located 1 mm apart and 2 mm from the interior edge of the CLNCL CHEMSTRY, Vol. 27, No. 3, 1981 391
TranscutonsouS 02 Electrode Lf Fiber LOV Optical Fibers 6 B C Electrode Rstaining Ring Fg. 1. Diagram llustrating the coupling of the Radiometer transcutaneous 02 electrode assembly with the laser Doppler velocimeter (LDV) probe () Transcutaneous electrode; (6) electrode attachment ring wth optical fibers ncorporated;(c) ranscutaneous electrode nstalled n the modif led attachment rng. The afferent (1) and efferent (2) optcal fbers of the wv probe are imbeddednthe electrode retaningring (4) perpendcularto and flush with the skin contact surface. The electrode hester (5) and the temperature probe (3) are located in the anode, which surrounds the cathode (6) ring, are secured with epoxy resin, then polished until they are flush with the ring s bottom surface. To secure the ring to the skin without obstructing the light beam, a small notch is cut in the double-sided adhesive rings used for attachment (item no. 2181; 3-M Co., St. Paul, MN 55101). The entire length of the glass fibers is encased in polyethylene tubing to protect them. The LDV apparatus was turned on for at least 20 mm before all experiments and was left in the operating mode between experiments, to maximize electrical stability. velocity calibration unit was fabricated by the Medical nstruments Facility of this institution. t consists of a synchronous clock motor (120 V, 7 W, 60 Hz), which drives a precision gear train with ratios of 1:10, 1:1,4.5:1, and 10.1. This four-speed transmission is linked to a solid acrylic cylinder 19.1 mm in diameter. Five millimeters above the cylinder s curved surface the laser probe holder is fixed so that when in place, the laser s beam strikes the surface at an 80_boo angle. The cylinder surface rotates at 0.4,4.0, 18.0, and 40 mm/mm, 4 respectively, with the above gears. These speeds were selected to bracket the estimated range of erythrocyte velocities in the capillary (9). Experimental Procedure nstrument calibration. We made 20 determinations of LDV at each of the four speeds. The gear ratios were changed between each LDV measurement. Neither the motor nor the laser system was turned off at any time during the experiment. LDV drift was assessed with the probe on a stationary surface for 24 h. n vivo experiments. Before each experiment the output of the laser Doppler instrument was set to zero while holding it stationary in the air 90 to 120cm from a fixed surface. This procedure was used to subtract the electronic noise and thus permit a zero reading for flow on a stationary target. fter this adjustment, the ring containing the flow sensor was attached to the flexor surface of the forearm within 5 cm of the antecubital fossa. hairless site was selected, to facilitate attachment and to eliminate potential optical interference from hair trapped near the laser beam. The initial LDV mean backscattered light level was observed as an index of adequate probe function. Values greater than 500 mv were acceptable. fter confirming the LDV performance subsequent to positioning, a calibrated oxygen electrode was screwed into the attachment ring with the heater off. The experiments were performed as soon as the po2 measurements stabilized. Subjects. Seventeen ostensibly healthy adult volunteers, 25-35 years of age and without excessive skin tanning at the test site, participated in 23 experiments. Each monitoring experiment lasted from 1.0-3.5 h, during which the subject was supine in a hospital bed with head and test arm in the horizontal position (except as noted) before the electrode was applied and during experimental maneuvers. The maneuvers were sequenced so that each individual would serve as his or her own control. Results n Vitro Experiments The precision data for the LDV calibration are based on 20 sets of measurements. Each set consisted of one measurement at each speed (Table 1). The data in Table 1 show that LDV is directly proportional to velocity to 18 mm/mm (r = 0.994). t 40 mm/mm the output fails to represent velocity because of the 17-kHz limit in frequency response. The average drift in the zero point is ±50 mv/h. During normal heated cutaneous flow this drift affects output by less than 1%, even though this prototype was not designed for optimal electrical stability. ( new system designed to meet more stringent drift criteria is already operational.) n Vivo Experiments The first set of 15 experiments on 11 subjects involved heating the electrode from the normal resting skin-surface temperature of 27-30 #{176}C directly to 44 #{176}C. Heater power was maximum until the 44 #{176}C temperature was reached. LDV increased gradually from 195 ± 90 mv, requiring 12 ± 6 mm Table 1. Precision Data for LDV Calibration Velocty, mm/mln LDV. my, mean (SD) 0-5(20) 4 243 (13) 18 1136(47) 40 1584 (40) 392 CLNCLCHEMSTRY,Vol. 27, No. 3, 1981
75 50 25 pto2, electrode temperature, HP, and LDV measured on one patient with a cuff rapidly inflated to 200 mmhg at point. The LDV decreased in 20s from an equilibrium value of 1400 mv to a value of 250 mv (in this instance 250 = zero flow) - during total occlusion. The HP current changed from 330 mw 75. at equilibrium before cuffing to 275 at equilibrium during #{231} cuffing. The pcj2 decreased from 92 too mmhg after 3 mm, of cuffing. When the cuff pressure was released suddenly and completely, the LDV increased rapidly (point B). n 50% of the experiments, the equilibrium value was restored within 3 a. n all other experiments, the equilibrium value was mi- N tially exceeded but was restored within 3 min. The over- shoot ranged up to + 125% of the initial reading. The 0-50% maximum response time of LDV is 1.0-1.5 s. The response times of HP and po2 calculated in the same fashion were 6-24 25 and 42-48 s, respectively. n seven subjects, progressive partial blood flow occlusions with cuffs were performed, starting at 20 mmhg applied #{163} 30 40 Temperature (#{176}C) Fig. 2. Effect of increasing electrode temperature on p2 and on WV Data from one experment on a single volunteer to reach a steady-state value of 400 ± 160 (± values are ranges), an increase of 105% over control values. Ptho2 also increased from 20±20 to 82± 15 mmhg but at a much slower rate, requiring 17 ± 5 mm. second set of experiments, closely related to the first, involved increasing the electrode temperature in three subjects from 30 to 45 #{176}C in stepwise increments to demonstrate the effect of increasing electrode temperature on po2, HP, and LDV. ll three variables were allowed to equilibrate before each successive increase in electrode temperature. Figure 2 illustrates the fmdings in one subject. puo2 remained zero below 35 #{176}C. t increased to 35 mmhg at 40 #{176}C and 52 mmhg at 42#{176}C, where it stabilized. t 45#{176}C the (cj.2 reached 95 mmhg and stabilized within 3 mm. LDV changed little between 30 and 42 #{176}C. t 45 #{176}C it increased from 150 mv to a new equilibrium of 1050 mv in less than 1 min. s shown in the comparative data in Table 2, HP went to 800 mw with each successive increase in electrode temperature. Subsequently, it established a new equilibrium that had no predictable relation to the previous or successive equilibria. Once the 44 #{176}C temperature had been reached, three different sets of experiments were performed: total arterial occlusion, partial occlusion, and modification of the external environment of the probe. n six subjects, total arterial occlusion was accomplished by using a blood-pressure cuff inflated to give a pressure mmhg greater than systolic pressure. typical recorder tracing (Figure 3) illustrates the pressure, and increasing in 20-mmHg increments every 20s up to 120 mmhg. Figure 4 shows results for two subjects, and B. LDV decreased about 20% in and 30% in B for each 20 mml-g increase in pressure up to about 80 mmhg, when zero LDV was attained in and 150 mv was attained in B. The small changes in heater power averaged <1% for each 20-mmHg increment in pressure. po2 diminished but only by about 5 mmhg with each 20 mmhg cuff pressure increase initially. t declined more rapidly to about 10-20 minhg at the higher pressures of 60 and 80 mmhg. The third set of experiments, at 44 #{176}C on five subjects, involved modulation of the environment surrounding the probe (Figure 5). n the first subset, hot air from a hair dryer was blown for 1.5-5.0 mm over the probe and skin from points to B, to generate an increase in ambient temperature. The probe temperature increased from 45 to 47#{176}C in <30 s. Under these conditions, HP decreased to zero and remained at or near that value until the hot air was discontinued. LDV in- 200 :3 t 40 400 20 200 Table 2. Effects of ncreasing Electrode Tempeiature on Ptco2, HP, and LDV p2, nital Equlibrum mmhg HP, mw HP, mw LDV, mv Temp. oc 30 35 0 0 0 800 40 36 800 425 42 52 800 260 45 95 800 330 0 50 95 50 150 1050 2 3 4 5 6 urns (mm) Fig. 3. Simultaneous recordings of transcutaneous 02 monitor, electrodetemperature, HP, and LDV measurements during a blood flowocclusionexperiment on one volunteer s arm Cuff pressure rapdly applied at. Cuff released at B. Zero flow equals 250 mv CLNCL CHEMSTRY, Vol. 27, No. 3, 1981 393
400 8 350 7 SO 80 t t B C 0 300 F. 0-4C 20 250 5j\ 4 200 4 B 50 3 4 1 B 2 2 3 4 5 6 Time (min 50 S. Fig. 5. Comparison of changes in LDV flow and heater power with changes in environmental temperature Hotair applied at and discontinuedat B. Unheated air applied at C anddiscontinued at 0 0 20 40 60 80 Cuff Pressure (mmhg) Fig. 4. Comparison of heater power and laser Doppler velocimetry (WV) measurements made on two patients ( and B), while sequentially increasing the arm cuff pressure to occlude blood flow The sensor assenl,ly was located on the flexor surface of the forearm, the cuff on the upper arm. Equlbrium was established at each pressure. Electrode temperature, 44 #{176}C creased by 150% and remained increased for 3-25 mm after heating was stopped. PtcO2 increased slightly during the experiment and fell to previous values once the hot air was discontinued. n the second subset, a stream of unheated air was passed over the probe from points C to D in the same manner as the heated air in the previous experimental subset. Under these conditions HP increased and a new equilibrium, 60% higher than baseline, was established within 15 a. LDV and PtcO2 remained essentially unchanged. n a final set of experiments, we turned the electrode heater off to test the effect of decreasing electrode temperature on PtcO2 and blood-flow indicators. PtcO2 and electrode temperature were linearly related over the range of 44-32#{176}Cin six of seven subjects. Data from a typical subject are presented in Figure 6. LDV remained high and steady until the electrode temperature fell below 33 #{176}C. s the temperature continued to decline, the LDV decreased dramatically, from 0 mv at 33 #{176}C to 250 mv at 31 #{176}C. However, this two-degree decrease in electrode temperature required 26 mm, so the decrease in LDV was, in fact, rather gradual. The interval required for this temperature decrease varied for the six subjects from 5 to 30 mm. Linear regression analysis of PtcO2 vs temperature (from 44 to 32 #{176}C) for the six showed correlation coefficients ranging from 0.978 to 0.998. The slopes of the regression lines varied from 1.77 to 5.85 and averaged 5.20. n these subjects there was no change in LDV for about 5-10 mm after the electrode temperature was allowed to decrease. LDV very gradually decreased during the subsequent 0-20 mm. n the seventh subject, although the electrode temperature decreased at the same rate as in the other six, the LDV decreased immediately, unlike the other six. The decrease in po in this subject was nonlinear in relation to the decrease in electrode temperature. Discussion There has long been a need for an instrument to measure cutaneous blood flow continuously and nonmnvasively. This is particularly true when measuring po2 transcutaneously, where the accuracy of the po2 as compared with Pa02 is determined by the adequacy of cutaneous perfusion. To achieve the high perfusion necessary to minimize the effect of cutaneous respiration and oxygen consumption, we incorporated a heater into the commercially available oxygen electrodes. The power delivered to this heater is commonly thought to vary directly with the extent of cutaneous perfusion. Because blood flowing through the dermis removes heat (thermal sink mechanism) the heater power has been viewed as a satisfactory indicator of cutaneous perfusion in the region of the transcutaneous oxygen electrode. Because heater power has many limitations and the laser Doppler velocimeter has numerous advantages, we have adapted the laser Doppler velocimeter to a Radiometer transcutaneous oxygen monitor to demonstrate the effective use of this new device in monitoring cutaneous perfusion while simultaneously monitoring PtcO2. The most serious objections to the LDV measurement of cutaneous blood flow relate to problems in calibration and drift. HP cannot be absolutely calibrated either, and its dependence on uncontrollable variables such as varying thermal conductivity of skin and changing environmental conditions is greater than is the case for LDV. The simple precision-gear device for calibrating the LDV signal against known velocities shows promise. The linearity shown over the range of signals 394 CLNCL CHEMSTRY, Vol. 27, No. 3, 1981
75, E c5 25 45 40 35 30 Temperature (#{176}C) Fig. 6. Effect of decreasing electrode temperature on p2 on WV Data from one experiment on a single volunteer and received under in vitro experimental conditions suggests that the calibration instrument is effective. One question remains: What does the signal from this calibration device represent? The cylinder does not compare to the complex geometry of the dermal capillary bed. t best, the calibration device, like the LDV device, is a method of making a relative measurement until a better standard of reference is developed. The results we have obtained in these experiments support our hypothesis that the LDV is more specific, more sensitive, and has a faster response time than does heater power as a measure of cutaneous perfusion. n three circumstances HP is unusable. When the heater is first turned on, the power output is maximum and it cannot be used as a monitor until thermal equilibrium with the skin is reached-a situation not always easily identified. Similarly, when the heater power is turned off to allow the electrode to cool, HP obviously cannot be used as an indicator of skin blood flow. n addition, HP cannot be used to compare blood flows at different sensor temperatures. LDV is interpretable under all these conditions. The cuffing experiments were undertaken to evaluate the relative rate and magnitude of response of the two blood-flow indicators to changes in cutaneous perfusion. Response times were calculated from the point of cuff release rather than the point of initial pressure application because inflating the cuff takes more time. s illustrated in Figure 3, the blood-flow indicators respond more rapidly than the po2. We defined the LDV response time as the time interval needed to achieve 50% of the equilibrium LDV after cuff release. This time varied among individuals but was consistent with an individual within an experiment. The response time of LDV following release of total occlusion is 1.5 s, whereas HP is 6-24 s and PtcO2 is 42-48 s. The LDV is at least four to 16 times faster than HP. The actual LDV response time may be faster, because cuff release time and vessel compliance are known limiting factors in this experiment. Severinghaus et al. (10) used Po2 values obtained during forearm cuffing experiments to estimate skin blood flow and metabolic rate. They used conditions similar to ours, except their subjects breathed % 02 rather than room air; they used a polypropylene membrane rather than Teflon; the oc- clusions lasted 10-20 min rather than 2 mix,; and the electrode temperature was 45 #{176}C rather than 44 #{176}C. Their mean 63% response time for prj2 after cuff release was 68.6 s. Our mean 50% response time was 45 s. The relative responses of LDV and HP to complete occlu- 75 -. sion of arterial inflow by blood-pressure-cuff inflation consistently show LDV decreasing to zero while HP decreases as #{231} little as 7% in some cases and up to 75% in others, presumably depending on environmental conditions. Heat losses through 50 the skin and electrode have a large direct and immediate effect on HP readings, but little if any direct effect on LDV. n our experience, changes in the immediate environment of the Li.. probe generated the largest discrepancies between LDV and. HP. Blowing hot air over the electrode caused a slight increase 25 C) in LDV and po2-presumably owing to an increase in blood a flow-but a marked decrease in HP. Conversely, blowing air at room temperature over the probe, a situation in which no increase in flow would be expected, resulted in no change in LDV or po2. However, HP increased markedly. Severinghaus et a). (10) estimate that under ideal conditions 70% of HP is lost to the tissues, and conclude that HP is a poor indicator of blood flow. When HP was turned off after hyperemia had been established, LDV remained stable for at least 5-10 mm in six of seven subjects, even though all heating of the electrode had stopped. LDV gradually declined, often rather slowly, thereafter. The pattern of LDV and PtcO2 after heat is turned off suggests the possibility that under carefully controlled conditions transcutaneous monitoring of po2 might be extended beyond the 3-h limit now imposed to avoid burning the patient. This would involve developing a new protocol for heating the transcutaneous electrode. Continuous transcutaneous monitoring would be contingent on an ability to make appropriate corrections for oxygen tension determinations at diminished temperatures. We are currently investigating the feasibility of such a Po2 monitoring protocol as one application of the po2-ldv coupled system. lthough our comparative analyses of LDV and HP as blood flow indicators deal with greater challenges than one would generally encounter in a clinical setting, we have touched on characteristics of each flow monitor that are relevant to clinical applications. recent article by Whitehead et al. (11) has aroused concern over the smaller po2 values seen when the electrode is placed over ribs or sternum. The LDV device would have signalled the lowered flow, alerting us to this problem with po2, whereas HP oould never provide this information. Decreases in po2 readings caused by pressure when patients have been rolled onto the electrode during surgical procedures were not attended by changes in HP but would be signalled by decreases in LDV. n neonatal intensive-care units, changes in HP must be interpreted cautiously in view of the measurement s extreme susceptibility to change with changes in incubator air temperature, drafts, and the patient s movements. HP susceptibility to noise (i.e., environmental factors that do not influence blood flow but do affect HP) make it much less valuable than the LDV in measuring blood flow. The relative sensitivity and specificity of LDV make it an excellent adjunct to quality control in po2 monitoring. HP will be of even more limited value than it is now if proposed extended monitoring permits HP to be turned off for intervals during continuous pcj2 monitoring. n summary, we conclude that the laser Doppler velocimeter can be successfully coupled to the Radiometer transcutaneous po2 monitor. When used in this manner the LDV is more specific, more sensitive, and has a more rapid response time than HP as a measure of cutaneous blood flow. We believe that simultaneous assessment of LDV and po2 will provide a more CLNCLCHEMSTRY,Vol. 27, No. 3, 1981 395
accurate and physiological understanding of changes in pj2 in the critically ill patient. The LDV data provide a valuable new dimension to Puo2 quality control under circumstances where HP is frankly erroneous or not available. dditionally, when cutaneous blood flow remains increased for considerable periods of time after the heater power has been turned off, cycling of the heater may be possible while po2 monitoring continues without adverse effect. These experiments show how LDV can be applied to the development of transcutaneous monitoring that is more convenient, safe, accurate, and reliable. This work was supported in part by NH grants HL23711, C06331, and HLO9 187. The transcutaneous oxygen monitor was provided by Radiometer /S. Copenhagen, Denmark. References 1. Graham, G., and Kenny, M.., Performance of a Radiometer transcutaneous oxygen monitor in a neonatal-intensive-care unit. Clin. Chem. 26, 629-633 (1980). 2. Eberhard, P., Continuous Monitoring of Newborns by Skin Sensors. Offset Press, Basel, Switzerland, 1976. 3. Lofgrenn, 0., On transcutaneous P02 measurements in humans. University of Lund, Malmo, Sweden, 1978. Thesis. 4. Peabody, J. L., Willis, M. M., Gregory, G.., and Severinghaus, J. W., Reliability of skin (te) P02 electrode heating power as a continuous noninvasive monitor of mean arterial pressure in sick newborns. Birth Defects 15, 127-133 (1979). 5. Stern, M. D., Lappe, D. L., Chimosky, J. E., et a)., Continuous measurement of tissue blood flow by laser-doppler spectroscopy. m. J. Physiol. 232, H441-H448 (1977). 6. Watkins, D., and Holloway, G.. Jr., n instrument to measure cutaneous blood flow using the Doppler shift of laser light. EEE Trans. Biomed. Eng. BME-25, 28-33 (1978). 7. Piraino, D. W., Zick, G. L., and Holloway, G.., Jr., n instrumentation system for the simultaneous measurement of transcutaneous oxygen and skin blood flow. n Frontiers of Engineering in Health Care, EEE Press, New York, NY, cat. 79CH1440-7, 1979, pp 55-58. 8. Stern, M. D., n vivo evaluation of microcirculation by coherent light scattering. Nature 254, 56-58 (1975). 9. Butti, P., ntaglietta, M., Reimann, H. eta!., Capillary red cell blood velocity measurements in the human nailfold by video densitometric methods. Microvasc. Res. 10, 220 (1975). 10. Severinghaus, J. W., Stafford, M., and Thunstrom,. M., Estimation of skin metabolism and blood flow with tcpo2 and tcpco2 electrodes by cuff occlusion of the circulation. cta naesthesiol. Scand., (Suppl.) 68,9-15 (1978). 11. Whitehead, M. D., Pollitzer, M. J., and Reynolds, E. 0., rtifactual hypoxemia during estimation of Pa02 by skin electrode. Lancet ii, 157, 21 (1979). Letter. 396 CLNCLCHEMSTRY,Vol. 27, No. 3, 1981