Effect of CO2 and blood media on laser probe temperature

Blood may limit laser ablation of arterial plaque by decreasing thermal energy transfer from metal-capped probes to arterial occlusions. Since a gas is a good insulator of heat, CO2 may be a better medium for laser recanalization. To study this possibility, a metal-capped fiber was positioned in a segment of blood-filled polyethylene tubing and activated with an argon laser. Probe temperatures were measured in blood and as the blood was displaced by flowing CO2 gas. Probe temperatures were higher at all powers studied in CO2 gas than in blood. Maximum probe temperatures averaged 518 +/- 24 degrees C after CO2 infusion versus 320 +/- 7 degrees C in blood, (P less than 0.0001). Blood aggregate formation was noted on the probe surface in blood but not in CO2 medium. Thus CO2 gas may be a preferable medium for laser recanalization, since higher probe temperatures are achieved, and the probe surface remains free of insulating blood coagulate.     

CO2 and argon laser vascular welding: acute histologic and thermodynamic comparison

CO2 and argon lasers have been used successfully for vascular welding in both experimental and clinical settings. This study compared the thermodynamics during CO2 and argon laser welding of 1-cm longitudinal arteriotomies in a canine model. Continuous recordings using an AGA 782 digital thermographic system with spatial and thermal resolution of +/-0.2 mm and +/-0.2 degree C, respectively, were analyzed. A HGM argon laser using a 300-microns optic fiber held at 1 cm from the vessel edges (spot diameter = 2.8 mm) with concomitant room temperature saline irrigation (1 drop/sec) was used for argon welds. Total exposure time was 150 sec/cm. CO2 welds were performed with a Sharplan CO2 laser (spot diameter = 0.22 mm) with no irrigation for total exposure time of 10 sec/cm. Thermodynamic results and laser parameters are summarized as follows: Argon-n = 20; power = 500 mW; energy fluence = 1,400 J/cm2; Tmax = 48.8 degrees C; T mean +/- S.D. = 45.1 +/- 2.7 degrees C; CO2-n = 20; power = 150 mW; energy fluence = 3,000 J/cm2; Tmax 84.0 degrees C; T mean +/- S.D. = 60.7 +/- 9.8 degrees C. There was a significant difference (P less than .05) in thermal measurements between successful CO2 and argon vascular welds. Temperature rise during the argon welds was limited by saline irrigation. In contrast, during CO2 laser welding, the temperature rose quickly to its maximum and was maintained at a relatively high level as the laser progressed (0.1 cm/sec) along the anastomosis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of blood flow on laser probe temperature in human arteries

Laser recanalization using metal capped fibers occurs by thermal vaporization of occluding plaque. However, little is known about the effects of blood and flow on the temperature of the laser probe or the arterial wall during lasing. To study this, probe and arterial wall temperatures were measured while a metal capped fiber, activated by an argon laser, was held stationary in a stenotic human peripheral artery. Arteries were perfused with saline and blood, and flow was varied from 0 to 140 cc/min. Probe temperatures were significantly higher in blood than in saline. However, the increased probe temperature achieved in blood was not transferred to the arterial wall. Increasing flow decreased probe temperature in both media, but again arterial wall temperatures were minimally affected. Thus, the presence of blood and flow may significantly affect heat generation and heat transfer during arterial recanalization using metal capped fibers.     

Thermal laser probe angioplasty: influence of constant tip temperature, plaque composition, and probe/vessel diameter ratio

Thermal laser angioplasty uses constant laser power, producing widely variable tip temperatures in vivo. Results have been suboptimal. We studied the effect of 50-400 degrees C tip temperatures on depth of ablation at 192 sites on plaqued and normal human aorta in vitro, and the angiographic and histologic response in vivo of 300-400 degrees C at probe/vessel ratios of 0.5-1.0, in 40 normal canine femoral artery segments. In vitro, there was a direct relationship between tip temperature and depth of ablation, r = 0.71 (all segments), r = 0.74 for fibrous plaque, but a poor correlation in fatty plaque r = 0.35. In fibrous plaque, there was proportionately more ablation at tip temperatures greater than 300 degrees C, mean depth 0.62 mm, than at 150-300 degrees C, mean 0.37 mm, (P less than .001). Ablation was similar in plaqued and normal aorta. In vivo, 300 degrees C, 350 degrees C, and 400 degrees C produced similar effects. At probe/vessel ratios less than 0.8, only disruption of internal elastic lamina was observed. At ratios greater than or equal to 0.8, spasm occurred in 39% (7/18), transmural damage in 28% (5/18), and perforation in one of 18. Ablation is not selective for plaque and is highly variable in fatty plaque. Tip temperatures above 300 degrees C produce greater ablation than at lower temperatures. In clinical applications, probe/vessel rations less than or equal to 0.7 may be most appropriate, and it appears that thermal remodeling may contribute more to outcome than plaque ablation.     

Thermal studies of in-vivo vascular tissue fusion by argon laser

There are conflicting opinions regarding the mechanism of welding or fusion of vascular tissue by lasers. In this study, we measured the effects of saline irrigation on tissue temperature and fusion produced by argon laser welding of eight femoral and four carotid arteriovenous fistulas. Temperatures were continuously recorded using a digital thermographic camera. Forty 1-cm. welds were performed using powers of 0.50 (n = 24), 0.75 (n = 8), and 1.00 (n = 8) watt (W), with an energy fluence of 1100 J/cm2 per 1 cm segment, and cooling of the anastomotic site by saline irrigation (3 ml/minute). The "success" of fusions was determined by testing integrity of the repairs by exposure to blood flow. At 0.50 and 0.75 W, successful welds were formed when the temperatures were 44.2 +/- 1.6 (n = 28) and 55.0 +/- 3.6 degrees C (n = 20), with maximum temperatures of 47.9 and 59.9 degrees C respectively. At 1 W, the tissue was desiccated and the welds disrupted when exposed to blood flow with temperatures measured at 63.7 +/- 10.0 degrees C (n = 22) and maximum of 88.0 degrees C. Eight welds were also attempted without saline irrigation at 0.25 (n = 4) and 0.50 W (n = 4). At 0.25 W, tissue fusion was achieved but disrupted when exposed to intraluminal pressures with temperatures 50.3 +/- 2.0 degrees C (n = 10) and maximum of 52.6 degrees C. At 0.50 W, the fusion failed after only minimal exposure to the laser energy because of tissue drying and retraction with temperatures measured at greater than 125 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)     

Transcranial Doppler-estimated versus thermodilution-estimated cerebral blood flow during cardiac operations. Influence of temperature and arterial carbon dioxide tension

The ability of the noninvasive continuous transcranial Doppler technique to reflect changes in cerebral blood flow during cardiac operations was evaluated in seven adults. Middle cerebral artery blood flow velocity changes were compared with simultaneous thermodilution measurements of venous blood flow in the ipsilateral internal jugular vein during 11 preset stages of the procedure. Cerebral blood flow was varied by changes in arterial carbon dioxide tension and temperature. High-dose fentanyl-droperidol anesthesia and alpha-stat pH management were employed. To facilitate comparisons between the two methods, the individual awake values of middle cerebral artery flow velocity (45.1 +/- 3.3 cm/sec, mean +/- standard error of the mean) and jugular venous blood flow (382 +/- 37 ml/min) were normalized (100%). Cerebral metabolic rate for oxygen was calculated as the product of jugular arteriovenous oxygen content difference and middle cerebral artery flow velocity or jugular venous blood flow, respectively. The individual correlations between the two flow estimates varied between 0.76 and 0.87 (median 0.83), and the correlation of the combined data from all seven patients was 0.77 (p less than 0.0001). Variations in arterial carbon dioxide tension induced significant changes in the two flow estimates both during normothermia before cardiopulmonary bypass and at deep hypothermia (20 degrees C) during cardiopulmonary bypass. The significant arterial carbon dioxide tension changes had no significant effects either on Doppler- or thermodilution-estimated cerebral metabolic rate for oxygen. Deep hypothermia (20 degrees C) reduced Doppler- and thermodilution-estimated cerebral metabolic rate for oxygen to 22.0% +/- 3.9% and 20.6% +/- 6.9% of the awake levels, respectively. The study supports the validity of using middle cerebral arterial flow velocity changes as an estimate of changes in volume flow through the brain during cardiac operations.     

Hemodynamics of vein graft stenosis

The hemodynamics of 56 femorodistal saphenous vein bypasses (in situ [n = 53] or reversed [n = 3]) identified to have residual or recurrent graft stenoses were characterized with Doppler-derived blood flow velocity and resting limb systolic pressure measurements. The magnitude and configuration of the graft blood flow velocity waveform were the best predictors of graft stenosis. Transformation of the graft blood flow velocity waveform from a triphasic to a monophasic or biphasic configuration coupled with a low (less than 45 cm/sec) or decrease (greater than 30 cm/sec) in peak systolic blood flow velocity relative to initial postoperative levels reliably predicted the presence of a remote occlusive lesion. In 20 (36%) of the 56 limbs, the ankle-brachial systolic pressure index (ABI) did not identify graft stenosis. The low sensitivity of ABI in the identification of graft stenosis was due to insignificant decrease (less than 0.15) of ABI (n = 11), incompressibility of the tibial arteries (n = 6), or residual occlusive lesions after surgery (n = 3). Duplex scanning of grafts with low blood flow velocity localized the site of stenosis in 31 (86%) of 36 patients examined. Graft revision increased systolic blood flow velocity from 33 +/- 9 to 77 +/- 18 cm/sec (mean +/- one standard deviation) a velocity similar to immediate postoperative levels (74 +/- 17 cm/sec). Resting ABI increased from 0.61 +/- 0.2 to 0.92 +/- 0.1 after graft revision. Four grafts with low systolic blood flow velocity (range 18 to 40 cm/sec) not revised occluded 5 to 45 days later.(ABSTRACT TRUNCATED AT 250 WORDS)     

Duplex ultrasound assessment of venous diameters, peak velocities, and flow patterns

Duplex ultrasound was used to study the diameters, flow patterns, and peak blood flow velocities of the common femoral vein (CFV) in 12 normal subjects (mean age 35 years). Each subject was supine and non-weight-bearing on a tilt table and rotated in 10-degree increments from -10 degrees (head down) to +30 degrees (head up). Cross-sectional B-mode image was used to monitor continuously CFV diameter for 5 minutes in each position. Doppler flow patterns were recorded in longitudinal axis; heart rate and respiratory movements were also noted. CFV flow was affected by respiratory and cardiac events. At -10 degrees flow was primarily related to cardiac events, with flow increasing during diastole. At +30 degrees flow varied minimally with the cardiac cycle and was primarily respiration dependent, stopping at peak inspiration. Proceeding from -10 to +30 degrees the mean maximal CFV diameter corrected for body surface area increased 92% (0.47 +/- 0.11 cm/m2 to 0.90 +/- 0.16 cm/m2, p less than 0.001), whereas peak flow velocity decreased from 41 +/- 10 cm/sec to 13 +/- 5 cm/sec, p less than 0.001. There was a linear, inverse relationship between mean peak velocity and mean corrected diameter, r = -0.99. The study confirms the multiple influences on venous flow patterns and establishes a quantitative relationship between venous diameters and flow velocities.     

Effect of transducer velocity on intramuscular temperature during a 1-MHz ultrasound treatment

STUDY DESIGN: A 3 x 2 repeated-measures design was used. The independent variables were transducer velocity (2-3 cm/s, 4-5 cm/s, and 7-8 cm/s) and time (pretreatment and posttreatment). OBJECTIVE: To determine if transducer velocity of a 1-MHz ultrasound treatment affects intramuscular tissue temperature. BACKGROUND: Most authors advocate ultrasound transducer velocities of 2 to 4 cm/s within an area of 2 to 3 times the effective radiating area or 2 times the size of the transducer head. However, a much faster rate of application (approximately 7-8 cm/s) is often observed in clinical settings. Methods and Measures: Eleven healthy screened volunteers (9 males, 2 females; mean +/- SD age, 22.6 +/- 1.7 years; mean +/- SD height, 175.7 +/- 13.7 cm; mean +/- SD body mass, 82.5 +/- 19.5 kg) were randomly assigned to a treatment order with all conditions administered during a single testing session. Each transducer velocity condition was administered for 10 minutes, using 1-MHz ultrasound with a 100% continuous duty cycle at an intensity of 1.5 W/cm2 over an area twice the size of the transducer head. After the first treatment, the 2 remaining subsequent velocity conditions were administered after the intramuscular temperature returned to within +/- 0.3 degrees C of the initial pretreatment temperature for 5 minutes. The dependent variable was left triceps surae muscle temperature measured at 3 cm below one half the measured skinfold thickness. Results: Temperature increase across the 3 velocities was within 0.4 degrees C (F2.20 = 0.07, P = .93). Posttreatment values (mean +/- SD) ranged from 42.7 degrees C +/- 2.3 degrees C for the slowest velocity to 43.1 degrees C +/- 1.4 degrees C for the fastest velocity. Temperature increase was significant for time (F1.01 = 155.68, P<.00001), increasing from 37.8 degrees C +/- 0.8 degrees C pretreatment to 42.9 degrees C +/- 1.9 degrees C after treatment. CONCLUSION: Very similar intramuscular temperature increases can be observed among ultrasound treatments (10-minute duration, 1-MHz frequency, 100% continuous duty cycle, 1.5 W/cm2 intensity, within an area twice the size of the transducer head), with transducer velocities of 2 to 3, 4 to 5, and 7 to 8 cm/s.     

Effect of cerebral embolization on regional autoregulation during cardiopulmonary bypass in dogs

BACKGROUND: Embolization during cardiopulmonary bypass probably alters cerebral autoregulation. Therefore, using laser Doppler flowmetry we investigated the cerebral blood flow velocity changes in response to changes in arterial pressure, before and after embolization in a canine bypass model. METHODS: After Institutional Animal Care and Use Committee approval, 8 anesthetized dogs had a laser Doppler flow probe positioned over the temporoparietal dura. During 37 degrees C cardiopulmonary bypass, the cerebral blood flow velocity response to changing mean arterial pressure (40 to 85 mm Hg in random order) was assessed before and after systemic embolization of 100 mg of 97-microm latex microspheres. RESULTS: Before embolization, cerebral blood flow velocity increased 39% as mean arterial pressure increased from 40 to 85 mm Hg. Following embolization, a 94% increase in cerebral blood flow velocity was demonstrated over the same mean arterial pressure range. The slopes of the curves relating cerebral blood flow velocity to mean arterial pressure were 0.21+/-0.74 and 1.31+/-0.87, before and after embolization (p = 0.016) respectively. CONCLUSIONS: Regional cerebral blood flow autoregulation may be impaired by microembolization known to occur during cardiopulmonary bypass, increasing the dependence of cerebral blood flow on mean arterial pressure.     

A low flow velocity predicts failure of femoropopliteal and femorotibial bypass grafts

The prognostic value of Doppler-derived blood flow velocity measurements for predicting the patency of femoropopliteal and femorotibial bypass grafts was analyzed. Peak systolic and end-diastolic blood flow velocities were measured in 42 femorotibial, 24 femoropopliteal, and three femoropopliteal (isolated segment) in situ saphenous vein bypasses at operation and serially in the postoperative period. At operation peak systolic flow velocity was greater (p less than 0.01) in femoropopliteal grafts (90 +/- 22 cm/sec) compared with femorotibial grafts (68 +/- 19 cm/sec) and isolated segment femoropopliteal (58 +/- 16 cm/sec) grafts. Diastolic forward flow, indicative of low outflow resistance, was present in all successful grafts at operation and in the immediate postoperative period, but decreased thereafter. Early graft occlusion was associated with a low peak systolic flow velocity (less than 40 cm/sec) and absent diastolic forward flow. Postoperative decrease in peak systolic velocity to less than 45 cm/sec identified grafts with impending failure due to intrinsic graft lesions or progression of atherosclerosis. A low blood flow velocity threatens graft patency and should prompt an angiographic evaluation to identify correctable graft lesions or an outflow tract suitable for sequential grafting for the purpose of augmenting flow velocity.     

Doppler color flow imaging for demonstrating changes in portal hemodynamic after Hassab procedure]

Doppler color flow imaging (DCFI) was used to observe portal hemodynamic changes in 40 patients with portal hypertension after Hassab procedure. Preoperative portal blood flow was hepatopetal in all patients, and in 30 of them gastric coronary vein was shown, with the hepatofugal blood flow of 235.32 +/- 56.24 ml/min, and the velocity of 8.62 +/- 2.33 cm/sec. Postoperatively, the gastric coronary vein was interrupted in 27 patients, the portal blood flow was significantly increased from 742.62 +/- 141.73 ml/min to 986.82 +/- 134.58 ml/min (P < 0.01) and the velocity was increased from 10.14 +/- 2.13 cm/sec to 13.41 +/- 1.91 cm/sec (P < 0.01). The results showed that the operation can stop abnormal blood flow in the gastrosplenic region and enhance liver perfusion. DCFI is useful for postoperative follow-up and observing portal hemodynamic changes.     

Noninvasive assessment of internal thoracic artery for reoperative coronary artery surgery.

To determine whether previous sternotomy alters internal thoracic artery (ITA) anatomy and flow characteristics, a duplex scanner was used for noninvasive evaluation of the ITA in 59 patients who were scheduled for reoperative coronary artery bypass surgery. The left ITA was insonated through the third intercostal space by use of a duplex scanner (5.0 MHz probe). Measurements of the ITA diameter (mm) and peak systolic velocity (cm/sec) were obtained; ITA flow was calculated from velocity and cross-sectional area. These findings were compared with the values obtained from 105 patients who were scheduled to undergo first-time (primary) coronary artery surgery during the same time period. In the reoperative group, preoperative mean ITA diameter was 2.26 +/- 0.06 mm; this was not significantly different from the primary group's mean ITA diameter of 2.15 +/- 0.04 mm (p = 0.09). Mean peak systolic velocity was 79.9 +/- 2.4 cm/sec and calculated systolic blood flow was 204.6 +/- 13.1 ml/min in the reoperative patients, as compared with 83.3 +/- 2.1 cm/sec and 189.5 +/- 8.6 ml/min in the primary group, respectively. Values were similar in both groups for the peak systolic velocity (p = 0.31) and calculated systolic blood flow (p = 0.32). These results suggest that previous heart surgery or sternotomy does not adversely affect ITA anatomy and flow characteristics. We conclude that ultrasonic imaging is an easily applicable technique for preoperative assessment of ITA in patients who have undergone previous sternotomy.     

Modulation of platelet aggregation after percutaneous transluminal angioplasty of the iliac artery for atherosclerosis obliterans

BACKGROUND: Platelet aggregation is modulated by blood flow. We investigated whether platelet function is altered during percutaneous transluminal balloon angioplasty in patients with atherosclerosis obliterans. METHODS: Blood samples were obtained from the iliac artery in 9 lower limbs of 7 patients undergoing percutaneous balloon angioplasty of the iliac artery. An agonists-induced platelet aggregation test was performed with an aggregometer. Femoral blood flow was measured with a Doppler velocimeter before and after the procedure. RESULTS: Before dilatation, the maximum platelet aggregation rates (+/- SEM) induced by adenosine phosphate, epinephrine, and arachidonic acid were 54.7% +/- 5.8%, 64.8% +/- 4.3%, and 60.5% +/- 6.1%, respectively. After angioplasty, these values reduced to 36.7% +/- 4.1%, 36.1% +/- 8.6%, and 40.1% +/- 5.0%, respectively (P < .05). The pre-procedural ankle-brachial pressure index, mean flow rate, mean velocity, and shear stress variation were 0.63 +/- 0.1, 218.1 +/- 32.1 mL/min, 9.4 +/- 1.1 cm/sec, and 60.6 +/- 17.7 dyne/cm2, respectively. The mean velocity at the stenotic lesion was 215.1 +/- 83.9 cm/sec, which was significantly greater than those of the distal artery or after angioplasty (P < .01). Both ankle-brachial pressure index and shear stress variation increased after angioplasty to 0.99 +/- 0.07 (P < .05) and 139.8 +/- 17.0 (P < .05) dyne/cm2, but the mean flow rate and the mean velocity (198.3 +/- 24.5 mL/min and 8.8 +/- 1.2 cm/sec after angioplasty) did not change significantly. CONCLUSIONS: These results indicate that activated platelet function at a stenosed artery was decreased after angioplasty, possibly because of normalized blood flow with reduction of stenotic lesion.     

Investigation of the Physiological Overflow Effect from Speed-Specific lsokinetic Activity

This article investigates the concept of physiological overflow during speed-specific isokinetic exercise. The idea was tested on 30 volunteers; 14 female, 16 male; mean age 24.5 years, age range 17-29 years; who were subjected to a repeated m o r e s experimental design. Testing involved 5 repetitions of knee extension/flexion at the velocity spectrum speeds of 80, 7 20, 180,240, and 300 degrees /sec preceded by a warm-up at 3 gradient submaximal repetitions and 1 maximal effort at each speed. The experimental treatment consisted of isokinetic exercise of the knee musculature to the level of 50% quadriceps fatigue at the speed of 180 degrees /sec in a format of 3sessions per week over a period of 8 weeks. Analysis of variance testing with repeated measures revealed statistically significant differences in quadriceps and hamstrings function for peak torque, torque acceleration energy, and watts average power at all velocity spectrum speeds over the period of experimentation. It was concluded that a +/-120 degrees /sec physiological overflow of exercise effects to both slower and faster speeds existed during the program of isokinetic activity. J Orthop Sports Phys Ther 1987;9(3):106-110.     

Accuracy of duplex scanning for measurement of arterial volume flow.

This study examined the accuracy of duplex ultrasound measurements of volume flow in a baboon model. Volume flow (Vf) through the external iliac artery was calculated from measurements of blood velocity averaged over several cardiac cycles (time-averaged velocity [TAV]) and vessel cross-sectional area (A) measured from the B-mode image: Vf = TAV x A. Fourteen anesthetized baboons were studied with a duplex scanner with a 7 MHz imaging transducer and 5 MHz pulsed Doppler. B-mode ultrasound measurements of external iliac artery diameters (2.5 +/- 0.2 mm) were used for calculation of cross-sectional area. Timed blood collections obtained through a cannula inserted into the common femoral artery and TAV measurements were obtained simultaneously during 6 to 15-second intervals. These measurements were repeated three to five times per animal with different flow rates each time. Flow rates ranged from 56 to 280 ml/min (170 +/- 54 ml/min). Average velocity was 55 +/- 17 cm/sec. There was no significant difference between the two methods of volume flow measurement (Student t test). Linear regression analysis revealed a high degree of correlation (r = 0.90, slope 0.95, and p = 0.0001). The absolute percentage error was 13% +/- 8%. Volume flow measured by duplex scanning correlates highly with timed blood collections. This method has potential application for the evaluation of diseased arteries and bypass grafts whose rates of flow and waveform patterns are similar to those of this experiment.     

In vitro effects of Nd:YAG laser radiation on blood: A quantitative and morphologic analysis

Use of the Neodymium: yttrium -aluminum -garnet (Nd:YAG) laser to recanalize stenosed arteries may require delivery of the beam through blood. To assess the degree of hemolysis and debris formation, 54 samples of citrated whole blood were exposed to Nd:YAG laser radiation of varying powers (10, 20 and 30 watts) and duration (1, 2.5 and 5 seconds). Compared to control samples which were not subjected to laser light, there was no significant decrease in hematocrit (41 to 40.5 +/- 5%), hemoglobin concentration (13.8 to 13.8 +/- .06 g/1OO ml), or increase in "free" hemoglobin concentration. Debris weight (from .45 +/- .002 to .45 +/- .002 mg), as well as the white blood cell count, was also not significantly changed (from 5,400 to 5,200 +/- 240 WBC/cm). Light microscopy examination of debris from samples of whole blood, washed erythrocytes, and platelet-rich plasma subjected to the laser at 30 watts for five seconds failed to demonstrate the presence of membrane denaturation of blood elements, as compared with the morphologic changes observed in whole blood samples exposed to a "hot tip" rather than Nd:YAG laser radiation. Nd:YAG laser can be used intravascularly without fear of hemolysis or debris "micro-embolization" up to a power of 30 watts for five seconds.     

Laser Doppler monitoring of replants using a small prism probe

Eighteen patients admitted to Duke University Medical Center for upper limb replantation or revascularization were studied. A small laser Doppler flow prism probe and a cutaneous temperature probe were attached to the distal pulp of replanted digits. Measurements were recorded hourly for the first 3 postoperative days. In the successful cases, the mean laser Doppler flow measurement was 1.76 units (lower 97% confidence limit 0.5 units) and the mean temperature was 34.1 degrees C (lower 97% confidence limit 32 degrees C). In 4 patients with vascular compromise, the laser Doppler flow correctly identified the problem. The reduced size of the new laser Doppler probe makes monitoring of distal pulp flow technically easier while maintaining accuracy.     

Laser thermal angioplasty probe ("hot tip") temperature: effects of flow

The temperature developed by the laser thermal ("hot tip") probe during arterial recanalisation is primarily dependent on the rate of energy delivery and the rate of dissipation to the surrounding medium. While higher probe tip temperatures enhance the efficacy of atheroma ablation, so too is the incidence of adverse effects increased. We studied the temperature developed in the probe tip in an artificial circulation using both saline and blood. In saline the peak probe temperatures were limited to 100 degrees C (boiling point), falling with each increment in flow. Small discrepancies in probes at different times and may be due to malalignment of the optical fibre-metal cap coupling, temperature measurement inaccuracy, tip insulation, or generator output instability. In blood, charring and clot formation insulated the tip raising the temperature (up to 700 degrees C within 5 seconds at 10 W) but also retarded dissipation of heat to the surroundings. The degree of clot and char formation was critical in determining subsequent thermal responses in any particular probe. The unknown rate and quantity of char buildup and changing blood flow during in vivo angioplasty are likely to be important obstacles to developing a reliable thermal feedback control system.     

Effectiveness of two radiofrequency ablation systems in atrial tissue.

OBJECTIVE: The efficacy of the left atrial radiofrequency ablation procedure, for the curative treatment of atrial fibrillation, is dependent upon obtaining a confluent transmural line of hyperthermic cellular death. We compare the in vitro effectiveness of obtaining transmural hyperthermic cellular death (>55 degrees C) of both the Osypka single electrode and Boston Scientific Thermaline multi-electrode radiofrequency systems. METHODS: Isolated cadaver porcine hearts were used to measure epicardial temperatures either 'central' or at the 'edge' in relation to an endocardial applied radiofrequency electrode. Reference set point was 70 degrees C, and 4-6-mm thick atrial tissue was used for all applications. 'Edge' temperatures with the Boston Scientific unit were measured whilst activating both adjacent electrodes. RESULTS: Boston Scientific: Probe temperature closely approximated the set point. 'Central' epicardial temperature was lower than probe temperature until after 40 s application (P<0.05), 55 degrees C was reached at 50 s, maximal mean temperature 63.0+/-8.9 degrees C was reached at 100 s. Epicardial 'edge' temperature remained lower than probe temperature for the entire 120 s (P<0.05). Osypka: Probe temperature tended to overshoot the set point. 'Central' epicardial temperature paralleled and occasionally exceeded probe temperature reaching 55 degrees C within 10 s, maximal mean temperature 76.3+/-12.7 degrees C was reached at 10 s and exceeded the set point thereafter. 'Edge' temperature was no different to probe temperature or 'central' epicardial temperature. The mean epicardial temperatures produced with a 65 degrees C set point was no different to that with the 70 degrees C set point, except for a lower final temperature at 60 s. CONCLUSIONS: The Boston Scientific system (70 degrees C set point) requires a minimum in vitro application of 40 s to transmurally increase 4-6 mm atrial tissue temperature above 55 degrees C, and 120-s duration per application would appear to be a reasonable clinical recommendation. The Osypka system transfers thermal energy more effectively, requiring less than 10 s in vitro to achieve a similar transmural temperature, and a 30-s application can be recommended. However, a tendency to overshoot both probe and set point temperature, suggests that a lower set point of 65 degrees C might be safer and as effective.