Evaluation of laser-Doppler flowmetry as a measure of tissue blood flow

In this study the technique of laser-Doppler flowmetry was evaluated for the measurement of tissue blood flow by comparing laser-Doppler flow (LDF) signal in the renal cortex, gracilis muscle, and cremaster muscle of anesthetized rats to whole-organ blood flow measured with an electromagnetic flowmeter or radioactive microspheres. In vitro, LDF signal was closely correlated (r = 0.99) to changes in erythrocyte velocity generated with a rotating wheel. Although individual LDF readings varied in situ, mean LDF signal calculated from multiple readings on the tissue surface were significantly correlated (r = 0.74-0.95) with tissue blood flows measured at various perfusion pressures. However, significant differences in the slope of the LDF signal vs. blood flow relationship were observed in different tissues and with different methods of measurement in the same tissue. This study indicates that mean laser-Doppler flow signal provides a good estimate of tissue blood flow, provided a sufficient number of points is scanned. However, there appears to be no universal calibration factor for the method.     

Monitoring acute skin-flap failure

Reliable and repeatable means for the objective postoperative monitoring of skin flaps is a necessity. If a failing free flap can be recognized early, it can be salvaged by revision of the appropriate anastomoses. For the threatened distal portion of a conventional flap, external factors, such as kinking or hematoma, may be corrected or drug therapy instituted. We have analyzed blood from stab wounds in experimental pig flaps for pO2, pCO2, pH, and hematocrit. The results were compared with fluorescein penetration and flap surface temperature. The most significant finding was hematocrit readings of threatened flaps (54 percent) elevated above those of control flaps (35 percent). pH readings in the jeopardized flaps were 0.4 units below control. These two measures proved to be more reliable than intermittent temperature readings. In contrast to the fluorescein test, which can be used only once, stab wound analysis is repeatable at any time in the postoperative period. It can be effectively used to follow dynamic changes within a skin flap.     

Early circulatory and metabolic events in island skin flaps of the pig

Circulatory and metabolic skin-flap events were studied prior to and up to 6 hours after elevation of buttock island flaps in pigs. During the elevation, significant reductions in superficial skin blood flow, measured by laser Doppler flowmetry (LDF) and dermal flap temperature, were seen. Significant correlations were found between blood flow and temperature. Total flap blood flow, measured as venous outflow, also showed an initial transient decrease, but 2 hours after flap construction, venous outflow had returned to preoperative values. A significant increase in lactate release, together with increased oxygen consumption and glucose uptake, was seen 4 hours after the surgical intervention. Hypoxanthine release, indicating ischemia, was seen only during the first hour after flap elevation. Noradrenaline outflow was noted after 4 and 6 hours, but there was no parallel reduction in flap blood flow. A great deal of the flow reduction in acutely elevated island flaps may thus be due to primary hypothermia rather than to the degenerative release of noradrenaline, which seems to have no early effect on skin flap blood flow. On the other hand, the noradrenaline release may be linked to an increased metabolic activity in the skin flaps.     

Clinical use of amrinone (a selective phosphodiesterase III inhibitor) in reconstructive surgery.

Amrinone is a selective phosphodiesterase III inhibitor that increases cyclic adenosine monophosphate by preventing its breakdown. It is effective in the treatment of congestive heart failure because of its ability to increase myocardial contractility and vascular smooth muscle relaxation. This study was designed to clarify the potential efficacy of amrinone in plastic surgery by clinically assessing its ability to enhance flap blood flow after reconstructive surgery and relieve intraoperative vasospasm. Its effects were compared with those of prostaglandin E1 and lidocaine, which are widely approved agents for improving the hemodynamics of flaps. In the first clinical study, the effects on flap blood flow after flap transfers were investigated. Twenty-six patients underwent reconstructive surgery with vascularized free or pedicled flaps. Blood flow was measured before and 60 minutes after intravenous infusion of lactated Ringer solution (control), amrinone (10 microg/kg/min), or prostaglandin E1 (10 ng/kg/min) using a laser Doppler flowmeter. In the second study, the effects on relief of vasospasm during operation were evaluated. The blood flow of 28 island flaps was measured by laser Doppler flowmetry immediately after flap elevation and 10 minutes after topical application of saline (control), amrinone (5 mg/ml), or lidocaine (10%) to the pedicle in an attempt to resolve the vasospasm. In both clinical studies, the effects of amrinone were statistically no less than those of prostaglandin E1 and lidocaine. The results show that amrinone positively influences the microcirculatory blood flow of transferred flaps and relieves intraoperative vasospasm in clinical cases. The present study suggests that amrinone could be useful for postoperative and intraoperative care in reconstructive surgery.     

Monitoring free flaps using the laser Doppler flowmeter: five-year experience

Over a 5-year period, 232 microvascular composite-tissue transfers to the head and neck, trunk, and extremities were monitored using the laser Doppler flowmeter. Thirteen free flaps (5.6 percent) developed vascular complications, all within 4 days after surgery. The laser Doppler flowmeter detected vascular compromise in all cases with no false positives or negatives. Failure to monitor the flap according to protocol by nursing staff occurred in one patient, which led to a delay in detection of venous compromise and subsequent flap loss. The salvage rate was 69.2 percent, leading to an overall flap viability of 98.3 percent. Our series of free-flap monitoring using the laser Doppler flowmeter is the largest reported to date. Review of the English literature shows consistent support by numerous clinical series for the use of the laser Doppler as a valuable postoperative monitor after free-flap transfers.     

Circulatory and metabolic changes in expanded pig skin flaps

To evaluate circulatory and metabolic changes in pig skin during tissue expansion, the buttock skin of 12 pigs was expanded for 5 weeks. In a second operation, island buttock flaps were elevated bilaterally. Flap temperature, laser Doppler flow (LDF), and fluorescein penetration borders were recorded. Norepinephrine infusions were given twice. After cannulation of the external iliac veins, the total venous outflow from the flaps and metabolic parameters such as glucose and oxygen consumption and lactate production were measured bilaterally. No significant difference in fluorescein staining was found, but laser Doppler flow in the expanded tissue was higher than in nonexpanded skin, whereas the total flap blood flow was not significantly different. A flow reduction was seen in expanded flaps during norepinephrine infusion, whereas nonexpanded flaps showed a slightly increased blood flow. This adrenergic supersensitivity indicates that it is possible that not only surgical nerve section but also tissue expansion can result in sympathetic denervation. No differences in the metabolic parameters were observed.     

Differing flow patterns between ischemically challenged flap skin and flap skeletal muscle: implications for salvage regimens.

In this study, the authors tested the hypothesis that there is a significant difference in spatial patterns of reflow in skin as opposed to skeletal muscle after an ischemic insult. The authors believe that this pathophysiologic difference between the two flap types has significant implications for flap salvage strategies. Bilateral buttock skin flaps (10 x 18 cm) and latissimus dorsi myocutaneous flaps (10 x 20 cm) were elevated in Landrace pigs (n = 7). Flaps on one side of the animal were randomly assigned to 6 hours of arterial occlusion, with the contralateral side acting as control. At 15 minutes, 1 hour, and 4 hours after reflow, radioactive microspheres (15 microm) were injected into the left ventricle. After 18 hours of reperfusion, skin and muscle viability were estimated by intravenous fluorescein and soaking in nitroblue tetrazolium, respectively. Flow rates in the skin with an ischemia-reperfusion injury were significantly reduced (30 to 53 percent), at all time intervals, compared with controls. The flow rate in the fluorescent skin with ischemia-reperfusion injury of the latissimus dorsi flaps (0.037 ml/min/g at 15 min) was greater than in that of the buttock flaps (0.018 ml/min/g). The muscle flaps with ischemia-reperfusion injury had significantly higher flow rates than control muscle flaps at all time intervals studied (at 1 hour, 0.32 ml/min/g compared with 0.16 ml/min/g, respectively). In flap skeletal muscle, an early hyperemic phase during reperfusion maintains a significant blood flow to all regions, including the area of the flap that is destined for necrosis. In flap skin, however, there is a marked decrease in flow rates. These differences have important implications for the intravascular delivery of therapeutic agents to the damaged portions of the flap. Transdermal drug delivery systems should be explored as an alternative to intravascular regimens for the salvage of flap skin with ischemia-reperfusion injury.     

Efficacy of conventional monitoring techniques in free tissue transfer: an 11-year experience in 750 consecutive cases

Conventional free flap monitoring techniques (clinical observation, hand-held Doppler ultrasonography, surface temperature probes, and pinprick testing) are proven methods for monitoring free flaps with an external component. Buried free flaps lack an external component; thus, conventional monitoring is limited to hand-held Doppler ultrasonography. Free flap success is enhanced by the rapid identification and salvage of failing flaps. The purpose of this study was to compare the salvage rate and final outcomes of buried versus nonburied flaps monitored by conventional techniques. This study is a retrospective review of 750 free flaps performed between 1986 and 1997 for reconstruction of oncologic surgical defects. There were 673 nonburied flaps and 77 buried flaps. All flaps were monitored by using conventional techniques. Both buried and nonburied flaps were used for head and neck and extremity reconstruction. Only nonburied flaps were used for trunk and breast reconstruction. Buried flap donor sites included jejunum (n = 50), fibula (n = 16), forearm (n = 8), rectus abdominis (n = 2), and temporalis fascia (n = 1). Overall flap loss for 750 free flaps was 2.3 percent. Of the 77 buried flaps, 5 flaps were lost, yielding a loss rate of 6.5 percent. The loss rate for nonburied flaps (1.8 percent) was significantly lower than for buried flaps (p = 0.02, Fisher's exact test). Fifty-seven (8.5 percent) of the nonburied flaps were reexplored for either change in monitoring status or a wound complication. Reexploration occurred between 2 and 400 hours postoperatively (mean, 95 hours). All 44 of the salvaged flaps were nonburied; these were usually reexplored early (<48 hours) for a change in the monitoring status. Flap compromise in buried flaps usually presented late (>7 days) as a wound complication (infection, fistula). None of five buried flaps were salvageable at the time of reexploration. The overall salvage rate of nonburied flaps (77 percent) was significantly higher than that of buried flaps (0 percent, p<0.001, chi-square test). Conventional monitoring of nonburied free flaps has been highly effective in this series. These techniques have contributed to rapid identification of failing flaps and subsequent salvage in most cases. As such, conventional monitoring has led to an overall free flap success rate commensurate with current standards. In contrast, conventional monitoring of buried free flaps has not been reliable. Failing buried flaps were identified late and found to be unsalvageable at reexploration. Thus, the overall free flap success rate was significantly lower for buried free flaps. To enhance earlier identification of flap compromise in buried free flaps, alternative monitoring techniques such as implantable Doppler probes or exteriorization of flap segments are recommended.     

Deepithelialized turnover flaps in burns

Acute and chronic burns leave behind a full-thickness defect that always requires a flap cover. Such defects are common in electrical burn injuries of the limbs. This paper deals with 35 patients with full-thickness defects following burns in whom deepithelialized turnover dermis flaps and deepithelialized turnover flaps with deep fascia have been used. This flap is an extension of Hynes's reversed dermis graft and Smahel's deepithelialized turnover flap where there is a larger area of blood supply on the deeper aspect of the dermis. If a good hinge is provided for safe blood supply, such a flap settles well in the defect, and cumbersome multistaged procedures can be avoided. If there is less fatty tissue in the area of flap used, then reversed dermis flaps are ideal because split-skin graft take is good. When there is a lot of fatty tissue on the undersurface of dermis, the fascia is also included to make it a reversed fasciocutaneous flap to augment the blood supply and for better split-skin graft survival. Advantages of the procedure and complications are elaborated.     

Continual measurement of intramedullary blood perfusion with laser Doppler flowmetry in intact and ostectomized tibiae of rabbits

Blood flow is important for the healing of bone fractures. Until now, however, there have been no publications on the daily, continual measurement of intramedullary blood perfusion using laser Doppler flowmetry (LDF) in the conscious animal. In this study, a model for the daily, continual measurement of intramedullary blood perfusion by LDF and the temperature near the cortex both in intact and ostectomized tibiae in the conscious rabbit is described. The probes for blood perfusion and temperature measurement were implanted permanently at three different localizations into the right tibia of 10 adult New Zealand White rabbits. The probes were held in place by a bilateral, single-plane external fixator. In five of these animals, a midshaft tibial ostectomy was created in order to simulate a fracture. Intramedullary blood perfusion and temperature were measured daily over 49 days. While in intact tibiae no significant (P > 0.05) differences were found in blood perfusion readings taken at various time points, for mean values or for blood perfusion over time, in ostectomized tibiae the differences were significant: various time points (P = 0.0056), mean values (P = 0.0034) and blood perfusion over time (P = 0.0337). Blood perfusion readings at the centre probe were elevated compared with those at the proximal and distal probes. Thus, a revascularization in the ostectomy gap during the fracture healing was proven by means of the LDF. No influence of the blood perfusion on the temperature in the ostectomy area could be determined during healing of the ostectomy. The described model seems suitable for the continual measurement of intramedullary blood perfusion both in intact and ostectomized tibiae in the conscious rabbit.     

Effects of sodium pentobarbital anesthesia on blood flow in skin, myocutaneous, and fasciocutaneous flaps in swine.

Drug effect on flap blood flow is most commonly determined in anesthetized animals, yet the effect of the anesthetic is often poorly understood. Halothane and nitrous oxide cause profound changes in skin blood flow and thus provide an unsuitable anesthetic technique for use in measuring drug effects on skin and myocutaneous flaps in swine. The goal of this study was to determine the effects of sodium pentobarbital anesthesia on cardiovascular parameters and blood flow in skin, myocutaneous, and fasciocutaneous flaps in pigs. In seven pigs, 7 forelimb skin flaps, 7 forelimb fasciocutaneous flaps, 14 arterial buttock flaps, and 14 latissimus dorsi flaps were created. Blood flow was measured at 2-cm intervals along each flap while the animal was awake and anesthetized. A cardiac depressant effect of pentobarbital was observed, but no change in blood flow could be demonstrated in control skin or control muscle. However, pentobarbital did significantly increase blood flow in all viable portions of arterial and random skin flaps, fasciocutaneous flaps, and the cutaneous segments of the latissimus dorsi flap. These demonstrated effects of pentobarbital should be taken into consideration in designing and analyzing studies of flap blood flow in the acute postoperative phase.     

Fat necrosis in free transverse rectus abdominis myocutaneous and deep inferior epigastric perforator flaps

A series of 310 breasts reconstructed by a single surgeon using free transverse rectus abdominis myocutaneous (TRAM) and deep inferior epigastric perforator (DIEP) flaps was reviewed to see if there were any differences in the incidence of fat necrosis and/or partial flap loss between the two techniques. During the study period, 279 breasts were reconstructed with free TRAM flaps and 31 breasts were reconstructed with DIEP flaps. In the breasts reconstructed with free TRAM flaps, the incidence of partial flap loss was 2.2 percent and the incidence of fat necrosis was 12.9 percent. The DIEP flaps were divided into two groups. For the first eight flaps, patients were selected using the same criteria normally used to choose patients for free TRAM flaps. In this unselected early group, the incidence of partial flap loss was 37.5 percent and the incidence of fat necrosis was 62.5 percent. Because of the high incidence of partial flap loss and fat necrosis in the first eight flaps, subsequent selection was modified to limit the use of DIEP flaps to patients who had at least one sufficiently large perforator in each flap (a palpable pulse and a vein at least 1 mm in diameter) and who did not require more than 70 percent of the flap to create a breast of adequate size. In this later (selected) group, fat necrosis (17.4 percent) and partial flap loss (8.7 percent) were reduced to a level only moderately higher than that found in the free TRAM flap group. From these data, it can be concluded that the incidence of partial flap loss and fat necrosis is higher in DIEP flaps than in free TRAM flaps, probably because the blood flow to the former flap is less robust. This difficulty can be circumvented to some extent, however, by careful patient selection. Factors that should be considered include tobacco use, size of the perforators (especially the vein), and (in unilateral reconstructions) the amount of flap tissue across the midline needed to create an adequately sized breast. If these factors are properly considered when planning the operation, fat necrosis and partial flap loss can be reduced to an acceptable level. For selected patients, the DIEP flap is an excellent technique that can obtain a successful, autologous tissue breast reconstruction with minimal donor-site morbidity. For patients who are not good candidates for reconstruction with this flap, the free TRAM flap remains a good alternative.     

The coincidence of TRAM flaps and prostheses in the setting of breast reconstruction

It is well known that transverse rectus abdominis myocutaneous (TRAM) flaps can be used to replace unsatisfactory prosthetic breast reconstructions; however, little has been written about the scope of breast implant use in TRAM flap patients. In this study, to ascertain the range of such therapeutic options, their frequency, and their clinical outcomes, the authors retrospectively reviewed the senior author's breast reconstruction experience from 1989 to 2000 with patients in whom both a TRAM flap and an implant were used for breast reconstruction. The authors examined the surgical indications, body habitus, bra size, chest wall irradiation history, flap type, implant type, complications, and outcomes for those patients with TRAM flap and breast implant combinations.Thirty-two women who had 50 (various) combinations of a TRAM flap and a breast implant were identified. There were more clinical scenarios than patients because many of the women had multiple scenarios. The 50 combination scenarios were then divided into six groups. Group I consisted of 14 patients who had elective prostheses placed beneath simultaneous TRAM flaps; group II consisted of 10 patients who had TRAM flaps with contralateral prosthetic reconstruction (in which two implants were received before the TRAM flaps, five implants were received simultaneously with the TRAM flaps, and three implants were received after the TRAM flaps); group III consisted of eight patients who had contralateral augmentation in addition to their TRAM flaps; group IV consisted of 11 patients who had TRAM flaps that were used to cover or replace previous prosthetic reconstructions; group V consisted of four patients in whom prostheses were used to augment or improve previous TRAM flap reconstructions; and group VI consisted of three patients who required prostheses to either reconstruct or salvage total or near-total TRAM flap failures. A broad range of implant types was used, although anatomic saline implants predominated. Forty-one percent of the patients in the review had undergone irradiation during the course of their treatment for breast cancer. Eight of the 32 patients experienced a total of twelve complications, four of which were related to the implants and eight of which involved the TRAM flaps and abdominal donor sites.Although complex, the wide variety of potential TRAM flap/breast implant combinations can be useful for patients with challenging reconstructive scenarios, particularly those that involve radiation therapy. In the group of patients reviewed by the authors, TRAM flaps were most often used in successful partnership either on the same side as or opposite to an implant reconstruction. A TRAM flap was used to salvage or replace an unsatisfactory implant reconstruction in less than a third of the patients. From a risk point of view, implants used opposite a TRAM flap reconstruction had a lower incidence of complication than did implants used beneath TRAM flaps.     

Monitoring muscle viability using evoked M waves

The experiments described reveal the direct relationship between blood flow and evoked electrical activity in muscle flaps. It is demonstrated that monitoring of EMWs will detect vascular occlusion to a muscle flap within 1 hour. Detecting failure this soon provides the surgeon an opportunity for reexploration and salvage of a muscle flap before irreversible change has occurred. Correlations with observed muscle contraction and intramuscular temperature changes are also made.     

Establishment of normal ranges of laser Doppler blood flow in autologous tissue transplants.

Over a 3-year period, 136 patients were monitored following free autologous tissue transplantation using a laser Doppler flowmeter linked to a computerized data-acquisition system. This monitoring system has indicated perfusion compromise in free flaps more rapidly than clinical observation alone. Most important, this has resulted in an increase in salvage rate from 50.0 to 82.4 percent. In addition, our overall success rate has increased from 92.6 to 97.8 percent since introducing this monitor clinically. Computerization also has facilitated the collection of data, which has enabled us to establish expected values for postoperative blood flow in several types of donor tissues used for microvascular reconstruction. Finally, this computerized monitoring system has relieved personnel from basing decisions on subjective data.     

Involvement of sympathetic nerve activity in skin blood flow oscillations in humans

We have used the wavelet transform to evaluate the time-frequency content of laser-Doppler flowmetry (LDF) signals measured simultaneously on the surfaces of free microvascular flaps deprived of sympathetic nerve activity (SNA), and on adjacent intact skin, in humans. It was thereby possible to determine the frequency interval within which SNA manifests itself in peripheral blood flow oscillations. The frequency interval from 0.0095 to 2 Hz was examined and was divided into five subintervals: I, approximately 0.01 Hz; II, approximately 0.04 Hz; III, approximately 0.1 Hz; IV, approximately 0.3 Hz; and V, approximately 1 Hz. The average value of the LDF signal in the time domain as well as the mean amplitude and total power in the interval from 0.0095 to 2 Hz and amplitude and power within each of the five subintervals were significantly lower for signals measured on the free flap (P < 0.002). The normalized spectral amplitude and power in the free flap were significantly lower in only two intervals: I, from 0.0095 to 0.021 Hz; and II, from 0.021 to 0.052 Hz (P < 0.05); thus indicating that SNA is manifested in at least one of these frequency intervals. Because interval I has recently been shown to be the result of vascular endothelial activity, we conclude that we have identified SNA as influencing blood flow oscillations in normal tissues with repetition times of 20-50 s or frequencies of 0.02-0.05 Hz.     

Fasciocutaneous flaps: an experimental model in the pig

No experimental studies have substantiated the claim that fasciocutaneous flaps are superior to skin flaps. Using fasciocutaneous flaps designed in the pig, both flap survival and blood flow were assessed. The forelimb and hindlimb fasciocutaneous flaps survived to 8.2 +/- 0.3 cm and 7.9 +/- 0.3 cm, respectively, compared with 7.3 +/- 0.3 cm and 6.7 +/- 0.3 cm for the comparable cutaneous flaps, a statistically significant finding (p less than 0.01). Random fasciocutaneous flaps survive 12 to 18 percent longer than skin flaps. Using the radioactive microsphere technique, blood flow was measured after flap elevation, and flap survival was estimated using fluorescein. Again, a significant difference in flap survival was found, but there was no significant difference in measured blood flow. This can be explained by the relatively large interval between blood flow measurements (2 cm) compared with the observed difference in survival length (1.0 +/- 0.3 cm).     

Electromagnetic flowmetry--an experimental method for continuous blood flow measurement using a new island flap model

We describe a reliable experimental method for direct, continuous measurement of the rate of blood flow in an island skin flap, using an electromagnetic flowmeter applied to the artery of the flap. The canine saphenous island flap model, developed as part of this study, is a large (11 x 14 cm) island flap, based solely on the saphenous artery (2 mm in diameter). We describe the anatomy of the flap and the method of raising it. Electromagnetic flowmetry is the only method of blood flow determination that provides immediate, continuous, and quantitative measurement of flow. We describe the principles and pitfalls of the electromagnetic flowmeter and the numerous rules of practice that must be observed to obtain consistent results. A validation study was carried out, in which we simultaneously measured arterial inflow with the flowmeter and venous outflow from the flap using direct collection. This study was based on the assumption that at any point in time the arterial and venous rate of flow were equal. We present the results of the study, which show an extremely close linear relationship between the measured and actual rates of flow. We intend to use this experimental method to study the factors that affect the rate of blood flow in free skin flap transfer.     

Myocutaneous flap ischemia: flow dynamics following venous and arterial obstruction.

To further clarify the pathogenesis of the poorer prognosis in skin flaps exposed to venous stasis compared with arterial insufficiency, a microsphere study was conducted in bilateral rectus abdominis island flaps in seven pigs. The relationship between capillary blood flow and arteriovenous (A-V) shunting was studied during progressive 1-hour intervals of arterial insufficiency and venous stasis and during 3 hours of reperfusion. Under controlled conditions, total blood flow was reduced from 100 percent to both 50 and 25 percent by application of an adjustable clamp on the artery supplying one flap and on the vein draining the contralateral flap. The relative distribution between A-V shunt flow and capillary blood flow was different in arterial insufficiency when compared with venous stasis at both the 50 percent and the 25 percent blood flow levels. In the arterial insufficiency flaps, the A-V shunt flow and capillary blood flow shared the total blood flow in the following percentages: 64/36 (at 100 percent total blood flow), 44/56 (at 50 percent total blood flow level), and 22/78 (at 25 percent total blood flow level). In the venous stasis flaps, the A-V shunt flow and the capillary blood flow shared the total blood flow in percentages of 70/30, 66/34, and 55/45, respectively. Hence, in arterial insufficiency flaps, capillary blood flow was spared by a relatively greater decline in A-V shunting compared with venous stasis flaps. Redistribution of capillary blood flow from subcutaneous tissue to muscle was observed, whereas blood flow was equally distributed throughout the length of the flaps at all flow levels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Measurement of airway wall blood flow in sheep by laser-Doppler flowmetry: interpretation and problems

We have used laser-Doppler flowmetry (LDF), a technique that detects movement of erythrocytes, to measure tracheal and bronchial wall blood flow in anesthetized open-chest sheep. LDF derives continuous measurements noninvasively, although fiber-optic bronchoscopy is necessary to introduce the LDF probe into the airways. The response of the LDF flow signals at four regions of the airway walls to varying bronchial arterial flow rates was examined in both live and dead sheep by cannulation and subsequent perfusion of the common bronchial artery at different flow rates by use of a roller pump. In the live sheep, variations in bronchial arterial blood flow resulted in variations in LDF signals in the principal bronchus and in lobar and segmental bronchi but not in the trachea. In the dead sheep, variations in bronchial arterial blood flow resulted in variations in LDF signals in all four regions. Within regions, the average response of the LDF signals to varying bronchial blood flow rates was approximately linear in both live and dead sheep, but considerable site-to-site variation in response was observed. In the live sheep, significant LDF signals were observed when the bronchial arterial flow was set to zero and when the bronchial artery was perfused with dextran solution, which would in theory be expected to produce no LDF signal. A small LDF signal was also detected under zero flow conditions in the dead sheep. These observations suggest that the LDF technique, in addition to detecting blood flow from the bronchial artery also detects background noise and/or collateral circulation.(ABSTRACT TRUNCATED AT 250 WORDS)