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Blood flow autoregulation in pedicled flaps
Journal of Plastic, Reconstructive & Aesthetic Surgery (2009) 62, 1671e1676
Blood flow autoregulation in pedicled flaps* Christian T. Bonde a,*, Niels-Henrik Holstein-Rathlou b, Jens J. Elberg a a
Department of Plastic Surgery and Burns Unit, Center of Head and Orthopedics, Copenhagen University Hospital, Rigshospitalet, Blegdamsvej 9, DK-2100 Copenhagen Ø, Denmark b Department of Medical Physiology, The Panum Institute, University of Copenhagen, DK-2200, Copenhagen N, Denmark Received 26 November 2007; accepted 25 July 2008
KEYWORDS Microcirculation; Calcium channels; Nimodepine; Papaverine; Pig
Summary Introduction: Clinical work on the blood perfusion in skin and muscle flaps has suggested that some degree of blood flow autoregulation exists in such flaps. An autoregulatory mechanism would enable the flap to protect itself from changes in the perfusion pressure. The purpose of the present study was to evaluate if, and to what extent, a tissue flap could compensate a reduction in blood flow due to an acute constriction of the feed artery. Further, we wanted to examine the possible role of smooth muscle L-type calcium channels in the autoregulatory mechanism by pharmacological intervention with the L-type calcium channel blocker nimodipine and the vasodilator papaverine. Material and methods: Pedicled flaps were raised in pigs. Flow in the pedicle was reduced by constriction of the feed artery (n Z 34). A transit time flow probe measured the effect on blood flow continuously. Following this, three different protocols were followed: (1) Time control (n Z 10): the procedure described above was repeated in the same flap to determine whether autoregulatory efficiency changed over time. (2) Nimodipine infusion (n Z 13): continuous intra-arterial infusion of nimodipine (0.2 mg/ml, 0.5 ml/min) started when the flow had returned to the initial value. After stabilisation, the flow was reduced. When the flow had been stable for at least 5 min, the constriction was removed. (3) Nimodipine and papaverine (n Z 8): the infusion of nimodipine was followed by an intra-arterial bolus of papaverine (10 mg). After stabilisation, the flow in the pedicle was reduced and the flow was recorded. Results: The flaps showed a strong autoregulatory response with complete compensation for flow reductions of up to 70e80%. Infusion of nimodipine caused a 28 10% increase in blood flow and removed the autoregulation. Papaverine caused a further increase in blood flow by 61 19%. The time control experiments proved that the experimental procedure was reproducible and stable over time. Conclusions: A tissue flap can nearly completely compensate for repeated flow reductions of up to 70e80%. This is due to a decrease in the peripheral resistance, mediated by a local intrinsic mechanism. Nimodipine (a blocker of L-type voltage-activated calcium channels) abolishes the autoregulation, but a significant vasodilatory reserve exists, as an additional
*
Presented at the IV Congress of the World Society for Reconstructive Microsurgery, WSRM, Athens, Greece, June 24e26, 2007. * Corresponding author. Tel.: þ45 35458963, þ45 32542333 (private); fax: þ45 35452667. E-mail address: [email protected] (C.T. Bonde).
1748-6815/$ - see front matter ª 2009 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.bjps.2008.07.039
1672
C.T. Bonde et al. injection of papaverine (a smooth muscle relaxant) results in a further increase in the blood flow. This strongly suggests a direct role for voltage-activated calcium channels in the autoregulatory process. ª 2009 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved.
In reconstructive plastic surgery, free vascularised flaps are a common tool, designed to move tissue from one part of the body to a defect in another. Success rates for free tissue transfers of more than 95%1,2 are common, but many questions about the regulation of blood flow in tissue flaps remain unanswered. What is the minimal perfusion needed for a flap to survive? To what extent can a tissue flap compensate for constrictions or vasospasms in the feed artery? If we are to further improve our treatment outcomes answers to these and other questions regarding regulation of blood flow in flaps are required. It is well known that blood flow to both the brain and the kidney are subject to autoregulation. Despite a wide variation in the perfusion pressure, blood flow to these organs is maintained at a nearly constant level. The constancy of blood flow is due to autoregulatory mechanisms intrinsic to the organ or tissue, which mediate compensatory changes in vascular resistance in response to changes in perfusion pressure. Despite much research the mechanisms underlying autoregulation are still not fully resolved. The issue is further complicated by the fact that the different mechanisms seem to vary in significance between different vascular beds. Recently published clinical work on perfusion in skin and muscle flaps has shown that some form of blood flow autoregulation seems to exist in tissue flaps.3,4 Despite a 50% reduction in the diameter of the pedicle artery due to vasospasm, arteriolar blood flow was maintained at the normal level.3 The purpose of the present study was to evaluate if, and to what extent, a tissue flap consisting of skin and muscle could compensate for a reduction in blood flow due to an acute constriction of the feed artery. Further, we wanted to examine the possible role of smooth muscle L-type calcium channels in the autoregulatory mechanism by pharmacological intervention with the L-type calcium channel blocker nimodipine and the vasodilator papaverine.
Materials and methods Animal model The pedicled rectus abdominis musculocutaneous flap is well described in the pig, and is an accepted model of the human counterpart.5,6 Adult LYD (Landrace, Yorkshire, Duroc) pigs weighing 50 kg were used for the experiment. The animals were kept in a standard environment 1 week prior to the procedures. The animals did not receive any food or water the night before surgery. The animals were pre-anaesthetised with 25 mg midazolam. They were then anaesthetised using 1.5 mg/kg/h Propofol and 250 mg/h of Fentanyl. Intra-arterial blood pressure was monitored continuously. The experiments were carried out according
to the declaration of Helsinki II (Convention for Animal Experiments) and the national rules for the care and use of laboratory animals. The experimental protocol was approved by the Danish Animal Experiments Inspectorate. Bilateral musculocutaneous pedicled flaps were raised, based on the superior epigastric artery and vein, as shown in Figure 1. The sympathetic nerve fibres around the vessels were removed during skeletonisation of the pedicle. A side branch of the main artery was spared and cannulated with a polypropylene tube (PP10). A 7/0 suture was tied around the side branch and tube, keeping the tube in place. This tube was connected to an infusion pump allowing for direct intra-arterial administration of drugs.7 Finally, a small rubber loop was placed around the feed artery allowing a controlled and maintained constriction of the artery.
Transit time blood flow measurements Arterial blood flow was measured continuously throughout the experiments by a transit time flowmeter (Medi-Stim Butterfly Flowmeter BF 2004). A pre-calibrated transit time flow probe (Medi-Stim AS, Oslo, Norway; 2, 2.5 or 3 mm) was placed around the feed artery distal to the site of constriction. Special care was taken to avoid the flow probe causing compression or kinking of the artery.
Experimental set up When arterial blood flow had been stable for 5 min, blood flow in the feed artery was reduced by constriction of the rubber loop. When blood flow had been stable for an additional 5 min, the constriction was removed, and the effect of this recorded. This procedure was performed in 34 flaps. Following this, three different protocols were followed: (1) Time control (10 flaps): when blood flow had stabilised following removal of the constriction, the entire procedure described above was repeated to determine whether autoregulatory efficiency changed over time. (2) Nimodipine infusion (13 flaps): continuous intra-arterial infusion of nimodipine (0.2 mg/ml given at 0.5 ml/min) was started when flow had returned to the initial value. After stabilisation, the flow in the pedicle was reduced again. When the flow had been stable for at least 5 min, the constriction was once again removed. (3) Nimodipine and papaverine (eight flaps): the infusion of nimodipine was followed by an intra-arterial bolus of papaverine (10 mg). After stabilisation, the flow in the pedicle was once again reduced by constricting the loop and the flow recorded.
Statistics Comparisons of flow values were performed with consecutive paired Student’s t-tests and P values less than 0.05 were
Flap autoregulation
1673
Figure 1 The experimental set up. Shown are the bilateral flaps and the relationship between the infusion system, the constriction and the flow probe.
considered statistically significant. Correlation analysis between initial reduction and final compensation were done using standard methods. All data are given as mean SE.
Results The recorded flow values for constriction and release in a typical experiment are shown in Figure 2. Initially, the blood flow is stable. Constriction of the feed artery and the subsequent release of the constraint results in four distinct phases: a reduced flow followed by a gradual return to the control value (compensation); and following the release there is hyperaemia with a gradual return of blood flow towards the control value. The results in the control experiments are summarised in Figure 3. The constriction reduced the flow to 46 3% (P < 0.05) (n Z 34) of the control value. After approximately 3 min (range 1 to 42 min) the blood flow had returned to 94 2% (NS) of the control value. Following the removal of the constriction, hyperaemia ensued with an increase in flow to 126 3% (P < 0.05) of control. Finally, after a few minutes blood flow returned to a value not significantly different from that of the control (100 0%) (NS).
There was no statistically significant correlation (r2 Z 0.03, NS) between the degree of reduction in flow and the compensation achieved, although there was a tendency for the compensation to be lowest at the most pronounced degrees of flow reduction (Figure 4). The time control experiments (n Z 10) showed the experimental procedure to be reproducible and stable over time (Figure 5). Thus, it was possible to achieve nearly identical constrictions in the two experimental periods, and the degree of compensation achieved was similar in the two periods. Continuous intra-arterial infusion of nimodipine caused a 28 10% (P < 0.05) (n Z 13) increase in blood flow (Figure 6). Following acute reduction of blood flow by 34 3% (P < 0.05) (n Z 13), the blood flow remained at the reduced level, and consequently no compensation was observed. When the constriction was released, flow returned to a value not significantly different from the preconstriction value. In eight of the flaps nimodipine infusion was followed by an intra-arterial injection of papaverine. This caused a further 61 19% (P < 0.05) increase in blood flow (Figure 6). When flow was reduced acutely by 35 5% (P < 0.05) (n Z 8) it remained low during the period of
Figure 2 Recordings of blood flow in a typical experiment. Left panel: Initially, the blood flow is stable at 17 ml/min. Flow is reduced at w30 s and compensation starts. Right panel: Following the release, the hyperaemic phase (24 ml/min) appears before a gradual return of blood flow towards the control value.
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Figure 3 Control experiment (n Z 34) showing the initial reduction in blood flow upon arterial constriction (Reduction) followed by a compensatory phase, and the subsequent hyperaemia after release of constriction followed by a return of the blood flow to the control value (Final). Means SE.
constriction, and no compensation was observed. Again, when the constriction was removed, flow returned to the pre-constriction value (Figure 6). The papaverine-induced increase in blood flow lasted until the experiments were terminated.
Discussion The present study shows that pedicled flaps have a considerable autoregulatory potential. Despite acute reductions of blood flow in the range from 30 to 80%, blood flow was nearly completely normalised after a few minutes. The autoregulatory process appears to involve L-type voltageactivated Ca2þ channels as it was completely inhibited by intra-arterial infusion of the calcium channel blocker nimodipine. Detailed knowledge of the physiology and haemodynamics governing the perfusion of musculocutaneous
Figure 4 Scatter plot showing corresponding values of initial reduction and final compensation of blood flow (n Z 34).
C.T. Bonde et al. pedicled or free flaps is limited. That a flap may have autoregulatory potential was suggested by a clinical study by Lorenzetti et al.4 They examined the blood flow changes that occurred in a free transverse rectus abdominis myocutaneous (TRAM) flap when transferred to either the thoracodorsal artery or the internal mammary artery.4 The average blood flow in the donor artery before flap harvest (inferior epigastric artery) was 11 ml/min. When the flap was transferred to the thoracodorsal artery, which had a blood flow of 5 ml/min, flap flow increased to 14 ml/min. When transferred to the internal mammary artery, with a blood flow of 25 ml/min, flow decreased to 12 ml/min. The authors concluded that flap flow is independent of the flow in the recipient vessel, but dependent on the tissue components and requirements of the flap. Similar findings have been published in another study of muscle flaps and radial forearm flaps.8 The results of the present experimental study confirm and extend these observations. When blood flow was reduced acutely over a wide range of values, we saw a nearly complete return of blood flow to the control value. In fact, the flaps could compensate for even a 70e80% reduction in blood flow. This shows a remarkable autoregulatory potential in the isolated flap. In fact, the average blood flow compensation of 94% found in this study is comparable to the 93% reported in the kidney.9 In comparison, autoregulation of blood flow in the rat hindlimb, consisting primarily of skin and muscle, only reached about 35%.9 We were not able to detect a lower limit for autoregulation, although there did appear to be some loss of compensation when flow was reduced to 20% of control (see Figure 4). However, technical difficulties prevented us from achieving stable constrictions lower than that, and it is therefore not possible from the present results to determine the lower limit of autoregulation. The time needed for the autoregulatory response to stabilise varied from less than 1 min to 42 min for the slowest. In 75% of the flaps, full autoregulatory response was achieved within 10 min, with a median of 3 min for all experiments. We speculate that this difference in autoregulatory response rate could be caused by different contributions from the autoregulatory components. Autoregulation in general is viewed as being composed of a fast myogenic component and a slower metabolic component. The wide range of variation could be caused by a difference in the balance of the myogenic and metabolic components. Thus, in a flap with a dominant metabolic component, the autoregulatory response would be expected to be slower than that in a flap with a less dominant metabolic component. This is illustrated in Figure 2, and was observed to a lesser or greater degree in all flaps. After 30 sec, the flow reduction is performed (Figure 2). In the following 30 seconds the flow is quickly autoregulated until it reaches about 80% of the original value. This likely represents the myogenic response. Following this, a slower phase appears where the remaining 20% is regulated, the metabolic response. Further studies are needed to elucidate these interesting observations. At present the mechanism(s) underlying autoregulation is poorly understood. The finding of an increased blood flow after transfer of a free flap has been ascribed to the sympathectomy relieving arteriolar vasoconstriction, thus causing vasodilation and increased perfusion, but this fails
Flap autoregulation
Figure 5 The time control experiments showing the experimental procedure to be reproducible and stable over time. Black columns represent the initial experimental period; the grey columns represent the second experimental period (n Z 10). *A value significantly different from the control value (P < 0.05).
to explain how the flap down regulates the blood flow when anastomosed to a high flow recipient artery. The observation in the present study that the calcium channel blocker nimodipine completely abolishes autoregulation indicates that voltage-activated L-type calcium channels may be involved in the process. Intracellular calcium is a key regulator of vascular smooth muscle contraction, and changes in calcium influx through calcium channels cause corresponding changes in smooth muscle tone.10 Studies in pressurised cerebral arterioles have shown that a decrease in perfusion pressure is associated with a hyperpolarisation of the cell membrane and, thus, with a closure of voltageactivated calcium channels. The reduced calcium influx will lead to a relaxation of the vascular smooth muscle cells and, consequently, to vasodilation.11,12 The present results
Figure 6 The effect on flow and flow compensation by continuous nimodipine infusion and bolus papaverine injection. Both drugs result in an increase of blood flow and abolish autoregulation. Con: control period; Nim: infusion of nimodipine; Red: flow reduction; Comp: compensation; Rel: release of arterial constriction; Pap: bolus injection of papaverine.
1675 suggest that a similar mechanism may operate in skin and muscle flaps. Nimodipine caused a significant vasodilation when administered intra-arterially, and it could be argued that the lack of autoregulation following nimodipine was simply due to the vessels already being maximally vasodilated. However, this was clearly not the case, since intra-arterial injection of papaverine caused a further significant increase in blood flow. This shows that the vessels maintained significant tone despite the administration of nimodipine. It therefore appears that the inhibitory effect of nimodipine on autoregulation was not due to the vasodilation per se, but rather to the specific action on the calcium channels. The vasodilatory actions of nimodipine and papaverine show that, despite the sympathectomy of the flap vessels, they still maintain significant tone. A similar result was reported by Banic et al., in an experimental model comparable to ours, when local injection of sodium nitroprusside into the flap increased total flap flow by 20% and decreased the vascular resistance by 20%.13 The nimodipine dose used in this experiment was calculated to achieve a plasma concentration similar to the therapeutic range in the treatment of hypertension (1e10 mM). The systemic effect of the L-type calcium antagonists is vasodilation. However, due to the local intra-arterial application of the agent, we observed no effect on CVP (Central venous pressure) or CO (Cardiac output). All L-type calcium antagonists (e.g. amlodipine, felodipine, isradipine, lacidipine, lercanidipine, nifedipine, nitrendipine) would be expected to have a similar effect on flap blood flow. The pedicled flap is an inhomogeneous tissue containing skin, subcutaneous fat and striated muscle. The present study does not allow any conclusions as to the degree of autoregulation in the different constituent tissues. However, in a recent study, Wettstein et al. found that even when the diameter of the pedicle artery was reduced to 50% of its original value, blood flow was restored to normal levels in the anatomically perfused arterioles, and this was associated with an increased diameter of third order arterioles (A3).3 The study also showed that flap areas which already had impaired flow did not posses the ability to restore flow levels when further reduced by vasospasm. Together with our observation of preserved total flow, the observation of preserved flow at the level of the individual vessels suggests good autoregulation in all the constituent tissues of the flap. Other groups have reported experiments with partial flow reductions and staged compressions of the vascular pedicle in animal models.14 Hjortdal et al. showed that at a 50% reduction in arterial blood flow, capillary blood flow was spared by a decline in A-V shunting.15 In a study of critical flow values in a rat cremaster muscle, Nanhekhan and Siemionow found that a 60% flow reduction was incompatible with long-term survival.16 Neither group reported autoregulation, but the failure to observe autoregulation is most likely due to differences in models and techniques used for flow reductions. For example, Hjortdal et al. performed the flow reductions in a way that prevented autoregulation, as they continuously constricted the artery until blood flow (venous output) reached a predetermined level. However, it has been shown that a 180 kinking of the artery has no influence on survival
1676 rate,17 probably because the reduction in flow is compensated by an autoregulatory mechanism. The clinical implication of the present study is that muscle flaps may be relatively resistant towards even quite pronounced vasospasm or surgically-induced constrictions at the site of vascular anastomosis. We did not find a lower limit of autoregulation despite up to an 80% reduction of flow. Whether the autoregulatory response is maintained for longer time periods (i.e. hours or days) cannot be determined from the present study. To answer this important question longer lasting studies are necessary. Several observations, however, suggest that flaps may not only show acute compensation, but that the vasodilatory processes may continue over several days or weeks. Thus, blood flow in a free flap increases in the postoperative period,18 and the postoperative arterial blood flow in the TRAM flap has been shown to increase from the fourth to the 30th day.19 In a prospective clinical study using free latissimus dorsi (LD) muscle flaps, Lorenzetti et al.20 demonstrated that blood flow in the pedicle and in the recipient artery of a free muscle flap increases after surgery. The same group also published similar findings for the free lower limb muscle flaps both with and without vein grafts.21,22 In conclusion, this is the first investigation of blood flow autoregulation in an experimental skin and muscle flap model. A tissue flap can nearly completely compensate for flow reductions of up to 70e80%. This is due to a decrease in peripheral resistance, mediated by a local intrinsic mechanism. Nimodipine (a blocker of L-type voltage-activated calcium channels) abolishes autoregulation, but a significant vasodilatory reserve exists, as additional injection of papaverine (a smooth muscle relaxant) results in a further increase in blood flow. This strongly suggests a direct role for voltage-activated calcium channels in the autoregulatory process.
Acknowledgements This work was supported by The L.F. Foghts Fund, The Sophus Jakobsen and wife Astrid Jakobsens Fund, The Danish Medical Association Research Fund/The Vibe A. Linholter Estate, The Jacob Madsen and wife Olga Madsens Foundation, The P. A. Messerschmidt and wife Fund and by the University of Copenhagen.
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References 20. 1. Schusterman MA, Kroll SS, Miller MJ, et al. The free transverse rectus abdominis musculocutaneous flap for breast reconstruction: one center’s experience with 211 consecutive cases. Ann Plast Surg 1994;32:234e41. 2. Blondeel PN. One hundred free DIEP flap breast reconstructions: a personal experience. Br J Plast Surg 1999;52:104e11. 3. Wettstein R, Wessendorf R, Sckell A, et al. The effect of pedicle artery vasospasm on microhemodynamics in
21.
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anatomically perfused and extended skin flap tissue. Ann Plast Surg 2000;45:155e61. Lorenzetti F, Kuokkanen H, von Smitten K, et al. Intraoperative evaluation of blood flow in the internal mammary or thoracodorsal artery as a recipient vessel for a free TRAM Flap. Ann Plast Surg 2001;46:590e3. Kerrigan CL, Zelt RG, Thomson JG, et al. The pig as an experimental animal in plastic surgery research for the study of skin flaps, myocutaneous flaps and fasciocutaneous flaps. Lab Anim Sci 1986;36:408e12. Cordeiro PG, Santamaria E, Hu QY, et al. Effects of vasoactive medications on the blood flow of island musculocutaneous flaps in swine. Ann Plast Surg 1997;39:524e31. Nalbantoglu U, Kusza K, Chick L, et al. Harmful effects of invasive animal monitoring on muscle flap microcirculation. Ann Plast Surg 1996;37:367e74. Lorenzetti F, Suominen S, Tukiainen E, et al. Evaluation of blood flow in free microvascular flaps. J Reconstr Microsurg 2001;17:163e7. Just A, Arendshorst WJ. Nitric oxide blunts myogenic autoregulation in rat renal but not skeletal muscle circulation via tubuloglomerular feedback. J Physiol 15-12-2005;569:959e74. Ogut O, Brozovich FV. Regulation of force in vascular smooth muscle. J Mol Cell Cardiol 2003;35:347e55. Harder DR. Pressure-dependent membrane depolarization in cat middle cerebral artery. Circ Res 1984;55:197e202. Harder DR, Gilbert R, Lombard JH. Vascular muscle cell depolarization and activation in renal arteries on elevation of transmural pressure. Am J Physiol 1987;253:F778e81. Banic A, Krejci V, Erni D, et al. Effects of sodium nitroprusside and phenylephrine on blood flow in free musculocutaneous flaps during general anesthesia. Anesthesiology 1999; 90:147e55. Cummings CW, Trachy RE. A model for staged compression of the vascular pedicle in a porcine myocutaneous flap. Head Neck Surg 1985;7:212e6. Hjortdal VE, Hansen ES, Hauge E. Myocutaneous flap ischemia: flow dynamics following venous and arterial obstruction. Plast Reconstr Surg 1992;89:1083e91. Nanhekhan LV, Siemionow M. Microcirculatory hemodynamics of the rat cremaster muscle flap in reduced blood flow states. Ann Plast Surg 2003;51:182e8. Biglioli F, Rabagliati M, Gatti S, et al. Kinking of pedicle vessels and its effect on blood flow and patency in free flaps: an experimental study in rats. J Craniomaxillofac Surg 2004;32: 94e7. Siemionow M, Andreasen T, Chick L, et al. Effect of muscle flap denervation on flow hemodynamics: a new model for chronic in vivo studies. Microsurgery 1994;15:891e4. Lorenzetti F, Ahovuo J, Suominen S, et al. Colour doppler ultrasound evaluation of haemodynamic changes in free tram flaps and their donor sites. Scand J Plast Reconstr Surg Hand Surg 2002;36:202e6. Lorenzetti F, Salmi A, Ahovuo J, et al. Postoperative changes in blood flow in free muscle flaps: a prospective study. Microsurgery 1999;19:196e9. Lorenzetti F, Tukiainen E, Alback A, et al. Blood flow in a pedal bypass combined with a free muscle flap. Eur J Vasc Endovasc Surg 2001;22:161e4. Salmi AM, Tierala EK, Tukiainen EJ, et al. Blood flow in free muscle flaps measured by color doppler ultrasonography. Microsurgery 1995;16:666e72.
Blood flow autoregulation in pedicled flaps* Christian T. Bonde a,*, Niels-Henrik Holstein-Rathlou b, Jens J. Elberg a a
Department of Plastic Surgery and Burns Unit, Center of Head and Orthopedics, Copenhagen University Hospital, Rigshospitalet, Blegdamsvej 9, DK-2100 Copenhagen Ø, Denmark b Department of Medical Physiology, The Panum Institute, University of Copenhagen, DK-2200, Copenhagen N, Denmark Received 26 November 2007; accepted 25 July 2008
KEYWORDS Microcirculation; Calcium channels; Nimodepine; Papaverine; Pig
Summary Introduction: Clinical work on the blood perfusion in skin and muscle flaps has suggested that some degree of blood flow autoregulation exists in such flaps. An autoregulatory mechanism would enable the flap to protect itself from changes in the perfusion pressure. The purpose of the present study was to evaluate if, and to what extent, a tissue flap could compensate a reduction in blood flow due to an acute constriction of the feed artery. Further, we wanted to examine the possible role of smooth muscle L-type calcium channels in the autoregulatory mechanism by pharmacological intervention with the L-type calcium channel blocker nimodipine and the vasodilator papaverine. Material and methods: Pedicled flaps were raised in pigs. Flow in the pedicle was reduced by constriction of the feed artery (n Z 34). A transit time flow probe measured the effect on blood flow continuously. Following this, three different protocols were followed: (1) Time control (n Z 10): the procedure described above was repeated in the same flap to determine whether autoregulatory efficiency changed over time. (2) Nimodipine infusion (n Z 13): continuous intra-arterial infusion of nimodipine (0.2 mg/ml, 0.5 ml/min) started when the flow had returned to the initial value. After stabilisation, the flow was reduced. When the flow had been stable for at least 5 min, the constriction was removed. (3) Nimodipine and papaverine (n Z 8): the infusion of nimodipine was followed by an intra-arterial bolus of papaverine (10 mg). After stabilisation, the flow in the pedicle was reduced and the flow was recorded. Results: The flaps showed a strong autoregulatory response with complete compensation for flow reductions of up to 70e80%. Infusion of nimodipine caused a 28 10% increase in blood flow and removed the autoregulation. Papaverine caused a further increase in blood flow by 61 19%. The time control experiments proved that the experimental procedure was reproducible and stable over time. Conclusions: A tissue flap can nearly completely compensate for repeated flow reductions of up to 70e80%. This is due to a decrease in the peripheral resistance, mediated by a local intrinsic mechanism. Nimodipine (a blocker of L-type voltage-activated calcium channels) abolishes the autoregulation, but a significant vasodilatory reserve exists, as an additional
*
Presented at the IV Congress of the World Society for Reconstructive Microsurgery, WSRM, Athens, Greece, June 24e26, 2007. * Corresponding author. Tel.: þ45 35458963, þ45 32542333 (private); fax: þ45 35452667. E-mail address: [email protected] (C.T. Bonde).
1748-6815/$ - see front matter ª 2009 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.bjps.2008.07.039
1672
C.T. Bonde et al. injection of papaverine (a smooth muscle relaxant) results in a further increase in the blood flow. This strongly suggests a direct role for voltage-activated calcium channels in the autoregulatory process. ª 2009 British Association of Plastic, Reconstructive and Aesthetic Surgeons. Published by Elsevier Ltd. All rights reserved.
In reconstructive plastic surgery, free vascularised flaps are a common tool, designed to move tissue from one part of the body to a defect in another. Success rates for free tissue transfers of more than 95%1,2 are common, but many questions about the regulation of blood flow in tissue flaps remain unanswered. What is the minimal perfusion needed for a flap to survive? To what extent can a tissue flap compensate for constrictions or vasospasms in the feed artery? If we are to further improve our treatment outcomes answers to these and other questions regarding regulation of blood flow in flaps are required. It is well known that blood flow to both the brain and the kidney are subject to autoregulation. Despite a wide variation in the perfusion pressure, blood flow to these organs is maintained at a nearly constant level. The constancy of blood flow is due to autoregulatory mechanisms intrinsic to the organ or tissue, which mediate compensatory changes in vascular resistance in response to changes in perfusion pressure. Despite much research the mechanisms underlying autoregulation are still not fully resolved. The issue is further complicated by the fact that the different mechanisms seem to vary in significance between different vascular beds. Recently published clinical work on perfusion in skin and muscle flaps has shown that some form of blood flow autoregulation seems to exist in tissue flaps.3,4 Despite a 50% reduction in the diameter of the pedicle artery due to vasospasm, arteriolar blood flow was maintained at the normal level.3 The purpose of the present study was to evaluate if, and to what extent, a tissue flap consisting of skin and muscle could compensate for a reduction in blood flow due to an acute constriction of the feed artery. Further, we wanted to examine the possible role of smooth muscle L-type calcium channels in the autoregulatory mechanism by pharmacological intervention with the L-type calcium channel blocker nimodipine and the vasodilator papaverine.
Materials and methods Animal model The pedicled rectus abdominis musculocutaneous flap is well described in the pig, and is an accepted model of the human counterpart.5,6 Adult LYD (Landrace, Yorkshire, Duroc) pigs weighing 50 kg were used for the experiment. The animals were kept in a standard environment 1 week prior to the procedures. The animals did not receive any food or water the night before surgery. The animals were pre-anaesthetised with 25 mg midazolam. They were then anaesthetised using 1.5 mg/kg/h Propofol and 250 mg/h of Fentanyl. Intra-arterial blood pressure was monitored continuously. The experiments were carried out according
to the declaration of Helsinki II (Convention for Animal Experiments) and the national rules for the care and use of laboratory animals. The experimental protocol was approved by the Danish Animal Experiments Inspectorate. Bilateral musculocutaneous pedicled flaps were raised, based on the superior epigastric artery and vein, as shown in Figure 1. The sympathetic nerve fibres around the vessels were removed during skeletonisation of the pedicle. A side branch of the main artery was spared and cannulated with a polypropylene tube (PP10). A 7/0 suture was tied around the side branch and tube, keeping the tube in place. This tube was connected to an infusion pump allowing for direct intra-arterial administration of drugs.7 Finally, a small rubber loop was placed around the feed artery allowing a controlled and maintained constriction of the artery.
Transit time blood flow measurements Arterial blood flow was measured continuously throughout the experiments by a transit time flowmeter (Medi-Stim Butterfly Flowmeter BF 2004). A pre-calibrated transit time flow probe (Medi-Stim AS, Oslo, Norway; 2, 2.5 or 3 mm) was placed around the feed artery distal to the site of constriction. Special care was taken to avoid the flow probe causing compression or kinking of the artery.
Experimental set up When arterial blood flow had been stable for 5 min, blood flow in the feed artery was reduced by constriction of the rubber loop. When blood flow had been stable for an additional 5 min, the constriction was removed, and the effect of this recorded. This procedure was performed in 34 flaps. Following this, three different protocols were followed: (1) Time control (10 flaps): when blood flow had stabilised following removal of the constriction, the entire procedure described above was repeated to determine whether autoregulatory efficiency changed over time. (2) Nimodipine infusion (13 flaps): continuous intra-arterial infusion of nimodipine (0.2 mg/ml given at 0.5 ml/min) was started when flow had returned to the initial value. After stabilisation, the flow in the pedicle was reduced again. When the flow had been stable for at least 5 min, the constriction was once again removed. (3) Nimodipine and papaverine (eight flaps): the infusion of nimodipine was followed by an intra-arterial bolus of papaverine (10 mg). After stabilisation, the flow in the pedicle was once again reduced by constricting the loop and the flow recorded.
Statistics Comparisons of flow values were performed with consecutive paired Student’s t-tests and P values less than 0.05 were
Flap autoregulation
1673
Figure 1 The experimental set up. Shown are the bilateral flaps and the relationship between the infusion system, the constriction and the flow probe.
considered statistically significant. Correlation analysis between initial reduction and final compensation were done using standard methods. All data are given as mean SE.
Results The recorded flow values for constriction and release in a typical experiment are shown in Figure 2. Initially, the blood flow is stable. Constriction of the feed artery and the subsequent release of the constraint results in four distinct phases: a reduced flow followed by a gradual return to the control value (compensation); and following the release there is hyperaemia with a gradual return of blood flow towards the control value. The results in the control experiments are summarised in Figure 3. The constriction reduced the flow to 46 3% (P < 0.05) (n Z 34) of the control value. After approximately 3 min (range 1 to 42 min) the blood flow had returned to 94 2% (NS) of the control value. Following the removal of the constriction, hyperaemia ensued with an increase in flow to 126 3% (P < 0.05) of control. Finally, after a few minutes blood flow returned to a value not significantly different from that of the control (100 0%) (NS).
There was no statistically significant correlation (r2 Z 0.03, NS) between the degree of reduction in flow and the compensation achieved, although there was a tendency for the compensation to be lowest at the most pronounced degrees of flow reduction (Figure 4). The time control experiments (n Z 10) showed the experimental procedure to be reproducible and stable over time (Figure 5). Thus, it was possible to achieve nearly identical constrictions in the two experimental periods, and the degree of compensation achieved was similar in the two periods. Continuous intra-arterial infusion of nimodipine caused a 28 10% (P < 0.05) (n Z 13) increase in blood flow (Figure 6). Following acute reduction of blood flow by 34 3% (P < 0.05) (n Z 13), the blood flow remained at the reduced level, and consequently no compensation was observed. When the constriction was released, flow returned to a value not significantly different from the preconstriction value. In eight of the flaps nimodipine infusion was followed by an intra-arterial injection of papaverine. This caused a further 61 19% (P < 0.05) increase in blood flow (Figure 6). When flow was reduced acutely by 35 5% (P < 0.05) (n Z 8) it remained low during the period of
Figure 2 Recordings of blood flow in a typical experiment. Left panel: Initially, the blood flow is stable at 17 ml/min. Flow is reduced at w30 s and compensation starts. Right panel: Following the release, the hyperaemic phase (24 ml/min) appears before a gradual return of blood flow towards the control value.
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Figure 3 Control experiment (n Z 34) showing the initial reduction in blood flow upon arterial constriction (Reduction) followed by a compensatory phase, and the subsequent hyperaemia after release of constriction followed by a return of the blood flow to the control value (Final). Means SE.
constriction, and no compensation was observed. Again, when the constriction was removed, flow returned to the pre-constriction value (Figure 6). The papaverine-induced increase in blood flow lasted until the experiments were terminated.
Discussion The present study shows that pedicled flaps have a considerable autoregulatory potential. Despite acute reductions of blood flow in the range from 30 to 80%, blood flow was nearly completely normalised after a few minutes. The autoregulatory process appears to involve L-type voltageactivated Ca2þ channels as it was completely inhibited by intra-arterial infusion of the calcium channel blocker nimodipine. Detailed knowledge of the physiology and haemodynamics governing the perfusion of musculocutaneous
Figure 4 Scatter plot showing corresponding values of initial reduction and final compensation of blood flow (n Z 34).
C.T. Bonde et al. pedicled or free flaps is limited. That a flap may have autoregulatory potential was suggested by a clinical study by Lorenzetti et al.4 They examined the blood flow changes that occurred in a free transverse rectus abdominis myocutaneous (TRAM) flap when transferred to either the thoracodorsal artery or the internal mammary artery.4 The average blood flow in the donor artery before flap harvest (inferior epigastric artery) was 11 ml/min. When the flap was transferred to the thoracodorsal artery, which had a blood flow of 5 ml/min, flap flow increased to 14 ml/min. When transferred to the internal mammary artery, with a blood flow of 25 ml/min, flow decreased to 12 ml/min. The authors concluded that flap flow is independent of the flow in the recipient vessel, but dependent on the tissue components and requirements of the flap. Similar findings have been published in another study of muscle flaps and radial forearm flaps.8 The results of the present experimental study confirm and extend these observations. When blood flow was reduced acutely over a wide range of values, we saw a nearly complete return of blood flow to the control value. In fact, the flaps could compensate for even a 70e80% reduction in blood flow. This shows a remarkable autoregulatory potential in the isolated flap. In fact, the average blood flow compensation of 94% found in this study is comparable to the 93% reported in the kidney.9 In comparison, autoregulation of blood flow in the rat hindlimb, consisting primarily of skin and muscle, only reached about 35%.9 We were not able to detect a lower limit for autoregulation, although there did appear to be some loss of compensation when flow was reduced to 20% of control (see Figure 4). However, technical difficulties prevented us from achieving stable constrictions lower than that, and it is therefore not possible from the present results to determine the lower limit of autoregulation. The time needed for the autoregulatory response to stabilise varied from less than 1 min to 42 min for the slowest. In 75% of the flaps, full autoregulatory response was achieved within 10 min, with a median of 3 min for all experiments. We speculate that this difference in autoregulatory response rate could be caused by different contributions from the autoregulatory components. Autoregulation in general is viewed as being composed of a fast myogenic component and a slower metabolic component. The wide range of variation could be caused by a difference in the balance of the myogenic and metabolic components. Thus, in a flap with a dominant metabolic component, the autoregulatory response would be expected to be slower than that in a flap with a less dominant metabolic component. This is illustrated in Figure 2, and was observed to a lesser or greater degree in all flaps. After 30 sec, the flow reduction is performed (Figure 2). In the following 30 seconds the flow is quickly autoregulated until it reaches about 80% of the original value. This likely represents the myogenic response. Following this, a slower phase appears where the remaining 20% is regulated, the metabolic response. Further studies are needed to elucidate these interesting observations. At present the mechanism(s) underlying autoregulation is poorly understood. The finding of an increased blood flow after transfer of a free flap has been ascribed to the sympathectomy relieving arteriolar vasoconstriction, thus causing vasodilation and increased perfusion, but this fails
Flap autoregulation
Figure 5 The time control experiments showing the experimental procedure to be reproducible and stable over time. Black columns represent the initial experimental period; the grey columns represent the second experimental period (n Z 10). *A value significantly different from the control value (P < 0.05).
to explain how the flap down regulates the blood flow when anastomosed to a high flow recipient artery. The observation in the present study that the calcium channel blocker nimodipine completely abolishes autoregulation indicates that voltage-activated L-type calcium channels may be involved in the process. Intracellular calcium is a key regulator of vascular smooth muscle contraction, and changes in calcium influx through calcium channels cause corresponding changes in smooth muscle tone.10 Studies in pressurised cerebral arterioles have shown that a decrease in perfusion pressure is associated with a hyperpolarisation of the cell membrane and, thus, with a closure of voltageactivated calcium channels. The reduced calcium influx will lead to a relaxation of the vascular smooth muscle cells and, consequently, to vasodilation.11,12 The present results
Figure 6 The effect on flow and flow compensation by continuous nimodipine infusion and bolus papaverine injection. Both drugs result in an increase of blood flow and abolish autoregulation. Con: control period; Nim: infusion of nimodipine; Red: flow reduction; Comp: compensation; Rel: release of arterial constriction; Pap: bolus injection of papaverine.
1675 suggest that a similar mechanism may operate in skin and muscle flaps. Nimodipine caused a significant vasodilation when administered intra-arterially, and it could be argued that the lack of autoregulation following nimodipine was simply due to the vessels already being maximally vasodilated. However, this was clearly not the case, since intra-arterial injection of papaverine caused a further significant increase in blood flow. This shows that the vessels maintained significant tone despite the administration of nimodipine. It therefore appears that the inhibitory effect of nimodipine on autoregulation was not due to the vasodilation per se, but rather to the specific action on the calcium channels. The vasodilatory actions of nimodipine and papaverine show that, despite the sympathectomy of the flap vessels, they still maintain significant tone. A similar result was reported by Banic et al., in an experimental model comparable to ours, when local injection of sodium nitroprusside into the flap increased total flap flow by 20% and decreased the vascular resistance by 20%.13 The nimodipine dose used in this experiment was calculated to achieve a plasma concentration similar to the therapeutic range in the treatment of hypertension (1e10 mM). The systemic effect of the L-type calcium antagonists is vasodilation. However, due to the local intra-arterial application of the agent, we observed no effect on CVP (Central venous pressure) or CO (Cardiac output). All L-type calcium antagonists (e.g. amlodipine, felodipine, isradipine, lacidipine, lercanidipine, nifedipine, nitrendipine) would be expected to have a similar effect on flap blood flow. The pedicled flap is an inhomogeneous tissue containing skin, subcutaneous fat and striated muscle. The present study does not allow any conclusions as to the degree of autoregulation in the different constituent tissues. However, in a recent study, Wettstein et al. found that even when the diameter of the pedicle artery was reduced to 50% of its original value, blood flow was restored to normal levels in the anatomically perfused arterioles, and this was associated with an increased diameter of third order arterioles (A3).3 The study also showed that flap areas which already had impaired flow did not posses the ability to restore flow levels when further reduced by vasospasm. Together with our observation of preserved total flow, the observation of preserved flow at the level of the individual vessels suggests good autoregulation in all the constituent tissues of the flap. Other groups have reported experiments with partial flow reductions and staged compressions of the vascular pedicle in animal models.14 Hjortdal et al. showed that at a 50% reduction in arterial blood flow, capillary blood flow was spared by a decline in A-V shunting.15 In a study of critical flow values in a rat cremaster muscle, Nanhekhan and Siemionow found that a 60% flow reduction was incompatible with long-term survival.16 Neither group reported autoregulation, but the failure to observe autoregulation is most likely due to differences in models and techniques used for flow reductions. For example, Hjortdal et al. performed the flow reductions in a way that prevented autoregulation, as they continuously constricted the artery until blood flow (venous output) reached a predetermined level. However, it has been shown that a 180 kinking of the artery has no influence on survival
1676 rate,17 probably because the reduction in flow is compensated by an autoregulatory mechanism. The clinical implication of the present study is that muscle flaps may be relatively resistant towards even quite pronounced vasospasm or surgically-induced constrictions at the site of vascular anastomosis. We did not find a lower limit of autoregulation despite up to an 80% reduction of flow. Whether the autoregulatory response is maintained for longer time periods (i.e. hours or days) cannot be determined from the present study. To answer this important question longer lasting studies are necessary. Several observations, however, suggest that flaps may not only show acute compensation, but that the vasodilatory processes may continue over several days or weeks. Thus, blood flow in a free flap increases in the postoperative period,18 and the postoperative arterial blood flow in the TRAM flap has been shown to increase from the fourth to the 30th day.19 In a prospective clinical study using free latissimus dorsi (LD) muscle flaps, Lorenzetti et al.20 demonstrated that blood flow in the pedicle and in the recipient artery of a free muscle flap increases after surgery. The same group also published similar findings for the free lower limb muscle flaps both with and without vein grafts.21,22 In conclusion, this is the first investigation of blood flow autoregulation in an experimental skin and muscle flap model. A tissue flap can nearly completely compensate for flow reductions of up to 70e80%. This is due to a decrease in peripheral resistance, mediated by a local intrinsic mechanism. Nimodipine (a blocker of L-type voltage-activated calcium channels) abolishes autoregulation, but a significant vasodilatory reserve exists, as additional injection of papaverine (a smooth muscle relaxant) results in a further increase in blood flow. This strongly suggests a direct role for voltage-activated calcium channels in the autoregulatory process.
Acknowledgements This work was supported by The L.F. Foghts Fund, The Sophus Jakobsen and wife Astrid Jakobsens Fund, The Danish Medical Association Research Fund/The Vibe A. Linholter Estate, The Jacob Madsen and wife Olga Madsens Foundation, The P. A. Messerschmidt and wife Fund and by the University of Copenhagen.
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