PHASE 1 CME: FLAP PHYSIOLOGY BY LAM HUI YUAN 1 Flap Vasculature 2 3 The Microcirculation components Figure 1 : The Microciculation ( Reference Chapter 30 ,Review of medical physiology , Ganong) Components Functional Morphology Arterioles Function The walls of arterioles contain less elastic tissue but much more smooth muscle Arterioles are mainly implicated in the regulation of blood flow The muscle is innervated by noradrenergic and cholinergic fibres and tissue perfusion The arterioles divide into smaller muscle walled vessels ‘’ metarterioles ‘’ and in turn feed into capillaries. Capillaries It consists of a thin wall which are made up of a single layer of endothelial cells capillaries are the site of oxygen and nutritional exchange Small bands of vascular smooth muscle, the precapillary sphincters, are located at the arterial end of many capillaries and are responsible for the control of blood flow within the capillaries. Arteriovenous shunts : Short channels connect arterioles to venules bypassing the true arteries Thermoregulation Figure 2 : Diagramatic representation of microcirculation AVA Whose radius twice that of capillary would carry 16 times as much blood per unit length . Venules They contain relatively little smooth muscles, but considerable vasoconstriction is produced by activities in noradrenergic nerves to vein and circulating vasoconstrictors such as endothelin 4 Regulation of blood flow Systemic control Neural control vasoconstrictors o Alpha-adrenergic receptors Humoral regulation Mediators of vasoconstriction o Epinephrine o Norepinephrine o Serotonin o Arachidonicacid metabolites (Thromboxane A2, Prostaglandin F2a) (Reference: The arterial anatomy skin skin flap by Cormack and lamberty) vasodilators o Beta-2 adrenargic receptor Mediators of vasodilation o Histamine o Bradykinin o Prostaglandin E1 o Prostacyclin/Prostaglandin I2 5 Regulation of blood flow: Local Factors ▪ a. Metabolic factors When the metabolic rate of skeletal muscle is increased by exercise, tissue levels of oxygen decrease, but those of carbon dioxide, H+, and K+ increase. Muscle tissue osmolarity also increases during exercise. All these chemical alterations cause arteriolar dilation. Figure 3 .local metabolites hypothesis adenosine It is formed from cellular AMP acted upon by 5'-nucleotidase. The AMP is derived from hydrolysis of intracellular ATP and ADP. A potent vasodilator, its formation increases during hypoxia and increased oxygen consumption. Hypoxia Hypoxia-induced vasodilation may be ▪ direct: inadequate O2 to sustain smooth muscle contraction. ▪ indirect: via the production of vasodilator metabolites. Potassium ion It is released by contracting cardiac and skeletal muscle. Small increases in extracellular K+ produces hyperpolarization of vascular smooth muscle and relaxation through stimulation of the electrogenic Na+/K+-ATPase pump and increasing membrane conductance to K+ (K+ activated K+ channels). Reference: Potassium Channels in the Peripheral Microcirculation by William F. Jackson. Microcirculation, 12: 113–127, 2005 Carbon dioxide Carbon dioxide formation increases during states of increased oxidative metabolism. 6 It readily diffuses from parenchymal cells in which it is produced to the vascular smooth muscle of blood vessels where it causes vasodilation. Hydrogen ion Hydrogen ion increases when CO2 increases or during states of increased anaerobic metabolism, which can produce metabolic acidosis. Like CO2, increased H+ (decreased pH) causes vasodilation Figure 4 .the partial pressure of carbon dioxide in arterial blood (PaCO2) changes extracellular pH, which is the initial step leading to changes in vascular smooth muscle (VSM) intracellular calcium concentration and vascular tone. Inorganic phosphate Inorganic phosphate is released by the hydrolysis of adenine nucleotides. It may have some vasodilatory activity in contracting skeletal muscle. b. Physical factors I. Myogenic factor II. Local hypothermia III. Increased blood viscosity Myogenic factor Myogenic reflex, which triggers vasoconstriction in response to distention of isolated cutaneous vessels and thereby maintains capillary flow at a constant level and control the amount of fluid filtered out of the capillaries 7 Local hypothermia Local thermoregulation Central Thermoregulation Figure 4 : Overview of thermoregulatory control of skin blood flow (Mayo Clin Proc. 2003;78:603-612 Skin Blood Flow in Adult Human Thermoregulation: How It Works, When It Does Not, and Why ) 1. Local warmimg of the skin causes vasodilation by stimulating local neuropeptide release from sensory nerves (including calcitonin gene–related peptide [CGRP], substance P [SP], and neurokinin A [NKA]) and by nonneural local vasodilation caused by nitric oxide (NO). 2. Local cooling of the skin stimulates localized neurotransmission from noradrenergic nerves to cause vasoconstriction. 3. Mechanisms for reflex control of skin blood flow include sympathetic adrenergic vasoconstrictor nerves and sympathetic vasodilator nerves, the latter of which are responsible for 80% to 90% of the substantial cutaneous vasodilation during whole body heat stress. 8 1. Viscosity : A quantity that express the magnitude of the internal frictional force that arises between adjacent layers of fluid that are in relative motion. Blood viscosity 2. The determinants of flow are summarized by the Hagen–Poiseuille equation: where: DP is the pressure difference across the tube, r is the radius of the vessel, h is viscosity and l is the length of the tube. 3. Plasma is 1.8 times as viscous as water ; whole blood is 3-4 times as viscosu as water . 4. Thus , viscosity depends on hematocrit , i.e. the percentage of the volume of blood occupied by red blood cells I. A lower hematocrit ( < 30 % ) : This do not provide much more advantage because the curve of viscosity versus haematocrit flattens off markedly. If the haematocrit falls further, the marginally improved flow characteristics from a lower viscosity may then be offset by a reduction in oxygen delivery: 9 II. Hematocrit of 30 % -35 % : It apears to be one that offers the best balance between viscosity and O2-carrying capacity and tissue perfusion III. Hematocrit of 40-60 % : It increases the resistance to blood flow and thereby increases the work of the heart and impairs organ perfusion. Local injury 1. The effects of local injury to a part of the arterial wall can completely override basal vascular tone and cause spasm even in the absence of sympathetic innervation. 2. For instance, a pin prick elicits a persistent isolated ring contraction locally, and extensive crushing or tearing can induce a widespread and prolonged spasm distantly. 10 SUMMARY : REGULATION OF BLOOD FLOW NERVOUS Vasoconstrictors Alpha adrenergic HUMORAL Norepinephrine METABOLIC PHYSICAL Viscosity Epinephrine Hypothermia Serotonin Myogenic Prostaglandin F2 reflex Thromboxane A2 Endothelin Vasodilators Beta adrenergic Nitric oxide Hypoxia Cholinergic Bradykinin Acidosis Histamine Hypercarbia Hyperthermia Prostacyclin Adenosine Diphosphate Prostaglandin Thrombin (Reference : Vedder NB. Flap Physiology, Chapter 20. In Mathes SJ (Ed.). Plastic Surgery, Volume 1, 2nd edition, 2006: 483-506.) 11 FLAP PHYSIOLOGY ANATOMIC CHANGES Impairment of blood supply 1. This results in a local decrease perfusion pressure to the skin, the result is that peripheral portions of the flap become acutely ischemic. “fresh flaps are always both viable and ischemic.” Myers, B. Understanding flap necrosis . Plast Reconstr Surg. 1986; 78:813 2. Depending on the degree of ischemia and the amount of time before recovery of nutrient blood flow, the flap will either die or recover. 3. If flap survive , o If the flap is in a favourable recipient site, a fibrin layer forms within the first 2 days. o Neovascularization of the flap begins 3 to 7 days after flap transposition. 12 o Neovascularisation: Direct Ingrowth or inosculation o Direct ingrowth Inosculation (Reference : Local Flaps in Facial Reconstruction by Shan R Baker, MD a. Direct Ingrowth: 1. Ischaemic regions of tissue release angiogenic cytokines, 2. This cause the breakdown of the vessel walls of arterioles and venules, and sprouting of cells, including pericytes in the vessel wall. 3. Endothelial cells proliferate and migrate out of the vessel into the tissue. 4.They reorganise to form capillaries which interconnect (anastomosis) and link to venules, thereby forming a new capillary network. o Angiogenic growth factors can stimulate capillary growth over distances of 2 to 5 mm. b. Inosculation: o capillaries join pre-existing flap vessels. 13 Denervation 1.Both cutaneous and sympathetic nerves are severed in the process of flap elevation. A. loss of sensation may limit the usefulness of the flap after transfer. B. Adrenergic denervation has implications for flap survival Banbury et al describe a muscle flaps’ triphasic response to sympathectomy and denervation. Acute hyperadrenergic o 0-24 hour phase Nonadrenergic phase o 24-48Hr: Significant vasodilation o The stored transmitter is depleted within 24 to 48 hours. o blood flow increases as the concentration of norepinephrine declines. Sensitised phase o 2 weeks after denervation : increased capillary perfusion : hyperresponsiveness to vasoactive substances. Impairment of Lymphatic drainage Reduction of the cutaneous lymphatic drainage results in an increase in interstitial fluid pressure that is compounded by increased leakage of intravascular protein associated with inflammation. The resulting edema can decrease capillary perfusion by increasing the intravascular resistance. 14 II. The Hemodynamic Changes (Hoopes 1976) o decrease circulatory efficiency for the 6 hours. o plateau at 6-12 hours o marked congestion and oedema during initial 24 hours. o oedema is generated by ischemia and inflammation, interstitial pressure increases. • 0-24 Hrs Figure 1. Within seconds of hypoxia, levels of ATP begin to decrease, and cells begin to swell intracellular movement of sodium and an increase in intracellular osmotic pressure. Relatively brief periods of ischemia result in a reversible swelling of the cell. If the ischemic insult is severe and prolonged, cell lysis and flap necrosis occur. 1-3 Days 3-7 Days o Little or no improvement in circulation during initial 48 hrs. o increase in number and calibre of longitudinal anastomoses. o increase in the number of small vessels in the pedicle. o vascular anastomoses between flap and recipient bed present at 2- 3 days o it becomes functionally significant in 5-7 days. o Increase in size and number of functioning vessels. o Reorientation of vessels along the long axis of the flap o o circulatory function well established pulsatile blood flow approaches preoperative levels 1 week Pedicle ligation beyond day 6 in rat and day 3 in pig did not produce flap necrosis , indicating adequate neovascularization for flap survival . Tsur, H., Daniller, A., & Strauch, B. (1980). Neovascularization of Skin Flaps. Plastic and Reconstructive Surgery, 66(1), 85–93. 15 7-14 Days o o no further significant increase in vascularization arterial pattern becomes normal. o o 2 weeks: continuous maturation of anastomoses between flap and recipient 3 weeks - flap achieves 90 % of its final circulation. - Vascular pattern = preoperative state - Full development of vascular connection with pedicle and recipient site o 4 weeks: all vessels decreased in diameter. III. METABOLIC CHANGES Tissue Oxygen Tension Tissue oxygen tensions are significantly higher in musculocutaneous flaps than in random pattern flaps up to 6 days after elevation. Differences in patterns of oxygen delivery to random versus musculocutaneous flaps may in part explain the greater reliability of musculocutaneous flaps when they are used in ▪ ▪ Inflammatory response the presence of infection and provide better bacterial killing function in the setting of infection The surgical trauma associated with an acutely raised flap results in an inflammatory response. Histamine, serotonin, and kinins are released into the extracellular compartment, increasing the permeability of the microcirculation. The result is an increase in the concentration of proteins and cells within the extracellular space. It may have deleterious effects due to the resultant edema formation. Reperfusion Injury The source of Superoxide radicals o Inadequate tissue oxygenation → conversion from aerobic to anaerobic metabolism. Superoxide is a byproduct of adenosine triphosphate production in the mitochondria and other oxidation reduction reactions. o Polymorphonuclear cells are a second source of superoxide radicals that are released in response to bacterial inflammation. This byproduct of reperfusion can cause damage at both the cellular and subcellular levels, contributing to postischemic tissue necrosis. 16 III. Pathophysiology of vessel healing following anastomosis. 1. A thin layer of platelets forms at the anastomotic site immediately after repair. 2. Exposed collagen triggers the platelet adhesion. 3. Platelets Accumulate over 48-72 hours before regressing. If no intima damage.. 4. the platelet aggregations disappear between 24 to 72 hours. 5. pseudo intima forms at the anastomosis site within 5 days 6. new endothelium covers the anastomosis site within 1-2 weeks. If intima damage occurs , 4. platelet aggregation continues and after reaching a certain critical mass it will trigger a cascade of events leading to thrombus formation in the vessel. 5. The critical period of thrombus formation in the anastomosis is the first 3-5 days of healing. Factors that contribute to intima damage and anastomotic thrombus o Rough vessel dissection o Diathermy close to the vessel o Use large needles. o Repeated needle stabs o Unequal spacing of sutures o Too many sutures o Application of vascular clamps with closing pressure > 30g/mm2 1. 2. 3. References: Harris et al. Endothelialization after arterial and venous micro-anastomosis. CAN J PLAST SURG VOL 3 NO 3 FALL 1995. Lidman et al. The Normal Healing Process of Microvascular Anastomoses. Scandinavian journal of plastic and reconstructive surgery 15(2):103-10 · February 1981 17 SUMMARY : FLAP PHYSIOLOGY ▪ ▪ ▪ Impairment of blood supply Denervation Impairment of lymphatic supply (Reference: Mathes Textbook of Plastic Surgery) 2.Vessel healing after anastomosis Immediately A thin layer of platelets forms at the anastomotic site 48-72 Hours Platelet accumulation First 3-5 days Thrombus formation can occur if intima damage Within 5 days Pseudointima occur 1- 2 weeks New intima covering the anastomotic site (Reference: Grab and Smith Plastic Surgery 8 th Edition) 18 DISCUSSION : FLAP PHYSIOLOGY ✓ The fear of vasopressor use: stems from the presumption that they can potentially lead to peripheral vasospasm, thrombosis, reduced flap perfusion, and ultimately flap failure. o A survey among microsurgeons revealed that 70% of respondents would not permit the use of vasopressors in nonemergent situations. o Another survey revealed that almost one-quarter of respondents attributed free flap loss to the administration of vasopressors. o Even among anaesthesiologists, norepinephrine uses in free flap surgery was deemed contraindicated by 46%. ✓ Meta-Analysis of Flap Outcomes: The majority of the studies in this review concluded that. vasopressors did not have a negative effect on free flap outcomes irrespective of dose, timing of administration, and method of delivery. 1. vasopressors were associated with significantly less pedicle thrombosis. ✓ These findings have been theorized to be due to the disruption of autonomic nerve fibres following adventitial stripping and pedicle division during free flap surgery. While chronic sympathetic denervation is characterized by adrenergic super sensitivity, α-adrenergic-mediated vasoconstriction is inhibited immediately after surgical adventitectomy. 2. Perioperative use of vasopressor: ✓ General anaesthetic drugs tend to induce systemic hypotension which can result in hypoperfusion of critical organs and the flap itself. ✓ Large fluid volumes can lead to progressive edema of the flap which may impair flap microcirculation by mechanical compression of the microvasculature. ✓ Due to the lack of lymphatic drainage and denervation-related decrease in interstitial fluid reabsorption, free flaps are more prone to graft edema . ✓ Wei FC, Mardini S. Flaps and Reconstructive Surgery. 1st ed. Philadelphia, PA: Saunders (Imprint), Elsevier; 2009 Sigurdsson GH. Perioperative fluid management in microvascular surgery. J Reconstr Microsurg 1995;11(01):57–65 19 3. Few studies have evaluated the use of vasopressors postoperatively. o Only postoperative theodrenaline/cafedrine were associated with adverse flap outcomes, possibly due to α1-mediated vasoconstriction. o Postoperative dopamine infusion at 5 to 10 μg/kg/min increased mean arterial pressure without compromising free flap outcomes. o Intraoperative and postoperative use of dobutamine, was not associated with free flap thromboembolic complications but instead increased head and neck free flap perfusion. o In breast reconstruction, dobutamine and dopamine both increased cardiac output and mean arterial pressure. However, only dobutamine increased blood flow across the free flap pedicle and was thus recommended over dopamine should vasoactive agents be required in microvascular surgery. Dobutamine Dopamine systematically increased CO and heart rate while infused at 3 and 12 g/kg/minute increased the it decreased SVR CO, increased vasoconstriction at the higher dose, and had no effect of flap blood flow. A simultaneous increase in donor and recipient artery flow was recorded. Blood flow in the recipient arteries remained unchanged Suominen, S., Svartling, N., Silvasti, M., Niemi, T., Kuokkanen, H., & Asko-Seljavaara, S. (2004). The Effect of Intravenous Dopamine and Dobutamine on Blood Circulation During a Microvascular TRAM Flap Operation. Annals of Plastic Surgery, 53(5), 425–431. 4.there are likely regional differences in terms of microvascular physiology and regulatory. mechanisms. ✓ Vasoconstrictive medications may preferentially target the peripheral circulation, leading to more adverse outcomes when involving the extremities as donor or recipient sites. ✓ Even with denervation-related insensitivity of the flap pedicle, there is theoretically a risk of overall reduction in distal flow to the extremity, leading to decreased flap perfusion. Kotsougiani D, Banz CM, Hundepool CA, et al. Influence of post-Operative vasoactive agent administration on free flap outcomes.Eur J Plast Surg 2016;39:421–428 5. At present, evidence-based recommendations regarding the use of specific vasopressors in free flap surgery are limited. ✓ Dobutamine ,dopamine and norepinephrine appear to have beneficial effects on flap blood flow; hence, their indications and safe dosing limits need to be further clarified. ✓ In contrast,epinephrine can decrease flap flow and should be avoided. Eley KA, Young JD, Watt-Smith SR. Epinephrine, norepinephrine, dobutamine, and dopexamine effects on free flap skin blood flow.Plast Reconstr Surg 2012;130(03):564–570 Eley KA, Young JD, Watt-Smith SR. Power spectral analysis of the effects of epinephrine, norepinephrine, dobutamine and dopex- amine on microcirculation following free tissue transfer. Microsurgery 2013;33(04):275–281 20 1. The Compromised Flap The 15th principle postulated by Sir Harold Gillies. ‘The after care is as important as the planning’. ‘How futile it is to lose flap or graft for the lack of a little postoperative care. If in any doubt about the progress slip your hands out of your pockets and get down to the haematoma.’ Outline ▪ Causes of a Failing Flap ▪ Pathophysiology of Flap Failure ▪ Management of Flap Failure 21 Flap Failure Causes of a Failing Flap Extraluminal factors Intraluminal factors Reference: Anaesthesia for microvascular free tissue transfer, British Journal of Anaesthesia | Volume 3 Number 2 2003 Chapter 24. Principles and management of microsurgery. Neligen Plastic Surgery 4th edition Singh, B., Cordeiro, P. G., Santamaria, E., Shaha, A. R., Pfister, D. G., & Shah, J. P. (1999). Factors Associated with Complications in Microvascular Reconstruction of Head and Neck Defects. Plastic and Reconstructive Surgery, 103(2), 403–411 22 ▪ Pathophysiology of Flap Failure 1. Vasospasm ▪ Vasospasm occurs in 5- 10 % of microsurgery procedures ,and plays an important role in pathogenesis of hypoperfusion ,promoting thrombosis ▪ It may be seen intraoperatively and up to 72 hours postoperatively ▪ The pathophysiology is not clear but it is thought to occur secondary to general and local factors : I. General factors: low core temperature, hypotension and sympathetic response to pain II. Local factors: 1 2 3 4 Figure 6 :Vasospasm in the pathogenesis of pedicle and free flap failure (Reference : Chapter 23 Flap pathophysiology , Neligen 4th Edition Surgical Trauma Sympathetic nerve endings release vasoactive compounds Traumatised Releases endothelium-derived contracting factors (EDCFS) such as thromboxane a 2 (TXA2), Vascular and endothelin-1 (ET-1) which raise vascular tone Endothelial Cells 23 The rate of endothelial degradation of norepinephrine (Ne )and serotonin (5HT2 )by catechol-o-methyl transferase and monoamine oxidase, respectively, is reduced in situations of impaired endothelial function. The synthesis and release of EDRFs such as PGI2 and NO from the traumatized vascular endothelium are depressed. Hematoma Haemoglobin from haemolyzed red blood cells (e.g., hematoma) is a potent vasoconstrictor. ✓ The hematoma could stretch the skin flap and place tension on the subdermal plexus interfering with dermal perfusion resulting in ischemic necrosis. ✓ The released iron in the hemolysate was found to be a significant factor in the conversion of Superoxide (02) to the very toxic hydroxide (OH-) radical. This free radical production ultimately correlated with flap necrosis. ✓ iron bound by deferoxamine is unable to participate in free radical mechanisms. Reference: Diaz, D. D., Freeman, S. B., Wilson, J. F., & Parker, G. S. (1992). Hematoma-Induced Flap Necrosis and Free Radical Scavengers. Archives of Otolaryngology - Head and Neck Surgery, 118(5), 516–518. 24 Free Radical Damage of Vascular Wall In reperfusion of ischemic blood vessels, superoxide radicals ( ) are produced by platelets, neutrophils, and endothelial cells and these free radicals can damage vascular walls during reperfusion. 2. Thrombogenesis (Reference: Peter C. Neligan. Plastic surgery third edition, volume one, 2013. 573-588) ▪ Causes: a) Changes in intraluminal blood flow b) Endothelial damage c) The state of coagulability a. Changes in blood flow External obstruction: Internal Obstruction a 25 I. a. b. c. d. II. External obstruction: Mechanical compression from bandages, Closure of the wound under tension, The weight of the flap, tension/twisting /vasospasm of the vascular pedicle after the anastomosis is completed. Intraluminal turbulence: a. Irregularities of intima from technical error b. The result of size mismatch B. Endothelial Damaged endothelium produces a highly thrombogenic state, resulting in damage platelet aggregation and the initiation of complex clotting cascade. c. I. Hypercoagula Hypercoagulability state: Pregnancy, active cancer, and recent trauma bility II. Hypercoagulability disorders : Activated protein C, hyperfibrinogenaemia , antiphospholipid syndrome and reactive thrombocytosis 3. Xanthine dehydrogenase/xanthine oxidase enzyme system in pathogenesis of ischemia– reperfusion injury in free flap surgery 1 2 4 5 3 1. In free flap surgery, skin and muscle are subjected to warm global ischemia under vascular clamp control during transfer from donor site to recipient site prior to reanastomosis. 26 muscle skin 2–2.5 hours of warm global ischemia 6–8 hours of warm global ischemia 2. During prolonged ischemia, adenosine triphosphate (ATP) in skin and muscle is catabolized 2+ stepwise to hypoxanthine, with concomitant increase in cytosolic Ca . 3. At the same time, a cytosolic protease is activated by intracellular Ca xanthine dehydrogenase to xanthine oxidase. 4. During reperfusion, the xanthine oxidase generates superoxide ( of molecular oxygen in the presence of hypoxanthine. 5. The unstable 2+ and it converts ) by univalent reduction also interacts with H2O2 in the presence of a transition metal (e.g., iron) to • form the most potent cytotoxic hydroxyl radical (OH ) through the Haber–Weiss (Fenton) reaction 6. These radicals destroy proteins, membranes, and DNA. 4. Neutrophilic nicotinamide adenine diphosphate (NADPH) and myeloperoxidase (MPO) enzyme system in pathogenesis of ischemia/reperfusion injury in free flap surgery Figure 9. Schematic representation of important inflammatory mediators with cytotoxic potential released from activated neutrophils. O2− indicates superoxide anion; HOCl, hypochlorous acid; H2O2, hydrogen peroxide; MPO, myeloperoxidase; E, elastase; C, collagenase; LTB4, leukotriene B4; and PAF, platelet-activating factor. (Reference: Role of neutrophils in ischemia and reperfusion, Circulation. 1995 | Volume 91, Issue 6: 1872– 1885) 1. Polymorphonuclear leukocytes (PMNs) are integrated into the acute inflammatory response to tissue injury and possess the capacity to produce oxygen-derived free radicals (OFRs) when activated by appropriate stimuli. 2 Activated neutrophils produce: a. large amounts of via NADPH oxidase, and these dismutates yield high concentration of H2O2 and • OH , causing tissue damage. b. Release of myeloperoxidase (MPO) from the azurophil granules which is unique and abundant in neutrophils, catalyzes the conversion of H2O2 to hypochlorous acid (HOCl), a potent cytotoxic oxidizing agent − + (H2O2 + Cl + H → HOCl + H2O) 27 5. Intracellular Ca2+ overload in pathogenesis of ischemia–reperfusion injury in free flap failure Figure 10 . Cellular effects of ischaemia and reperfusion. The reduced ATP during ischaemia leads to increased intracellular hydrogen ion and calcium. At reperfusion the formation of reactive oxygen species (ROS), changing pH and raised calcium lead to opening of the mitochondrial permeability pore (MPP), leading to cell death. (Reference: Kharbanda, R. K. (2010). Cardiac conditioning: a review of evolving strategies to reduce ischaemia-reperfusion injury. BMJ Journal Heart, 96(15), 1179– 1186.) 1. In sustained ischemia, mitochondrial ATP synthesis ceases and glycolysis ensues, resulting in a net breakdown of + ATP and an accumulation of lactate and intracellular H ,causing intracellular acidosis. 2. + + + This build-up of intracellular H activates the Na /H exchange isoform-1 (NHE-1) antiporter, resulting in extrusion + + of H and accumulation of intracellular Na to restore intracellular pH. + 3. Elevation of intracellular Na concentration causes an increase in intracellular Ca + Na /Ca 2+ exchanger causing Ca 2+ influx. If these events continue, the cystolic Ca 2+ 2+ by activation of the will be overloaded, and 2+ significant uptake of Ca from the cytosol to the mitochondria will occur, resulting in mitochondrial Ca which causes depolarization of mitochondria and impairs ATP synthesis, resulting in cell necrosis. 2+ overload + 4. At reperfusion, the rapid washout of the extracellular H reactivates the NHE-1, resulting in further extrusion of + + intracellular H , and further accumulation of intracellular Na , causing further cystolic Ca + 2+ Na /Ca exchange. Again, cytosolic Ca and resulting in cell death. 2+ overload causes mitochondrial Ca 2+ 2+ overload through overload, impairing ATP synthesis 28 6. Pathogenesis of no-reflow phenomenon in free flap surgery Three pathogenic mechanisms have been suggested to play a central role in the development of no-reflow phenomenon in the skeletal muscle of laboratory animals Cell membrane damage 2+ allowing Ca influx, resulting in intracellular overload and lead to swelling of the endothelial and parenchymal cell, narrowing of the capillary lumen . Oxygen-derived free radicals causing damage in the endothelial and parenchymal cells; Change in arachidonic acid metabolism resulting in synthesis of less vasodilating and antithrombotic PGI2 by the endothelium and increased synthesis of vasoconstricting and thrombotic TXA2 by platelets. This pathology increased with the increase in length of ischemic time from 1 to 8 hours and the obstruction of blood flow reached a point of irreversibility after 12 hours of ischemia, leading to no reflow and ultimate death of the flap. 29 Treatment for failing flap b hysical Interventions lap urvival harmacology treatment Experimental Attempts Reference: Local flaps in facial reconstruction by Shan R. Baker 30 A. Thrombolytic Agents History of pharmacology thrombolysis Indications: o established extensive clot in either the arterial or the venous system. o no sufficient restoration of blood flow after blood clot evacuation o failure to re-establish venous outflow after establishing good arterial inflow. Contraindications: o o o o o recent stroke or malignancy (particularly if brain metastases are likely) renal insufficiency allergy cardiac thrombus diabetic retinopathy and coagulopathy Streptokinase o Dosage ranging from 50000 – 125000 units o slowest rate of clot lysis among the three agents o Stimulates antibody production making retreatment difficult. o Higher incidence of allergic reaction urokinase Urokinase is obtained from human fetal kidney cells. The advantages of urokinase over streptokinase include: 31 o less antigenicity, o direct plasminogen activation, o allowing use in a high concentration as distinguished from streptokinase, decreased systemic effects reported clinically Tissue plasminogen activator (t-PA) o Most flaps were slowly injected in 1 minute via the arterial pedicle with a dose between 2 and 10mg diluted in saline at a concentration of 1mg/mL. o Khansa et al. reported on infusion of up to 40mg of t-PA in some cases but prevented the thrombolytic drug from entering the systemic circulation. o t-PA has several beneficial attributes that include specific affinity for fibrin-bound plasminogen, enhancement of enzyme activity in the presence of fibrin, rapid onset of activity, and a short half-life (range 3.6 - 4.6 minutes). B. Anticoagulant Agents Heparin o The intraoperative use of heparin as a bolus and for irrigation o A retrospective clinical evaluation of free flap failures demonstrates safety and efficacy of both intraoperative bolus heparin (5000 IU) and low dose intravenous standard heparin (2000–3000 IU bolus, continued by 100–400 IU per hour) o More frequent haematoma rates were observed when high dose heparin was given. S.S. Kroll, M.J. Miller, G.P. Reece, B.J. Baldwin, G.L. Robb, B.P. Bengtson, M.D. Phillips, D. Kim and M.A. Schusterman, Anticoagulants and hematomas in free flap surgery, Plast. Reconstr. Surg. 96 (1995), 643–647 Aspirin o Aspirin is an inhibitor of both prostaglandin synthesis and platelet aggregation. o It was also reported that a low oral dose of aspirin (325 mg/day) did not cause postoperative hematoma formation in clinical free flaps. o One retrospective clinical study aims at aspirin treatment and free flaps survival in head and neck reconstruction. o In this trial, oral aspirin (325 mg daily) was found to be safe and equivalent compared with subcutaneous heparin (5000 IU bid). 32 Dextran o Dextran is a heterogeneous polysaccharide used in clinical hemorheology as a volume expander. o It is effective as an antithrombotic agent, as well, by decreasing factor VIII and von-Willebrand-factor activity and by inhibiting platelet function. o Dextran 40 is the most popular dextran used to decrease platelet aggregation and to improve blood flow in free flap surgery. o However, dextran 40 also has undesirable side-effects such as anaphylaxis, pulmonary and cerebral edema, and renal failure. o clinical evidence is accumulating to indicate that pre- or postoperative low-molecular-weight dextran treatment may not be effective in augmenting free flap viability C. Antispasmodic agents 1.Local 2.Calcium Channel Blocker anesthesia 3.Alpha Antagonist 4.phosphodiest erase inhibitors Direct Vasodilators In general, antispasmodic agents can be classified into five pharmacologic categories based on their primary mechanisms. Fig. 11 Mechanisms of topical vasodilation. 33 Drug class Example characteristics phosphodiesterase Papaverine, Papaverine inhibitors pentoxifylline o time to effect with papaverine was less, between 1 and 5 minutes after topical application. Advantages. o it is one of the most studied drugs to treat microvascular vasospasm. o it is an effective spasmolytic agent with a quick onset and a reasonable duration of effect. Disadvantages: o Of note, available papaverine solutions are typically quite acidic (pH, 3 to 4.5) and can be caustic to the vascular endothelium, inducing apoptosis of vascular endothelial and smooth muscle cells in animal investigations. o For this reason, systemic administration is contraindicated, and use of papaverine as an intraluminal irrigate has been questioned. Local anaesthetics Lidocaine, o bupivacaine The precise mechanism by which lidocaine exerts its local vasoactive properties is unclear. o Some authors have postulated that vasodilation may result from inhibition of voltage gated sodium channels, producing a decrease in intracellular sodium and calcium concentration within vascular smooth muscle cells. o Doses Evaluated: 1%, 2%, 4%, 8%, 10%, 12%, 20% o The ideal concentration of lidocaine to resolve microvascular vasoconstriction has been reported to be 12% because this dose was as effective as, but less toxic than, 20% lidocaine in a rat tail artery study that evaluated ergotamine-induced vasospasm. 34 o However, the safetyand efficacy of lidocaine greater than 2% have not been systematically examined. o For humans, the safety of doses in excess of 2% has not been conclusively established. o Calcium channel Nicardipine, Calcium channel blockers work by blocking transmembrane blockers nifedipine, voltage-gated calcium channels, effectively decreasing the verapamil, intracellular calcium concentration in vascular smooth magnesium muscle cells and ultimately producing vasodilation. sulfate o Advantages: o Extraluminal verapamil and papaverine were demonstrated to be effective antispasmodic agents following NE vasoconstriction, but verapamil showed higher flow rates across the porcine gastroepiploic artery (GEA) compared with papaverine o Verapamil was a found superior to 2% lidocaine at preventing flap marginal necrosis in a rat abdominal wall skin flap model following injection adjacent to the arterial anastomosis Direct vasodilators o Disadvantages: Insufficient study in humans Nitroprusside, o Advantages: Non apparent prostaglandin o Disadvantages: insufficient study in humans E1 , o Major systemic side effects: Hypotension, tachycardia o Mechanism of Vasodilation: Alpha-1 receptor antagonist o Advantages: Encouraging in vivo results o Disadvantages: Insufficient study in humans o Major systemic effects: Reflex tachycardia nitroglycerin, hydralazine Alpha antagonists Phentolamine, chlorpromazine (Reference: Vargas, Christina R.; Iorio, Matthew L.; Lee, Bernard T. (2015). A Systematic Review of Topical Vasodilators for the Treatment of Intraoperative Vasospasm in Reconstructive Microsurgery. Plastic and Reconstructive Surgery, 136(2), 411–422.) 35 Which is the preference among plastic surgeons? Between May and August 2008, an email questionnaire was sent to all 281 consultants in the 49 ‘main’ Plastic Surgery Units listed in the BAPRAS Members & Associates 2008 Booklet. 36 1. The choice of agent and dose to use is often institution- or surgeon-specific and there is no clear consensus regarding the optimum drug or dose. 2. Based on the studies examined, CCBs (nicardipine/nifedipine/GTN-verapamil) appear to have the greatest efficacy in preventing vasospasm and inducing vasodilation following microsurgical anastomosis compared with other agents 3. Future studies need to further compare various CCBs, both topically and systemically, to identify the most effective antispasmodic agent in this class D. Free Radical Scavengers Superoxide dismutase Hawkes, Young, and Cleland note anaphylactic reactions in a pig model associated with the use of superoxide dismutase. iron chelator deferoxamine ● Fe2+ + H2O2 → e3+ + OH− + •OH (Haber-Weiss reaction). ● Inhibit hydroxyl radical (OH) formation. Reduces haematoma-related flap necrosis. These benefits, however, occur with significant toxicity that can be ameliorated (in pigs) by the conjugated form, deferoxamine-hespan (DFO-H). DFO-H has a longer half-life, but with reduced efficacy in augmenting flap survival. This was postulated to decrease its ability to reach the intracellular oxygen free radicals. Allopurinol Elevated levels of xanthine oxidase have been noted to be elevated in ischaemic flap tissue in animals. Allopurinol is a xanthine oxidase inhibitor, reduce SOR production. A beneficial effect has yet to be shown in humans. PicardAmi and colleagues, however, note that xanthine oxidase levels in human tissue are 1/40th of those in rats, casting doubt on XO as a major source of free radicals responsible for tissue injury and flap necrosis in human skin. 37 B. Physical Moist diminished the depth of tissue loss and increased flap survival, presumably by minimizing desiccation of ischemic tissue. (McGrath in 1981) Warm Warming of the flap prevent vasoconstriction and prevent increased blood viscosity, with resultant increase in skin blood flow Surgical Delay The delay procedure is a preliminary surgical intervention wherein a portion of the vascular supply to a flap is divided before the definitive elevation and transfer of the flap. The resulting benefit, termed the delay phenomenon, is extension of the longitudinal reach of a flap’s vascular pedicle, creating a greater flap area due to the survival of a more extended random cutaneous component distally. Purpose: 1. Increase the surviving length of a flap. 2. Improve the circulation of a flap to diminish the insult of transfer. Mechanism of delay phenomenon: 2 school of thoughts 1. Delay conditions tissue to ischaemia, allowing it to survive on less nutrient blood flow than normally needed. 2. Delay improves or increases vascularity. *Most likely a combination of both mechanisms. Theories have been proposed to explain delay phenomenon: 1.Increased axiality of blood flow ▪ Removal of blood flow from the periphery of a random flap promotes development of an axial blood supply from its base. ▪ Axial flaps have improved survival compared to random flaps. 38 2.Tolerance to ischaemia • Cells become accustomed to hypoxia after the initial delay procedure. • Less tissue necrosis therefore occurs after the second operation. (Reference: Dhar, S. C., & Taylor, G. I. (1999). The Delay Phenomenon: The Story Unfolds. Plastic and Reconstructive Surgery, 104(7), 2079–2091) 3.Sympathectomy vasodilatation theory 39 • Dividing sympathetic fibres at the borders of a flap results in vasodilatation and improved blood supply. Figure : Active dilatation of choke vessels due to relaxation of the sympathetic tone on the vascular smooth muscle . choke vessels reorient themselves and increase blood flow Reference : Ghali S, Butler P EM, Tepper O M, Gurtner G C. Vascular delay revisited. Plast Reconstr Surg. 2007;119(6):1735–1744. 4.Intraflap shunting hypothesis • Postulates that sympathectomy dilates AVAs, resulting in an increase in non-nutritive blood flow bypassing the capillary bed. • A greater length of flap will survive at the second stage as there are fewer sympathetic fibres to cut and therefore less of a reduction in nutritive blood flow. 5.Hyperadrenergic state Figure :Biphasic response to sympathetic nerve division (Pearl, R. M. (1981). A Unifying Theory of the Delay Phenomenon– Recovery from the Hyperadrenergic State. Annals of Plastic Surgery, 7(2), 102–112.) • Surgery results in increased tissue concentrations of vasoconstrictors, such as epinephrine and norepinephrine. • After the initial delay procedure, the resultant reduction in blood supply is not sufficient to produce tissue necrosis. 40 ∘ The level of vasoconstrictor substances returns to normal before the second procedure. • The second procedure produces another rise in the concentration of vasoconstrictor substances. ∘ This rise is said to be smaller than it would be if the flap were elevated without a prior Delay -- The flap is therefore less likely to undergo distal necrosis after a delay procedure. Leeches Background: The word ‘leech’ is supposed to be derived from an old English word for physician,laece 1500 BC: Leeches were used for treatment in Egypt- treat ailments, like nosebleeds and gout. 1981: They have been shown to be useful in distal digital replantation (Foucher et al., 1981) and in the replantation of other tissues (Henderson et al., 1983). July 2004: the FDA approved leeches as a medical device in the area of plastic and reconstructive surgery Mechanism: • Mechanical- Blood suction following a bite will temporarily improve tissue perfusion by actively draining blood from congested tissue (mechanism demonstrated by laser Doppler analysis by Knobloch et al) • Biological effects-Once active suction is complete, passive blood loss will occur. Anticoagulants, inhibitors of platelet aggregation, and other vasodilators produced by leeches will allow blood flow at the bite site to continue even after the leech is detached. Side effects: • Excessive blood loss may necessitate a blood transfusion. • Allergic responses, including anaphylaxis. • bacterial infection from the gram-negative rod Aeromonas hydrophilia (which is the leech enteric organism responsible for red cell digestion) -Infections can arise 2 to 11 days after therapy begins and can result in abscesses and cellulitis - Prophylactic antibiotics are usually recommended: double coverage (two antibiotics) during therapy and single coverage (one antibiotic) for two weeks afterward [ 41 Limitations: 1. flap volume. The success rate falls to around 30% for high-volume flaps. such as TRAM or DIEP 2. Studies on hirudotherapy have a relatively low-level evidence. 42 3. FLAP MONITORING o The goal of postoperative free flap monitoring is to detect microvascular complications before permanent injury. o The ultimate success of microsurgical free-tissue transfer rests on optimizing the ability to identify and salvage a failing free flap o When thrombosis does occur, success of salvage is highly dependent on early clinical detection, with free flap salvage rates dropping from 62.2 to 21.4% if recognized after 16 h ( Young 2007) o Creech and Miller outlined essential criteria for an ideal monitoring technique should be: – Simple, and harmless to the patient and free flap – Rapid, repeatable, reliable, recordable, and rapidly responsive – Accurate and inexpensive – Objective and applicable to all kinds of flaps – Equipped with a simple display that could alert relatively inexperienced personnel to the development of circulatory impairment. Non-invasive Invasive Clinical assessment (gold standard) Implantable Doppler/ venous coupler Acoustic cutaneous Doppler Oxygen tension monitoring Color Doppler sonography Tissue pH monitoring Laser Doppler flowmetry Microdialysis Microlight-guided spectrophotometry Technetium-99m sestamibi scintigraphy Surface temperature monitoring Contrast-enhanced Doppler Tissue oximetry Biochemical markers from free-flap blood (e.g., glucose, lactate) Fluorescence imaging Perfusion-weighted MRI 43 ▪ Non-Invasive Monitoring Technique Description Gold standard – Clinical Assessment Advantages Disadvantages 1. Standard technique May be unreliable, impossible for some flaps 2.cheap (cost) 3. The effectiveness of clinical monitoring is high: one meta-analysis found flap success rates with clinical assessment alone to range. from 85 to 95% Handheld doppler An 8 MHz hand-held Doppler probe is used to perform periodic detection of venous and arterial Doppler signals cheap and portable not possible in buried flaps and flaps with difficult access e.g., the oropharynx. Colour Duplex Ultrasonography o This is a combination of b -mode ultrasound and bidirectional Doppler imaging. o It can be useful in the assessment of buried flaps. o US$30,000 to US$225,000 =~ 2 x Honda city car o Colour duplex sonography combines the recording of blood flow velocity with the recording of blood flow direction. o Excellent positive and negative predictive values o Combined colour flow and spectral Doppler imaging within both flap and recipient vessels enables accurate assessment of anastomotic patency o Requires for an ultrasound technician, radiologist, and 44 the microvascular surgeon to each be present to perform and interpret the examination. Laser Doppler Flowmetry o LDF measurements are relative and measured in blood perfusion units which is proportional to the velocity and concentration of RBCs within the tissues studied. o Using this technique, a laser light is emitted onto a tissue surface. The light penetrates to a depth of 1-2 mm causing its wavelength to shift by moving particles. o Moving red cells contribute to more than 99.9% of the total amount of moving cells within a blood vessel (Doppler effect). The shifted and un-shifted light is returned to photo detectors where it is processed and amplified. o Easy to use for surface flaps. o o The probe can be attached to the skin by using double-sided adhesive rings; sutures can also be used when a firmer attachment is required. Difficult to use in buried flaps, probe can be fragile and easily damaged. o The price of the laser Doppler flowmeter ranges upward from $5460, o the price of the probe ranges upwards from $1015. The probe can be used for at least 10 cases or half a year before it needs to be replaced 45 Surface temperature Monitoring o o o a reduction of 3°C in the centre of the flap indicated arterial occlusion. a reduction of 1°C to 2°C venous compromise The temperature can be monitored using: a) touch b) temperature probes c) temperature-sensitive tape o Simple o Not suitable for most oral or pharyngeal reconstruction o Kaufman’s study of muscle flaps suggested that temperature monitoring was not a reliable indicator of flap perfusion unless all environmental influences were monitored and kept absolutely constant so that perfusion was the only variable d) infrared thermometry 46 ii. INVASIVE MONITORING Implantable Doppler (Cook- Swartz Doppler ) o o The use of an implantable Doppler probe as a method for free-flap monitoring was first described in 1988 by Swartz; the device is commonly referred to as a Cook–Swartz Doppler. o Excellent positive and negative predictive values o A recent systematic review comparing the implantable Doppler to clinical monitoring—for head and neck, breast, and extremity reconstruction—found that overall, the implantable Doppler was associated with significantly greater flap salvage rates and overall flap success rates. The device consists of a piezoelectric crystal mounted on a silicon cuff that is wrapped around the anastomosed blood vessel— either artery, vein, or both— with wires extending to the surgical wound where they are connected to the equivalent of a cutaneous acoustic Doppler. o o By this means, the implantable Doppler provides continuous information about intravascular flow through the anastomosed vessel(s). Vascular compromise can therefore be detected instantaneously, allowing, in theory, a more prompt return to the operating room and better chances at flap salvage. o o Learning curve, difficulty detaching lead £1500-£2000 for monitor and £200-£300 for probe 47 o Microdialysis Micro dialysis involves insertion of a semipermeable micro dialysis catheter Excellent positive and negative predictive values o Labour intensive o Cost: £35,000.00 for the analyzer/monitor plus £350.00 per case for reagents and consumables. o = 2x into the flap (fat or muscle) intraoperatively. 48 o This is perfused using Ringers Solution at a constant rate using a batteryoperated pump to monitor the metabolism of a flap. o The analysis of each sample takes about 15 min and needs a minimum of 0.5_L of sample fluid and 15_L of reagent. o The perfusate is normally collected every hour so flap monitoring is not continuous. Ischaemia can be detected by monitoring the changes in glucose, lactate, and pyruvate levels in the interstitial fluid of monitored tissue. Fluorescein angiography o Fluorescein angiography was first described in 1977 to evaluate the viability of arterialized flaps after harvest. o Newer trials have employed indocyanine green (ICG) dye instead of fluorescein, stating its low side-effect profile and lack of vascular extravasation (compared with fluorescein which does not remain strictly intravascular) o Fluobeam™ system set-up to explore perforator vessels of the thigh o The technique is relatively straightforward and inexpensive o require additional investigation (larger trials, randomized headto-head comparisons, and cost analyses) to determine whether or not they truly provide any additional benefit over the traditional monitoring techniques discussed previously. 49 Summary Clinical monitoring remains the gold standard method of postoperative free-flap evaluation. Other adjuncts can prove useful to the surgeon and allied health team, but no definite consensus exists regarding their proper use. As with many postoperative regimens, free-flap monitoring techniques are surgeon-dependent and can vary significantly from institution to institution. Thank You. 50