Critical Ischemia Time Research Articles

Effect of clamp versus anastomotic‐induced ischemia on critical ischemic time and survival of rat epigastric fasciocutaneous flap

Effect of clamp versus anastomotic‐induced ischemia on critical ischemic time and survival of rat epigastric fasciocutaneous flap

An integrated theory of the no-reflow phenomenon and the beneficial effect of vascular washout on no-reflow.

No-reflow is failure of perfusion in free tissue transfer despite adequate arterial inflow. The objectives of this study were to construct a theory of interactive mechanisms of the no-reflow phenomenon and to determine whether preischemic vascular washout could increase flap ischemia tolerance. The evidence for the role of various mechanisms in the development of no-reflow is reviewed, and an integrated network proposed. A rat-groin free flap model is used to test preischemic vascular washout with normal saline, heparinized normal saline, lactated Ringer's solution, Tis-U-Sol, and Viaspan. The mean ischemia tolerance of this flap without any therapeutic intervention was first determined, using 22 animals. An additional 50 animals were used to compare with the control group the ischemia tolerance of flaps washed out with the above fluids before their ischemic period. The critical ischemia time 50 (time after which half of the flaps are expected to survive and half, die) of the untreated flap is 23.4 hours in this model (P<.05). Flaps washed out with normal saline or lactated Ringer's solution have significantly worse ischemia tolerance (P<.0001). Flaps washed out with Tis-U-Sol or Viaspan behave similarly to the control group (P>.57). Flaps receiving preischemic washout with heparinized normal saline (4,000 units/L) had a significantly better outcome than the control group (P<.027). Preischemic washout with normal saline, lactated Ringer's solution, or heparinized Tis-U-Sol is detrimental for flap survival after ischemia, Tis-U-Sol- and Viaspan-treated flaps do have ischemia tolerance similar to the control group, and flaps washed out with heparinized normal saline have a survival advantage in this model.

Ischemic preconditioning of a rat adipomusculocutaneous flap: which duration of ischemia/reperfusion works best?

In this study a comparison was made to the efficacy of three cycles of various duration of ischemic preconditioning (IPC) on flap survival after the critical ischemic time. Method: In the first study, 20 Sprague-Dawley rats were used (6 groups of rats; flap ischemia ranging from 6 to 14 h) to determine the critical ischemia time for the rat groin flap (6×3 cm). In the second study the rats (n=31 were divided into nine groups; ischemia and reperfusion times being 5, 10 or 15 min) a groin flap was elevated. Thereafter IPC was induced to the flap by clamping its pedicle prior to the critical ischemic time (14 h). Three cycles of IPC were applied. After seven days, the flap was retraced for viability assessment and the surviving flap area was calculated with planimetry. Results: Without pretreatment with IPC flap survival after 8, 10, 12, 13, and 14 h global ischemia was respectively 100%, 77±13%, 78±15%, 42±16%, and 2±4%. When the flaps were treated with three cycles of IPC before 14 h global ischemia the significant flap survival was noted in the 10/5 min and 15/10 min IPC groups (p<0.05). Overall, the 5 min reperfusion groups increased flap survival (38±26%; p<0.05), and had fewer total flap losses (2/9) than the 10 or 15 min reperfusion groups. Only if ischemia time was longer than reperfusion time the flaps had increased survival area (40±22%, p<0.05). Conclusion: This study demonstrated the efficacy of IPC to improve the viability of a composite flap even after a long critical ischemic time. The optimal IPC duration was 10/5 min combination, yet the 15/10 schedule was also acceptable.

Ischemic preconditioning improves the survival of skin and myocutaneous flaps in a rat model.

Inadequate blood supply of pedicle flaps results in partial necrosis, and prolonged ischemia during free-tissue transfer can result in partial or complete flap necrosis. Recent research in the field of cardiovascular surgery has shown that ischemic preconditioning (repeated brief episodes of coronary artery occlusion followed by reperfusion) improves myocardial muscle survival when the heart is subsequently subjected to prolonged ischemia. Preconditioning of skin or myocutaneous flaps as either pedicle or free flap models has never been studied. The goal of this investigation was to measure the effect of ischemic preconditioning on myocutaneous and skin flap survival areas and total necrosis rates after variable periods of global ischemia. In 220 rats, 100 transverse rectus abdominis myocutaneous flaps and 120 dorsal cutaneous flaps were randomized into treatment and control groups. The treatment flaps underwent preconditioning by three cycles of 10 minutes of pedicle clamping followed by 10 minutes of reperfusion for a total preconditioning period of 1 hour. The control flaps were perfused without clamping for 1 hour. Both control and treatment flaps then underwent global ischemia for 0, 2, 4, 6, 10, or 14 hours by pedicle clamping. Flap survival area was measured on the fifth postoperative day. Statistical analysis was performed with analysis of variance, student's t tests, and probit analysis. Preconditioning improved survival areas of pedicle myocutaneous flaps (0-hour group) from 47 +/- 16 percent (mean percent area surviving +/- SD) to 63 +/- 5 percent. This difference was statistically significant (t test, p < 0.04). There was no statistically significant improvement in pedicle skin flap survival. For free flap models (flaps undergoing global ischemia), preconditioning increased the survival areas of skin and myocutaneous flaps (analysis of variance, p < 10(-5)). For the skin flap model, statistical significance of the survival area difference was reached at 6, 10, and 14 hours of ischemia (t test, p < 10(-4)). The magnitude of this effect was higher in the myocutaneous flap model and reached statistical significance at 2, 4, 6, and 10 hours of ischemia (p < 10(-3)). Preconditioned flap survival areas were increased by two to five times that of non-preconditioned flaps at these ischemia times. Preconditioning lowered total necrosis rates at all ischemia times for both flap models. The critical ischemia time when 50 percent of skin flaps became totally necrotic (CIT50) improved from 6.9 to 12.4 hours by preconditioning. Similarly, preconditioning improved the CIT50 of myocutaneous flaps from 3.6 to 9.2 hours. For the first time, statistically significant improvements of partial necrosis areas and total necrosis rates have been demonstrated through intraoperative ischemic preconditioning of skin and myocutaneous flaps. In clinical practice, application of this technique may lead to improved survival during pedicled or free transfer of myocutaneous flaps and free transfer of skin flaps.

Primary critical ischemia time in the hairless mouse ear.

The primary critical ischemia time of the hairless mouse ear was determined using a probit analysis. An atraumatic clamp was used across the pedicle base of the ear to induce total ischemia at 0, 2, 4, 6, 7, 7.5, 8.0, 8.5, 9.0, 9.5, and 12 hours of normothermic ischemia. Seventy-seven ears were examined 7 days after the ischemic interval for evidence of necrosis. The median critical ischemia time was determined to be 8.2 hours with 95 percent fiducial limits of 7.74 hours and 8.69 hours.

Use of urokinase during secondary ischemia in experimental skin flaps.

The ability of urokinase to salvage experimental flaps after a secondary ischemic insult was investigated in a rat model. Unilateral abdominal island skin flaps based on the superficial inferior epigastric vessels were raised and subjected to either 4 or 6 hours of primary ischemia followed by 12 hours of reperfusion and varying lengths of secondary ischemia. At the conclusion of secondary ischemia, the flaps were perfused with either lactated Ringer's solution or urokinase. One group of flaps served as a control and received no postischemic perfusion washout. The secondary critical ischemia time at which 50% of the flaps failed clinically was greater for flaps perfused with urokinase. Furthermore, the survival rates for all flaps perfused with urokinase were significantly greater than either control flaps or flaps perfused with lactated Ringer's solution (p < 0.05). Flap survival decreased significantly in all groups with increasing primary and/or secondary ischemia time (p < 0.05).

Prolongation of Secondary Critical Ischemia Time of Experimental Skin Flaps Using UW Solution as a Normothermic Perfusate

A myriad of investigations have been published on the pharmacologic manipulation of flaps to enhance tolerance to ischemia. We recently reported a threefold increase in ischemic tolerance of the rat abdominal skin flap pedicle after 6 hours of primary ischemia and 12 hours of reperfusion. Flaps underwent normothermic perfusion washout with lactated Ringer's or U.W. solution, a newly developed organ preservation medium. Perfusion washouts were performed at one of three different points in the protocol: (1) onset of primary ischemia; (2) onset of secondary ischemia; or (3) 2 hours after onset of secondary ischemia. The last group was used to simulate the clinical situation in which flaps are discovered and salvage procedures instituted at a delayed time interval. This is the longest normothermic ischemic interval reported. We undertook the present study to determine the utility of the U.W solution in prolonging the tolerance of the flap to a second ischemic insult after a period of reperfusion. Seventy-five unilateral rat abdominal skin flaps were raised. Secondary ischemia was produced by placing a microvascular clamp across the inferior epigastric pedicle. Flap survival was assessed at 1 week postoperatively. While none of the nonperfused flaps survived 8 hours of secondary ischemia, at least 50% of the U.W. perfused flaps survived an average of 14 hours of secondary ischemia. Lactated Ringer's perfusion washout only modestly increased the ischemic tolerance. Perfusion washout in the secondary ischemic phase improved the ischemic tolerance to a significantly greater degree than in the primary ischemic interval.

Uw solution as an experimental microvascular skin flap perfusate

UW solution has been found to be an effective organ perfusate for transplantation. Initial studies in experimental pedicle skin flaps have also demonstrated its unique effectiveness in prolonged ischemia. To understand better the limits of its preservation properties without the influence of endothelial clamp damage, we have undertaken to study the properties of UW solution in experimental microvascular free flaps. Control, lactated Ringer's, and UW solutions were utilized in pedicle and microvascular free flaps in Sprague-Dawley rats over varying periods of ischemia. UW solution demonstrated a clear superiority over all other solutions in both flap models. In addition there also was a significant prolongation of critical ischemia time in UW-treated free flaps compared to pedicle flaps.

When Does a Random Flap Die?

A random flap can be constructed, its circulation determined, and the ischemic portion identified. Left untreated for a period, the critical ischemia time, the ischemic portion will die and is clinically recognized several days later. What is not known is when this tissue, destined to die, actually dies. To ascertain this time, we compared the percent necrosis of a distal 3 x 3 cm segment of a 10 x 3 cm reverse McFarlane random flap with a known distribution of necrosis to the percent necrosis of the distal 3 x 3 cm of full-thickness skin grafts taken from a similar reverse McFarlane flap at 0, 4, 8, 12, and 16 hours after pedicle construction. Implicit in this experiment is the assumption that necrosis of the full-thickness skin grafts in excess of that of control animals represented skin no longer viable. Sometime between 8 and 12 hours, the percent necrosis of the full-thickness skin grafts surpassed that of the control, and it was concluded that this graft was dead prior to grafting. Thus it is suggested that critical ischemia time and death of the flap tissue are nearly identical, and the latter occurs at between 8 and 12 hours.

Temporal Factors Affecting the Secondary Critical Ischemia of Normothermic Experimental Skin Flaps

In free tissue transfer surgery all flaps are subjected to a primary ischemic episode prior to reperfusion. Some flaps may fail due to a second ischemic insult in the postoperative period. The maximum allowable time for salvage of such failing flaps is referred to as the secondary critical ischemia time. Unilateral abdominal island skin flaps based on the superficial inferior epigastric vessels were raised in 96 Sprague-Dawley rats. Animals were divided into 24 groups of four rats each. Normothermic ischemia was produced by applying microvascular clamps to the vascular pedicles. The flap groups were subjected to combinations of primary ischemia, reperfusion, and secondary ischemia. Flap survival was assessed on the seventh postoperative day. Flap survival decreases significantly with increased primary and/or secondary ischemia time and decreased reperfusion period. Moreover, a longer primary ischemia and/or shorter reperfusion decrease the tolerance of the flap to a second ischemic insult.

The Use of a New Perfusate in Experimental Microvascular Flaps: A Threefold Increase in Ischemic Tolerance

The benefit of perfusion washout in both experimental and clinical skin flaps has long been debated. By perfusing ischemic rat pedicled flaps with UW solution, a recently developed, high-molecular-weight, organ-preservation medium, a 170 percent increase in the critical ischemia time of treated versus untreated control flaps was demonstrated. Sixty rats were used in this study. A 3- x 6-cm unilateral abdominal skin flap based on the superficial inferior epigastric artery and vein was raised. The flaps were divided into three groups: Group 1 (control--no perfusion washout (n = 15); Group 2 (LR)--perfusion washout with lactated Ringer's solution (n = 15); Group 3 (UW)--perfusion washout with UW solution (n = 30). Flaps were subjected to varying periods of ischemia, ranging between 8 and 30 hr. The primary ischemia time at which 50 percent of the flaps survived clinically was 10 hr for Group 1, 15 hr for Group 2, and 27 hr for Group 3. The differences between the survival rates for flaps in Groups 1, 2, and 3 were statistically significant (p less than .0005). By bathing the vascular and parenchymal cells in an impermeant preservation solution, it was hypothesized that cellular swelling would be inhibited, thereby significantly improving a skin flap's tolerance to warm ischemia. Furthermore, after reviewing the pertinent literature, it is evident that the primary critical ischemia time of 27 hr is the highest reported to date for the normothermic experimental rat pedicled flap. Clinical application of these findings, as well as the need for further studies, are discussed.

Monitoring in Microvascular Surgery

The importance of monitoring in microvascular surgery is underscored by the high reported salvage rates of failing free flaps and replants. In this overview, we begin by defining the physiology of ischemic tissue with emphasis given to the no-reflow phenomenon and the secondary critical ischemia times. Based on the physiological changes accompanying ischemia, several variables are defined that can be monitored to reflect the vascular state of a free flap or replant. Multifarious monitoring systems are then reviewed, including clinical observation, temperature, isotope clearance, ultrasonic Doppler, laser Doppler, transcutaneous oxygen tension, reflection plethysmography, dermofluorometry, pH, electromagnetic flowmetry, serial hematocrits, interstitial fluid pressure, and magnetic resonance imaging.

Ischemic Tolerance of Human Skeletal Muscle

Until now, the ischemic tolerance of muscle tissue has not been adequately understood. Even when muscle vitality is lost, the perfusion matrix of the muscle flaps is retained. Because of toxic decomposition, however, irreversibly damaged muscle cells almost certainly increase the rate of complications. The retention of the vitality of the transplanted muscle tissue is absolutely essential for the myokinetic substitute operations, currently in the development stage, involving the free transplantation of muscles. Investigations into vitality reserves were carried out on skeletal muscle specimens. Nuclear magnetic resonance spectroscopy was used to establish that, in ischemia, the ATP pool remained topped up to a large extent as long as phosphocreatine was available. As long as the ATP pool was retained, rearterialization led to the complete restoration of the essential preischemic metabolite concentrations. After the ATP had been exhausted, biochemical restitution through arterial reperfusion did not occur. The time by which the established vitality threshold was reached because of the loss of the ATP pool is called the critical ischemia time; it depends on muscle temperature. The critical ischemia time of human skeletal muscles was determined between 26 degrees and 38 degrees C. A normothermia of 34 degrees C yielded a critical ischemia time of 2.25 hours, which is shorter than that previously reported in the literature. An ischemic tolerance of 5 hours presupposes a muscle temperature of less than 26 degrees C.

Critical Ischemia Times and Survival Patterns of Experimental Pig Flaps

Previous work on critical ischemia time suggested (1) a greater susceptibility of myocutaneous flaps over skin flaps to the ischemia reperfusion injury and (2) that duration of ischemia may affect the survival area of a flap. Using a pig model, 55 animals were operated on and the critical ischemia times and survival patterns of the buttock skin (n = 85) and latissimus dorsi myocutaneous (n = 88) island flaps were determined after being submitted to 0, 2, 4, 6, 8, 10, 12, 14, and 16 hours of normothermic ischemia. The average critical ischemia times (CIT50) were determined to be 9 and 10 hours for the buttock skin and latissimus dorsi myocutaneous flaps, respectively. Percentage of total area surviving (%TAS) in those flaps which did survive was adversely affected by increases in the ischemic interval in both flap models. A statistically significant decrease in percentage of total area surviving was found after 6 and 8 hours of ischemia for the buttock skin and latissimus dorsi myocutaneous flaps, respectively.

Critical Ischemia Times and Survival Patterns of Experimental Pig Flaps

Previous work on critical ischemia time suggested (1) a greater susceptibility of myocutaneous flaps over skin flaps to the ischemia reperfusion injury and (2) that duration of ischemia may affect the survival area of a flap. Using a pig model, 55 animals were operated on and the critical ischemia times and survival patterns of the buttock skin (n = 85) and latissimus dorsi myocutaneous (n = 88) island flaps were determined after being submitted to 0, 2, 4, 6, 8, 10, 12, 14, and 16 hours of normothermic ischemia. The average critical ischemia times (CIT50) were determined to be 9 and 10 hours for the buttock skin and latissimus dorsi myocutaneous flaps, respectively. Percentage of total area surviving (%TAS) in those flaps which did survive was adversely affected by increases in the ischemic interval in both flap models. A statistically significant decrease in percentage of total area surviving was found after 6 and 8 hours of ischemia for the buttock skin and latissimus dorsi myocutaneous flaps, respectively.

Sensory denervation with capsaicin does not influence the critical ischemia time in rat island flaps.

Capsaicin causes release and long-term depletion of neuropeptides from sensory neurons. Recently, critical rat pedicle flaps pretreated with capsaicin have shown decreased survival. In the present study, the effect of systemic capsaicin pretreatment on the no-reflow phenomenon was investigated in the rat. Effective depletion of neuropeptides was confirmed with functional tests and immunohistochemistry. Groin island flaps were exposed to 4, 6, 8, 10 or 12 hours of warm ischemia, and flap survival was judged one week postoperatively. In contrast to previously reported negative effects on pedicle flap survival after pretreatment with capsaicin, the present study showed no difference in flap survival between capsaicin-pretreated rats and controls. It is concluded that depletion of neuropeptides from sensory neurons does not have negative effects on the tissue's ability to withstand prolonged complete ischemia. As this contrasts with the findings in critical pedicle flaps, the influence of sensory neuropeptides and the necrosis mechanisms are likely to be different in these two types of ischemic conditions.

Primary and secondary critical ischemia times of myocutaneous flaps.

The primary critical ischemia time of the latissimus dorsi myocutaneous flap model was determined in the pig. Latissimus dorsi flaps were subjected to a primary ischemic insult of 2 hours (mimicking the ischemic event of free-tissue transfer). Following 12 hours of normal flow, the flaps were subjected to a second ischemic insult ranging from 0 to 12 hours. The secondary critical ischemia time (11.3 hours) was found to be statistically comparable to the primary critical ischemia time (9.1 hours). Questions are raised concerning the mechanism of action of this phenomenon and its clinical relevance.

Secondary critical ischemia time of experimental skin flaps.

Secondary ischemia time represents the interval between a postoperative vascular thrombosis of a free flap and its successful revascularization. Using an island-flap model in pigs, the skin was found to tolerate an average secondary ischemia time of 7.2 hours. The safe secondary critical ischemia time (10 percent probability of necrosis) is 4.7 hours. This compares with the primary ischemia times of 13.1 hours (average) and 7.0 hours (10 percent necrosis). The discrepancies between these observed values are discussed.

Effects of Delay in Administration of Potassium Cardioplegia to the Isolated Rat Heart

Ischemic injury to the heart in the period between aortic cross-clamping and administration of cardioplegic solution was evaluated in the normothermic rat heart model. After isolation and control perfusion with oxygenated Krebs-Henseleit bicarbonate buffer, the hearts were given lactated Ringer's cardioplegic solution (30 mEq of K + per liter) for 2 minutes at three different intervals following aortic clamping: no delay, 2-minute delay, and 5-minute delay. Thereafter, the hearts were left unper-fused and the time to initiation of ischemic contracture was recorded. Adenosine triphosphate (ATP) and creatine phosphate levels were measured in all groups prior to and at the conclusion of cardioplegia administration. A 2-minute delay in the administration of cardioplegic solution resulted in significantly lower ( p < 0.001) ATP levels that were restored after 2 minutes of cardioplegia administration. Contracture times were not significantly altered. A 5-minute delay resulted in significantly shorter ( p < 0.001) contracture times and significantly lower ( p < 0.001) ATP levels that were not restored to preischemic levels by 2 minutes of cardioplegia administration. The fate of the myocardium may be insensitive to events that occur during the earliest moments of ischemia provided that rapid administration of oxygenated potassium cardioplegia follows the ischemic period and restores preischemic high-energy phosphate stores. However, there is a critical ischemic time during the initial interval before cardioplegia that is associated with an impaired ability of the myocardium to tolerate subsequent ischemia.