BASICS OF MICROVASCULAR SURGERY
SOURCE: UTMB Dept. of Otolaryngology Grand Rounds
DATE: February 26, 1997
RESIDENT PHYSICIAN: Christopher Thompson, M.D.
FACULTY PHYSICIAN: Karen Calhoun M.D.
SERIES EDITOR: Francis B. Quinn, Jr., M.D.
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Head and neck defects are often inhospitable, requiring contact with saliva, nasal secretions, and tissues previously exposed to radiation and surgery. Well perfused free flaps are suited to these conditions. Pedicled flaps frequently have less perfusion at the margins, which may be very distant from the blood supply. Skull base defects may require a water-tight closure to prevent CSF leakage. Again, excellent perfusion at the wound edges make the free tissue transfers more likely to live up to these expectations.
Bony reconstruction is now virtually synonymous with free tissue transfer. Resorption, which plagued non-viable bony transfers is eliminated. Unlike other reconstructive techniques, primary osseointigration is now possible. Transferring well perfused tissues incites a strong union with the surrounding bone in as little as 1 to 2 months, eliminating the long term use of reconstruction plates.
Both functional and aesthetic advantages are abundant with free tissue transfers. Flap transfers capable of sensation are plausible with the use of neurofasciocutaneous free tissue transfers, unlike any of the pedicled flap transfers. Pedicle flaps are often less than perfect when the defect requires the extremes of massive bulk or thin, pliable tissue. Free flaps are not limited by these constraints. Along the same lines, pedicled flaps often transfer skin of a poor match to the host site. Free transfers provide a much wider range of skin characteristics.
The principal disadvantage in free tissue transfer is the technical demands required by the technique. A great deal of additional expertise and equipment is required intraoperatively, as well as perioperatively. This drives the costs of the patientís care up significantly. Although the technique is usually performed with a two-team approach, an average of 4 hours are added to an already lengthy surgical procedure. Even in the most experienced hands, one can expect a 5 to 10 percent flap failure rate, usually due to thrombosis. Flap failure necessitates a second operative procedure as well as additional donor site morbidity if a second flap is required. This problem is encountered less frequently with pedicled flap reconstructions.
There are no absolute indications for free flap reconstruction, and for virtually every instance in which free transfer is used, an argument for a pedicled graft can be made. However, certain defects have proved to be better indications for free transfer. Reconstruction of mandibular defects seem particularly suited to free tissue transfers as do reconstructions that require a thin, pliable consistency such as in the floor-of-mouth. Massive skull base defects present problems that free flaps seem to solve better than pedicled flaps. Also, because of the potential for neural anastomosis, facial reanimation has recently seen the application of this technology.
Concerning contraindications, there are a number of patient factors that severely limit the likelihood of successful free tissue transfer. Age in and of itself may not be important; however, many of the following patient factors are more often found in patients of advanced age. Severe cardiovascular disease and atherosclerosis obviously compromise the flap vascularity and strongly advise against such a procedure.
Diabetes is also a detriment to vascularity and often impairs wound healing. Connective tissue disorders may compromise the cardiovascular system and should be strongly considered. One must also consider the requirements for surgical and nursing personnel; because not only will they be required for the initial procedure, they must be available in the early postoperative period in the event of flap failure.
As in all microsurgical situations, maneuvers must be delicate and accurate. The vascular intima is easily damaged or sent into spasm with excessive traction. Attention to vessel drying is necessary to avoid spasm and thrombosis. Vessels should be constantly bathed in 37 degree centigrade normal saline or Ringer's solution. Excessive vessel cooling will also induce spasm, as will contact with fresh blood.
Great care is needed when lining the vessels up for anastomosis. The vessels should be adequately mobilized with as long a pedicle as possible. The greater the mobilization, the easier the anastomosis. Obviously, one wants to avoid any tension at the anastomotic site. But just as important is care in avoiding redundancy in vessel length. This will encourage kinking or twisting of vessels and ultimately thrombosis. Insetting the flap must be considered before the anastomosis. Flap geometry is often sited as a common cause of thrombosis postoperatively.
The process of microvascular suture is considered to be the most important determinant of vessel patency. Accurate alignment of the intima is of utmost importance. Suture placement must always be symmetrical. Knots must lie flat, and cannot be placed with excessive tension, as intimal damage will result.
The technical and time requirements necessitate a team approach. The resection team does some identification of suitable vasculature, but the reconstruction team will prepare the recipient area. Ideally, this is done while another team is harvesting the donor flap. Although such an approach requires up to 6 surgeons, the procedure moves efficiently and quickly.
End to end anastomosis is the simplest, most reliable, and most-widely used method. To prepare the vessels, 2-3mm of adventitia is stripped from the end of each vessel. This is done with the jeweler's forceps and microscissors under 10 - 16x magnification. This prevents adventitia from being caught in the lumen during anastomoses. The framed approximator clamp is then applied, bringing the two ends of the vessels close enough so that there is little tension during the anastomosis. It is important to remember that the clamps will be flipped after the anterior anastomosis is complete. Blood in the vessel lumen should be flushed, using heparinized saline via a #22 angiocath. Vessel spasm can be reversed with topical lidocaine or gentle dilation.
The needle is grasped using a two-handed technique by grasping the suture with one hand and the needle with the other. The needle is held just beyond its midpoint, 1-2 mm back from the end of the needle holder. Three guide sutures are placed 120 degrees apart, two on the anterior, and one in the posterior wall. . The tails are left long. It is imperative that these be accurately place, as guide sutures not exactly 120 degrees will result in twisting at the anastomoses. With the guides in place, equal number of interrupted sutures are placed between each suture. Enough sutures are placed so that there is no anastomotic leak. Usually a total of 9 sutures are necessary.
Needle placement must be accurate and symmetric. The needle entry point should be twice the thickness of the vessel wall away from the edge. Symmetrical entry should be taken on the opposite edge. Uneven placement leads to vessels overlapping and thrombus. Needle placement should utilize a two-handed action under 20x to 30x magnification. The needle lumen is cannulated with microforceps (in larger vessels the adventitia is grasped) to provide counter pressure as the needle is placed between. After the needle penetrates the wall, the needle is pulled along its arch. A two-pass technique is usually used, unless the edges are approximated.
Tying the suture correctly also impacts on the likelihood of vascular patency. Knots are generally tied under 16x magnification. They need to lie flat. The proper amount of tension must be applied each time. Excessive tension damages the vascular intima, inadequate tension hampers the vessel approximation. Surgeons knots are thrown first, followed by a simple square knot. After the anterior wall is sutured, the clamp is flipped, and the process is repeated.
As stated previously, the chance of thromboses is greatest at the site of anastomoses 15 to 20 minutes following the closure. It is therefore customary to observe the anastomosis and test its patency during this period of time. If partial obstruction occurs, gently squeezing the vessel with forceps, or massaging the vessel may break up the thrombus. A complete thromboses necessitates resection of the damaged area, and repeat anastomoses.
Vascular thrombosis is most commonly due to technical error in suture placement, or the use of a vessel with damaged intima. Venous rather than arterial thrombosis is the most common cause of flap failure. The thinner venous wall makes the anastomosis more fragile, more compressible, and more likely to twist and kink. After the first 20 minutes, the next critical period is postoperative days 1 - 3. In most cases, a flap that is successful at day 5, will not thrombose. However, the vascular pedicle cannot be safely divided for at least 8 days.
Flap geometry has been demonstrated in some studies to be an important determinant of flap survival. In retrospective reviews, experienced surgeons have noted problems with flap circulation due exclusively to poor choice of flap geometry. Before the anastomosis is begun, one must visualize how the flap will be inset to ensure an ideal path for the pedicle. Lack of attention here may result in vessel kinking and subsequent thrombosis. The most favorable geometry allows for the vessels to lie longitudinally in the neck without tension, yet without excessive pedicle length.
Hemodynamics and blood volume must be monitored closely. Although scant scientific evidence exists to support an ideal hematocrit in postoperative free flap patients, the consensus among experienced surgeons is somewhere between 27 and 29. It is important to inform the ICU personnel to avoid transfusing these patients without notifying the surgeon. Close surveillance for hematoma formation is necessary to avoid the deadly consequences of vascular compression. Blood pressure should be maintained appropriately.
Pharmacotherapy has become a routine is free tissue transfers, and much of the basis is borrowed from organ transplantation data. Aspirin therapy is initiated after the surgery using 5 - 10 grains daily for 2 to 3 weeks in order to inhibit platelet and endothelial cyclooxygenase. Dextran 40 is also part of most protocols with its viscosity lowering properties and inhibition of rouleaux formation. Antibiotics are given as usual for head and neck procedures. Delerium tremens prophylaxis is often necessary in this patient population.
It is self-explanatory that early detection of flap compromise allows for earlier intervention, and improved survival. It is with this in mind, that we have the development of so many different methods of monitoring. Temperature measurements have demonstrated reliability, and a recent review of 600 free flaps indicated a sensitivity of 98% and a predictive value of 75% using this technique. They continue to be plagued by interference from ambient temperatures when used in the oral cavity. A more recently introduced technique uses near infra-red spectroscopy to non- invasively monitor the concentrations of oxy and deoxyhemoglobin. Animal studies indicate accurate measurements through as much as 10cm of tissue. The device has yet to be used in humans. Transcutaneous and intravascular devices which measure oxygen tension have seen some enthusiasm, but expense continues to be an obstacle. The laser doppler flowmeter also holds promise, but is not applicable to deep flaps or those in the oral cavity. As in many cases in medicine, multiple different solutions to a problem indicate lack of a good solution. Clinical assessment will remain the standard until the expense and reliability problems of the others improve.
Once the patient is returned to the operating room, the flap is quickly dismantled and the anastomosis examined. Extrinsic pressure from hematoma formation, kinking, or twisting is logically performed first. Unless successful, the next step is takedown of the anastomosis. Once this is accomplished, the flap vessels are irrigated with heparinized saline, and the anastomosis is repeated. Failure of free irrigation indicates a long standing thrombosis in the microvasculature. Plasminogen activators such as strepto and urokinase, and recombinant tissue plasminogen activator (TPA) have been used successfully in such instances. These are applied with the venous drainage from the flap disconnected, to avoid systemic doses. This allows high concentrations of the drug. Once free irrigation is accomplished, the anastomosis are redone in the usual manner.
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