Angiosomes, perforasomes and the delay phenomenon

The angiosome: a three-dimensional territory

An angiosome is the three-dimensional block of tissue — skin, fat, fascia, muscle and bone — supplied by one source artery and its accompanying veins. Taylor and Palmer's injection studies mapped about 40 across the body, fitting together like a jigsaw in every layer, so the skin territory of a source artery overlies its deep territory.

Their anatomical observations still guide flap design. Vessels follow the connective tissue framework, travel with cutaneous nerves and radiate from fixed points such as septa and bony attachments; where one perforator is large its neighbours tend to be small, and vice versa. Venous territories mirror the arterial ones and are linked by avalvular ‘oscillating’ veins that allow flow in either direction across the boundary.

Choke vessels and true anastomoses

Adjacent angiosomes are connected in two ways: by choke vessels, which narrow at the boundary, and by true anastomoses, which cross between territories with no change in calibre. Choke vessels are the norm in skin, and their resistance is what limits how far a flap can be extended.

The working rule: a flap based on one angiosome reliably captures the adjacent angiosome across one choke zone, but the territory beyond a second choke zone is at risk of necrosis. Where territories are joined by true anastomoses, survival extends further. That is why partial flap loss is almost always distal, and why it reflects a design that outran its perfusion rather than a failed pedicle.

Perforators: direct and indirect

Perforators reach the skin either directly, with skin supply as their main purpose, or indirectly, as spill-over branches of vessels whose main job is to feed muscle. Direct perforators — direct cutaneous and most septocutaneous vessels — tend to be larger, longer and quicker to dissect; indirect perforators are the musculocutaneous vessels that must be teased out through muscle. Terminology varies between authors, and the Gent consensus simply calls them septal and muscle perforators.

Perforasomes

A perforasome is the territory of a single perforator, and Saint-Cyr's work showed that each is linked to its neighbours by direct linking vessels in the fat and indirect linking vessels through the subdermal plexus. A flap raised on one perforator therefore captures neighbouring perforasomes through these links, filling those from the same source artery before those of an adjacent one.

• Orientate along the linking vessels. They run axially in the limbs and roughly perpendicular to the midline on the trunk, so skin paddles are designed in those directions.
• Flow direction is predictable. Perforators near a joint direct their flow away from it; those near the trunk midline flow away from the midline.
• Size the paddle to the perforator. A large perforator carries a large perforasome; a small one may need a second perforator to support a big paddle.

Mapping perforators before surgery turns these principles into a plan. A handheld Doppler is quick and cheap but cannot tell a perforator from an underlying axial vessel and over-calls signals. CT angiography shows calibre, the point where each vessel pierces the fascia and its intramuscular course, which is why it is widely used before abdominal perforator flaps; colour duplex ultrasound gives similar information without radiation but depends on the operator.

The delay phenomenon

Delay is a planned partial interruption of a flap's blood supply, made before transfer, that increases the area the flap can carry. Incising the margins or dividing a secondary pedicle forces flow through the remaining source, and the tissue adapts.

• Choke vessel dilatation. Choke vessels enlarge by hypertrophy and hyperplasia until they behave more like true anastomoses; this begins within about 48–72 hours.
• Reorientation of the vascular network along the flap's long axis.
• Sympathectomy effect. Dividing sympathetic fibres causes an early catecholamine surge followed by depletion and loss of vasoconstrictor tone.
• Ischaemic preconditioning. The tissue becomes metabolically more tolerant of ischaemia.

The effect is roughly maximal at 10–14 days, so transfer is usually planned about two weeks after the delay. Examples include the delayed pedicled TRAM, in which the deep inferior epigastric vessels are ligated in advance, and long random or distally based flaps.

Supercharging and turbocharging

Both techniques add a second inflow or outflow to a flap; the difference is where the extra vessel is joined. Supercharging anastomoses it to a recipient vessel at the defect — the deep inferior epigastric vessels of a pedicled TRAM joined to the thoracodorsal vessels, or the superficial inferior epigastric vein of a congested DIEP drained to a second recipient vein. Turbocharging joins it to the flap's own primary pedicle or a branch, so no extra recipient vessel is needed — classically a bipedicled abdominal flap in which the contralateral pedicle is anastomosed to a side branch of the ipsilateral one.

From territory to monitoring

Flap monitoring asks whether the pedicle is still flowing. Global compromise of the whole flap is a pedicle problem and needs urgent return to theatre; patchy distal loss in an otherwise healthy flap is a territory problem and is managed on its merits.

Venous problems are more common than arterial ones, and most thromboses occur in the first 48–72 hours, which is when monitoring is most intensive. Clinical assessment is backed by handheld or implantable Doppler and, in some units, near-infrared spectroscopy; early re-exploration gives the best chance of salvage.