What is the most likely cause of microvascular flap ischemia post-operatively with intact arterial connection, no congestion, and no bleeding upon needle pricking?

No-Reflow Phenomenon is the Most Likely Cause

The most likely cause in this clinical scenario is no-reflow phenomenon (option b), given the prolonged 5-hour ischemia time, intact arterial anastomosis on re-exploration, absence of bleeding on needle pricking, and lack of venous congestion.

Clinical Reasoning

The key diagnostic features pointing to no-reflow phenomenon include:

• Prolonged ischemia time of 5 hours creates the pathophysiologic substrate for microvascular obstruction, as no-reflow becomes progressively irreversible after extended ischemia periods, with experimental models showing irreversibility after 12 hours 1

• Patent arterial anastomosis on re-exploration definitively excludes macrovascular arterial occlusion or thrombosis at the anastomotic site 1, 2

• No bleeding on needle pricking indicates failure of microvascular perfusion despite patent inflow vessels, which is the hallmark of no-reflow 1, 2

• Absence of venous congestion rules out venous thrombosis or outflow obstruction, which would present with characteristic congestion and dark discoloration 3

Pathophysiology of No-Reflow in Free Flaps

No-reflow phenomenon represents microvascular obstruction that occurs despite technically adequate arterial anastomoses 1, 2. The underlying mechanisms include:

• Endothelial cell swelling and dysfunction from ischemia-induced cellular injury, creating physical obstruction to capillary blood flow 1, 2

• Intravascular aggregation of blood cells within the microvasculature during the ischemic period 1

• Interstitial edema from leakage of intravascular fluid, causing external compression of capillaries 1

• Intracellular calcium overload, oxygen-free radical damage, and altered arachidonic acid metabolism as the three central pathophysiologic processes 2

Why Other Options Are Less Likely

Atheromatous plaque (option a) is unlikely because:

• Free flap harvest typically involves healthy donor vessels without significant atherosclerotic disease 3
• The patient demographics and vessel selection in microsurgery make this improbable

Primary ischemia (option c) is excluded because:

• The arterial anastomosis was confirmed patent on re-exploration 1
• Primary ischemia would indicate ongoing arterial insufficiency, which was ruled out

Secondary ischemia (option d) typically refers to:

• Ischemia from venous congestion or outflow obstruction 3
• The absence of congestion makes this diagnosis incompatible with the clinical presentation

Microvascular thrombosis (option e) at the anastomotic level is excluded because:

• Re-exploration confirmed intact arterial connection with no occlusion 1
• While microthrombi may contribute to no-reflow, the primary pathology is endothelial dysfunction rather than discrete thrombotic occlusion 2

Clinical Implications and Management

The 5-hour ischemia time is critical because experimental studies demonstrate that no-reflow becomes progressively severe and potentially irreversible with prolonged ischemia 1. The threshold for irreversibility in experimental models is approximately 12 hours, but significant microvascular damage begins much earlier 1.

Management strategies for no-reflow in free flaps should include:

• Intraoperative steroids to address reperfusion injury and inflammation, used in 70.2% of salvage cases in one large series 3

• Vasodilators such as calcium channel blockers, prostaglandin analogs, and thromboxane synthesis inhibitors have shown benefit in preventing no-reflow 2

• Thrombolytic agents like tissue plasminogen activator may be considered, used in 55.3% of salvage attempts in breast reconstruction 3

• Postoperative therapeutic anticoagulation to prevent propagation of microthrombi, employed in 61.3% of salvage cases 3

Common Pitfalls

Do not assume arterial thrombosis without direct visualization - the intact anastomosis on re-exploration confirms this is a microvascular rather than macrovascular problem 1, 2.

Recognize that no-reflow can occur despite perfect surgical technique - this is an ischemia-time dependent phenomenon related to tissue-level pathophysiology rather than technical error 1, 2.

Early recognition is paramount - the progressive nature of microvascular obstruction means that delayed intervention reduces salvage potential 1, 4.