Skin flap physiology from Cummings

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Skin Flap Physiology — Cummings Otolaryngology Head and Neck Surgery

Core Principle

The most important determinant of flap success is vascular supply. All flaps — pedicled or free tissue transfer — depend on maintaining adequate perfusion to meet the metabolic demands of mobilized tissue. The distal flap is both the most critical for defect coverage and the most likely to fail.

Physics of Flow

Flow (Q) in a vessel is governed by the Hagen-Poiseuille equation:

Q = πr⁴(ΔP) / 8ηl

r⁴ = fourth power of vessel radius (dominant factor)
ΔP = perfusion pressure = P_diastolic − P_tissue
η = dynamic viscosity
l = vessel/flap length

Perfusion pressure (PP) = Diastolic BP − Tissue pressure

All clinical efforts target two goals: maintain normal PP and minimize resistance to flow.

Factor	Clinical Action
Systemic BP	Avoid hypotension and vasoconstriction
Tissue pressure	Avoid excess tension, edema, fluid overload
Vessel radius	Base flaps on large named vessels; use flap delay; free tissue transfer
Blood viscosity	Maintain hydration; target low-normal hematocrit
Flap length	Longer flaps = more resistance = greater risk of distal necrosis

Critical Closing Pressure

Critical closing pressure is the point at which tissue pressure exceeds intracapillary pressure, causing capillary collapse and cessation of flow.

In rigid tubes, flow rises linearly with pressure (Hagen-Poiseuille)
In distensible blood vessels, flow rises exponentially with pressure (LaPlace's law)
When tissue pressure rises above capillary pressure → vessels collapse → flow stops

LaPlace's law also explains why wider vessels dilate preferentially and how collateral flow develops over time (rationale for flap delay).

Effect of Flap Length on Viability

Fig. 77.1 Effect of flap length and tissue pressure on flap blood flow

Fig. 77.1 — As flap length increases, perfusion pressure falls. Where it intersects the critical closing pressure threshold, the flap becomes "at risk," then "necrotic." Falling systemic BP or rising tissue pressure shifts this intersection proximally.

Key myth dispelled: A wider base in a random flap adds vessels with the same perfusion pressure — it does NOT change the relationship between perfusion pressure and critical closing pressure, and does NOT increase survival length.

Zones of Perfusion

Flap perfusion is conceptualized in three zones:

Zone	Compartment	Key Determinants
Zone I — Macrocirculatory	Large vessels, pedicle, anastomosis	Systemic BP, vessel patency
Zone II — Microcirculatory	Arterioles, capillaries, venules	Tissue pressure, sphincter tone, A-V shunts
Zone III — Interstitial	Extracellular matrix, lymphatics	Starling forces, edema, diffusion distances

Zone II — Microcirculation

Terminal arterioles → metarterioles → precapillary sphincters → capillary beds
Arteriovenous shunts allow direct arteriole→venule bypass (thermoregulation)
Preshunt sphincters: regulate thermoregulation & systemic BP via vasoactive substances
Precapillary sphincters: regulate nutritive (capillary) blood flow
Elevated interstitial pressure compresses capillaries → decreased flow
Autoregulation = normalization of capillary flow in response to pressure changes
Lymphatics run parallel to blood capillaries; impaired by inflammation and loss of pulsation

Zone III — Interstitial System

Filled with proteoglycans, collagen, hyaluronic acid filaments — high resistance to fluid movement in normal hydration
Fluid movement occurs by two mechanisms:
1. Diffusion — dominant for small molecules; distance increases with edema → ↓ nutrient delivery
2. Convective flow (bulk flow) — fluid swept along microchannels; large molecules use this route

Starling equation governs transcapillary flow:

Jv/A = Lp[(Pi − Pi) − σ(πi − πi)]

Where Lp = membrane water permeability, P = hydrostatic pressures, π = osmotic pressures, σ = osmotic (reflection) coefficient.

Classification of Flaps by Vascular Supply

Fig. 77.5 Classification of skin flaps based on vascular supply

Fig. 77.5 — (A) Random, (B) Arterial cutaneous (axial), (C) Fasciocutaneous, (D) Musculocutaneous

Flap Type	Blood Supply	Plane of Dissection	Notes
Random cutaneous	Subdermal plexus from unnamed musculocutaneous perforators	Subcutaneous fat	Survival = function of PP, NOT length:width ratio
Arterial (axial) cutaneous	Named septocutaneous artery along longitudinal axis	Must include septocutaneous vessel	Better survival than random; distal extension = random territory
Fasciocutaneous	Septocutaneous vessels → deep fascial plexus → subdermal plexus	Deep fascia included	Relies on skin vascular territories; 4 subtypes based on fascial supply pattern
Musculocutaneous / Myocutaneous	Segmental vessels to muscle → musculocutaneous perforators	Muscle incorporated	Best survival; leaves local perforators intact; named for donor muscle

Examples in H&N:

Deltopectoral flap — anterior perforators of internal mammary artery
Paramedian forehead flap — supratrochlear vessels
Pectoralis myocutaneous flap — pectoral branch of thoracoacromial artery
Latissimus dorsi flap

Flap Failure

Failure is most detectable by the time it is irreversible — hence early monitoring is critical in at-risk cases.

Monitoring and salvage by zone:

Zone	Monitoring	Salvage
Zone I (macrocirculatory)	Doppler, near-infrared spectroscopy, temperature	Explore pedicle/anastomosis; thrombosis treatment
Zone II (microcirculatory)	Doppler, tissue oxygenation, tissue pressure	Vascular washout; reduce edema
Zone III (interstitial)	Clinical assessment	Address Starling forces, reduce edema

Risk factors for failure: diabetes, prior irradiation, hypotension, excess tension, edema.

Methods to Augment Flap Survival

Flap Delay

Creates collateral flow by preliminary interruption of part of the blood supply
Dilates remaining vessels via LaPlace's law over time
Extends viable flap length

Hyperbaric Oxygen (HBO)

Increases blood oxygen-carrying capacity by ~20% (oxygen dissolved in plasma)
Greater benefit when hematocrit is low
Also increases oxygen diffusion from perfused surrounding tissue into ischemic zones
Beneficial effects: ↓ leukocyte adherence, ↓ edema (vasoconstriction effect), ↑ neovascularization in irradiated tissue
Must be given early — little benefit if >24 hours after flap elevation
HBO challenge test: transcutaneous O₂ rise ≥50 mmHg predicts successful outcome
Not cost-effective for every flap; reserve for high-risk cases

Source: Cummings Otolaryngology Head and Neck Surgery, Chapter 77 — Skin Flap Physiology (pp. 1388–1395)

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Skin Flap Physiology

Presentation · PPTX

11 slides with a deep navy + surgical red palette:

Slide	Content
1	Title
2	Overview / Agenda
3	Physics of Flow — Hagen-Poiseuille, clinical goals
4	Critical Closing Pressure — flap length graph (Fig. 77.1)
5	Three Zones of Perfusion (macro/micro/interstitial)
6	Starling Forces & Capillary Exchange (Zone III)
7	Flap Classification — with the 4-type vascular diagram (Fig. 77.5)
8	H&N Flap Examples (deltopectoral, forehead, pec major, LD, radial forearm, fibula)
9	Flap Failure — risk factors, monitoring & salvage table by zone
10	Augmenting Survival — Flap Delay vs. HBO side-by-side
11	Key Takeaways + source citation

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