Hypoxia and Ischemia

Slide 1 · Hypoxia and Ischemia

Cerebral perfusion · energy failure · excitotoxicity · tissue fate

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain the metabolic dependence of the brain on uninterrupted oxygen delivery.

Distinguish reduced oxygen availability from inadequate cerebral blood flow.

Trace ischemic ATP depletion, calcium dysregulation, and excitotoxicity.

Interpret collateral perfusion, tissue at risk, and the double-edged effect of reperfusion.

Slide 3 · Where hypoxia–ischemia fits within cerebrovascular disease

Ischemic mechanism

Impaired oxygenation or blood supply

May be focal or widespread

Can lead to tissue infarction

Hemorrhagic mechanism

Blood escapes from ruptured CNS vessels

Distinct initial vascular event

Secondary tissue ischemia can accompany injury

Mechanistic point: Different vascular insults converge on CNS dysfunction and tissue injury.

Slide 4 · Neural tissue requires continuous oxidative metabolism

Perfusion

Delivers oxygen and substrates

Mitochondria

Generate ATP

Ion homeostasis

Maintains neuronal function

Viable tissue

Preserves signaling

Interrupting this chain threatens high-demand neurons.

Slide 5 · The brain’s disproportionate metabolic demand

1–2%

Of body weight

≈15%

Of resting cardiac output

≈20%

Of total oxygen use

A small organ receives high perfusion because its aerobic energy demand is continuous.

Slide 6 · Why cerebral metabolic dependence is distinctive

The brain has disproportionately high oxygen consumption relative to its mass.

Constant energy requirements arise from neuronal and glial cellular functions.

Strict reliance on aerobic metabolism for sustained demand.

Slide 7 · Two routes to insufficient cerebral oxygen supply

Reduced blood oxygen availability

Blood flow may be maintained

Less oxygen is delivered per unit of blood

Tissue hypoxia can occur without occlusion

Reduced cerebral perfusion

Blood delivery decreases to affected tissue

Oxygen and substrate delivery both decline

Washout of metabolic products is compromised

Slide 8 · Autoregulation stabilizes cerebral blood flow

Changing perfusion pressure

Vascular resistance adjusts

Flow relatively stable

Autoregulation

Active regulation of vascular resistance

Buffers changes in blood pressure and intracranial pressure

Limits

Flow can fall when compensation is insufficient

Vascular occlusion can defeat local regulation

Slide 9 · What autoregulation does—and does not—guarantee

Vascular resistance adjusts over a range of blood and intracranial pressures.

The goal is relatively steady cerebral blood flow despite pressure variations.

Autoregulation cannot guarantee adequate flow during severe hypotension or occlusion.

Slide 10 · Cerebral ischemia: Inadequate blood flow

Inadequate cerebral perfusion is the defining hemodynamic defect.

Reduced perfusion pressure, arterial occlusion, or both can be responsible.

Downstream tissue receives an insufficient oxygen and nutrient supply.

Slide 11 · Hypoxia and ischemia are related but not identical

Hypoxia

Primary limitation: Available oxygen

Perfusion and glucose delivery may persist

Partial anaerobic ATP formation remains possible

Ischemia

Primary limitation: Blood supply

Oxygen and fuel availability fall together

Waste metabolite removal also deteriorates

Slide 12 · Why ischemia is more damaging than isolated hypoxia

Hypoxia with flow preserved

Oxygen availability falls

Some substrate delivery persists

Anaerobic glycolysis can continue temporarily

Ischemia

Oxygen supply declines

Glucose delivery is limited

Metabolites are poorly cleared

Consequence: Energy failure usually progresses more rapidly in ischemia.

Slide 13 · Reduced perfusion pressure can produce ischemia

Hypotension

Lower driving pressure

Cerebral flow

May fall despite compensation

Oxygen delivery

Becomes inadequate

Injury

Depends on severity and time

Slide 14 · Arterial occlusion produces a downstream perfusion deficit

Narrowed vessel

Limited luminal blood delivery

Distal tissue

Flow falls in supplied area

Metabolic stress

Oxidative ATP production drops

Tissue risk

Viability depends on residual flow

Slide 15 · Small and large vessels can both be involved

Both small-vessel and large-vessel occlusion as sources of reduced perfusion.

The vascular territory shapes the distribution of injury.

The functional deficit follows the anatomic distribution of damaged tissue.

Slide 16 · Perfusion-pressure failure versus vessel occlusion

Pressure-driven deficit

Reduced systemic or regional perfusion pressure

Affects flow by diminishing the driving gradient

May be prominent during severe hypotension

Occlusion-driven deficit

Physical limitation within a vascular lumen

Territory distal to blockage becomes vulnerable

Collateral routes may partly compensate

Slide 17 · Determinants of tissue fate during flow reduction

Collateral flow

Alternate route for oxygen and nutrients

Duration

Length of threatened perfusion

Magnitude

Severity of flow decline

Rapidity

How abruptly flow is lost

Together: They determine lesion site, size, and neurologic deficit.

Slide 18 · Collateral circulation modifies ischemic injury

Alternative vascular routes can continue to supply a threatened territory.

The amount of remaining flow influences whether tissue survives.

Limited collateral pathways increase vulnerability when the primary route fails.

Slide 19 · Autoregulation and collateralization serve different roles

Autoregulation

Adjusts resistance within the arterial circulation

Buffers pressure variation

Acts to preserve flow through existing vessels

Collateralization

Provides alternative routes around compromised supply

Determines residual supply to threatened territory

May limit the extent of infarction

Slide 20 · Ischemia duration is biologically decisive

Brief hypoxic or ischemic episodes may cause reversible functional injury.

Longer exposure raises the probability of irreversible cellular damage.

Duration must be interpreted together with severity and collateral flow.

Slide 21 · Magnitude and rapidity also determine outcome

A deeper fall in blood flow restricts oxidative ATP production more severely.

An abrupt reduction gives less opportunity for compensatory adjustments.

Magnitude and rapidity separately among determinants of survival.

Slide 22 · Cerebral ischemia links vascular injury to clinical deficit

Vascular event

Reduces local perfusion

Tissue damage

Varies in site and size

Neural circuit loss

Disrupts regional function

Clinical deficit

Reflects location

The same vascular mechanism can produce different deficits depending on where it occurs.

Slide 23 · Focal and global insults: Two spatial patterns

Cerebral oxygen / flow failure

Focal

Region supplied by a compromised vessel

Extent depends on duration and collateral supply

Global

Generalized perfusion/oxygen-delivery failure

Potentially widespread neuronal involvement

This lecture covers shared mechanisms; Focal and global patterns have separate Robbins units.

Slide 24 · Focal ischemia: Localization matters

Partial or complete arterial occlusion reduces flow to a defined region.

The anatomic territory and collateral supply influence lesion configuration.

Sustained severe focal flow loss may progress to infarction.

Slide 25 · Global hypoxic–ischemic injury: Diffuse exposure

Marked systemic hypotension or cardiac arrest reduces cerebral perfusion generally.

Reduced oxygen-carrying capacity is another route to widespread cerebral hypoxia.

The extent of injury is determined in part by insult duration and severity.

Slide 26 · How oxygen loss becomes cellular energy failure

Oxygen tension ↓

Terminal oxygen acceptor limited

Oxidative phosphorylation ↓

Mitochondrial ATP generation falls

ATP depletion

Energy-dependent processes fail

Cell injury

Swelling may precede death

Slide 27 · Mitochondrial oxidative phosphorylation is central

Brain cells rely heavily on mitochondrial generation of ATP.

Hypoxia directly compromises the oxygen-dependent process.

Mitochondrial damage itself can reinforce subsequent ATP failure.

Slide 28 · Why anaerobic glycolysis is insufficient in ischemia

Some oxygen-deficient states

Perfusion can continue to deliver glucose

Anaerobic glycolysis can contribute limited ATP

Metabolites may still be cleared

Sustained ischemia

Substrate delivery is interrupted

Metabolites accumulate locally

Anaerobic energy formation becomes inadequate

Slide 29 · Loss of ATP disrupts ion gradients

ATP depletion

Active transport loses energy

Na⁺/K⁺-ATPase

Pump activity decreases

Cation imbalance

Intracellular sodium increases

Water uptake

Cells swell osmotically

Slide 30 · ATP depletion produces cytotoxic cell swelling

Nucleus

ATP ↓

Na⁺/K⁺ pump ↓

Na⁺ and water influx

Osmotic water accumulation explains reversible cell and organelle swelling.

Slide 31 · Cytotoxic swelling is an early energy-failure response

Transport failure produces intracellular sodium accumulation.

Osmotically driven water entry enlarges the cell and its organelles.

At this stage, the injury may remain reversible if the insult abates.

Slide 32 · Severe or sustained ATP failure advances to necrosis

Persistent energy depletion compromises multiple cellular systems.

Membrane failure and irrecoverable mitochondrial injury can follow.

Necrosis as a terminal result of sufficiently severe ischemic injury.

Slide 33 · Calcium dysregulation amplifies cellular injury

Hypoxic–ischemic stress

Ion gradients deteriorate

Intracellular Ca²⁺ ↑

Homeostasis is lost

Mitochondrial damage

ATP synthesis declines further

Amplification

Injury becomes self-reinforcing

Slide 34 · Mitochondrial permeability transition and ATP loss

Calcium overload can favor opening of the mitochondrial permeability transition pore.

Loss of mitochondrial membrane potential further limits oxidative phosphorylation.

ATP depletion thus becomes both a cause and an effect of injury.

Slide 35 · Multiple injury pathways interact rather than act alone

Energetic failure

ATP depletion

Transport defects

Ion and water shifts

Mitochondrial injury

Further energy failure

Cell death

Necrotic injury may result

An injured neuron is not governed by one isolated biochemical pathway.

Slide 36 · Reversible injury and irreversible tissue injury

Potentially reversible

Residual cellular integrity

Swelling and dysfunction may subside

Restored substrate delivery can aid recovery

Irreversible

Persistent severe biochemical failure

Loss of membrane/mitochondrial integrity

Necrosis leaves tissue loss

Slide 37 · Hypoxic stress can trigger adaptive transcription

Induction of hypoxia-inducible factor-1 (HIF-1).

HIF-1 supports angiogenic, survival, and glycolytic responses.

An adaptive response does not guarantee neuronal preservation during severe ischemia.

Slide 38 · Adaptation has limits in a high-demand organ

Protective responses require time and surviving metabolic capacity.

The brain’s continuous aerobic demand makes protracted deprivation hazardous.

Injury severity reflects the balance between inadequate supply and available compensation.

Slide 39 · Excitotoxicity is a CNS-specific amplification mechanism

Ischemia can provoke inappropriate release of excitatory amino-acid neurotransmitters.

Glutamate is the example highlighted.

The resulting receptor-mediated calcium influx can injure neurons.

Slide 40 · Glutamate–NMDA receptor excitotoxicity

Extracellular space

Excess glutamate

NMDA receptor

Ca²⁺ influx ↑

Outcome: Neuronal damage from excess intracellular calcium

Slide 41 · Excitotoxicity connects neurotransmission to cell injury

Ischemic stress

Energy homeostasis perturbed

Glutamate release

Excitatory signaling increases

NMDA receptors

Excess calcium influx

Neuronal injury

Calcium-dependent damage

Slide 42 · Energetic failure and excitotoxicity are complementary

Primary energetic failure

Oxygen-limited ATP generation

Failure of ion pumps

Cell swelling and metabolic collapse

Excitotoxic amplification

Excess extracellular excitatory transmitter

NMDA-type glutamate receptor activation

Calcium influx further injures neurons

Slide 43 · Excess calcium is a biochemical convergence point

Calcium overload is implicated in both general ischemic injury and neuronal excitotoxicity.

Mitochondria are vulnerable to cytosolic and mitochondrial calcium elevation.

This reinforces the connection between signaling failure and energy depletion.

Slide 44 · One insult may induce more than one route to cell death

The penumbral discussion recognizes both necrosis and apoptosis in ischemic neurons.

Animal-model rescue by antiapoptotic interventions supports an apoptotic component.

This observation does not mean all ischemic neurons die by apoptosis.

Slide 45 · Core, penumbra, and adjacent viable brain

Core

Penumbra

Viable brain

Interpretation

Core: Critically injured tissue

Penumbra: Threatened but potentially recoverable

Outside: Tissue with adequate perfusion

Schematic: Nested zones show a concept—not a fixed anatomic boundary.

Slide 46 · The ischemic penumbra is threatened—not necessarily dead—tissue

The penumbra at the transition from necrotic to relatively normal brain.

It is described as an area of brain at risk rather than established infarct.

Its potential reversibility provides the biological rationale for rescue.

Slide 47 · Core and penumbra are different biological states

Necrotic core

Injury has exceeded tissue viability

Restoration of flow may not reverse cell death

Infarction represents established tissue loss

Penumbra

Neurons remain vulnerable to worsening flow failure

Some tissue may recover if conditions improve

Potential survival is not guaranteed

Slide 48 · What determines whether the penumbra survives?

Collateral delivery

Maintains residual perfusion

Insult severity

Determines energetic stress

Insult duration

Limits reversible window

Final fate

Recovery or progression to injury

Slide 49 · Why rescue of penumbral tissue is plausible

A transition zone implies an intermediate state between normal and necrotic brain.

In experimental models, various antiapoptotic manipulations can rescue cells.

Clinical translation of experimental rescue is not established by this passage alone.

Slide 50 · Tissue at risk is dynamic, not a fixed ring

The conceptual penumbra is defined by viability, not by a standard thickness.

Blood-flow duration, severity, and alternative supply can change tissue fate.

An illustrative diagram must not be mistaken for a measured patient-specific border.

Slide 51 · The time-sensitive logic of ischemic tissue rescue

Perfusion falls

Threat to oxygen/ATP supply

At-risk tissue

Cellular stress evolves

Flow restoration

May rescue viable cells

Delayed restoration

Less tissue may remain viable

Slide 52 · Reperfusion has competing effects

Restored blood flow

Potential benefit

Reoxygenates viable tissue

Restores substrates to reversibly injured cells

Potential injury

Reactive species and calcium overload

Inflammation and complement activation

Clinical principle: Salvage and reperfusion injury are both possible.

Slide 53 · Reoxygenation is not invariably harmless

Resuming perfusion can revive cells whose injury is reversible.

Reoxygenation may initiate additional damaging reactions.

The total injury can therefore include ischemic and reperfusion components.

Slide 54 · Oxidative stress during reperfusion

Oxygen restored

Damaged cells encounter O₂

ROS/RNS generation

Oxidants increase

Antioxidant defense

May be impaired by ischemia

Cell injury

Oxidative damage increases

Slide 55 · Calcium overload can worsen after flow restoration

Acute ischemia initiates intracellular and mitochondrial calcium loading.

Reperfusion can exacerbate calcium entry across damaged membranes.

Mitochondrial permeability transition may intensify ATP failure.

Slide 56 · Inflammation adds an injury pathway

Injured cells

Release danger signals

Resident cells

Produce inflammatory mediators

Endothelium

Supports leukocyte recruitment

Reperfused tissue

May sustain added injury

Slide 57 · Complement can contribute to reperfusion injury

Complement activation among proposed mechanisms.

Some IgM deposits in ischemic tissue may interact with complement after flow returns.

The resulting response can amplify inflammation and parenchymal injury.

Slide 58 · Major biochemical pathways: Integrated comparison

Pathway

Initiating change

Consequence

ATP depletion

Reduced oxidative phosphorylation

Pump failure, swelling, necrosis

Excitotoxicity

Excess glutamate / NMDA activation

Neuronal Ca²⁺ overload

Oxidative stress

Reactive species on reperfusion

Additional macromolecular injury

Inflammation

DAMPs and leukocyte recruitment

Secondary tissue damage

Complement

Activation after reperfusion

Amplification of injury

Slide 59 · Regional tissue injury explains neurologic manifestations

The site and size of ischemic injury to the clinical deficit.

Localization reflects the affected vascular territory and the functions of that neural tissue.

The hemodynamic mechanism alone does not define a single stereotyped presentation.

Slide 60 · Nonhemorrhagic and hemorrhagic infarction: Boundary concept

An arterial infarct can initially be nonhemorrhagic.

Secondary hemorrhage may accompany subsequent reperfusion of damaged small vessels.

Detailed infarct morphology and hemorrhagic transformation belong to the focal ischemia lecture.

Slide 61 · Application: Low oxygen availability

Clinical vignette

A patient develops reduced blood oxygen availability while cerebral arterial perfusion remains present.

Which mechanism is primarily impaired?

Reasoning: This is principally an oxygenation defect; Flow-dependent glucose delivery may initially persist.

Slide 62 · Application: Occluded cerebral artery

Clinical vignette

An arterial branch becomes obstructed, reducing flow in its distal territory.

Which variables predict tissue survival?

Reasoning: Collateral supply, duration, magnitude, and rapidity of flow reduction govern tissue fate.

Slide 63 · Application: Return of blood flow

Clinical vignette

A threatened region is reperfused after a transient period of severe ischemia.

Why can reperfusion have two effects?

Reasoning: Viable cells may recover, while oxidant, calcium-mediated, and inflammatory injury may add damage.

Slide 64 · Synthesis: Determinants and mechanisms

Hemodynamic determinants

Oxygen availability and cerebral blood flow

Collateral supply and perfusion pressure

Magnitude, rapidity, and duration of reduction

Cellular mechanisms

Failure of oxidative phosphorylation and ATP

Ion-pump failure, swelling, mitochondrial injury

Glutamate excitotoxicity and reperfusion pathways

Slide 65 · From vascular compromise to clinical injury

Low oxygen / flow

Cerebral supply crisis

Energy failure

ATP and ionic gradients fall

Amplified damage

Excitotoxicity / reperfusion

Tissue consequence

Reversible injury or infarction

Tissue fate—and therefore neurologic outcome—depends on the residual viability of the affected region.

Slide 66 · Concept check: Three noninterchangeable terms

Hypoxia: Tissue oxygen availability is insufficient.

Ischemia: Blood flow is inadequate, restricting oxygen, substrates, and clearance.

Penumbra: Threatened peri-infarct tissue that may still be recoverable.

Interpretation: Excitotoxicity is a cellular mechanism, not a synonym for infarction.

Slide 67 · Reference and source boundaries

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026), Chapter 28, pp. 1136–1137.

Foundational mechanisms drawn from Chapter 2, pp. 45–51 (ATP failure and reperfusion injury).

Focal and global cerebral ischemia are introduced only as context; each has a separate, more detailed topic.