Altered Perfusion

Core Concepts of Altered Perfusion

• Definition of Altered Perfusion: The inability to adequately oxygenate tissues at the capillary level.

• Primary Factors: There are two fundamental factors that can alter perfusion:

  • Inadequate ventilation.

  • Inadequate perfusion.

Ventilation-Perfusion Mismatching

• Overview: Problems with the ventilation–perfusion ratio are identified as the most common cause of hypoxemia.

• Inadequate Ventilation: This occurs when there is a lack of airflow to well-perfused areas of the lungs. Examples include:

  • Asthma

  • Pneumonia

  • Pulmonary edema

• Inadequate Perfusion: This occurs when there is a lack of blood flow to well-ventilated areas of the lungs. This is typically caused by vascular obstructive processes, such as a pulmonary embolus.

Impaired Circulation and Mechanisms of Vessel Injury

• Circulation Problems: Impaired circulation refers to inadequate or excess blood flow to tissues or organs. It is caused by impaired circulatory patency and functioning through four primary mechanisms:

  1. Injury to the vessels.

  2. Obstructive processes.

  3. Inadequate movement of blood.

  4. Inadequate blood volume.

• Vessel Injury Components:

  • Loss of vessel integrity.

  • Loss of blood (haemorrhage) through the vessel wall.

• Secondary Causes of Hemorrhage:

  • Aneurysms.

  • Coagulation disorders.

  • Degradation of the vessel by neoplasms.

Obstructive Processes in Circulation

• Obstruction Mechanism: Obstructions block the free movement of blood through the circulatory system.

  • Occluded Arteries: Prevent oxygenated blood from reaching peripheral tissues.

  • Occluded Veins: Restrict venous return and lead to circulatory congestion.

• Thrombus: A blood clot that is a common source of obstruction. Thrombus formation is a response to injury and is essential for wound healing, but "undesired" thrombi can form in both arteries and veins.

• Virchow’s Triad: Three major factors responsible for thrombus formation in both arteries and veins:

  1. Vessel wall damage (Vasculature): This is the most common cause for thrombus formation in arteries and contributes to venous thrombosis, specifically through injury to the vessel endothelium resulting in atherosclerosis.

  2. Excessive clotting (Blood): Hypercoagulability.

  3. Alterations in blood flow (Flow): Turbulence or sluggish/stagnant blood movement (stasis).

Pathogenesis of Atherosclerosis

• Definition: A condition of irregularly distributed lipid deposits in the inner lining (intima) of large or medium-sized arteries, which may eventually occlude the artery.

• Step-by-Step Development:

  1. Injury to the intima.

  2. Low-density lipoproteins (LDL) become trapped and accumulate in the injured intima.

  3. Inflammatory cells migrate to the injury site; macrophages engulf the lipoproteins, producing foam cells.

  4. Foam cells accumulate and combine with lipids to form fatty streaks.

  5. Fatty streaks develop into fibrous plaques.

  6. Fibrous plaques accumulate and are covered by a platelet cap, known as red thrombus deposition.

Alterations in Blood Flow and Hypercoagulability

• Common Sites of Altered Blood Flow:

  • Bifurcations: Regions where a vessel branches.

  • Aneurysms: Local outpouchings in the vessel wall caused by structural weakness.

  • Venous Stasis: A condition of slow blood flow specifically in the veins.

• Hypercoagulability: The unregulated formation of thrombi throughout the body. Causes include:

  • Congenital: Genetic mutations causing excess coagulant factors or deficient anticoagulant mechanisms.

  • Acquired Mechanisms:

    • Autoimmune mechanisms activating platelets and altering coagulation factors.

    • Certain cancers or myeloproliferative disorders like thrombocythemia (excess platelets).

    • Sickle cell anemia.

    • Polycythemia vera (overproduction of Red Blood Cells/RBCs).

    • Use of oral contraceptives.

    • Vascular changes occurring in the late stages of pregnancy.

Thromboembolus Outcomes and Impact

• Potential Thrombus Outcomes:

  • A. Growth until the vessel is completely occluded.

  • B. Degradation by enzymes, leading to a decrease in size.

  • C. All or part of the thrombus breaks off and travels through the circulation.

• Embolus Classification:

  • Thromboembolus: A thrombus that has broken off and is traveling.

  • Embolus: Any plug of material traveling in the circulation capable of obstructing a lumen. Types include thrombi, air/foreign material, neoplasm fragments, microorganisms, and amniotic fluid.

• Embolus Origins and Destinations:

  • Venous Thromboemboli: Often originate in the deep veins of the legs, travel through the veins, and lodge in pulmonary arteries at points of narrowing or bifurcation.

  • Arterial Thromboemboli: Often originate in the heart as atherosclerotic plaques and travel to the brain, intestines, kidneys, or lower extremities (legs).

• Infarction:

  • An infarct is an area of necrosis resulting from a sudden insufficiency of blood supply due to vessel obstruction.

  • Process: Loss of blood supply $\rightarrow$ necrosis formation $\rightarrow$ loss of function in affected tissue.

  • Severity Factors: Depends on the size and location of the emboli.

  • Small Emboli: Result in less significant necrotic areas; areas with collateral circulation can perfuse tissue around the obstruction.

  • Larger Emboli: Occlude large vessels and their tributaries, which can cause sudden death.

Inadequate Cardiac Output

• Mechanism: The heart cannot successfully eject the necessary amount of blood into pulmonary and systemic circulation.

• Five Major Causes:

  1. Changes in blood volume, composition, or viscosity.

  2. Impaired ventricular pumping.

  3. Structural heart defects (e.g., valve defects causing leaking or regurgitation).

  4. Conduction defects leading to unresponsive heart rate and rhythm.

  5. Excessive or significantly reduced peripheral vascular resistance.

• Volume and Viscosity Particulars:

  • Altered by dehydration, hemorrhage, or hypercoagulation (Disseminated Intravascular Coagulation (DIC)).

  • Composition alterations: Anemia (fewer RBCs for oxygen transport). The heart overtaxes itself by trying to move fewer cells at a faster rate.

• Ventricular Pumping and Failure:

  • Loss of muscle activity prevents effective forward movement of blood through arterial circulation.

  • Results in venous blood congestion and impaired venous return.

  • Heart Failure: Defined by impaired ventricular pumping and venous insufficiency. Failure of one ventricle increases the workload on the other, often leading to total failure.

• Structural and Valvular Defects:

  • Congenital or acquired (infection, inflammation, trauma, degeneration).

  • Stenosis: Narrowing of the valve.

  • Regurgitation: Incompetence of the valve.

  • Most commonly affected: Bicuspid (mitral) and aortic valves.

• Conduction Defects:

  • Cardiac dysrhythmias indicate problems maintaining efficient rhythm.

  • Sites of dysfunction: SA node, AV node, cardiac cells joining the nodes, or the conduction systems in the atria/ventricles.

Excessive Perfusion Demands

• Mechanism: Tissue demands for oxygen exceed the supply, even when ventilation and diffusion are functioning optimally.

• Causes:

  • Extreme and prolonged exertion.

  • Metabolic alterations (e.g., hyperthyroidism).

  • Anemia.

General Manifestations of Altered Perfusion

• Impaired Cardiac Output Signs: Cyanosis, edema, shortness of breath, impaired growth, tachycardia, tachypnea, fatigue.

• Volume/Resistance Signs: Hypotension or hypertension.

• Obstructive Processes:

  • Myocardial or cerebral infarction: Loss of function, ischemia, pain, edema.

  • Deep vein thrombosis (DVT): Tenderness in the calf, specifically with dorsiflexion of the foot.

  • Total occlusion of veins: Edema, coolness, pallor, and cyanosis of the lower extremity.

• Hemorrhage (Skin manifestations):

  • Ecchymoses: Bruises from superficial bleeding into the skin.

  • Petechiae: Pinpoint hemorrhages.

  • Purpura: Diffuse hemorrhages of the skin or mucous membranes.

  • Hematoma: Large accumulation of blood in the tissue.

• Heart Sounds (Lub-Dup):

  • First sound (S1): AV valves close; marks the beginning of systole.

  • Second sound (S2): SL valves close; marks the beginning of ventricular diastole.

  • Pause: Indicates heart relaxation.

  • Murmurs: Abnormal sounds typically indicating incompetent or stenotic valves.

Diagnostic Aids to Detect Altered Perfusion

• Echocardiography (cardiac ultrasound): Identifies cardiovascular/valvular lesions, heart size/position changes, and wall motion abnormalities. Transesophageal echo uses an esophageal probe for clearer views of great vessels.

• Cardiac Catheterization: Insertion of a catheter into a vein or artery to withdraw blood samples, measure pressures, or inject contrast (coronary angiography). Also used for surgical corrections.

• Chest Radiograph: Shows heart borders, size, position, and detects pulmonary congestion/pleural effusion.

• Electrocardiography (ECG): Measures electrical impulses. Detects conduction disturbances, chamber size increases, ischemia, or infarction. Holter monitor provides $24$-hour portable recording.

• Pressure Measurements:

  • Sphygmomanometry: Indirect/noninvasive measurement of systolic/diastolic BP.

  • Invasive: Threading a catheter (usually femoral) to measure structural pressure directly.

• Stress Test: Exercise (treadmill/bicycle) with monitoring to detect myocardial ischemia, chest pain, or imaging changes. Pharmacologic agents can induce stress if exercise is not possible.

• Cardiac Nuclear Scanning: Uses intravenous radioactive compounds and a gamma camera. Myocardial perfusion scanning visualizes flow patterns. Cardiac gating synchronizes images with the ECG.

• Doppler Ultrasonography: Measures sound frequency changes to detect movement of red blood cells. Doppler color flow uses computer images where different colors represent different flow directions.

• Right Heart Catheterization: Required to confirm Pulmonary Arterial Hypertension (PAH).

  • Criteria for PAH: $mPAP \ge 25\,mmHg$ at rest and $PCWP \le 15\,mmHg$.

Treatment Measures

• Lifestyle Modifications: Weight reduction, BP management, stress reduction, smoking cessation, exercise, nutrition, and diabetes management.

• Pharmacologic Interventions:

  • Medications to dilate vessels, regulate BP, control rate/rhythm, increase contractility, or reduce workload.

  • ASA (Aspirin): Reduces platelet aggregation and clot formation.

  • Thrombolytic agents: Break through obstructions to revascularize tissue.

  • Pain medications: Specific to acute MI.

  • Oxygen therapy: To reduce hypoxemia.

  • Intravenous Fluids: Fluid or blood replacement for severe hemorrhage or dehydration.

• Surgical Interventions:

  • Coronary Artery Bypass Grafting (CABG): Bypasses obstructed arteries.

  • Percutaneous Transluminal Coronary Angioplasty (PTCA): Compresses fatty deposits.

  • Laser Angioplasty: Vaporizes fatty deposits with a hot-tip laser.

  • Repair of valve defects, removal of varicose veins, or drainage of fluid from the pericardial cavity.

  • Pacemaker Placement: To mechanically control heart rate and rhythm.