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VETERINARY CLINICAL CARDIOLOGY
CARDIOLOGY CONCEPTS
Cardiovascular Physiology
Vascular Control
1. What are the factors that control BP?
Hemodynamic factors:
  • BP = CO x Arterial resistance
  • Recall factors that affect CO (HR and SV)
  • Factors that affect peripheral arterial resistance
  • 2. What affects organ flow?
    Blood flow = perfusion pressure / vascular resistance

    Perfusion pressure = mean arterial pressure - mean venous pressure

    3. What factors affect peripheral vascular resistance (also called total peripheral resistance or systemic vascular resistance)?
    Autoregulatory factors (local mechanisms): due to local mechanisms within the tissue. These intrinsic mechanisms predominate over the extrinsic mechanisms for control of blood flow to critical organs (heart, brain, and working skeletal muscle)
  • Myogenic response: The form of auto-regulation in circumstances characterized by changes in blood pressure without changes in blood flow. This is well recognized in the brain, heart, and working skeletal muscle. Blood flow is altered in response to perfusion pressure in the absence of a change in metabolic rate. The resultant effect of the change in blood flow on metabolites changes arteriolar resistance to normalize blood flow in spite of an altered perfusion pressure.
  • Metabolic response: The increase in tissue blood flow in response to increased metabolic rate is called active hyperemia.
    • Increased metabolic rate causes increased tissue metabolites (especially CO2, adenosine, lactic acid, and K) and decreased O2
    • Increased metabolism causes local arteriole vasodilation with increased blood flow to the tissue
    • As the increased blood flow removes these metabolic products the stimuli for increased flow is removed so a reduction in blood flow back to resting state occurs.
    • Another form of metabolic response to tissue perfusion is known as reactive hyperemia, which refers to a temporary increase in blood flow to tissue after a period of restricted flow to this tissue. This occurs due to the accumulation of metabolites as described for active hyperemia.

    Extrinsic control factors (nerves/hormones): these mechanisms predominate over the intrinsic mechanism to control blood flow to non-critical organs such as the kidneys, skin, splanchnic, and resting skeletal muscles.

  • Factors promoting vasoconstriction
    • Sympathetic stimulation:Alpha1 and alpha2 adrenergic stimulation
    • Angiotensin II receptor stimulation
    • Endothelin receptor stimulation
    • Arginine vasopressin stimulation
  • Factors promoting vasodilation
    • Parasympathetic stimulation (vagal): Muscarinic (M3) receptors cause vasodilation via NO
    • Sympathetic stimulation:
      • Beta2 stimulation: causes vasodilation of skeletal muscle arterioles
    • Natriuretic peptides
    • Adenosine
    • Prostacyclin
    • Endothelium derived relaxin factor
    • Bradykinin
    4. What is the role of Baroreceptors (carotid/aortic arch) in BP control?
    Baroreceptors are stretch receptors in the carotid sinus and aortic arch that maintain BP within a normal range and respond to changes in BP to normalize it.

  • Increase in BP results in an inhibition of the sympathetic outflow from the vasomotor center and increase in vagal tone causing vasodilation as well as a decrease in HR and contractility, which reduce BP
  • A reduction in BP has the opposite effects.
  • 5. Role of atrial volume receptors:
    Cardiopulmonary (stretch) receptors: These receptors located in the left atrium, right atrium, pulmonary arteries, and ventricular endocardium, are activated by increased volume. They send signals to the brain to inhibit sympathetic outflow and increase vagal activity to reduce arterial vasomotor tone and decrease BP.

    The converse is also true.

    6. What is syncope?
    Syncope refers to a sudden and transient loss of consciousness due to the temporary loss of cerebral perfusion. The metabolism of the brain, unlike other organs, is exclusively dependent on perfusion. In contrast to skeletal muscle, for example, storage of high-energy phosphate in the brain is limited, and energy supply depends largely on the oxidation of glucose extracted from the blood. Thus, cessation of cerebral blood flow causes a loss of consciousness within about 10 seconds.
    7. What are the causes of syncope?
  • Sudden changes in heart rate: bradycardia, tachycardia
  • Obstruction to blood flow: aortic stenosis, pulmonic stenosis, mitral stenosis, tricuspid stenosis, pulmonary artery hypertension, pulmonary artery embolism, obstructive HCM, cardiac tamponade with pericardial effusion
  • Right to left shunting: Tetralogy of Fallot, Eisenmenger's Syndrome
  • Reduced preload: dehydration, hemorrhage, hypotensive drugs, cardiac tamponade
  • Vascular or neurogenic dysfunction: carotid sinus hypersensitivity (vasovagal), post-micturition, post-tussive
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