Lecture Notes on Preload, Contractility, Blood Pressure, and Circulation

Hemodynamics and Perfusion

Preload

• Preload is the volume of blood in the ventricles at the end of diastole (end-diastolic volume).
• Dehydration leads to decreased preload.
• Signs of decreased preload in dehydrated patients include a thready pulse.
• Fixing Preload: Increasing volume (e.g., with IV fluids).
• Typically, low preload is the problem.

Contractility

• Contractility refers to the force of ventricular contraction (the "swish").
• Impact of Heart Attack (Myocardial Infarction - MI):
  • Can impair the muscle of the heart.
  • Can disrupt the electrical activity of the heart.
  • Either issue will compromise contractility (the "squish").

Stroke Volume and Cardiac Output

• Stroke volume is influenced by preload, afterload, and contractility.

• Compensation for Dehydration: In dehydrated patients, heart rate increases to maintain cardiac output.

  • Cardiac output (CO) is the product of heart rate (HR) and stroke volume (SV).
    $CO = HR \, x \, SV$
  • When stroke volume decreases (due to dehydration), the heart rate increases to compensate.

• Myocardial Infarction (MI): Also known as a heart attack.

Blood Pressure and Perfusion

• Arterial blood pressure is essential for organ perfusion.

• Formula for Mean Arterial Pressure (MAP):

  • $MAP = \frac{2 \times \text{Diastolic BP} + \text{Systolic BP}}{3}$

• Organ Response to Low Blood Pressure: If blood pressure drops too low, organs will not perfuse adequately, leading to organ damage or death.

• Vulnerable Organs: Kidneys are often among the first to suffer damage from inadequate perfusion.

• Body's Response to Hypotension: The body prioritizes blood flow to the brain, heart, and kidneys, potentially sacrificing perfusion to extremities, the gut, and the liver.

  • Cold extremities are a sign of this compensatory mechanism.

• Shock: Prolonged inadequate perfusion leads to shock and multisystem organ failure.

  • Organ damage from shock may be irreversible.

Blood Pressure Regulation

• Blood pressure is regulated by multiple mechanisms.

• Neurological Control: Neurons in the brain regulate blood pressure via the sympathetic and parasympathetic nervous systems.

  • Sympathetic nervous system increases blood pressure and heart rate.
  • Parasympathetic nervous system decreases blood pressure and heart rate.

• Kidney's Role: Receptors in the kidneys monitor blood pressure and fluid balance.

  • The kidneys activate the renin-angiotensin-aldosterone system (RAAS) to increase blood pressure when perfusion is inadequate.
  • Kidney injury or failure often leads to blood pressure dysregulation (either too high or too low).

• ACE Inhibitors: Angiotensin-converting enzyme (ACE) inhibitors are medications that block the conversion of angiotensin I to angiotensin II, thus lowering blood pressure.

  • These medications target the renin-angiotensin system.

• Aldosterone: A hormone produced by the adrenal glands and kidneys that promotes sodium and water retention to increase blood pressure.

Altered Perfusion: Causes and Mechanisms

• Altered perfusion refers to impaired blood flow to tissues and organs.

• Underlying Respiratory Issues: Inadequate oxygenation due to respiratory problems impairs perfusion.

• Circulation Problems: Hemorrhage or blood clots obstruct blood flow.

• Inadequate Pumping: Heart failure or other cardiac conditions reduce cardiac output.

• Excessive Demand: Increased metabolic demands can outstrip the body's ability to perfuse tissues adequately.

• Anemia: Reduced oxygen-carrying capacity impairs oxygen delivery to organs.

  • Anemia reduces the amount of oxygen that can be delivered to organs, impacting perfusion.

• VQ Mismatch: VQ mismatch is a condition in which ventilation and perfusion are not properly matched in the lungs.

Impaired Circulation

• Hemorrhage:
  • Injury to a blood vessel results in hemorrhage.
  • The "Stop the Bleed" campaign aims to educate people on how to control bleeding from deep wounds, particularly in mass shooting events.
  • Death from deep wounds is often due to exsanguination (bleeding out).
  • Controlling hemorrhage helps maintain preload and cardiac output.
• Obstruction:
  • Obstructions impair circulation by impeding blood flow.
  • Thrombus: A blood clot that forms in a blood vessel.
  • Pulmonary Embolus (PE): A blood clot that travels to the lungs, blocking pulmonary circulation.
  • If a clot detaches, it becomes a thromboembolus.
    • Venous Stasis: Stagnant blood flow in the veins, promoting clot formation.
    • Prolonged standing can lead to venous stasis and varicose veins.
    • Compression socks prevent venous stasis by promoting venous return.
    • Sequential compression devices (SCDs) mimic the effect of compression socks in hospitalized patients.
    • Veins have valves to prevent backflow, but prolonged pressure can damage these valves, leading to varicose veins.
  • Hypercoagulability: Increased tendency to form blood clots.
    • Thrombocythemia: An abnormally high number of platelets in the blood.
    • Platelets are involved in blood clotting.
  • Virchow's Triad: Describes the three broad categories of factors that are thought to contribute to thrombosis
    • Endothelial injury (damage to the blood vessel lining).
    • Hypercoagulability (increased tendency to clot).
    • Venous stasis (slow or stagnant blood flow).
    • All three contribute to increase risk of clot formation.
• Atherosclerosis: Deposition of fat in the arteries.
• Aneurysms and Bifurcations: Weakened blood vessel walls or abnormal branching can impede blood flow.

Cardiac Output Impairment

• Inadequate cardiac output results in inadequate perfusion.
• Factors that Change Cardiac Output:
  • Blood volume and viscosity.
  • Electrical and pumping function of the heart.
  • Heart rate and rhythm.
  • Blood pressure regulation.
  • Structural defects in the heart (e.g., atrial septal defect (ASD) or ventricular septal defect (VSD)).
    • ASD: Hole between the atria.
    • VSD: Hole between the ventricles.
    • Structural defects cause abnormal blood flow patterns.