5.0 Shock - hemodynamics - video 2

Feb 8, 2025

Lecture Notes on Shock and Tissue Perfusion

What is Shock?

  • Shock is characterized by decreased perfusion of tissues.
    • In simple terms, it's the decreased delivery of oxygen to different organs.
  • Perfusion is measured as blood flow to the organs over the mass of tissue.
    • Flow: volume per unit time (e.g., liters/minute).
    • Tissue: measured in mass (e.g., grams of kidney).

Factors Influencing Perfusion

  1. Cardiac Output

    • Proportional to perfusion.
    • Determined by:
      • Stroke Volume: Amount of blood ejected per beat (liters/beat).
      • Heart Rate: Beats per minute.
      • Cardiac Output Units: liters/minute.

  2. Systemic Vascular Resistance

    • Resistance of blood vessels affects perfusion.
    • Blood vessels act like pipes; resistance is the opposition to flow.
    • Smaller diameter vessels increase resistance and thereby perfusion.

  3. Oxygen Content in Blood

    • Higher oxygen content leads to better tissue perfusion.

Detailed Breakdown of Cardiac Output

  • Stroke Volume Components:
    • Preload: Blood volume in the heart at the start of contraction.
    • Afterload: Blood volume remaining after contraction.
    • Contractility: Heart's ability to contract and eject blood.
    • Stroke Volume Equation: Preload - Afterload.

Systemic Vascular Resistance

  • Blood vessels are compared to trampolines in terms of resistance.
  • Diameter changes influence resistance.

Hypovolemic Shock Example

  • Low blood volume leads to decreased preload.
  • Less blood into the heart means less can be pumped out.

Blood Pressure and Perfusion

  • Mean Arterial Blood Pressure (MAP):
    • MAP = Cardiac Output x Systemic Vascular Resistance + Central Venous Pressure (CVP).
    • CVP is typically low and often neglected in calculations.
    • Blood Pressure also measured via a sphygmomanometer.
    • MAP Calculation: 2/3 Diastolic BP + 1/3 Systolic BP.

Summary

  • Shock involves decreased tissue perfusion and oxygenation.
  • Key factors in managing shock include cardiac output, vascular resistance, and oxygen content.
  • Understanding these factors helps in managing and diagnosing shock.
  • There's a direct relationship between blood pressure and tissue perfusion. Patients in shock typically have low blood pressure and inadequate tissue oxygenation.