Hemodynamic Physiology

Hemodynamic Physiology

David Ray Velez, MD

Table of Contents

Hemodynamic Homeostasis
Cardiac Output (CO)
Systemic Vascular Resistance (SVR)
Cardiovascular Receptors
Normal Hemodynamic Values

Hemodynamic Homeostasis

Hemodynamic Homeostasis: Regulation of Blood Circulation to Meet the Demands of Different Organ and Tissue Systems

Major Variables Reflecting Homeostasis

  • Mean Arterial Pressure (MAP)
    • MAP = CO x SVR
  • Cardiac Output (CO): Driving Force from the Heart to Circulate Blood
  • Systemic Vascular Resistance (SVR): Resistance Within the Circulatory System Used to Create Blood Pressure

Cardiac Output (CO)

Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV)

Determinants of Stroke Volume

  • Preload: End-Diastolic Volume
    • Increased Preload Increases Stroke Volume
    • Factors that Increase Preload:
      • Venous Return
      • Ventricular Compliance
    • Atrial Kick Contributes 20%
  • Contractility: Strength of the Heart’s Systolic Contraction
    • Increased Contractility Increases Stroke Volume
    • Anrep Effect: Sudden Increases in Afterload Causes Increased Contractility
    • Bowditch Effect: Increase in Heart Rate Causes Increased Contractility
      • HR 120-150: CO Increases with HR
      • HR > 150: CO Decreases with HR (Due to Decreased Diastolic Filling Time)
  • Afterload: Resistance the Left Ventricle Must Overcome to Circulate Blood
    • Increased Afterload Decreases Stroke Volume
    • Factors that Increase Afterload:
      • Systemic Vascular Resistance (SVR)
      • Blood Viscosity
      • Aortic Impedance

Cardiac Output Distribution

OrganAt Rest (5 L/min)Heavy Exercise (25 L/min)
Kidney20-25%2-4%
GI Tract & Liver20-25%3-5%
Muscle15-20%70-85%
Brain10-15%3-4%
Heart4-5%4-5%
Skin4-5%5-20%
Bone3-5%1%

Systemic Vascular Resistance (SVR)

Systemic Vascular Resistance (SVR): Resistance Within the Circulatory System Used to Create Blood Pressure

Hagen-Poiseuille Equation

  • ∆P = 8μlQ / πr4
    • ∆P: Change in Pressure
    • μ: Blood Viscosity
    • l: Length of the Vessel
    • Q: Flow Rate Through the Vessel
    • r: Radius of the Vessel
  • *Pronounce Poiseuille: “pwah-zoo-ee”

Resistance (R)

  • R = ∆P/Q
    • Therefore: R = 8μl / πr4
  • Radius is the Strongest Factor (r4)
    • Decreased Radius (Vasoconstriction) Significantly Increases Resistance
  • Increased Blood Viscosity (Hemoconcentration) Also Increases Resistance
  • Vessel Length Generally Does Not Change

Cardiovascular Receptors

Adrenergic Receptors

  • α-1: Vascular Smooth Muscle Constriction (Increase BP)
  • α-2: Venous Smooth Muscle Constriction
  • β-1: Heart (Increase HR/Contractility)
  • β-2: Vascular Smooth Muscle Relaxation and Bronchial Smooth Muscle Relaxation (Decrease BP)
  • β-3: Relax Bladder and Increases Lipolysis in Adipose Tissue
    • Unrelated to Cardiovascular System
  • Adrenergic Effects Can Be Blunted by Hypoxia, Acidosis, and Persistent Sympathetic Activation

Other Receptors

  • Dopamine Receptors: Renal and Splanchnic Smooth Muscle Relaxation
  • Vasopressin Receptors:
    • V-1 Receptor: Arterial Constriction
    • V-2 Receptor: Renal Water Resorption
    • V-3 Receptor: Mediates Factor VIII & vWF Release

Normal Hemodynamic Values

Cardiac Parameters

  • Cardiac Output (CO): 4-8 L/min
    • CO = HR x SV
  • Cardiac Index (CI): 2.5-4.0 L/min/m2
    • CI = CO / Body Surface Area (BSA)
  • Stroke Volume (SV): 60-100 cc/beat
  • Stroke Volume Index (SVI): 33-47 cc/m2/beat
    • SVI = SV / BSA
    • Also: SVI = CI / HR
  • Stroke Volume Variation (SVV): 10-13%
    • SVV (%) = (Maximum SV – Minimum SV) / Average SV
    • Accurately Measured Only if Mechanically Ventilated and in Normal Sinus Rhythm

Vascular Pressures

  • Systemic Arterial Blood Pressure
    • Systolic Blood Pressure (SBP): 90-140 mmHg
    • Diastolic Blood Pressure (DBP): 60-90 mmHg
    • Pulse Pressure (PP): 40-60 mmHg
      • PP = SBP – DBP
    • Mean Arterial Pressure (MAP): 70-105 mmHg
      • MAP = 1/3 SBP + 2/3 DBP
      • Also: MAP = DBP + 1/3 PP
  • Central Venous Pressure (CVP): 2-8 mmHg
    • Measured at End-Expiration – When Intrathoracic Pressure Equals Atmospheric Pressure
  • Pulmonary Artery Pressure (PAP): 25/10 mmHg
  • Pulmonary Capillary Wedge Pressure (PCWP): 11 mmHg
    • Estimates Diastolic Pressures of the Left Atrium and Left Ventricle

Vascular Resistance

  • Pulmonary Vascular Resistance (PVR): 25-1.6 mmHg/min/L (30-250 dynes/sec/cm-5)
    • PVR = 80 x (mean PAP – LAP) / CO
  • Systemic Vascular Resistance (SVR): 800-1,200 dynes/sec/cm-5
    • SVR = 80 x (MAP – CVP) / CO
  • Systemic Vascular Resistance Index (SVRI): 1,970-2,390 dynes/sec/cm-5/m2
    • SVRI = SVR / BSA
    • Also: SVRI = 80 x (MAP – CVP) / CI

Other Parameters

  • Mixed Venous Saturation (SvO2): Oxygen Saturation from the SVC, IVC, and Coronary Sinus
    • Measured by a Pulmonary Artery Catheter
    • Normal: 80%
  • Central Venous Oxygen Saturation (ScvO2): Used to Approximate SvO2
    • Measured by a Central Line
    • Does Not Include Coronary Sinus Return
  • Vascular/Venous Capacitance (VC): Ability of Blood Vessels (Mostly Veins) to Increase the Volume of Blood Held without a Large Change in Blood Pressure
    • VC = Change in Volume / Change in Pressure