Surgical Critical Care: Hemodynamic Physiology

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 & Bronchial Smooth Muscle Relaxation (Decrease BP)
  • β-3: Relax Bladder & Increases Lipolysis in Adipose Tissue (Unrelated to Cardiovascular System)

Other Receptors

  • Dopamine Receptors: Renal & 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

Cardiac Output & Hemodynamic Parameters

Cardiac Output Distribution

Organ At Rest (5 L/min) Heavy Exercise (25 L/min)
Kidney 20-25% 2-4%
GI Tract & Liver 20-25% 3-5%
Muscle 15-20% 70-85%
Brain 10-15% 3-4%
Heart 4-5% 4-5%
Skin 4-5% 5-20%
Bone 3-5% 1%

Determinants of Cardiac Output

  • Cardiac Output (CO) = Heart Rate (HR) x Stroke Volume (SV)
  • Determinants of Stroke Volume:
    • Preload: End-Diastolic Volume
      • Increases Stroke Volume
      • Factors that Increase:
        • Venous Return
        • Ventricular Compliance
      • Atrial Kick Contributes 20%
    • Contractility: Strength of the Heart’s Systolic Contraction
      • 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
      • Decreases Stroke Volume
      • Factors that Increase:
        • Systemic Vascular Resistance (SVR)
        • Blood Viscosity
        • Aortic Impedance

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 / BSA
  • Stroke Volume (SV): 60-100 cc/beat
  • Stroke Volume Index (SVI): 33-47 cc/m2/beat
    • SVI = SV / BSA = CI / HR
  • Stroke Volume Variation (SVV): 10-13%
    • SVV (%) = (Maximum SV – Minimum SV) / Average SV
    • Accurately Measured Only if Mechanically Ventilated & In Normal Sinus Rhythm

Vascular Pressures

  • Systemic Arterial Blood Pressure
    • Systolic Blood Pressure (SBP): 90-140 mmHg
    • Diastolic Blood Pressure (DBP): 60-90 mmHg
    • Mean Arterial Pressure (MAP): 70-105 mmHg
      • MAP = 1/3 SBP + 2/3 DBP
  • Pulmonary Capillary Wedge Pressure (PCWP): 11 mmHg
    • Estimates Diastolic Pressures of Left Atrium & Left Ventricle
  • Central Venous Pressure (CVP): 2-8 mmHg
  • Pulmonary Artery Pressure (PAP): 25/10 mmHg

Systemic Vascular Resistance

  • 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 = 80 x (MAP – CVP) / CI

Other Measures

  • IVC Diameter on US: 12-17 mm & Not Completely Compressible
  • 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