Exam

Cardiac Output

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Cardiac Output, Cardiac Index, Stroke Volume, Stroke Index, Clinical Assessment of Cardiac Output, Cardiac Function

  • Definitions
  1. Cardiac Output (CO)
    1. Quantity of blood pumped per minute through the aorta and into the peripheral circulation
    2. Cardiac Output is proportional to (arterialPressure / totalPeripheralResistance)
    3. CO = SV * HR
      1. Where SV = Stroke Volume
      2. Where HR = Heart Rate
    4. Normal Adult
      1. Range: 4-8 L/min (typical 70 kg male = 5.25 L/min)
  2. Cardiac Index (CI)
    1. Average stroke output of heart per minute (Cardiac Output) adjusted for body surface area
    2. CI=CO/BSA
      1. Where CO = Cardiac Output
      2. Where BSA = Body Surface Area
    3. Normal Adult
      1. Range: 2.4 - 4 L/min/m2 (60% of csardiac output in the normal, average sized adult)
  3. Stroke Volume (SV)
    1. Volume of blood pumped from the heart in one cycle of diastole and systole
    2. Stroke Volume is affected by Preload, contractility and Afterload
    3. SV = EDV - ESV
      1. Where EDV = End-diastolic volume
      2. Where ESV = End-systolic volume
    4. Normal Adult
      1. Range: 55-100 ml (typical 70 kg male = 70 ml)
  4. Stroke Index (SI)
    1. Average stroke output of heart per cycle (Stroke Volume) adjusted for body surface area
    2. SI = CI/HR
      1. Where CI = Stroke Index
      2. Where HR = Heart Rate
    3. Normal Adult
      1. Range: 20-40 ml/m2
  5. Ejection Fraction (EF)
    1. Reflects the percentage of blood ejected from the ventricle
    2. EF = SV/EDV
      1. Where SV = Stroke Volume
      2. Where EDV = End-diastolic volume
    3. Normal Adult
      1. Left Ventricular Ejection Fraction (LVEF): 55-70%
  • Images
  1. Cardiac Cycle Volumes and Pressures (Wiggers Diagram)
    1. cvCardiacCycleWiggers.png
  • Physiology
  1. Stroke Volume components
    1. Ventricular Preload (based on venous return of Circulatory Volume and ventricular compliance)
      1. End-diastolic volume (EDV)
      2. Increases Cardiac Output in the normal heart
      3. Results in Fluid Overload and third spacing in Congestive Heart Failure
    2. End-systolic volume (ESV) depends on ejection fraction (percentage of EDV ejected from left ventricle)
      1. Ventricular Contractality
        1. Force of ventricular contraction or inotropy
        2. Increased contractility increases Stroke Volume and Cardiac Output
      2. Ventricular Afterload (systemic Blood Pressure)
        1. Force against which the left ventricle pumps
        2. Increased Afterload decreases Stroke Volume and Cardiac Output
  2. Cardiac Output components
    1. Product of Heart Rate and Stroke Volume
    2. Heart Rate is the first compensatory mechanism to increase Oxygen Delivery to the tissues
    3. Heart is most efficient with lower Heart Rate and higher Stroke Volume
  3. Myocardial Oxygen Demand
    1. Oxygen requirements increase with venticular muscle wall tension
    2. Ventricular muscle wall tension is related to Ventricular Preload and Ventricular Afterload
      1. Ventricular Preload is the increasing ventricular wall tension that increases with ventricular filling
      2. Ventricular Afterload is the constant tension in ventricular muscle as it contracts and shortens
    3. Wall tension increases with ventricular volume (ventricular radius) and ventricular pressure (Afterload)
      1. Laplace Equation = Tension (T) = Pr
      2. where P = ventricular pressure (Ventricular Afterload)
      3. where r = ventricular radius
  4. Mediators of Cardiac Output
    1. Positive Inotropes increase contractility
    2. Positive Chronotropes increase Heart Rate
    3. Positive Dromotropes increase cardiac conduction velocity
  • Diagnostics
  • Clinical Assessment of Cardiac Output (and SV, Preload, Afterload and Contractility)
  1. Heart Rate and rhythm
    1. Palpated pulse
    2. Pulse Oximetry
    3. Telemetry or Electrocardiogram
  2. Right Ventricular Preload
    1. Central Venous Pressure
    2. IVC Ultrasound for Volume Status (Caval Aorta Index)
  3. Left Ventricular Preload
    1. Symptoms (Orthopnea or Dyspnea on exertion)
    2. Signs (Pulmonary Edema, rales)
    3. Imaging (Lung Ultrasound)
  4. Right Ventricular Afterload
    1. Mean pulmonary artery pressure or wedge pressure
  5. Left Ventricular Afterload
    1. Mean arterial pressure
  6. Cardiac Contractility
    1. Echocardiogram (ejection fraction, Stroke Volume)
  7. Cardiac Output
    1. Pulmonary artery catheter
    2. Arterial catheter (arterial waveform analysis)
    3. Esophageal doppler (descending aorta flow)
  • Resources
  • References
  1. Goldberg (2014) Clinical Physiology, Medmaster, Miami, p. 49-50
  2. Killu and Sarani (2016) Fundamental Critical Care Support, p. 93-114
  3. Marino (2014) ICU Book, 4th Ed Wolters-Kluwer p. 144-6