Determinants of Cardiac Output and Principles of Oxygen Delivery by chenboying

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									Determinants of Cardiac Output
   and Principles of Oxygen
   Scott V Perryman, MD PGY-III
• Principle of Continuity:

     •   Conservation of mass in a closed hydraulic system
     •   Blood is an incompressible fluid
     •   Vascular system is a closed hydraulic loop
     •   Vol ejected from left heart = vol received in R heart
• Preload: load imposed on a muscle before
  the onset of contraction

• Muscle stretches to new length

• Stretch in cardiac muscle determined by
  end diastolic volume
• At bedside, use EDP as surrogate for
  ventricular preload

  – i.e. assume EDV = EDP
• How can we measure EDP?

Pulmonary Capillary Wedge Pressure
• How does wedge pressure work?

  – A balloon catheter is advanced into PA
  – Balloon at the tip is inflated
  – Creates static column of blood between
    catheter tip and left atrium
  – Thus, pressure at tip = pressure in LA
• Only valid in Zone 3 of lung where:

  – Pc > PA

     •   Catheter tip should be above left atrium
     •   Not usually a problem since most flow in Zone 3
     •   Can check with lateral x-ray
     •   Will get high respiratory variation if in Zone 1 or 2
• Ventricular function is mostly determined
  by the diastolic volume

• Relationship between EDV/EDP and
  stroke volume illustrated by ventricular
  function curves
        Ventricular Compliance
• Cardiac muscle stretch determined by EDV

• Also determined by the wall compliance.

• EDP may overestimate the actual EDV or true
Cardiac Output and EDV
        Effect of Heart Rate

• With increased heart rate, we get
  increased C.O….to a point.

• Increased HR also decreases filling time
• The ability of the cardiac muscle to
  contract (i.e. the contractile state)

• Reflected in ventricular function curves
• Afterload: Load imposed on a muscle at the
  onset of contraction

• Wall tension in ventricles during systole

• Determined by several forces

  – Pleural Pressure
  – Vascular compliance
  – Vascular resistance
           Pleural Pressure
• Pleural pressures are transmitted across
  the outer surface of the heart

  – Negative pressure increases wall tension.
    Increases afterload
  – Positive pressure Decreases wall tension.
    Decreases afterload
• Impedence = total force opposing flow

• Made up of compliance and resistance

• Compliance measurement is impractical in
  the ICU

• Rely on resistance
       Vascular Resistance
• Equations stem from Ohm’s law: V=IR

Voltage represented by change in pressure
Intensity is the cardiac output


            Oxygen Transport
• Whole blood oxygen content based on:

     • hemoglobin content and,

     • dissolved O2

  Described by the equation:

  CaO2 = (1.34 x Hb x SaO2) + (0.003 x PaO2)
          Oxygen Content
• Assuming 15 g/100ml Hb concentration
• O2 sat of 99%

Hb O2 = 1.34 x 15 x 0.99 = 19.9 ml/dL

For a PaO2 of 100

Dissolved O2 = 0.003 x 100 = 0.3 ml/dL
           Oxygen Content
• Thus, most of blood O2 content is
  contained in the Hb

• PO2 is only important if there is an
  accompanying change in O2 sat.

• Therefore O2 sat more reliable than PO2
  for assessment of arterial oxygenation
          Oxygen Delivery

• O2 delivery = DO2 = CO x CaO2

• Usually = 520-570 ml/min/m2
            Oxygen Uptake
• A function of:

  – Cardiac output

  – Difference in oxygen content b/w arterial and
    venous blood

  VO2 = CO x 1.34 x Hb (SaO2 – SvO2) 10
     Oxygen Extraction Ratio
• VO2/DO2 x 100

• Ratio of oxygen uptake to delivery

• Usually 20-30%

• Uptake is kept constant by increasing
  extraction when delivery drops.
      Critical Oxygen Delivery
• Maximal extraction ~ 0.5-0.6

• Once this is reached a decrease in delivery =
  decrease in uptake

• Known as ‘critical oxygen delivery’

• O2 uptake and aerobic energy production is now
  supply dependent = dysoxia
        Tissue Oxygenation
• In order for tissues to engage in aerobic
  metabolism they need oxygen.

• Allows conversion of glucose to ATP

• Get 36 moles ATP per mole glucose
        Tissue Oxygenation

• If not enough oxygen, have anaerobic

• Get 2 moles ATP per mole glucose and
  production of lactate

• Can follow VO2 or lactate levels

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