determinants of cardiac output and principles of oxygen
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Determinants of Cardiac Output
and Principles of OxygenDelivery
Scott V Perryman, MD PGY-III
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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
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Preload
Preload: load imposed on a muscle before
the onset of contraction
Muscle stretches to new length
Stretch in cardiac muscle determined byend diastolic volume
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Preload
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Preload
At bedside, use EDP as surrogate for
ventricular preload
i.e. assume EDV = EDP
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Preload
How can we measure EDP?
Pulmonary Capillary Wedge Pressure
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PCWP
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
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PCWP
Only valid in Zone 3 of lung where:
Pc > PA
Catheter tip should be above left atrium
N
ot 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
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Preload
Ventricular function is mostly determined
by the diastolic volume
Relationship between EDV/EDP and
stroke volume illustrated by ventricular
function curves
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Ventricular Compliance
Cardiac muscle stretch determined by EDV
Also determined by the wall compliance.
EDP may overestimate the actual EDV or true
preload
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Cardiac Output and EDV
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Effect ofHeart Rate
With increased heart rate, we get
increased C.O.to a point.
Increased HR also decreases filling time
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Contractility
The ability of the cardiac muscle to
contract (i.e. the contractile state)
Reflected in ventricular function curves
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Afterload
Afterload: Load imposed on a muscle at theonset of contraction
Wall tension in ventricles during systole
Determined by several forces
Pleural Pressure
Vascular compliance
Vascular resistance
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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
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Impedence
Impedence = total force opposing flow
Made up of compliance and resistance
Compliance measurement is impractical inthe ICU
Rely on resistance
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Vascular Resistance
Equations stem from Ohms law: V=IR
Voltage represented by change in pressureIntensity is the cardiac output
SVR = (MABP CVP)/CO
PVR = (MPAP LAP)/CO
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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)
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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
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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
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Oxygen Delivery
O2 delivery = DO2 = CO x CaO2
Usually = 520-570 ml/min/m2
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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
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Oxygen Extraction Ratio
VO2/DO2 x 100
Ratio of oxygen uptake to delivery
Usually 20-30%
Uptake is kept constant by increasingextraction when delivery drops.
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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
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Tissue Oxygenation
If not enough oxygen, have anaerobic
metabolism
Get 2 moles ATP per mole glucose and
production of lactate
Can follow VO2 or lactate levels