1. Determinants of Cardiac Output and Principles of Oxygen Delivery
Scott V Perryman, MD PGY-III

2. 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

3. Preload
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

4. Preload
This curve is known as the Frank-Starling curve. On the Y-axis we have the pressure changes within the ventricle. On the X-axis, we have the diastolic volume. The upper curve represents systolic performance and the lower curve, diastolic performance. This graph demonstrates several important changes. We note that with small changes in diastolic volume, there are large consequent changes in systolic pressure. The area under the curve A, B, C and D represents the stroke work.

5. Preload At bedside, use EDP as surrogate for ventricular preload
i.e. assume EDV = EDP

6. Preload
How can we measure EDP?
Pulmonary Capillary Wedge Pressure

7. 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

8. PCWP 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

9. Preload
Ventricular function is mostly determined by the diastolic volume
Relationship between EDV/EDP and stroke volume illustrated by ventricular function curves

10. Ventricular Compliance
Cardiac muscle stretch determined by EDV
Also determined by the wall compliance.
EDP may overestimate the actual EDV or true preload

11. Cardiac Output and EDV

12. Effect of Heart Rate
With increased heart rate, we get increased C.O….to a point.
Increased HR also decreases filling time

14. Contractility
The ability of the cardiac muscle to contract (i.e. the contractile state)
Reflected in ventricular function curves

15. Afterload
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

16. 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

17. Impedence Impedence = total force opposing flow
Made up of compliance and resistance
Compliance measurement is impractical in the ICU
Rely on resistance

18. Vascular Resistance Equations stem from Ohm’s law: V=IR
Voltage represented by change in pressure
Intensity is the cardiac output
SVR = (MABP – CVP)/CO
PVR = (MPAP – LAP)/CO

19. 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)

20. 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 = x 100 = 0.3 ml/dL

21. 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

22. Oxygen Delivery O2 delivery = DO2 = CO x CaO2
Usually = ml/min/m2

23. 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

24. 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.

25. Critical Oxygen Delivery
Maximal extraction ~
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

26. 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

27. 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