Determinants of Cardiac Output and Principles of Oxygen

8/8/2019 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

Determinants of Cardiac Output and Principles of Oxygen

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