Cardiac Output Measurement
18/4/11
DEFINTIONS
Q = SV x HR (L/min)
CI = SV x HR/BSA (L/min/m2)
Normal CI = 2.5-4.2
SV = end-diastolic volume – end systolic volume
EF = (SV/EDV) x 100%
Shock = failure of tissue perfusion -> end organ injury
THE FICK PRINCIPLE
- Adoph Fick (1870)
- assumes oxygen consumption is a function of rate of blood flow and rate of oxygen pick
pick up by RBC’s.
- involves measurement of oxygen concentration of arterial and venous blood and
subsequent calculation of O2 consumption.
- Q can then be derived
Measurements
- VO2 = oxygen consumption/min (from spirometer with subject rebreathing air through a
CO2 absorber)
- Cv = oxygen content of blood taken from pulmonary artery (deoxygenated)
- Ca = oxygen content of blood taken from a peripheral artery (oxygenated)
Calculations
VO2 = (Q x Ca) – (Q x Cv)
Therefore,
Q = VO2/(Ca-Cv)
- impractical
- assumes no shunt (pulmonary blood flow = systemic blood flow)
- assumes arterial blood is equal to pulmonary venous blood
Jeremy Fernando (2011)
DILUTION TECHNIQUES
- known quantity of tracer substance introduced into a space to be measured
- concentration measured after complete mixing
C1 x V1 = C2 x (V1 + V2)
C1
C2
V1
V2
=
=
=
=
initial concentration of indicator
final concentration of indicator
volume of indicator
volume to be measured
- marker injected proximally to right ventricle and concentration measure distally (pulmonary
artery or a peripheral artery)
- concentration vs time plotted -> integration allow calculation of area under curve (SV x HR
= Q)
- suitable substances: radioiosotope, dye, cold water, temperature of blood.
TECHNIQUES
Clinically Used
Non-invasive BP monitoring
Central venous monitoring
Arterial monitoring
Pulmonary arterial monitoring
ECHO: TOE and TTE
Pulse contour analysis (PiCCO)
Oesophageal Doppler
Cardiac catheterisation and angiography
Experimental
Aortovelography – dopper U/S probe in suprasternal notch to measure blood velocity
and acceleration in ascending aorta.
Ballistocardiography – detection of body motion due to movement of blood within
body with each heart beat.
Electromagnetic flow meters
Jeremy Fernando (2011)
Oxygen consumption estimation (Fick)
Impedance plethymography
TIPS WHEN USING CARDIAC OUTPUT MONITORS
- there is no ‘normal’ CVP or wedge -> follow trend and look at the response to treatment
- abnormal hearts (ischaemic, fibrotic, contused) are less compliant so require higher filling
pressures to reach ‘normal’ SV.
- use SV rather than Q as a response to treatment as Q is calculated from HR which may be
fast and mask a poorly performing ventricle.
- low SvO2 usually indicates under-resuscitation.
- the first treatment for all shock (including cardiogenic) = volume, volume and more volume.
- a little extravascular lung water is less harmful than vasoactive drugs.
- there is no formula to calculate the effect of PEEP on PCWP and CVP -> if kept constant,
trend should be consistent.
- during resuscitation if becomes apparent what CVP the patient ‘likes’ -> aim for this.
- be cautious of all derived variables, particularly SVR.
Jeremy Fernando (2011)