Cardiac Output Measurement

DEFINITIONS

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

• Adolf Eugen Fick (1829-1901) in 1870, was the first to measure cardiac output
• 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)

Issues:

• impractical
• assumes no shunt (pulmonary blood flow = systemic blood flow)
• assumes arterial blood is equal to pulmonary venous blood

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 = initial concentration of indicator
C2 = final concentration of indicator
V1 = volume of indicator
V2 = 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
• 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.