Cardiac output measurement by indicator dilution

1. Fick Technique

a) Requires the simultaneous collection of arterial & mixed venous (pulmonary artery) blood samples while a sample of expired gas is taken

b) The difference in oxygen content of the arterial & venous blood is the A—V O₂ difference. Oxygen consumption is calculated from the oxygen content of the inspired minus the expired gas and the respiration rate

c) Accurate in low cardiac output states, valvular regurgitation and shunts

2. Indicator Dilution — Dye

a) Technique

Addition of 1 cc of dye solution with a known concentration of 2000 dye particles per cc into an unknown volume of solution. If the final dye concentration becomes 1 particle per cc then:

2000 particles x(1cc)=1 particle x unknown vol

1 cc 1cc

Unknown vol = 2000 x 1cc/particle

Unknown volume = 2000 cc

i) A dye of known volume and concentration is injected into the blood via the right atrium or pulmonary artery

ii) The concentration of the dye after equilibration measured at a downstream site; usually the femoral of radial artery

iii) The greater the final concentration, the smaller the flow (volume)

iv) A densitometer spectrophotometrically determines the moment to moment dye concentration producing a resulting curve

v) Cardiac output = 60 x the amount of dye injected divided by the area under the curve (average dye concentration x time)

b) Sources of error

i) As the blood is in motion, the addition of the dye must be rapid and the measurement of the dye concentration downstream site continuous until all of the added dye flows past the sampling site

ii) Inaccurate measurement

iii) Prolonged injection time

iv) Inadequate mixing of dye & blood

v) Intracardiac shunts, low cardiac output, valvular insufficiency

vi) Rapid recirculation may affect calculation

c) Indicator dilution — thermal

i) Technique

a) An indicator dilution technique which uses temperature change as the indicator

b) A chilled solution is added to the blood (usually via right atrium), and the resultant drop in temperature is recorded at a downstream site(usually in pulmonary artery)

c) Cardiac output is determined from 4 variables: volume of injectate (often iced water to increase signal to noise ratio), the area under the curve (determined by computer), temperature of the blood (use patient’s temperature), and the correction factor for the injectate warming

ii) Sources of error

a) Not accurate in presence of shunts, or pulmonary or tricuspid regurgitation

b) May be inaccurate in low cardiac output states

c) Specific heat and gravity of blood changes with Hct (a factor in calculation of cardiac output)

d) Patient’s respiration

e) Poor injection technique