Cardiac Output Monitoring Mark Vivian 465y23

Cardiac Output Monitoring The hard stuff

Mark Vivian

Contents • Cardiac Output • The Fick Principle • PA Catheters • Thermodilution and dye-dilution measurement • (Transoesophageal) Doppler • LiDCO / PICCO

Cardiac Output, CO • Volume of blood pumped by heart in a

unit time • CO is a measurement of FLOW • Average value of 70kg ♂ is approx 5Lmin-1 • Affected by – Natural physiology (eg respiration) – Pathology (organ dysfunction, shock, trauma etc)

• Measured using invasive and non-invasive techniques

CO • CO = heart rate (HR, bpm) x stroke volume (SV, mL)

= volume per unit time

• CO = SBP (systemic BP) ÷ systemic vascular resistance (SVR)

The Fick Principle • Adolf Fick (1829-1901) • German physiologist • 1870 - Developed principle for measuring cardiac output

• Can be utilised in variety of clinical situations

Fick Principle - 1 • blood flow to / from an organ can be measured using a marker substance

• Relies on observation:

– ‘total uptake/release of a substance by an organ is equal to the

product of bloodflow to that organ and the arterial-venous concentration gradient of marker substance’

• Several substances can be used

• note – – – –

Total uptake of substance = Vsub Blood flow = CO Arterial concentration of substance = CAsub Venous concentration = CVsub

Fick Principle - 2 • Can estimate CO if we measure Substance taken up by organ per unit time, V

Organ Concentration of substance Supplying organ, CVsub

Concentration of substance leaving organ, CAsub

• Fick’s principle: Vsub = CO (CAsub – CVsub)

Fick Principle - 3 • Use Oxygen = substance • Use pulmonary system (receives 100% CO) Oxygen taken up by organ per unit time, VO2

Pulmonary artery Concentration of oxygen Supplying lungs, CVO2

Lungs

LA / aorta / arterial system

Concentration of oxygen leaving lungs, CAO2

Fick Principle - 4 • Measure VO2 using spirometry – Approx 250mL O2 min-1

• CVO - from pulmonary artery • CAO – from peripheral artery 2

2

• From before: VO2 = CO (CAO2 – CVO2)

• Rearranging to: CO = VO2 _ (CAO2 – CVO2)

units = mLmin-1

Pulmonary Artery Flotation catheter • Applies Fick principle when used to measure CO • CO measured by Thermodilutional methods and • Dye dilutional methods

• Use a known volume / concentration of

substance • Measure effects at a distal point over time

Dilutional CO measurement • Utilises the PA catheter • Current gold standard • 4-5 lumens • 10cm apart

Dilutional CO measurement - 2 • Discrete volume ‘cold’ fluid or dye injected in RA

• Temperature/concentration is

measured at a known distance (usually in PA) and recorded over time

• CO is calculated from the

resultant concentration or temperature curve (‘thermodilution curve’)

Dilutional CO measurement - 3 • CO is calculated using the Stewart-Hamilton equation

• CO is a flow – volume per unit time • A known ‘mass’ of marker is injected and its’ concentration is measured over time

• The volume into which it was given can be derived using Vol = Mass ÷ Conc V =

M_ C

• Concentration is measured using an appropriate sensor

Dilutional CO measurement – 4 • The flow of the volume is calculated using Flow = Mass ÷ (Concentration x change in time) or

CO =

M _ C . Δt

• This is the equation used for dyedilutional measurement of CO

• Thermodilutional method works similarly, with an adaption

Dilutional CO measurement - 5 • The equation adapted for thermodilutional CO measurement -

CO = Vinj (Tb – Tt). K Tblood (t) Δt Where Vinj = Tb = Tt = K= Tblood (t) Δt =

volume of injected fluid (injectate) blood temperature temperature of injectate constant derived from multiplication of density and a constant change in blood temperature over time

Dilutional CO measurement - 6 • The (thermo)dilution curve produced is an inverse ‘hump’ since Temp fall is recorded versus time

• The graph is usually presented with temperature decrease on y-axis so the deflection becomes positive:

Dilutional CO measurement - 7 • This is an example of a washout curve • It describes an exponential process: • the rate of change of a quantity at any time  the quantity at that time

• The rate at which the injectate is removed is

equal to the flow of blood x concentration of the injectate in the blood Q = V x [inj] note: V is flow; Q = rate of removal of injectate

Dilutional CO measurement - 8 • However, V is constant, therefore Q

(rate of removal injectate)

 [inj]

V is the flow, or volume of blood between the injection site and the thermistor sensor ie the parts of the heart and vasculature = constant

• And, the actual amount of injectate in the blood volume at any time is proportional to its’ concentration, so Mass injectate  injectate concentration - an exponential.

Dilutional CO measurement – 9 • This exponential (proportional) relationship is exploited to derive the CO

• The graph is converted by a semi-log transformation

• log10 Temp decrease y axis • time x axis

• Area under the curve is used to measure CO

Dilutional CO measurement - 10 • The Area under the curve is calculated to give the CO

CO = _dose injectate_ area under curve

= __mass___ = concn . Time

• Similar techniques for both Dye and thermodilutional methods

• a computer works the values out! • , CO = flow = volume per unit time

Doppler measurement of CO • Probe in oesophagus • Anatomically close to aorta • Measures velocity of blood in descending aorta • Velocity enables calculation of volume per unit time, ie CO

• Some additional data is needed, such as

diameter of aorta (calculated via normograms)

Doppler Effect • CO measurement relies on Doppler Effect • phenomenon by which frequency of

transmitted sound is altered as it is reflected from a moving object there is an increase in the observed frequency of a signal when the signal source approaches the observer

e.g. ambulance siren

Doppler Effect - 2 • Electromagnetic wave equation: v=f.λ Where v = velocity of wave motion, f = frequency, λ = wavelength

• Doppler effect: Wavelength ↓ = frequency ↑ = pitch ↑

Wavelength ↑ = frequency ↓ = pitch ↓

Doppler Effect - 3 • Doppler effect represented by: V = _ΔF . c _ 2 F0 cos θ Where V = velocity of object ΔF = frequency shift c = speed of sound in medium (body tissue here) F0 = frequency of emitted sound cos θ = angle between sound wave and flow (RBC)

• To measure CO transoesophageally, we

need to know the diameter of the aorta

Doppler Effect - 4 • F0 = frequency emitted • FR = frequency reflected off RBCs

moving at u m/s • Θ = angle of incidence (transmitter to direction of flow) Frequency (phase) shift = FR – F0

= ΔF

and ΔF  velocity of RBCs

If a measurement – or estimate (by normogram) – of the cross-sectional area of the aorta is known, flow can be derived as area x velocity (m2.m.s-1 = m3s-1) i.e. a volume per unit time = CO

Doppler output

PiCCO / LiDCO • Continuous CO analysis

• PiCCO – pulse contour cardiac output • LiDCO – Lithium dilution cardiac output • Both systems employ very complicated algorithms based on the Fick principle

LiDCO • Bolus isotonic LiCl (150mM) injected peripheral/central vein

• CLiCl over time is monitored via an ion selective electrode on the arterial line manometer

• CO is calculated from the Li dose and the area under the concentration-time curve prior to recirculation using: CO =

Li dose (mmol) x 60 Area x (1-PCV) (m mol/sec)

Where PCV = packed cell volume = Hb (g/dL) /34

LiDCO -2 • Blood flows into the sensor assembly at the rate of a peristaltic pump.

• Beat to beat CO by calibrating an arterial BP trace algorithm

• Additionally, LiDCO can be used in conjunction with another device to calibrate and provide continuous arterial waveform CO analysis

PiCCO • Cold bolus as described previously • Injected via central line • Injectate travels through multiple compartments

• Variables are indexed according to patient parameters • Complicated algorithms used to calculate CO

PiCCO - 2 • Pulse contour continuous cardiac output • Displayed following calibration • SV is calculated from AUC from arterial trace

• CO calculated from SV (CO = HR . SV)

Summary • Invasive, semi-invasive methods • Based on the Fick and Doppler Principles

• Fick Principle is gold standard but practically hard to measure

• Dilutional methods give CO via washout curves

and analysis of ‘area under the curve’ (i.e. mass  concentration)

• Doppler and LiDCO / PiCCO methods provide continuous waveform analysis

• CO = flow = volume per unit time!