Measurement of CVS FunctionRavi

SyllabusOutline the physics of blood flowGive a detailed account of the various methods of measuring blood pressureGive a detailed account of the various methods of measuring cardiac output as well as their limitationsOutline methods and principles used to measure regional blood flow

Non-Invasive Blood PressureCuff:Single cuff containing an inflatable bladderPositioned so that the centre of the bladder is medial - over the brachial arteryWidth should be 20% greater than the diameter of the armAirpumpBleed ValvePressure transducerMicroprocessor and electrical timerPressure display

NIBP

TechniqueSingle cuff appliedPressure in cuff inflated to above systolicCuff pressure monitored constantly via pressure transducerPressure is slowly released by the bleed valve (2-3mmHg/sec) and then held and so onSmall regular oscillations in pressure recorded (size of fluctuations above base line recorded)Cuff then deflated fully

Technique continued

Systolic Pressure - When oscillations in the pressure rise significantlyMAP - when oscillations in the pressure are maximumDiastolic Pressure - when pressure oscillations drop significantlyHeart rate - frequency of oscillations

ResultsMAP is most accurateDiastolic least accurateDiastolic sometimes calculated using systolic and MAP.Systolic sometimes calculated using diastolic and MAP

Sources of ErrorInappropriate cuff sizeIrregular heart rhythm (AF) - BP varies greatly with each contractionLow output states - inaccurate below 50mmHg - ?oscillations too smallPatient movement (shivering) - ?artifact in pressure oscillationsInaccurate calibration

Exam Question03A13 - Briefly describe the principles and sources of error in the measurement of systemic arterial blood pressure using an automated oscillometric non-invasive monitor.

Invasive Blood PressureArtery is canulated for a direct measurement of bpFixed and securely flushed with heparinised saline through a 3-way tapHeparinised saline in a pressurised container at a pressure > systolic passes through a constriction so flow cannot exceed 4 mL/Hr

EquipmentPressure transducer, amplifier and recorder

TransducerFibreoptic catheter tip transducerMirror coated diaphragm reflects light carried to tip by optical fibrePosition of diaphragm determines the fraction of light that is reflected back down a second fibreConverted to an electrical signal

Transducer

ResonanceTransducer diaphragm, catheter and saline column all oscillate naturally at their resonant frequencyIf less than 40 Hz - falls within the range of frequencies in bp wave formOscillations at resonant frequency produce a sine wave which is superimposed on the bp wave form

Resonance

ResonanceShorter, wider and stiffer catheters increase the resonant frequencyCurrent transducers have a very high resonant frequencyResonant frequency should be several fold the pulse not to have an effect

DampingWhen an elastic system is disturbed it oscillates before settling on a new valueDamping decreases these oscillations and allows the system to eventually come to rest at a new value

DampingAny restriction to transmission of blood pressure from the artery to transducer will cause the trace to be damped or smoothed out so sharp changes are not displayed

Damping CoefficientDepends on the reduction in amplitude of each oscillation

Critical DampingDamping Coefficient >1.0No overshootTime taken to reach new rest position is much greaterNot clinically useful as time taken is too long. Hence, systolic and diastolic inaccurate

Optimal DampingDamping Coefficient = 0.64Compromise where overshoot & oscillation minimised but response is still rapid3 advantages:1. Amplitude distortion is minimised2. Frequency response obtained minimal (ie. Accuracy up to 2/3 ressonant frequency)3. Phase distortion minimised

Damping

DampingCan be caused by:Air bubbles (absorb pressure changes in the saline column)Clot formation in canula (restricts movement in the saline column)

DampingCan alter the values for systolic and diastolic blood pressuresLess effect on MAP

Exam Question07A15 - Describe the effects of resonance and damping on an invasive arterial blood pressure tracing.

Fick PrincipleExpression of the law of conservation of massUptake=Blood flow x diff in concentrationPrinciple of calculating pulmonary blood flow from the oxygen uptake in the lungsPt rebreathes oxygen into a Benedict Roth spirometer through a soda lime absorber

Fick Principle cont.Rate of oxygen uptake calculated (250mL/min)Catheter in RA measures oxygen content in mixed venous blood (150mL/L)ABG - oxygen content (200mL/L)Blood gains - 50mL/LCO=uptake/difference in concentration=250/50=5L/min

Non-Invasive CO measurementFick Prinicple - elimination of CO2CO=rate of CO2 elimination/(venousCO2 - arterial CO2)If gas rebreathed - CO2 rises and 2 different equations:Equations..

Dye dilution techniqueKnown amount of inocyanine green injection into RH via catheterConcentration measured in peripheral artery using a spectometric techniqueConcentration vs time plotted on a semilogarithmic paper (y axis logarithmic)

Dye dilution techniqueDye dilution graphFlow=mass added/AUC

Thermal Dilution TechniqueFick Principle - expression of law of mass conservationIndicator - thermal mass - injected upstream and measured down streamAssuming no indicator is lost:Flow=amount injected/average downstream concentration

Thermal dilution techniquePAC with 2 channels inserted into RH via IJVInjecting channel - SVC & RA junction2nd channel with thermister - Pul artery10mL 5% Dextrose zero degrees - mixes and cools blood that passes thermisterTemperature vs time curve plottedCO calculated using AUC similar to dye dilution technique

Exam Question00A1 - Explain how cardiac output is measured using the thermodilution technique.

Exam Question99B8 - Draw both Aortic root and radial artery pressure wave forms on the same axes. Explain the differences between them.

Old exam questions1992 Write short notes on the principles of oscillometry1991 Write short notes on the ideal properties of a pressure transducer