Cardiology measurement Flashcards
Describe the oscillometric measurement of blood pressure
The cuff is coupled to an oscillometer which measures the pulse of the brachial artery transmitted through the air-filled tubing
As it inflates, the oscillometer stores the maximal amplitude of oscillations.
The cuff inflates above the systolic (i.e. when the oscillometer no longer sees any oscillations)
The cuff is gradually deflated until oscillations are 25-50% of their maximal amplitude: this is the systolic.
The cuff is gradually deflated until maximal amplitude is reached: this is the MAP
The cuff is deflated until the amplitude decreases again by 80% or more; this is the diastolic.
What sources of innacuracy are there in NIBP measurements
Wildly inaccurate: 95% CI for NIBP within the normal range is 15mmHg
Even more inaccurate in the extremes of blood pressure; over-estimates low blood pressure and under-estimates high blood pressure
Impossible to calibrate
Cuff size is a major influence of measurement
Oscillometer is confused by arrhythmia, shivering, or tremor.
What 2 methods of calibration are involved in using arterial lines
Static calibration
Dynamic calibration
What are the essential steps to static calibration of an arterial line
Zeroing the transducer
Checking and adjusting the gain
Checking for time stability
How do you zero an arterial line transducer
Zeroing - sets the zero reference point for pressure measurement
Technique
- Line off to patient - 3 way tap
- Position the tap at the level which is to be used as the zero reference point
- Open the tap in the transducer dome to air so the transducer is exposed to atmospheric pressure
- press the zero button the monitor
- Close the tap on the dome
- Open the 3 way tap so there is a continuous column of fluid between the tranducer and arterial lumen
What is involved in dynamic calibration of arterial lines
Resonant frequency
Damping coefficient
What are the advantages of optimal damping?
What is the otimal damping coefficent
0.64
If the pulse rate was 60bpm, what is the fundamental frequency? What therefore must the resonant frequency of the system be above in order to avoid resonance?
1 Hz
10Hz is the upper limit of the frequency of the 10th harmonic; as the resonant frequency must be >0.64 of this the answer is 15Hz
brandis 264
What is the SI unit of pressure
pascal
What is the typical pressur ein a hospital gas pipeline
400kPa
4 atmospheres
60psi
What is the difference between gauge pressure and absolute pressure
What is an ECG
graphic representation of myocardial electric potential against time used for monitoring and diagnosis of heart disorders
Components to producing an ECG
ECG electrodies
Cables
Amplifier
Processor
Monitor and recording device
How many ECG electrodes are used
3, 5, 10
What makes up an ECG electrode - why is this important
◦ Electrodes are disposable, thin layers of silver electrode and silver chloride on its surface covered in a gel that is rich in chloride ions - this combination results in a stable electrode potential that odes not interfere with recording
◦ 10mm diameter thin and broad, conducting gel to improve skin contact, high sampling rate 10000 - 15000 Hz to detect pacing spikes
What happens at the level of an ECG electrode to cause detection?
◦ Small changes in potential difference at the skin surface cause polarisation fo the silver/silver chloride electrode
What properties of ECG cables are important
each electrode will have a cable that returns the signal to the monitor
◦ Insulated to avoid eddy currents from surrounding electrical/magnetic sources
Why si amplification necessary for ECGs?
- Amplification of electrode signals is required as although a change in myocardial potential may be 120mV (from -90mV to +30 mV during an action potential intracellularly, and extracellular charge the inverse), the amplitude of a QRS at the skin surface is 1-2mV
What is an ECG lead
◦ Lead is a measure of the potential difference between two electrodes examining the heart’s potential difference changes at different angles
‣ The electrode at one end of the lead acts as the positive terminal while the other a negative terminal e.g Lead 1 LA is positive and RA negative
‣ Depolarisation towards the + terminal or repolarisation away results in positive deflection in the ECG
‣ E.g. Lead 1 measuring potential difference between LA and RA
What direction does depolarisation and repolarisation run in leads? Which is postivie and which is negative?
◦ Lead is a measure of the potential difference between two electrodes examining the heart’s potential difference changes at different angles
‣ The electrode at one end of the lead acts as the positive terminal while the other a negative terminal e.g Lead 1 LA is positive and RA negative
‣ Depolarisation towards the + terminal or repolarisation away results in positive deflection in the ECG
‣ E.g. Lead 1 measuring potential difference between LA and RA
What is the indifferent electrode? What charge does it have>
◦ Use of the indifferent electrode allows for unipolar analysis against a single lead and this is utilised for the chest leads but also for the augmented leads; it is always the negative electrode
Describe some of the processing elements to an ECG that aims to reduce interference
‣ This is also the site of common mode rejection of interference - where sources of electrical noise which affect each electrode equally are eliminated
‣ Differential amplicfication amplifies the difference between electrode elads rather than absolute voltages
‣ High frequency filters reduce - muscle and mains current interference
‣ Low frequency filters - screen respiratory movements
‣ Wider range frequency used for diagnostic 0.05 - 100Hz
‣ Reduced frequency used for monitoring 0.5 - 40Hz
What does 1cm represent vertically on an ECG?
1mV
Draw the bipolar limb leads including their direction of current and + vs -
Draw the augmented limb leads and their +/- and direction of current
What is common mode rejection in the context of an ECG
- A ground electrode - the activity of the lead of interest is compared to the ground electrode and anything in common is discarded as noise - “common mode rejection”
What role does a ground electrode play in interference and artefact in ECGs
- A ground electrode - the activity of the lead of interest is compared to the ground electrode and anything in common is discarded as noise - “common mode rejection”
What role does an amplifier play in reducing interfeerence and artefact with ECGs
ECG frequency is 0.5 - 100Hz and frequency outside this range can be ignored using high or low pass electrical filters
◦ Monitor mode - amplifier only responds to frequencies 0.5 - 40Hz
‣ Loses resolution but reduced interference due to narrow bandwidth
◦ DIagnostic mode - 0.5 -100Hz
◦ More accurate
◦ Increased interference
What factors play a role in ECG interference and artefact reduction?
- A ground electrode - the activity of the lead of interest is compared to the ground electrode and anything in common is discarded as noise - “common mode rejection”
- Amplifiers - ECG frequency is 0.5 - 100Hz and frequency outside this range can be ignored using high or low pass electrical filters
◦ Monitor mode - amplifier only responds to frequencies 0.5 - 40Hz
‣ Loses resolution but reduced interference due to narrow bandwidth
◦ DIagnostic mode - 0.5 -100Hz
◦ More accurate
◦ Increased interference - ECG cables are shielded with an insulator to reduce induction currents from external sources
- Conductive paste connecting the electrodes to the skin
- Patient factors
◦ Remaining still - relax without movement to avoid EMG interference; avoiding shivering
◦ Skin preparation -dry and no hair
◦ Removing electronic devices from patient near the leads if possible
‣ Diathermy plate - DIsplay - accurate with printer at correct speed and appropriate amplification (vertical calibration)
What methods are there of measuring blood pressure non invasively
- Oscillometric - measures MAP, estimates SBP and DBP
- Auscultatory - measures SBP and DBP, estimates MAP
- Pulse palpation measures/estimates SBP only
- Flush measures SBP only - exsanguinating a tourniquetted limb by tight pressure bandage then gradually deflating the cuff until the pale bloodless limb flushes pink again
- Ultrasound - measures SBP and DBP, estimates MAP
Compare the methods of measuring NIBP in their measurement and estimation of SBP, DBP and MAP
- Oscillometric - measures MAP, estimates SBP and DBP
- Auscultatory - measures SBP and DBP, estimates MAP
- Pulse palpation measures/estimates SBP only
- Flush measures SBP only - exsanguinating a tourniquetted limb by tight pressure bandage then gradually deflating the cuff until the pale bloodless limb flushes pink again
- Ultrasound - measures SBP and DBP, estimates MAP
Oscillometric BP measures and estimates which parameters?
- Oscillometric - measures MAP, estimates SBP and DBP
Manual ascultatory BP measures and estimates which parameters?
- Auscultatory - measures SBP and DBP, estimates MAP
What compoennts are there to a sphygomomanometer?
‣ Cuff - width 20% greater than diameter of the arm
‣ Manometer - either Bourdon gauge, or a fluid manometer column
‣ Inflating bulb to elevate cuff pressure
‣ Deflating valve
Cuff width to arm ratio for NIBP?
‣ Cuff - width 20% greater than diameter of the arm
‣
What position should the arm be in NIBP?
Phlebostatic axis
Describe the sounds heard with auscultatory NIBP measurement, what do each of these sounds represent
- As the cuff is deflated blood released into the distal limb makes characteristic sounds which can be related to pulse pressure range - as turbulent flow is present as flow only occurs in brief spurts when pressur exceeds occlusion pressure
◦ Phase 1 - loud clear snapping tone with repeated tapping - short bursts of blood flow aat systolic pressure
◦ Phase 2 - Murmurs/soft swishing - low flow blood; occasionally an auscultatory gap after phase 1 before phase 2
◦ 3 - Disappearance of murmurs and appearance of tone resembling the first phase but less marked - increased blood flow
◦ 4 - less clear quality or dull tones thumping/blowing occuring 10mmHg above diastolic
◦ 5 - disappearance - steady laminar flow resumes at diastolic pressure
NIBP vs IABP in hypotension?
- NIBP ausculatory Underestimates BP in hypotension, may be unable to detect BP in low cardiac output states
◦ Whereas the oscillometric overestimates it in shock
What sources of error does NIBP auscultatory methods have?
- Inaccuracy from
◦ Interference with measurement
‣ User auditory sensitivity
‣ Stephoscope quality
‣ Ambient noise
◦ User related
‣ Cuff size
‣ stethoscope bell misplaced
‣ Deflation of cuff too fast
‣ Arm positioned too far above or below phlebostatic axis
‣ Arm not relaxaed
◦ Unavoidable errors
‣ Stress/white coat syndrome
‣ Underestimation fo systolic - as the cuff needs to be deflated BELOW the systolic to measure it
‣ Underestimates diastolic
Draw a graph of cuff pressure against time to describe oscillometric BP
Define oscillometer
Oscillometer - an instrument for measuring the changes in pulsations in the arteries
What components are there to an oscillometer
- Components
◦ Cuff
◦ Microprocesser to inflate with an air pump
◦ Bleed valve to slowly deflate
◦ Pressure sensor monitor
Describe the method of measurement of an oscillometer
◦ Cuff applied to a measuring site over an artery typically the brachial and inflated above systolic pressure where there is no flow - the pressure in the cuff is monitored as the arterial pulse will convey changes in the volume of the limb and therefore air filled tubing causing a change in presssure
◦ The cuff pressure is then gradually decreases 2-3mmHg per secnod and the pressure is held for a brief period and this continues - during each holding period the machine detects where the pressur eis constant or if there are small regular oscillations and the size of the fluctuations from baseline is measured
◦ The maximum size of oscillations above the baseline rpessure is the MAP
◦ The cuff inflates above the systolic (i.e. when the oscillometer no longer sees any oscillations)
‣ This was thought to originally be the point at which systolic blood pressure should be measured however there is no distinct transition above the systolic or below the diastolic as some oscillations are still measured
‣ The cuff is gradually deflated until oscillations are 25-50% of their maximal amplitude: this is the systolic. Each brand will have a slightly different proprietary way of calculating the systolic
* % of maximum amplitude
* When the small oscillations rise significantly ‘
* When they are first heard
◦ The cuff is gradually deflated until maximal amplitude is reached: this is the MAP
◦ The cuff is deflated until the amplitude decreases again by 80% or more; this is the diastolic. This is the least accurate measurement
WHat is the least reliable variable in oscillometric BP
DBP
How is DBP potentially calculated in oscilometric BP
◦ The cuff is deflated until the amplitude decreases again by 80% or more; this is the diastolic. This is the least accurate measurement
‣ Other measures will utilise equations e.g. MAP = diastolic + (systolic - diastolic)/3
‣ MAP = 2 x DBP/3 + SBP/3
Describe measures of calculating MAP
◦ MAP = diastolic + (systolic - diastolic)/3
‣ MAP = PP/3
‣ MAP = 2 x DBP/3 + SBP/3
Sources of inaccuracy of oscillatory BP measurement
- Sources of innaccuracy
◦ Technique
‣ Cuff size is a major influence of measurement
‣ Deflation too rapid
◦ Interference with measurement
‣ Oscillometer is confused by arrhythmia, movement e.g. shivering, or tremor.
◦ Unavoidable
‣ Wildly inaccurate: 95% CI for NIBP within the normal range is 15mmHg
* Due to use of coefficients and constats
‣ Even more inaccurate in the extremes of blood pressure; over-estimates low blood pressure and under-estimates high blood pressure
‣ Impossible to calibrate
Advantages generlaly of NIBP
- No invasive procedure
- Cheap
- Reusable
- Minimal training
- No monitoring equipment or electronics
- Durable, robust
- Does not require regular recalibration
Disadvantages generlaly of NIBP
- Less reliable at extremes
- Continuous monitoring not possible
- Painful if repeated freqeuntly
- Pressure areas
- Maximum accuracy requires manual operation
What is the Penaz tehcnique
Describe the components of an invasive arterial measurement set
◦ Arterial catheter 20g (radial) 18g (femoral)- short, parallel walled, rigid, patent. Balance of thin enough to reduce clot risk but thick enough to reduce damping
◦ Incompressible tubing - rigid, non compiant, wide bore and fluid filled with tap for zeroing and sampling
◦ Pressure transducer - fluid filled tubing and a counterpressure bag
‣ 300mmHg with flow of 3-5ml/hr
◦ Electrical transducer - wheatstone bridge strain gauge. Electrical connection to the silicon chip is isolated from the saline by a compliant silicone elastomer gel allowing pressure to be transmitted from liquid to chip but prevents electric shock from sensor to patient or alternatively circuit destruction from defribrillation of the aptient
◦ Monitoring -
Arterial catheters for arterial lines should have what properties
◦ Arterial catheter 20g (radial) 18g (femoral)- short, parallel walled, rigid, patent. Balance of thin enough to reduce clot risk but thick enough to reduce damping
What characteristics of arterial line tubing are important>
◦ Incompressible tubing - rigid, non compiant, wide bore and fluid filled with tap for zeroing and sampling
◦ Pressure transducer - fluid filled tubing and a counterpressure bag
‣
What rate of flow does an arterial line have into the artery?
3-5ml/hr
What length of tubing maximum should an arterial line have?
1.2m
How is the wave on the screen calclated and made?
Fourier analysis from sine wave harmonics
Indications for arteirla line 5
- Indications
◦ Rapidly or dangerously fluctuating BP
◦ Prolonged course of BP moniotring - neuropraxias, tissue injury
◦ Titration of vasoactive agents
◦ Frequent blood sampling
◦ Inaccuracy of non invasive - massive obesity, burns
Advantages to IABP
◦ Continuous - bea ot beat variation, close monitoring of those who rapidly change
◦ Sampling
◦ Waveform itself a source of information
◦ Gold standard
Disavantages of IABP
◦ Arterial puncture
◦ Non reusable
◦ Moniotring requirepemtn required and expensive
◦ Re-zeroing and re-levelling
◦ Transducers can drift and fail
Sources of inaccuracy to IABP
◦ Device
‣ Cannula or tubing kinked
‣ Air bubbles or clots
‣ Inadequate freqency response of the transducer
‣ Inappropriately zero’d or levelled
‣ Faulty calibration
◦ Patient
‣ Damaged artery or spasm
What is zeroing?
transducer sets the atmospheric pressure to 0
What is levelling in IABP
The transducer is set at a particular height along a fluid column with the transducer at the reference point to avoid additional hydrostatic pressure leading to inaccuracy - phlebostatic axis
What advantages and disadvantages are there of radial arterial lines
‣ Easily accessable
‣ Generous collateral circlation
‣ No collateral damage with surroudning structures
‣ does not restrict movement
‣ Disadvantages
* Pulse amplification making systolic and diastolic less accurate
* Highly mobile site - easy to dislodge
* Small
* Anatomical variation
What advantages and disadvantages are there of brachial arterial line insertion
‣ Large and proximal - more accurate
‣ Much larger than radial therefore easier to access
‣ Easily compressible
‣ Disadvantages
* Difficult to access during CPR
* End artery - limb ischaemia risk
* Next to median nerve
* Kink and occludes with aptient movement
What advantages and disadvantages are there of femoral arterial line insertion
‣ Largest and most proximal
‣ Most accurate reading - least affected by PVD and pulse amplification
‣ Easily palpable
‣ Accessable in crisis
‣ Disadvantaes
* Retroperitoneal haematoma which si not compressible
* AV fistula if through and through puncture
* Higher risk of infection
* Mobilistion issue
What factors account for discrepancies between invasive and non invasive measurements of BP
- Device factors
◦ Non-invasive measurement error
‣ The cuff is the wrong size
‣ The oscillometric measurement is confused by an arrhythmia
‣ The patient is moving around too much
◦ Invasive measurement error
‣ The transducer is zeroed incorrectly
‣ The zero level is incorrectly selected
‣ The transducer system is incorrectly set up - Patient factors
◦ The artery being measured is in spasm
◦ There is peripheral vascular disease, which is unequally distributed
◦ The patient has subclavian artery stenosis
◦ There is aortic pathology which influences flow into the limbs (eg. aneurysm) - Relative reliability
◦ Invasive measurement is the “gold standard” of BP measument overall
◦ Mercury sphygmomanometer is the gold standard of non-invasive measurement
◦ Peripheral and central invasive measurements of arterial pressure tend to show good agreement, but in context of severe shock the peripheral lines tend to overestimate the blood pressure.
In a haemodynamically stable patient is a IABP or NIBP measurement more representative of aortic root preassure?
nIBP - more proximal artery
Draw a wheatstone bridge circuit diagram
What is a piezoelectrical pressure transducer
◦ Common piezoelectrical pressure transducers convert kinetic energy (pressure) into a change in electrical resistance
What is a piezoresistaive strain gauge
Usually a thin semiconductive silicone membrane 0.02mm in diametre
‣ This membrane increases its resistance whenever it undergoes deformation - high sensitivity - property changes with temperature but accurate between 15-40 degrees
◦ It is usually built into an integrated circuit which contains the Wheatstone bridge circuit and other electronic components
What is a wheatstone bridge - draw a diagram, describe it and give more modern configurations
- Wheatstone bridge
◦ an electrical circuit used to measure an unknown electrical resistance where the resistance of the unknown resistor is determined by pressure thus becoming a pressure gauge
◦ This is a circuit with four resistors:
‣ Three have a known resistance and the fourth (Rx) is the semiconducting membrane.
‣ If the ratio of resistance in the R1/R2 limb is the same as the resistance of the R3/Rx limb, there should be no current flowing through the galvanometer VG.
‣ The variable resistor R2 is adjusted until the current drops to zero (which is when the resistance of R2 is the same as the resistance of Rx)
‣ Alternatively, you can calculate what the Rx resistance is using Kirchhoff’s circuit laws, which is what ends up happening in the common hospital-grade blood pressure monitor.
‣ Thus, the resistance of Rx, and therefore pressure, is determined.
◦ component of the pressure transducer that allows the transducer to calculate the change in resistance resulting from the change in pressure
◦ Alternatively
‣ Classically, these were arranged with three resistors of known resistance and one of variable resistance (the
strain gauge). When the ratio of the resistors on the known side of the circuit (R2/R1) equals the ratio on the
other side of the circuit (R3/Rx) the bridge is balanced, no current will flow and no potential difference will be
measured by the galvanometer (VG). When the resistance of the strain gauge (Rx) changes due to pressure applied to the attached diaphragm, the two sides of the bridge become unbalanced and a current flows. The
resulting potential difference is measured by the galvanometer and is proportional to the magnitude of the
pressure applied
Describe this diagram
◦ This is a circuit with four resistors:
‣ Three have a known resistance and the fourth (Rx) is the semiconducting membrane.
‣ If the ratio of resistance in the R1/R2 limb is the same as the resistance of the R3/Rx limb, there should be no current flowing through the galvanometer VG.
‣ The variable resistor R2 is adjusted until the current drops to zero (which is when the resistance of R2 is the same as the resistance of Rx)
‣ Alternatively, you can calculate what the Rx resistance is using Kirchhoff’s circuit laws, which is what ends up happening in the common hospital-grade blood pressure monitor.
‣ Thus, the resistance of Rx, and therefore pressure, is determined.
Define wheatstone bridge
◦ an electrical circuit used to measure an unknown electrical resistance where the resistance of the unknown resistor is determined by pressure thus becoming a pressure gauge
Describe the relative changes in arterial line trace as you progress through the vasculature
- The further you get from the aorta
◦ The taller the systolic peak - higher systolic pressure
◦ Further the dicrotic notch is from the peak
◦ Lower the end diastolic pressure - wider pulse pressure
◦ Later the arrival of the pulse - 60msec delayed in radial
◦ MAP pretty much the same
Where is the phlebostatic axis
phlebostatic axis corresponding to RA. Axis intersection of midazillary line and 4th IC space. Minimised hydrostatic pressure ensuring accuracy. for every 2.5cm change the pressure is 1.87 mmHg
What effect does raising the pressure transducer or dropping the pressure transducer have on measured arterial blood pressure
phlebostatic axis corresponding to RA. Axis intersection of midazillary line and 4th IC space. Minimised hydrostatic pressure ensuring accuracy. for every 2.5cm change the pressure is 1.87 mmHg
What is the augmented pressure in an arterial line
Draw a dorsalis pedis and an aortic waveform on top of each other
Does the arterial waveform represent blood transitting from the heart?
No conducted pressure - pressure wave travels much faster than the blood ejected
Describe the phases of an arterial wave form
◦ Systolic - rapid increase to peak, followed by rapid decline. LV ejection. The peak represents the systolic
◦ Dicrotic notch
◦ Diastolic phase - run off of blood into peripheral circulation - and the trough is the diastolic BP
◦ MAP - area under the curve
What measurements can you derive from an arterial line trace
- Heart rate
- Systolic pressure
- Diastolic pressure (coronary filling)
- Mean arterial pressure (systemic perfusion)
- Pulse pressure (high in AR, low in cardiac tamponade or cardiogenic shock)
- Changes in amplitude associated with respiration (pulse pressure variation) - >10mmHg thought to be significance
- Slope of anacrotic limb associated with aortic stenosis, contractility
- Area under the curve - stroke volume index
What time delay is there between QRS complex and systolic upstroke? What does this delay represent
160-180msec at the aortic root
Isovolumetric contraction
How fast does the blood get ejected in the aorta
6-10m/s
Why does a pressure triggered aortic balloon pump struggle
Because the transmitted pressure wave is 160-180msec after the QRS and contraction begins and if this is based on peripheral arterial line it is even more delayed not even accounting for circuit related delays in conduction back to the aortic balloon
What is the anacrotic limb of the arterial line
systolic upstroke
What does the anacrotic limb represent? What information can potentially be derived form this?
◦ Ventricular ejection - initial fast moving 10,/sec wave correpsonding to peak aortic blood flow accelaration. The slope of this segment has some relationship with rate of LV pressure change and aortic valve –> if slurred = AS (poor LV contraction however has not shown relationship)
Draw a aortic stenosis arterial waveform
◦ ventricle struggles to squeeze blood out so systolic upstroke less steep
◦ Systolic peak may be loewr as generating high aortic pressures higher
◦ Pulse pressure can be narrowed especially if no aortic regurgitation concurrently
◦ Incisura lost and dicrotic notch may disappear
How does an AR trace look on an arterial line
◦ Sharp systolic upstroke as instead of contracting isovolumetrically pressure from early contraction is transmitted to the aorta and steep systolic decline, low end diastolic pressure and a widened pulse pressure
◦ The more incompetent the valve the closer diastolic pressure gets to LV end disastolic presssure - 5mmHg
◦ No dicrotic notch, small incisura
What is the equation for natural frequency
Using the equation denoting the natural frequency interpret the effect modifying the length, diametre, elasticity of the system will have on the natural frequency
- Equation denoting the natural frequency of a fluid filled pressure transducer system –>
◦ Lengthening tube = increased elasticity –> decreased natural frequency
◦ Increasing the diameter of the tubing - increased natural frequency
◦ Stretchy tubing increases elasticity, decreasing the natural frequency
◦ Air bubbles increase elasticity and decrease natural frequency
Graphically represent what damping means to a system? Why is it therefore important
From this, the beneficial effects of damping become clear. The damped system has a larger range over which there is little amplitude increase with increasing frequency, and even at its natural frequency the amplitude change is smaller. As a result, the measured pressure waves will not be overestimated as much.
Draw a circulatory flow model diagram for dye dilution and explain its key features
- The diagram of actual circulatory flow is seen below
◦ Dye injected at T0
◦ Dye detected later as it takes time to travel from injection to sensor
◦ Rapid rise in oncentration leading to a rounded peak due to the fact laminar flow is occuring so dye travels at a range of velocities from injection to sensor; and also reflects the eddies occuring in circulation
◦ Exponential decay of concentration then occurs - second peak due to recirculation of fast moving dye
◦ Cardiac output calculated by: amount of dye injected / atrea under the graph
‣ the recirculation must be removed
What is cardiac output equal to in the circulatory flow model diagram dye dilution technique
Amount of dye injected / area under graph
Draw the equation for thermodilution calculations of cardiac output
What is the equation for the relative shift in frequency caused by a sound wave reflected from a blood cell moving
f is sound wave freuqency in hertz
V is blood cell velocity
C is velocity of sound in tissue (1500m/s)
As C is so much greater than V the denominator can be simplified
Note if velocities do not all act along the same line we add an angle factor (cos angle) to the numerator - the angle is the angle between the probe and the direction of flow
Draw Eindhovens triangle including lead names, vectors, + and - nodes
Fick principle
What is the Stewart Hamilton equation
What different methods of indicator dilution are there? What underlying prinicple is there?
Stewart Hamilton equation underlies
Methods
* Thermodilution by PAC or PICCO
* Lithium dilution
* Conductivity dilution using saline
* Indicator dye dilution
Outl
- Giving a known dose of a substance IV can be used to measured cardiac output by measuring the rate of transit of the substance at a downstream detector fr
◦ The area under the concnetration time curve which is = mean concentration x time interval - Washout curve - a graph of the concentration of a subtance against time if the substance is removed or washed away by a flow of liquid. The most simple being an exponential decay
- If a simple bucket with inflow and outflow rates that are steady has a dye added then the following graph applies ->
- Rate of dye removal from an insertion site can be measured as a reflection of flow x dye concentration
- The concentration will decrease more quickly at higher flow rates such that
◦ Flow rate = amount of dye added / area under the graph - These principles can be applied using indocyanine green dye via a large vein and then measurement in peripheral arterial tree using optical based sensor
What is the equation for cardiac output when an indicator dye is used?
Draw a graph reflecting the indicator dye dilution technique, the equation to calculate cardiac output and describe the timing of the cycle where injections occur
- The diagram of actual circulatory flow is seen below
◦ Dye injected at T0 - 10mL
◦ Dye detected later as it takes time to travel from injection to sensor
◦ Rapid rise in oncentration leading to a rounded peak due to the fact laminar flow is occuring so dye travels at a range of velocities from injection to sensor; and also reflects the eddies occuring in circulation
◦ Exponential decay of concentration then occurs - second peak due to recirculation of fast moving dye - logarithmic transformation of concentration is used and this makes area under the first curve easier to measure
◦ Cardiac output calculated by: amount of dye injected / atrea under the graph
‣ the recirculation must be removed
What is the equationf or cardiac output when temperature change is used?
What additional constants are used in calculating cardiac output int he temperature dilution technique?
What is the density constant in reference ot cardiac output measurement via thermodilation
What is the calibration constant in reference to thermodilution methods of measuring cardiac output
How is stroke volume derived from pulse contour analysis? How is it calibrated? What is the calibration factor?
What is the equation for SVR
BSA calculation
What is LVSWI? How is it caclulated? Normal value?
What is DaO2 and how is it calculated
What is VO2 and how is it calculated?
When can a PiCCO not be used
