Cardiac output monitoring Flashcards
What are the different methods for assessing cardiac output?
Cardiac output can be assessed indirectly and directly.
Indirectly via both qualatitive and quantative methods.
INDIRECT
Clinical assessment of heart rate, peripheral perfusion (capillary refil), urine output, character of pulse, lactate, blood pressure. ETCO2 e.g. in an arrest.
DIRECT
- there are a number of direct methods including invasive and non invasive..
- invasive:
- the dye dilution
- thermodilution method
- using Ficks principle
- arterial waveform pulse contour analysis (PiCCO/ LiDCO)
- non invasive
- using oesophageal dopplers
- transthoracic dopplers
- electrical thoracic bioimpedance
what are the features of an ideal cardiac output monitor?
- cheap
- easy to use - minimal training
- accurate
- allows continous measurements
- non invasive and safe
indications for cardiac output monitoring?
critically ill paitents to find out if they need ionotropic or vasopressive support
pre-op assessment for cardiac surgery or non cardiac in those suspected with cardiac issues
research and clinical trials
what is the gold standard for CO measurements?
thermodilution method using pulmonary artery floatation catheter = PAFC
Swan Ganz catheter
How are transoesophageal dopplers used for cardiac output monitoring
this method relies on the fact that sound waves change frequency as a object moves closer/ further away. Hence the frequency change seen is related to velocity.
The oesophageal probe is lubricated and placed 35-40cm (from the teeth) into the oesophagus such that its end can be placed on the left wall in contact with descending aorta.
It is angulated to 45 degrees such that the velocity of blood can be calculated via the equation relating velocity to frequency shift and angle of contact.
the doppler monitor then creates a graph for velocity change overtime.
the area in the curve is the stroke distance
by finding the cross sectional area of the aorta this can be muliplied by stroke distance to give a volume i.e. stroke volume.
Although descending aorta = 70% of total stroke volume
so this can be divided by 0.7 to give total SV
finally CO = HR x SV
what is the formula used by oesophageal doppler to calculate the velocity of blood
frequency shift x speed of sound in blood
divided by 2x original freq x cos 90.
Frequency shift is the difference between transmitted and received frequency
how is aortic cross sectional area measured?
2 methods
either directly via the doppler - some dopplers will measure the diameter of the aorta.
or using nonograms to give an estimate based on gender, age, height, weight.
what information is available from an oesophageal doppler monitor?
SV and CO
info on contractility..
peak velocity - peak of the curve
accelaration - gradient of upstroke
Flow time corrected
pressure gradient across heart valves
what is the flow time corrected and what can a short and long FTc indicate
Flow time corrected (FTc) = flow time is the width of the waveform. this is then corrected for HR by dividing flow time by the square root of QT interval
normal range 330 - 360ms
short = impaired filling of LV e.g. mitral stenosis, excess vasopressors, hypovolaemia
long = vasodilation e.g. sepsis
how is the pressure gradient across the heart valves calculated?
ΔP = 4V^2
what can the peak velocity tell you about afterload/ preload?
peak velocity will be reduced with
* reduced preload
* increased afterload
what is the cardiac index?
CI = CO / BSA
(BSA = body surface area in m^2)
it is used for easier comparison of CO values as it removes body size as a factor influencing CO.
what is a normal cardiac index?
2.5 to 3.6 L/min/m2
how can the oesophageal doppler be used in fluid management assesment?
measure SV using doppler
give fluid bolus
if >10% increase in SV , suggests fluid responsivenes.
which scenarios may oesophageal doppler be innaccurate?
aortic pathologies
- coarctation of aorta - measuring in descending after narrowing
- thoracic aortic aneurysm - especially if the aortic diameter measurements are measured directly via the doppler rather than nonograms
oesophageal pathology
-stricture or tumour - obsecures view
technical
- improper positioning of probe
- patient movement / inadequate sedation
when are oesophageal dopplers contraindicated?
oesophageal varices
clotting abnormalities
recent oesophageal surgery
patient refusal
what are the pros and cons of the oesophageal doppler method for CO measurement?
pros
- minimally invasive
- safe in most people
- easy to set up , no extra lines etc needed
- can give useful info on CO, contractility, FTc
- quick
- real time measurements
cons:
- still requires a GA
- risk of oesophageal perforation
- requires skilled user for positioning and interpretation - hence variation
- can result in errors from incorrect positioning
- makes a few assumptions
what errors is the oesophageal doppler prone to?
incorrect placement of probe
suboptimal position
measures coeliac artery instead - blood flow in diastole
measures pulmonary artery instead - opposite direction of blood flow.
what assumptions does the oesophageal doppler make?
descending aorta always 70% of CO
the size of aorta does not change in systole
all blood is flowing with same velocity
for each of the following state the abnormalities
- can see there is a low FTc (normal 330-360) which suggests inadequate filling of LV. also low peak and slow upstoke and reduced CI. overall suggest hypovolaemia or vasopressors.
- small SV, low peak velocity, normal FTc. suggest LV failure.
- variation in peak height = AF
- flow towards and away from probe - aortic regurgitation. reverse flow seen in diastole. also large SV and prolonged FTc
- prolonged FTc, high CO, high peak velocity, likely sepsis
- reverse flow, likely measure pulmonary artery.
what is a normal peak velocity?
age related
in adults 90 cm/s and 120 cm/s.
as low as 30cm/s in elderly
what is the Ficks principle for measuring CO?
Ficks states that uptake of O2 by a tissue relies on the CO x (CaO2-CvO2)
hence we can rearrange this to make CO the subject…
VO2/ (CaO2-CvO2) = CO
each component can be measured to measure CO…
* VO2 = spirometry
* CaO2 - arterial blood gas sampling, Hb conc, sats
* CvO2 - venous O2 from pulmonary artery via pulmonary artery catheter.
what are the pros and cons of the Ficks method for CO measurement?
PROS
* very accurate
CONS
* very invasive - requires pulmonary artery catheter
* time consuming
what is meant by the assumed FICKs determination?
due to time consuming nature of FICKs, sometimes it can be assumed that O2 consumption (VO2) is 250ml/min or 125ml/min/M2
less accurate but quicker - no need to do spirometry.
what is the modified FICKs method using CO2?
instead of using O2 consumption , CO2 production is used via ventilating patient and measuring MV and EtCO2.
No need for CVO2 measurements now
less invasive but requires intubation.
less accurate as CO2 much more suscpetible to changes in ventilation e.g. hypoventilation
describe the dye dilution method for CO measurements…
dye injected into a central vein
conc of dye measured in peripheral artery over time
log dye conc vs time is plotted (semi log)
stewart hamilton equations used to derive CO.
simple formula Q = m/AUC
m= amount given at the beggining
also if a very tall thin curve = high CO.
dye used = lithium indicator OR Indocyanine green dye.
second hump = recirculation
what is the purpose of using a semi log scale in dye dilution method?
easier interpretation
improves accuracy
describe the thermodilution method for CO measurements?
same principle as dye dilution but instead a cold volume of saline injected into central vein and peripheral artery used to measure temp over time.
usually 15ml of 8 degrees saline
thermistor in arterial cannula measures temp change
plot log temp decrease on y vs time on x
use stewart hamilton equations and AUC to derive CO.
this time no recirculation hump equilibrates before this.
what are the advantages of the thermodilution method for CO measurement?
Rapid
no issue with redistribution of dye and hence can be repeated frequently.
in fact modern methods allow continous data
what are the problems of thermodilution method?
very invasive
the sudden drop in temp can reduce HR and alter accuracy of CO measurement
the initial bolus and temp measurements need to be very accurate for accurate results.
What is the difference between LiDCO and PiCCO?
these methods use arterial pressure waveform analysis to estimate CO but using calibrations with either dye dilution or thermodilution methods..
PiCCO = thermodilution (needs larger artery e.g. femoral)
LiCCO = dye dilution (can use standard art line)
calibrations 8 hourly
both correlate well with gold standard PAFC method - hence accurate
how is arterial pulse pressure waveform used to estimate cardiac output?
the AUC of an arterial pulse pressure waveform is related to SV. (only arterial section)
SV x HR = CO
the relationship between SV and AUC first needs to be determined via calibration either via
- direct method = thermodilution/ dilution - once or twice daily.
- indirect = using nonograms for age/height, sex etc (Flowtrac device) - much less reliable
once a mathematical relation between AUC and SV is determined it can be applied to AUC to determine consequent SVs for continous CO measurement.
what are the advantages and disadvantages of arterial pulse pressure waveform analysis?
PROs
* beat to beat info on CO
* awake patient
* if central and arterial lines used anyway, not anymore invasive
CONs
* relies on good quality waveform - affected by damping
* complications of invasiveness of lines
* PiCCO requires femoral arterial line - more invasive
* not reliable with arrhythmias
which drug interferes with LiDCO?
atracurium interfers with lithium dye conc
describe the transcutaneous doppler method for CO monitoring…
instead of using descending aorta
the probe is placed on suprasternal notch on the chest and the ascending aorta flow is measured - total CO not 70%.
same doppler principle and algorithms used to measure CO
how does transthoracic electrical bioimpedance work to measure CO?
the thoracic electrical impedence varies with increased blood volume in the chest during systole.
by measuring changes in this electrical impedance SV can be determined and hence CO.
uses electrodes to measure this placed on neck and thorax and a low current is passed between them. changes in resistance relate to CO.
pros and cons of transthoracic bioimpedance method of CO measurement
non invasive
easy to set up
not very accurate as impedence is altered by many other factors and really patient needs to be ventilated for ventilation to be controlled/ accounted for.
how can parameters of the oesophageal doppler measurement be used to indicate treatments..
- low SV
- low FTc
- low Peak velocity
- low peak velocity and low FTc
Actions from readings (need to be interpreted in clinical context)
low SV -> fluid
low FTc -> indicates poor filling - fluid
low PV -> indicates poor contractility - inotrope
low PV + low FTc -> decrease afterload
how is oesophageal doppler inserted?
via oral or nasal route
lubricated
up to 35-40cm (T5/6)
what are the methods for calibration of CO?
either via invasive CO measurements e.g. LiDCO/ PiCCO
non invasive - best estimate using nonograms for height, age, gender, BSA
How can the pulmonary artery floatation catheter be used to measure CO?
PAFC is inserted via the IJV into the pulmonary arterial system.
It can be used to measure CO via FICKs method OR thermodilution..
FICKs method relies on O2 uptake and CaO2- CvO2. the PAFC is used to measure the venous O2 sats to estimate the CvO2 and hence CO.
thermodilution method - relies on cold saline injected via PAFC into proximal lumen and temp change measured via a thermistor at distal end. the rate of change in temp is a measure of CO and using stewart hamilton equations CO can be measured. This is the gold standard measurement for CO monitoring
draw a graph to show the record temp change in thermodilution method ..
how does PICCO measure CO vs PAFC?
PICCO stands for pulse index continous cardiac output monitoring
uses a distal arterial thermistor e.g. in femoral artery
PAFC measures the temp change at the pulmonary artery - more accurate
what is the doppler effect?
Change in frequency of sound waves depending on whether the source of sound is approaching/ moving away from the detector.
motorbike
this can be used to calculate velocity of moving blood via measuring the frequency of sound waves via the following equation
velocity = change in freq x speed of sound in blood / original freq x 2 x cos angle.
explain the steward hamilton equation..
flow = amount at the start / AUC
AUC for conc vs time or change in temp vs time.
hence the smaller the AUC the smaller the CO
draw a graph to show high and low CO in thermodilution method
red = high CO
AUC is smaller
how does a PAFC allow continuous CO measurements..
new devices have a heating element at the proximal end
the distal end measures temp change (increase in temp rather than decrease)
continous - no need to inject cold saline anymore
draw a graph to show oesophageal doppler waveform…
what can you say about this..
normal for older person
SV = normal
CO = 7L (slightly high)
Peak velocity = normal in elderly (90-120 in young adults)
FTc normal (330-360ms)
what can you say about this response to fluid?
both FTc and CO changed
>10%
hence fluid responsive
suggest patient was underfilled
describe how changes to preload, afterload and contractility will affect doppler waveform
is PICCO accurate?
strongly related to CO measurements with PAFC thermodilution (gold standard) so yes
how do we compare methods of measuring CO e.g. PAFC to PICCO
bland altman plot
a.k.a mean-difference plot
on y axis = difference betwen measurements
on x axis = average of the 2 readings
if they are comparable/ in agreement , the results will cluster around 0 line
