Cardiovascular and Cardiac Output Measurements Flashcards
Define cardiac output
Volume of blood expelled by either ventricle per unit time (L/min)
What is the CO for a 70kg person?
5L/min
What is cardiac index?
CO related to different individuals
L/min/m2
Normal cardiac index at rest
2.6-4.2
Below 2.2 = cardiogenic shock
How long does a diastole cycle usually last?
750ms
How long does a systole cycle usually last?
270ms
Equation for CO
= HR x SV
Normal HR and SV
70bpm
70ml
Equation for SV
EDV-ESV
End diastolic volume and end systolic volume difference
Equation for Ejection Fraction
SV/EDV
% of blood leaving ventricles
SV
- volume ejected during ventricles
- difference between EDV and ESV
Cardiac reserve volume
What is left after systole
- equal for both ventricles
Which side of the heart do we use for measurements most commonly?
- left side for Doppler
- oxygenated
- easier to see
- for fermodilution use right side
What ejection fraction values are healthy/unhealthy?
>55% = healthy <50% = reduced health
Causes of change in HR
- exercise
- stress (adrenaline, cortisol, fight or flight)
- electrolyte balance (K+, Na+)
- oxygen (reduces HR in hypoxia)
- body temperature
How does HR affect SV
- increase HR reduces SV
What does sodium do to HR?
- bradycardia
- takes longer to repolarise
How does body temp change HR?
- hyperthermia increases heart rate and contraction strength
- hypothermia = reduces HR and strength of contraction
What 3 factors primarily affect stroke volume?
- preload
- afterload
- contractility
Define preload
- degree of myocardial stretch before contraction
- cannot be directly measured so measured by EDV and pressure
- altered by venous return and filling time
Define afterload
- amount of force stopping blood ejecting from heart
- altered by systemic vascular resistance (high BP)
Define contractility
- tension developed and shortening velocity of myocardial fibres
What changes preload?
- increased venous return and increased filling time increases it
- passive stretch as larger volume
- this increases stroke volume and muscle fibres stretched more passively so exert greater force for heart pump
- increased ventricular compliance as allow myocyte stretching and reduced resistance to venous return
- reduced by gravity as less fill/hypoperfusion/inflow valve stenosis
What increases afterload?
- higher aortic pressure (aortic stenosis or high BP) = increased resistance
- lower SV in increased resistance
Frank Starling Mechanism
- associated with contractility
- ability to change force of contraction and SV due to change in venous return
- occurs up to a point then contractility will not increase above this
How do we measure CO?
- use blood flow to indicate CO
- due to principles of mass transport
- C = indicator quantity/volume
- as fluid is constantly moving out and replaced, to maintain fixed change in concentration, fixed quantity needs to be added of an indicator per unit of time
- then calculate flow
How is flow calculated?
= change in additional indicator quantity overtime/ change in concentration
Fick’s Principle
total consumption of a substance by peripheral tissues equals the product of blood flow and arterial systemic concentration difference
How does Fick’s principle allow us to measure CO?
CO = rate of oxygen consumption/(systemic arterial O2 - systemic venous O2)
- invasive technique as catheter to measure O2 concentrations
- inhalation/exhalation done by a spirometer
- arterial access any artery
- venous use pulmonary artery
- venous blood analyser for oxygen concentrations
Pros and assumptions of using Fick’s principle for measuring CO
- pros = +-5% accuracy
- assumed that there is a steady state output as pulmonary gas exchange needs to maintain constant throughout
Other techniques of measuring CO?
- indicator dilution technique
- thermodilution (gold standard, invasive)
- doppler US
- blood pressure
Cons of indicator dilution technique
- indicators toxic
- dye’s cannot be fully removed quickly from system so repeat measurements difficult
- uses a dye instead of cold saline like thermodilution
What is the thermodilution technique based on?
Modified Stewart Hamilton equation
What is the modified stewart hamilton equation?
- law of conservation of mass
- Concentration change in indicator added to moving liquid using to calculate flow
- Q = flow rate
- T = temperature of indicator and blood
- D is density
- S is specific heat
- dt is change in time
- look at area under curve which is inversely proportional to CO (this is the integral)
What does a high CO look like in the thermodilution technique?
Rapid dilution of cold injectate within warm blood = smaller area under temperature time curve
What does a low CO look like in the thermodilution technique?
Slower dilution of cold injectate within warm blood
How is the thermodilution technique performed?
- use a Swan-Ganz catheter
- often use trans-pulmonary thermodilution set up
- most commonly on pulmonary side but can use aortic side
- have a thermistor at end of catheter to sense temperature, inserted by peripheral vein into pulmonary artery
- injection of cold saline of known temperature into pulmonary artery via port proximal catheter
- saline bolus needs to injected at a consistent timepoint in cardiac/respiratory cycle as CO affected by this, normally at end of expiration
- have a distal port for pulmonary-arterial pressure
- proximal port around 20cm from catheter tip (bolus injection given here)
- thermistor put on display, and is proximal to a balloon which allows inflation to allow catheter advancement, can be deflated and reinflated
- bolus injection repeated and CO averaged
Which peripheral vein is usually used in thermodiltuion?
Femoral vein
Ideal requirements for thermistor for thermodilution
- highly sensitive = small temperature change measured
- minimal size
- linear response
- infinite working life
Doppler US technique requirements
2 requirements
- CSA of ascending aorta (measure diameter by sternal notch, can use descending aorta but would be more invasive using oesophageal US)
- blood flow velocity (velocity-time integral as changes, measured at aorta at same place - sternal notch)
Assumptions made in Doppler US technique
- when measuring diameter of sternal notch for CSA, we assume it is cylindrical but actually more close to elliptical
What are some cardiovascular measurements?
- arterial blood pressure
- myocardial contractions
- heart sounds
- heart murmurs
What is the difference between heart sounds and murmus?
Sounds = vibrations that occur as blood accelerates/decelerates Murmurs = vibrations due to blood turbulence flowing rapidly through heart
Strain gauge system for myocardial contraction
- contact methods using a series of strain gauges
- transducer weight = 25g
- frequency response up to 30Hz
- however today most often sonomicrometry is used
Sonomicrometry
- non contact ultrasound transducers
- transmitter and receiver either side of vessel/heart chamber
- converts sound to an electrical signal
- can remain in place up to 5-6 months
- resolution up to 0.01mm can be achieved
Phonocardiography
- all the sounds made in cardiac cycle
- compares heart sounds & electrical signals to provide a overview of cardiac cycle
S1 heart sound
- during isovolumetric contraction
- closure of mitral and tricuspid valves
S2 heart sound
- during isovolumetric relaxation
- closure of aortic and pulmonary valves
S3 heart sound
- during early ventricular filling
- normal in children
- in adults associated with ventricular dilation (ventricular systolic failure)
S4 heart sound
- during atrial contraction
- associated with stiff ventricular compliance (ventricular hypertrophy, ischemic ventricle)
Why do we need to amplify heart sounds?
Sounds travel through heart from body and acoustical properties of body results in attenuation not reflection
Amplitude and frequency of heart sounds
Small amplitudes
Large frequency range = 0.1-2000Hz
How do we amplify heart sounds?
STETH!
Parts of a stethoscope
- bell & chest piece
- diaphragm
- tubing
- headset (binaural/tubing)
- aural tube
- eartips
Cons of a sethoscope
Not perfect, attenuates lower frequencies more than higher frequencies
Use of diaphragm vs. bell
- bell detects low frequency sounds better as skin becomes taut with rim of bell with pressure (diastole aspects, mitral stenosis)
- diaphragm better for higher frequency sounds
Normal BP
120/80 mmHg
When does systole occur?
Between 1st and 2nd heart sounds
Pulmonary arterial pressures
Make up 1/4 of those in the systemic system
Central venous pressure
Pressure close to the right atrium
Mean arterial pressure
Average pressure over a single cardiac cycle
Calculation for mean arterial pressure
MAP = CO x SVR x CVP
But often:
MAP = CO x SVR
(because CVP is often around 0)
SVR = systemic vascular resistance
What can affect SVR?
- vasoconstriction
- vasodilation
- changes in viscosity of blood
2 main methods of blood pressure measurement
indirect
direct
Indirect methods of BP measuring
- auscultation and sphygmomanometer
- oscillometric
Direct methods of BP measuring
- catheterisation
2 types of sphymanometer
- mercury (less transportable)
- without liquid (spring device and metal membrane)
Method of auscultatory method
- inflate cuff to above systolic pressure (200mmHg)
- stethoscope placed on brachial artery to listen to Korotkoff sounds
- listen to when sounds appear and disappear as slowly relieve pressure valve
- relieve pressure valve at around 3mmHg/second
Why do we get these changes in sounds when measuring BP?
Change from no flow as artery is occluded -> turbulent flow -> laminar flow
Oscillometric method
- instead of auscultation, use transducer in monitor
- measure cuff pressure oscillation
- parts include: pressure sensor -> MAP determined by analysis oscillometric pulse -> SBP and DBP estimated -> electronic display
- no clear cutoff of diastolic BP
Catheterisation method
- direct
- invasive
- arterial catheter connected to pressure sensor
- large bore needle
- cannula connected to sterile fluid filled system which is connected to electronic pressure transducer
- pressure measured depends on location of catheter tip in vascular system
- needed for dynamic circumstances and essential if significant blood loss is expected
- catheter with balloon tip carried by blood flow into arteries for measurement
Bernouli
Ideal fluid
See lecture
Don’t understand
Blood flowing at a higher velocity has a higher ratio of kinetic energy to potential pressure energy
Standardisation of catheterisation method
- have to standardize posture, location for attaching monitor
- measurements taken at a level with the right atrium
What if we do need to take pressure at a different height using catheterisation?
- if we do need to take pressure at a different height, this should be corrected for:
Equivalent heart level pressure = distance above/below heart (mm)/12.9
Correcting for kinetic energy term
- can also correct for kinetic energy term (more complex, dictated by blood velocity)
- patient kept at rest as kinetic component very small
- aorta (kinetic energy <3% which is not significant as static pressures are so high)
- pulmonary system (kinetic energy <25%)
- optimise location of sensor in vessel
Most common measurement sites of direct measurement
- brachial or radial arteries
Intravascular measurement system
directly by a transducer at the tip of a catheter placed into vascular system
Extravascular measurement system
pressure transducer is coupled to measurement site, by a catheter filled with saline
- transducer can be outside the body
Extravascular sensors
- diaphragm displacement transducer
- catheter must be stiff with no air bubbles to distort or dampen frequency response
- transducer must then be located directly connecting with catheter dome
What can cause problems?
- flow resistance
- inertia
- elasticity
- incorrect damping (small cathter leads to more damping, damping needed to compensate)
Frequency components of normal pressure pulse
- DC component
- heart rate component (60-90bpm)
- 6-20 harmonics suggested as significant (up to first 10 harmonics usually used)
- upper frequency response at least 15Hz
- working range must be well below natural frequency
Types of damping adjustment
- optimally damped = rapid response to a change in signal by allowing a small amount of overshoot
- critically damped = no overshooting occurs but system response is too slow
- under-damped = resonance occurs and signal oscillates
- over-damped = take far too long to reach equilibrium to give a true reading (may occur due to soft tubing/bubble forming)
Catheter tip transducers
- used in intravascular sensor
- greater accuracy as direct measurement so better than extravascular measurement
- transducer in tip of catheter, inserted directly into blood stream
- more complex as requires vent tube from rear side of diaphragm to atmospheric pressure so it can take relative measurements
Intravascular sensors
- not routinely used
- much more expensive
- simultaneous blood sampling not generally possible
Diaphragm transducers
- geometry dictates pressure-strain relationship
- small diaphragms manufactured using silicon micromachining technology
- commercially available transducers work over 50-300mmHg
- most are stable over an 8 hour period
Capacitative Method
- pressure moves diaphragm which moves plate
- plate should be as small and stiff as possible for optimal high frequency response
- alternatively an iron core moving in and out of a coil is used for inductor systems
Optical methods
- fibre optic catheter
- lower cost
- pressure = membrane distorts
- reflection off back of diaphragm
- varies coupling between LED source & photodetector, changing the output
General considerations
- care to prevent coagulation of blood
- catheter must be often monitored during insertion
- sterility is extremely important for an implantable transducer (disposable thin plastic membrane over catheter is often used for this, also increases lifespan)
Pros of thermodilution technique
- accurate
- allows continuous monitoring
- gold standard
Cons of thermodilution technique
- invasive
- reliable as long as minimum 3 bolus injections
- assumes constant blood flow
Pros of Doppler US technique
- non-invasive
- compact size (portability)
- can be performed bedside in critical patients
- rapid
Cons of Doppler US technique
- accuracy depends on user skill
- prone to errors compared to thermodilution
- assumption that LVOT (left ventricular outflow tract) is circular, in fact most people it is elliptical
- validation studies limit usefulness
