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Clinical Measurements > Cardiovascular and Cardiac Output Measurements > Flashcards

Cardiovascular and Cardiac Output Measurements Flashcards

1
Q

Define cardiac output

A

Volume of blood expelled by either ventricle per unit time (L/min)

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2
Q

What is the CO for a 70kg person?

A

5L/min

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3
Q

What is cardiac index?

A

CO related to different individuals

L/min/m2

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4
Q

Normal cardiac index at rest

A

2.6-4.2

Below 2.2 = cardiogenic shock

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5
Q

How long does a diastole cycle usually last?

A

750ms

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6
Q

How long does a systole cycle usually last?

A

270ms

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7
Q

Equation for CO

A

= HR x SV

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8
Q

Normal HR and SV

A

70bpm

70ml

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9
Q

Equation for SV

A

EDV-ESV

End diastolic volume and end systolic volume difference

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10
Q

Equation for Ejection Fraction

A

SV/EDV

% of blood leaving ventricles

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11
Q

SV

A
  • volume ejected during ventricles

- difference between EDV and ESV

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12
Q

Cardiac reserve volume

A

What is left after systole

- equal for both ventricles

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13
Q

Which side of the heart do we use for measurements most commonly?

A
  • left side for Doppler
  • oxygenated
  • easier to see
  • for fermodilution use right side
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14
Q

What ejection fraction values are healthy/unhealthy?

A 
>55% = healthy
<50% = reduced health
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15
Q

Causes of change in HR

A
  • exercise
  • stress (adrenaline, cortisol, fight or flight)
  • electrolyte balance (K+, Na+)
  • oxygen (reduces HR in hypoxia)
  • body temperature
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16
Q

How does HR affect SV

A
  • increase HR reduces SV
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17
Q

What does sodium do to HR?

A
  • bradycardia

- takes longer to repolarise

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18
Q

How does body temp change HR?

A
  • hyperthermia increases heart rate and contraction strength

- hypothermia = reduces HR and strength of contraction

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19
Q

What 3 factors primarily affect stroke volume?

A
  • preload
  • afterload
  • contractility
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20
Q

Define preload

A
  • degree of myocardial stretch before contraction
  • cannot be directly measured so measured by EDV and pressure
  • altered by venous return and filling time
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21
Q

Define afterload

A
  • amount of force stopping blood ejecting from heart

- altered by systemic vascular resistance (high BP)

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22
Q

Define contractility

A
  • tension developed and shortening velocity of myocardial fibres
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23
Q

What changes preload?

A
  • 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
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24
Q

What increases afterload?

A
  • higher aortic pressure (aortic stenosis or high BP) = increased resistance
  • lower SV in increased resistance
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25
Q

Frank Starling Mechanism

A
  • 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
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26
Q

How do we measure CO?

A
  • 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
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27
Q

How is flow calculated?

A

= change in additional indicator quantity overtime/ change in concentration

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28
Q

Fick’s Principle

A

total consumption of a substance by peripheral tissues equals the product of blood flow and arterial systemic concentration difference

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29
Q

How does Fick’s principle allow us to measure CO?

A

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
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30
Q

Pros and assumptions of using Fick’s principle for measuring CO

A
  • pros = +-5% accuracy

- assumed that there is a steady state output as pulmonary gas exchange needs to maintain constant throughout

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31
Q

Other techniques of measuring CO?

A
  • indicator dilution technique
  • thermodilution (gold standard, invasive)
  • doppler US
  • blood pressure
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32
Q

Cons of indicator dilution technique

A
  • indicators toxic
  • dye’s cannot be fully removed quickly from system so repeat measurements difficult
  • uses a dye instead of cold saline like thermodilution
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33
Q

What is the thermodilution technique based on?

A

Modified Stewart Hamilton equation

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34
Q

What is the modified stewart hamilton equation?

A
  • 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)
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35
Q

What does a high CO look like in the thermodilution technique?

A

Rapid dilution of cold injectate within warm blood = smaller area under temperature time curve

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36
Q

What does a low CO look like in the thermodilution technique?

A

Slower dilution of cold injectate within warm blood

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37
Q

How is the thermodilution technique performed?

A
  • 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
38
Q

Which peripheral vein is usually used in thermodiltuion?

A

Femoral vein

39
Q

Ideal requirements for thermistor for thermodilution

A
  • highly sensitive = small temperature change measured
  • minimal size
  • linear response
  • infinite working life
40
Q

Doppler US technique requirements

A

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)
41
Q

Assumptions made in Doppler US technique

A
  • when measuring diameter of sternal notch for CSA, we assume it is cylindrical but actually more close to elliptical
42
Q

What are some cardiovascular measurements?

A
  • arterial blood pressure
  • myocardial contractions
  • heart sounds
  • heart murmurs
43
Q

What is the difference between heart sounds and murmus?

A 
Sounds = vibrations that occur as blood accelerates/decelerates
Murmurs = vibrations due to blood turbulence flowing rapidly through heart
44
Q

Strain gauge system for myocardial contraction

A
  • contact methods using a series of strain gauges
  • transducer weight = 25g
  • frequency response up to 30Hz
  • however today most often sonomicrometry is used
45
Q

Sonomicrometry

A
  • 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
46
Q

Phonocardiography

A
  • all the sounds made in cardiac cycle

- compares heart sounds & electrical signals to provide a overview of cardiac cycle

47
Q

S1 heart sound

A
  • during isovolumetric contraction

- closure of mitral and tricuspid valves

48
Q

S2 heart sound

A
  • during isovolumetric relaxation

- closure of aortic and pulmonary valves

49
Q

S3 heart sound

A
  • during early ventricular filling
  • normal in children
  • in adults associated with ventricular dilation (ventricular systolic failure)
50
Q

S4 heart sound

A
  • during atrial contraction

- associated with stiff ventricular compliance (ventricular hypertrophy, ischemic ventricle)

51
Q

Why do we need to amplify heart sounds?

A

Sounds travel through heart from body and acoustical properties of body results in attenuation not reflection

52
Q

Amplitude and frequency of heart sounds

A

Small amplitudes

Large frequency range = 0.1-2000Hz

53
Q

How do we amplify heart sounds?

A

STETH!

54
Q

Parts of a stethoscope

A
  • bell & chest piece
  • diaphragm
  • tubing
  • headset (binaural/tubing)
  • aural tube
  • eartips
55
Q

Cons of a sethoscope

A

Not perfect, attenuates lower frequencies more than higher frequencies

56
Q

Use of diaphragm vs. bell

A
  • 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
57
Q

Normal BP

A

120/80 mmHg

58
Q

When does systole occur?

A

Between 1st and 2nd heart sounds

59
Q

Pulmonary arterial pressures

A

Make up 1/4 of those in the systemic system

60
Q

Central venous pressure

A

Pressure close to the right atrium

61
Q

Mean arterial pressure

A

Average pressure over a single cardiac cycle

62
Q

Calculation for mean arterial pressure

A

MAP = CO x SVR x CVP

But often:
MAP = CO x SVR
(because CVP is often around 0)

SVR = systemic vascular resistance

63
Q

What can affect SVR?

A
  • vasoconstriction
  • vasodilation
  • changes in viscosity of blood
64
Q

2 main methods of blood pressure measurement

A

indirect

direct

65
Q

Indirect methods of BP measuring

A
  • auscultation and sphygmomanometer

- oscillometric

66
Q

Direct methods of BP measuring

A
  • catheterisation
67
Q

2 types of sphymanometer

A
  • mercury (less transportable)

- without liquid (spring device and metal membrane)

68
Q

Method of auscultatory method

A
  • 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
69
Q

Why do we get these changes in sounds when measuring BP?

A

Change from no flow as artery is occluded -> turbulent flow -> laminar flow

70
Q

Oscillometric method

A
  • 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
71
Q

Catheterisation method

A
  • 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
72
Q

Bernouli

A

Ideal fluid
See lecture
Don’t understand
Blood flowing at a higher velocity has a higher ratio of kinetic energy to potential pressure energy

73
Q

Standardisation of catheterisation method

A
  • have to standardize posture, location for attaching monitor
  • measurements taken at a level with the right atrium
74
Q

What if we do need to take pressure at a different height using catheterisation?

A
  • 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

75
Q

Correcting for kinetic energy term

A
  • 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
76
Q

Most common measurement sites of direct measurement

A
  • brachial or radial arteries
77
Q

Intravascular measurement system

A

directly by a transducer at the tip of a catheter placed into vascular system

78
Q

Extravascular measurement system

A

pressure transducer is coupled to measurement site, by a catheter filled with saline
- transducer can be outside the body

79
Q

Extravascular sensors

A
  • 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
80
Q

What can cause problems?

A
  • flow resistance
  • inertia
  • elasticity
  • incorrect damping (small cathter leads to more damping, damping needed to compensate)
81
Q

Frequency components of normal pressure pulse

A
  • 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
82
Q

Types of damping adjustment

A
  • 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)
83
Q

Catheter tip transducers

A
  • 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
84
Q

Intravascular sensors

A
  • not routinely used
  • much more expensive
  • simultaneous blood sampling not generally possible
85
Q

Diaphragm transducers

A
  • 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
86
Q

Capacitative Method

A
  • 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
87
Q

Optical methods

A
  • fibre optic catheter
  • lower cost
  • pressure = membrane distorts
  • reflection off back of diaphragm
  • varies coupling between LED source & photodetector, changing the output
88
Q

General considerations

A
  • 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)
89
Q

Pros of thermodilution technique

A
  • accurate
  • allows continuous monitoring
  • gold standard
90
Q

Cons of thermodilution technique

A
  • invasive
  • reliable as long as minimum 3 bolus injections
  • assumes constant blood flow
91
Q

Pros of Doppler US technique

A
  • non-invasive
  • compact size (portability)
  • can be performed bedside in critical patients
  • rapid
92
Q

Cons of Doppler US technique

A
  • 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

Clinical Measurements (11 decks)

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