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Year 3 Cardiac physiology > CC7: Cardiac Output 1 > Flashcards

CC7: Cardiac Output 1 Flashcards

1
Q

What is cardiac output?

A

-The flow per minute from the heart into the systemic circulation

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2
Q

How does flow compare between left and right side?

A

-Flow through left heart (systemic) = Flow through right heart (pulmonary)
Qs= Qp

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3
Q

Example where flow is not equal between left and right side

A

-Shunts
-ASD/VSD

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4
Q

Effects of low cardiac output

A

-Cellular hypoxia
-Loss of cellular viability
Poor cerebral perfusion which leads to:
-agitation
-confusion
-unconsciousness
-metabolic acidosis
-rapid respiratory rate

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5
Q

What is the formula for CO

A

CO = HR x SV

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6
Q

What are features of autonomic innervation?

A

-Vagus nerves on SA and AV node (parasympathetic)
-Sympathetic on SA and AV node
-Autonomic tone (acetylcholine and noradrenaline)
-Cardiac reflexes (baroreceptors and chemoreceptors)

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7
Q

What is stroke volume?

A

-Amount of blood pumped out of heart with each contraction
-SV=EDV-ESV

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8
Q

What factors affect SV?

A

-Preload (atrial pressure)
-Afterload (peripheral vascular resistance)
-Contractility (vigour of contraction)

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9
Q

What are the limitations of estimating SV by echo/angiogram?

A

-They are 2 dimensional
-This could impede accurate measurement

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10
Q

What is preload?

A

-The load (stretch, filling) on the ventricle before ejection
-Regulated by venous return to the heart

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11
Q

What are measures of preload?

A

-EDV
-EDP
-Right atrial pressure

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12
Q

What happens as afterload increases?

A

-As afterload increases, it takes longer before SL valves open and thus less blood will be ejected: ESV increases and SV decreases
-Increased by any factor that restricts arterial blood flow (atherosclerosis)

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13
Q

What is afterload?

A

-Tension the ventricle must produce to open the semilunar valve and eject blood to the great vessels

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14
Q

What causes increase and decrease in SV?

A

Increase:
-Slow heartbeat
-Exercise

Decrease:
-Blood loss
-Rapid heartbeat

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15
Q

What is EDV affected by?

A

-Filling time (duration of ventricular diastole)
-Rate of venous return (rate of blood flow during ventricular diastole)

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16
Q

What is ESV affected by?

A

-Preload
-Afterload
-Contractility

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17
Q

What is contractility affected by?

A

Sympathetic activity
-Adrenaline and noradrenaline cause ventricles to contact with more force
-This increases EF and decreases ESV

Parasympathetic activity
-Acetylcholine released by Vagus nerve reduces force of contractions

Hormones

Drugs mimic hormone actions
-Stimulate/block beta 1 receptors (beta blockers)
-Affect calcium ions, decreasing contractility (CCBs)

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18
Q

How do EDV and stroke volume change with rest and exercise?

A

At rest:
-EDV is low
-myocardium stretches little (low preload)
-stroke volume is low, ESV is high

With exercise:
-EDV increases (increased venous return)
-myocardium stretches more
-stroke volume increases, ESV decreases

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19
Q

What is the unit of cardiac output?

A

L/min

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20
Q

What is EDV?

A

-max filling after atrial systole (~130ml)

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21
Q

What is ESV?

A

-residual volume after ventricular systole (~50ml)

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22
Q

What is EF?

A

-Percentage of EDV represented by SV

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23
Q

What are the methods of measuring stroke volume?

A

Geometric methods
-Angiographic
-Echocardiographic (single/biplane)

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24
Q

Measuring SV on echo

A

-Relies upon simple shape modelling of LV
-Assumption of symmetry
-Limited regional wall motion abnormalities
-Can accurately detect contour edges

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25
Q

What are other ways of measuring cardiac output?

A

Radionuclide gated studies
-level of activity via volume

Bioimpedance methods
-electrical properties of the thorax alters with blood volume

Ultrasound Doppler
-Measure Time-velocity integral
-B mode echo gives cross sectional area of conduit
-calculate SV

Respiratory rebreathing techniques
-uses CO2 or acetylene

Dye dilution
-Indocyanine Green or Cardio blue

Thermodilution
-uses cold as a marker

Oxygen uptake measurements
-The Fick principle

26
Q

What are the 3 principle phases of thermodilution for measuring cardiac output?

A

-Injection - indicator is brought into circulation
-Mixing and dilution - indicator mixes with the bloodstream
-Detection - Concentration of the indicator is determined downstream

27
Q

What is the gold standard for CO measurement?

A

Thermodilution

28
Q

How to do thermodilution in vascular system?

A

-Choose accessible injectate site (right atrium)
-Choose practical sampling site (pulmonary artery)
-Inject known dose of marker (idocyanine green)
-Continuously monitor distal blood concentration via light absorbance

29
Q

Describe the thermodilution method downstream

A

-Injectate of known volume and temperature injected into right atrium and cooled
-Blood transverses a thermistor in a major vessel downstream over a duration of time
-CO is inversely proportional to mean blood-temperature depression and duration of transit of cooled blood

30
Q

What are the features of thermodilution kit?

A

-5% Dextrose in water injectate (D5W)
-Swan-Ganz floatation catheter
-Injectate system

31
Q

What are the system connections in thermodilution kit?

A

-Cooling coil
-Injectate thermistor connection
-Insulated syringe
-Valved ‘T’ piece
-Distal thermistor connection

32
Q

What is special about Swan Ganz catheter?

A

-It has 10cm depth markers to show how far in the body

33
Q

What pressures are shown here?

A

-Right atrium
-Right ventricle
-Pulmonary artery
-PCW
-Systolic pressure between RV and PA should be the same
-Diastolic pressure between RA and RV should be the same
-Diastolic pressure between PA and PCW should be the same

34
Q

List the steps for the procedure of thermodilution

A

-Use 10ml of 5% dextrose in water as injectate
-Pass injectate through cooling coil into insulated syringe
-Inject fairly rapidly through one-way valve into proximal (RA) catheter lumen
-Distal thermistor (PA) records temperature changes
-Temperature variables input to micro-computer which calculates parameters

35
Q

How does thermodilution calculate CO?

A

-Area under dilution curve calculates cardiac output
-Can’t do too many thermodilutions as it lowers patient’s temperature

36
Q

Advantages of thermodilution?

A

-Readily repeatable
-Injectate is non-toxic
-Simple measurement technique
-Good correlation with earlier methods
-Gold standard for CO measurement

37
Q

Limitations of thermodilution?

A

-Reproducibility of measurements
-Limits on the frequency and number of measurements
-Complications with PA catheter
-PA based CO not clinically useful during periods of haemodynamic instability as it is not real-time

38
Q

Sources of measurement error and variability in thermodilution?

A

-Variation of injectate temperature and volume
-Loss of indicator prior/during/after injection
-Cyclic changes in CO
-Truncation and extrapolation of of TD curves
-Decreased HR during cold indicator injection

39
Q

What are some problems and errors in thermodilution?

A

Technical factors with injectate
-errors in injectate temperature
-errors in injectate volume
-injection too slow

Physiologic and pathophysiologic variations
-Changes in pulmonary temperature with respiration
-Arrhythmia
-Tricuspid regurgitation

Analytic factors
-Low CO
-Changes in baseline core temperature
-Position of wedge catheter

40
Q

What are 4 causes of shunt?

A

-ASD
-VSD
-PFO (patent foramen ovale)
-PDA (patent ductus arteriosus)

41
Q

What is a shunt?

A

-Movement of blood between the left and right side of the heart
-Blood flows from high pressure to low
-Blood flows to region of high compliance

42
Q

Where is the shunt?

A

-Right ventricle

43
Q

3 causes of a bi-directional shunt?

A

-Eisenmenger
-Coughing
-Valsalva

44
Q

3 way to detect intra-cardiac shunts?

A

-Indicator dilution method
-Contrast angiography
-Oximetry run

45
Q

Explain Oximetry run

A

-Measured by taking O2 saturations from varying chambers and vessels of the heart
-Samples acquired from patients breathing room air, <30% O2
-If shunt exists, arterial and venous blood mixes, altering SO2 measurement

46
Q

How is shunt expressed?

A

-Ratio of the pulmonary flow to the systemic flow (e.g. 2:1, 3:1)
-Qp to Qs

47
Q

What is a basic technique for detecting shunts?

A

-Measuring O2 saturation of blood samples drawn sequentially from PA->RV->RA->SCV/IVC
-Left to right shunt may be detected if there is significant increase in O2 saturation

48
Q

Mechanism of right to left shunt

A

Needs downstream obstruction
-Valvular (pulmonary stenosis, RVOT obstruction)
-Vascular (pulmonary hypertension)

Causes desaturation in systemic circulation
-Cyanosis
-Causes increase in Hb level

49
Q

Is Qp/Qs affected by haemoglobin?

A

No

50
Q

How many binding sites does one molecule of Hb have?

A

-4 binding sites for O2
-Hb molecule is saturated when all 4 sites are bound to O2

51
Q

What is the formula for Qp/Qs?

A

Systemic arterial - Mixed venous
/Pulmonary vein - Pulmonary artery
(OXYGEN SATS)

52
Q

What is the formula for Mixed venous?

A
53
Q

What step-up value indicates left to right shunt?

A

-Step up in O2 Sat more than 7%

54
Q

What is the criteria for Qp:Qs ratio?

A

Qp:Qs<1.5 Surgery not necessary
Qp:Qs 1.5-3.0 Mild to moderate shunt
Qp:Qs>3.0 Significant shunting

55
Q

What are the limitations of shunting?

A

-Loses accuracy when calculating small shunts
-Step up can vary
-High CO underestimates AVO2 difference

56
Q

Where is step up?

A

-right atrium

57
Q

What is the Qp/Qs

A

1.5:1 left to right

58
Q

What is Qp/Qs value in right to left shunt?

A

Qp/Qs < 1
(e.g. 1:1.5)
<1.5 = small shunt
>2.0 = significant

59
Q

In what case could you have right to left shunt and what are signs?

A

-Eisenmenger syndrome
Primary detection
-Presence of cyanosis
-Arterial hypoxaemia

60
Q

What are the normal saturations?

A

-Right side = 60-75
-Left side = 95-99

61
Q

What factors can affect CO

A

-Autonomic nervous system: sympathetic and parasympathetic affect HR
-Circulating hormones: catecholamines affect HR
-Venous return and stretch receptors: Preload, afterload and contactility affect SV

Year 3 Cardiac physiology (30 decks)

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