CC7: Cardiac Output 1 Flashcards
What is cardiac output?
-The flow per minute from the heart into the systemic circulation
How does flow compare between left and right side?
-Flow through left heart (systemic) = Flow through right heart (pulmonary)
Qs= Qp
Example where flow is not equal between left and right side
-Shunts
-ASD/VSD
Effects of low cardiac output
-Cellular hypoxia
-Loss of cellular viability
Poor cerebral perfusion which leads to:
-agitation
-confusion
-unconsciousness
-metabolic acidosis
-rapid respiratory rate
What is the formula for CO
CO = HR x SV
What are features of autonomic innervation?
-Vagus nerves on SA and AV node (parasympathetic)
-Sympathetic on SA and AV node
-Autonomic tone (acetylcholine and noradrenaline)
-Cardiac reflexes (baroreceptors and chemoreceptors)
What is stroke volume?
-Amount of blood pumped out of heart with each contraction
-SV=EDV-ESV
What factors affect SV?
-Preload (atrial pressure)
-Afterload (peripheral vascular resistance)
-Contractility (vigour of contraction)
What are the limitations of estimating SV by echo/angiogram?
-They are 2 dimensional
-This could impede accurate measurement
What is preload?
-The load (stretch, filling) on the ventricle before ejection
-Regulated by venous return to the heart
What are measures of preload?
-EDV
-EDP
-Right atrial pressure
What happens as afterload increases?
-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)
What is afterload?
-Tension the ventricle must produce to open the semilunar valve and eject blood to the great vessels
What causes increase and decrease in SV?
Increase:
-Slow heartbeat
-Exercise
Decrease:
-Blood loss
-Rapid heartbeat
What is EDV affected by?
-Filling time (duration of ventricular diastole)
-Rate of venous return (rate of blood flow during ventricular diastole)
What is ESV affected by?
-Preload
-Afterload
-Contractility
What is contractility affected by?
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)
How do EDV and stroke volume change with rest and exercise?
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
What is the unit of cardiac output?
L/min
What is EDV?
-max filling after atrial systole (~130ml)
What is ESV?
-residual volume after ventricular systole (~50ml)
What is EF?
-Percentage of EDV represented by SV
What are the methods of measuring stroke volume?
Geometric methods
-Angiographic
-Echocardiographic (single/biplane)
Measuring SV on echo
-Relies upon simple shape modelling of LV
-Assumption of symmetry
-Limited regional wall motion abnormalities
-Can accurately detect contour edges
What are other ways of measuring cardiac output?
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
What are the 3 principle phases of thermodilution for measuring cardiac output?
-Injection - indicator is brought into circulation
-Mixing and dilution - indicator mixes with the bloodstream
-Detection - Concentration of the indicator is determined downstream
What is the gold standard for CO measurement?
Thermodilution
How to do thermodilution in vascular system?
-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
Describe the thermodilution method downstream
-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
What are the features of thermodilution kit?
-5% Dextrose in water injectate (D5W)
-Swan-Ganz floatation catheter
-Injectate system
What are the system connections in thermodilution kit?
-Cooling coil
-Injectate thermistor connection
-Insulated syringe
-Valved ‘T’ piece
-Distal thermistor connection
What is special about Swan Ganz catheter?
-It has 10cm depth markers to show how far in the body
What pressures are shown here?
-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
List the steps for the procedure of thermodilution
-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
How does thermodilution calculate CO?
-Area under dilution curve calculates cardiac output
-Can’t do too many thermodilutions as it lowers patient’s temperature
Advantages of thermodilution?
-Readily repeatable
-Injectate is non-toxic
-Simple measurement technique
-Good correlation with earlier methods
-Gold standard for CO measurement
Limitations of thermodilution?
-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
Sources of measurement error and variability in thermodilution?
-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
What are some problems and errors in thermodilution?
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
What are 4 causes of shunt?
-ASD
-VSD
-PFO (patent foramen ovale)
-PDA (patent ductus arteriosus)
What is a shunt?
-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
Where is the shunt?
-Right ventricle
3 causes of a bi-directional shunt?
-Eisenmenger
-Coughing
-Valsalva
3 way to detect intra-cardiac shunts?
-Indicator dilution method
-Contrast angiography
-Oximetry run
Explain Oximetry run
-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
How is shunt expressed?
-Ratio of the pulmonary flow to the systemic flow (e.g. 2:1, 3:1)
-Qp to Qs
What is a basic technique for detecting shunts?
-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
Mechanism of right to left shunt
Needs downstream obstruction
-Valvular (pulmonary stenosis, RVOT obstruction)
-Vascular (pulmonary hypertension)
Causes desaturation in systemic circulation
-Cyanosis
-Causes increase in Hb level
Is Qp/Qs affected by haemoglobin?
No
How many binding sites does one molecule of Hb have?
-4 binding sites for O2
-Hb molecule is saturated when all 4 sites are bound to O2
What is the formula for Qp/Qs?
Systemic arterial - Mixed venous
/Pulmonary vein - Pulmonary artery
(OXYGEN SATS)
What is the formula for Mixed venous?
What step-up value indicates left to right shunt?
-Step up in O2 Sat more than 7%
What is the criteria for Qp:Qs ratio?
Qp:Qs<1.5 Surgery not necessary
Qp:Qs 1.5-3.0 Mild to moderate shunt
Qp:Qs>3.0 Significant shunting
What are the limitations of shunting?
-Loses accuracy when calculating small shunts
-Step up can vary
-High CO underestimates AVO2 difference
Where is step up?
-right atrium
What is the Qp/Qs
1.5:1 left to right
What is Qp/Qs value in right to left shunt?
Qp/Qs < 1
(e.g. 1:1.5)
<1.5 = small shunt
>2.0 = significant
In what case could you have right to left shunt and what are signs?
-Eisenmenger syndrome
Primary detection
-Presence of cyanosis
-Arterial hypoxaemia
What are the normal saturations?
-Right side = 60-75
-Left side = 95-99
What factors can affect CO
-Autonomic nervous system: sympathetic and parasympathetic affect HR
-Circulating hormones: catecholamines affect HR
-Venous return and stretch receptors: Preload, afterload and contactility affect SV
