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Cardiac output monitoring
Dr Karen OrrST6 anaesthetics/ICM
Altnagelvin ICM study day 7/11/13
Cardiac output
• Volume of blood ejected from the left ventricle per minute
• Depends on preload, contractility, heart rate and afterload
• CO = HR x SV
• MAP = CO x SVR
Methods of measuring CO
• Clinical
• Minimally-invasive
• Invasive
Clinical• Assess adequacy rather than "numbers"
• End organ perfusion
• Brain (confusion, altered consciousness)
• Kidney (UO)
• Tissues (lactate)
• Skin (CRT)
• BP correlates poorly...but...narrowed pulse pressure may have some value
• Increased intrathoracic pressure during inspiration
• Reduced venous return therefore reduced SV and BP
• More pronounced during MV
Minimally invasive
Efficacy compared to PAFC?????
Fick principle • Amount of a substance taken up by an organ per unit
time is equal to the arterial minus the venous concentration multiplied by blood flow
• CO = VCO2/ CaCO2- CvCO2
• CO2 production can be measured via sensors on breathing circuit
• CO2 content in mixed venous and arterial blood
• Reduced accuracy in sicker patients, severe chest trauma, intra pulmonary shunt, low MV and high CO
Thoracic bio-impedance
• Ejection of blood from LV into aorta is associated with changes in electrical impedance of the thoracic cavity
• High frequency, low voltage AC is applied
• Adv- minimally invasive
• Correlates relatively well in healthy people but not in unwell patients (0.29L/min)
• Reduced reliability in advanced age, perioperative fluid shifts, pulmonary oedema, MI, patient movement and electrical interference
Oesophageal Doppler• Continuous, real time monitoring
• Shift in frequency of reflected sound waves changes proportionally with change in velocity
• V = 2 x transmitted frequency/ velocity of US in blood x Doppler shift x cosine theta
Assumptions• 70% of blood enters descending aorta
• Blood flow is uniform and maximal
• Cross sectional area is constant (calculated using formula dependent on age, sex and height)
Measured variables• CO
• SV (stroke distance x aortic root diameter)
• Stroke distance is AUC x HR
• FTc (corrected flow time): indicates preload
• Peak velocity: indicates contractility
• HR
Contraindications• Oesophageal varices
• IABP
• Severe coarctation
• Known oesophageal pathology
Limitations • Intubated patients
• Probe must be as close as possible to parallel to aorta
• Operator dependent
• Learning curve for operator
• Probe displacement
Evidence for use• Reduced post operative complications, cost,
CVC use and hospital LOS when used in high risk surgical patients
• No change in mortality in either surgical or ICU pts
• Recommended by NICE for high risk surgical pts
• Small study (12 pts) comparing PAFC and OD in adult sepsis in ICU: good correlation with CI but poor with preload and SVR
Pulse contour analysis
• Relate the contour of the arterial pressure waveform to SV and SVR
• An algorithm is use to determine CO and produce a continuous readout
• Provide info on CO/ SVR etc but also SVV as a measure of fluid responsiveness
• SVV is the difference between max and min SV across the respiratory cycle
PiCCO• Thermistor tipped femoral arterial line
• Standard central line is used to calibrate using thermodilution
• Correlates strongly with PAFC readings in both controls and patients with abnormal physiology (less than 0.29L/min)
FloTrac• Standard arterial catheter
• Algorithm is used based on age, height, gender, weight and waveform characteristics
• No external calibration
• Conflicting evidence for accuracy compared with PiCCO and PAFC
• One study showed CO underestimated by up to 2L/min in 40% of readings
• Main advantage is ease of use
LiDCO• Pulse power analysis (based on law of conservation of
mass)
• Assumes that net power equates to net flow
• Standard arterial line +/- CVL
• Calibrated using lithium dilution
• Good correlation with PAFC across a range of values (error of <0.11L/min)
• Contraindicated in chronic lithium use, recent NDNMB, early pregnancy
Limitations
• Rely on optimal arterial signal
• Arrhythmias
• IABP
• Severe aortic regurg
• Changes in SVR
Invasive
Pulmonary artery flotation catheter
• Dye dilution (known quantity of indocyanine green with timed samples)
• Thermodilution (continuous- heated coil or intermittent-known volume of cold saline injected via RA with distal thermistor at the tip)
• PACWP (surrogate for LVEDP)
• Modified Stewart Hamilton equation
Limitations • Right and left ventricular output may differ in the
presence of an cardiac shunt
• Tricuspid or pulmonary valve regurg may cause underestimation of CO
• MV causes variation in CO depending on point in respiratory cycle
• Tip of catheter in west zone 1 or 2
• Mitral stenosis
Advantages
• Right and left sided pressures
• SvO2
• Core temperature
• Multi lumen infusion port
• Provides angiographic access
Complications • Associated with CVL (arterial puncture, nerve
injury, embolism, pneumothorax)
• Associated with catheterisation (arrhythmias, RV rupture)
• Associated with prolonged catheter insertion (PA rupture, pulmonary infarction, thrombosis, stenosis)
• PACMAN trial 10% incidence, ESCAPE trial 5%
The evidence
• No effect on mortality, LOS, or cost of care in either general ICU or high risk surgical patients
• No effect on surgical outcomes when used pre-operatively to optimise haemodynamics
What about cardiac surgery?
• Increased mortality and end organ complications in propensity matched obs study
• Authors recognise need for RCT
Device comparison