Cardiac output monitoring

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