Minimally invasive cardiac output monitors

Question 1

What is the most invasive method of monitoring cardiac output?

Answer 1

Pulmonary artery catheter, however this is considered the gold standard for cardiac output measurement

Question 2

Why is cardiac output monitoring useful in anaesthesia?

Answer 2

• marker of O2 delivery to tissues
• can identify patients at high risk of significant morbidity, mortality or both
• guides treatment for fluid resuscitation
• guides use of vasoactive/inotropic drugs

Question 3

What are the disadvantages of a pulmonary artery catheter?

Answer 3

• infection
• pulmonary artery rupture
• arrhythmias on insertion
• thrombosis
• embolism

Question 4

What is the pulse contour analysis PiCCO and what is it’s limitations?

Answer 4

Thermistor-tipped arterial line in a central vessel (proximal artery) to measure the aortic trace waveform morphology.

An algorithm is used to determine CO by integrating the area under the curve of the arterial pressure vs time trace. Has to be calibrated using transpulmonary thermodilution.

Requires intermittent recalibration and a central arterial catheter. Less accurate with significant aortic regurgitation, arrhythmia or intraaortic balloon pump.

Question 5

What does pulse contour analysis LiDCO need? What are it’s disadvantages?

Answer 5

Requires an arterial line +/- CVC.

Uses pulse power analysis rather than pulse contour. Algorithm is based on law of conservation of mass for continuous CO calculation

Requires intermittent recalibration with lithium dilution.

Question 6

What are the limitations of pulse contour analysis FloTrac/Vigileo?

Answer 6

It requires an arterial line with a blood flow sensor attached. Uses age, height, gender and weight to measure the patient’s vascular compliance.

CO is calculated every 20s using an algorithm. Multiplies arterial pulsatility and a constant (K - from vascular compliance) resulting in a SV to multiply by HR for calculation of CO.

It’s less accurate for absolute measurement than calibrated pulse wave devices or with significant aortic regurg, arrhythmia or intra-aortic balloon pump.

Does not require calibration.

Question 7

What are the disadvantages of the oesophageal doppler?

Answer 7

Poorly tolerated unless tracheal tube present.

Relies on assumed proportion of blood flow through the descending aorta

Question 8

What does USCOM need and what are it’s limitations?

Answer 8

Transthoracic doppler probe. Probe is place suprasternally to measure flow through the aorta or left chest to measure transpulmonary flow.

Is completely non-invasive. Uses nomogram for valve area estimation, not accurate with significant valve stenosis.

Question 9

What does gas rebreathing require?

Answer 9

Tracheal intubation and stable tidal volumes during measurements. Rebreathing circuit.

Question 10

What are the limitations of transpulmonary thermodilution?

Answer 10

It’s invasive, it requires a thermistor tipped A-line.

It’s less accurate with pulmonary congestion and in the presence of shunting.

Question 11

What does lithium dilution require?

Answer 11

It’s invasive- needs an arterial line +/- CVC.

Inaccurate with intercurrent lithium use.

Can’t be used in patients <40kg or 1st trimester of pregnancy

Question 12

What are the limitations of thoracic bioimpedance?

Answer 12

Cutaneous electrodes only so not invasive.

Accuracy with haemodynamic instability is not well tested. Limited usefulness in awake patients.

Question 13

What is stroke volume variation? (SVV)

Answer 13

It’s the difference between the maximum and minimum stroke volumes over the respiratory cycle and is caused by changes in preload with alterations in intrathoracic pressure.

Question 14

What is SVV an indicator of?

Answer 14

Fluid responsiveness. If the patient has a SVV <10% they’re unlikely to be fluid responsive, but if they’re >15% they’re likely to benefit from fluid resuscitation.

Question 15

In general, what can cause inaccuracies in pulse contour analysis?

Answer 15

• over/under damped traces
• arrhythmias
• aortic regurg
• use of intra-aortic balloon pump
• changes in SVR may also lead to inaccuracies

Question 16

What does the oesophageal aortic doppler meausure? How does it calculate CO?

Answer 16

Blood flow through the descending aorta.

CO is calculated from the VTI (AUC), HR and CSA of the aorta

Question 17

What is the doppler equation?

Answer 17

V - velocity of blood

Fd - the doppler shift in frequency

c - the speed of ultrasound in tissue (1540 m/s)

Fo - the initial US frequency

Ø = the angle of US beam in relation to blood flow

Question 18

What is the VTI in the oesophageal doppler?

Answer 18

Velocity-time-integral - it’s calculated from the area under the velocity time curve and used as the stroke distance

Question 19

What is CSA in the oesophageal doppler?

Answer 19

Aortic cross sectional area. It’s taken from a nomogram using height, age and weight.

Question 20

What is the equation for calculating CO using the oesophageal doppler?

Answer 20

CO = CSA x VTI x HR

(although a correction factor must be used as the descending aorta only carries 70% of the cardiac output)

Question 21

What can cause inaccuracies in the oesophageal doppler?

Answer 21

• estimation of CSA because this can change based on volume status or vascular tone
• probe position as even small misalignments can cause underestimation of flow
• any turbulent flow, as the equation assumes laminar flow

Question 22

How does gas rebreathing determine cardiac output?

Answer 22

It uses the indirect Fick equation.

A rebreathing apparatus is attached to the patient’s tracheal tube and serial measurements are taken every 3 mins. At steady state, the CO2 entering the lungs via the pulmonary artery is proportional to the cardiac output and equals the amount exiting the lungs via expiration and pulmonary veins.

It requires intubation and fixed ventilator settings.

Question 23

What limits the accuracy of gas rebreathing as a measure of CO?

Answer 23

• severe chest trauma
• significant intra pulmonary shunt
• low minute ventilation
• high cardiac output
• assisted spontaneous breathing patients

All reduce accuracy

Question 24

How does transpulmonary thermodilution work?

Answer 24

Cold injectate is introduced into the SVC via a CVC.

An arterial line with a thermistor is placed in a major artery (femoral, axillary, brachial) and the change in temperature of the blood is measured after the injectate has traversed the right heart, pulmonary circulation and left heart.

The change in temperature over time curve begins later and has a lower peak compared to the PAC curve.

Thermodilution equation is then used to calculate CO.

Question 25

What are the possible errors with transpulmonary thermodilution?

Answer 25

• thermodilution via PAC measures R heart output, wherease transpulmonary measures L heart output - usually these should be equal but if the patient has an intra-cardiac or intra-pulmonary shunt this will lead to errors
• indicator can be lost into the lungs esp in pulmonary oedema
• indicator recirculation where more cold injectate enters the tissues and reenters the blood later can cause an abnormal prolongation of the thermodilution curve and lead to underestimation of CO

Question 26

How does lithium dilution work for measuring cardiac output?

Answer 26

0.5-2mls boluses (max 20ms cumulative) of lithium chloride as an indicator injected via a CVC or peripheral venous line and measured via aspiration of blood through an arterial catheter with an attached disposable electrode selective for lithium, at a constant rate of 4ml/min.

The change in voltage is electronically converted to plasma lithium concentration and the resulting conc vs time curve is used to calculate plasma flow which can then be converted to blood flow.

Question 27

What will reduce the accuracy of lithium dilution?

Answer 27

• long term lithium treatment
• non-depolarizing neuromuscular blockers
  
  • CI in patients <40kg and 1st trimester pregnancy

Question 28

What is the least invasive method of measuring CO?

Answer 28

Transthoracic electrical bioimpedance (TEB).

It measures the electrical resistance of the thorax to a high frequency, low magnitude current. 6 electrodes are placed and the resistance to current flowing from outermost to innermost electrodes is measured. The bioimpedance is indirectly proportional to the content of thoracic fluid. Changes in CO will change the amount of aortic blood and be reflected by a change in TEB.

Stroke volume is calculated using the formula:

SV = VEPT x VET x EPCI

VEPT = the volume of electrically participating tissue

VET = the ventricular ejection time taken from R-R interval

EPCI = the ejection phase contractility index which is indirectly proportional to TEB

Question 29

What are the limitations with transthoracic electrical bioimpedance?

Answer 29

• interference by electrocautery
• very sensitive to movement
• inaccurate in arrhythmias
• may be less accurate in haemodynamically unstable patients