Cardiac output monitoring

Question 1

What forms of cardiac output monitoring are there?

Answer 1

Invasive and non-invasive.

Non-invasive:

• Fluid challenge and response in SVV (LiDCO rapide)
• TTE

Invasive:

• Pulse contour wave analysis
  i) LiDCo - requires arterial line
  ii) PiCCO - requires special arterial line
• TOE
• Pulmonary artery catheter
• Oesophageal doppler

Question 2

What are the principles of pulse contour wave analysis?

Answer 2

The arterial waveform in combination with the dilution of the injectate can be used to interpret cardiac function.
The stroke volume is proportional to the area under the curve up to the dichrotic notch.

Measured values include:

• Heart rate
• blood pressure
• MAP

Calculated information include:

• stroke volume
• cardiac output

Derived information includes:

• SVRI
• cardiac index
• oxygen delivery index

Question 3

What is stroke volume and what is stroke volume variability and how can its interpretation guide patient care?

Answer 3

Stroke volume is the volume of blood ejected from the left ventricle during systole (EDV - ESV).
SVV is the variability in stroke volume during the ventilatory cycle.
Normal SVV is 5-10%
If elevated trial response to fluid bolus until SVV in normal range.

Question 4

What is Fick’s principle with respect to cardiac output?

Answer 4

The amount of oxygen taken up by the tissues is dependent upon the delivery (CO) and the concentration difference (the arterial-venous gradient)
Therefore VO2 = (CO x Ca) - (CO x Cv).
Ca is arterial line O2
Cv is pulmonary catheter O2
VO2 is the difference between inspired and expired oxygen concentrations.

This can be rearranged to get CO= VO2 / (Ca - Cv)

Question 5

Describe the features of a Swan-Ganz/pulmonary artery catheter?

Answer 5

8Fr catheter usually 110cm long

Working distally to proximally:

1. Distal lumen used to measure PCWP and mixed venous sampling
2. Balloon for inflation when floating the catheter (1.5ml air).
3. Thermistor
4. Heating coil
5. Proximal lumen for measuring CVP and for cold injectate for thermodilution.

Question 6

What information can we obtain from using pulmonary artery catheters?

Answer 6

1. Measured variables
   - CVP
   - right atrial pressure
   - right ventricular pressure
   - pulmonary artery pressure
   - PCWP
   - SvO2
   - core temperature
2. Derived variables
   - Cardiac output
   - Stroke volume
   - Stroke volume index
   - systemic vascular resistance
   - systemic vascular resistance index
   - pulmonary vascular resistance
   - pulmonary vascular resistance index

Question 7

What are the complications of pulmonary artery catheters?

Answer 7

1. Complications from central access
   - introducing infection
   - arterial puncture/damage
   - pneumothorax
   - venous thrombosis formation
2. Complications from floating the catheter
   - arrhythmias
   - valvular trauma
   - misplacement
   - knotting of catheter
3. Complications from the PAC being insitu
   - pulmonary artery rupture
   - PE
   - pulmonary infarction

PAC-man trial 2005 showed 10% complication rate and no benefit in outcomes.

Question 8

Describe how oesophageal doppler works?

Answer 8

A probe is inserted into the oesophagus with a doppler probe pointing to the descending thoracic aorta at about 35-40cm.
The doppler probe produces a doppler trace by tracking red cell movement to produce a velocity-time trace giving:
- peak velocity
- mean acceleration
- stroke distance (AUC)

From these derived values can be obtained:

• stroke volume (by multiplying stroke distance by cross sectional area)
• flow time corrected (by knowing the velocity and distance)

Question 9

Describe the important features of a central venous pressure trace.

Answer 9

/\
/ \/\ /\
___/ \_/ \_

a = highest peak, resulting from atrial contraction at end-diastole.
c = close of the tricuspid valve
x = atrial relaxation
v = v(f)illing of the atria
y = opening of tricuspid valve

Question 10

Describe the important features of a central venous pressure trace.

Answer 10

a
c
x
v
y