**8.10. Making sense of the outcomes**

these limits concord. Positive changes in cardiac output (∆CO) (right half) and negative ∆CO (left half) are presented on op‐

Our earliest description of polar plots used a full 360-degree circle to show both positive and negative directional changes (Figure 16). The data points are seen to lie within narrow ±30 degree sectors about the polar axes signifying good trending ability. When 30-degree limits are used the allowable differences in size of ∆CO are limited to a ratio of 1 to 2, rather than just

The half moon plot was later developed to show positive and negative ∆CO changes together

**i.** The mean polar angle which shows the deviation in agreement from the polar

**ii.** The radial limits of agreement which are 95% confidence intervals of the polar angles.

**iii.** The polar concordance rate which for comparisons against thermodilution are set at 30-degrees, but there is currently limited data to support these limits.

differ by less that 1 to 2 (i.e. half to double) in 95% of paired readings.

axis zero-degrees. It is a measure of difference in scale between the test and

If the angles lie within the 30-degree boundaries the original x-y ΔCO values will

posite halves of the plot. The mean polar angle and radial limits of agreement for data have been omitted.

The plot provides several parameters that describe trending:

reference methods.

direction of change.

(Figure 17).

72 Artery Bypass

If evidence based approaches are to be adopted when using MICOM devices in ones clinical practice then data from clinical validation studies will need to be critically reviewed. Market‐ ing information from most manufactures of MICOM devices provide lists of publications that they claim support their product. In reviewing such data one needs to ask the following ques‐ tions:


Study design is critical. (a) A sufficient number of patients should have been studied, though calculating the power of validation studies is not easy. Comparison of study size with other similar validation studies may help. (b) Type of patients and clinical setting effects results. Situations where a wide range of cardiac outputs and conditions (i.e. peripheral resistance) are encountered provide a rigorous test of performance. (c) Some of the early and more favourable validation studies using pulse contour devices were performed in cardiac surgery patients in whom haemodynamics were kept relatively stable. It was only when the same devices were tested in more labile liver transplant patients with cirrhosis that the problem with these devices and peripheral resistance became apparent [51].

The different statistical methods used in validation have been systematically covered previ‐ ously. (a) If a simple test versus reference method comparison has been performed then only Bland-Altman analysis is needed, but make sure the outcomes of the analysis are properly presented, including the percentage error. (b) If a sophisticated study design that allows trending to be assessed has been used, then concordance analysis using the four quadrant plot, and possibly a polar analysis should have been used to show trending. Check that central exclusions zones have been applied to the ∆CO data. (c) Animal studies are slightly different because of extent and quality of data that can be collected, and it is reasonable to use regression analysis.

When interpreting the results of Bland-Altman analysis: (a) Make sure the precision error of the reference method is correct. Normally for thermodilution it is ±20%, but other modalities may have different precisions and criteria may need correcting, like the 30% for percentage error. (b) Make sure all the outcomes of the Bland-Altman analysis have been presented. The key to interpreting Bland-Altman is the percentage error which needs the mean cardiac output and limits of agreement to be calculated. (c) Make sure that the limits of agreement have been correct for repeated measures [46,47].

than the agreement between methods. Either r or R2

**9.3. Current status of technology in 2012**

range of clinical areas.

protocols.

patient monitoring.

**Nomenclature**

MICOM – Minimally invasive cardiac output monitoring

TOE – Transoesophageal Echocardiography

VEPT – Volume of electrically participating tissue

PAC – Pulmonary Artery Catheter

LVET – Left ventricular ejection time

CSA – Cross sectional area

PEP – Pre ejection period

ECG – Electrocardiogram

exists between the two methods. The correlation coefficient (R2

validate the supra-sternal Doppler method in anaesthetized dogs [52].

are quoted. R2

Minimally Invasive Cardiac Output Monitoring in the Year 2012

value > 0.9 signifies good correlation. Ideally, if the test and reference (i.e. flow probe) methods are correctly calibrated, their data should lie along the line of identity y=x and correlation can also be performed along this line, which is known as Lin's concordance. Alternatively, the interclass correlation coefficient (ICC) is used. These methods were used in our 2005 paper to

Bioimpedance is no longer used clinically. Bioreactance (NICOM, Cheetah Medical) has only recently been released and still needs further clinical evaluation. It is being promoted in a wide

Pulse contour methods have not proved universally successful because of issues with the current algorithms failing to cope with swings in peripheral resistance. The PiCCO has a role in intensive care for continuous cardiac output monitoring in combination with transpulmo‐ nary thermodilution. The other modalities seem more useful when used to measure "func‐ tional haemodynamic variables" such as stroke volume variation in response to the straight leg raise test and fluid challenge. They are now being promoted to drive fluid optimization

Oesophageal Doppler (CardioQ, Deltex Medical) appears to be a useful intra-operative and in‐ tensive care monitor of haemodynamic status. It has been used successfully to drive goal di‐ rected fluid therapy protocols in high risk surgical patients. It has recently become popular in Britain as part of enhanced surgical recovery programs. External Doppler (USCOM) is less commonly used but appears useful in a number of clinical settings including paediatrics.

Other MICOM technology does exist but none currently have a major role to play in developing

is used when a relationship

http://dx.doi.org/10.5772/54413

75

) ranges from 0 to 1, where a

When interpreting the results of concordance analysis: (a) Make sure central exclusion zones have been used. These should be shown on the four quadrant plot. (b) Make sure the exclusion criteria used in the plot are appropriate, usually set at 15% or 0.75 L/min when mean cardiac output is 5 L/min. (c) Make sure the precision error of the reference method is known as this will affect the threshold criteria for good trending. (d) When thermodilution is the reference method a concordance rate of above 90-95% signifies good trending ability of the test method.

Polar plots are relatively new to trend analysis so their usefulness and threshold criteria for good trending still need to be set. However, they are an excellent method of showing trend data from multiple patients and for good trending data should lie within the 30 degree radial limits [50].

When reading authors conclusions regarding their validation study data, be skeptical about what is written, as the statistical analyses is often incomplete and authors tend to exaggerate their findings. In general the percentage error should be less than 30% for good agreement and the concordance rate above 90-95% for good trending ability.
