Best Evidence Topics


Baumann UA et al
Estimation of central venous pressure by ultrasound
  • Submitted by:Simon Carley - Consultant in Emergency Medicine
  • Institution:Manchester Royal Infirmary
  • Date submitted:3rd June 2005
Before CA, i rated this paper: 6/10
1 Objectives and hypotheses
1.1 Are the objectives of the study clearly stated?
  Yes, this is a study to examine the ability of USS to detect CVP. They used invasive CVP as a gold standard. They found that it failed to accuratley describe CVP. They used stable ITU patients for the measurements. They also looked at the effect of PEEP in healthy volunteers.
2 Design
2.1 Is the study design suitable for the objectives
  Yes, this is a diagnostic study. The test is the USS results. The gold standard is invasive CVP.
2.2 Who / what was studied?
  2 studies. In patients, 32 haemodynamically stable patients with CVP monitors in situ were used. Arguably this is not the group who might be the most useful to use the test on in the future. However, you have to start somewhere and if it does not work in this group it is unlikely to do so in sicker patients.
The also used 6 healthy volunteers without CVP lines to examine the effects of PEEP on measurement.
2.3 Was this the right sample to answer the objectives?
  At this stage yes, it looks like a pilot study at this stage and it is therefore applicable to use a relatively well controlled test popluation.
2.4 Is the study large enough to achieve its objectives? Have sample size estimates been performed?
  No sample size calculation. Is it large enough? Don't know but possibly due to the dissapointing nature of the results.
2.5 Were all subjects accounted for?
2.6 Were all appropriate outcomes considered?
  I think so as they accept the CVP is an imperfect clinical parameter, but that is what they use and therefore that is all they were looking for.
2.7 Has ethical approval been obtained if appropriate?
2.8 Was an independent blinded gold standard test applied to all subjects?
  It appears that the operator was not able to see the test result, but there is no mention of blinding to the invasively measured CVP. One must presume that blinding was inadequate.
3 Measurement and observation
3.1 Is it clear what was measured, how it was measured and what the outcomes were?
  Yes, the measurement of bearing pressure (when vein deformation occurs) and collapsing pressure (when vein occluded) are described well. Some of the bias in their results probably results from some assumptions made on Laplace's law which I think are incorrect. Most notably that the pressure measured at the skin equates to that applied across the vessel wall (which it does not).
3.2 Are the measurements valid?
  For the gold standard - yes.
For the USS - no
3.3 Are the measurements reliable?
  For the gold standard - presumably yes (though I would like to have seen more information on calibration prior to testing).
For the USS, then no. There are both systematic differences (a general under-reading of CVP) and variable, see the Bland-Altman plots. Variability probably increases at higher pressures.
The differences found are clinically important.
3.4 Are the measurements reproducible?
  Reproducibility in the well, stable, volunteers was quite low.
4 Presentation of results
4.1 Are the basic data adequately described?
4.2 Are the results presented clearly, objectively and in sufficient detail to enable readers to make their own judgement?
4.3 Are the results internally consistent, i.e. do the numbers add up properly?
5 Analysis
5.1 Are the data suitable for analysis?
5.2 Are the methods appropriate to the data?
  yes, good to see Bland Altman as a representation of the results rather than simple correlation.
5.3 Are any statistics correctly performed and interpreted?
6 Discussion
6.1 Are the results discussed in relation to existing knowledge on the subject and study objectives?
  The discussion addresses all the issues but is, in my opinion, a bit biased in favour of the USS, which on the basis of the results is pretty poor.
6.2 Is the discussion biased?
  See above.
7 Interpretation
7.1 Are the authors' conclusions justified by the data?
  They are OK. A bit favourable I think.
7.2 What level of evidence has this paper presented? (using CEBM levels )
  Not sure
7.3 Does this paper help me answer my problem?
  Yes, for the time being it tells me that USS is an inadequate method of estimating CVP
After CA, i rated this paper: 8/10
8 Implementation
8.1 Can the test be implemented in practice?
  Yes - but it shouldn't
8.2 What aids to implementation exist?
  None - but it shouldn't
8.3 What barriers to implementation exist?
  Lack of efficacy!!!!!
8.4 Are my patients the same as the patients tested?
  No, they are generally sicker.
8.5 Will the test improve diagnosis in my patients?