This chapter is relevant to Section G7(iii) of the 2017 CICM Primary Syllabus, which asks the exam candidate to "describe the invasive and non-invasive measurement of blood pressure, including limitations and potential sources of error". This chapter deals with the misguided use of CVP as some sort of an indicator of preload, which has never been asked about in the CICM Part One. Fellowship exam candidates had previously been asked about this, for example in Question 16 from the first paper of 2001 and Question 8 from the first paper of 2014. Because it seemed logical to keep all the physiology in the Primary Exam section, the topic is dissected here and briefly revisited in the Fellowship Exam revision resources.
In short, in days gone by, people relied on the CVP as a simple means of predicting fluid responsiveness. But it turns out the CVP is really bad at predicting the patients' responsiveness to fluid challenges. There are too many variables governing central venous pressure. This has become evident from some high-quality evidence, and it has been known for some time. Indeed, so obvious the uselessness of CVP in this scenario, and so entrenched the practice of its use, that prominent authors have described a recent meta-analysis as a plea for common sense.
The significance of the CVP in terms of right ventricular preload
The CVP, as measured in the SVC, should be the same pressure as in in the right atrium; and if the tricuspid valve is doing its job properly, the right ventricular end-diastolic pressure should be equal to the CVP (because after all the right ventricle fills with the blood from the right atrium, under right atrial pressure).
Thus, CVP = right ventricular diastolic pressure… …right?...
Ventricular compliance and the relationship of filling volume and filling pressure
The filling pressure of the right ventricle is far from being a straightforward measure of its volume.
Increasing the pressure which fills a poorly compliant fibrotic ventricle will not result in much more filling. C
Conversely, a supple young ventricle will respond to increasing pressure by gladly filling with more blood.
End-diastolic pressure only predicts preload when right ventricular compliance is normal.
Changes in end-diastolic pressure only predict changes in preload when right ventricular compliance is constant.
Relationship of central venous pressure to central venous volume
The central veins have a certain elasticity to them, and their stretchy response to fluid boluses was once thought to be predictive of right heart preload.
Some thought that this "delta CVP" might have some sort of predictive value when it comes to fluid responsiveness.
Surely, if you keep dumping fluid into your patient and the CVP fails to change dramatically, that must mean they have plenty of space left - plenty of "central venous reserve" - and thus more fluid boluses are required.
The CVP is a poor predictor of fluid responsiveness.
It turns out the CVP is really bad at predicting the patients' responsiveness to fluid challenges.
There are too many variables governing central venous pressure; it is never a perfect picture like this graph, where central venous compliance is predictable and constant. This has become evident from some high-quality evidence, and it has been known for some time. Indeed, so obvious the uselessness of CVP in this scenario, and so entrenched the practice of its use, that prominent authors have described a recent meta-analysis as a plea for common sense.
The detailed scholarly arguments against the use of CVP in assessing fluid responsiveness were sufficiently interesting to merit a whole chapter of their own.