This chapter is relevant to Section G7(iv) of the 2017 CICM Primary Syllabus, which asks the exam candidate to "describe the methods of measurement of cardiac output including calibration,
sources of errors and limitations".
This is the measure of “swing” in the art line trace; the measurements are compared over 30 seconds.
This only works in positive-pressure-ventilated patients; the theory is that the lower on the Frank-Starling Curve you are, the more stroke volume will vary depending on the phase of ventilation.
(This is one of the known and well-exploited haemodynamic effects of positive pressure ventilation)
I.e. the decrease in preload from mechanical inspiration = decrease in stretch = decrease in stroke volume.
You aim for an SVV of under 10%; any greater variation than this warrants a fluid bolus.
It still applies in spontaneously breathing patients; however it is a poorer predictor of fluid responsiveness.
Overall, SVV appears to be a good predictor of fluid responsiveness. The patients in whom this would be most accurate are those being ventilated with large volumes (8ml/kg or higher). This rests on the premise that certain predictable conditions for validity are met, eg. the patient is not breathing spontaneously nor is suffering atrial fibrillation, and so on.
The details of using SVV (and its poorer cousin, PPV) in the assessment of fluid responsiveness is discussed in greater detail in the Fluid Resuscitation section.