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 hemodynamic 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.
Why does Stroke Volume Variation only apply to positive pressure ventilated patients?
It still applies in spontaneously breathing patients; however it is a poorer predictor of fluid responsiveness.
- The sensitivity is decreased: its only 63%. The spontaneous breathing efforts draw a smaller tidal volume, and from such minor changes in thoracic pressure there would insufficient change in ventricle loading; so there may still be changes to stroke volume, but they would be tiny and difficult to measure.
- If there is profound hypovolemia, the IVC can collapse on inspiration. Obviously this decreases preload, and confuses your SVV. You cannot predict fluid responsiveness this way, because you never get an accurate impression of preload.
- In spontaneous respiration, inspiration increases the right ventricular preload, which means the right ventricular filling is likely still appear adequate even if there is some hypovolemia. In spite of low overall volume, the right ventricular preload remains adequate, and thus at least one of the ventricles is likely to be operating in the preload-independent straight part of the Frank-Starling curve.
Utility of this parameter
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.