Example 1: Pulse pressure ("the strength of a heart beat") varies with ventilation (mechanical). We can use a cyclic spline to represent the repeating effect of ventilation on pulse pressure.
This is especially useful when we only have a few beats per ventilation (high respiratory rate, low heart rate), where the pulse pressure variation in any individual ventilation does not represent the "true" pulse pressure variation.
Example 2: A steady central venous pressure (CVP) curve can be decomposed into separate cyclic effects: heart cycle, respiratory cycle, and the interaction of the two.
We can present the sum of these three effects as a plane (mainly for fun).
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