Figure A1.

Schematic representation of the interactions between respiratory, cardiac and vascular activity, together with the corresponding recordings: respiratory effort signal (RES), electrocardiogram (ECG), laser Doppler flowmetry (LDF) and pulse plethysmography (PPG). In the resting state, the concentration of red blood cells can be considered constant, and so a Doppler shift in the velocity signal provides a measure of microvascular flow. In contrast, the finger PPG provides a measure of changes in arterial volume proportional to changes in arterial blood pressure.