Table 1.

Respiratory modulations comparison in ECGs vs. PPGs

	ECGs	PPGs
Baseline wander	Changes in the orientation of the heart’s electrical axis relative to the electrodes and changes in thoracic impedance	Changes in tissue blood volume
Amplitude modulation	Changes in the orientation of the heart’s electrical axis relative to the electrodes and changes in thoracic impedance	Changes in intrathoracic pressure that reduce pulse amplitude and stroke volume during inhalation
Frequency modulation	Manifestation of respiratory sinus arrhythmia, the spontaneous increase in heart rate during inspiration, and decreases during exhalation	Manifestation of respiratory sinus arrhythmia, the spontaneous increase in heart rate during inspiration, and decreases during exhalation

ECG, electrocardiogram; PPG, photoplethysmogram.