8.1 Introduction

The respiratory signal is usually recorded with techniques like spirometry, pneumography, or plethysmography. These techniques require the use of cumbersome devices that may interfere with natural breathing, and which are unmanageable in certain applications such as ambulatory monitoring, stress testing, and sleep studies. Nonetheless, the joint study of the respiratory and cardiac systems is of great interest in these applications and the use of methods for indirect extraction of respiratory information is particularly attractive to pursue. One example of application would be the analysis of the influence of the respiratory system in heart rate variability (HRV) during stress testing, since it has been observed that the power in the very high frequency band (from 0.4 Hz to half the mean heart rate expressed in Hz) exhibits potential value in coronary artery disease diagnosis [1], and HRV power spectrum is dependent on respiratory frequency. Another field of application would be sleep studies, since the diagnosis of apnea could be based on fewer and simpler measurements, like the ECG, rather than on the polysomnogram, which is expensive to record.

It is well known that the respiratory activity influences electrocardiographic measurements in various ways. During the respiratory cycle, chest movements and changes in the thorax impedance distribution due to filling and emptying of the lungs cause a rotation of the electrical axis of the heart which affects beat morphology. The effect of respiration-induced heart displacement on the ECG was first studied by Einthoven et...