Fig. 3. Simultaneous recordings of instantaneous ECG (black) and ventilation, recorded as intra-peritoneal pressure (red; the upward spike indicates lung expiration). The interval tachogram (blue) describes the change in timing between heart beats (nominal scale), and shows that the heart beat is increased during lung inflation. The recordings were made from unrestrained, conscious animals, and unknown factors other than ventilation will also be affecting the timing of each heart beat. Confirmation that an oscillatory component exists in heart rate that is in phase with the ventilation cycle was made using power spectral analysis on a more extensive data set (512 heart beat intervals). The results are shown in Fig. 4.