Cardioventilatory coupling, a transient temporal alignement between the heartbeat and inspiratory activity, has been studied in animals and humans mainly during anesthesia. The origin of the coupling remains uncertain, whether or not ventilation is a main determinant in the CVC process and whether the coupling exhibits chaotic behavior. In this frame, we studied sedative free, mechanically ventilated patients experiencing rapid sequential changes in breathing control during ventilator weaning (during a switch from a machine-controlled assistance mode (assist-controlled ventilation ACV) to a patient driven mode (inspiratory pressure support (IPS) and unsupported spontaneous breathing (USB)). Time series were computed as R to start inspiration (RI) and R to the start of expiration (RE). Chaos was characterized with the noise titration method (noise limit), largest Lyapunov exponent (LLE) and correlation dimension (CD). All the RI and RE time series exhibit chaotic behavior. Specific coupling patterns were displayed in each ventilatory mode and these patterns exhibited different linear and chaotic dynamics. When switching from ACV to IPS, inspiratory loading decreases the noise limit value, the LLE and the correlation dimension of the RI/RE time series in parallel whereas decreasing intra-thoracic pressure from IPS to USB has the opposite effect. Coupling with expiration exhibits higher complexity than coupling with inspiration during mechanical ventilation either during ACV or IPS probably due to active expiration. Only 33% of the cardiac time series (RR interval) exhibit complexity either during ACV, IPS or USB making the contribution of the cardiac signal to the chaotic feature of the coupling minimal. We conclude: (i) Cardioventilatory coupling in human exhibits a complex dynamic that can be chaotic. (ii) Ventilatory mode has major effects on the linear and chaotic features of the coupling. Taken together these findings reinforce the role of ventilation in the CVC process.