Postoperative respiratory complications are common complications of patients after cardiac surgery and increase morbidity and mortality and hospital length of stay. Diaphragmatic dysfunction accounts for between 2 and 15% of these complications. Diaphragmatic paresis is one of these dysfunctions and could be due to an intra-operative phrenic nerve injury or harvesting of a mammary artery responsible for diaphragmatic devascularization. It alters the ventilatory mechanics and causes acute respiratory distress often requiring the use of mechanical ventilation. The diagnosis of this dysfunction can be made by thoracic ultrasound with assessment of diaphragmatic excursion. For patient with paresis, ultrasound criteria is an excursion < 25 mm after deep inspiration for at least one of the two hemidiaphragms. This dysfunction is most often transient in the postoperative period, but it can also become persistent.

Contract-Relax (CR) physical therapy technique can be applied to any muscle, providing muscle strengthening, neuromotor stimulation, and a gain in joint amplitude.

Currently, post-cardiac surgery management of respiratory physiotherapy is the same for a patient with or without paresis. Moreover, the CR technique of the diaphragm is not part of this "standard" rehabilitation.

The objective of this study is to determine if the CR technique associated with the current respiratory management allows an early rehabilitation of patients with diaphragmatic paresis after cardiac surgery.

This is a single-center, prospective, comparative, randomized, controlled, parallel group, single blind study, trial assessing the efficacy of the association of CR with a "standard" respiratory rehabilitation for patient with diaphragmatic paresis after cardiac surgery.

This study compares two group :

• "Control" group : Standard rehabilitation (4 rehabilitation sessions a day in Intensive Care Unit (ICU) and 2 sessions in cardiac surgery unit).
• "Interventional' group : Standard rehabilitation + 3 CR during each session. A stratification of the randomization is planned according to diaphragmatic involvement (unilateral versus bilateral).

Diaphragmatic excursion will be assessed by thoracic ultrasound in time motion (TM) mode at D3 and D5, before the first physiotherapy session of the day.

The probe is placed on the mid-clavicular line under the costal grill, with an orientation at 90° of the diaphragmatic dome. The aim is to see the diaphragm through an acoustic window: the liver on the right and the spleen on the left. The diaphragm appears as a hyper echogenic line, the excursion is measured with the TM mode.

Oxygen saturation SpO2 will be taken before and after each respiratory physiotherapy session taking place at D3 and D4. A measurement will be taken on D5 before the first rehabilitation session of the day.

Inclusion Criteria:

• Cardiac surgery under extracorporeal circulation,
• Postoperative diaphragmatic paresis (Diaphragmatic excursion <25mm),
• Consent for participation,
• Affiliation to the social security system

Exclusion Criteria:

• History of respiratory pathologies,
• History of neurological pathologies,
• Post-operative cardiac and circulatory complications,
• Pregnant or breastfeeding women,
• Unable to understand,
• Guardianship, curators or safeguard of justice.

from Baseline (Day 0) through Day 30

To minimize assessment bias, investigators and evaluators (doctor and ultrasound operator) will be unaware of the intervention group. Only the physiotherapist, who will realize the procedure, and the patient will know the arm of randomization.