Does PEP Compensate the Reduction of Tidal Volume During One Lung Ventilation?

Approval Status: Approved Approval Number: 2007-06 Board Name: CPP Sud Ouest et Outre-Mer 3 Board Affiliation: French Ministry of Health (DGS) Phone: Email: cpp.soom3@orange.fr No France: Afssaps - French Health Products Safety Agency

During general anesthesia, airway closure and the formation of atelectasis impair oxygenation. During one-lung ventilation, large tidal volumes are used to resume atelectasis with a risk of regional over distension and Ventilator-Induced Lung Injury (VILI). The reduction in TV should reduce the occurrence of VILI but lead to a consistent alveolar derecruitment. This harmful effect may be counteracted by PEP. We, therefore, study the impact on oxygenation, of increasing PEP during OLV, in order to maintain alveolar recruitment when TV is reduced.

A recent study reported that mechanical ventilation with large intraoperative TV is associated with an increased risk of post-pneumonectomy respiratory failure. Indeed, large TV during OLV may lead to Ventilator-Induced Lung Injury (VILI) with the creation of alveolar stretch injury and the development of permeability pulmonary edema. The reduction in TV should reduce the occurrence of VILI but lead to a consistent alveolar derecruitment. This harmful effect may be counteracted by PEP. Therefore, once TV is reduced, PEP may play a key role in minimizing lung collapse and preventing lung units from repeated opening and collapse phases. Such "protective" ventilatory strategy may be proposed if it does not lead to hypoxemia during exclusion. We, therefore, study the impact on oxygenation, of increasing PEP in order to maintain recruitment, keeping Pplat constant when TV is reduced. We will compare, in each patient without severe pulmonary obstructive disease (FEV1 and FEV1 /CV > 70%), two strategies of ventilation with two different levels of TV and PEP, but keeping the same Pplat :

After induction of anesthesia, fiberoptic bronchoscopy confirms the correct position of the tube. Anesthesia is maintained with sevoflurane with a BIS® objective between 45 and 55. Boluses of sufentanyl and cisatracurium are done when clinically necessary. Patients are ventilated in VCV with a ZEUS® respiratory device (Dragger, Germany). Before incision, patients are switched to one-lung ventilation in the lateral position. The tidal volume (TV) is 8 ml/kg of ideal body weight, with a maximal plateau pressure limited to 32 cm H2O. The ventilatory frequency is adjusted in order to maintain end tidal concentrations of carbon dioxide (PetCO2) between 30 to 35 mmHg. 5 cmH2O of positive end expiratory pressure (PEP) is used, and the inspired oxygen fraction is adjusted in order to maintain the pulse oxymetry above 95%. During OLV, if SpO2 decreases to less than 90% with 100% of inspired oxygen fraction, surgery is temporarily stopped to resume two lungs ventilation until SpO2 recover at least 95%. If necessary, a continuous positive airway pressure (CPAP) with 5 cm H2O of oxygen is maintained to provide the non dependent lung. Datas are recorded when the chest is opened. The alveolar pressures and the inspiratory and expiratory flow time curves are monitored. After a period of 15 min, the two strategies are compared in a random order :

• TV of 8 ml/kg of ideal body weight and a PEP of 5 cmH2O during 10 minutes
• TV of 5 ml/kg and a PEP level in order to keep the same plateau pressure during 10 minutes.

Arterial blood gases are measured after 10 minutes using each strategy and before any vessels are ligated. The occurrence of PEPi is detected on the expiratory flow time curve.