Influence of the End-inspiratory Pause on Mechanical Ventilation and Its Correlation With Electrical Impedance Tomography

Prolonging the EIP while maintaining an adequate expiratory time has shown benefits in terms of improving alveolar effective ventilation and enhancing gas exchange in surgical and intensive care patients. However, there are no published studies addressing the effects of different EIP times on the respiratory mechanics and gas distribution of surgical patients when associated with OLA strategies for ventilation.

Assuming the benefits of OLA, these investigators hypothesized about the potential effects of increasing the EIP when ventilating patients in a volume control mode. In the present study the investigators evaluated the influence of two different EIP (10% and 30% of the inspiratory time) on the respiratory mechanics of patients submitted to scheduled abdominal surgery under general anesthesia and ventilated with a protective lung strategy. The investigators studied the influence of EIP on driving pressure (Pdriv), plateau pressure (Pplat), respiratory static compliance (Crs) and open lung PEEP (OL-PEEP). We also assessed gas distribution by means of electric impedance tomography and studied its influence on gas exchange measured by means of serial gasometries.

Study protocol. A forced spirometry was performed in all patients after accepting their inclusion.

On the day of surgery, standard monitoring was initiated on arrival in the theatre, including electrocardiography, pulse oximetry, and noninvasive blood pressure monitoring. After light sedation with 1-2 mg of midazolam, a thoracic epidural catheter was placed under local anesthesia on anesthesiologist's criteria. A remifentanil infusion 0.03 mcg/kg/min was started before left radial artery catheterization under local anesthesia. After recording basal data during full consciousness on 21% inspired oxygen, all participants were preoxygenated via a facial mask for 5 min on spontaneous ventilation with fraction of inspired oxygen (FiO2) of 0.7 and fresh gas flow of 6 L/min. After induction with propofol ((1-1.5 mg/kg of predicted body weight (PBW)), 0.8 mg/kg of PBW of rocuronium were administered and proceeded with tracheal intubation. Patients were ventilated via a Primus (Drager, Telford, PA, USA) using a tidal volume of 7 ml/kg of PBW; volume control mode comprised an inspiration: expiration ratio of 1:2 and a respiratory rate of 12-14 breaths per minute to maintain the etCO2 at 35-40 mmHg, with an initial PEEP of 5 cmH2O. EIP was programmed according to randomization group. Fresh gas flow of 2 L/min with FiO2 of 0.7 was used throughout the procedure. Anesthesia was maintained with remifentanil 0.03-0.1 mcg/kg/min and sevoflurane, with minimal alveolar concentration (MAC) of 0.7-1 adjusted for patient´s age. Bispectral Index monitoring was used throughout the entire procedure (BIS Quatro, Covidien Ilc, Mansfield, MA, USA). All ventilation parameters remained stable throughout the study except the EIP (diverted in function of study protocol assignation) and the PEEP, which was tailored according the principles of OLA ventilation previously published. A central venous line was inserted in all cases and continuous cardiac output monitoring, systemic vascular resistance and systolic volume variation were monitored throught out all procedure by means of FloTrac sensor (Edwards Lifesciences, Irvine, California, USA). Other monitoring included train of four (TOF) for neuromuscular relaxation.

Dräger Primus (Dräger Medical, Lübeck, Germany) was used for ventilation with continuous monitoring of peak pressure (Ppk), Pplat, PEEP, Crs, FiO2, fraction of expired oxygen (FeO2), end-tidal CO2 (etCO2). For blood gases an ABL90 FLEX PLUS analyzer (Radiometer Medical, Copenhagen, Denmark) was used. If hemodynamic instability occurred during the ARM (fall> 20% of the cardiac index or mean arterial pressure), maneuver was discontinued and ephedrine or phenylephrine was administered and registered, restoring ARM on haemodynamics recovering.