This study was conducted at Pirajussara and Sao Paulo Hospitals of the Federal University of Sao Paulo, Sao Paulo, Brazil. After our institutional ethics committee approval and achievement of the written informed consent, a total of 488 patients underwent to elective CABG, from February of 2005 to March of 2010 were prospectively included. Inclusion criteria were age 18 years or older, oral endotracheal intubation, and conventional mechanical ventilation (MV). Patients with obesity, chronic respiratory disease, laryngeal disease or anomaly, and difficult intubation (two or more trials), were excluded from the study.
The lung function indicators of forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) were evaluated at the bedside on the day before the operation, using a portable spirometer (Spirobank G, MIR, Rome, Italy), according to the standards of the American Thoracic Society to exclude chronic respiratory disease .
Anesthesia and operative technique
Anesthesia was induced in a routine fashion with etomidate and midazolam and maintained with sufentanil and isoflurane. Endotracheal tubes with high residual volume, low-pressure cuff, with an inner diameter of 7.5 mm for female and 8.0 mm for male, were used. The anesthesiologists were free to inflate the ETT cuff as per their clinical judgment and assessment, and blinded to the nature of the study. The patients were ventilated to maintain normocapnia without positive end-expiratory pressure (PEEP) and the inspiratory oxygen fraction (FiO2) was maintained between 50% and 60%.
The operation was done on-pump or off-pump, through a midline sternotomy and using the left internal thoracic artery, complemented with additional saphenous vein grafts. In all cases where the pleural cavity was incidentally opened, a pleural drain was inserted. In all patients, a mediastinal tubular drain was also left at the subxyphoid region.
After the operation, the patients were transferred to the intensive care unit (ICU), and ventilated in pressure-assist-control mode at 14 breaths/min, inspiratory time of 1.2 seconds, PEEP of 5 cmH2O, inspiratory pressure to promote a tidal volume of 8 ml/kg of predicted body weight, and FiO2 for keep arterial oxygen saturation above 90%.
ETT CP interventions
Thirty minutes upon ICU arrival and started MV, a 20 mL syringe was attached to the pilot balloon and the cuff was completely deflated. Sequentially, the cuff was progressively inflated by air injection, to promote a minimal volume to occlude the trachea. To assist the cuff inflation and identify air leakage, the graphical monitoring of the volume-time curve was adopted. The volume-time curve is able to demonstrate the presence of air leak. The existence of an air leak causes decrease of ETV when compared to inspiratory volume. In the presence of air leakage, the descending branch of the volume-time curve does not reach the zero value, with a flattening, being abruptly interrupted by the beginning of the next inspiration . The time of flattening occurrence of the expiratory branch of the curve was used to determine the air leakage. The inflation was performed until the descending branch of the volume-time curve came back to zero, stopping the flattening earlier on. The volume-time curve analysis was performed using a specific device for respiratory mechanics evaluation (Ventcare 9505 VSF, Takaoka, Sao Paulo, Brazil), and utilizing a pressure transducer interposed between the mechanical ventilator circuit and ETT.
Twenty minutes after adjusting of air volume injected into the cuff, ETV was recorded; sequentially the CP was measured and designated as P1. The CP was measured using a manometer graduated in cmH2O (VBM Medizintechnik GmbH, Sulz am Neckar, Germany) that was connected to the inflating channel of the pilot balloon. Twenty minutes after P1 measurement, the ETV was monitored. At this point, another measurement of CP, designated P2, was performed to evaluate the actual CP.
After P2 measurement the cuff was re-inflated with air through a 20 mL syringe according to a previously described technique, to avoid any complication resulting from these maneuvers.
During all study period, the patients were maintained in supine position. The head of the bed was kept in elevation of 30 degrees, and the patient's head and neck were maintained in midline with no flexion, extension or rotation.
Data are expressed as means ± standard deviation. The values of ETV post-P1 were expressed as percentage of values obtained pre-P1, as well as the CP values were expressed in percentage (P2 in relation to P1). Data were analyzed using paired Student's t test. Statistical analysis was performed with GraphPad Prism 3.0 software (GraphPad Software Inc, San Diego, CA). A value of p < 0.05 was considered statistically significant.