Effects of Preoperative Respiratory Physical Therapy on Postoperative Respiratory Function in Morbidly Obese Patients Undergoing Bariatric Surgery. A Randomized, Controlled, Clinical Trial

Background. Morbidly obese patients show an increased risk of hypoxemia during the postoperative period resulting in a higher incidence of postoperative pulmonary complications and prolonged hospital length of stay when compared with normal weight subjects. Mechanisms involved in this predisposition of morbidly obese patients for develop this physiopathologic scheme include a greater impairment on the respiratory mechanics induced by general anesthesia and paralysis -greater reduction in pulmonary and chest compliances and higher airway resistance- and an increment in ventilatory requirements. That leads to an increased work of breathing (WOB) and oxygen consumption (VO2). These effects are exaggerated in patients undergoing laparoscopic surgery because the direct effects of the pneumoperitoneum on the pulmonary mechanics. In addition, in morbidly obese patients respiratory muscles show a reduction in both strength and endurance probably a cause of the fatty tissular infiltration and the exacerbated inflammatory response usually observed in these patients. Respiratory physical therapy has shown to be effective and reduce postoperative pulmonary complications both, as postoperative treatment and when given preoperatively. When applied postoperatively respiratory physiotherapy has proved to be able to expand pulmonary volumes and to improve arterial oxygenation, leading to decreases in postoperative pulmonary complications, specially after abdominal surgery. In the specific setting of morbidly obese patients submitted to bariatric surgery, postoperative inspiratory muscle training (IMT ) improves inspiratory muscle strength and endurance allowing an earlier recovery of pulmonary airflows. There are scanty data about the results of applying respiratory physiotherapy on the preoperative period. However it has proved to be able to reduce the postoperative incidence of postoperative pulmonary complications in some different settings: (1) In patients after thoracic surgery, (2) In patients undergoing CABG surgery, specially in those considered at high risk for pulmonary complications and (3) after upper abdominal surgery. At our knowledge no studies aimed to determine wether preoperative respiratory physiotherapy could improve the postoperative respiratory function in morbidly obese patients have been published. The objective of this study was to evaluate the effects of a preoperative respiratory physical therapy program on the postoperative respiratory function in morbidly obese patients undergoing bariatric laparoscopic surgery. Fifty morbid obese patients (BMI≥40%) consecutively scheduled for laparoscopic bariatric surgery were considered eligible. All subjects were treated by the same surgical team and were submitted to the same anesthetic procedures. All the patients were informed about the objectives and interventions of the study and signed an informed consent form. The protocol was approved by the local Human Research Ethics Committee. The sample size was calculated using the repeated measures of the PaO2/FiO2 ratio in the postoperative period as the primary endpoint. Assuming that an increment of 25% or more in this parameter (effect size of 0.25) would be of clinical importance, with an alfa error probability of 0.05 and a power of 0.95, a required sample size of 44 patients was obtained. To account for attrition a proportion of 15% was added, reaching a total sample size of 50 patients. The calculation was carried out by means of the GPower 3.1.5 application for statistics. After inclusion, the informed consent procedure, and evaluation of baseline characteristics, the study patients were randomly and blindly divided into two groups: control group (CG) and intervention group (RPT). Randomization was done with a computer generated randomization table and individual closed sealed envelopes. An investigator blinded to the allocation sequence picked consecutive allocation envelopes for consecutive participants. Surgery was scheduled on the day 32 to 36 from the baseline day for all patients. Anesthetic management was standardized. Patients received no premedication. In the operating room patients were placed in reverse trendelemburg (RTDL) position (30º). Continuous positive airway pressure (CPAP) at 10 cmH2O was applied to all patients during the administration of oxygen in air (FiO2 0.8) for 5 minutes. After preoxygenation anesthesia was induced with propofol, 3 mg.kg-1 based on ideal body weight (IBW= X + 0.91* height (cm) - 152,4-1; X = 50 for men; X= 45 for women) and fentanyl 0,1 microg.kg-1, followed by rocuronium 0.6 mg. kg-1. Then all the patients were ventilated using pressure support ventilation (PSV) with a inspiratory pressure of 10 cmH2O, PEEP 10 cmH2O and a minimum respiratory rate of 10 breaths.min-1 (Ventilator Engström CS, GE Healthcare, Finland). When neuromuscular blockade was complete tracheal intubation was performed maintaining the RTDL position at 30º. The radial artery was cannulated for continuous monitoring of arterial blood pressure and arterial blood gas measurements. Anesthesia was maintained by continuous infusion of propofol and remifentanil to target a bispectral index ™ (XP version 3.0, Aspect Medical Inc. Norwood, MA) between 40 and 50. Neuromuscular blockade was maintained with rocuronium and continuously monitored (TOF Watch, Bluestar Enterprises, Inc., Chanhassen, MN). Remifentanil was discontinued at the time of the removal of the laparoscope. Residual neuromuscular blockade was reversed with neostigmine, 0.05 mg/kg IV (IBW). Normothermia was maintained intraoperatively using a forced warm air system (Bair Hugger®, Arizant Healthcare Inc., Eden Prairie, MN, USA). Intraoperative mechanical ventilation was the same for both groups of patients. The lungs were ventilated with volume controlled ventilation (VCV) with a mixture of 50% oxygen in air, and a tidal volume of 6 mL.kg-1 (IBW), inspiratory to expiratory ratio 1:2, and PEEP 10 cmH2O. Respiratory rate was adjusted to maintain end-tidal carbon dioxide partial pressure (etCO2) between 30 and 35 mmHg. An alveolar recruitment strategy (ARS) was applied after the onset of pneumoperitoneum. ARS was performed by increasing PEEP in increments of 5 cmH2O from 0 to 20 cmH2O using pressure controlled ventilation with a driving pressure similar to the airways plateau pressure obtained during VCV. Once PEEP reached 20 cmH2O the driving pressure was augmented to reach 45 cmH2O. After 10 breaths at maximal airways pressure, PEEP was decreased in steps of 2 cmH2O and static compliance of the respiratory system was measured at each step in order to determine the lung's closing pressure. Then a second ARS was applied and PEEP was set at 2 cm H2O higher than the closing pressure. At the end of surgery a new ARS was applied, before the extubation. In this case PEEP after the ARS was kept in 10 cmH2O until the extubation. Tracheal extubation was performed in the RTDL position (30º). Fully monitored patients were then transferred to the PACU in a semi-sitting position, while receiving a high flow of oxygen in air at a FiO2 of 0.5. Postoperative analgesia in the first 12 h was intravenous paracetamol (1 g every 6 h) and intravenous dexketoprofen (50 mg every 8 h) for all patients. In order to know the training effects we recorded the patient follow-up to the physical therapy program and measured forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1), maximal inspiratory pressure (PiMAX), and maximal expiratory pressure (PeMAX) (Datospir-600D, SIBEL SA Spain) before and at the end of the 30 days of training. All the patients were also instructed and trained about these measurements, aiming to avoid any learning effect during the study. The measurements were performed with the patients on a sitting position and wearing a nasal clip. PiMAX was measured from residual volume and PeMAX from total lung capacity. Five maneuvers were performed for each patient, and the three most acceptable were registered. The highest value obtained was considered for calculations. Spirometry was performed according to American Thoracic Society (ATS) recommendations, employing the above mentioned spirometer (Datospir-600D) previously calibrated . Three acceptable curves and two replicable ones were considered, allowing a maximum of eight attempts to each patient. The choices of the best values were done according to ATS criteria . The obtained values were expressed as percentage of normal values calculated according to Knudson et al. All the functional data were obtained by the same investigator, namely, the physical therapist.