The effect of individualized titration of positive end expiratory pressure guided by electrical impedance tomography on ventilation distribution during laparoscopic radical resection of colorectal cancer in the elderly

Interventions: A group:Intervention: The PEEP incremental method for lung recruitment starts with PEEP 5 cmH2O, increasing PEEP 5 cmH2O every 5 breaths until PEEP reaches 20cmH2O and maintaining breathing 10 times. Then decrease PEEP to 5 cmH2O by the reversed operation. After lung recruitment, PEEP titration was performed. PEEP gradually decreased from 20 cmH2O to 4 cmH2O, and decreased by 2 cmH2O every 5 breaths. The PEEP value corresponding to the lowest driving pressure was the optimal PEEP value. The method of lung recruitment is the same as before every 1 hour in the future.;Bgroup:Intervention: The PEEP incremental method of lung recruitment starts with PEEP 5 cmH2O and increases PEEP 5cmH2O every 5 breaths until PEEP reaches 20 cmH2O and maintains breathing for 10 times. Then reverse the operation to decrease PEEP to 5 cmH2O. After lung recruitment, PEEP titration was performed. PEEP gradually decreased from 20 cmH2O to 4 cmH2O, and decreased by 2cmH2O every 5 breaths. The PEEP value corresponding to the lowest driving pressure was the optimal PEEP value. No lung recruitment in the future.;C group:Constant 5cmH2O PEEP
Primary outcome(s): global impedance curve amplitude
Study Design: Parallel