In 1967, the term "respirator lung" was coined to describe the diffuse alveolar infiltrates and hyaline membranes that were found on postmortem examination of patients who had undergone mechanical ventilation.This mechanical ventilation can aggravate damaged lungs and damage normal lungs. In recent years, Various ventilation strategies have been used to minimize lung injury, including low tide volume, higher PEEPs, recruitment maneuvers and high-frequency oscillatory ventilation. which have been proved to reduce the occurrence of lung injury.

In 2012,Needham et al. proposed a kind of lung protective mechanical ventilation, and their study showed that limited volume and pressure ventilation could significantly improve the 2-year survival rate of patients with acute lung injury.Volume controlled ventilation is the most commonly used method in clinical surgery at present.Volume controlled ventilation(VCV) is a time-cycled, volume targeted ventilation mode, ensures adequate gas exchange. Nevertheless, during VCV, airway pressure is not controlled.Pressure controlled ventilation（PCV） can ensure airway pressure,however minute ventilation is not guaranteed.Pressure controlled ventilation-volume guarantee(PCV-VG) is an innovative mode of ventilation utilizes a decelerating flow and constant pressure. Ventilator parameters are automatically changed with each patient breath to offer the target VT without increasing airway pressures. So PCV-VG has the advantages of both VCV and PCV to preserve the target minute ventilation whilst producing a low incidence of barotrauma pressure-targeted ventilation.

Current studies on PCV-VG mainly focus on thoracic surgery, bariatric surgery and urological surgery, and the research indicators mainly focus on changes in airway pressure and intraoperative oxygenation index.The age of patients undergoing laparoscopic colorectal cancer resection is generally higher, the cardiopulmonary reserve function is decreased, and the influence of intraoperative pneumoperitoneum pressure and low head position increases the incidence of intraoperative and postoperative pulmonary complications.Whether PCV-VG can reduce the incidence of intraoperative lung injury and postoperative pulmonary complications in elderly patients undergoing laparoscopic colorectal cancer resection, and thereby improve postoperative recovery of these patients is still unclear.

One hundred patients undergoing elective laparoscopic colorectal cancer resection (age > 65 years old, body mass index(BMI）18-30 kg/m2, American society of anesthesiologists(ASA )grading Ⅰ - Ⅲ ) will be randomly assigned to volume control ventilation(VCV)group and pressure controlled ventilation-volume guarantee(PCV-VG)group.General anesthesia combined with epidural anesthesia will be used to both groups.

Ventilation settings in both groups are VT 8 mL/kg,inspiratory/expiratory (I/E) ratio 1:2,inspired oxygen concentration (FIO2) 0.5 with air,2.0 L/min of inspiratory fresh gas flow,positive end-expiratory pressure (PEEP) 0 millimeter of mercury （mmHg）,respiratory rate (RR) was adjusted to maintain an end tidal CO2 pressure (ETCO2) of 35 -45 mmHg.

In operation dates will be collected at the following time points: preanesthesia, 1 hour after pneumoperitoneum,2 hours after pneumoperitoneum ,30 minutes after admission to post-anaesthesia care unit (PACU) .The dates collected or calculated are the following:1)peak airway pressure,plate airway pressure, mean inspiratory pressure, dynamic compliance, RR,Exhaled VT andETCO2，2) Arterial blood gas analysis: arterial partial pressure of oxygen (PaO2), arterial partial pressure of carbon dioxide (PaCO2),power of hydrogen（PH）, and oxygen saturation (SaO2)，3) Oxygenation index (OI) calculation; PaO2/FIO2, 4) Ratio of physiologic dead-space over tidal volume(Vd/VT) (expressed in %) was calculated with Bohr's formula ; Vd/VT = (PaCO2 - ETCO2)/PaCO2,5) Hemodynamics: heart rate, mean arterial pressure (MAP),and central venous pressure （CVP），6) lung injury markers ：Interleukin 6(IL6）,Interleukin 8（IL8）,Clara cell protein 16（CC16）,Solution advanced glycation end products receptor（SRAGE）,tumor necrosis factor α(TNFα) .

Investigators will collect the following dates according to following-up after surgery: the incidence of postoperation pulmonary complications(PPC) based on PPC scale within seven days , incidence of pneumonia within seven days after surgery,incidence of atelectasis within seven days after surgery,length of hospital days after surgery, the incidence of postoperative unplanned admission to ICU, the incidence of operation complications within 7 days after surgery, the incidence of postoperative systematic complications within 7 days after surgery.

Inclusion Criteria:

1. scheduled for Laparoscopic colorectal cancer resection
2. age >65 years
3. body mass index(BMI) 18-30kg / m2
4. ASA gradingⅠ-Ⅲ

Exclusion Criteria:

1. history of lung surgery
2. severe restrictive or obstructive pulmonary disease (preoperative lung function test: forced vital capacity(FVC)< 50% predictive value of FVC,forced expiratory volume at one second(FEV1)< 50% predictive value of FEV1
3. Acute respiratory failure, pulmonary infection, ALI/ARDS, and acute stage of asthmaAcute respiratory failure, pulmonary infection, acute lung injury(ALI),acute respiratory distress syndrome(ARDS), and acute stage of asthma (bronchodilators were needed for treatment) were found within 1 month before surgery
4. Patients at risk of preoperative reflux aspiration
5. Preoperative positive pressure ventilation (as obstructive sleep apnea hypopnea syndrome patients) or long-term home oxygen therapy were performed
6. Serious heart, liver and kidney diseases: heart function class more than 3, severe arrhythmia (sinus bradycardia (ventricular rate < 60 times/min), atrial fibrillation, atrial flutter, atrioventricular block, frequent premature ventricular and polyphyly ventricular early, early to R on T, ventricular fibrillation and ventricular flutter), acute coronary syndrome, liver failure, kidney failure
7. Neuromuscular diseases affect respiratory function, such as Parkinson's disease, myasthenia gravis and cerebral infarction affect normal breathing
8. Mental illness, speech impairment, hearing impairment
9. Contraindications for spinal anesthesia puncture
10. Refuse to participate in this study or participate in other studies -