Feasibility Of Target Controlled PEEP To Reduce Pulmonary Shunt During One Lung Ventilation

Objective:The technology of one lung ventilation（OLV）can effectively avoid the spread of harmful substances into the healthy lungs and provide a good surgical field,so it is widely used in the thoracic surgery.Meanwhile,the thoracic surgical operation in the lateral decubitus position during one lung ventilation would increase the intrapulmonary shunt,so as to increase the risk of perioperative hypoxemia in those patients.This paper was designed to compare the four different methods for setting the positive end expiratory pressure（PEEP）during one lung ventilation in patients undergoing thoracic surgeries,to determine the optimal PEEP setting,by which to improve lung compliance,reduce intrapulmonary shunt and dead space and decrease the incidence of hypoxemia.Methods:75 cases of patients with ASA status I or II,aged 18⁷0,BMI20²8kg/m²,undergoing thoracic surgery in general anesthesia were chosen to admitted in the study.The patients were randomly divided into 5groups（n=15 each）.Group A:P-V curve was determined at 3 times of two-lung ventilation（TLV）,the lower inflection point（LIP）was measured and the pressure at LIP(P_LIP)was recorded as Pa₀.The best PEEP value（P_a）of this group is P_LIP+2 cm H₂O;Group B:the mean value of 3 P_LIPIP of TLV was recorded as P_b0,the basic titration value,and then the PEEP value was adjusted by increasing 1cmH₂O every 5 minutes,based on the static lung compliance.when the static lung compliance was achieved to maximum,the PEEP value was regarded as the best PEEP value（P_b）;Group C:taking 4cmH₂O as the starting point,increasing the PEEP value in 1cmH₂O in every 5 minutes according to the lung static compliance,when lung static compliance was achieved to maximum,the PEEP value was set as the best PEEP value（P_c）;Group D:taking 4cmH₂O as the starting point,increasing the PEEP value in 1cmH₂O in every 5minutes according to the lung dynamic compliance,when the lung dynamic compliance was achieved to maximum,the PEEP value was determined to be the best PEEP value（P_d）;Group E:setting the PEEP value as 0 cmH₂O.Heart rate,blood pressure,end tidal carbon dioxide partial pressure（ETCO₂）,peak airway pressure（Ppeak）,plateau airway pressure（Pplat）,and lung compliance were measured and recorded at the beginning of OLV（T₁）,15 minutes（T₂）,30 minutes（T₃）,45 minutes（T₄）,and50minutes after OLV（T₅）.Arterial and venous blood samples were taken at each time points for blood gas analysis.The intrapulmonary shunt（Q_s/Q_t）and physiologic dead space/tidal volume ratio（V_D/V_T）was calculated.Results:With the extension of time,intrapulmonary shunt,dead space increased andarterial oxygen pressure had no obvious change in the control group（Group E,p<0.05）,;In Group A:intrapulmonary shunt,dead space increased（p<0.05）,arterial oxygen pressure had no obvious change;dead space volume was significantly lower thanthat in Group E in each time point（p<0.05）;In Group B:dead space volume increased（p<0.05）,intrapulmonary shunt,arterial oxygen pressure had no obvious change;dead space volume was significantly less than that in Group E in T₁,T₂ time point（p<0.05）;In GroupC:intrapulmonary shunt,dead space volume,arterial oxygen pressure all increased（p<0.05）;pulmonary shunt was higher than that in Group E in T₄（p<0.05）,and arterial oxygen pressure was higher than that in Group E in T₃ and T₄（p<0.05）.In Group D:intrapulmonary shunt,dead space volume did not change significantly,arterial oxygen pressure increased（p<0.05）;arterial oxygen pressure was significantly higher than that in the Group E in T₅（p<0.05）.Conclusion:Dead space and intrapulmonary shunt would increase in patients undergoing thoracic surgeries in general anesthesia during the period of one lung ventilation.Giving appropriate PEEP could increase the arterial oxygen,the best PEEP should be 8 cmH₂O.However,the method of titrating PEEP value according to the dynamic compliance could significantly reduce the increasing of intrapulmonary shunt and dead space volume,meanwhile increase the arterial oxygen pressure,which should be suggested as the optimal protective lung ventilation strategy.