Both groups were similar in terms of gender, age, weight, body mass index, ASA physical status and type of surgery performed. In terms of comorbidities, both groups were similar, except for the presence of chronic obstructive pulmonary disease. There was 100% compliance to the designated PaCO2 intraoperative targets. The median (IQR) PaCO2 in the TMH group and TN groups were 51.5 mm Hg (46.9–60.9) and 34.8 mm Hg (32.8–38.1), respectively (p<0.001). With regards to surgical characteristics, the duration of surgery was longer in the TMH group, with a median (IQR) duration of 219 min (124–304) versus 144 min (108–218) in the TN group, although this was not significant at the 5% level (p=0.121). PaO2 was similar between the two groups: 156.8 mm Hg (146.3–217.2) in the TMH group and 142.5 mm Hg (122.5–199.1) in the TN group (p=0.380) cena largefriends. Oxygen saturation was similar: 98.5% in the TMH group (98.1–99.0) and 98.5% in the TN group (97.9–99.0) (p=0.834). Both groups also had similar mean arterial pressure (MAP) intraoperatively (p=0.307), similar total Hb (130.5 vs 122.2 g/L; p=0.132) and received similar total doses of intravenous morphine equivalent opioid, mg in the TMH group (–) and mg in the TN group ( – ) (p=0.22). In terms of intraoperative positioning of patients, one patient from each group was positioned in steep reverse Trendelenburg with minimal tilt. All other patients were positioned in the supine position with a neutral head position.
Interestingly, the incidence of postoperative delirium after surgery was lower in the TMH group, while LOS remained similar between the groups. Patients who suffered from postoperative delirium were all in the TN group, but they were also older (median (IQR) age=72 (59.5–77)) and had higher ASA scores (ASA scores of 3, 2, 1, 4 and 4). Their baseline medical comorbidities and duration of surgery (median (IQR) duration of surgery=171 min (83.5–254.5)) were similar to other study participants. 2 and LOS on postoperative cognitive performance. Cognitive outcomes were similar in groups with or without NIRS-based rSO2 optimisation in a recent randomised controlled trial.14 35 On the other hand, Murkin et al found that monitoring and reacting to cerebral desaturation during coronary artery bypass surgery was associated with clinical benefits.13 Patients with shorter LOS (<10 days) had a higher mean rSO2. Intraoperative NIRS rSO2 monitoring led to a significant reduction in postoperative cognitive disturbance, confirmed by Trafidlo et al. 36 Casati et al also reported that higher rSO2 led to shorter LOS and improved Mini-Mental State Examination scores in elderly patients undergoing major abdominal surgery,37 and Schoen et al found that low preoperative rSO2 was associated with a higher incidence of postoperative delirium. Among patients who started at a normal rSO2 level, those who developed delirium had a larger intraoperative drop in rSO2.38 Our findings were consistent with those of Schoen et al.; however, they need to be interpreted with caution, as the ASA scores and age were slightly higher in the TN group, and our study was not designed to quantitatively investigate postoperative cognitive performance in hypercapnia.
Supplemental thing
We did not measure cardiac output, stroke volume and systemic vascular resistance. Therefore, the effects on changes in intrathoracic pressure on cardiac output are unknown. Changes in intrathoracic pressure may have adversely impacted cardiac output, which may in turn have affected the EtCO2. However, given that the positive end-expiratory pressure was held constant in both groups, and the changes in lung tidal volumes were relatively small, the impact of intrathoracic pressure on cardiac output is likely to be small. Finally, our findings of a greater incidence of early postoperative delirium in the TN group need to be interpreted with caution, as confounders of postoperative delirium were not controlled, our study was not powered to investigate postoperative delirium, and mental state was only assessed by CAM, once preoperatively and once postoperatively. Accordingly, our findings for delirium should be viewed as hypothesis generating. Nevertheless, if we were to consider that our effect size observed (ie, risk difference of 0.3) could be due to chance and a smaller effect would be observed in a larger study, an appropriate powered randomised controlled trial for this outcome would be very feasible. If the proportion of patients with delirium in the intervention group is 10%, to achieve 90% power, the required sample size for each group would be 92.