Introduction
Intra-operative hypotension is common during general anaesthesia, but its association with postoperative pneumonia remains uncertain, partly because hypotension has been defined using different thresholds. We evaluated whether absolute thresholds or relative reductions from baseline mean arterial pressure were associated more strongly with postoperative pneumonia after non-cardiac surgery.
Methods
We conducted a retrospective multicentre cohort study of 151,036 adults undergoing non-cardiac surgery under general anaesthesia at three Chinese hospitals. Intra-operative hypotension was assessed using 12 metrics across nine thresholds, including absolute mean arterial pressure thresholds of < 50 mmHg to < 70 mmHg and relative reductions of 20–50% from baseline. The primary outcome was postoperative pneumonia within 30 days.
Results
Of 151,036 patients, postoperative pneumonia occurred in 3486 (2.3%). Relative reductions in mean arterial pressure were more consistently associated with postoperative pneumonia than absolute thresholds. At a 20% reduction from baseline, the number of intra-operative hypotension episodes was associated with postoperative pneumonia (subdistribution hazard ratio (sHR) 1.079, 95%CI 1.050–1.109, p < 0.001), as was the presence of intra-operative hypotension (sHR 1.108, 95%CI 1.068–1.149, p < 0.001). Absolute thresholds were not consistently associated with postoperative pneumonia after adjustment. Duration-related metrics were generally less consistent than depth- or recurrence-related metrics. Dose–response analyses showed that risk increased rapidly at low exposure levels and then plateaued across relative thresholds of 20–40%.
Discussion
Intra-operative hypotension was associated with postoperative pneumonia when defined as a relative reduction from baseline mean arterial pressure, whereas conventional absolute thresholds were less informative. These findings suggest that maintaining intra-operative blood pressure close to baseline may be more relevant to pulmonary risk than relying on fixed absolute thresholds alone.
Plain Language Summary
What we did
We studied 151,036 adults who had non-heart surgery under general anaesthesia at three hospitals in China. We looked at their blood pressure during surgery and checked whether they developed pneumonia within 30 days. We compared different ways of defining low blood pressure, including a fall below a set blood pressure level and a fall compared with the patient’s normal blood pressure.
Why we did it
Low blood pressure is common during anaesthesia. However, it was not clear whether low blood pressure during surgery increases the chance of getting pneumonia afterwards. We wanted to find out which way of measuring low blood pressure was most closely linked to pneumonia.
What we found
About 1 in 43 patients developed pneumonia after surgery. Patients were more likely to develop pneumonia when their blood pressure fell by 20% or more below their usual level during surgery. The more often this happened, the higher the risk. Measuring blood pressure against each patient’s normal level was more useful than using a fixed blood pressure number for everyone. The results suggest that keeping a patient’s blood pressure close to their usual level during surgery may help reduce the risk of pneumonia afterwards.