Defining intra‐operative hypotension – a pilot comparison of blood pressure during sleep and general anaesthesia*

The scientific justification for particular values of intra-operative hypotension is poorly substantiated. To provide a rationale for appropriate values we recorded blood pressure measurements at home for 24 h using an automated non-invasive ambulatory blood pressure measurement device. These blood pressures were compared with blood pressure measured before and during general anaesthesia in 18 subjects undergoing elective day surgery. We confirmed that a pre-operative reading taken upon admission to hospital is significantly elevated compared to a usual daytime blood pressure in the same patient. The median (IQR range) increases in systolic and mean arterial pressures were 10 (2-15 -5 to 59) mmHg, p = 0.003 and 10 (5-14 -5 to 35) mmHg, p = 0.002, respectively. When using this admission blood pressure measurement as a 'baseline', systolic and mean arterial pressures decreased during sleep by 41 (30-46 6-83) mmHg and 34 (26-36 6-58) mmHg, respectively (p = 0.001). This decreased even further intra-operatively: systolic blood pressure by 49 (36-64 15-96) mmHg and mean arterial pressure by 36 (26-46 8-66) mmHg (p = 0.001).