Blood pressure – during general anaesthesia versus sleep

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A person’s normal blood pressure changes throughout the day, typically being higher during the awake hours and lower during the hours when a person is asleep. There are many factors that could also change the blood pressure during the day: level of activity, pain, stress, disease, hormones, hydration etc. One reliable way of measuring all the daily changes in blood pressure is a device called Ambulatory Blood Pressure Monitor (ABPM). ABPM units indirectly measure blood pressure through vibrational signals associated with blood flow in the artery in the elbow, when a blood pressure cuff is inflated and then deflated. These are non-invasive, automated, lightweight, and are typically battery powered, belt-worn, and of a size and shape similar to that of a walkman or portable CD player. There may be mild discomfort when the blood pressure is being measured, however, this is only brief. With ABPM, multiple automatic measurements of blood pressure are obtained at specific intervals throughout a 24 to 48 hour period, and this information is recorded onto a computer chip, which could be printed out for analysis by a doctor. Blood pressure is one of the parameters routinely monitored when an anaesthetic is given for any operation or procedure. When a person undergoes an anaesthetic, there are many factors that could decrease blood pressure, including the medications that are given. It is usually treated with fluid, or other medications, when the blood pressure is lower than the initial blood pressure measured. However, there has been no study that has looked at how much a person’s blood pressure normally decreases when a person is asleep in comparison with their blood pressure under anaesthesia. Patients may well be treated for low blood pressure during anaesthesia (albeit unnecessarily) despite the possibility that their blood pressure may actually reach lower levels during sleep. To investigate this further, we propose to follow a group of fit and well patients (ie. No pre-existing blood pressure problems) undergoing minor general surgery. They will be fitted with an ABPM for 24 hours when they come to hospital for their pre-admission appointments. When they re-present for operation, their blood pressure under anaesthesia will be recorded (by both the ABPM, and routine BP monitoring), and compared to their normal “asleep” blood pressure. The presence of the ABPM will not affect the patient’s surgery or other management in any way. It is hoped that by establishing a relationship between a person’s normal asleep blood pressure and that during anaesthesia will better enable doctors to manage their patients.

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