Why? you might be wondered how this two situations happen alternately during operation. Why not! Imagine you are taking care of a hypertensive patient, because you have seen his/her high systolic BP, so you decided to administer more amount of anaesthetics to attenuate the stress response occurred from the operation, just a while your patient will end up with a hypotensive episode, as if he/she has not been seen by you before or you will not remember who this patient is, not the same as I have known. After the blood pressure is reduced from whatever the course would be, you then lighten the plane of anaesthetics, not so long the patient's blood pressure will shoot to where it was to the hypertensive level or much more than it had been in the early part of the anaesthetics management.
You have to realise what pathophysiologic changes occur in a hypertensive patient, the high BP does not mean that your patient has adequate blood volume, when we measure blood pressure, we usually assume that pressure and volume should be moved in the same direction, or high pressure reflects high volume. However, this assumption is definitely not true, because high blood pressure does not have to result from high blood volume/ high cardiac output but it can arise from the vessel itself, the vascular resistance. The more resistance the vessel has, the higher blood pressure you will get from the reading.
Therefore, the flip-flop of hypertension and hypotension is quite common during anaesthesia, and that would impose more complications to your patient as well, because the autoregulation of the cerebral blood flow, renal blood flow are used to be familiar with longstanding hypertensive level, when a hypotensive episode occurs, it would definitely harm your patient, and the perioperative complication can lead to mortality.
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