Pre-operative hypertension and intra-operative hypotension.

Abstract

Crowther et al. conclude that pre-operative hypertension is not associated with an increased risk of intra-operative hypotension 1. However, several factors must be considered in order to reach this conclusion, and we invite the authors to comment on these. Firstly, ‘intra-operative hypotension’ requires a more precise definition, particularly in association with end-organ damage. Wesselink et al. 2 found that the risk of end-organ damage (acute kidney injury, myocardial injury or stroke) increased in patients undergoing non-cardiac surgery when intra-operative mean arterial pressure (MAP) fell below 80 mmHg for > 10 min, < 60–65 mmHg for > 5 min or < 50–55 mmHg at any time. Could the authors provide further information about these thresholds and durations, and comment on any consequent risk, in their patient population? Secondly, Crowther et al.'s study required a number of conditions for an episode of hypotension to be recorded. The patient had to have an automated MAP < 55 mmHg, followed by a second automated ‘check’ recording of a MAP < 55 mmHg, both recorded accurately by the treating anaesthetist on a paper data-capture report form. There are any number of reasons any of these conditions may not have been met. Could the authors provide further information about ‘failed’ recording rates, with reasons, as well as commenting on the likely number of patients that they think had a MAP < 55 mmHg for > 1 min? Wesselink's systematic review includes papers that have been available to read for a number of years 2, and it is likely that anaesthetists monitor and intervene in the haemodynamics of those with hypertension more aggressively than those without hypertension, especially in the setting of a clinical trial; the hypertensive group, for example, were administered significantly higher amounts of vasopressors intra-operatively. Moreover, anaesthetists might be less likely to record dangerously low blood pressures on the data-capture form; the authors acknowledge this as a limit of their study, but to what extent do they think that any Hawthorne effect might have biased their results? Could the authors confirm whether any of the patients had continuous invasive or non-invasive blood pressure monitoring, which may have ameliorated the rate of hypotension in recipients? When connecting 65 patients undergoing elective caesarean section to continuous non-invasive blood pressure monitoring using a CNAP device, but blinding the treating anaesthetist to its readings, Illies et al. found that 376 CNAP hypotensive episodes occurred, only 208 of which were detected by the intermittent blood pressure cuff, supporting the view that intermittent blood pressure monitoring is insufficient to estimate incidence of hypotension 3. While we are pleased with increased recognition about the risks of intra-operative hypotension, we are not certain that Crowther et al.'s study adequately answers the question of whether hypertensive patients are subject to an increased risk of intra-operative hypotension, or not. Future studies must define hypotension thresholds and duration along a continuum of risk, rather than as single incidences, measured using continuous, automated, (non-)invasive blood pressure-recording devices.