As guardians of patient safety during the surgical procedure, anesthetists are tasked with the prevention of adverse intraoperative events. Of the many risks of surgery, death and stroke are two of the most devastating events that can occur. Ischemic stroke occurs in 0.1-3% of patients undergoing general anesthesia. Thus, maintenance of cerebral perfusion is essential during the perioperative period.
In an article by Bijker JB et al titled Intraoperative Hypotension and Perioperative Ischemic Stroke After General Anesthesia (Anesthesiology. 2009 Dec;111(6):1217-26) the relationship between intraoperative hypotension and stroke is evaluated. The purpose of the study was not only to validate a correlation between hypotension and stroke, but also to determine the degree of hypotension and the length of time associated with an adverse outcome.
The study found that the incidence of ischemic events was increased when the blood pressure dropped 30% below baseline. The longer the blood pressure was below the critical level, the greater the incidence of adverse outcome. In the words of the author:
“Our results suggest that intraoperative hypotension accounts for an increase in stroke risk of approximately 1.3% per minute hypotension (i.e., the risk is increased 1.013 times for every minute of hypotension), depending on the definition of IOH that is used (in this case a decrease in mean blood pressure more than 30% from baseline). For example, a cumulative duration of 10 min of hypotension will result in a 1.14 times increased stroke risk (1.01310). If applied to the POISE trial, this would mean an increase in absolute stroke risk from 0.5% (POISE trial control patients) to 0.57%.”
Again, in the words of the authors: “In conclusion, the most widely proposed mechanism of a postoperative stroke is arterial embolism. Nonetheless, the results of the current study support the hypothesis that hypotension can influence the evolution of a postoperative stroke by compromising (collateral) blood flow to ischemic areas. In this context, hypotension is best defined as a decrease in mean blood pressure relative to a preoperative baseline, rather than an absolute low blood pressure value.”
Since patients present with a wide variety of baseline blood pressures, there is no magic number for a mean blood pressure to be maintained during surgery. The anesthetist is advised to calculate each patients lowest acceptable blood pressure based on maintaining the blood pressure within 30% of baseline.
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