What if clinicians could move beyond the current one-size-fits-all approach to monitoring a patient’s blood pressure in the operating room? What if we were given the tools for personalizing each patient’s definition of hypotension? As you are fully aware, determining a true “baseline” blood pressure is challenging in the operating room — as anesthesiologists, it is one of the constant pressures of our profession.
While the 2018 Wesselink et al. systematic review of the literature was important for synthesizing what we know about blood pressure cutoffs in relation to adverse patient outcomes after noncardiac surgery, its synthesis does not provide evidence that a single threshold for hypotension applies to all patients.
Why Do We Need a New Method?
Why do we need a better method for defining and approaching intraoperative hypotension? Because, as you know, intraoperative hypotension is connected to poor patient outcomes. For example, hypotension can be a cause of organ injury. It can also “exacerbate injury from other sources such as embolism (i.e., thrombo- or atheromatous embolism).” There are also data showing a relationship between low mean arterial pressure (MAP) “during cardiac surgery using cardiopulmonary bypass (CPB) and adverse patient outcomes including stroke and acute kidney injury.”
“Simply raising contemporary MAP targets during surgery may result in a MAP that is higher than the upper limit of autoregulation in some patients. In this situation, cerebral blood flow will increase proportionally with MAP, potentially leading to cerebral hyperperfusion, a situation that is associated with cerebral edema and brain dysfunction in nonsurgical patients.”
And the list of complications and poor patient outcomes continues.
Seeking to address the fact that applying a single threshold for hypotension to all patients is unsubstantiated by the data, a new study argues against the current industry standard: maintaining a mean arterial pressure (MAP) of “more than 80 mmHg in all patients in all clinical situations.”
“Personalizing the Definition of Hypotension to Protect the Brain” — a study funded in part by a grant from the National Heart, Lung, and Blood Institute of the National Institutes of Health — advances a new method for “potentially defining hypotension individually based on monitoring cerebral blood flow autoregulation.”
Their method advocates for “defining hypotension based on arbitrary or population-driven thresholds.”
The research was conducted mainly in cardiac surgery settings in which an arterial catheter was available for direct blood pressure monitoring. Their methods included “correlating low frequency changes in cerebral blood flow with MAP not systolic or diastolic blood pressure.”
This study has accomplished the following:
- validated clinically feasible methods of cerebral blood flow autoregulation in laboratory animals and humans;
- demonstrated a relationship between a MAP outside the autoregulation boundaries and brain dysfunction and other organ injury; and
- shown that acting on MAP that is lower than the lower limit of autoregulation during cardiac surgery reduces the frequency of delirium after surgery.
It should be noted that the majority of this research was conducted in cardiac surgery settings and may not be applicable in other settings. Given those limitations, as well as the fact that a clinically available autoregulation monitor does not currently exist, the researchers note:
“We have found that the ‘optimal MAP’ (i.e., MAP with the most robust autoregulation or the lowest mean velocity index) during CPB in adults is (mean ± SD) 78 ± 11 mmHg (g. 2). The results of our studies of 617 patients undergoing cardiac surgery found that the average (±SD) lower limit of autoregulation was 65 ± 12 mmHg, and the average upper limit of autoregulation was 84 ± 11 mmHg.”
While current data continues to suggest that there are limitations to deploying “a single blood pressure cutoff for a wide population of patients because the individual’s lower limit of autoregulation may or may not be above this arbitrary cutoff,” the study’s authors admit that in the meantime, like all of us, they will continue to use this method until an FDA-approved monitor for autoregulation data in the operating room can be developed, tested and put into practice. As always, the researchers call for more research to “confirm and extend” these promising results.
At Allied, patient safety is our top priority. We do everything we can to ensure proper cerebral or coronary blood flow (CBF) in our patients and continually monitor our patients to ensure the best of outcomes. When necessary, our Allied Anesthesiologists will monitor beat-to-beat fluctuations in blood pressure using an arterial catheter to ensure blood pressure never falls or rises to dangerous levels; and we treat these fluctuations when appropriate.
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