Postoperative neurocognitive dysfunction (PND), which includes postoperative delirium (POD) and postoperative cognitive dysfunction (POCD), is a frequent complication in patients undergoing cardiac surgery, with reported incidences of 14-35% for POD and up to 80% for POCD. Both conditions have been associated with prolonged intensive care unit (ICU) and hospital stays, long-term cognitive decline, and increased mortality.
POD is defined by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), as an acute and fluctuating disturbance of attention and awareness, accompanied by cognitive or perceptual changes, typically occurring in the first days after surgery. The most commonly used assessment tool is the Confusion Assessment Method for the ICU (CAM-ICU), which demonstrates high sensitivity and specificity and is suitable for intubated patients or those in critical care.
POCD is considered a persistent reduction in cognitive function, particularly memory, attention, processing speed, and executive functions. It usually appears after the first postoperative week and can last from weeks to months, being more common in older adults. The Mini Mental State Exam (MMSE) is recommended as a screening and predictive tool for POCD.
There is evidence that significant decreases in cerebral oxygen saturation (rSO₂), measured non-invasively by near-infrared spectroscopy (NIRS) (either a drop >20% from baseline or absolute values <50%), are associated with higher rates of postoperative neurocognitive dysfunction. Prolonged intraoperative hypotension, defined as mean arterial pressure (MAP) <65 mmHg, is also linked to worse postoperative neurological outcomes.
Several studies have compared whether rSO₂ or MAP better detect postoperative neurocognitive dysfunction, with some showing that rSO₂ is a more sensitive predictor, reflecting global perfusion and immediate prognosis. Clinical practice guidelines recommend monitoring rSO₂ to improve neurological outcomes after cardiac surgery. Decreases in rSO₂ can indicate microcirculatory dysfunction even when blood pressure appears normal, correlating with adverse clinical outcomes such as elevated lactate, acute kidney injury, prolonged mechanical ventilation, or extended vasopressor use.
The time-weighted average (TWA) allows quantification not only of the magnitude but also the duration of hemodynamic or hypoxic deviations. TWA is calculated as:
TWA = ∑ (depth of deviation × duration in minutes) / total procedure duration
For TWA-MAP, the depth is the number of mmHg below 65 mmHg, multiplied by the duration, and adjusted to the total procedure time. The same principle applies to TWA-rSO₂. Elevated TWA-MAP indicates macrohemodynamic dysfunction, whereas elevated TWA-rSO₂ reflects cerebral microcirculatory dysfunction.
Previous studies have correlated TWA-MAP with acute kidney injury, ICU stay, and mortality. TWA-rSO₂ has been used in observational studies and correlated with lactate levels, creatinine, or duration of vasopressor therapy. However, no studies have evaluated the predictive value of TWA-MAP and TWA-rSO₂ for postoperative neurocognitive dysfunction (delirium or cognitive impairment) after cardiac surgery. Critical gaps remain in establishing validated TWA thresholds for predicting brain injury, determining whether neurological impairment develops beyond the intraoperative period, and evaluating the diagnostic sensitivity and specificity of this tool.
