Active Temperature Management After Cardiac Surgery and Its Effect on Postoperative Cognitive Dysfunction

As one of the more common hospital-based major interventional procedures, 180,476 CABG surgeries were performed in the United States in 2016, according to the Society of Thoracic Surgeons Adult Cardiac Surgery Database (STS ACSD). Of these, 23,545 operations also involved aortic or mitral valve repair or replacement. The demographic characteristics of patients undergoing cardiac surgery have changed over time, with a higher proportion of elderly patients undergoing increasingly complex procedures. The number of patients with neurological disease prior to surgery has nearly doubled from 1.4% in 2001 to nearly 2.8% as of 2010, and is rising as surgical care improves and includes more such patients .

The recognition of neurological complications associated with cardiac surgery has been report-ed widely, and neurological dysfunction can be categorized as type 1 (brain death, non-fatal stroke, and new transient ischemic attack) or type 2 (delirium and postoperative cognitive dys-function) . The incidence postoperative neurological complications, defined as "permanent stroke" by the STS ACSD, was noted in 2016 to be 1.3% following isolated CABG operations, but was noted to be higher, between 2-3% in CABG patients also receiving aortic or mitral valve surgery as part of their operation. Notably, this statistic does not capture POCD, although steps are being taken to better capture this, perhaps as a patient-reported outcome (PRO) in the future.

As early as 1955, POCD was described by Bedford in the Lancet under the designation "adverse cerebral effects of anesthesia on old people" . It can affect any age group, but does seem to dis-proportionately affect patients age 60 or older, causing difficulty in daily life and their ability to return to work . Short-term POCD, up to 6 weeks postoperatively, occurs in 20-50% of patients undergoing cardiac surgery. Longer-term POCD, defined as a subtle deterioration in cognitive function 6 months following surgery, occurs in 10-30% of cardiac surgery patients . In a longitudinal study with 8.5 years of follow-up, Steinmetz et al. found POCD to be associated with higher mortality (Cox proportional hazard ratio 1.63, 95% confidence interval 1.11-2.38; p = 0.01), earlier retirement, and greater utilization of social financial assistance . Monk et al. found that the risk of death within one year was greater among patients who had POCD when they were discharged from the hospital .

CABG and valve surgery most commonly use the extracorporeal cardiopulmonary bypass machine to perfuse the body during cardiac surgery. The aorta is cross-clamped to allow work and suturing in a near bloodless field. The major blood branches to the brain come off the aorta. When the aorta is cross-clamped, the brain undergoes a period of ischemia. While the patient is on bypass, the body and brain are kept at low temperatures to quell the effects of ischemia. Temperature elevation has been proposed to have an all-or-none response with a defined threshold beyond which the increased temperature aggravates ischemic injury. Hyperthermia leads to physiological and structural changes in the brain which include alterations of enzyme activity and damage to cytoskeletal proteins found in the brain. In addition, hyperthermia causes a release of excitatory neurotransmitters, the production of free radicals, an increase in blood-brain barrier permeability, and increase in intracellular acidosis after ischemic reperfusion all of which have all been proposed as mechanisms through which hyperthermia leads to brain tissue injury and can exacerbate ischemic brain injury .

Intraoperative temperature during cardiac surgery has long been thought to have an association with postoperative neurological outcomes, and has been well studied. In mitigating neuroinflammation and subsequent brain injury, intraoperative hypothermia as compared with normothermia has been the subject of much debate, and outcomes have varied considerably in various studies. The process of rewarming patients as they gradually disengage from cardiopulmonary bypass has been noted as another important process in modulating neuroprotection, and has al-so been studied extensively. Nearly all studies have associated a slower rate of intraoperative rewarming with a decreased incidence of POCD.