DALLAS — Mild hypothermia reduced the incidence of postoperative cognitive deficits in patients undergoing coronary bypass surgery, according to results from two studies with 448 patients.
“Mild hypothermia is a new potential strategy to prevent postoperative cognitive deficits in patients undergoing coronary artery bypass surgery,” Dr. Munir Boodhwani said at the annual scientific sessions of the American Heart Association.
Other factors were associated with an increased risk of postoperative cognitive deficits (POCD), but they are not easily modified. They were prolonged stay in the intensive care unit following surgery, abnormal left ventricular function, a high serum level of serum creatinine preoperatively, and a higher education level. Although no specific interventions can help a patient who needs coronary bypass surgery but has impaired ventricular or renal function (the second causes a high level of serum creatinine), patients with several of these high-risk markers might benefit from special surgical precautions such as off-pump surgery or a “no-touch” technique, said Dr. Boodhwani, a cardiothoracic surgeon at Beth Israel Deaconess Medical Center in Boston.
The analysis combined data from two studies at the University of Ottawa Heart Institute that were designed to assess the efficacy of mild hypothermia. One study, which was done during 1995–1998, randomized 223 patients and tested the efficacy of treating patients at either 34° C or 37° C during the rewarming phase of coronary bypass surgery. The second study, which was done during 2001–2004, compared surgery at 34° C or 37° C during the entire operative period in 263 patients. The results were pooled for all 486 patients; postoperative neurocognitive assessments were not done on 38 patients, leaving 448 for this analysis.
The neurocognitive analyses used a battery of 11 tests and categorized deficits into three domains: memory and learning, attention, and psychomotor speed and dexterity. The tests were administered an average of 15 days before surgery and again postoperatively but prior to hospital discharge, an average of 7.7 days after surgery. A patient was considered to have a deficit if one or more of the patient's scores dropped by at least 0.5 standard deviations, compared with preoperative levels.
The overall incidence of POCD was 59%. Of the 265 patients with POCD, 61% had a deficit in a single domain, 30% had a deficit in two domains, and 9% had deficits in all three domains.
In a multivariate analysis that controlled for baseline differences in clinical and demographic measures, normothermia during surgery was associated with a statistically significant 15% increased risk of POCD, compared with patients who were treated with hypothermia.
Other statistically significant determinants of POCD in the multivariate model included stay in the intensive care unit following surgery for more than 24 hours, a marker of more severe underlying disease, which was linked with an 88% increased rate of POCD. Abnormal left ventricular function prior to surgery was linked with a 53% increased risk of POCD. For every 1 mmol/L increase in serum creatinine level prior to surgery, the risk of POCD rose by 1%. And patients with an education level that was above the 12th grade were 52% more likely to have POCD.
The impact of elevated serum creatinine on the rate of POCD was not uniform. At preoperative levels up to 125 mmol/L (equal to 140 mg/dL), the risk of POCD rose linearly as the level of serum creatinine increased. But above 125 mmol/L the risk of POCD increased exponentially, Dr. Boodhwani said.