Logic dictates that assuring oxygen perfusion of organs and tissues during open heart surgery benefits the patient. Therefore, inducing some degree of hyperoxia during such surgeries has entered the realm of best practice.
Recently, concerns about organ injury from oxidative stress caused by the higher oxygen levels have prompted some anesthesiologists to revert to maintaining normal oxygen levels during surgery.
At Vanderbilt University Medical Center, cardiac anesthesiologist Frederic T. (Josh) Billings, IV, M.D., M.Sc., launched a study to measure oxidative stress in patients under hyperoxic conditions and those maintained on normal oxygen levels and compare their surgical outcomes, both post-operatively and after one year.
With hyperoxia known to decrease risk of regional hypoxia and insufficient perioperative perfusion, Billings was interested in whether it came with any detriments – such as risk of acute kidney injury, delirium, myocardial injury, atrial fibrillation, or other unwanted outcomes.
“While we found evidence that hyperoxia during surgery increases oxidative stress, oxygen treatment did not make a significant difference in organ injury,” he concluded.
The question of which approach is superior has been examined in some past studies, but more broadly in Billings’ study.
“Our study is the first to compare normoxia with high levels of hyperoxia maintained throughout the intraoperative period,” Billings said. “We did this to maximize our ability to identify the effects hyperoxia might have.”
“We acknowledged that kidney and other organ injuries are common in patients having cardiac surgery, that prior work demonstrated a relationship between increased oxidative stress and organ injury, and that oxygen administration impacts oxidative stress,” Billings said. “We wanted to determine if providing normal oxygen concentrations during surgery, such as those as low as air, would decrease oxidative stress and organ injury in comparison to oxygen concentrations as high as 100 percent.”
Equivalent Outcomes
To test the hypothesis, Billings prospectively randomized 201 adult Vanderbilt patients having elective open-heart surgery to either normoxia levels of oxygen saturation at between 95 and 97 percent or to hyperoxia conditions. All were blinded except for the anesthesiologist and perfusionist.
The researchers assessed oxidative stress through biomarkers and measured acute kidney injury, delirium, myocardial injury, atrial fibrillation and additional secondary outcomes.
The primary clinical endpoint was change in serum creatinine concentration from baseline through postoperative day two, and it was found to be low in both groups. During surgery, markers of concentrated oxidative stress were higher in the hyperoxia group.
“While we found evidence that hyperoxia during surgery increases oxidative stress, oxygen treatment did not make a significant difference in organ injury.”
These effects, however, did not extend into the postoperative period. When treatment had been completed patients were administered similar concentrations of oxygen.
The researchers looked at acute kidney injury, neuronal injury, delirium, atrial fibrillation, myocardial infarction and injury, stroke, transient ischemic attack, pneumonia, surgical site infection, death, mechanical ventilation, and time to first ICU discharge. They found that neither postoperative rate of organ injury or other deleterious side effects were significantly affected by intraoperative oxygen treatment.
Of particular note is the fact that the number of patients developing acute kidney injury was identical in the two groups – 21 in each. Subgroup analyses by age, BMI, diabetic status, and smoking status showed no significant differences.
A year later, data gathered on kidney function, cognitive function and activities of daily living – both at day two and after one year – also demonstrated no significant discrepancies.
Outlier Findings
There was some evidence that normoxia might reduce postoperative atrial fibrillation rates and delirium, though these differences had resolved by the time of follow-up.
The researchers did not see differences in cognitive test results, despite reduced oxygen levels in the brain under normoxia. Various demographics and diabetic status did not modify the effects of treatment on the clinical outcomes.
“Our conclusion was that intraoperative hyperoxia compared to normoxia increased oxidative stress during surgery but did not affect kidney injury or other postoperative measurements of morbidity,” Billings said.
Clinical Decisions
While maintaining normoxia by blending oxygen and air is more labor-intensive, Billings has opted to maintain this level of oxygen in his practice based on these results and the strong evidence that oxidative stress increases organ injury.
“In training, I was taught to give high concentrations of oxygen during surgery, and in particular cardiac surgery,” he said. “If you can minimize oxidative stress with a similar safety profile, this option makes sense to me.”
