Even if a patient undergoes non-cardiac surgery, there is still a risk of side effects for the cardiovascular system. Now, researchers have found statistically significant evidence that having severe, unrecognized obstructive sleep apnea (OSA) may increase this risk even further.
OSA causes a person’s breathing to stop and start repeatedly during sleep and is thought to affect around 14% of adult men and 5% of adult women in the US. The condition is a known risk factor for poor long-term cardiovascular health because sudden drops in blood oxygen levels ramp up blood pressure and strain the cardiovascular system.
Surgical anaesthetics and postoperative analgesics have potent effects on the respiratory system, which in turn exacerbates OSA.
In an accompanying editorial, the authors state that the majority of people with OSA remain undiagnosed and the potential for increasing postoperative risk isn’t well understood. Previous work examining this risk has had mixed results.
“Uncertainty remains whether unrecognized OSA adversely affects postoperative outcomes,” say the authors.
The team conducted a prospective cohort study of 1,218 patients aged 45 or older, without a prior OSA diagnosis, and scheduled for non-cardiac surgery. The study’s primary outcome included conditions such as myocardial injury (a heart attack), cardiac death, heart failure, thromboembolism, atrial fibrillation and stroke.
All patients had at least one risk factor for a postoperative cardiac event and underwent an overnight sleep study to check for OSA, either at home within 30 days of the surgery (34.1%) or at the hospital the night before (65.9%).
For the first three nights after surgery, all patients underwent overnight oximetry monitoring, a test which allows doctors to see if you have enough oxygen in your blood as you sleep. A higher risk of experiencing complications was associated with a longer duration of postoperative oxygen desaturation (lower blood oxygen levels) below 80%.
This level of oxygen desaturation was an important risk factor, with patients experiencing cardiac complications dropping below it for an average of 23.1 minutes. By contrast, patients who did not experience complications only fell below 80% oxygen desaturation for an average of 10.2 minutes.
Out of the total cohort, 19.3% experienced complications within a month after surgery. Most striking, however, was the gap between those with no OSA and those with severe OSA: 14.2% of patients without OSA suffered complications, while this shot up to 30.1% for those with severe OSA.
Severe OSA was also linked to infective outcomes, unplanned tracheal intubation, and readmission to the ICU.
Babak Mokhlesi from the University of Chicago, who was not involved in the study, praised its quality but noted that solutions are not as clear as they seem.
“Most reasonable people would say we should give them oxygen for their obstructive sleep apnea, because that would seem to make sense and solve the problem. But the fact is that oxygen can worsen people’s outcomes if they don’t need it,” he said in an interview with TCTMD.
Mokhlesi pointed to a recent study in which patients, some of whom had surgery recently, experienced adverse outcomes in the wake of liberal oxygen therapy.
The authors believe the next step is to conduct randomized trials to determine what effect treatments like oxygen therapy, continuous positive airway pressure (CPAP, which involves a machine that increases pressure in the throat so the airway remains clear while you are asleep), or more intensive monitoring would have on cardiovascular complications.
Mokhlesi agreed with this idea, emphasizing that it remains unknown whether any of these potential solutions are actually dangerous or not: “We can’t just go with the notion that there’s no way that CPAP or a little bit of oxygen is bad for you, because we don’t know,” he adds.
OSA is a difficult disease to study because the population tends to have other comorbidities (such as obesity and hypertension) that make it hard to isolate the impact of OSA on complications. Whatever the next step may be, the consensus is that OSA needs recognition as a postoperative risk.
“[This study] should raise awareness about the association between unrecognized OSA in the presurgical population and adverse postoperative cardiovascular outcomes,” say the authors.
“The study results provide further evidence to support preoperative screening for OSA, yet they also raise many important questions. Among these is how best to manage the care of patients identified as being at high risk for OSA or who have a preoperative diagnosis made by objective sleep testing.”
