Patients with obstructive sleep apnea (OSA) who are administered oxygen therapy postoperatively may develop elevated levels of carbon dioxide (CO2), according to researchers.
The study, which used transcutaneous CO2 to measure CO2, found that 11.3% of study patients (14/123) had elevated CO2 in their blood, exceeding 55 mm Hg, “indicating a degree of respiratory depression in postoperative nights, mostly in the first post-op night,” said senior author Frances Chung, MBBS, FRCPC, professor of anesthesiology at University Health Network, University of Toronto.
“When the health care team gives oxygen supplementation to patients in wards after discharge from the PACU, we may not think about CO2 levels,” Dr. Chung said. “Most of us monitor oxygen saturation by oximetry and do not monitor respiratory rate and CO2 on a regular basis.”
Dr. Chung said this is the first study to evaluate the effect of oxygen on untreated OSA patients during the first three postoperative nights.
“Surgical patients may have OSA, which is associated with higher risks of complications,” Dr. Chung said. “If we give oxygen therapy to these patients with untreated OSA postoperatively, how does that impact oxygen saturation, respiratory adverse events and the CO2 level when they are asleep?”
The patients in the study had an apnea–hypopnea index (AHI) of greater than five events per hour on preoperative polysomnography. Patients were then equally randomly assigned to the oxygensupplementation group (n=62) or the no-oxygen group (control group, n=61). Findings were published in the journal Chest (2017;151:597-611).
The oxygen group received oxygen at a rate of 3 L per minute with nasal prongs for three consecutive post-op nights. A total of 13 patients in the oxygen group and one patient in the control group had elevated CO2 that was more than 55 mm Hg at certain times during post-op nights 1, 2 and 3. One patient had episodes of CO2 at 96 mm Hg, and six patients had episodes of CO2 greater than 60 mm Hg.
“Higher inspired oxygen may result in greater opioid-induced respiratory depression. Our findings raise concerns about whether we should be monitoring the respiratory rate and the end-tidal or transcutaneous CO2regularly, in addition to oxygen saturation,” Dr. Chung said. “If you give oxygen to a patient with OSA, especially if they are on opioids, the supplemental oxygen could lead to a longer apnea event, or it might cause hypercapnia and sustained respiratory depression or hypoventilation.”
Similarly, by giving oxygen, “patients may not desaturate quickly,” Dr. Chung said. “Oxygen masks the ability of pulse oximetry to detect the abnormalities in the level of ventilation. The patient seems to be doing well, but may not be breathing. As a result, the CO2 rises and the respiratory rates decrease. In fact, the CO2 can rise to such a high level that the patient becomes comatose.”
The authors noted that the effect of supplemental oxygen on sleep respiratory events has been inconsistent in previous studies of nonsurgical patients with OSA. At least one study has demonstrated that breathing oxygen reduced the frequency of apnea, whereas other studies found that the duration of apnea was increased, or that AHI was not reduced by breathing oxygen.
Another study concluded that nocturnal oxygen therapy decreased hypopnea and central apnea in peritoneal dialysis patients with OSA. Moreover, one study found that patients with eucapnia benefited from oxygen by reducing central sleep apnea, but oxygen did not affect obstructive and mixed apneas.
Use Capnography Postoperatively?
In the current study, the decrease in AHI was mostly attributed to a drop in the hypopnea index and, to a lesser degree, central apnea. However, because the hypopnea index was not separated into central or obstructive hypopnea, the role played by each type of hypopnea could not be determined.
A decrease in hypopnea events also may be ascribed to an improvement in oxygenation by supplemental oxygen, thus having fewer events meet the hypopnea criterion.
Although some centers use capnography to measure respiratory rate and end-tidal CO2, “the technology is not perfect yet. There are a fair amount of false alarms,” Dr. Chung said. “That is why people are reluctant to use capnography postoperatively.”
Dr. Chung and her colleagues recommended larger follow-up studies and studies to define which OSA phenotypes would benefit from post-op supplemental oxygen, and to identify which patients should be monitored for respiratory depression by respiratory rate or capnography.
Meanwhile, for OSA patients, “we need to be aware that a certain number of surgical patients may have respiratory depression secondary to opioids, and therefore we should look into further monitoring of hypoventilation besides oximetry,” Dr. Chung said.