High flow nasal oxygen therapy in patients after cardiac surgery

Abstract

Background: Following cardiac surgery, patients are admitted routinely to the Intensive Care Unit (ICU). They are mechanically ventilated until ready for extubation following which oxygen therapy is administered until adequate oxygenation is maintained. There are many devices available to the clinician by which oxygen may be administered. One therapy, nasal high flow oxygen therapy, has been shown to improve oxygenation and be better tolerated when compared to other methods. However, there is little evidence to describe the mechanisms of action of nasal high flow or to demonstrate efficacy. Aims: To investigate the effect of nasal high flow oxygen therapy on patients after cardiac surgery. Methods 1. Review of the literature – Pubmed, Medline and CINAHL were searched for all descriptions and reports of nasal high flow oxygen therapy use in patients. This included case reports, observational studies and randomised controlled trials. Nasopharyngeal pressure measurements were performed using nasal high flow oxygen therapy and continuous positive airway pressure (CPAP) delivered via a sealed facemask. Pressures generated over the whole of the respiratory cycle were determined. 2. An evaluation study assessed anonymised chest x-rays of 50 consecutive patients which were scored by a blinded radiologist using both an existing and a modified scoring system. Chest x-ray (CXR) scores were also compared with oxygenation indices at the time of CXR. Scores were assessed for their ability to predict day 3 oxygenation indices. 3. A prospective randomised controlled trial was undertaken in adult patients undergoing cardiac surgery. Participants were assigned to receive either nasal high flow or standard oxygen therapy from the time of extubation through until 0900 hours day 2 postoperative. The primary outcome was number of patients with a ratio of peripheral oxygen saturation to fraction of inspired oxygen (SpO2/FiO2) > 445 on post-operative day 3. 4. A multi-centric, point prevalence study was undertaken to describe the current practices with regards oxygen therapy in non-intubated patients in 40 ICUs in Australia and New Zealand. Results There was no evidence available to guide clinicians providing care to patients following cardiac surgery as to whether or not the use of routine nasal high flow oxygen therapy would lead to improved outcomes and reduced incidence of pulmonary complications. Mean (SD) nasopharyngeal airway pressures of 1.4 (0.6), 2.2 (0.8) and 3.0 (1.0) were recorded at 30, 40 and 50 L/min using nasal high flow. Analyses also determined the mean peak expiratory and mean expiratory plateau pressures. Expiratory pressures during NHF were significantly higher than the mean pressures previously reported for this therapy. When evaluating two x-ray scoring systems it was found that the modified score demonstrated better ability to detect atelectasis on chest x-ray and better specificity than the existing score when comparing the CXR findings with the clinical oxygenation status of the patients. This modified scoring method performed better as an outcome measure for atelectasis in studies of patients following cardiac surgery. The routine use of NHF following cardiac surgery was not associated with an increase in SpO2/FiO2 ratio on day 3 postoperative but it may be associated with a reduction in escalation of respiratory therapy and a significantly lower partial pressure of carbon dioxide (PaCO2). It was found that oxygen was administered to 86% of non-intubated adult patients in 40 ICUs surveyed. The most common method of oxygen delivery on the day was simple nasal cannulae. Only 24.4% of patients had a documented prescription for oxygen therapy, of which only 7% would be considered complete and comprehensive. Conclusion Nasal high flow oxygen therapy delivers positive airway pressure across the whole of the respiratory cycle but is not associated with an increase in SpO2/FiO2 ratio on day 3 following cardiac surgery when compared to routine care.