Comparison of Mask Oxygen Therapy and High-Flow Oxygen Therapy after Cardiopulmonary Bypass in Obese Patients

Background To clarify the efficiency of mask O2 and high-flow O2 (HFO) treatments following cardiopulmonary bypass (CPB) in obese patients. Methods During follow-up, oxygenization parameters including arterial pressure of oxygen (PaO2), peripheral oxygen saturation (SpO2), and arterial partial pressure of carbon dioxide (PaCO2) and physical examination parameters including respiratory rate, heart rate, and arterial pressure were recorded respectively. Presence of atelectasia and dyspnea was noted. Also, comfort scores of patients were evaluated. Results Mean duration of hospital stay was 6.9 ± 1.1 days in the mask O2 group, whereas the duration was significantly shorter (6.5 ± 0.7 days) in the HFO group (p=0.034). The PaO2 values and SpO2 values were significantly higher, and PaCO2 values were significantly lower in patients who received HFO after 4th, 12th, 24th, 36th, and 48th hours. In postoperative course, HFO leads patients to achieve better postoperative FVC (p < 0.001). Also, dyspnea scores and comfort scores were significantly better in patients who received HFO in both postoperative day 1 and day 2 (p < 0.001, p < 0.001 and p=0.002, p=0.001, resp.). Conclusion Our study demonstrated that HFO following CPB in obese patients improved postoperative PaO2, SpO2, and PaCO2 values and decreased the atelectasis score, reintubation, and mortality rates when compared with mask O2.


Introduction
According to World Health Organization, obesity is dened as excessive and abnormal fat accumulation that creates risk for health, and a person with a body mass index (BMI) greater than or equal to 30 kg/m 2 is considered obese [1]. Incidence of sedentary lifestyle with high-fat diet intake and comorbidities including metabolic syndrome, diabetes mellitus, and hypertension are common among obese patients. Also, it is well known that obesity and obesityrelated disorders are risk factors for severe coronary artery diseases which may require cardiopulmonary bypass (CPB) [2]. Additionally, obese patients are more vulnerable to pulmonary complications following CPB due to decreased total lung capacity, functional residual capacity, forced vital capacity, and expiratory reserve volume.
Moreover, anesthetic agents and sedatives aggravate the respiratory instability in obese patients [3].
Previous studies had demonstrated that severe hypoxemia, hypercapnia, and prolonged apnea periods deteriorate healing after CPB. To avoid these undesirable conditions, some authors recommended oxygen therapy which had a key role in improving respiratory functions and patient comfort by decreasing desaturation episodes and reintubation rates after cardiac surgery [4,5]. In a prospective randomized study, Zhu et al. showed signi cant reduction of the reintubation rate after cardiac surgery with mask O 2 [5]. In contrast, Stéphan et al. reported insu ciency of mask O 2 therapy for oxygen treatment following cardiac surgerysupported high-ow O 2 (HFO) treatment [6]. erefore, the most appropriate method of oxygen therapy is still under investigation, especially in obese patients who are at high risk for respiratory problems following CPB.
Although previous studies investigated the role of mask O 2 and HFO treatments following CPB, none of these studies have compared these two di erent treatment modalities in patients with BMI more than 30 kg/m 2 . In this study, we, for the rst time, aimed to clarify the importance of mask O 2 and HFO treatments following CPB in obese patients.

Materials and Methods
In this prospective randomized study, charts of patients who underwent CABG in a tertiary academic center between January 2015 and January 2017 were analyzed. Ethical approval for the study was obtained from the Haseki Teaching and Research Hospital's Regional Ethical Committee, with study ID number 499. Patients with BMI > 30 kg/m 2 were enrolled in the study. Randomization was done by a computer-based random number-sequencing program. Exclusion criteria were hemodynamic instability, patients younger than 18 years of age, and patients with tracheostomy, obstructive sleep apnea, and active pulmonary disease. Also, patients with low cardiac output and operations who were held under emergency conditions were excluded from the study. Written consent was obtained from patients and/or relatives.

Study protocol.
In Cardiovascular Surgery Intensive Care Unit (ICU), every patient, who underwent CABG, was followed up by a well-trained ICU nurse and a cardiovascular surgeon in the postoperative period. Hemodynamically stable patients, who had a su cient oxygenation (SpO 2 (peripheral oxygen saturation) > 92, FiO 2 (fraction of inspired oxygen) ≤ 0.4, PEEP (positive end-expiratory pressure) ≤ 8 mmHg, and PaO 2 (arterial pressure of oxygen)/FiO 2 ≥ 150), were weaned by the spontaneous breathing trial (SBT) with low-level pressure support or oxygen T-piece for 90-120 minutes. Endotracheal extubation was performed in patients who have tolerated the SBT.
After endotracheal extubation, patients were randomly divided into two groups. In patients (n � 50) who received HFO treatment, high-ow humidi ed oxygen (44 mm/H 2 O/L and 37°C) was released through a nasal cannula continuously with Opti ow (Vapotherm, New Hampshire, USA). e preliminary ow rate was 25-40 L/min, and the initial FiO 2 was 50% to maintain SaO 2 > 93. In the other group (n � 50 patients) who underwent oxygen therapy, oxygen was delivered from a simple face mask (Orya Medikal, Istanbul, Turkey) with a ow of 2-4 liters in a minute to maintain SpO 2 > 93%. Active respiratory physiotherapy was performed for all patients during the postoperative period. e patients were encouraged for early mobilization.

Atelectasis Scoring.
Presence of atelectasis was evaluated by a postoperative chest X-ray and classi ed according to the radiological atelectasis score system (RAS) [7]. e RAS is divided into ve categories (0: clear lung elds, 1: plate-like atelectasis or slight in ltration, 2: partial atelectasis, 3: lobar atelectasis, and 4: bilateral atelectasis), and chest X-rays were evaluated by a single radiologist who was blinded to mask O 2 and HFO outcomes.
Discontinuation of mask O 2 or HFO therapy due to the side e ects, requirement of additional treatment, or necessity of reintubation for mechanical ventilation was accepted as failure of the current treatment. Reintubation criteria were cardiovascular instability, respiratory arrest or respiratory acidosis (pH < 7.30 and PaCO 2 ≥ 50 mmHg), encephalopathy, and clinical ndings of exhaustion and refractory hypoxemia (arterial oxygen saturation < 88% with FiO 2 � 100%). Reintubation was performed according to the physician's decision, if required. e length of ICU and hospital stay was recorded. Lastly, respiratory complications, extrapulmonary complications, and presence of mortality were documented.
During statistical analyses, values were evaluated as numbers, means, percentages, and intervals. Numbers and percentages were compared using the chi-square test. Before the comparison of means of values, the values were evaluated for homogeneity. Homogeneously distributed values were compared using Student's t-test, and heterogeneously distributed values were compared using the Mann-Whitney U test.

Results
During the study period, 137 patients underwent the CABG procedure, and 100 patients were enrolled in the study. Other 37 patients were excluded from the study according to study exclusion criteria ( Figure 1). ese patients were divided into two groups; 50 patients were treated with mask O 2 , and 50 patients were treated with HFO. e mean age was 61.3 ± 8.5 in patients who received mask O 2 and 62.0 ± 6.7 in patients who received HFO (p � 0.660). Sex, BMI, and history of smoking were similar between groups (p � 1.000, p � 0.259, and p � 0.842, resp.). e operative parameters in terms of duration of surgery, myocardial ischemia period, and extubation time were comparable between patients who received mask O 2 and HFO (p � 0.709, p � 0.740, and p � 0.529, resp.). Preoperative and operative parameters are summarized in Table 1.
Among the patients who have received mask O 2 , 11 patients required continuous positive airway pressure (CPAP) and four patients required reintubation due to the increased arterial carbon dioxide value and mental deterioration despite extensive pulmonary rehabilitation maneuvers. In patients who have received HFO, CPAP was required in six patients; however, none of these patients required reintubation (p � 0.187 and p < 0.001). e duration of ICU stay was 2.8 ± 1.7 days in patients with mask O 2 and 2.4 ± 0.5 days in patients with HFO (p � 0.130). e duration of hospital stay was 6.9 ± 1.1 days in the mask O 2 group, whereas it was signi cantly shorter (6.5 ± 0.7 days) in the HFO group (p � 0.034). Moreover, the atrial brillation rate and mortality rate were signi cantly lower in patients who were treated with HFO (p < 0.001 and p < 0.001, resp.). Death was observed in only two patients who have received mask O 2 ( Table 2). e respiratory parameters PaO 2 value and SpO 2 value were signi cantly higher and PaCO 2 value was signicantly lower in patients who have received HFO. e PaO 2 value was 104.3 ± 5.6, 96.2 ± 7.4, 96.6 ± 6.7, 97.1 ± 6.3, and 99.4 ± 7.1 after 4th, 12th, 24th, 36th, and 48th hours in patients with mask O 2 and 112.3 ± 8.8, 106.9 ± 7.5, 100.0 ± 4.5, 104.9 ± 5.9, and 106.0 ± 6.9 after 4th, 12th, 24th, 36th, and 48th hours in patients with HFO (p < 0.001, p < 0.001,

Discussion
Acute respiratory failure is still a challenging and lifethreatening complication in patients who underwent open heart surgery with CPB. During respiratory insu ciency, respiratory support is a critical step to maintain patient comfort, prevent invasive mechanical ventilation, and decrease mortality [8]. erefore, choosing the most appropriate device for oxygen therapy is a crucial decision to maintain adequate oxygenation especially in patients who are at high risk for acute respiratory failure such as patients with sleep apnea syndrome, advanced stage heart failure, and high BMI. Obese patients are prone to hypoxia development due to several risk factors. Obese patients have extensive adipose tissue with high metabolic activity that is associated with increased oxygen consumption and carbon dioxide production. Due to breathing di culties in obese cases, supportive muscles spend more oxygen and produce more carbon dioxide to overcome the workload. Additionally, compliance of respiratory organs and lung volumes are decreased in obese patients [9]. Due to the lower resting functional residual capacity, obese patients have an increased respiratory rate to compensate the ventilationperfusion mismatch especially at the base of the lungs. Furthermore, postoperative atelectasis is common and more prominent in obese patients when compared with nonobese patients [10].
us, the best treatment modality to overcome postoperative oxygenation problems in obese patients is a critical problem and still under investigation. e simple face mask and HFO nasal cannula are used for oxygen delivery in patients with hypoxia and/or hypercarbia. However, there is a trend in use of HFO especially in high-risk patients. Costello et al. stated that delivering oxygen through a face mask leads to dryness of the mouth and respiratory tract, and this situation leads to displacement of the face mask. Also, displacement of the mask is associated with the decrease in oxygen concentration [11]. On the other hand, HFO generates a dead space and creates a reservoir for oxygen in the respiratory system. In patients with tachypnea, delivered O 2 may decrease due to the increased respiratory frequency, and HFO overcomes this by adequate and stable oxygenation. Moreover, HFO with positive pressure prevents the formation of atelectasis [12].
Previous reports on the e ciency of mask O 2 and HFO in the management of acute respiratory failure had controversial results. Lemiale et al. investigated the e ect of HFO in immunocompromised patients and found that HFO neither decreased the need for mechanical ventilation nor improved patients' quality of life [13]. In contrast, Schwabbauer et al. found signi cantly better dyspnea and patient comfort scores with HFO [12]. In another study, Maggiore et al. compared HFO and mask O 2 in patients who were mechanically ventilated for more than 24 hours. ey stated that high-ow O 2 provided better oxygenation, fewer desaturation episodes, and lower reintubation rates [14]. Rittayamai et al. suggested HFO to overcome dyspnea and improve physiologic parameters [15]. In the present study, we achieved better PaO 2 , SpO 2 , and PCO 2 levels in the rst 48 hours with HFO in our obese patient cohort.
Pulmonary atelectasis is a common and undesired condition which is associated with oxygen impairment, decrease in lung compliance, increase in vascular resistance, and infectious complications [16]. To achieve more accurate classi cation of atelectasis and to evaluate treatment response, the Radiological Atelectasis Score was developed and validated. Parke et al. evaluated the e ect of mask O 2 and HFO by using Radiological Atelectasis Score, in 340 patients who underwent heart surgery. ey claimed that prophylactic HFO improved the atelectasis scores [7]. Zarbock et al. analyzed 500 patients who underwent elective cardiac surgery and were supported by HFO to prevent atelectasis in the postoperative period [17]. In accordance with the literature, we have obtained better atelectasis scores in patients  [19]. Similarly, Calderini found mask discomfort as a reason for the cessation of treatment [20]. In our study, patients who received HFO had signi cantly better dyspnea and comfort scores. We analyzed only a speci c group with BMI > 30 kg/m 2 , and this is emphasized to be the explanation of better dyspnea and comfort scores with HFO. Obese patients spend more e ort on breathing and are more vulnerable to hypoxia. us, adequate oxygenation is mandatory to improve symptoms. Moreover, the treatment failure rate was higher with mask O 2 and may have contributed to lower dyspnea and comfort scores in our patients.
Although the current study is a prospective randomized study in which we investigated the e ciency of HFO and compared HFO with conventional mask O 2 in the postoperative period of obese patients who underwent open heart surgery, the research has certain limitations. e sample size in both groups in the study is modest. e cost of mask O 2 and HFO treatments in obese patients was not investigated. Lastly, we have only evaluated the short-term results of both treatment modalities in statistically similar groups, and further researches with long-term follow-up outcomes may be helpful to strongly support the superiority of the technique to another.
In conclusion, we demonstrated that HFO therapy following CPB in obese patients improved postoperative PaO 2 , SpO 2 , and PaCO 2 values and decreased atelectasis scores, reintubation, and mortality rates when compared

Conflicts of Interest
e authors declare that they have no con icts of interest.