Abstract
Hypoxemia is common in postoperative patients and is associated with prolonged hospital stays, high costs, and increased mortality. This review discusses the postoperative management of hypoxemia in regard to the use of conventional oxygen therapy, high-flow nasal cannula oxygen therapy, CPAP, and noninvasive ventilation. The recommendations made are based on the currently available evidence.
- postoperative hypoxemia
- oxygen therapy
- high-flow nasal cannula
- continuous positive airway pressure
- noninvasive ventilation
- incentive spirometry
Introduction
Hypoxemia is common in postoperative patients and mainly caused by atelectasis, ventilation/perfusion mismatch, or pulmonary edema. Postoperative hypoxemia is associated with increased mortality, prolonged hospital stays, and increased costs, especially in patients who have multiple risk factors.1-4 Patients at risk often face prolonged respiratory support and re-intubation, which leads to poor overall outcomes. There is current evidence that early identification of risk factors of postoperative hypoxemia is imperative for the prevention or treatment of the condition. The purpose of this paper is to discuss pertinent findings from recent publications that pertain to postoperative management of hypoxemia. The recommendations are made based on the current evidence (Fig. 1).
Algorithm for respiratory support in postoperative patients. SBT = spontaneous breathing trial; NIV = noninvasive ventilation; HFNC = high-flow nasal cannula.
Incidence and Outcome of Postoperative Hypoxemia
The incidence of postoperative hypoxemia ranges from 3% to 65%, depending on definitions, the presence of risk factors, and type of surgery.2,4-6 Postoperative hypoxemia was variably defined in the publications included in this review. Definitions were a peripheral capillary oxygen saturation () value of <93% on room air, or the ratio of the
/
of < 300 mm Hg.7 Severe postoperative hypoxemia was defined as the need for an
of 1.0 to maintain
≥ 85%.8 However, recorded
values might underestimate the severity of postoperative hypoxemia if done manually and periodically. Sun et al9 used the
data that were recorded by a monitor at 1-min intervals in 833 postoperative adult subjects. More than one-fifth of the subjects in their study were found to have a
< 90% for >10 min/h.9 The inconsistent use of definitions in research papers and variable practices that relate to pulse oximetry complicates the understanding of the incidence of postoperative hypoxemia.
Postoperative hypoxemia has been reported to compromise wound healing and cause other severe complications, such as brain dysfunction, dysrhythmias, and myocardial ischemia. These complications are particularly noted within the first week after surgery. Of the 1,202 subjects with abdominal, orthopedic, and neurologic procedures reported by Fernandez-Bustamante et al,10 19.6% required prolonged oxygen therapy, whereas 17.1% developed atelectasis. These subjects also had significantly more ICU admissions, a longer ICU and/or hospital stay, and higher early mortality. Moderate and severe postoperative hypoxemia within the first 3 postoperative days has also been shown to be independently associated with increased postoperative mortality at 1 year.4 Therefore, prevention and management of postoperative hypoxemia is necessary to improve patient outcomes.
Etiologies and Risk Factors of Postoperative Hypoxemia
The etiologies of postoperative hypoxemia include reduced chest wall and diaphragmatic activity caused by surgical-site pain, hemodynamic impairment, and anesthetic drugs. These factors may lead to ventilation/perfusion mismatch and alveolar hypoventilation.11 Risk factors for postoperative hypoxemia are generally categorized as patient related or surgery related (Fig. 2). Understanding and identifying these risk factors can aid in the selection of appropriate respiratory care interventions.
Risk factors of postoperative hypoxemia. BMI = body mass index; OSA = obstructive sleep apnea; ASA = American Society of Anesthesiology; CABG = coronary artery bypass grafting; aTAAD = acute Stanford A aortic dissection.
Patients who are morbidly obese, with a body mass index (BMI) > 30 kg/m2, experience more frequent oxygen desaturation episodes after surgery compared with patients with normal weight.12 Patients with obstructive sleep apnea are at high risk of developing postoperative hypoxemia due to hypoventilation.13 These subjects with a preoperative apnea-hypopnea index of ≥ 15 were reported to be independently associated with postoperative hypoxima.14 Other patient-related risk factors include being elderly (generally > 65 years), an American Society of Anesthesiology physical status classification of IV, a preoperative dependent-living status, preoperative sepsis, moderate-to-severe cardiorespiratory disease, ineffective cough or poor lung function, and history of heavy smoking (>10 pack-years).15
In general, high-risk surgeries that are associated with the development of postoperative hypoxemia include brain, aortic, cardiac, thoracic, and upper abdominal surgery.2 In addition to the surgical procedures themselves, other risk factors contribute to the development of hypoxemia. Preoperative acute myocardial infarction and elevated free fatty acid concentrations are risk factors of postoperative hypoxemia after a coronary artery bypass grafting procedure.16-19 In the subjects who underwent an acute Stanford A aortic dissection surgery, preoperative /
≤ 300 mm Hg, long cardiac arrest time, and massive blood transfusion (after surgery, >6 units in 24 h or >3,000 mL) were predictors of postoperative hypoxemia.20,21 In the subjects after surgical aortic valve replacement, age, COPD, congestive heart failure, and bleeding disorders were associated with 30-d re-intubation.22 Similarly, in subjects after thoracic surgery, American Society of Anesthesiology physical status classification of ≥ III, surgery duration > 80 min, fluid balance during operation > 4,000 mL were found to be independent risk factors of postoperative hypoxemia.23-28 During abdominal surgery, including hepatectomy, upper or upper and lower (vs lower) abdominal incision, multiple procedures (vs one), crystalloid replacement > 6 L, and total surgery duration > 5 h were found to be risk factors.29-31
Efforts to predict the likelihood of postoperative pulmonary complications have been made. Canet et al32 developed a scoring tool, the ARISCAT (assessed respiratory risk in surgical patients in Catalonia) score, to predict postoperative pulmonary complications based on their surgical cohort. Components of this score include age, preoperative and hemoglobin, previous respiratory infection within 1 month of surgery, surgical incision (abdominal or intrathoracic), surgery duration, and planned versus emergency status of the surgery; the risk level was considered as moderate with an ARISCAT score ≥ 26.32 This scoring tool has been used in several randomized controlled trials (RCTs) that sought to better understand how to prevent or manage postoperative hypoxemia. To the best of our knowledge, a scoring tool specific for the prediction of postoperative hypoxemia has not yet been published.
Prophylactic Versus Curative Use of Respiratory Support for Postoperative Patients
Oxygen therapy has been recommended in the perioperative period to reduce surgical-site infections by the World Health Organization.33 In postoperative patients, standard O2 therapy, such as low-flow nasal cannula, a simple face mask, or an air-entrainment mask is routinely applied after extubation. In recent years, high-flow nasal cannula (HFNC) oxygen therapy has been increasingly used for postoperative patients after extubation. HFNC oxygen therapy provides a constant and generates some degree of PEEP and has been shown to improve oxygenation for patients who are hypoxemic.34 The PEEP generated by HFNC is variable and depends on gas-flow settings, nasal cannula size, the patient’s breathing pattern, and whether the mouth is open or closed.35 In contrast, CPAP and noninvasive ventilation (NIV) provide constant positive airway pressure, which recruits alveoli or maintains alveolar recruitment. By providing 2 levels of positive pressure, NIV augments tidal volume and reduces the work of breathing. Importantly, both CPAP and NIV can be used for patients who tend to mouth breathe because they can be connected with an oronasal mask, total face mask, or helmet. An important consideration with regard to respiratory support devices is timing. Respiratory support can be given prophylactically as a way to prevent extubation failure or curatively when signs of respiratory compromise are apparent. In addition, respiratory support can be provided as a way to facilitate extubation in patients at high risk, such as those with COPD.
Prophylactic Use of HFNC Versus Standard O2 Therapy Versus NIV or CPAP
The aim of the prophylactic use of respiratory support is to reduce pulmonary complications, prevent respiratory failure, and avoid re-intubation.36 Ten RCTs assessed the effects of HFNC and standard O2 therapy in preventing respiratory failure and re-intubation in the immediate postoperative period (Table 1).37-46 Of the 10 RCTs, 5 were conducted in subjects after cardiac surgery,37-41 and 4 were conducted in subjects after thoracic surgery.42-45 Only one trial was completed in subjects after major thoracic and abdominal surgery.46 In the most recent systematic review and meta-analysis, which included these 10 RCTs, HFNC was associated with significant reductions in re-intubation and escalation of respiratory support when compared with standard O2 therapy.7 These effects were noted in cardiothoracic subjects, and a post hoc subgroup analysis suggested that the subjects with high-risk factors such as BMI ≥ 30 kg/m2, ARISCAT score ≥ 26, or chronic pulmonary disease benefited the most from HFNC.7 There were no significant effects on other important clinical outcomes, such as mortality, ICU length of stay, and hospital length of stay. The prophylactic use of HFNC was recommended in patients with high-risk factors after cardiothoracic surgery (Fig. 1). When considering that the only RCT of the subjects who underwent abdominal surgery did not find any significant differences between HFNC and standard O2 therapy;46 currently, no recommendation is made for patients after abdominal surgery.
Characteristics of Randomized Control Trials With Postextubation Preventive Use: HFNC vs Standard O2 Therapy
Similarly, multiple RCTs compared the prophylactic use of NIV or CPAP versus standard O2 therapy for postoperative subjects (Table 2), but all were completed before 2009 and most were the comparison of CPAP versus standard O2 therapy.47-66 Of the 20 RCTs, only 5 reported re-intubation rates,56,59,60,64,66 in which one compared CPAP and standard O2 therapy.56 In the CPAP versus standard O2 therapy study, no significant differences in re-intubation rates were found. In the remaining 4 studies,59,60,64,66 2 studies compared the continuous versus intermittent use of CPAP and found lower re-intubation rates with continuous CPAP;59,60 however, the clinical implication of this finding was questionable due to the concerns of patient comfort and complications of continuous CPAP, for example, skin breakdown. In addition, the low incidence of re-intubation rates in those subjects is also of concern. In the study reported by Zarbock et al,60 the re-intubation rates were reduced from 2.5% to 1.3% by using continuous CPAP, which might not be clinically meaningful.
RCTs Comparing the Prophylactic Use of NIV or CPAP vs Standard O2 Therapy Immediately Postextubation for Postoperative Subjects
Stéphan et al67 conducted a unique RCT to compare HFNC with NIV for subjects after cardiothoracic surgery, who were divided into 3 groups: 1) subjects who passed a spontaneous breathing trial (SBT) and had one of 3 high-risk factors (BMI > 30 kg/m2, left-ventricular ejection fraction of <40%, or failure of previous extubation); 2) subjects who passed an SBT but developed hypoxemic respiratory failure after extubation; 3) subjects in whom an SBT failed but were still extubated. Randomization was stratified based on the 3 groups. Interestingly, even though the outcomes between the 2 overall groups were not significantly different, the subgroup analysis on the prophylactic use of HFNC versus NIV for the subjects with high-risk factors showed lower rates of treatment failure in the HFNC group (5.7% vs 12.6%; P =.04).67 This result might be explained by the better compliance and longer use of HFNC than NIV due to patient comfort and convenience with the 2 devices. This is the only RCT that compared the prophylactic use of HFNC with NIV for subjects after surgery. Future RCTs with larger sample size are needed to confirm this finding.
Curative Use of NIV or CPAP Versus Standard O2 Therapy Versus HFNC
In a recently published European Society of Anaesthesiology and European Society of Intensive Care Medicine guideline,68 compared with standard O2 therapy, NIV or CPAP is recommended to treat patients with perioperative or periprocedural hypoxemia to improve oxygenation. So far, 8 RCTs compared the use of NIV or CPAP and standard O2 therapy in subjects after surgery who had already developed hypoxemic respiratory failure (Table 3).69-76 Of the 8 studies, 5 were completed in subjects after cardiothoracic surgery70-72,74,76 and 3 were completed in subjects who underwent abdominal surgery.69,73,75 NIV was used in 6 RCTs and significantly reduced the re-intubation rates in the subjects after cardiothoracic or abdominal surgery.69-74 Interestingly, Yang et al74 compared the use of NIV with a helmet versus an oronasal mask versus standard O2 therapy in their subjects after Stanford type-A aortic dissection; only NIV with the helmet was found to significantly reduce re-intubation rates compared to standard O2 therapy. Conversely, no significant difference was found between the groups that used NIV with oronasal mask and standard O2 therapy. The superiority of the helmet over the oronasal mask agrees with the findings by Patel et al77 that the lower intubation rate was found in the group of subjects with acute hypoxemia and receiving NIV via the helmet than those receiving NIV via the oronasal mask. However, its benefits in postoperative patients with hypoxemic respiratory failure still need future studies with larger sample sizes to confirm.
RCTs Compared the Curative Use of NIV or CPAP and Standard O2 Therapy in Postoperative Subjects Who Developed Hypoxemic ARF
Only 2 RCTs compared CPAP with standard O2 therapy.75,76 No significant differences were found in the re-intubation rates of subjects after cardiac surgery, with a /
of 100–250 mm Hg.76 In contrast, Squadrone et al75 found a lower re-intubation rate with CPAP compared with standard O2 therapy in 209 subjects who developed hypoxemia within 1 h after abdominal surgery. Using CPAP in patients who have undergone gastrointestinal surgery should be done cautiously due to the concerns of anastomotic leakage caused by gas aspiration. The findings from the 2 RCTs might suggest that CPAP is most effective as a preventive strategy rather than a therapeutic modality.75,76 It seems that, for patients who develop hypoxemic respiratory failure after cardiothoracic or abdominal surgery, NIV reduces re-intubation rates compared with standard O2 therapy (Fig. 1) and NIV with the helmet might be more beneficial than an oronasal mask. The early use of CPAP might be helpful but only in patients after abdominal surgery.
To our knowledge, no study has been done to compare the use of HFNC and standard O2 therapy to treat postoperative patients with hypoxemic respiratory failure. As reported in the aforementioned section, one of the 3 subgroups in the RCT by Stéphan et al67 compared the curative use of HFNC and NIV for subjects after cardiothoracic surgery in whom extubation failed, no significant differences of treatment failure rates were found between HFNC and NIV (27.4% vs 27.8%; P = .93). This suggests that HFNC might be considered as an alternative to NIV to treat hypoxemic respiratory failure in patients who undergo cardiothoracic surgery, especially for those who do not tolerate NIV.
Facilitative Extubation With NIV Versus HFNC
Patients in whom traditional weaning attempts fail usually continue invasive ventilation until passing an SBT. However, the risks of continuing invasive ventilation are significant for some patients, such as those that are immunocompromised.68 Thus, early extubation for those patients might play an important role in their outcomes.36 The European Respiratory Society and American Thoracic Society guideline78 suggests using NIV to facilitate weaning from invasive ventilation in patients with hypercapnic respiratory failure, whereas no recommendation was provided for patients who are hypoxemic. Recently, Vaschetto et al79 conducted an RCT in a group of highly selected subjects who were hypoxemic and found that early extubation followed by immediate NIV application reduced the days on invasive ventilation without affecting the length of ICU stay.
Similarly, in a historical comparison study implemented by Liu et al,80 early extubation followed by subsequent NIV significantly reduced the duration of invasive ventilation and the length of ICU stay in postoperative subjects in whom the first SBT failed. A small subgroup of postoperative subjects in the Stéphan et al67 RCT were evaluated on the effects of NIV versus HFNC to facilitate weaning in postoperative patients. A trend toward higher treatment failure was found in patients who were extubated to HFNC versus those who were treated with NIV (40.7% vs 28.0%; P = .33). The evidence supporting early extubation to NIV after surgery is still lacking. The use of NIV to facilitate early extubation for postoperative patients in whom an SBT failed should be done so with caution.
Optimize Respiratory Support
Appropriate settings to achieve optimal treatment effects are essential for treatment success when respiratory support is used. For patients who developed atelectasis after cardiac surgery, Pasquina et al66 compared the use of NIV and CPAP in an RCT, and found that more subjects in the NIV group had radiologic improvement of atelectasis (60% vs 40%; P = .02). This finding supports the use of inspiratory pressure provided by NIV rather than a constant positive pressure that does not change between the phases of the breath. More importantly, the key to NIV success seems to be sufficient driving pressure. Joris et al48 compared the inspiratory and expiratory positive pressure settings of 12 and 4 cm H2O, respectively, with pressure settings of 8 and 4 cm H2O, and no NIV in subjects with obesity who underwent gastroplasty. They found only the subjects in the NIV settings of 12 and 4 cm H2O had significant improvement in pulmonary function after surgery, whereas no significant differences were found in the groups of NIV setting at 8 and 4 cm H2O and no NIV.48 For the utilization of HFNC, flow settings play a key role in treatment success. However, no consensus has been achieved in the flow settings for patients with different etiologies and situations. Particularly, the individual patient’s inspiratory flow may vary breath by breath. One universal setting does not fit all; an individualized setting and timely adjustment should be considered.
Clinical Monitoring During Respiratory Support
Close monitoring is necessary to ensure the success of respiratory support, particularly the first 1 h of initiating treatment because most patients who improve with treatment will do so within the first hour. Although delaying intubation is associated with increased mortality, escalation of therapy is warranted if the patients do not respond to treatment in the first hour.81 Common monitoring variables include breathing frequency, accessory muscle use, or work of breathing, , and
/
.11 More recently, the ROX index, defined as the ratio of
/
to frequency, has been described and prospectively validated to predict the failure of HFNC in patients with acute respiratory failure caused by pneumonia, and the cutoff value was determined as 4.88.82-84 However, the sensitivity and specificity of the ROX index in patients who were hypoxemic after surgery and the threshold to determine HFNC failure require further investigation.
When NIV is used, special attention should be paid to risk factors for NIV failure, such as copious secretions with ineffective cough ability, hemodynamic instability, and the intolerance to the interface or positive pressure. Duan et al85 developed the HACOR score (heart rate, acidosis, consciousness, oxygenation, and respiratory rate) to predict NIV failure among patients who are hypoxemic. The scale seemed effective in predicting NIV failure in a separate cohort of subjects with hypoxemia. An HACOR score > 5 after 1 h use of NIV was a cutoff point to determine NIV failure, and intubation was recommended.
Making the determination of treatment failure and knowing when to escalate therapy, such as endotracheal intubation and mechanical ventilation, are challenging. We summarized the intubation criteria from all the RCTs by comparing HFNC and NIV or CPAP versus standard O2 therapy in Table 4. The most common criteria are 1) tachypnea with a frequency > 35 breaths/min and accessory muscle use; 2) respiratory acidosis, with pH < 7.30 and > 50 mm Hg; 3) altered mental status; 4) hemodynamic instability; and 5) loss of the ability to protect the airway. The only controversial criterion is for refractory hypoxemia, which varies greatly in RCTs.
Intubation Criteria Used in the RCTs that Compared HFNC, CPAP, NIV vs Standard O2 Therapy
Other Treatments
Incentive Spirometry
Incentive spirometry (IS) encourages patients to perform deep breathing exercises independently, with visual feedback of inspiratory effort.86 Since the introduction in 1970, IS has been broadly used for postoperative patients to prevent and treat pulmonary complications, such as atelectasis or pneumonia.87,88 However, data with regard to its effectiveness are conflicting, and high-quality evidence is lacking.89 In a recently published systematic review and meta-analysis of 95 RCTs, no significant differences in patient outcomes were found between IS and standard medical care (risk ratio 1.06, 95% CI 0.85–1.34).90 However, in the RCT of subjects after coronary artery bypass grafting, Eltorai et al91 found that the use of IS significantly improved the radiographic atelectasis severity score, the need for NIV, length of stay in the ICU and hospital, and 6-month mortality in a subgroup of subjects after non-elective surgery. Importantly, in this study, a reminder bell was used in the experimental group. This increased adherence to IS when compared with the control group, which had no reminder bell. 91 This finding suggests that using IS correctly may be the key to treatment success. In a large national survey of health-care providers, respondents reported that patients might forget to use their incentive spirometer, which contributed to therapy nonadherence.92 To accurately assess and harness the true value of IS for postoperative patients, efforts aimed at improving IS adherence are warranted.93
Inhaled Pulmonary Vasodilators
Inhaled pulmonary vasodilators have been increasingly used to improve oxygenation for patients with hypoxemia by correcting a ventilation/perfusion mismatch.94 They are also used in patients with hypoxemia and with pulmonary hypertension and/or right heart failure to reduce pulmonary arterial pressure. Small cohort studies demonstrated that inhaled pulmonary vasodilators via HFNC or NIV can reduce pulmonary arterial pressure and/or improve oxygenation in patients after cardiac surgery,95-99 but larger RCTs are needed to validate these findings.100 Inhaled pulmonary vasodilators can improve hypoxemia but did not reverse the underlying condition.
Non-Opioid Analgesics
Adverse events due to respiratory depression often occur on the first postoperative day due to the administration of opioids.101 Non-opioid analgesics have been proposed as a way to control pain while reducing the use of opioids. This would, at least in theory, reduce the incidence of postoperative hypoxemia.101 However, in a recent double-blind RCT with 570 subjects after abdominal surgery, no significant difference in the duration of postoperative hypoxemia was found between the groups of subjects treated with opioids and with non-opioids.102 Future studies are needed to investigate the role of non-opioid analgesics in postoperative hypoxemia.
Areas of Uncertainty and Future Research
Some uncertainties remain in the prevention and management of postoperative hypoxemia. A comprehensive scoring system that uses risk factors specific for postoperative hypoxemia is needed to determine patients at high risk. For those postoperative patients who are considered high risk, the prophylactic use of HFNC versus NIV needs to be determined. The role of HFNC for patients after abdominal surgery compared with standard O2 therapy needs further investigation. For patients for whom a planned extubation failed, whether HFNC or NIV is more effective to prevent re-intubation is still unknown. NIV has theoretic superiority over CPAP for reducing work of breathing and prevention of atelectasis in postoperative patients, but whether this translates to better clinical outcomes is still unclear. The combination of HFNC and NIV, meaning the utilization of HFNC during NIV breaks, has shown to be more effective than NIV alone in reducing re-intubation in nonsurgical patients at high risk.103 Whether this combination has similar benefits in patients after surgery and with high-risk factors warrants further study. Also, whether the combined use of oxygen therapy with adjunct therapy, for example, lung expansion therapy and inhaled pulmonary vasodilators, would generate better outcomes for patients after surgery is unknown.
Summary
Postoperative hypoxemia is common in clinical practice. Compared with patients without postoperative hypoxemia, these patients have longer lengths of stay and higher mortality in an ICU and hospital. Special attention needs to be paid to patients with risk factors, specifically age > 65 years, BMI ≥ 30 kg/m2, American Society of Anesthesiology physical status classification ≥ III, ARISCAT score ≥ 26, preoperative dependent living status, preoperative sepsis, or chronic pulmonary disease such as moderate-to-severe COPD, asthma, or obstructive sleep apnea. Analysis of the currently available evidence suggests that the prophylactic use of HFNC benefits patients at high risk of hypoxemic respiratory failure after cardiothoracic surgery. For patients in whom planned extubation failed after surgery, NIV should be used to treat hypoxemic respiratory failure and to avoid re-intubation, whereas early CPAP should only be considered for patients after abdominal surgery. If a patient is intolerant of NIV or CPAP, HFNC might be considered as an alternative. Close monitoring of the patient’s response to treatment within the first hour of initiation is essential in treatment success and escalation of therapy should be considered when no improvement is observed.
Footnotes
- Correspondence: Jie Li PhD RRT RRT-ACCS RRT-NPS FAARC, Division of Respiratory Care, Department of Cardiopulmonary Sciences, Rush University, 600 S Paulina St, Suite 765, Chicago, IL 60612. E-mail: jie_li{at}rush.edu
Dr Li discloses relationships with Fisher & Paykel Healthcare, Aerogen, Rice Foundation, and the American Association for Respiratory Care. She is also Section Editor for Respiratory Care. Dr Scott discloses a relationship with Teleflex. The remaining authors declare no conflict of interest.
Dr Li presented a version of this paper at the New Horizons Symposium: Care of the High Risk Surgical Patient at AARC Congress 2020 LIVE!, held virtually on November 18, 2020.
- Copyright © 2021 by Daedalus Enterprises
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