Systemic Complications: Respiratory

Patient-Related Risk Factors

Patient factors conferring high risk of developing respiratory complications in patients undergoing noncardiac surgery include advanced age, American Society of Anesthesiology (ASA) score ≥2, smoking history, male gender, obesity, obstructive sleep apnea (OSA), chronic obstructive pulmonary disease (COPD), congestive heart failure (CHF), and asthma. ^, A smoking history is known to increase the risk of pulmonary complications by nearly 2.5 times. ^, A systematic review of 9354 studies, including 107 in a meta-analysis, found that the risk of postoperative respiratory complications was 73% higher among smokers than non-smokers. Older patients have anatomical changes to the lung parenchyma and chest wall that lead to increased work of breathing and diminished gas exchange. In combination with complex comorbidities and hemodynamic changes, this can cause increased atelectasis and suboptimal ventilation and oxygenation.

The physiology of obesity leads to reduced lung volumes, impaired ability to clear secretions, and decreased air flow. Specifically, as BMI increases, functional residual capacity and residual volume decrease. Thus, obese patients are 4.5 times more likely to suffer from respiratory complications. A related disease is obstructive sleep apnea (OSA), which has been shown by numerous studies to be an independent risk factor for postoperative respiratory complications. ^, The prevalence of OSA is higher in surgical patients compared with the general population and most of these patients are undiagnosed at the time of operation. Perioperatively, patients with OSA have an increased incidence of adverse events including oxygen desaturations, respiratory failure, and reintubation. The use of opiates, sedatives, and anesthetics in the perioperative period are thought to decrease arousal, increase pharyngeal collapse, and impair ventilation. A meta-analysis analyzing 13 studies, comprising 3942 patients, found that patients with OSA had higher incidence of desaturations, respiratory failure, and ICU transfers. Other studies found a 2-fold increase in pulmonary complications and mechanical ventilation in OSA patients. ^, Preoperative treatment with continuous positive airway pressure (CPAP) has been shown in some studies to be associated with lower risk of cardiopulmonary complications compared with untreated patients. ^, Thus, guidelines suggest that preoperative use of CPAP should be considered in OSA patients.

Obstructive lung diseases including chronic obstructive pulmonary disease (COPD) and asthma cause impaired lung function and therefore confer increased risk for postoperative pulmonary complications. ^{, , ,} In patients undergoing aortic surgery, 40% of those with COPD suffer a respiratory complication. COPD is associated with increased adverse events and decreased long-term survival in EVAR patients. A study of 3979 patients undergoing elective endovascular aneurysm repair (EVAR) found that COPD was a preexisting condition more common in those requiring prolonged intubation. In patients with untreated asthma, the incidence of postoperative pulmonary complications is 24%. This number is reduced to 4.5% in those receiving perioperative systemic corticosteroids. A study of 48,218 patients showed that patients who were hospitalized for asthma in the 3 months prior to an operation had nearly 3 times the risk of contracting postoperative pneumonia. Thus, it is worth considering postponement of surgery in patients with recent exacerbations of asthma or COPD until symptoms have resolved and pulmonary function has returned to baseline. ^, Overall, patient-related risk factors should be carefully considered and modified as able in the preoperative period.

Procedure-Related Risk Factors

Risk factors for pulmonary complications related to the procedure itself include type of operation, surgical site, duration of surgery, anesthesia factors, transfusion requirements, and urgency of surgery. Many studies have shown that vascular surgery, in particular aortic surgery, places patients at elevated risk from pulmonary complications compared with other types of surgery. ^{, ,} In an analysis of 52,562 vascular surgery patients from the Vascular Quality Initiative, there was a 5.4% incidence of postoperative respiratory adverse events. Of those, certain vascular operations had the highest rates of pulmonary complications: open abdominal aortic aneurysm repair (17.6%), in situ suprainguinal bypass (9.68%), and thoracic endovascular aortic repair (TEVAR) (9.6%). A study on ARDS showed that 16% of patients undergoing aortic surgery developed acute lung injury. ^, Surgical approach for aortic aneurysm repair has been investigated as a possible variable in respiratory complication rates. In 3530 elective open abdominal aortic aneurysm repair cases, there was no difference in pulmonary complications between retroperitoneal and transperitoneal approaches. When comparing open repair with EVAR of 446 ruptured abdominal aortic aneurysms, respiratory complications were more common after open repair (40.5% vs. 25%). Surgical site, namely proximity of the incision to the diaphragm, is an important risk factor for postoperative pulmonary complications as well. Upper abdominal and thoracic operations decrease mobility of the diaphragm and therefore reduce ventilation. ^,

Operative duration longer than 2 hours is an important predictor of pulmonary complications. ^{, ,} Thus, it is wise to be cognizant of case length both during the operation itself and in surgical planning. Anesthesia considerations such as neuromuscular blockade, opioid use, and general anesthesia are risk factors for pulmonary complications. These factors cause impaired ability to cough and swallow as well as diminished muscular contraction. Secretions can accumulate, gastric contents are more likely to be aspirated, and lung ventilation is reduced. Some studies have suggested that reversal of neuromuscular blockade with sugammadex or neostigmine may reduce postoperative pulmonary morbidity. There have been conflicting reports of the impact of anesthesia types on vascular postsurgical pulmonary complications. In EVAR patients, one study found that only 0.72% of patients receiving local or regional anesthesia developed respiratory complications and the authors suggest that avoiding general anesthesia may be beneficial. Separate studies of 8141 and 6009 EVAR patients both found that general anesthesia was associated with more respiratory complications and higher pulmonary morbidity than spinal or local anesthesia. ^, However, a recent study of 9783 elective EVAR patients showed similar frequency of pulmonary complications in general, regional, and local anesthetic groups. Another analysis of 16,052 patients undergoing lower extremity bypass showed no significant difference in pulmonary complications between regional and general anesthesia.

Blood transfusion intraoperatively, as an indicator of hemodynamic instability, has been associated with increased rates of early re-intubation. In an analysis of 941,496 operative cases, transfusion of just one unit of packed red blood cells increased pulmonary and other complications. Another study of 10,100 patients undergoing vascular, general, and orthopedic operations showed similar results, with those receiving one or two units of blood more likely to have pulmonary complications (OR 1.76). Urgent and emergent procedures confer exceptionally high risk of respiratory complications. ^, Patients who require emergent procedures have advanced disease pathologies and do not have the option of preoperative optimization for preexisting conditions. In vascular cases, emergent procedures have a pulmonary complication incidence of 25.9%. While some of these procedure-related risk factors may not be modifiable, it is important to recognize them and provide prompt and effective management.

Laboratory Test Risk Factors

Although not a substitute for clinical evaluation, certain laboratory studies may be helpful in determining perioperative pulmonary risk. Several studies have shown low preoperative arterial oxygen saturation and anemia (hemoglobin <10 g/dL) to be predictors of pulmonary complications. ^, In fact, SpO ₂ on room air in supine position was the strongest patient-related risk factor for postoperative respiratory complications. Preoperative anemia confers almost a 3-fold risk increase for these complications. Blood urea nitrogen levels greater than 21 mg/dL and albumin less than 35 g/L are also risk factors.

Other tests such as pulmonary function tests (PFTs), arterial blood gas measurements (ABG), chest radiographs (CXR), and exercise testing may be relevant in certain cases, but should not be considered routine. The role of PFTs in preoperative risk stratification is not well established. Most studies indicate that PFTs do not independently predict postoperative pulmonary complications. Patients with COPD or asthma may benefit from PFTs to help optimize clinical status. Similarly, chest radiographs are commonly ordered in the preoperative setting, but rarely influence preoperative management. A meta-analysis found that only 10% of routine preoperative chest radiographs had abnormalities. Of these, only 1.3% were unexpected and 0.1% resulted in management changes. Guidelines indicate that it is reasonable to consider chest radiograph in patients with existing cardiopulmonary disease and those older than 50 years undergoing high risk surgery. There is no data to suggest that hypercapnia or hypoxemia, as identified by ABG, provide prediction of postoperative pulmonary morbidity.

Pulmonary Risk Indices

There have been numerous pulmonary risk indices developed to stratify and predict likelihood of postsurgical respiratory complications. One well-respected tool is the ARISCAT score, developed from 2464 patients in Spanish hospitals. It uses four patient-related factors (preoperative SpO ₂ , recent respiratory tract infection, age, hemoglobin level) and three procedure-related factors (intrathoracic or upper abdominal surgery, procedural length, and emergency surgery) to determine risk of postoperative pulmonary complications ( Table 45.2 ). The ARISCAT score has subsequently been externally validated in the PERISCOPE study across 63 European centers spanning 21 countries. The SLIP model analyzed 4366 patients for postoperative acute lung injury and classified them into low, intermediate, and high risk. The predictors were cardiac, vascular, or thoracic surgery, diabetes mellitus, COPD, GERD, and alcohol abuse. In patients with baseline risk factors for ARDS, the SLIP-2 model was found to perform better and identified nine predictive characteristics: aortic surgery, cardiac surgery, sepsis, emergency surgery, cirrhosis, admission location other than at home, increased respiratory rate, FiO ₂ greater than 35%, and SpO ₂ less than 95%.

Another risk index, SPORC, took the following factors into account: ASA ≥3, emergency procedure, high risk surgery, COPD, and congestive heart failure. A score of 0 conferred a 0.12% risk of reintubation, while scores of 7–11 had a 5.9% risk of reintubation. The SPORC-2 index added seven intraoperative factors to predict respiratory complications. The LAS VEGAS score also took intraoperative factors into account. This index showed independent association between postoperative pulmonary complications and six patient-related factors (age, ASA score, anemia, SpO ₂ , OSA, active cancer), two procedure-related factors (emergency procedure, surgical length), and five intraoperative characteristics (airways other than supraglottic, IV anesthetic with volatile agents, desaturations, high PEEP, and vasopressor use). Gupta et al. formulated two calculators using the NSQIP dataset, one for evaluating respiratory failure and one for predicting pneumonia. ^, Finally, the Arozullah respiratory failure index examined 81,719 men at 44 VA medical centers. It stratifies patients into five classes of risk based on surgical type, urgency of procedure, albumin level, BUN level, functional status, age, and COPD. These are all tools that can be used in conjunction with clinical assessment to estimate surgical risk.