Coronary artery disease (CAD) is one of the most common cardiac conditions, primarily caused by atherosclerosis. Studies have shown that environmental factors such as lifestyle, diet, genetics, and air pollution contribute to the increasing prevalence of atherosclerosis. Despite advances in non-invasive and medical therapies, coronary artery bypass grafting (CABG) remains a definitive treatment in many cases. CABG is commonly performed using cardiopulmonary bypass (CPB), which increases the risk of additional complications. Among postoperative complications, pulmonary events are the most frequent and are closely associated with increased morbidity and mortality.

Postoperative pulmonary complications (PPCs) significantly raise healthcare costs, prolong hospital stays, and worsen patient outcomes. Cardiac surgery increases susceptibility to PPCs through factors such as sternotomy, CPB, myocardial protection via hypothermia, and phrenic nerve injury. These factors impair thoracic compliance, diaphragm movement, and mucociliary function. Mechanical ventilation further contributes through inflammatory responses that may cause barotrauma and volutrauma.

The incidence of PPCs varies from 1.96% to 43.7%, depending on patient characteristics, surgical type, and diagnostic criteria. With an aging population and more high-risk patients undergoing surgery, the incidence of PPCs is expected to rise. Reported preoperative risk factors include advanced age, active or recent smoking, BMI >25, heart failure, prior myocardial infarction, chronic lung disease, type 2 diabetes mellitus, hypertension, and ASA score >2. Intraoperative factors include prolonged anesthesia, CPB duration >95 minutes, multiple grafts, and use of the internal mammary artery. Postoperative contributors include prolonged ventilation, nasogastric tube use, and insufficient nursing care.

Common PPCs include atelectasis, pneumonia, bronchospasm, pulmonary embolism, pleural effusion, acute respiratory distress syndrome (ARDS), pneumothorax, hemothorax, and transfusion-related acute lung injury (TRALI). Atelectasis is observed in 30-72% of cases on postoperative chest X-rays. TRALI is the leading cause of transfusion-related death and is characterized by hypoxia and bilateral pulmonary infiltrates. Pleural effusions are associated with low BMI, female sex, atrial fibrillation, heart failure, valve surgery, and anticoagulant use. Phrenic nerve injury, often resulting from internal thoracic artery dissection or cold-induced damage during myocardial protection, leads to unilateral diaphragmatic dysfunction.

Hospital-acquired infections, particularly pneumonia, are among the most common and costly non-cardiac complications following cardiac surgery. Pneumonia occurs in 2.4%-20% of cases, while ventilator-associated pneumonia affects 35.2% of patients intubated longer than 48 hours. ARDS is the most severe PPC, involving widespread alveolar injury, endothelial disruption, and non-cardiogenic pulmonary edema. Tension pneumothorax may also occur after chest tube removal.

Spirometry is a standard preoperative test used to assess lung function, primarily through FEV₁ and FVC values. These measurements help detect obstructive or restrictive defects. FEV₁/FVC <70% has been identified as an independent risk factor for PPCs. However, spirometry's predictive value is limited. A systematic review by Dankert et al. reported sensitivities ranging from 38% to 84% and specificities from 55% to 99%. Combined data yielded a sensitivity of 62%, specificity of 70%, and an area under the curve (AUC) of 0.74.

Pulse oximetry alone cannot reliably indicate PaO₂ above 100 mmHg or provide information on oxygen reserve. The Oxygen Reserve Index (ORI) is a novel, continuous, and non-invasive parameter that evaluates oxygenation status in the mild hyperoxic range (PaO₂ 100-200 mmHg). ORI values range from 0.00 to 1.00 and correlate with changes in PaO₂. ORI >0.24 corresponds to PaO₂ ≥100 mmHg when SpO₂ exceeds 98%, while ORI >0.55 may indicate PaO₂ ≥150 mmHg. A declining ORI trend approaching 0.24, despite SpO₂ >98%, can signal impending hypoxemia.

Given the substantial impact of PPCs on postoperative outcomes and the limitations of spirometry in accurately predicting these complications, there is a need for more reliable preoperative assessment tools. The present study aims to compare the diagnostic performance of the Oxygen Reserve Index and spirometry in predicting postoperative respiratory complications in patients undergoing coronary artery bypass surgery. The goal is to identify a more accurate and effective method for preoperative pulmonary risk stratification.

Coronary artery disease (CAD) is one of the most prevalent cardiac disorders caused by atherosclerosis. Comprehensive studies have shown that changes in environmental factors such as lifestyle, diet, genetics, and air pollution contribute to an increased incidence of atherosclerosis. Despite the continuous development of non-invasive and medical treatments, coronary artery bypass grafting (CABG) remains the final treatment option for many patients. In this procedure, open-heart surgery is performed with or without the use of cardiopulmonary bypass (CPB), during which vascular grafts are created. In most cases, CPB is utilized, which may lead to additional complications beyond those associated with the surgery itself. Following CABG, cardiovascular, pulmonary, renal, infectious, and psychiatric complications may occur. Among these, pulmonary complications are the most commonly encountered and are significantly associated with morbidity and mortality.

Postoperative pulmonary complications (PPCs) following cardiac surgery increase healthcare costs, prolong hospital stay, and elevate morbidity and mortality rates. Cardiac surgery predisposes to PPCs more than other surgical types due to median sternotomy/thoracotomy, CPB, myocardial protection via hypothermia, and phrenic nerve injury. It directly impairs thoracic compliance, diaphragm function, and mucociliary clearance. Additionally, mechanical ventilation activates local and systemic inflammatory responses, leading to volutrauma and barotrauma.

The incidence of PPCs following cardiac surgery ranges from 1.96% to 43.7%, depending on patient-related risk factors, the type of cardiac procedure performed, and the definition of PPC used. With the increasing number of high-risk patients undergoing cardiac surgery due to an aging population, a rise in PPC incidence is expected. Preoperative risk factors identified in various studies include age above 65 or 80 years, active smoking within the past 6 or 12 months, body mass index over 25, heart failure, history of myocardial infarction (MI), chronic lung disease, type 2 diabetes mellitus (DM2), hypertension (HT), and an American Society of Anesthesiologists (ASA) score greater than 2. Intraoperative risk factors include anesthesia duration longer than 270 minutes, CPB duration longer than 95 minutes, repair of four or more vessels, and use of the internal mammary artery (IMA) as a graft. Postoperative risk factors include mechanical ventilation for more than 10 hours, presence of a nasogastric tube, and lack of high-quality nursing care.

PPCs include atelectasis, pneumonia, bronchospasm, pulmonary embolism, pleural effusion, acute respiratory distress syndrome (ARDS), pneumothorax, hemothorax, mechanical ventilation longer than 48 hours, transfusion-related acute lung injury (TRALI), and phrenic nerve injury. Atelectasis is a common cause of hypoxemia and impaired gas exchange following cardiac surgery. Postoperative chest X-rays show atelectasis in 30-72% of patients, making it one of the leading causes of respiratory dysfunction. TRALI is the most common transfusion-related adverse event and the leading cause of transfusion-related deaths globally. It is characterized by acute hypoxia and bilateral pulmonary infiltrates following allogeneic blood transfusion. Pleural effusions after CABG are associated with low body mass index, female sex, history of atrial fibrillation, heart failure, concomitant valve surgery, and anticoagulant use. Surgical injury to the phrenic nerve often results in unilateral loss of diaphragmatic function and is commonly observed during dissection of the internal thoracic artery. Previous studies have shown that phrenic nerve injury may also occur due to cold exposure during myocardial protection strategies.

Healthcare-associated infections are a leading cause of non-cardiac morbidity after cardiac surgery. Among these, pneumonia is the most frequently observed, the most expensive, and the most resource-intensive infection. Pneumonia develops in 2.4% to 20% of patients following cardiac surgery. Additionally, ventilator-associated pneumonia occurs in 35.2% of patients who remain on mechanical ventilation for more than 48 hours. ARDS is the most severe pulmonary complication after cardiac surgery, characterized by pro-inflammatory injury to the alveolar structure, widespread endothelial damage, severe hypoxia, and non-cardiogenic pulmonary edema.

Mediastinal and pleural drains are commonly placed after cardiac surgery and are removed when fluid output is minimal and hemodynamic stability is achieved. However, recurrence due to tension pneumothorax following drain removal represents a life-threatening complication. This occurs when a one-way air flow between the lung parenchyma and the pleural space causes air to become trapped in the pleural cavity.

Spirometry is used to assess pulmonary function and is a widely accepted preoperative test in thoracic surgeries. It evaluates the forced expiratory volume in one second (FEV₁) and forced vital capacity (FVC), aiding in the diagnosis of obstructive or restrictive ventilatory defects. Spirometry is closely associated with chronic obstructive pulmonary disease (COPD), asthma, and interstitial lung diseases. An FEV₁/FVC ratio below 70% is considered an independent risk factor for postoperative complications in thoracic surgeries.

Although spirometry is a conventional respiratory function test, its sensitivity and specificity in predicting PPCs remain suboptimal. A systematic review by Dankert et al. reported sensitivities ranging from 38% to 84% and specificities from 55% to 99%. Based on the pooled data from the included studies, a ROC analysis was performed to assess spirometry's predictive value for PPCs. The average sensitivity was found to be 62%, specificity 70%, and area under the curve (AUC) 0.74.

While pulse oximetry is a standard monitoring tool, it does not reliably indicate whether PaO₂ exceeds 100 mmHg or whether oxygen reserves are increasing. Therefore, the Oxygen Reserve Index (ORI) may assist in identifying inadequate preoxygenation and provide preliminary information on pulmonary oxygen reserve. ORI is a non-invasive and continuous parameter that evaluates real-time oxygen reserve within a mild hyperoxic range (approximately PaO₂ 100-200 mmHg). It is measured using a finger sensor and varies between 0.00 and 1.00 depending on oxygen reserve status. Changes in ORI values reflect corresponding changes in PaO₂ within this range. Although ORI and PaO₂ do not show exact correspondence, a positive correlation has been observed within the specified range. Studies have shown that when SpO₂ exceeds 98%, an ORI >0.24 may indicate PaO₂ ≥100 mmHg. Similarly, an ORI >0.55 appears to correspond with PaO₂ ≥150 mmHg. A decline in ORI toward 0.24 while SpO₂ remains >98% may serve as an early indicator of PaO₂ approaching 100 mmHg.

Given the high morbidity and mortality associated with PPCs, especially in cardiac surgery cases, accurate preoperative prediction and appropriate risk stratification are of critical importance. Due to the limited predictive power of spirometry alone, there is a need for novel and updated approaches. The current study aims to evaluate the effectiveness of ORI and spirometry in predicting PPCs. Based on the findings, a more effective method for anticipating pulmonary complications in cardiac surgery may be developed.

Inclusion Criteria:

• Undergoing elective coronary artery bypass grafting (CABG) surgery.
• Aged 18 years or older at the time of enrollment.
• Provided written informed consent to participate in the study.
• Medically stable and cooperative for preoperative respiratory assessments (ORI and spirometry).

Exclusion Criteria:

• Refusal to participate or failure to provide informed consent.
• Age under 18 years.
• Left ventricular ejection fraction below 40% on preoperative evaluation.
• Presence of cognitive impairment or psychiatric disorders preventing cooperation (e.g., dementia, Alzheimer's disease).
• Severe anemia defined as hemoglobin level below 8 g/dL.
• Undergoing emergency surgery rather than elective CABG.
• Anatomical or mechanical obstruction that may impair nasal oxygenation (e.g., severe nasal septum deviation, presence of a nasogastric tube).