Comparison of Oxygen Reserve Index and Spirometry in Predicting Respiratory Complications After Coronary Artery Bypass Surgery

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.