Prediction of Postoperative Complications after Major Lung Resection: A Literature Review

Abstract

Background: Lung resection is the primary treatment option for many patients with lung cancer; however, it is a high-risk surgery with many potentially lethal perioperative complications. The aim of this review is to examine the capability of forced expiratory volume in one second (FEV1), diffusing capacity of the lung for carbon monoxide (DLCO), maximal oxygen uptake in exercise (VO_2max), and maximal inspiratory and expiratory pressures (PI_max and PE_max, respectively) to predict postoperative lung function. Methods: A literature review was performed using PubMed and the Preferred Reporting Items for Systematic Reviews and Metaanalyses (PRISMA) guidelines. The research included articles after 2000. Experimental studies on animals, studies before 2000, and studies in a language other than English were excluded. Results: A total of 11 studies were included in this review. The main findings were highlighted. In addition, the optimal threshold values of FEV1, DLCO, VO_2max, and PI_max as well as PE_max were discussed. Conclusions: Preoperative FEV1, DLCO, VO_2max, and PI_max as well as PE_max have all proven to be independent risk factors for the prediction of postoperative morbidity, mortality, and cardiopulmonary complications after lung resection surgery.

1. Introduction

Lung cancer is one of the most common causes of death worldwide. It is the second most common cancer in both men and women, following prostate cancer in men and breast cancer in women [1]. The American Cancer Society has estimated approximately 235,000 new cases of lung cancer (both small-cell and non-small-cell) in the USA and about 131,000 deaths [2].

Lung resection is a type of surgery used to diagnose and treat various lung disorders, especially lung cancer. During this procedure, part of a lobe, an entire lobe, or the whole lung is removed. Lung resection can be performed either with minimally invasive surgery or open surgery (thoracotomy) [3]. Lung resection is the primary treatment option for many patients with lung cancer; however, it is a high-risk surgery with many potentially lethal perioperative complications, such as acute respiratory distress syndrome (ARDS), pneumonia, atelectasis, anastomotic rupture, and bronchial stenosis [4]. The mortality rate for lung resection is usually below 1% when the procedure is performed on patients with good pulmonary function [5]; however, in patients with poor lung function, the risk of complications is much higher, and the mortality rate can be as high as 4–6% [5].

Moreover, most of these operations are performed under general anesthesia, and the technique of one-lung ventilation (OLV) is commonly used. OLV is associated with several intraoperative and postoperative complications due to the increased strain on the ventilated lung. Postoperative complications can range from hypoxemia to ARDS and respiratory failure [6].

It has been reported that predicting the likelihood of the aforementioned complications is possible based on clinical factors and specific laboratory measurements. Utilizing these factors allows for the identification of high-risk patients, who may benefit from preventative or additional therapeutic interventions instead of lung resection surgery [7]. Additionally, this approach helps select patients who would benefit from preoperative preparation techniques aimed at improving respiratory function.

Some well-known clinical factors that determine the long-term survival of patients with lung cancer include cancer stage, histological type, patient age, and type of therapy. Beyond these clinical factors, several reports have identified additional determinants [8].

Pulmonary function tests (PFTs), including forced expiratory volume in one second (FEV1) and diffusing capacity of the lung for carbon monoxide (DLCO), are used to predict postoperative complications and mortality in lung cancer patients undergoing resection [9]. FEV1 measures the amount of air expelled in the first second of a forceful breath after full inhalation. Typically, healthy individuals can expel 80% of their forced vital capacity (FVC) within this timeframe. For men aged 20–60, average FEV1 values range from 4.5 to 3.5 L, and for women in the same age group the range is from 3.25 to 2.5 L. FEV1 is a key predictor of overall survival [10]. Introduced in 1909, DLCO assesses the efficiency of oxygen transfer from the alveoli to the blood [11]. A normal DLCO value is over 75% of the predicted value, up to 140% [12]. This measurement is crucial for determining long-term outcomes after lung resection.

Furthermore, apart from the fundamental preoperative functional assessments, the utilization of exercise testing has been suggested for the evaluation of potential candidates for lung resection due to its ability to offer a thorough evaluation of both respiratory and cardiovascular functions [13]. The term maximal oxygen consumption (VO_2max) refers to the maximum amount of oxygen that an individual can utilize during high-intensity or maximal physical activity. Considered a key measure of cardiovascular health and aerobic endurance, this parameter presents valuable insights into an individual’s physiological capabilities [14].

With the exception of pulmonary function, the efficacy of respiratory muscles holds significant importance in determining the postoperative progression of a patient [15]. The presence of respiratory muscle insufficiency amplifies the respiratory burden, resulting in clinical manifestations like breathlessness and compromised physical performance, and can be detected through the assessment of maximal inspiratory and expiratory pressures (PImax and PE_max, respectively). These pressures are produced at the oral cavity, and their evaluation serves as a non-invasive clinical approach to gauging the potency of respiratory muscles [16].

The aim of this review is to study various prospective and retrospective clinical trials as well as existing reviews that studied and compared the capability of these factors [pulmonary function tests (FEV1 and DLCO), maximal oxygen uptake in exercise (VO_2max), and maximal inspiratory as well as expiratory pressures (PImax and PE_max, respectively)] to predict postoperative lung function, so that we can determine which is the best option to identify patients that would benefit more from a lung resection without major pulmonary complications.

2. Materials and Methods

This investigation incorporates publications that analyzed the variables capable of accurately forecasting postoperative pulmonary capacity after a lung excision procedure. Specifically, the significance of forced expiratory volume in the first second (FEV1), diffusing capacity of the lung for carbon monoxide (DLCO), maximal oxygen uptake in exercise (VO_2max), and maximal inspiratory as well as expiratory pressures (PImax and PE_max, respectively) in relation to the prognosis of postoperative pulmonary function was assessed. The scrutiny has been officially documented on the Open Science Framework. The DOI assigned to this examination is https://doi.org/10.17605/OSF.IO/DWQ7R (accessed on 19 June 2024).

2.1. Inclusion Criteria

The inclusion criteria were defined according to PICOS as follows: (1) population: adult patients (>18 years old) undergoing lung resection surgery; (2) intervention: preoperative and postoperative FEV1, DLCO, VO_2max, and PI_max as well as PE_max; (3) comparison: predictive FEV1, DLCO, VO_2max, and PI_max as well as PE_max compared with actual postoperative measurements; (4) outcomes: length of hospital stay, postoperative complications, and postoperative mortality; and (5) study design: prospective and retrospective cohort studies as well as reviews after 2000.

2.2. Exclusion Criteria

The exclusion criteria encompassed the following: experimental studies involving children, experimental studies on animals, surgical procedures other than lung resections, studies including patients with chronic obstructive disease with an FEV1 < 60% of the predicted volume, studies conducted prior to the year 2000, and studies published in languages other than English.

2.3. Search Strategy

Articles were searched for in the databases of PubMed and Cochrane Library. This study includes articles that were published in the last 20 years using the following key words: “lung resection”, “predictive”, “lung function”, “postoperative complications”, “postoperative mortality”, “FEV1”, “DLCO”, “VO_2max”, and “PI_max and PE_max”.

The initial search identified 88 publications related to the prediction of postoperative lung function, with an additional 3 studies obtained through hand-searched citations from relevant original articles. The studies were initially screened and reviewed by title and abstract by the first (L.R.) and second authors (G.D.). After excluding records from before 2000 and records written in a language other than English, the studies were initially screened and reviewed via their titles and abstracts. We excluded non-human research, studies involving children, and articles without accessible full texts, resulting in 18 studies being suitable for initial review. Full-text articles were then screened further, and 7 more studies were excluded because the outcome of interest was not included. Full-text articles were then screened by the first (L.R.), second (G.D.), and third authors (C.T.). Ultimately, we included 11 studies in our review. Figure 1 shows the flow diagram of the article screening process.

Table 1. Trial characteristics and tested parameters.

Study, Year	Country	Design	Sample	Place	Parameters Tested	NOS Score	Results
Almquist et al., 2018 [9]	USA	Retrospective study	149 patients undergoing lung resection for lung cancer	Operating room	FEV1 DLCO	8 stars	FEV1 and DLCO are correlated with length of hospital stay, but not postoperative complications
Brunelli et al., 2004 [17]	Italy	Prospective study	190 patients undergoing lung resection	Operating room	FEV1 DLCO	8 stars	Estimated and observed losses of FEV1 and DLCO have no significant differences
Alam N. et al., 2007 [18]	USA	Retrospective study	1428 patients undergoing lung resection	Operating room	FEV1 DLCO	8 stars	The possibility of postoperative lung injury increases, as FEV1 and DLCO drop
Ferguson M. et al., 2007 [19]	USA	Retrospective study	1046 patients undergoing lung resection	Operating room	DLCO	8 stars	Postoperative predictive DLCO predicts pulmonary morbidity and operative mortality
Ferguson M. et al., 2012 [20]	USA	Retrospective study	854 patients undergoing lung resection	Operating room	DLCO	8 stars	DLCO is an independent predictor of long-term survival
Licker M. et al., 2011 [21]	Switzerland	Observational study	210 patients undergoing lung resection for lung cancer	Operating room	VO2max	7 stars	Cardiopulmonary complications increase as VO2max decreases
Bobbio A. et al., 2009 [22]	Italy	Prospective study	73 patients undergoing lung resection	Operating room	VO2max FEV1	7 stars	Patients with lower preoperative VO2max and FEV1 have more postoperative complications
Refai M. et al., 2013 [23]	Italy	Prospective Cohort study	283 patients undergoing lung resection	Operating room	PImax PEmax	8 stars	Greater PImax reduction after exercise is associated with more complications
Algar et al., 2002 [24]	Spain	Retrospective study	242 patients undergoing pneumonectomy for lung cancer	Operating room	FEV1	6 stars	Patients with low ppo-FEV1 are at increased risk for PC after pneumonectomy
Cundrle Jr et al., 2022 [25]	Czech Republic	Prospective study	398 patients scheduled for lung resection surgery	Operating room	FEV1 DLCO	7 stars	9% of patients with normal FEV1 and DLCO developed PPC
Cerfolio et al., 2009 [26]	USA	Retrospective study	906 patients undergoing thoracic surgery	Operating room	FEV1 DLCO	7 stars	Identification of ppoDLCO% and ppoFEV1% as significant independent predictors of respiratory morbidity

Abbreviations: NOS: Newcastle–Ottawa scale; FEV1: forced expiratory volume in one second; DLCO: diffusing capacity of the lung for carbon monoxide; ppo: predicted postoperative; and PPC: postoperative pulmonary complications.

depicts the characteristics of each trial included in the review.

2.4. Bias Assessment

For the quality assessment of the studies included in the systematic review, the Newcastle–Ottawa Scale (NOS) was utilized. This scale is specifically applied to non-randomized observational studies. The studies are evaluated using a star rating system, with a maximum of nine stars, based on the selection of observational teams, the comparability of the groups, and the outcomes of interest. According to the results, one study received a score of 6 stars, four studies received 7 stars, and six studies received 8 stars.

3. Results

According to several reports (Table 1), preoperative pulmonary function tests (measurement of the predictive FEV1 and DLCO) and maximal oxygen uptake in exercise (VO_2max) can predict which patients can be scheduled for thoracic procedures dealing with an average operative risk.

Brunelli et al. [17] conducted a retrospective analysis of 190 patients submitted to a lobectomy or pneumonectomy. All patients performed preoperative and early postoperative PFTs. PFT results were expressed as percentages of predicted values for age, gender, and height, and the postoperative declines were expressed as percentage losses from preoperative values. At first, they documented no differences between observed and estimated values of FEV1 loss (28.2% vs. 28%) and DLCO loss (28.1% vs. 28.1%). Additionally, patients with a preoperative FEV1 < 70% of the predicted value had lower FEV1 loss (12.6% vs. 29.8%) and DLCO loss (20.3% vs. 28.3%) compared to patients with a preoperative FEV1 > 70%. Likewise, patients with a preoperative DLCO < 70% had lower DLCO loss (20.2% vs. 30.8%) and FEV1 loss (21.7% vs. 29.9%) compared to patients with a preoperative DLCO > 70%.

Almquist et al. [9] performed a retrospective analysis of 149 patients with lung cancer who underwent lung resection. Overall survival, hospital stay, and postoperative complications were studied. They found that while FEV1 and DLCO were significantly correlated with an increased length of hospital stay (>10 days compared with a median stay of 7 days), they were not correlated with postoperative complications or overall survival.

However, this finding is debatable and disagrees with that of other studies. Alam et al. [18] reviewed 1428 patients who underwent thoracic surgery for lung cancer. They studied the correlation between FEV1 and DLCO and postoperative lung injury, which was defined as pneumonitis, acute lung injury (ALI), or ARDS occurring in the immediate postoperative period during initial hospitalization. ALI and ARDS were defined as an acute onset of hypoxemia with an abnormal oxygenation ratio (PO₂/FiO₂ < 300 for ALI, PO₂/FiO₂ < 200 for ARDS). Using postoperative predicted FEV1, the possibility of developing lung injury increases as postoperative FEV1 decreases (the odds ratio is 1.10 for every 5% decrease in postoperative FEV1). Similarly, as postoperative DLCO decreases by 5%, there is a 9% greater possibility of developing ALI.

In 2007, Ferguson et al. [19] retrospectively analyzed 1428 patients who underwent lung resection between 1980 and 2006 to identify predictors of postoperative morbidity. The patients were categorized into two groups: those with chronic obstructive lung disease (COPD), defined as having a forced expiratory volume in the first second to forced vital capacity ratio of less than 0.7, and those without COPD, defined as having a ratio of 0.7 or greater. Later, in 2012, Ferguson et al. [20] studied the possibility of the prediction of long-term survival after lung resection. A retrospective analysis was conducted on 854 patients with a follow-up duration of 115 months. Out of the total number of patients, which was 854, there were 587 recorded deaths. The median survival period observed was 51.9 months. Various patient characteristics including age, disease stage, body weight, and smoking habits were assessed for their association with overall mortality risk. It was found that patients demonstrating a spirometric function of FEV1 < 80% of the predicted value exhibited an increased risk of mortality.

In 2011, Licker et al. [21] conducted a prospective data collection from 210 patients who had undergone lung resection due to lung cancer. The study cohort consisted of individuals with a forced expiratory volume in one second (FEV1) of less than 80%. Utilizing symptom-limited cardiopulmonary exercise testing (CPET), the researchers employed an upright electronically braked cycle ergometer. The exercise evaluation was concluded upon the patients reaching a point of exhaustion or presenting any electrocardiogram (ECG) abnormalities indicative of myocardial ischemia. Peak oxygen consumption (VO₂) was quantified in milliliters per minute per kilogram of body weight and milliliters per minute per kilogram of predicted body weight (PBW). The study’s analysis encompassed the assessment of operative mortality (defined as death occurring within 30 days after surgery or later if the patient remained hospitalized), cardiovascular incidents, as well as pulmonary issues.

Bobbio et al. [22] conducted a prospective cohort study with 73 patients, recording postoperative 30-day mortality after lung resection and both cardiac and pulmonary complications. The patients preoperatively underwent classical pulmonary function tests, including FEV1 measurement, DLCO measurement, and VO_2max measurement via a cycloergometer maximal incremental cardiopulmonary exercise test. The mean preoperative FEV1 was 66.7 ± 22.8% of that predicted, DLCO was 71 ± 24.9% of that predicted, and VO_2max was 18.7 ± 5.4 mL/min/kg when corrected for body weight. Upon preoperative evaluation, 14 patients had a postoperative predicted FEV1 < 35%. Postoperative mortality was 2.7%, with one death due to postoperative pneumonia and ARDS and one after myocardial infarction.

Refai et al. [23] found that patients with a lower PImax measured before exercise and those with a greater reduction in PImax after exercise were more vulnerable to pulmonary complications. They conducted PI_max and PE_max measurements preoperatively before and after a stair-climbing test in 283 candidates for lung resection. Patients unable to perform the test were excluded. Cardiopulmonary complications occurred in 74 patients (26%). Algar et al. [24] carried out a retrospective analysis of 242 individuals who had undergone pneumonectomy for lung carcinoma. Data pertaining to the perioperative period, encompassing clinical variables, pulmonary function tests, and details of the surgery, were gathered to ascertain the potential risk factors associated with postoperative complications through both univariate and multivariate analyses. The estimation of the predicted postoperative forced expiratory volume in 1 s (ppo-FEV1) was conducted by taking into account the preoperative FEV1 and pulmonary perfusion. In cases where the FEV1 exceeded 2 L/s, the ppo-FEV1 computations were as follows: ppo-FEV1 = 0.55 × FEV1 for left pneumonectomy and ppo-FEV1 = 0.45 × FEV1 for right pneumonectomy. Conversely, for patients with an FEV1 below 2 L/s, radionuclide perfusion scans were employed, with the ppo-FEV1 being determined as (1% of perfusion in the lung slated for resection) × FEV1.

Cundrle Jr et al. [25] conducted a study between 2017 and 2021. In this study, 398 patients were prospectively studied at two centers. Postoperative pulmonary complications (PPCs) were recorded during the first 30 days after surgery. Patients were divided into subgroups with and without PPCs, and factors with significant differences were analyzed using univariate and multivariate logistic regression. Among 188 patients with normal FEV1 and DLCO, 17 (9%) developed PPCs. Thoracotomy was strongly associated with PPCs in both models (OR 6.419; p = 0.005 and OR 5.884; p = 0.007). The study of Cerfolio et al. [27] retrospectively analyzed a database of 906 patients who underwent pulmonary function tests (PFTs) and pulmonary resection was performed. The PFTs assessed included FEV1%, and three types of diffusion capacity of the lung for carbon monoxide: DLCO%, DLCO adjusted for hemoglobin (DL adjusted%), and DLCO adjusted for alveolar volume (DLCO/VA%).

4. Discussion

Thoracic surgery, including lobar or lung resection, poses a significant risk for many patients due to the multitude of potential postoperative complications. Lung parenchyma is often reduced, sometimes to the point of insufficiency for normal postoperative lung function, which can be further compromised by general anesthesia, leading to reduced tidal volume and vital capacity. Consequently, not all patients are suitable candidates for lung resection, and clinicians must be able to predict which patients are capable of tolerating major lung resections.

The guidelines of the British Thoracic Society (BTS) recommend performing PFTs, including FEV1 and DLCO, for all patients undergoing thoracic surgery [27]. Patients are considered operable with an average operative risk when FEV1 is > 1.5 L (and > 2 for pneumonectomy) or when predictive postoperative FEV1 is > 40% and DLCO is > 40% of those predicted [28]. If one of these criteria is not met, according to the BTS guidelines, additional tests such as VO_2max should be performed. The threshold for VO₂ defined by the guidelines for an average operative risk is VO₂ > 15 mL/kg/min [27]. Otherwise, patients are classified as being high risk for lung resection surgery.

CPET is recommended by the European Respiratory Society (ERS) and the European Society of Thoracic Surgery (ESTS) as part of a functional algorithm for the evaluation of candidates for lung resection [28]. According to these guidelines, CPET should be performed in all patients with FEV1 < 80% or with DLCO < 80% of predicted values [28].

4.1. FEV1

In clinical practice, FEV1 is the most frequently used parameter for the preoperative evaluation of patients [29]. Predicted postoperative FEV1 (PPO-FEV1) has been proven to be an independent risk factor for postoperative morbidity and mortality. PPPO—FEV1 in absolute values (L) is calculated as follows:

PPO FEV1 = preoperative FEV1 × (19 segments—the number of segments to be removed) ÷ 19, [28].

Following anesthesia and thoracic surgery, there is a notable reduction in pulmonary function, with FEV1, FVC, and FRC decreasing by up to 50%. This substantial decline heightens the risk of atelectasis and hypoxia, making respiratory complications a leading cause of perioperative morbidity and mortality in these patients [30].

As per Brunelli et al. [17], it is highly significant to predict the early decline in FEV1 and DLCO following major lung resection surgery. Patients who undergo lobectomy and have a preoperative FEV1 value lower than 70%, indicating a case of moderate to severe COPD as per the classification by the European Respiratory Society, tend to experience around 58% less reduction in FEV1 compared to those with a preoperative FEV1 of 70% or higher.

Less encouraging results were presented in the study of Cundrle Jr et. al. [25]. They proved that 9% of patients with normal FEV1 and DLCO values, as assessed by current preoperative guidelines [28], developed postoperative pulmonary complications (PPCs). They highlighted that the ability of these parameters to discriminate is more robust in patients with low FEV1 and/or DLCO values, but the tests’ performance diminishes as these values increase.

It is worth noting that the study of Alam et al. [18] indicates that both higher perioperative fluid administration and reduced predicted postoperative lung function are independent risk factors for lung injury following lung cancer surgery. Multivariate analysis revealed that the odds of developing lung injury increase by 1.17 for each additional 500 mL of perioperative fluid administered and by 1.10 for every 5% decrease in predicted postoperative FEV1. Parquin et al. [31] reviewed 146 pneumonectomy patients and found that postoperative FEV1 below 45% was linked to the development of postpneumonectomy pulmonary edema.

4.2. DLCO

In patients with diffuse interstitial changes in the lungs, DLCO should be evaluated [32]. The severity of obstructive and restrictive lung diseases, pulmonary vascular disease, and preoperative risk can be assessed using DLCO [33]. To calculate the predicted postoperative DLCO (ppoDLCO), use the following formula:

ppoDLCO = preoperative DLCO × (1 − y/z),

where y is the number of functional or unobstructed lung segments removed and z is the total number of functional segments [34].

Ferguson et al. [19] proved that the risk of complications in the COPD and non-COPD groups was related to the degree of impairment of diffusing capacity. Pulmonary complication risk levels were somewhat higher for patients with COPD than for those without COPD for each ppoDLCO% value. Moreover, they retrospectively analyzed a group of 368 patients without COPD who underwent lung resection and recorded postoperative pulmonary complications (pneumonia, need for prolonged initial intubation or reintubation, and lobar collapse), overall complications (cardiovascular, infectious, and any other complications), and operative mortality (death during initial hospitalization or within 30 days of the operation). The analysis demonstrated that pulmonary complications were related to preoperative predictive DLCO (odds ratio of 0.728 for each 10-point increase). Overall complications were related to ppoDLCO (odds ratio of 0.772 for each 10-point increase). There were too few operative deaths in this group, and, as a result, an analysis of the predictive factors of mortality was not possible.

Alam et al. [18] discovered that the utilization of postoperative predicted DLCO was associated with an odds ratio of 1.09 for lung injury per 5% decrease in postoperative DLCO, indicating a 9% higher probability of lung injury occurrence with every 5% reduction in DLCO. These findings were further supported by the investigation conducted by Ferguson et al. [21]. A correlation between DLCO and all-cause mortality was observed, with a higher likelihood of mortality in individuals with DLCO values exceeding 80%. Subsequent examination revealed that the influence of DLCO was minimal in patients at an early disease stage but more pronounced in those at a late disease stage: patients at stage I/II displayed a 1.8% increase in risk per 10-point reduction, while patients at stage III/IV exhibited a 7.4% rise in risk per 10-point reduction.

Conflicting results were shown by the research of Almquist et al. [9]. They found that while DLCO was significantly correlated with an increased length of hospital stay (>10 days compared with a median stay of 7 days), it was not correlated with postoperative complications or overall survival. Moreover, Cerfolio et al. [26] proved that significant respiratory complications were observed in some patients whose ppoDlCO% was above 40%. Similar findings were reported in the research conducted by Cundrle Jr et al. [25], indicating that the discriminative effectiveness of these parameters is more pronounced in patients with low DLCO values.

4.3. VO_2max

In addition to basic preoperative pulmonary function assessments, more advanced investigations into cardiopulmonary function have been suggested for the anticipation of postoperative morbidity, mortality, and cardiopulmonary complications. The utilization of exercise testing during preoperative screening offers an assessment of the endurance capacities of both the respiratory and cardiovascular systems. The determination of maximal oxygen consumption during exercise (VO_2max) appears to serve as an autonomous indicator of cardiorespiratory risk subsequent to lung resection [35].

During physical workload, the physiological reserve of the heart and lungs can be quantified by monitoring ECG changes, hemodynamic changes (elevated or decreased blood pressure or heart rate), respiratory volumes, oxygen consumption as well as carbon dioxide production, and symptoms such as dyspnea or chest pain. Accordingly, the interpretation of abnormal results during CPET is helpful for the diagnosis of coronary artery disease, heart failure, and gas exchange or ventilation abnormalities [36].

In the study of Licker et al. [21], cardiopulmonary complications were more prevalent in patients with lower preoperative VO_2max, with the incidence of these complications increasing in parallel with the reduction in peak VO₂ during exercise testing. Compared to patients with VO_2max > 17 mL/min/kg, those with VO_2max < 10 mL/min/kg had a four-fold higher incidence of cardiac and pulmonary morbidity. The optimal cut-off values of peak VO₂ to predict cardiac and pulmonary complications were 13.6 mL/min/kg and 12.8 mL/min/kg, respectively. Patients with low peak VO₂ (< 17 mL/min/kg) were more likely to be older, female, hypertensive, and have a higher body mass index (BMI) as well as lower FEV1 (both preoperative and predicted postoperative). The 30-day mortality rate was 1.9%, with all four deaths occurring in patients with VO_2max < 17 mL/min/kg.

Bobbio et al. [22] provided evidence indicating that preoperative exercise capacity can assist in the identification of patients who are at risk for postoperative pulmonary complications. Nevertheless, it does not appear to autonomously forecast postoperative results. Patients with cardiac complications did not exhibit any significant difference in VO_2max values. Among the 30 patients, 19 encountered respiratory complications, while 17 experienced cardiac complications. Through univariate analysis, it was determined that the mean preoperative VO_2max and mean FEV1 were notably lower in patients who faced postoperative pulmonary complications in comparison to those who did not (p = 0.043; 95% CI = 0.8–5.3 and p = 0.013; 95% CI = 4.6–27.9, respectively). Notably, 6 out of 14 patients (43%) with a preoperative VO_2max < 15 mL/min/kg encountered a pulmonary complication, whereas only 2 out of 22 patients (9.1%) with a preoperative VO_2max > 20 mL/min/kg experienced such complications. It is essential to emphasize that patients with an exercise capacity below 15 mL/kg/min were recommended preoperative pulmonary rehabilitation, potentially introducing bias in the comparison of preoperative functional parameters.

4.4. PI_max and PE_max

With the exception of assessing lung parenchyma, the assessment of respiratory muscle strength stands out as another crucial variable that may impact postoperative respiratory capacity. Weakness in the respiratory muscles can lead to dyspnea and ultimately respiratory insufficiency. The determination of peak inspiratory and expiratory pressures (PI_max and PE_max) produced at the orifice is a recognized clinical approach for appraising muscle strength [37].

In a review that was conducted by Silveira et al. [38], the reliability and validity of maximal respiratory pressures were tested. Twenty-three studies were included in the final analysis. The level of evidence for test–retest reliability was moderate for both PI_max and PE_max, with an intraclass correlation coefficient (ICC) greater than 0.70 for each. Inter-rater reliability was low for PI_max and very low for PE_max, with both having an ICC greater than 0.70. The measurement error for both PImax and PE_max was very low. Additionally, concurrent validity showed a high level of evidence for both PImax and PE_max, with a correlation coefficient (r) greater than 0.80.

Concerning thoracic surgeries, Refai et al. [23] showed in their study that complicated patients had a four-fold greater reduction in their PImax after exercise compared to non-complicated ones (8.7% vs. 2.1%, p = 0.03). On the other hand, PE_max was similar in complicated and non-complicated patients (0.7% vs. 1.3%, p = 0.5). The optimal cut-off for predicting complications appears to be a reduction in PImax of 10%. Among 167 patients who showed a PImax reduction < 10%, 20% experienced postoperative complications, whereas 116 patients had a PImax reduction > 10% and 33% experienced complications.

5. Conclusions

Preoperative FEV1, DLCO, VO_2max, and PI_max as well as PE_max have all been demonstrated to be independent risk factors for predicting postoperative morbidity, mortality, and cardiopulmonary complications following lung resection surgery. Each parameter provides unique and valuable information about different aspects of pulmonary and cardiopulmonary function. The methodologies currently employed for measuring these predictive values are highly reliable, and there is minimal discrepancy between these values and the observed postoperative outcomes. Consequently, clinicians can utilize these parameters to effectively guide the selection of patients most suitable for lung resection.