The role of the BMI ≥40 kg/m² criterium in ASA-PS classification for metabolic surgery

doi:10.21203/rs.3.rs-5008929/v1

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Research Article

The role of the BMI ≥40 kg/m² criterium in ASA-PS classification for metabolic surgery

https://doi.org/10.21203/rs.3.rs-5008929/v1

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Purpose This study explores historical trends in ASA-PS scoring and evaluates whether the ASA-PS III classification based solely on a BMI ≥ 40 kg/m² effectively contributes to peri-operative risk stratification for patients undergoing metabolic surgery.

Methods Adult patients (January 2015-January 2023) were included from the Dutch Audit for the Treatment of Obesity registry. Logistic regression adjusted for confounders compared groups stratified by BMI and presence of systemic disease. Outcome measures: complication prevalence (peri-operative, ≤30days and >30days), Clavien-Dindo score, hospital stay (>2 days), ICU admission, readmission, and mortality rates.

Results A total of 75,871 patients (78.8% women, age 43.5 (SD 11.8) yrs, BMI 42.9 (SD 5.3) kg/m²) were included. The number of patients receiving ASA-PS III increased from 26% to 92% (BMI≥40 kg/m²), and from 20% to 64% (BMI<40 kg/m²), despite generally stable mean age, BMI, and concurrent diseases. From 2018, ASA-PS II was inaccurately assigned in 64%. Patients without severe systemic disease (BMI ≥ 40 kg/m²) were significantly less likely to experience complications, ICU admission, prolonged hospital stay, or readmission compared to patients with severe systemic disease (BMI < 40 kg/m²) (OR: 1.72, 95% CI: 1.43 – 2.08, p < 0.001).

Conclusion A large Dutch national registry on metabolic surgery showed that ASA-PS III patients with BMI ≥ 40 kg/m² only were less likely to experience complications after surgery compared to those with BMI < 40 kg/m² and severe systemic disease. This suggests that adhering to the BMI criterion may undermine the ASA-PS classification's effectiveness in risk stratifying these patients.

Increased amount of ASA-PS III in Dutch metabolic surgery patients (2015-2023)
Among ASA-PS III: Fewer complications in BMI ≥ 40 patients without severe disease
Sticking to the BMI criterion may undermine effectiveness in risk stratification
Revising ASA-PS strategies may improve peri-operative efficiency and resource use

Since its introduction in 1941, the ASA-PS classification system has been used to classify the patient's physical condition to assess the level of peri-operative risk. Although an increased BMI was added as a criterion in 2014, higher ASA-PS scores were often assigned to patients living with obesity in previous years prior to this update, potentially on theoretical or subjective grounds^1,2. However, the question remains whether assigning higher ASA-PS classifications solely based on a BMI ≥ 40 kg/m² is evidence-based and accurately reflects peri-operative risk for patients undergoing metabolic surgery. Metabolic Surgery in this group is divided in two techniques. Roux and Y gastric bypass and gastric sleeve.

Living with obesity is linked to metabolic abnormalities heightening the risk of diabetes mellitus type 2, cardiovascular disease or even death^3,4. Therefore, obesity-related complications are an important part of metabolic surgery indications and peri-operative risk assessment. BMI alone is still the most commonly used measure to estimate the degree of obesity and its associated metabolic diseases. Currently, patients with a BMI ≥ 40 kg/m² are assigned the same ASA-PS classification as those with a BMI less than 40 kg/m² along with the presence of concomitant diseases, suggesting a comparable cardio-metabolic vulnerability. Yet, there are different obesity-phenotypes, of which not all patients exhibit metabolic disturbances^5,6. This so called metabolically healthy obesity, is characterized by less visceral adipose tissue and less inflammation, and a better cardiorespiratory fitness than patients with metabolically unhealthy obesity^5,7. It is also associated with a lower risk of developing cardiovascular disease^5,8. Some patients with metabolically healthy obesity will eventually develop metabolic syndrome and become metabolically unhealthy obesity^5,8. Contributing factors are unique per individual and include male sex, a certain genetic profile for fat distribution, sedentary lifestyle, older age, higher total body fat content, and longer exposure to adipose tissue^7–13.

Although the severity of obesity and the presence of obesity-related complications can be independent risk factors for surgical blood loss, wound and urinary tract infections^14–16, increased BMI by itself cannot imply the presence of obesity-related complications, nor can it estimate the occurrence of peri-operative complications^17–20. Also, for anaesthesia, obesity-related peri-operative risks are probably due to metabolic disease, such as myocardial infarction¹⁶, rather than to technical limitations caused by an increased amount of adipose tissue, for example a larger neck circumference²¹ Previous research showed that compared with non-obese patients, ≥ class-II obesity was not associated with higher peri-operative morbidity or mortality after general surgery^14,18,19. However, there is a lack of investigation into the impact of a BMI ≥ 40 mg.kg-1, as per the BMI criterion for a higher ASA-PS classification. Additionally, there is a notable absence of differentiation between groups based on the presence or absence of (chronic) cardiac or metabolic diseases. Furthermore, in a high-volume centre specializing in the care of patients living with obesity, the influence of solely BMI on peri-operative complications may be less significant.

Besides providing insight into the behaviour of the ASA-PS classification following the introduction of the BMI criterion, this study aims to test the hypothesis that ASA-PS III class only based on a BMI ≥ 40 kg/m² exhibit lower complication or comorbidity rates compared to other patients in the same ASA-PS class after metabolic surgery and is thereby not contributing to peri-operative risk stratification.

Data in this retrospective study derived from the Dutch Audit for Treatment of Obesity (DATO)²². DATO is a nationwide mandatory quality register since 2015 that collects detailed information from 20 expertise centers in the Netherlands on patient, comorbidity, treatment, follow-up, and short- and long-term outcome characteristics of patients undergoing metabolic surgery. Details of the DATO regarding data collection, quality, and validation have been described elsewhere²². The DATO’s scientific committee unanimously approved the use of the data for this study (reference number DATO-2022-174), and the study was carried out in accordance with ethical standards for research integrity and the regulations of the Dutch Institute for Clinical Auditing. No informed consent was required because this is an opt-out quality registry and is performed in accordance with the ethical standards of Dutch law. A data analysis and statistical plan was written after the data were accessed.

Figure 1 shows a complete overview of data selection. General selection criteria included: age < 75 years, registered BMI ≥ 40 kg/m² or a BMI ≥ 30 kg/m² with an obesity-related comorbidity, registered ASA-PS classification and primary or first of two-stage metabolic surgery. The included centers follow standardized metabolic surgery and anesthesia practices in alignment with ERAS guidelines²³. Specifically for analyses of ASA-PS classification behaviour: procedures between January 1st 2015 and January 1st 2023. For scoring compliance and morbidity and mortality: complete Charlson Comorbidity Index²⁴ (procedures between January 1st 2018-januari 1st 2023). Patients were categorized by 3 groups based on their BMI level and severity of the concomitant systemic disease: 1) BMI 30–40 kg/m², no/mild systemic diseases (group one), 2) BMI ≥ 40 kg/m², no/mild concomitant systemic diseases (group two), and 3) BMI < 40 kg/m², severe systemic diseases (group three). The latter two groups reflect patients who should have received an ASA-PS score III by the current guideline, the first group an ASA-PS score II. For this study, severe systemic disease encompassed elements of the Charlson Comorbidity Index²⁴. The components: non-metastatic malignancies, dementia, gastric ulcer, paralyses, HIV, connective tissue disease, uncomplicated diabetes mellitus 2 were for this study categorized as mild, along with hypertension, sleep apnea syndrome, gastric esophageal reflux disease, musculoskeletal pain, and dyslipidemia as these conditions all should be well-controlled before availability for surgery.

Mortality was defined as any death related to the registered procedure. Morbidity was defined as any complication to each unique patient case. Additional outcome measures involved the cumulative count of recorded complications, encompassing cases with more than one complication registry: Prevalence of complication severity differentiated by Clavien-Dindo scores, prolonged hospital stays (> 2 days)²⁵, readmission rates and ICU admission rates (planned and unplanned). If available, the prevalence of specifically peri-operative, short-term surgical and general (< 30 days after surgery) and long-term complications (> 30 days after surgery) was analyzed. Table S1 shows an overview of the included complication-details. To test our hypothesis, statistical analyses will be performed between specifically group II and III.

Statistical Analysis

Data of patient characteristics were presented as mean with SD if normally distributed and as median with IQR if not normally distributed. Categorical data were described in an absolute number and or percentage. Differences within group characteristics were analyzed using ANOVA for numerical parametric, Kruskal-Wallis for numerical non-parametric data, and Chi-squared or Fishers’ exact test for binary data. Then, a multivariable regression model was used to analyze statistical differences of morbidity and mortality outcome measures between the group two and three. For analyses that encompasses the total count of complications, potentially confounding variables were included in the model if considered clinically relevant and if a significant difference between groups is observed. Tested confounding variables included age, sex, waist circumference and surgery type. The variables were tested for multicollinearity and effect modification. Group characteristic difference analyses were performed using R 4.2.1. and R studio (version 22.07.02 build 576) software, logistic regression analyses using SPSS software version 29.0.1.0 (171).

In total 75,871 patients (78.8% female) were included in the cohort for analyses of ASA-PS classification distribution. Their mean age was 43.5 (± 11.8) years, BMI 42.9 (± 5.3) kg/m² and waist circumference 128.7 (± 13.5) cm. Of these patients, 23% underwent Sleeve gastrectomy, 77% Roux-en-Y Gastric Bypass.

Distribution ASA-PS classification behaviour

In 2015, 77% of the patients with a BMI < 40 kg/m² and 70% with a BMI ≥ 40 kg/m² received ASA-PS II (Fig. 2). These numbers decreased over time towards respectively 35% and 7% in 2022. Simultaneously, the number of patients scoring ASA-PS III increased for both BMI < 40 kg/m² and BMI ≥ 40 kg/m² levels (respectively 20% and 26% in 2015 and to 64% and 92% in 2022). These changes were most pronounced between 2015 and 2018.

The proportion of patients undergoing metabolic surgery with a BMI ≥ 40 kg/m² decreased from 75.7% in 2015 to 68.6% in 2022. Patient characteristics remained largely consistent: in 2015, the average BMI was 43.5 ± 5.4 kg/m² and the average age was 43.1 ± 11 years, while in 2022 their values were 42.6 ± 5.2 kg/m² and 43.3 ± 12.3 years respectively. Among patients with a BMI < 40 kg/m², the prevalence of the sum of severe concomitant systemic disease decreased from 53% in 2018 to 26% in 2022 (Figure S1 for details).

ASA-PS classification compliance

In total 28,597 patient cases were available for this analysis (Fig. 1). From 2018 to 2023, an increased number of patients were assigned correctly ASA-PS III (averagely 77% in 2015 and 86.7% in 2022). Likewise, a decrease was encountered for patients receiving correctly ASA-PS II (averagely 41% in 2018 and 34% in 2022). The number of patients who received correctly ASA-PS III based on BMI ≥ 40 kg/m² was even higher than the patients who received this on the basis of underlying severe systemic disease (in 2022 93% versus 74%) (Fig. 3).

Morbidity and mortality

Characteristics of the subgroups are shown in Table 1, the prevalence of morbidity and mortality in Table 2. The groups differed statistically from each other regarding age, waist circumference, BMI level, sex, and type of surgery (all p < 0.05). Of patients with none/mild systemic disease, regardless of BMI level, the number of patients with a complication was generally equal and even slightly lower for patients with BMI ≥ 40 kg/m² (5.9% and 5.45%). This complication prevalence was highest for patients from group three (8.74%). This group had also the highest number of patients with > one registered complications.

After adjustment for confounding variables, patients from group two were significantly less likely to have a complicated course than patients from group three (OR 1.72 95% CI 1.43–2.08, p < 0.001). From the sum of all registered complications, this latter group were also significantly less likely to have a complication scored CD-II or III, a planned ICU-admission, a prolonged hospital admission and to be readmitted on short- and long-term. Information regarding specific surgical and general complications was not available for 118 out of 1093 records for patients from group two and for 11 out of 265 records for patients from group three. Analyses of these specified complications indicated a lower likelihood for patients from group two to experience short- and long-term peri-operative and surgical complications.

Table 1

**Patient characteristics of the three groups and the group differences.** Data presented as mean ± SD, or median [IQR].
BMI level (kg/m²) Systemic disease	I 30–40 None/Mild	II ≥ 40 None/Mild	III < 40 Severe	p-value
Patients (n) Age (yrs) Waist circumference (cm) BMI (kg/m²) Gender (female) Roux-en-Y-gastric bypass (%)	7887 46.4 ± 10.8 120.8 ± 9.8 37.6 [36.4–39.0] 78.6 81.7	18068 44.5 ± 12.9 132.4 ± 13.7 43.4 [41.4–46.7] 77.0 77.6	2642 50.2 ± 10.1 122.4 ± 9.4 37.5 [36.2–38.8] 71.5 81.9	p < 0.05 P < 0.05 P < 0.05 P < 0.05 P < 0.05

Table 2

**Group differences morbidity and mortality.** Results of statistical analysis are adjusted for confounding variables and presented in odds ratio (OR), 95% confidence interval (CI), * = significant results, UA = unadjusted result. # Analysis in case of available data.
Group	I	II	III	II vs III: OR 95% CI, p-value
Patients with complicated case Patients with > 1 complication	5.9 6.9	5.45 8.43	8.74 11.3	1.72 [1.43–2.08], p < 0.001*
Clavien-Dindo score I II III ICU admission Planned Unplanned Mortality Prolonged hospital stay (> 2) Readmission Short term Long term	1.18 1.47 3.05 0.13 0.25 0.03 3.98 2.26 1.50	1.14 1.67 2.67 0.23 0.17 0.01 3.60 2.26 1.32	1.59 3.23 4.13 0.57 0.38 0.04 7.34 3.41 1.86	1.56 [1.01–2.42] p = 0.05 1.79 [1.31–2.44] p < 0.001* 1.82 [1.41–2.34] p < 0.001* 2.61 [1.18–5.76] p = 0.02* 2.00 [0.81–4.91] p = 0.13 1.36 [0.16–11.64] p = 0.78 (UA) 1.97 [1.60–2.43] p < 0.001* 1.76 [1.30–2.37] p < 0.001* 1.40 [1.03–1.91] p = 0.03*
Peroperative Short term Surgical# General# Long term	0.46 4.18 3.39 0.88 1.81	0.33 3.61 2.76 0.85 1.51	0.83 6.70 5.33 1.55 2.50	2.43 [1.28–4.62] P < 0.001* 1.80 [1.45–2.23] p < 0.001* 1.86 [1.46–2.37] p < 0.001* 1.58 [0.99–2.50] p = 0.05 1.64 [1.20–2.25] p = 0.002*

This study revealed that patients undergoing metabolic surgery in the Netherlands from 2015 to 2023 were, irrespective of their BMI level, increasingly assigned an ASA-PS III score. Furthermore, it showed that in this selected populations, patients with a BMI ≥ 40 kg/m² without severe systemic disease, showed a significantly lower likelihood of experiencing surgical complications during their metabolic surgery -trajectory compared to patients with a BMI < 40 kg/m² and severe systemic disease. Additionally, they exhibited reduced probabilities of prolonged hospital stay, readmission, planned, and unplanned ICU admission. These results confirmed our hypothesis.

ASA-PS classification is a simple and fast way to gain insight into the patient's health status and estimate postoperative risk²⁶. The increase in ASA-PS III scores can largely be attributed to changes in the guidelines: Without taking into consideration co-morbidity or physical status, patients with a BMI ≥ 40 kg/m² switched from score II to III. Notably, also patients with a BMI < 40 kg/m² increasingly received an ASA-PS III. We have objectified that between 2018–2023, patients were incorrectly assigned an ASA-PS III score in 60–70% of cases. These patients had a BMI below 40 kg/m² and no severe concomitant systemic diseases. Even before 2018, the population did not suffer from concomitant diseases more frequently, which may suggest that the unjustified allocation had started earlier. Although the ASA-PS classification guidelines have been made as objective as possible, it is well known that it remains subject to interpretation^27–29. We hypothesize that by raising awareness about criteria for determining an ASA-PS classification through the introduction of a new guideline, stricter assessments and subjective up-coding leading to higher scores has occurred. Although the context differs, this effect bears some resemblance to the Hawthorne effect in terms of behavioural changes in response to external factors³⁰.

The presence of a serious systemic disease appeared to have a greater impact on the occurrence of complications during a metabolic surgery procedure compared to a BMI > 40 kg/m². Although both are categorized as ASA-PS III, the operative risk profiles as assessed by the ASA-PS classification were not identical. Considering the observation that the rate of complications among patients with a BMI > 40 kg/m² closely mirrored that of patients with a lower BMI, it indicates insufficient evidence to support the BMI-criterion in our study population. Using BMI > 40 kg/m² as sole criterion for ASA-PS III classification undermines the efficacy of risk stratification in clinical practice. Accurate ASA-PS classification at the individual level, would guarantee more precise allocation of hospital resources, better decisions regarding inpatient or outpatient surgery and timing of hospital discharge, and facilitating the delivery of personalized care efficiently while maintaining safety standards. Moreover, incorrect ASA-PS classification can result in research outcomes or quality evaluations that present a skewed perspective, rendering them unsuitable for reliable interpretation and implementation in clinical settings. Previous research in patients undergoing metabolic surgery showed a minimal difference in complications between ASA-PS II and ASA-PS III patients³¹. This could be explained by the fact that the results were based on the assigned ASA-PS classification and not on the actual underlying disease thereby reflect clinical practice inaccurately.

BMI has frequently faced criticism as a metric of obesity severity or the risk of comorbidities or complications^32–34. Instead, it is rather a static representation of a patient's body composition at a given point in time, while for example, long-term exposure to excess adipose tissue appears to be crucial in the development of obesity-related complications^35–38. Therefore, the inquiry arises regarding the suitability of the BMI metric for assessing health status altogether. Functional status or cardiorespiratory fitness, for instance a Metabolic Equivalent Score, is also widely used in preoperative screenings. It is a measure the amount of oxygen the body consumes during an activity compared to a resting state³⁹ and can be used to estimate whether the patient is able to withstand the physical demands of surgery regardless of age, body composition, or comorbidities⁴⁰. Functional status is correlated logically with the ASA-PS classification, as illness could lead to immobility and vice versa, however functional status has also been shown to be a better predictor than the ASA-PS classification itself³¹. It would be interesting to further explore the effect of incorporating functional status to the ASA-PS classification, instead of or in addition to BMI, as there are already indications that the latter is an independent predictor of mortality within each ASA-PS classification⁴¹.

The recategorization of patients for concurrent systemic diseases was dependent on a diagnosis code, which was not always distinctive in terms of severity of the condition. This could be seen as a limitation of the study. For instance, hypertension encompassed both treated stable hypertension and untreated unstable hypertension, posing a potential risk of underestimating research results. Nonetheless, this method still remains closely aligned with the current clinical practice, where the ASA-PS classification is not entirely comprehensive. In addition, information regarding smoking status or alcohol use was absent. Although these factors may not directly differentiate between ASA-PS classifications, they should be taken into account as they may increase the risk of postoperative complications independently^42,43. Due to the limited number of cases with multiple complication registrations, a multilevel analysis was not performed. This could lead to repeated-measure bias, affecting the accuracy of effect size estimation. Notably, both the highest complication prevalence and majority of patients who experienced more than one complication were identified in the subgroup characterized by BMI < 40 kg/m² and severe systemic disease. Given the direct link between repeated measurements and complications in this context, the exclusion from multilevel analysis can be considered less problematic.

This study was strengthened by the large size of the cohort. The results are representative of Dutch clinical practice and may also be applicable to other countries with similar patient populations levels of surgical selection or preparation and, importantly, a comparable healthcare system. The homogeneity of the cohort reduces the risk of bias in this setting. We suggest that future research should investigate the relationship between increasing BMI and peri-operative complications to determine whether different BMI thresholds may offer greater accuracy. Additionally, although beyond the scope of this study, exploring other metrics that might serve as better indicators of health status and risks for patients living with obesity would be valuable to consider. Also, our hypothesises could be examined in other cohorts undergoing metabolic surgery or in patients undergoing other surgery types.

We conclude that due to changed guidelines, patients undergoing metabolic surgery in the Netherlands were increasingly assigned ASA-PS III, irrespective of BMI or concomitant systemic disease suggesting subjective upcoding. However, patients from this ASA-PS class based on solely a BMI ≥ 40 kg/m² were less likely to have a complicated course after metabolic surgery than patients with a severe systemic disease. This finding suggests a devaluation of the value of ASA-PS in peri-operative risk stratification in patients living with obesity by sticking to a sole rigid criterium. It has significant implications for enhancing peri-operative resource utilization and improving efficiency without compromising safety. Correcting the overestimation of ASA-PS classification in patient assessments can not only contribute to more accurate risk stratification but also improve the overall validity of research results incorporating ASA-PS classification.

Conflict of interests:

The authors have no conflicts of interest to declare

Informed consent:

No informed consent was required because this is an opt-out quality registry and is performed in accordance with the ethical standards of Dutch law

Ethics approval:

The DATO’s scientific committee unanimously approved the use of the data for this study (reference number DATO-2022-174), and the study was carried out in accordance with ethical standards for research integrity and the regulations of the Dutch Institute for Clinical Auditing

Financial disclosure

This study did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors, and the authors have no financial interests related to it.

Author Contribution

E:E conceptualization, data curation, methodology, investigation, formal analysis, writing- original draft preparation. SN: conceptualization, writing - review & editing, supervision. AB: conceptualization, writing - review & editing, supervision. MB: conceptualization, writing - review & editing, supervision. DATO group: resources, data curation

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The role of the BMI ≥40 kg/m² criterium in ASA-PS classification for metabolic surgery

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Background

Methods

Statistical Analysis

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Distribution ASA-PS classification behaviour

ASA-PS classification compliance

Morbidity and mortality

Discussion

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