The utility of 6-minute walk distance in predicting waitlist mortality for lung transplant candidates

Profile of patients referred for lung transplant and their transplant-free survival

INTRODUCTION

Lung transplant (LTx) is a potential treatment option for all patients with chronic, end-stage respiratory disease, who are refractory to optimal medical therapy or where no medical therapy exists. In India, LTx is still in its evolving stages and published literature is sparse. The current study was carried out to study the selection criteria for lung transplant and to evaluate the clinical and socio-economic profile of patients referred for the same at a tertiary health care facility.

METHODS

The study was a descriptive, prospective, observational study. All adults referred for lung transplant were evaluated for clinical and laboratory profiles. All enrolled patients were assessed for presence of referral criteria, listing criteria, contraindications, and willingness for lung transplant. These patients were followed up for 2 years for transplant-free survival, and the Cox proportional hazards model was used to determine independent predictors of all-cause mortality.

RESULTS

A total of 103 were included in study. The most common diagnosis was interstitial lung disease (57.2%), followed by bronchiectasis (17.5%) and COPD (13.6%). Most patients were referred for LTx at an advanced stage as 90% met listing criteria. Fifty-four (52.4%) patients had an absolute or relative contraindication to transplant; however, the majority of those contraindications were modifiable. Patients with a lower socio-economic status were less likely to be willing for LTx. The median survival was 757 days. A 6-minute walk distance (6MWD) lesser than 250 m was found to be an independent predictor of mortality.

CONCLUSION

Making patients aware about lung transplant early in their treatment may give them sufficient time to come to terms with their disease and understand the risk and benefits associated. Efforts should be focused on screening and early treatment of reversible contraindications for the eligible patients. Patients with 6MWD < 250 m are at increased risk of mortality.

In-Person Versus Remote 6-Minute Walk and Incremental Shuttle Walk Distances in Advanced Lung Disease

BACKGROUND

Field-based walk tests conducted remotely may provide an alternative method to a facility-based assessment of exercise capacity for people with advanced lung disease. This prospective study evaluated the level of agreement in the distance walked between a 6-min walk test (6MWT) and an incremental shuttle walk test performed by using standard in-person procedures and test variations and settings.

METHODS

Adults with advanced lung disease underwent 4 study visits: (i) one in-person standard 6MWT (30-m corridor) and one in-person treadmill 6MWT, (ii) a remote 6MWT in a home setting (10-m corridor), (iii) 2 in-person standard incremental shuttle walk tests (10-m corridor), and (iv) a remote incremental shuttle walk test in a home setting (10-m corridor). A medical-grade oximeter measured heart rate and oxygen saturation before, during, and for 2 min after the tests.

RESULTS

Twenty-eight participants were included (23 men [82%]; 64 (57-67) y old; 19 with interstitial lung disease [68%] and 9 with COPD [32%]; and 26 used supplemental oxygen (93%) [exertional [Formula: see text] of 0.46 ± 0.1]). There was no agreement between the tests. Greater walking distances were achieved with standard testing procedures: in-person 6MWT versus treadmill 6MWT (355 ± 68 vs 296 ± 97; P = .001; n = 28), in-person 6MWT versus remote 6MWT (349 ± 68 vs 293 ± 84; P = .001; n = 24), and in-person incremental shuttle walk test versus remote incremental shuttle walk test (216 ± 62 vs 195 ± 63; P = .03; n = 22).

CONCLUSIONS

Differences in the distance walked may have resulted from different track lengths, widths, and walking surfaces. This should be considered in test interpretation if tests are repeated under different conditions.

Reproducibility of the 6-minute walk test in lung transplant recipients

PURPOSE

There is reason to believe that the favorable measurement properties of the 6‑minute walk test (6MWT) reported for retest reliability and its capability to detect a true change in healthy individuals or persons with chronic respiratory disease may not apply to lung transplant recipients (LuTXr). We therefore investigated retest reliability of the 6MWT and, in addition, made an attempt to explore whether the 6MWT was sensitive enough to detect important changes that occur with postacute rehabilitation in LuTXr after first time LuTX.

METHODS

Immediately before postacute rehabilitation, 50 LuTXr completed 6MWT testing twice, separated by 1-2 workdays (retest reliability), and were reassessed after completion of rehabilitation 2 months later (sensitivity to changes). Body function measures and health-related quality of life (HRQoL) assessments were collected at baseline.

RESULTS

Baseline retest 6‑minute walk distance (6MWD) and the age-related predicted walking distance (6MWD%pred) scores significantly increased before postacute rehabilitation. The intraclass coefficient of correlation ICC of the 6MWD was 0.93 (95% confidence interval, CI: 0.88-0.96) and its smallest real difference (SRD) 79 m (95% CI: 52;107). Receiver operating curve analyses revealed the rehabilitation associated changes in 6MWD/6MWD%pred to exceed the SRD/SRD% values in a highly accurate way.

CONCLUSION

The 6MWT overall represents a reliable functional performance tool in LuTXr that is sensitive to detect changes in physical performance as a result of medical postacute rehabilitation.

Relationship of Exercise Capacity, Physical Function, and Frailty Measures With Clinical Outcomes and Healthcare Utilization in Lung Transplantation: A Scoping Review

Measures of exercise capacity, frailty, and physical function are commonly used in lung transplant candidates and recipients to evaluate their physical limitations and the effects of exercise training and to select candidates for transplantation. It is unclear how these measures are related to clinical outcomes and healthcare utilization before and after lung transplantation. The purpose of this scoping review was to describe how measures of exercise capacity, physical function, and frailty are related to pre- and posttransplant outcomes.

Limitations of the dichotomized 6-minute walk distance when computing lung allocation score for cystic fibrosis: a 16-year retrospective cohort study

PURPOSE

The 2010 Lung Allocation Score (LAS) version considers the estimated survival benefit offered by lung transplantation (LTx) and uses 6-minute Walk Test (6MWT) distance as a dichotomous covariate of whether an individual can walk more than 150 ft or 45.7 m in 6 min. This study aimed to provide evidence that 6MWT gives no clinically meaningful information to be used in the current LAS for candidates to LTx with cystic fibrosis (CF).

MATERIALS AND METHODS

We collected data from 6MWTs performed since 2003 at our CF centre. A joint model was fitted to describe the effect of changes in walked distance on the hazard of LTx or death.

RESULTS

Up to 2019, 552 6MWTs were performed on 163 individuals with CF. None of the individuals included walked for less than 45.7 m during the 6MWT. Based on the joint modelling, the association of walked distance with the hazard ratio (HR) of LTx or death was significant (HR 0.99, 95% Credible Interval [CI]: 0.99 to 1.00).

CONCLUSIONS

When adopted dichotomously for LAS calculation, walked distance does not add any useful information about exercise capacity. Longitudinal trajectories of walked distance may provide complementary information about prognosis in individuals with CF.Implications for rehabilitationDichotomized walked distance does not contribute to lung allocation score in candidates to lung transplantation with cystic fibrosisChanges in the longitudinal trajectory of walked distance can be clinically meaningful for prognostication.Sensitive outcomes to be incorporated in the lung allocation scoring system for individuals with CF are yet needed to catch rapid falls in functional capacity.

Predicting Adverse Events During Six-Minute Walk Test Using Continuous Physiological Signals

Background and Objective: The 6-min walk test (6MWT) is a common functional assessment test, but adverse events during the test can be potentially dangerous and can lead to serious consequences and low quality of life. This study aimed to predict the occurrence of adverse events during 6MWT, using continuous physiological parameters combined with demographic variables.

Methods: 578 patients with respiratory disease who had performed standardized 6MWT with wearable devices from three hospitals were included in this study. Adverse events occurred in 73 patients (12.6%). ECG, respiratory signal, tri-axial acceleration signals, oxygen saturation, demographic variables and scales assessment were obtained. Feature extraction and selection of physiological signals were performed during 2-min resting and 1-min movement phases. 5-fold cross-validation was used to assess the machine learning models. The predictive ability of different models and scales was compared.

Results: Of the 16 features selected by the recursive feature elimination method, those related to blood oxygen were the most important and those related to heart rate were the most numerous. Light Gradient Boosting Machine (LightGBM) had the highest AUC of 0.874 ± 0.063 and the AUC of Logistic Regression was AUC of 0.869 ± 0.067. The mMRC (Modified Medical Research Council) scale and Borg scale had the lowest performance, with an AUC of 0.733 and 0.656 respectively.

Conclusion: It is feasible to predict the occurrence of adverse event during 6MWT using continuous physiological parameters combined with demographic variables. Wearable sensors/systems can be used for continuous physiological monitoring and provide additional tools for patient safety during 6MWT.

Extracorporeal Membrane Oxygenation as a Bridge to Lung Transplant

Extracorporeal membrane oxygenation (ECMO) is a cardiopulmonary technology capable of supporting cardiac and respiratory function in the presence of end-stage lung disease. Initial experiences using ECMO as a bridge to lung transplant (ECMO-BTLT) were characterized by high rates of ECMO-associated complications and poor posttransplant outcomes. More recently, ECMO-BTLT has garnered success in preserving patients' physiologic condition and candidacy prior to lung transplant due to technological advances and improved management. Despite recent growth, clinical practice surrounding use of ECMO-BTLT remains variable, with little data to inform optimal patient selection and management. Although many questions remain, the use of ECMO-BTLT has shown promising outcomes suggesting that ECMO-BTLT can be an effective strategy to ensure that complex and rapidly decompensating patients with end-stage lung disease can be safely transplanted with good outcomes. Further studies are needed to refine and inform practice patterns, management, and lung allocation in this high-risk and fragile patient population.

Lung transplantation: candidate selection and timing of transplant

PURPOSE OF REVIEW

Over the past two decades, lung transplant has become the mainstay of treatment for several end-stage lung diseases. As the field continues to evolve, the criteria for referral and listing have also changed. The last update to these guidelines was in 2014 and several studies since then have changed how patients are transplanted. Our article aims to briefly discuss these updates in lung transplantation.

RECENT FINDINGS

This article discusses the importance of early referral of patients for lung transplantation and the concept of the 'transplant window'. We review the referral and listing criteria for some common pulmonary diseases and also cite the updated literature surrounding the absolute and relative contraindications keeping in mind that they are a constantly moving target. Frailty and psychosocial barriers are difficult to assess with the current assessment tools but continue to impact posttransplant outcomes. Finally, we discuss the limited data on transplantation in acute respiratory distress syndrome (ARDS) due to COVID19 as well as extracorporeal membrane oxygenation bridge to transplantation.

SUMMARY

The findings discussed in this article will strongly impact, if not already, how we select candidates for lung transplantation. It also addresses some aspects of lung transplant such as frailty and ARDS, which need better assessment tools and clinical data.

Changes in 6-minute walking distance in lung transplant candidates while participating in a home-based pre-habilitation program-A retrospective chart review

Home-based exercise programs may offer a less costly alternative to providing exercise pre-transplant to a large number of patients. We describe the changes in 6-minute walk distance (6MWD) in lung transplant candidates who participated in a home-based exercise program and their relationship to post-transplant outcomes. Retrospectively, we investigated 159 individuals while awaiting transplantation who performed the surgery between 2011 and 2015. Primary outcome was 6MWD at time of assessment for transplant, last test prior to transplant and one-month post-transplant. 6MWD decreased by 28 ± 93.9 m between the time of assessment and the last 6MWD prior to transplantation (P < .001). Forty-one patients (25.8%) increased their 6MWD (mean + 85.8 ± 42.8 m); 72 patients (45.3%) decreased their 6MWD (mean -109.8 ± 71.2 m); and 46 patients (28.9%) had no change in 6MWD (-1.5 ± 15.7 m). There was a moderate correlation (r = .528; P < .001) between the last 6MWD prior to transplant and 6MWD post-transplant. Change in 6MWD prior to transplant weakly correlated with length of time on mechanical ventilation (r = -.185; P = .034). When adjusted for covariates, change in 6MWD prior to transplant was not associated with length of time on mechanical ventilation, total hospital LOS, or intensive care unit LOS. The majority of the patients were able to either increase or maintain their 6MWD while participating in a home-based pre-habilitation program during the waiting list period. Prospective research is needed to evaluate the effects of home-based pre-habilitation program for lung candidates.

A Comparison of Frailty Measures at Listing to Predict Outcomes After Lung Transplantation

BACKGROUND

Sarcopenia may be an important predictive factor of outcomes after lung transplantation (LTx). Serum albumin and the 6-minute walk distance (6MWD) have been shown to be a marker of LTx outcomes. We measured sarcopenia, albumin, and 6MWD in a cohort of LTx patients and analyzed the utility of these as markers of outcomes for LTx patients.

METHODS

We retrospectively identified LTx recipients from 2013-2018 at our institution who underwent computed tomographic imaging during their listing evaluation. From that image, we measured skeletal muscle cross-sectional surface area at the third lumbar vertebral level, and sarcopenia was diagnosed by established cutoffs. Associations between sarcopenia, albumin, 6MWD, and survival, and hospital length of stay, complications, readmissions, and discharge destination were evaluated.

RESULTS

Sarcopenia was found in 72% (95 of 132) of patients, 18% (24 of 131) of patients were hypoalbuminemic, and 41% had a low 6MWD. Survival was not associated with presence of sarcopenia (hazard ratio [HR], 1.06; 95% confidence interval [CI], 0.46-2.42) or low 6MWD (HR, 0.86; 95% CI, 0.410-1.83). Hospital length of stay, complications, readmissions, and discharge destination were not influenced by sarcopenia or 6MWD. In contrast, hypoalbuminemia was independently associated with decreased survival (HR, 2.25; 95% CI, 1.04-4.85) and a higher grade of postoperative complications (P = .04).

CONCLUSIONS

Sarcopenia is prevalent in LTx patients. Neither sarcopenia nor 6MWD predicted mortality or short-term outcomes after LTx. This is in contrast to albumin levels, which were inversely associated with survival and complications. Albumin shows promise as an important predictor of mortality and short-term outcomes after LTx.

Six-Minute Walk Test: Clinical Role, Technique, Coding, and Reimbursement

The 6-min walk test (6MWT) is a commonly used test for the objective assessment of functional exercise capacity for the management of patients with moderate-to-severe pulmonary disease. Unlike pulmonary function testing, the 6MWT captures the often coexisting extrapulmonary manifestations of chronic respiratory disease, including cardiovascular disease, frailty, sarcopenia, and cancer. In contrast with cardiopulmonary exercise stress testing, this test does not require complex equipment or technical expertise. In this low complexity, safe test, the patient is asked to walk as far as possible along a 30-m minimally trafficked corridor for a period of 6 min with the primary outcome measure being the 6-min walk distance (6MWD) measured in meters. There has been interest in other derived indexes, such as distance-desaturation product (the product of nadir oxygen saturation and walk distance), which in small studies has been predictive of morbidity and mortality in certain chronic respiratory conditions. Special attention to methodology is required to produce reliable and reproducible results. Factors that can affect walk distance include track layout (continuous vs straight), track length, oxygen amount and portability, learning effect, and verbal encouragement. The absolute 6MWD and change in 6MWD are predictive of morbidity and mortality in patients with COPD, pulmonary arterial hypertension, and idiopathic pulmonary fibrosis and patients awaiting lung transplant, highlighting its use in management decisions and clinical trials. As of January 2018, Current Procedural Terminology code 94620 (simple pulmonary stress test) has been deleted and replaced by two new codes, 94617 and 94618. Code 94617 includes exercise test for bronchospasm including pre- and postspirometry, ECG recordings, and pulse oximetry. Code 94618, pulmonary stress testing (eg, 6MWT), includes the measurement of heart rate, oximetry, and oxygen titration when performed. If 94620 is billed after January 2018 it will not be reimbursed.

Predictive Utility of Lung Allocation Score for Retransplantation Outcomes

BACKGROUND

Treatment of primary graft failure after lung transplantation (LTx) may include retransplantation (rLTx). The number of rLTx cases has doubled since implementation of the Lung Allocation Score in 2005. The Lung Allocation Score was intended to predict LTx outcomes, but its predictive utility has not been assessed in rLTx. We investigated whether 1-year outcomes of LTx and rLTX were equally well predicted by the Lung Allocation Score.

METHODS

Recipients of LTx and rLTx aged 18 years or more were identified in 2005 to 2015 United Network for Organ Sharing data. The Lung Allocation Score was entered in multivariable logistic regression models of 1-year retransplant-free survival. Areas under the receiver-operating characteristics curve summarized model predictive value. We examined whether the Lung Allocation Score and its components were differentially associated with outcomes of LTx and rLTx.

RESULTS

There were 16,837 LTx and 765 rLTx cases meeting inclusion criteria. Crude 1-year retransplant-free survival rates were 86% after LTx compared with 74% after rLTx. On univariate analysis, both LTx and rLTx cohorts showed poor predictive utility of the Lung Allocation Score (area under the curve 0.55 and 0.57, respectively; difference by transplant type, p = 0.307). Neither the Lung Allocation Score nor its components was differentially associated with LTx compared with rLTx outcomes.

CONCLUSIONS

The Lung Allocation Score achieved comparable, but poor, predictive utility for 1-year outcomes of primary LTx and rLTx. We found no evidence that Lung Allocation Score components should be weighted differently for rLTx candidates.