Cardiopulmonary exercise testing in lung transplantation: a review

The role of the microcirculation and integrative cardiovascular physiology in the pathogenesis of ICU-acquired weakness

Skeletal muscle dysfunction after critical illness, defined as ICU-acquired weakness (ICU-AW), is a complex and multifactorial syndrome that contributes significantly to long-term morbidity and reduced quality of life for ICU survivors and caregivers. Historically, research in this field has focused on pathological changes within the muscle itself, without much consideration for their in vivo physiological environment. Skeletal muscle has the widest range of oxygen metabolism of any organ, and regulation of oxygen supply with tissue demand is a fundamental requirement for locomotion and muscle function. During exercise, this process is exquisitely controlled and coordinated by the cardiovascular, respiratory, and autonomic systems, and also within the skeletal muscle microcirculation and mitochondria as the terminal site of oxygen exchange and utilization. This review highlights the potential contribution of the microcirculation and integrative cardiovascular physiology to the pathogenesis of ICU-AW. An overview of skeletal muscle microvascular structure and function is provided, as well as our understanding of microvascular dysfunction during the acute phase of critical illness; whether microvascular dysfunction persists after ICU discharge is currently not known. Molecular mechanisms that regulate crosstalk between endothelial cells and myocytes are discussed, including the role of the microcirculation in skeletal muscle atrophy, oxidative stress, and satellite cell biology. The concept of integrated control of oxygen delivery and utilization during exercise is introduced, with evidence of physiological dysfunction throughout the oxygen delivery pathway - from mouth to mitochondria - causing reduced exercise capacity in patients with chronic disease (e.g., heart failure, COPD). We suggest that objective and perceived weakness after critical illness represents a physiological failure of oxygen supply-demand matching - both globally throughout the body and locally within skeletal muscle. Lastly, we highlight the value of standardized cardiopulmonary exercise testing protocols for evaluating fitness in ICU survivors, and the application of near-infrared spectroscopy for directly measuring skeletal muscle oxygenation, representing potential advancements in ICU-AW research and rehabilitation.

Lung transplantation for COPD/pulmonary emphysema

COPD and α-1 antitrypsin deficiency emphysema remain one of the major indications for lung transplantation. If all other treatment possibilities are exhausted or not possible (including rehabilitation, oxygen therapy, noninvasive ventilation, lung volume reduction), patients may qualify for lung transplantation. Strict selection criteria are implemented with a lot of relative and absolute contraindications. Because of an ongoing donor shortage, only a minority of endstage COPD patients will finally get transplanted. The procedure may involve a single or a double lung transplantation, dependent on the experience of the centre, the waiting list, the availability of donor lungs and the patient's risk-benefit ratio. In general, the life expectancy as well as the health-related quality of life after lung transplantation for COPD are usually increased, and may be somewhat better after double compared with single lung transplantation. Several specific complications can be encountered, such as the development of solid organ cancer and chronic lung allograft dysfunction, which develops in up to 50% of patients within 5 years of their transplant and has a major impact on long-term survival, because of the current inefficient treatment modalities.

The importance of physical performance in the assessment of patients on haemodialysis: A survival analysis

Background

Physical performance is an important determinant of quality of life in patients on haemodialysis. An association between physical performance and survival could further enhance the importance of physical performance. We aimed to assess the association between different measures of physical performance and survival in dialysis patients.

Methods

117 patients on haemodialysis were included from December 2016 and followed up to September 2020. Muscle strength (quadriceps, handgrip strength, and sit-to-stand), exercise capacity (six-minute walking test, 6MWT) and the risk of falls (Dialysis Fall Index, Tinetti, and Frailty and Injuries: Cooperative Studies of Intervention Techniques) were measured at the time of inclusion. Hospitalisation, morbidity (Davies Stoke index) and death were recorded. Data were analysed by least squares linear regression models and competing risks survival hazard models.

Results

During the observation period (median 33, min 30 max 45 months), 45 patients died (= 38.5%), resulting in a mortality rate of 15% per year. Cardiovascular disease (42.9%) was the most common cause of death. All domains of physical performance were associated with mortality, with the highest hazards for an increased risk of falls (Hazard Ratio (HR) = 20.4, p = 0.003) and poor exercise capacity (HR = 7.4, p<0.001). A score lower than 298 meters (specificity = 0.583; sensitivity = 0.889) on the 6MWT was established as a haemodialysis-specific cut-off point for mortality risk. Each increase in 6MWT (m) corresponded with a 0.4% decrease in mortality risk (HR = 0.996, 95%CI [0.994; 0.998]). The 6MWT as also associated with comorbidity (F-value = 6.1, p = 0.015). Physical performance was not associated with hospitalisation.

Conclusions

The 6MWT is associated with mortality in patients on haemodialysis and can be considered as a valid assessment tool to identify high-risk patients.

Near-normal aerobic capacity in long-term survivors after lung transplantation

The clinical course of lung transplantation (LT) is diverse: some patients present chronic lung allograft dysfunction (CLAD) and progressive decline in pulmonary function, but others maintain normal spirometric values and active lives.

This is a multicentre study reporting, for the first time, near-normal peak V′_O2 values during cardiopulmonary exercise testing and normal exercise capacity in long-term lung transplant recipients without CLADhttps://bit.ly/35ftce3

Dyspnea, effort and muscle pain during exercise in lung transplant recipients: an analysis of their association with cardiopulmonary function parameters using machine learning

Background

Despite improvement in lung function, most lung transplant (LTx) recipients show an unexpectedly reduced exercise capacity that could be explained by persisting peripheral muscle dysfunction of multifactorial origin. We analyzed the course of symptoms, including dyspnea, muscle effort and muscle pain and its relation with cardiac and pulmonary function parameters during an incremental exercise testing.

Methods

Twenty-four bilateral LTx recipients were evaluated in an observational cross-sectional study. Recruited patients underwent incremental cardio-pulmonary exercise testing (CPET). Arterial blood gases at rest and peak exercise were measured. Dyspnea, muscle effort and muscle pain were scored according to the Borg modified scale. Potential associations between the severity of symptoms and exercise testing parameters were analyzed using a Forest-Tree Machine Learning approach, which accomplishes for a ratio between number of observations and number of screened variables less than unit.

Results

Dyspnea score was significantly associated with maximum power output (WR, watts), and minute ventilation (VE, L/min) at peak exercise. In a controlled subgroup analysis, dyspnea score was a limiting symptom only in LTx recipients who reached the higher levels of WR (≥ 101 watts) and V_E (≥ 53 L/min). Muscle effort score was significantly associated with breathing reserve as percent of maximal voluntary ventilation (BR%MVV). The lower the BR%MVV at peak exercise (< 32) the higher the muscle effort perception. Muscle pain score was significantly associated with VO₂ peak, arterial [HCO₃⁻] at rest, and V_E/VCO₂ slope. In a subgroup analysis, muscle pain was the limiting symptom in LTx recipients with a lower VO₂ peak (< 15 mL/Kg/min) and a higher V_E/VCO₂ slope (≥ 32).

Conclusions

The majority of our LTx recipients reported peripheral limitation as the prevalent reason for exercise termination. Muscle pain at peak exercise was strictly associated with basal and exercise-induced metabolic altered pathways. The onset of dyspnea (breathing effort) was associated with the intensity of ventilatory response to meet metabolic demands for increasing WR. Our study suggests that only an accurate assessment of symptoms combined with cardio-pulmonary parameters allows a correct interpretation of exercise limitation and a tailored exercise prescription. The role and mechanisms of muscle pain during exercise in LTx recipients requires further investigations.

The Utility of the Sit-to-Stand Test for Inpatients in the Acute Hospital Setting After Lung Transplantation

OBJECTIVE

Measurement of physical function is important to guide physical therapy for patients post-lung transplantation (LTx). The Sit-to-Stand (STS) test has proven utility in chronic disease, but psychometric properties post-LTx are unknown. The study aimed to assess reliability, validity, responsiveness, and feasibility of the 60-second STS post-LTx.

METHODS

This was a measurement study in 62 inpatients post-LTx (31 acute postoperative; 31 medical readmissions). Interrater reliability was assessed with 2 STS tests undertaken by different assessors at baseline. Known group validity was assessed by comparing STS repetitions in postoperative and medical groups. Content validity was assessed using comparisons to knee extensor and grip strength, measured with hand-held dynamometry. Criterion validity was assessed by comparison of STS repetitions and 6-minute walk distance postoperatively. Responsiveness was assessed using effect sizes over inpatient admission.

RESULTS

Median (interquartile range) age was 62 (56-67) years; time post-LTx was 5 (5-7) days postoperative and 696 (244-1849) days for medical readmissions. Interrater reliability was excellent (intraclass correlation coefficient type 2,1 = 0.96), with a mean learning effect of 2 repetitions. Repetitions were greater for medical at baseline (mean 18 vs 8). More STS repetitions were associated with greater knee extensor strength (postoperative r = 0.57; medical r = 0.47) and 6-minute walk distance (postoperative r = 0.68). Effect sizes were 0.94 and 0.09, with a floor effect of 23% and 3% at baseline (postoperative/medical) improving to 10% at discharge. Patients incapable of attempting a STS test were excluded, reducing generalizability to critical care. Physical rehabilitation was not standardized, possibly reducing responsiveness.

CONCLUSIONS

The 60-second STS demonstrated excellent interrater reliability and moderate validity and was responsive to change postoperatively.

IMPACT

The 60-second STS represents a safe, feasible functional performance tool for inpatients post-LTx. Two tests should be completed at each time point.

Effect of high-intensity training on peak oxygen uptake and muscular strength after lung transplantation: A randomized controlled trial

BACKGROUND

Peak oxygen uptake (VO_2peak) remains low after lung transplantation (LTx). We evaluated the effect of high-intensity interval training (HIIT) on VO_2peak, muscular strength, health-related quality of life (HRQOL), pulmonary function, and physical function after LTx.

METHODS

In this randomized controlled trial, 54 participants were enrolled from 6 to 60 months after LTx. The HIIT group (n = 25) followed a supervised HIIT program, consisting of endurance and strength trainings 3 times a week for 20 weeks. The control group (n = 29) received usual care. The primary outcome was a change in VO_2peak measured by cardiopulmonary exercise testing. The secondary outcomes were changes in 1-repetition maximum (1RM) for arm press and leg press, HRQOL (36-Item Short-Form Health Survey [SF-36]), pulmonary function (forced expiratory volume in 1 sec, diffusing capacity of the lungs for carbon monoxide), and physical function (1RM in handgrip, 15-sec stair run, and 30-sec chair stand).

RESULTS

A total of 46 participants completed the study, including 23 of 25 in the intervention group. For the primary outcome, the intention-to-treat analysis revealed a non-significant between-group difference for change in VO_2peak of 0.7 ml/(kg.min) (95% CI = ‒0.3, 1.8) (p = 0.17). The between-group differences for 1RM arm press and leg press and mental aspect of SF-36 were 4.9 kg (95% CI = ‒0.1, 9.9) (p = 0.05), 11.6 kg (95% CI = 0.1, 23.0) (p < 0.05), and 5.7 kg (95% CI = 0.9, 10.4) (p = 0.02), respectively. There were no between-group differences in pulmonary function or physical function. When excluding participants with an attendance of <70% (n = 16), the between-group difference for VO_2peak was 1.2 ml/(kg.min) (95% CI = 0.1, 2.4) (p = 0.032).

CONCLUSIONS

HIIT improved muscular strength and HRQOL but did not improve VO_2peak more than usual care after LTx. However, with acceptable adherence, HIIT appears to have beneficial effects on VO_2peak.

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

BACKGROUND

The lung allocation score (LAS) has led to improved organ allocation for transplant candidates. At present, the 6-minute walk distance (6MWD) is treated as a binary categorical variable of whether or not a candidate can walk more than 150 feet in 6 minutes. In this study, we tested the hypothesis that 6MWD is presently under-utilized with respect to discriminatory power, and that, as a continuous variable, could better prognosticate risk of waitlist mortality.

METHODS

A retrospective cohort analysis was performed using the Organ Procurement and Transplantation Network/United Network for Organ Sharing (OPTN/UNOS) transplant database. Candidates listed for isolated lung transplant between May 2005 and December 2011 were included. The population was stratified by 6MWD quartiles and unadjusted survival rates were estimated. Multivariable Cox proportional hazards modeling was used to assess the effect of 6MWD on risk of death. The Scientific Registry of Transplant Recipients (SRTR) Waitlist Risk Model was used to adjust for confounders. The optimal 6MWD for discriminative accuracy in predicting waitlist mortality was assessed by receiver-operating characteristic (ROC) curves.

RESULTS

Analysis was performed on 12,298 recipients. Recipients were segregated into quartiles by distance walked. Waitlist mortality decreased as 6MWD increased. In the multivariable model, significant variables included 6MWD, male gender, non-white ethnicity and restrictive lung diseases. ROC curves discriminated 6-month mortality was best at 655 feet.

CONCLUSIONS

The 6MWD is a significant predictor of waitlist mortality. A cut-off of 150 feet sub-optimally identifies candidates with increased risk of mortality. A cut-off between 550 and 655 feet is more optimal if 6MWD is to be treated as a dichotomous variable. Utilization of the LAS as a continuous variable could further enhance predictive capabilities.

Decreased Hypercapnic Ventilatory Response in Long-Term Lung Transplant Recipients is Associated with Exercise Impairment

PURPOSE

Bilateral lung transplantation results in complete denervation of the lung and might impair hypercapnic ventilatory response (HCVR). However, experimental and clinical findings are scarce and conflicting. Therefore, this study investigated the relationship between HCVR and exercise capacity after long-term bilateral lung transplantation.

METHODS

This cross-sectional analysis enrolled 46 bilateral lung transplant recipients between October 2011 and July 2012 who underwent cardiopulmonary exercise testing to evaluate maximum workload, and carbon dioxide (CO2) rebreathing. CO2 rebreathing was also evaluated in 35 control subjects.

RESULTS

In lung transplant recipients age was 54 ± 11 years, body mass index (BMI) 25.4 ± 4.1 kg/m(2), and time after transplantation 4.5 ± 2.5 years (range 9 months to 10 years). Controls were aged 41 ± 12 years and had a BMI of 24.9 ± 4.0 kg/m(2). There were significant differences between lung transplant recipients and controls in forced expiratory volume in 1 s (76 ± 22 vs. 94 ± 12 % predicted, p < 0.001) and inspiratory vital capacity (91 ± 20 vs. 105 ± 14 % predicted, p = 0.001). Blood gases did not differ significantly in patients versus controls. HCVR in lung transplant recipients was 1.44 ± 1.07 L/min/mmHg compared with 2.09 ± 1.14 L/min/mmHg in controls (p = 0.001). Exercise capacity in lung transplant recipients (73 ± 24 W) was 49 % predicted. Linear regression analysis showed that exercise capacity was significantly associated with HCVR. A 1 L/min/mmHg decrease in HCVR decreased exercise capacity by 50 W.

CONCLUSION

HCVR is reduced in long-term bilateral lung transplant recipients and this might explain the observed impairment of exercise capacity.