Limited Functional Performance in Chronic Obstructive Pulmonary Disease: Nature, Causes and Measurement

Abstract

Patients with chronic obstructive pulmonary disease (COPD) frequently describe limitations in functional performance. These limitations predict mortality, adversely affect health-care burden and impair health-related quality of life. The optimal method for quantifying the functional performance in COPD subjects has not been established. This paper discusses the (i) nature of limited functional performance reported by individuals with COPD, (ii) mechanisms that contribute to these limitations, (iii) assessment techniques available to provide markers of functional performance and (iv) areas for further research in measuring functional performance of COPD subjects.

Keywords::

• Activity
• Assessment
• Dyspnea
• Exercise

INTRODUCTON

Chronic obstructive pulmonary disease (COPD) is a major cause of mortality and burden worldwide ([1]). Functional performance, which has been defined as the physical, psychological, social, occupational and spiritual activities that people actually do in the course of their lives as they attempt to meet basic needs, fulfill usual roles and maintain their health and well being, is reduced in individuals with COPD ([2]). It is important to distinguish functional capacity (what an individual can do) from functional performance (what an individual actually does), the difference between the two being the functional reserve ([3]). Quantifying the impairment in functional performance resulting from a disease process is a challenge for clinicians and researchers.

In COPD, “markers” ([4]) of impaired functional performance are commonly derived from questionnaires or measurements of exercise capacity or physical activity ([5]). Several disease-specific questionnaires have been developed to measure the difficulties with, symptoms during, or participation in, activities during daily life ([2], [6], [7], [8], [9]). Likewise, several protocols have been described to measure functional exercise capacity ([10], [11], [12]). However, the extent to which the measurements obtained using these techniques represent a true estimate of an individual's functional performance has only recently started to receive attention.

Accurate measurement of the limitation in functional performance experienced by individuals with COPD has important implications for mortality, health-care burden and the health-related quality of life resulting from this condition. Patients who usually require physical assistance for going up or down stairs or for walking modest distances during daily life have an increased length of stay following an acute exacerbation of COPD (AECOPD) ([13]). Low levels of physical activity following an AECOPD characterize those patients experiencing one or more admissions within a 12-month period ([14]). Patients describing high usual levels of physical activity have a reduced risk of readmission ([15]). Limited functional performance is a strong predictor of survival ([16]) and low self-reported measures of daily activity have been associated with worse physical and mental scores on quality of life as measured by the 12 item Short-Form Health Survey ([17]).

The following discussion describes the (i) nature of the limited functional performance reported by individuals with COPD, (ii) mechanisms that contribute to these limitations, (iii) assessment techniques available that provide markers of functional performance and (iv) areas for further work in measuring the functional performance of COPD patients.

NATURE OF LIMITED FUNCTIONAL PERFORMANCE

In 2001, the International Classification of Functioning, Disability and Health defined impairments as “problems with body function or structure,” activities as “the execution of a task or action by an individual” and participation as “the involvement in a life situation.” In COPD patients, limitations in activities and restrictions in participation are the result of interplay between the primary disease process in the airways and lung parenchyma, and secondary impairment of both skeletal muscle function and psychological status, with environmental and personal contextual factors ([18]).

Large observational multi-centre studies have reported limited functional performance among patients with COPD ([13], [19]). In 305 patients admitted to hospital because of an AECOPD, Incalzi and co-workers ([13]) described pre-admission dependency (need for physical assistance) in basic activities of daily living required for self care such as bathing and continence as well as those instrumental activities of daily living required to live independently, such as housework and managing finances. Approximately 10% of patients described dependency in 3 or more basic activities of daily living and 30% described dependency in 3 or more activities of daily living required for independent living. Dependency was described in both lower limb (walking) and upper limb (laundry) activities. Of particular importance, 17% of patients reported difficulty taking medications, a potential factor contributing to the risk of hospital admission with AECOPD ([20]).

In a random telephone survey of COPD patients in North America and Europe, 60% of individuals described limited participation in sports, recreation and normal physical exertion and 40% in social and family life, sleep, household chores and sexual activity ([19]). Work loss during the prior 12 months was reported by 45% of patients aged less than 65 years. Although dyspnea was more prevalent among older patients, activity restriction could not be separated by age. The disparity between symptoms and activity restriction might represent age-specific contextual factors such as work and retirement activities.

In COPD patients, self-reports of limited functional performance derived from questionnaire-based assessments are in concordance with objective measures of physical daily activity obtained using activity monitors. Less time spent walking and standing and more time spent sitting or lying ([5]) results in the daily physical activity measured in COPD patients being half to a third that of their age matched healthy peers ([21], [22]). This sedentary lifestyle is most likely a strategy for coping with dyspnea ([23]).

CAUSES OF LIMITED FUNCTIONAL PERFORMANCE

COPD patients, tested using standard protocols, tend to require a higher rate of oxygen consumption during walking on level ground, with or without a load and a significantly greater rate of oxygen consumption during stair climbing, compared with healthy volunteers ([24]). This finding is likely to reflect the high oxygen consumption of the respiratory muscles and poor mechanical efficiency demonstrated in COPD patients ([25], [26]). Intolerable dyspnea is commonly reported as the symptom limiting performance during both walking and upper limb activity in COPD patients ([27], [28]).

The pathophysiological origin of dyspnea in COPD is multifactorial ([29]). As ventilation increases during exercise, the faster respiratory rate, in the presence of expiratory airflow obstruction, leaves insufficient time for exhalation resulting in an increase in end-expiratory lung volume ([30]). This dynamic pulmonary hyperinflation increases the elastic and threshold loads borne by the inspiratory muscles ([31]) in addition to placing them at a mechanical disadvantage, reducing their force-generating capacity and efficiency ([32], [33]). This process is a major contributor to the sensation of dyspnea during activity ([31]). Secondary impairments of skeletal muscle aerobic capacity ([34]) result in an early increase of blood lactate during exercise, serving to heighten ventilatory demand ([35]). The imbalance between ventilatory capacity and ventilatory demand has been postulated to be perceived as dyspnea ([36]).

Activities that require upper limb exercise pose a particular challenge for patients with COPD ([27]). Sustained upper limb positioning (90 degree shoulder flexion) tends to evoke greater ventilatory and metabolic responses in COPD patients than in healthy volunteers ([37]). Overhead positioning increases end-expiratory lung volumes ([38]) and the reliance on the diaphragm to generate inspiratory airflow ([39]). It also deprives the respiratory system of accessory muscles of respiration, usually available to assist in generating inspiratory flow. These factors associated with upper limb activity give rise to dyspnea by increasing the load on the ventilatory system while impairing its capacity to function.

In addition to dyspnea, other factors such as age, depression, anxiety, impaired cognition and balance, as well as the use of long-term oxygen therapy (LTOT) contribute to the limitations in functional performance. COPD patients who report independence in mobility-related activities (e.g., negotiating stairs, walking modest distances) are younger and have less cognitive impairment than those who report dependence ([13]). COPD patients with lower forced expiratory volumes in one second (FEV₁) have greater reductions in balance and coordination ([40]). The association between measures of balance and coordination and disease severity is likely to represent the deleterious effects on the neuromuscular system of the reduced time that people with more advanced disease spend walking ([5]). Depression and anxiety are common in COPD ([41], [42]) and heighten the perception of dyspnea ([43]), thereby increasing the likelihood of adopting a sedentary lifestyle. Compared with COPD patients with similar impairment in lung function, those receiving LTOT are less active, most likely reflecting the house-bound lifestyle adopted by COPD patients on LTOT ([22]).

MARKERS OF FUNCTIONAL PERFORMANCE

Questionnaire-based assessments

The functional performance of COPD patients is most commonly estimated using questionnaire-based assessments which require subjects to quantify (i) the number of activities undertaken during daily life, (ii) dyspnea during exertion, or (iii) the level of difficulty or assistance required to undertake activities during daily life. Disease-specific questionnaires developed to quantify the functional performance of COPD patients include the London Chest Activity of Daily Living Scale ([8], [44]), the Pulmonary Functional Status and Dyspnea Questionnaire ([6]), the Pulmonary Functional Status Scale ([7]), the Functional Performance Inventory ([2]) and the Manchester Respiratory Activities of Daily Living Questionnaire ([9]). Although the reliability, validity and responsiveness of some of these instruments have been investigated, to date, their interpretability remains unclear as the thresholds for minimal clinically important differences have not been elucidated. It is important to consider that functional performance is affected by environmental and personal contextual factors such as culture, gender or age in addition to the disease process. Failure to consider the influence of these other factors may distort the estimate of the impact of the disease on functional performance.

Subscores of disease-specific health-related quality of life measures such as the dyspnea and fatigue domains of the Chronic Respiratory Disease Questionnaire ([45]) and the symptoms, activity and impact subscores of the St George's Respiratory Questionnaire ([46]) may also provide markers of functional performance, although at present, their application as markers of functional performance remains to be defined.

Quantifying physical activity during daily life

Activity monitors such as accelerometers are increasingly used to quantify physical activity during daily life ([5], [22], [47]), especially as self-report techniques appear to be unreliable in patients with COPD ([48]). A detailed description of such devices is available elsewhere ([49]). However, even accelerometers with sophisticated software do not detect isometric exercise or upper limb activity. They also lack sensitivity to differentiate changes in cycling intensity and walking on an incline from walking on level ground ([48], [49]). Furthermore, such devices do not provide information regarding the level of assistance required or capability to complete individual tasks necessary during daily life ([50]).

Measuring exercise capacity

Measures of exercise capacity can be used as markers of physical activity during daily life. The 6-minute walk distance (6MWD) is correlated with total daily physical activity ([51], [52]), time spent walking, time spent standing and movement intensity during walking ([5]). In one study 6MWD was found to account for 56% of the variance in the measurement of time spent walking in daily life ([5]). Although the remaining 44% of the variance could not be accounted for by measures of resting lung function, respiratory or peripheral muscle strength, corticosteroid use or co-morbid conditions, it may be attributable to other variables such as symptoms and psychological manifestations of the disease. The relationship between measurements of daily physical activity using accelerometers and other tests of functional exercise capacity such as the incremental shuttle walk test or the endurance shuttle walk have not been examined.

Upper limb ADL such as brushing hair, cleaning teeth and putting away groceries, generally require unsupported upper limb exercise. The Unsupported Upper Limb Exercise Test uses a standardized assessment protocol designed to mimic upper limb activities performed during daily life ([53]). This reproducible, responsive test, correlates with measures obtained using incremental arm ergometry ([53], [54]). Whether an increase in exercise time achieved during the Unsupported Upper Limb Exercise Test is associated with a change in upper limb physical activity during daily life remains to be established.

BODE

The limitation in functional performance demonstrated in patients with COPD is multifactorial in origin. It is conceivable therefore that an aggregate index such as the BODE, which comprises measures of dyspnea during activity and functional exercise capacity ([55]), represents a better marker of functional performance in COPD patients than the individual measures used in isolation. To date, no study has associated the BODE index with measures of physical activity during daily life.

AREAS FOR FURTHER WORK

Although many therapeutic interventions provided to patients with COPD aim to improve daily function ([56]), the optimal marker of an individual's true functional performance is unclear. Head-to-head comparisons are needed of the ability of disease-specific questionnaires to (i) discriminate the impairment of functional performance between patients and (ii) evaluate changes over time in functional performance following intervention. The extent to which measures of functional exercise capacity, other than the 6MWD, relate to physical activity during daily life as measured by an accelerometer presents important opportunities for further research. Upper limb activity during daily life needs to be quantified for those with COPD. The extent to which measures of upper limb exercise capacity correlate with limitations in functional performance also requires clarification. Addressing these issues will allow the effects of therapeutic strategies, such as pulmonary rehabilitation, to be described not only in terms of functional capacity, but perhaps more importantly, functional performance.