CAUSE OF LOW ARTERIAL OXYGEN SATURATION IN PULMONARY FIBROSIS

The Exercising Brain: An Overlooked Factor Limiting the Tolerance to Physical Exertion in Major Cardiorespiratory Diseases?

“Exercise starts and ends in the brain”: this was the title of a review article authored by Dr. Bengt Kayser back in 2003. In this piece of work, the author highlights that pioneer studies have primarily focused on the cardiorespiratory-muscle axis to set the human limits to whole-body exercise tolerance. In some circumstances, however, exercise cessation may not be solely attributable to these players: the central nervous system is thought to hold a relevant role as the ultimate site of exercise termination. In fact, there has been a growing interest relative to the “brain” response to exercise in chronic cardiorespiratory diseases, and its potential implication in limiting the tolerance to physical exertion in patients. To reach these overarching goals, non-invasive techniques, such as near-infrared spectroscopy and transcranial magnetic stimulation, have been successfully applied to get insights into the underlying mechanisms of exercise limitation in clinical populations. This review provides an up-to-date outline of the rationale for the “brain” as the organ limiting the tolerance to physical exertion in patients with cardiorespiratory diseases. We first outline some key methodological aspects of neuromuscular function and cerebral hemodynamics assessment in response to different exercise paradigms. We then review the most prominent studies, which explored the influence of major cardiorespiratory diseases on these outcomes. After a balanced summary of existing evidence, we finalize by detailing the rationale for investigating the “brain” contribution to exercise limitation in hitherto unexplored cardiorespiratory diseases, an endeavor that might lead to innovative lines of applied physiological research.

Measurement and Interpretation of Exercise Ventilatory Efficiency

Cardiopulmonary exercise testing (CPET) is a method for evaluating pulmonary and cardiocirculatory abnormalities, dyspnea, and exercise tolerance in healthy individuals and patients with chronic conditions. During exercise, ventilation (V˙_E) increases in proportion to metabolic demand [i.e., carbon dioxide production (V˙CO₂)] to maintain arterial blood gas and acid-base balance. The response of V˙_E relative to V˙CO₂ (V˙_E/V˙CO₂) is commonly termed ventilatory efficiency and is becoming a common physiological tool, in conjunction with other key variables such as operating lung volumes, to evaluate exercise responses in patients with chronic conditions. A growing body of research has shown that the V˙_E/V˙CO₂ response to exercise is elevated in conditions such as chronic heart failure (CHF), pulmonary hypertension (PH), interstitial lung disease (ILD), and chronic obstructive pulmonary disease (COPD). Importantly, this potentiated V˙_E/V˙CO₂ response contributes to dyspnea and exercise intolerance. The clinical significance of ventilatory inefficiency is demonstrated by findings showing that the elevated V˙_E/V˙CO₂ response to exercise is an independent predictor of mortality in patients with CHF, PH, and COPD. In this article, the underlying physiology, measurement, and interpretation of exercise ventilatory efficiency during CPET are reviewed. Additionally, exercise ventilatory efficiency in varying disease states is briefly discussed.

Exercise Pathophysiology in Interstitial Lung Disease

Interstitial lung disease (ILD) is a heterogeneous group of disorders that primarily affect the lung parenchyma. Patients with ILD have reduced lung volumes, impaired pulmonary gas exchange, and decreased cardiovascular function. These pathologic features of ILD become exacerbated during physical exertion, leading to exercise intolerance and abnormally high levels of exertional dyspnea. In this review, the authors summarize the primary pathophysiologic features of patients with ILD and their effect on the integrative response to exercise.

Advances in the Evaluation of Respiratory Pathophysiology during Exercise in Chronic Lung Diseases

Dyspnea and exercise limitation are among the most common symptoms experienced by patients with various chronic lung diseases and are linked to poor quality of life. Our understanding of the source and nature of perceived respiratory discomfort and exercise intolerance in chronic lung diseases has increased substantially in recent years. These new mechanistic insights are the primary focus of the current review. Cardiopulmonary exercise testing (CPET) provides a unique opportunity to objectively evaluate the ability of the respiratory system to respond to imposed incremental physiological stress. In addition to measuring aerobic capacity and quantifying an individual's cardiac and ventilatory reserves, we have expanded the role of CPET to include evaluation of symptom intensity, together with a simple "non-invasive" assessment of relevant ventilatory control parameters and dynamic respiratory mechanics during standardized incremental tests to tolerance. This review explores the application of the new advances in the clinical evaluation of the pathophysiology of exercise intolerance in chronic obstructive pulmonary disease (COPD), chronic asthma, interstitial lung disease (ILD) and pulmonary arterial hypertension (PAH). We hope to demonstrate how this novel approach to CPET interpretation, which includes a quantification of activity-related dyspnea and evaluation of its underlying mechanisms, enhances our ability to meaningfully intervene to improve quality of life in these pathologically-distinct conditions.

Lung diffusing capacity for nitric oxide as a marker of fibrotic changes in idiopathic interstitial pneumonias

Lung diffusing capacity for carbon monoxide (DLCO) is decreased in both usual interstitial pneumonia-idiopathic pulmonary fibrosis (UIP-IPF) and nonspecific interstitial pneumonia (NSIP), but is moderately related to computed tomography (CT)-determined fibrotic changes. This may be due to the relative insensitivity of DLCO to changes in alveolar membrane diffusive conductance (DMCO). The purpose of this study was to determine whether measurement of lung diffusing capacity for nitric oxide (DLNO) better reflects fibrotic changes than DLCO. DLNO-DLCO were measured simultaneously in 30 patients with UIP-IPF and 30 with NSIP. Eighty-one matched healthy subjects served as a control group. The amount of pulmonary fibrosis was estimated by CT volumetric analysis of visually bounded areas showing reticular opacities and honeycombing. DMCO and pulmonary capillary volume (VC) were calculated. DLNO was below the lower limit of normal in all patients irrespective of extent and nature of disease, whereas DLCO was within the normal range in a nonnegligible number of patients. Both DLNO and DLCO were significantly correlated with visual assessment of fibrosis but DLNO more closely than DLCO. DMCO was also below the lower limit of normal in all UIP-IPF and NSIP patients and significantly correlated with fibrosis extent in both diseases, whereas VC was weakly correlated with fibrosis in UIP-IPF and uncorrelated in NSIP, with normal values in half of patients. In conclusion, measurement of DLNO may provide a more sensitive evaluation of fibrotic changes than DLCO in either UIP-IPF or NSIP, because it better reflects DMCO.

Factors limiting exercise tolerance in chronic lung diseases

The major limitation to exercise performance in patients with chronic lung diseases is an issue of great importance since identifying the factors that prevent these patients from carrying out activities of daily living provides an important perspective for the choice of the appropriate therapeutic strategy. The factors that limit exercise capacity may be different in patients with different disease entities (i.e., chronic obstructive, restrictive or pulmonary vascular lung disease) or disease severity and ultimately depend on the degree of malfunction or miss coordination between the different physiological systems (i.e., respiratory, cardiovascular and peripheral muscles). This review focuses on patients with chronic obstructive pulmonary disease (COPD), interstitial lung disease (ILD) and pulmonary vascular disease (PVD). ILD and PVD are included because there is sufficient experimental evidence for the factors that limit exercise capacity and because these disorders are representative of restrictive and pulmonary vascular disorders, respectively. A great deal of emphasis is given, however, to causes of exercise intolerance in COPD mainly because of the plethora of research findings that have been published in this area and also because exercise intolerance in COPD has been used as a model for understanding the interactions of different pathophysiologic mechanisms in exercise limitation. As exercise intolerance in COPD is recognized as being multifactorial, the impacts of the following factors on patients' exercise capacity are explored from an integrative physiological perspective: (i) imbalance between the ventilatory capacity and requirement; (ii) imbalance between energy demands and supplies to working respiratory and peripheral muscles; and (iii) peripheral muscle intrinsic dysfunction/weakness.

Exercise alveolar-arterial oxygen pressure difference in interstitial lung disease

Abnormality of gas exchange is best evaluated by the exercise alveolar-arterial oxygen pressure difference, P(A-a)O2. We studied the P(A-a)O2 in 168 patients with sarcoidosis, desquamative interstitial pneumonia (DIP), usual interstitial pneumonia (UIP), berylliosis, and asbestosis who were seen for clinical and disability consultations. The increase of P(A-a)O2 with exercise was greatest in UIP (mean 16 mm Hg), least in sarcoidosis (mean 1 mm Hg), and intermediate in DIP, berylliosis, and asbestosis (means 9, 9, and 7 mm Hg, respectively). The increase was best predicted by the single breath diffusing capacity (Dsb), and it occurred in patients with sarcoidosis and DIP if the Dsb was less than 50 percent predicted and in patients with the other diseases if the Dsb was less than 70 percent predicted. However, the magnitude of the increase could not be predicted from resting tests, even when multilinear regression equations were used. We conclude that for clinical evaluation of patients with interstitial lung disease, the exercise test with arterial blood gas measurement adds important information if the Dsb is less than 70 percent predicted. For disability evaluation, the invasive exercise study may be helpful when there is a wide discrepancy between clinical findings and resting physiologic studies.

Morphologic-physiologic correlates of the severity of fibrosis and degree of cellularity in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive disease of lung parenchyma characterized by a chronic inflammatory cellular infiltration and varying degrees of interstitial fibrosis. Current data indicate that the severity of fibrosis and the degree of cellularity determine, in part, the prognosis of IPF and the response to therapy. Whereas lung biopsy gives the best assessement of fibrosis and cellularity, physiologic studies are used to stage and monitor the disease process. To determine which physiologic studies correlate best with severity of fibrosis and degree of cellularity, these parameters were graded in lung biopsies of 23 patients with IPF and compared with a variety of physiologic studies. Although vital capacity, total lung capacity, and diffusing capacity are commonly used as objective monitors of the disease process, none of these parameters correlated with either the severity of fibrosis or the degree of cellularity in biopsy specimens. In contrast, almost all parameters of lung distensibility correlated with the morphologic assessment of degree of fibrosis; compliance had the best correlation. Parameters of distensibility, however, correlated poorly with the degree of cellularity. In comparison, gas exchange during exercise correlated with both morphologic parameters; the exercise-induced changes in arterial oxygen pressure per liter of oxygen consumed had a high correlation with the degree of fibrosis (r = 0.89; P less than 0.001) and correlated to a lesser extent with the degree of cellularity (r = 0.56; P = 0.009). In contrast, neither the resting arterial oxygen tension nor the arterial oxygen tension at maximal exercise correlated with the morphologic assessment of degree of fibrosis or the degree of cellularity. These morphologic-physiologic comparisons suggest that (a) lung volumes and diffusing capacity are poor monitors of both the degree of fibrosis and the degree of cellularity; (b) the fibrotic process contributes, at least in part, to parameters of lung distensibility, and both fibrosis and cellularity contribute to gas exchange alterations during exercise; and (c) parameters of lung distensibility and exercise-induced gas exchange alterations may be useful in staging the severity of disease in IPF.

Ultrastructural morphometry of the blood-air barrier in pulmonary sarcoidosis

Stereologic techniques were utilized in an electron microscopic study of biopsy samples obtained from the lungs of seven patients with pulmonary sarcoidosis. Relative fractional volumes of alveolar septal components and the arithmetic mean thickness and harmonic mean thickness of alveolocapillary membranes (blood-air barrier) were compared with values for normal lungs. Based on morphometric analysis, increases in the arithmetic mean thickness and the harmonic mean thickness of the alveolocapillary membranes appeared too small to account for the reduction in gas transfer present; however, there was a quantitative relative increase in interstitial tissue in alveolar septa, which does not take part in gas exchange, at the expense of the capillary bed, which is critical to this function.

Application of ultrastructural morphometry to lung biopsy specimens in pulmonary histiocytosis X

Stereological techniques were applied to an electron microscopic study of biopsy samples from nine human lungs with diffuse pulmonary histiocytosis X, and the results were compared with values for normal lungs. This made possible a morphometric analysis of the tissue changes associated with the measurable abnormalities in gas transfer present in this disease. The small increases in arithmetic mean thickness of the alveolar-capillary membranes appeared insufficient to account for the reduction in gas transfer present. When compared with normal lung, a threefold increase in volumetric fraction of septal intercapillary tissue was found along with a corresponding decrease in septal capillaries. While uniformity of distribution cannot be determined by this method, it appears that abnormalities of blood gas transfer in this disease result primarily from a decrease in the available diffusing surface and the ventilation-perfusion distrubances with which these tissue changes are associated.

Determination of distribution of diffusing capacity in relation to blood flow in the human lung

A method for appraising the distribution of diffusing capacity of the lungs (D(L)) in relationship to pulmonary capillary blood flow ([unk]Q(C)) in normal human subjects was derived from measurements of oxygen diffusing capacity (D(LO2)) and carbon monoxide diffusing capacity (D(LCO)) performed during breath holding. This method utilizes the fact that the observed D(LO2) is considerably reduced in value if uneven distribution of D(L) with respect to [unk]Q(C) (uneven D(L)/[unk]Q(C)) is present. In contrast, D(LCO) is barely affected by uneven D(L)/[unk]Q(C), and from its measured value one can calculate the value D(LO2) would have if no uneven D(L)/[unk]Q(C) were present (true D(LO2)). Once observed D(LO2) and true D(LO2) are known, the degree of uneven D(L)/[unk]Q(C) in the lung can be calculated. In five normal, resting, sitting subjects average values for true D(LO2) were 57 ml per (minute x mm Hg), and the directly measured D(LO2) was 33 ml per (minute x mm Hg). These values could be explained if one-half of total [unk]Q(C) were distributed to approximately 15% of total D(L). These measurements did not permit the determination of the alveolar to end capillary O(2) gradient, but calculations demonstrate that an important factor in determining its size may be the pattern of uneven D(L)/[unk]Q(C) present in the lungs. Estimations of the alveolar-end capillary O(2) gradient from measurements of D(LCO) or D(LO2) that do not take into account uneven D(L)/[unk]Q(C) may underestimate its size.