Regional and overall inequality of ventilation and blood flow in patients with chronic airflow obstruction

Beyond Spirometry: Linking Wasted Ventilation to Exertional Dyspnea in the Initial Stages of COPD

Exertional dyspnea, a key complaint of patients with chronic obstructive pulmonary disease (COPD), ultimately reflects an increased inspiratory neural drive to breathe. In non-hypoxemic patients with largely preserved lung mechanics - as those in the initial stages of the disease - the heightened inspiratory neural drive is strongly associated with an exaggerated ventilatory response to metabolic demand. Several lines of evidence indicate that the so-called excess ventilation (high ventilation-CO2 output relationship) primarily reflects poor gas exchange efficiency, namely increased physiological dead space. Pulmonary function tests estimating the extension of the wasted ventilation and selected cardiopulmonary exercise testing variables can, therefore, shed unique light on the genesis of patients' out-of-proportion dyspnea. After a succinct overview of the basis of gas exchange efficiency in health and inefficiency in COPD, we discuss how wasted ventilation translates into exertional dyspnea in individual patients. We then outline what is currently known about the structural basis of wasted ventilation in "minor/trivial" COPD vis-à-vis the contribution of emphysema versus a potential impairment in lung perfusion across non-emphysematous lung. After summarizing some unanswered questions on the field, we propose that functional imaging be amalgamated with pulmonary function tests beyond spirometry to improve our understanding of this deeply neglected cause of exertional dyspnea. Advances in the field will depend on our ability to develop robust platforms for deeply phenotyping (structurally and functionally), the dyspneic patients showing unordinary high wasted ventilation despite relatively preserved FEV1.

Cardiopulmonary performance at rest and during exercise in seven patients with increased transradiancy of one lung ("Macleod's syndrome")

A group of seven patients with increased transradiancy of one lung included four with chronic bronchitis. The function of the two lungs combined was assessed by measurement of the lung volumes, airway resistance, and carbon monoxide uptake at rest, and by measurement of pulmonary ventilation, gas exchange, blood gases, and cardiac output at rest and during steady state exercise. The contribution of the hypertransradiant lung to the overall functional abnormalities at rest was estimated using a radioactive gas technique.

All the patients had airway obstruction. Pulmonary gas exchange was only mildly affected at rest because of equal reduction of ventilation and perfusion in the abnormal lung, but during exercise ventilation-perfusion inequality increased in some of the patients. Cardiac output was low in five patients during exercise and in four there was excessive lactate production. The greatest physiological abnormalities were seen in two of the chronic bronchitics who had abnormalities of ventilation and perfusion in both lungs.

We suggest that a pulmonary vascular abnormality resulting in reduced cardiac output, as well as ventilatory impairment due to airway obstruction, may contribute to the limited exercise capacity of these patients.

Regional lung function in patients with obstructive lung diseases

Regional lung function was measured, using radioactive xenon-133, in a group of normal subjects and in three carefully defined groups of patients with obstructive lung disease. When compared with the normal subjects, patients in the emphysematous group showed a relative reduction of ventilation and perfusion to the upper zones, while patients having chronic bronchitis without cardiac or respiratory failure showed a predominantly lower zone defect. In the group of patients with chronic bronchitis with cardiac and respiratory failure no consistent pattern was found.

Effect of bronchoconstrictor aerosols on the alveolar plateau of the single breath O2 test

In 17 subjects with bronchial asthma and chronic bronchitis measurements of the uneven ventilation using the single breath oxygen test, F.E.V._1·0 and V.C. were performed before and after acetylcholine or allergen nebulization. An increase of the alveolar slope was noticed in five out of the seven patients with induced bronchospasm and a decrease in the remaining two. In two other subjects moderate dyspnoea not reflected by spirographic tests was accompanied by a definite increase of the N₂ gradient. One patient experienced a severe asthmatic attack after acetylcholine without significant increase of the N₂ gradient. In the last seven subjects the spirographic tests were not impaired but the N₂ gradient rose to a significant level in three. The various implications of these results are discussed. In one subject the alveolar nitrogen concentration decreased after acetylcholine with a concomitant increase in the residual volume. This finding was assumed to be the result of either a failure of inspired oxygen to mix with alveolar nitrogen or a decreased available lung space.