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Exercise intolerance in chronic airflow limitation: Variation in the ventilatory responses to exercise (2)

Variations in the response of alveolar ventilation: Even in the healthy population, arterial PCO2 varies between 35 and 45 mmHg, which at a moderate work load (carbon dioxide of 2 L/min) implies an alveolar ventilation (VA) of 38 to 50 L/min. The variation in patients with COLD is much larger, PaC02 during exercise being less than 30 mmHg in some and above 70 in others. Variations in gas exchange capacity and ventilation-perfusion matching account for some of these variations, but differences in the work of breathing and in respiratory control mechanisms are also known to be important.

Variations in dead space ventilation: The concept of dead space, influencing total ventilation for given carbon dioxide outputs and VA, has been used to explain variations in the ventilatory response to exercise in terms of ventilation-perfusion mismatching. Christie well understood the implications: “The emphysematous patient is indeed in an unfortunate position. Both inspiration and expiration have to be executed by unnatural respiratory efforts, and a considerable proportion of the air which is inspired is wasted by not coming into proper contact with the pulmonary blood”. Thus, patients with well-ventilated, poorly perfused areas in the lung tend to have higher ventilation; also, a shallow pattern of breathing leads to wastage of ventilation, a high dead space ventilation to tidal volume ratio (Vd/Vt), resulting merely from a low Vt. Normally, Vd/Vt falls during exercise from around 0.3 to less than 0.1, but in many patients the ratio is as high as 0.6 and may not fall during exercise. Vd/Vt was shown to contribute to dyspnea independent of the degree of airflow obstruction (Figure 4). You will always enjoy getting viagra super active online, being 100% sure you are safe.

Figure 4. Exercise intolerance
Figure 4) Dead space/tidal volume ratio (Vd/Vt) at rest and exercise in patients with chronic obstructive lung disease grouped according to severity of dyspnea (United Kindgom Medical Research Council grades 1 to 4), showing increasing Vd/Vt at rest, and with lessening reduction in exercise in patients with increasing dyspnea grade (21). N Normal subject

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