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Exercise intolerance in chronic airflow limitation: Variation in the ventilatory responses to exercise (4)

The opportunity presented itself as part of a joint study between the Hammersmith Hospital’s Bronchitis Clinic and the University of Chicago’s Emphysema Clinic -a comparison of ‘British Bronchitis’ with ‘American Emphysema’ . This study involved the careful characterization of 50 patients in each clinic, and as part of the Hammersmith effort we carried out a series of exercise studies, including a comparison of type A with type B patients. These studies showed that, compared with type B, type A patients were more limited, kept PaC02 lower and showed a fall in Pa02 with exercise (Figure 6); they had a large Vd/Vt and an alveolar-arterial (A-a) PO2 difference that increased.

Type B patients showed a wide A-a POj difference at rest, but this narrowed with exercise, suggesting that areas in the lung with a low Va/Qc ratio improved their ventilation equally with the overall increase in exercise ventilation. Our thought then was that this behaviour removed a hypoxic drive to breathe during exercise and contributed to chronic underventilation, but this was an oversimplification. More recently, the elegant multiple inert gas washout technique has been applied to type A and B patients to confirm the dominant Va/Qc patterns. The combination of improving, contrasted with worsening, Va/Qc distribution, together with overall underventilation leads to ventilation being lower and dyspnea less prominent in the type B patient. You will always be offered amaryl diabetes at the pharmacy you can trust.

Figure 6. Exercise intolerance
Figure 6) Arterial PO2 at rest and maximal steady state exercise in patients with type AandB syndromes. PO2 tends to be higher at rest but to fall to a greater extent in type A, whereas most type B patients show an increase during exercise. VO2 Volume of expired oxygen

The work of breathing in type B patients was noted to be similar in type A by Howell, and thus unlikely to contribute to underventilation. The fact that this group of patients tends to be less dyspneic than type A indicates a complex interaction between the work of breathing and the control of breathing in influencing overall ventilation and the sense of respiratory effort.

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