Attenuation of hypercapnic carbon dioxide chemosensitivity after postinfarction exercise training: possible contribution to the improvement in exercise hyperventilation

Objective: To elucidate the responsible mechanisms of increased slope of minute ventilation relative to carbon dioxide production (V̇E/V̇co2) during exercise after acute myocardial infarction without overt signs of heart failure, patients who had an acute myocardial infarction were examined after participating in a three month supervised exercise training programme. Design: Exercise testing, hypercapnic CO2 chemosensitivity measurement (rebreathing method), and pulmonary function test were repeated at entry and after three months in 50 acute myocardial infarction patients with neither symptoms nor signs of heart failure who completed the training programme. Ten patients who performed initial inhospital training served as controls. Results: Age, peak oxygen uptake, left ventricular ejection fraction, CO2 chemosensitivity, respiratory parameters (percentage of predicted normal vital capacity (%VC), forced expiratory volume in one second, and carbon monoxide transfer factor (%Tlco)) were all significantly correlated with V̇E/V̇co2 slope. Multivariate regression analysis showed that age (β = 0.29, p = 0.01), %Tlco (β = −0.27, p = 0.01), and CO2 chemosensitivity (β = 0.49, p < 0.001) were independent determinants of V̇E/V̇co2 slope. After three months, there was no significant change in these parameters in the control group. Peak oxygen uptake, %Tlco, and %VC and attenuation in CO2 chemosensitivity increased significantly in the training group. The V̇E/V̇co2 slope decreased marginally (p = 0.11). The changes in V̇E/V̇co2 slope were correlated only with those in CO2 chemosensitivity (r = 0.50, p < 0.001). Conclusion: After acute myocardial infarction, exercise hyperventilation is seen in association with aging, enhanced hypercapnic CO2 chemosensitivity, and reduced Tlco, even in the absence of overt heart failure. The correlation of V̇E/V̇co2 attenuation after training with the reduction in CO2 chemosensitivity suggests that exercise training may reduce increased V̇E/V̇co2 slope, at least partially by reducing CO2 chemosensitivity.