Enhanced parasympathetic neural activity lowers exercise heart rate at high altitude, while hypoxia mediates the attenuation of peak exercise cardiac output.
Enhanced parasympathetic neural activity accounts for the lowering of HR during exercise at ALT without influencing Q. The abrupt restoration of peak exercise Q in chronic hypoxia to maximal SL values when arterial PO(2) and SO(2) are similarly increased suggests hypoxia-mediated attenuation of Q.
Boushel et al. (2001) studied this question.