On the inaccuracy of breath-by-breath metabolic gas exchange systems.

This technical note is presenting and discussing a severe limitation of the breath-by-breath (BBB) determination of pulmonary gas exchange routinely used as a surrogate for the metabolic gas exchange rate at rest and during exercise. We are presenting the view that continuous airway gas sampling at the mouth used for the determination of O and CO content is inaccurate in the range of a low-to-medium expiratory flows, due to the discrepancy between the cross section surfaces of the sampling line and the expiratory tube (mask or mouth piece). This difference results in the sampling of a mixed exhaled gas at low expiratory flow, for which any temporal relationship between the instantaneous expired CO2 and O signals and instantaneous expiratory flow is lost.

Sensing vascular distension in skeletal muscle by slow conducting afferent fibers: neurophysiological basis and implication for respiratory control.

This review examines the evidence that skeletal muscles can sense the status of the peripheral vascular network through group III and IV muscle afferent fibers. The anatomic and neurophysiological basis for such a mechanism is the following: 1) a significant portion of group III and IV afferent fibers have been found in the vicinity and the adventitia of the arterioles and the venules; 2) both of these groups of afferent fibers can respond to mechanical stimuli; 3) a population of group III and IV fibers stimulated during muscle contraction has been found to be inhibited to various degrees by arterial occlusion; and 4) more recently, direct evidence has been obtained showing that a part of the group IV muscle afferent fibers is stimulated by venous occlusion and by injection of vasodilatory agents. The physiological relevance of sensing local distension of the vascular network at venular level in the muscles is clearly different from that of the large veins, since the former can directly monitor the degree of tissue perfusion.