Influence of respiratory motor neurone activity on human autonomic and haemodynamic rhythms

Although humans hold great advantages over other species as subjects for bi omedical research, they also bring major disadvantages. One is that among t he many rhythmic physiological signals that can be recorded, there is no su re way to know which individual change precedes another, or which change re presents cause and which represents effect. In an attempt to deal with the inherent complexity of research conducted in intact human subjects, we deve loped and used a structural equation model to analyse responses of healthy young men to pharmacological changes of arterial pressure and graded inspir atory resistance, before and after vagomimetic atropine. Our model yielded a good fit of the experimental data, with a system weighted R-2 of 0.77, an d suggested that our treatments exerted both direct and indirect influences on the variables we measured. Thus, infusions of nitroprusside and phenyle phrine exerted all of their direct effects by lowering and raising arterial pressure; the changes of R-R intervals, respiratory sinus arrhythmia and a rterial pressure fluctuations that these drugs provoked, were indirect cons equences of arterial pressure changes. The only direct effect of increased inspiratory resistance was augmentation of arterial pressure fluctuations. These results may provide a new way to disentangle and understand responses of intact human subjects to experimental forcings. The principal new insig ht we derived from our modelling is that respiratory gating of vagal-cardia c motor neurone firing is nearly maximal at usual levels of arterial pressu re and inspiratory motor neurone activity.