Hierarchy levels in the control of blood flow to the hind limb of the dog

Abstract

In order to quantitatively assess the dynamic interactions between different levels of the O₂ transport system control hierarchy from the dynamics of the relative contribution by individual cardiovascular and respiratory variables to the response of the system to stress, flow perturbations were introduced in the hind leg of greyhounds by:1) passive motion of the biceps muscle, 2) electrical stimulation (el. stim.) of the muscle nerve, 3) el. stim. of the femoral and sciatic nerve, 4) el. stim. of the proximal end of both nerves, 5) el. stim. of the peripheral end of both nerves, 6) peripheral stimulation after autonomic blockers and 7) peripheral stimulation after Flaxedil. The following variables were continuously measured: upper and lower inflow in the biceps, inflows into the stimulated and the contralateral leg, O₂ saturation of the venous outflow from the muscle and the leg, arterial pressure, heart rate, respiratory rate and tidal volume; and their responses to perturbations were characterized by four parameters [delay time, initial slope, maximum (or minimum) and average response]. Of these, the slope was found to be the most sensitive index of stimulus specificity, while the traditionally measured steady state response, although an important consideration for performance adaptation to stress, does not permit a differentiation between the various control mechanisms which mediate the response. These results indicate that the low frequency dynamics of cardiovascular and respiratory performance contain essential information about the type of control arrangement existing at a given moment. Marked differences in both time course and magnitude of flow responses in vascular beds arranged in parallel and in series were observed with corresponding differences in the changes of the O₂ content of their respective venous outflows. From the ranking of the four parameters a numerical estimate of the overall response as well as of its peripheral and central components was obtained. Between the different experimental conditions the spectrum of the overall responses ranged from predominance of centrally mediated control mechanisms (as judged by the magnitude of the contribution by “central” indicators) to predominance of locally mediated control mechanism. The latter depended on the presence of increased metabolic activity. If hierarchy levels are defined functionally (rather than structurally) on the basis of the magnitude of the response of the different indicators our results show a complete reversal of the hierarchical ordering between stimulation of the proximal and peripheral end of the sciatic and femoral nerve. An appropriate choice of sufficient and adequate parameters for the characterization of integrated response pattern is a crucial prerequisite for this type of analysis.