Hemodynamics Laboratory

The Hemodynamics Laboratory is used to understand better how blood pressure, or more broadly the cardiovascular system, is controlled. A major focus in this lab is understanding how specific conditions, such as chronic exercise, age or gender, affect the cardiovascular responses to an orthostatic stress. A goal here is to elucidate mechanisms underlying differences in these responses. For example, women tend to be less tolerant than men to severe orthostatic stress. Is this reduced tolerance due to differences in blood volume, organ-specific differences in the vasoconstrictor responses to the stress, differences in venous compliance of the lower extremities, autonomic differences or something else?

A more recent focus has been determining the nature and extent to which baroreceptor resetting, seen with acute exercise, can be a trained phenomenon. For example, baroreceptor resetting is commonly seen with acute bouts of either aerobic or resistance exercise. Chronic exercise training results in a number of physiologic adaptations. Is an altered baroreceptor response to acute exercise one of these adaptations?

LBNP

 

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LBNP is a laboratory technique used to mimic orthostatic stress. Cardiovascular responses, such as changes in forearm blood flow and stroke volume relative to the magnitude of LBNP, can be used to make inferences about how the blood pressure response to orthostatic stress is regulated. Blood samples can be acquired serially to determine if differences in the adrenergic responses to this stress contribute to differences in the cardiovascular responses.

 

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With this technique, the venous valves in the dependent arm become incompetent. A continuous column of blood between the catheter insertion site and the right atrium forms. Thus, changes in central venous pressure, due to reductions in central venous return caused by graded LBNP, can be inferred by changes in pressure at the catheter site.

We use all these techniques to assess how blood pressure is regulated in the presence of orthostatic stress.

Other techniques used in this line of research are assessments of cerebral blood flow and venous compliance. These are not pictured. Future projects will include assessments of splanchnic and renal blood flow.

 

 

Isometric exercise

 

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Isometric exercise can evoke pronounced increases in blood pressure. With exercise, the brain, or “central command”, makes appropriate adjustments such that blood pressure is regulated around the new higher value; this is commonly called baroreceptor resetting. Peripheral feedback, e.g. from the exercising muscles, enables the cardiovascular system to fine-tune this blood pressure response. The neck collar is used to manipulate the carotid baroreceptors via imposition of positive or negative pressure on the neck. With this technique, baroreceptor resetting can be assessed. We used this technique to ascertain the extent to which chronic exercise training affects baroreceptor resetting with an acute bout of exercise.

 

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Changes in leg vascular conductance (leg blood flow is assessed with the equipment on his right leg while concurrent blood pressure is assessed with that on his left arm) reflect changes in muscle sympathetic nerve activity. Trapping metabolites, with an occlusion cuff, in the exercising arm increases peripheral feedback. Central command can be manipulated by modulating the effort required to perform the exercise (e.g. changing the force produced). Thus, selectively manipulating either central command or peripheral feedback while indirectly assessing sympathetic nerve activity, heart rate, and stroke volume can give us further insights into how the blood pressure response to exercise is regulated. We used this technique to explore how the cardiovascular system increases blood pressure when the “preferred” method for doing so, increasing heart rate, is inhibited via b-blockade.

 

 

Dr. Warren D. Franke is the director of this laboratory. He can be contacted at wfranke@iastate.edu.