Measuring the baroreflex: equivalency, repeatability, and its relationship with sex, age, and orthostatic tolerance

The lifetime prevalence of fainting is estimated at 20-40%. Fainting, or syncope, is caused by reduced cerebral perfusion, often secondary to a reduction in blood pressure. During standing, the movement of blood into the lower limbs stimulates the arterial baroreflex to increase cardiac output and constrict peripheral blood vessels to maintain blood pressure and blood flow to the brain. Syncope is more common in older adults and women; however, it is unclear whether this might relate to age- and/or sex-related changes in baroreflex function. Different methods of baroreflex stimulation and measurement may not be comparable, and the repeatability of these techniques is uncertain. In Chapter 3, I quantify the agreement and repeatability of different baroreflex testing methods. Cardiac baroreflex tests have higher agreement and repeatability than sympathetic vascular tests, highlighting the need for better methods of vascular baroreflex measurement. In Chapter 4, I examine the relationship between aging, sex and both the cardiac and vascular arms of the baroreflex using novel sympathetic vascular baroreflex measurement techniques. In Chapter 5, I examine the influence of carotid sinus massage (CSM) methodology on the cardiovascular responses, and recommend that CSM be applied to the area of maximal pulsatility when ultrasound-guidance is unavailable. Lastly, in Chapter 6, I refute the hypothesis that large responses to carotid baroreceptor stimulation in older adults are related to chronic denervation of the sternocleidomastoid muscles. Thus, the pathophysiology of hypersensitive responses to CSM remains unknown. This work demonstrates that careful consideration should be taken when selecting baroreflex measurement techniques. Our novel sympathetic vascular baroreflex testing methods are critical for a thorough description of baroreflex control of blood pressure in younger and older men and women. While the vascular responses to orthostasis are large in older individuals, the vascular responses to acute carotid baroreceptor stimulation with the neck collar decline with age. The standard clinical methodology of CSM may not accurately deliver mechanical stimulation to the carotid baroreceptors. We have shown that the previously reported association with sternocleidomastoid denervation is likely coincidental, not causal. Overall, this thesis reiterates the importance of sympathetic vascular baroreflex function in the maintenance of blood pressure.