Effects of Vestibular Loss on Orthostatic Mechanisms

当直立时，保持稳定的血压受到立位(静水)柱加长的挑战。我们假设，前庭耳石器官提供短潜伏期定位信息，有助于在姿势变化期间自主神经心肺调节。这一假设将首先通过比较不同方向的直立倾斜时的血压和循环的非侵入性测量来检验，这些方向对耳石提供相同的立位应力但不同的刺激。我们预测，由于对耳石的不同刺激，健康受试者对直立倾斜的早期心肺反应将随倾斜方向的不同而不同，而在直立倾斜后期的反应将是相似的，无论倾斜方向如何，因为激活了其他心肺(如压力感受器反射)机制。其次，我们将通过比较已知的双侧前庭缺陷患者的倾斜数据和年龄和性别匹配的健康个体的相应数据，来评估前庭缺失对直立机制的影响。我们预测，无论倾斜的方向如何，前庭缺失患者对直立倾斜的早期心肺反应都将受到影响。12名双侧前庭功能丧失的受试者和12名健康受试者将在一次测试中进行四次10分钟的倾斜，眼睛从水平方向闭合到80度直立(仰卧倾斜、俯仰倾斜、右耳下滚倾斜和左耳下滚倾斜)。在倾斜刺激过程中测量动脉血压、心率、每搏量、外周血管阻力、呼气末二氧化碳分压、呼吸频率和容量、脑血流速度和脑血管阻力的变化。在第二组实验中，当受试者以不同的频率绕地球水平轴旋转时，耳石对立位反应的贡献将被评估，头部朝前或转向一侧。我们预测，在健康受试者中，心肺反应将被调节为头部朝向相对于重力和频率的函数，而在双侧前庭缺陷症受试者中则不会。这项研究将提供关于前庭缺失如何增加立位不耐受易感性的新信息，并将为耳石自主受试者提供新的见解。

When standing upright, the maintenance of stable blood pressure is challenged by the lengthening of the orthostatic (hydrostatic) column. We hypothesize that the vestibular otolith organs provide short-latency orientation information that contributes to autonomic cardio-respiratory regulation during changes in posture. This hypothesis will be tested by first comparing non-invasive measurements of blood pressure and circulation during upright tilts from different orientations that provide the same orthostatic stress but different stimuli to the otoliths. We predict that due to varying stimuli to the otoliths, the early cardio- respiratory responses to upright tilt in healthy subjects will differ depending on the direction of tilt whereas responses during the later portions of upright tilt will be similar regardless of the direction of tilt due to activation of other cardio-respiratory (e.g., baroreflex) mechanisms. Second, we will assess the effects of vestibular loss on orthostatic mechanisms by comparing tilt data from subjects with known bilateral vestibular deficiency to corresponding data from healthy individuals matched for age and gender. We predict the early cardio-respiratory responses to upright tilt in patients with vestibular loss will be compromised regardless of the direction of tilt. Twelve subjects with bilateral loss of vestibular function and twelve healthy subjects will be tested during one session consisting of four 10 min tilts performed with eyes closed from a horizontal orientation to 80 deg upright (supine - pitch tilt, prone - pitch tilt, right ear down - roll tilt and left ear down - roll tilt). Modulation of arterial blood pressure, heart rate, stroke volume, peripheral vascular resistance, end-tidal C02, respiratory rate and volume, cerebral blood flow velocity and cerebrovascular resistance will be measured during the tilt stimuli. During a second set of experiments, the contribution of the otoliths to orthostatic responses will be assessed as subjects are continuously rotated about an earth horizontal axis at varying frequencies with head facing straight ahead or turned to the side. We predict that cardio- respiratory responses will be modulated as a function of head orientation relative to gravity and frequency in healthy subjects but not in subjects with bilateral vestibular deficiency. This study will provide new information on how vestibular loss might increase susceptibility for orthostatic intolerance, and will provide new insights on otolith- autonomic human subjects.