Anatomic localization of trigger sites initiating stretch-induced hypotension in humans: a prospective observational study
A Shabbir, J L Reyes, M Kosmopoulos, C Keller, R Sutton, J G van Dijk, D G BendittAbstract
Background
Stretch-induced syncope (SIS) is a rare situational syncope. The different proposed mechanisms for which include: carotid baroreceptor stimulation, vertebral artery compression, and a Valsalva equivalent. Recently, however, we demonstrated that upper back and neck muscle stretching may trigger transient hypotension and relative cardio-inhibition suggesting a neural reflex mechanism. This study examined magnitude of change in beat-to-beat blood pressure and heart rate using autonomic testing in sites within the neck and upper back muscles to determine the primary origin for this neural reflex.
Purpose
To characterize the anatomic trigger sites and chronotropic responses underlying stretch-induced hypotension in humans, and to determine whether isolated neck and upper back stretching produce distinct and additive reflex cardiovascular effects consistent with a neural mediated mechanism.
Methods
We prospectively studied 28 healthy adults (21 female, mean age 32.7 ± 1.5 years) being evaluated for lightheadedness or syncope unrelated to stretching. All subjects underwent autonomic testing including active standing (AS), Valsalva maneuver and ratio (VR), respiratory sinus arrhythmia (RSA), tilt-table testing, and three stretching maneuvers in randomized order: isolated neck, isolated shoulder-back stretching, and a combined neck–back stretching maneuver. Beat-to-beat heart rate (HR) and blood pressure (BP) were recorded using electrocardiography and finger plethysmography. The primary outcome was magnitude of stretch-induced BP reduction and HR change for each test.
Results
AS, VR, and RSA were normal in all participants. However, stretching maneuvers induced a vaso-depression with BP fall in all maneuvers, but with site-dependent differing magnitudes. Neck extension caused least BP fall (ΔSBP −13.5 (-17, -13) mmHg) while shoulder-back stretching yielded greater BP reduction (ΔSBP −29 (-47.3, -19) mmHg, p < 0.05 vs neck). The combined maneuver triggered greatest BP drop (ΔSBP −37 (-50.3, -25) mmHg, p < 0.01 vs neck, p < 0.05 vs back alone). Further, for comparable magnitude of BP fall in each subject, the compensatory HR response was less with stretching than with AS, a finding consistent with the additional presence of cardio-inhibition.
Conclusion
SIS is a neural reflex triggered primarily by shoulder and back muscle stretching, with minimal contribution from isolated neck movement, thereby largely excluding other proposed mechanisms such as carotid baroreceptor stimulation or vertebral artery compression.FIgure 1Figure 2:BP responses in individual subjects