Cardiac and Vascular Sympathetic Baroreflex Control during Orthostatic Pre-Syncope

Abstract

We hypothesized that sympathetic baroreflex mediated uncoupling between neural sympathetic discharge pattern and arterial pressure (AP) fluctuations at 0.1 Hz during baroreceptor unloading might promote orthostatic pre-syncope. Ten volunteers (32 ± 6 years) underwent electrocardiogram, beat-to-beat AP, respiratory activity and muscle sympathetic nerve activity (MSNA) recordings while supine (REST) and during 80° head-up tilt (HUT) followed by -10 mmHg stepwise increase of lower body negative pressure until pre-syncope. Cardiac and sympathetic baroreflex sensitivity were quantified. Spectrum analysis of systolic and diastolic AP (SAP and DAP) and calibrated MSNA (cMSNA) variability assessed the low frequency fluctuations (LF, ~0.1 Hz) of SAP, DAP and cMSNA variability. The squared coherence function (K²) quantified the coupling between cMSNA and DAP in the LF band. Analyses were performed while supine, during asymptomatic HUT (T₁) and at pre-syncope onset (T₂). During T₂ we found that: (1) sympathetic baroreceptor modulation was virtually abolished compared to T₁; (2) a progressive decrease in AP was accompanied by a persistent but chaotic sympathetic firing; (3) coupling between cMSNA and AP series at 0.1 Hz was reduced compared to T₁. A negligible sympathetic baroreceptor modulation during pre-syncope might disrupt sympathetic discharge pattern impairing the capability of vessels to constrict and promote pre-syncope.

Keywords: MSNA variability; baroreceptors; blood pressure variability; heart rate variability; muscle sympathetic nerve activity; power spectrum analysis; syncope; vasovagal.

Figure 1

Individual traces of arterial pressure (AP), integrated muscle sympathetic nerve activity (MSNA), heart rate (HR) and respiratory activity (Respiration), recorded in a representative subject while supine (REST), during the asymptomatic phase of a 80° head-up tilt (T₁) and during late tilt just before the pre-syncope onset (T₂). The right squared window highlights the pre-syncope changes: notice the enduring but chaotic neural sympathetic discharge activity as compared with MSNA trace at T₁ characterized by rhythmic sympathetic firing at 0.1 Hz (i.e., a cycle every 10 s); also, please note the concomitant progressive hypotension in spite of the compensatory tachycardia and sympathetic neural activity to the vessels (see above for further explanations).

Figure 2

Individual values of heat rate (HR), systolic arterial pressure (SAP) and MSNA while supine (REST), during asymptomatic 80° head-up tilt (T₁) and during pre-syncope (T₂), in the upper panels. In the lower panels values of the low frequency spontaneous oscillation of SAP (LF_SAP) and MSNA variability (LF_cMSNA) are depicted. Notice hypotension in the presence of persistently elevated HR and MSNA during T₂. In addition, T₂ was characterized by a decrease of the power of the low frequency components of both SAP and cMSNA variability. * p < 0.05 REST vs T₁. # p < 0.05 T₁ vs T₂. § p < 0.05 REST vs T₂.