Mechanisms of tilt‐induced vasovagal syncope in healthy volunteers and postural tachycardia syndrome patients without past history of syncope

Figures

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  Figure 1. Head‐up tilt table test for a vasovagal syncope patient. Mean arterial pressure (MAP) is shown in the top panel and heart rate (HR) in the bottom panel. Fiducial points are marked at the top. MAP and HR follow a characteristic time course comprising four stages: Early blood pressure (BP) stabilization (Phase 1, fiducial point 2); “Progressive early hypotension” with BP slowly declining as HR increases (fiducial points 3–5); Syncope with hypotension and bradycardia (Phase 3, fiducial point 6). Phase 4 recovery of BP occurs within 30 sec once supine.

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  Figure 2. Blood pressure, heart rate, cardiac output, and systemic vascular resistance for representative subjects during head‐up tilt table testing. Figure 2 shows representative data from left to right for Control‐noFaint, Control‐Faint and POTS‐Faint subjects. Each data panel contains graphs of BP, HR, CO, and SVR in top to bottom order for representative subjects belonging to each group. HR increased in all groups during orthostatic stress. Following initial pressure stabilization there was a gradual fall off in BP in fainters representing Phase 2. During Phase 2, CO increased above baseline for Control‐Faint while SVR decreased and CO progressively decreased throughout tilt in POTS‐Faint while SVR increased. Phase 3 ensued with rapid hypotension followed by bradycardia associated with decreased SVR and CO in all fainters.

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  Figure 3. Averaged heart rate, mean arterial pressure, and pulse pressure during upright tilt. Heart rate (HR) appears in the upper panel, mean arterial pressure (MAP) in the middle panel and pulse pressure (PP) in the lower panel at designated fiducial time points. Control‐noFaint data are shown as open circles (Ο), Control‐Faint data are shown as black boxes (■), and POTS‐Faint are shown as black triangles (▲).HR was increased above control (P < 0.001) in all faint groups prior to faint. HR fell precipitously in all Fainters at the time of faint. MAP and PP decreased significantly throughout tilt in all VVS (P < 0.05) compared to Control‐noFaint subjects.

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  Figure 4. Regional blood flows during upright tilt. The top panel shows the cardiac output (CO) in L/min, the second shows percent change in splanchnic blood flow (%∆ Splanchnic Blood Flow), the third panel shows percent change in pelvic blood flow (%∆ Pelvic Blood Flow), and the bottom panel shows percent change in calf blood flow (%∆ Calf Blood Flow). Control‐noFaint data are shown as open circles (Ο), Control‐Faint data are shown as black boxes (■), POTS‐Faint are shown as black triangles (▲). CO decreased progressively in POTS‐Faint compared to Control‐noFaint (P < 0.001). Splanchnic blood flow was increased in Control‐Faint (P < 0.001).

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  Figure 5. Percent change in regional blood volumes during upright tilt. The top panel shows the percent change in central blood volume (%∆ Central Blood Volume), the second shows percent change in splanchnic blood volume (%∆ Splanchnic Blood Volume), the third panel shows percent change in pelvic blood volume (%∆ Pelvic Blood Volume) and the bottom panel shows percent change in calf blood volume (%∆ Calf Blood Volume). Control‐noFaint data are shown as open circles (Ο), Control‐Faint data are shown as black boxes (■), POTS‐Faint are shown as black triangles (▲). Central, splanchnic, pelvic, and calf blood volumes are expressed as percent change from supine, measured before subjects were tilted upright to 70°. The Percent Central Blood Volume (%CBV) decreased in all subjects with tilt. %CBV decreased significantly more in fainting subjects than Control‐noFaint (P < 0.01). The Percent Splanchnic Blood Volume (VolSpl) increased during tilt in all subjects, was similar to Control‐noFaint in Control‐Faint, but was significantly larger for POTS‐Faint compared to Control‐noFaint (P < 0.01). The Percent Pelvic Blood volume (VolPelv) increased similarly in all groups during tilt. %Calf Blood Volume…

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  Figure 6. Percent change in regional arterial vascular resistance during upright tilt. The top panel shows the percent change in systemic vascular resistance (SVR), the second shows percent change in splanchnic resistance (%ΔRSplanchnic), the third panel shows percent change in pelvic resistance (%ΔRPelvic) and the bottom panel shows percent change in calf resistance (%ΔRcalf). Control‐noFaint data are shown as open circles (Ο), Control‐Faint data are shown as black boxes (■), POTS‐Faint are shown as black triangles (▲). SVR and %ΔRSplanchnic were markedly reduced in Control‐Faint and slightly increased in POTS‐Faint. Splanchnic, pelvic, and calf blood vascular resistances are expressed as percent change from supine, measured before subjects were tilted upright to 70°. SVR increased by 1 min following tilt for all subjects. SVR then decreased progressively in Control‐Faint (P < 0.05). SVR remained similar to Control‐no‐Faint for POTS‐Faint SVR until syncope supervened. Splanchnic Resistance increased similarly for Control‐noFaint and POTS‐Faint until syncope occurred, while decreasing throughout tilt in Control‐Faint (P < 0.005).