Aim: 

We studied sympathetic activity changes in relation to myocardial contractility, haemodynamic parameters and hormones in subjects undergoing orthostatic challenge.

Methods: 

We used a graded orthostatic stress paradigm consisting of head – up tilt combined with lower body negative pressure up to a presyncopal end-point in 15 healthy males. Low – frequency and high-frequency oscillations were estimated from heart period and diastolic pressure values.

Results: 

From supine control to presyncope, heart rate increased by 74 ± 11% (p < 0.0001) whereas stroke index decreased by 46 ± 4% (p < 0.0001). Thoracic impedance rose by 12 ± 1% (p < 0.0001), total peripheral resistance index decreased by 12 ± 5% (p = 0.03) and end diastolic index by 33 ± 4% (p < 0.0001). Baroreflex sensitivity decreased significantly by 62 ± 7% (p = 0.0004) as well as baroreflex effectiveness index by 56 ± 6% (p = 0.0004) but the sympathetic tone increased significantly by 39 ± 9% (p = 0.0004). Significant decreases in index of contractility (51 ± 4%; p < 0.0001), acceleration index (13 ± 7%; p = 0.02) and left ventricular working index (27 ± 4%; p < 0.0001) were observed. Increased plasma norepinephrine (86 ± 16%, p = 0.0001), epinephrine (460 ± 266%; p = 0.06), cortisol (10 ± 6%; p = 0.02), plasma renin activity (147 ± 26%; p = 0.002) and aldosterone (24 ± 21%; p = 0.2) were noted.

Conclusion: 

In spite of reduction in myocardial contractility observed sympathetic activity increased sixty seconds before presyncope.