Abstract

We investigated non-invasively cardiac contractility and autonomic nervous activity during presyncopal orthostatic stress induced in healthy humans. A graded orthostatic stress (GOS) paradigm, consisting of head-up tilt (HUT) combined with lower body negative pressure (LBNP) of increasing magnitude, was used to reach a presyncopal end-point in 15 healthy adults. Continuous beat-to-beat hemodynamic and autonomic parameters were recorded. From supine control (C1) to presyncope (PS), total peripheral resistance index (TPRI) decreased from 2300+/-500 to 1910+/-320 dyne*s*m(2)/cm(5) (p=0.004), index of contractility (IC) from 59+/-14 to 27+/-6 1000/s (p<0.0001), left ventricular working index (LVWI) from 5.2+/-1.3 vs. 3.6+/-0.6 mmHg*L/(min*m(2)) (p=0.0001) and acceleration index (ACI) from 65+/-18 vs. 54+/-15 100/s(2) (p=0.04). Low frequency variation of diastolic blood pressure (LF(nu)dBP) increased from 51+/-14 to 67+/-11 % (p=0.0006) and of systolic blood pressure (LF(nu)sBP) from 50+/-6 vs. 67+/-8 % (p<0.0001). High frequency variation of RR-interval (HF(ms(2))RRI) decreased from 385+/-320 to 38+/-43 ms(2) (p=0.001). From late GOS (G3) to PS, TPRI decreased from 2540+/-640 to 1910+/-320 dyne*s*m(2)/cm(5) (p=0.003), IC from 35+/-6 to 27+/-6 1000/s (p=0.003), LVWI from 4.6+/-0.9 to 3.6+/-0.6 mmHg*L/(min/m(2)) (p=0.003), LF(nu)sBP from 71+/-8 to 67+/-8 % (p=0.03), LF(mmHg(2))dBP from 6.6+/-4.0 to 4.8+/-2.9 mmHg(2) (p=0.0001), LF(mmHg(2))sBP from 9.7+/-7.8 to 7.4+/-4.8 mmHg(2) (p=0.01). HF(nu)RRI increased from 19+/-8 to 28+/-13 % (p=0.008). Myocardial contractility indices and parameters of sympathetic activity were reduced in the presyncopal state, while parasympathic activity was increased. This suggests a decrease in cardiac contractility during orthostatically induced presyncope in healthy subjects.