Carr, D. L. | Search NASA

NASA NTRS1998

Mechanisms underlying very-low-frequency RR-interval oscillations in humans

BACKGROUND: Survival of post-myocardial infarction patients is related inversely to their levels of very-low-frequency (0.003 to 0.03 Hz) RR-interval variability. The physiological basis for such oscillations is unclear. In our study, we used blocking drugs to evaluate potential contributions of sympathetic and vagal mechanisms and the renin-angiotensin-aldosterone system to very-low-frequency RR-interval variability in 10 young healthy subjects. METHODS AND RESULTS: We recorded RR intervals and arterial pressures during three separate sessions, with the patient in supine and 40 degree upright tilt positions, during 20-minute frequency (0.25 Hz) and tidal volume-controlled breathing after intravenous injections: saline (control), atenolol (0.2 mg/kg, beta-adrenergic blockade), atropine sulfate (0.04 mg/kg, parasympathetic blockade), atenolol and atropine (complete autonomic blockade), and enalaprilat (0.02 mg/kg, ACE blockade). We integrated fast Fourier transform RR-interval spectral power at very low (0.003 to 0.03 Hz), low (0.05 to 0. 15 Hz), and respiratory (0.2 to 0.3 Hz) frequencies. Beta-adrenergic blockade had no significant effect on very-low- or low-frequency RR-interval power but increased respiratory frequency power 2-fold. ACE blockade had no significant effect on low or respiratory frequency RR-interval power but modestly (approximately 21%) increased very-low-frequency power in the supine (but not upright tilt) position (P<0.05). The most profound effects were exerted by parasympathetic blockade: Atropine, given alone or with atenolol, abolished nearly all RR-interval variability and decreased very-low-frequency variability by 92%. CONCLUSIONS: Although very-low-frequency heart period rhythms are influenced by the renin-angiotensin-aldosterone system, as low and respiratory frequency RR-interval rhythms, they depend primarily on the presence of parasympathetic outflow. Therefore the prognostic value of very-low-frequency heart period oscillations may derive from the fundamental importance of parasympathetic mechanisms in cardiovascular health.

Non-NASA Center↗

NASA NTRS1981

Ion streams in the magnetotail

Ion mass spectrometer observations of low-temperature streaming plasmas in the earth's magnetotail are reported. Measurements in the energy per charge range 0 to 17 keV/e were made at geocentric radial distances less than 23 earth radii from the ISEE 1 spacecraft. Ion streams of solar wind origin in the magnetotail boundary layer and of ionospheric origin in the tail lobes and plasma sheet are described. A statistical study of some of the characteristics of the streams allows the inference that the central tail lobe plasmas are primarily constituted of ion streams of ionospheric origin and that the ionosphere is a significant contributor to the hot plasmas that form the plasma sheet.

Sharp, R. D.↗

NASA NTRS1971

Coordinated auroral-electron observations from a synchronous and a polar satellite.

The ATS 5 satellite in synchronous orbit and the OV1-18 satellite in a low-altitude polar orbit carried similar experiments to measure auroral-particle fluxes in the energy range from 1/2 to 50 keV. Simultaneous electron flux observations are presented from 5 instances when the OV1-18 passed within a few hundred kilometers of the computed conjugate point to ATS 5. The integral flux levels were found to be generally comparable at the two locations with no evidence for angular distributions highly peaked along the field line. The spectrums were significantly harder at ATS 5 with average energies approximately a factor of 2 higher than at OV1-18. Evidence is presented for a dramatic difference in the shapes of the spectrums at the two locations with smoothly falling relatively featureless spectrums observed at ATS 5 and spectrums with a peak in the few keV range observed at OV1-18. Based on the 5 passes described here, there is evidence that the trapped component of the flux increases more rapidly than the precipitating component during more active times.

Sharp, R. D.↗