Orthostatic intolerance and motion sickness after parabolic flight.

Because it is not clear that the induction of orthostatic intolerance in returning astronauts always requires prolonged exposure to microgravity, we investigated orthostatic tolerance and autonomic cardiovascular function in 16 healthy subjects before and after the brief micro- and hypergravity of parabolic flight. Concomitantly, we investigated the effect of parabolic flight-induced vomiting on orthostatic tolerance, R-wave-R-wave interval and arterial pressure power spectra, and carotid-cardiac baroreflex and Valsalva responses. After parabolic flight 1) 8 of 16 subjects could not tolerate 30 min of upright tilt (compared to 2 of 16 before flight); 2) 6 of 16 subjects vomited; 3) new intolerance to upright tilt was associated with exaggerated falls in total peripheral resistance, whereas vomiting was associated with increased R-wave-R-wave interval variability and carotid-cardiac baroreflex responsiveness; and 4) the proximate mode of new orthostatic failure differed in subjects who did and did not vomit, with vomiters experiencing comparatively isolated upright hypocapnia and cerebral vasoconstriction and nonvomiters experiencing signs and symptoms reminiscent of the clinical postural tachycardia syndrome. Results suggest, first, that syndromes of orthostatic intolerance resembling those developing after space flight can develop after a brief (i.e., 2-h) parabolic flight and, second, that recent vomiting can influence the results of tests of autonomic cardiovascular function commonly utilized in returning astronauts.

Inspiratory CO2 increases orthostatic tolerance during repeated tilt.

INTRODUCTION: The partial pressure of end tidal CO2 (PetCO2) is known to decrease with head-up tilt. Decreases in arterial CO2 reduce cerebral blood flow (CBF) and may increase the incidence of presyncope. We measured cerebral and central cardiovascular responses to repeated tilt where: 1) PetCO2 was allowed to change with tilt (eucapnic): and 2) PetCO2 was clamped at supine levels (isocapnic). METHODS: In eight healthy subjects breath-by-breath measurements were made of ventilation (VE) and PetCO2 along with beat-by-beat measurements of blood pressure (BP), heart rate (HR) and middle cerebral artery mean flow velocities (MFV). Following 30-min in the supine position, a series of six 10-min 90 degrees head-up tilts were performed, with 30-s of supine between each. Presyncopal subjects were returned immediately to the supine position. RESULTS: Statistical comparisons were made between the supine, and the first and last minute of the first tilt. BP, HR responses were not different between the eu- and isocapnic conditions; however, by the end of the first tilt VE was significantly higher than supine. MFV and BP at brain level decreased and HR increased from supine to tilt. MFV was higher in the isocapnic compared with the eucapnic condition but decreased from the beginning to the end of the first tilt in both conditions (i.e., tilt #1: eucap. 49.4 to 46.7; isocap. 65.0 to 59.6 cm s(-1); p < 0.05) while the BP remained constant. Five subjects were presyncopal in the study. With isocapnic tilt, presyncopal time was not reduced but was extended in four of the five subjects (2.2, 5.5, 6.3 and 31 min) yet at presyncope the values for MFV, BP and HR were the same in both conditions. CONCLUSIONS: Inspiratory CO2 contributed to increased MFV at the beginning of tilt and increased orthostatic tolerance.

Complexity of middle cerebral artery blood flow velocity: effects of tilt and autonomic failure.

We examined spectral fractal characteristics of middle cerebral artery (MCA) mean blood flow velocity (MFV) and mean arterial blood pressure adjusted to the level of the brain (MAPbrain) during graded tilt (5 min supine, -10 degrees, 10 degrees, 30 degrees, 60 degrees, -10 degrees, supine) in eight autonomic failure patients and age- and sex-matched controls. From supine to 60 degrees, patients had a larger drop in MAPbrain (62 +/- 4.7 vs. 23 +/- 4.5 mmHg, P < 0.001; means +/- SE) and MFV (16.4 +/- 3.8 vs. 7.0 +/- 2.5 cm/s, P < 0.001) than in controls. From supine to 60 degrees, there was a trend toward a decrease in the slope of the fractal component (beta) of MFV (MFV-beta) in both the patients and the controls, but only the patients had a significant decrease in MFV-beta (supine: patient = 2.21 +/- 0.18, control = 1.99 +/- 0.60; 60 degrees: patient = 1.46 +/- 0.24, control = 1.62 +/- 0.19). The beta value of MAPbrain (MAPbrain-beta; 2.19 +/- 0.05) was not significantly different between patients and controls and did not change with tilt. High and low degrees of regulatory complexity are indicated by values of beta close to 1.0 and 2.0, respectively. The increase in fractal complexity of cerebral MFV in the patients with tilt suggests an increase in the degree of autoregulation in the patients. This may be related to the drop in MAPbrain. The different response of MFV-beta compared with that of MAPbrain-beta also indicates that MFV-beta is related to the regulation of cerebral vascular resistance and not systemic blood pressure.

Cerebrovascular and cardiovascular responses to graded tilt in patients with autonomic failure.

BACKGROUND AND PURPOSE: Patients with autonomic nervous system failure often experience symptoms of orthostatic intolerance while standing. It is not known whether these episodes are caused primarily by a reduced ability to regulate arterial blood pressure or whether changes in cerebral autoregulation may also be implicated. METHODS: Eleven patients and eight healthy age- and sex-matched control subjects were studied during a graded-tilt protocol. Changes in their steady state middle cerebral artery mean flow velocities (MFV), measured by transcranial Doppler, brain-level mean arterial blood pressures (MABPbrain), and the relationship between the two were assessed. RESULTS: Significant differences between patients and control subjects (P < .05) were found in both their MFV and MABPbrain responses to tilt. Patients' MFV dropped from 60 +/- 10.2 cm/s in the supine position to 44 +/- 14.0 cm/s at 60 degrees head-up tilt, whereas MABPbrain fell from 109 +/- 11.7 to 42 +/- 16.9 mm Hg. By comparison, controls' MFV dropped from 54 +/- 7.8 cm/s supine to 51 +/- 8.8 cm/s at 60 degrees, whereas MABPbrain went from 90 +/- 11.2 to 67 +/- 8.2 mm Hg. Linear regression showed no significant difference in the MFV-MABPbrain relationship between patients and control subjects, with slopes of 0.228 +/- 0.09 cm.s-1.mm Hg-1 for patients and 0.136 +/- 0.16 cm.s-1.mm Hg-1 for control subjects. CONCLUSIONS: The present study found significant differences between patients and control subjects in their MFV and MABPbrain responses to tilt but no difference in the autoregulatory MFV-MABPbrain relationship. These results suggest that patients' decreased orthostatic tolerance may primarily be the result of impaired blood pressure regulation rather than a deficiency in cerebral autoregulation.

Transfer function analysis of cerebral autoregulation dynamics in autonomic failure patients.

BACKGROUND AND PURPOSE: Autonomic nervous system diseases affect systemic blood pressure regulation. Patients with autonomic nervous system diseases have consistently larger drops in blood pressure associated with standing than the normal population. Autonomic dysfunction and/or these changes in blood pressure may affect dynamic cerebral autoregulation. METHODS: Heart rate, mean blood flow velocity (MBFV) of the middle cerebral artery via transcranial Doppler ultrasound, mean arterial blood pressure adjusted to brain level (MABPbrain) via Finapres, and end tidal CO2 were measured continuously during graded tilt (after 5 minutes in supine position as baseline, -10 degrees, +10 degrees, +30 degrees, +60 degrees, -10 degrees, and supine recovery) in autonomic failure patients and their age- and sex-matched control subjects. The dynamic response of MBFV to spontaneous variations in MABPbrain was investigated by cross-spectral analysis. The transfer gain and phase relationships between MBFV and MABPbrain were determined from the final 256 beats of each 5-minute-tilt segment. The transfer gain was normalized to mean MABPbrain and MBFV and then converted to decibels (dB). RESULTS: MBFV variation (0.03 to 0.14 Hz) preceded MABPbrain by similar phase angles in patients and control subjects and in all tilt conditions (patients: 31 +/- 5 degrees; control subjects: 30 +/- 5 degrees; mean +/- SEM). Patients had a higher supine gain than control subjects (P < .05). Both patients and control subjects showed a significant decrease in gain with tilt and by 60 degrees the patients were not different from the control subjects (supine to 60 degrees: patients = 5.23 +/- 0.77 to -1.65 +/- 0.89 dB; control subjects = 1.74 +/- 0.82 to -1.80 +/- 0.62 dB). CONCLUSIONS: These data indicate an altered, yet present, autoregulatory response with autonomic failure.

Vertical shift in cerebral autoregulation curve: a graded head-up tilt study.

According to the classical theory of cerebral autoregulation, cerebral blood flow (CBF) will be maintained at a near-constant level for cerebral perfusion pressure (CPP) within the range of 60-150 mmHg. In recent years, however, studies with contradicting results have demonstrated a change in CBF when the level of CPP had not changed significantly. It is suggested that a shift in the autoregulation curve may have taken place along the horizontal axis or along the vertical axis. This paper describes previously unreported findings of a graded head-up tilt (HUT) study which was designed as a preliminary experiment to test the protocol for possible use on astronauts upon returning from space-flight. Data from this study tend to support the findings of previous studies regarding possible shifts in the cerebral autoregulation curve. Five female and four male healthy volunteers were exposed to HUT for 5 minutes at each angle of 30 degrees, 60 degrees, and 90 degrees. Throughout the test, cerebrovascular and cardiovascular responses were evaluated by use of continuous acquisition of mean flow velocity (MFV) from the right middle cerebral artery with transcranial Doppler sonograhy, mean arterial blood pressure (MABP, Finapres) and heart rate (HR, ECG). Based on the last 60 seconds of data at each tilt angle and baseline, the percentage changes in MFV from baseline were found to be similar to those in MABP at the level of the brain (MABPbrain), and both were significant (p<0.05) at the 60 degrees (-9.1 +/- 7% for MABP brain, -9.8 +/- 5% for MFV) and 90 degrees (-13 degrees +/- 8%, -12.0 +/- 6%) positions. Heart rate (HR) increased significantly (p<0.05) from the baseline at 30 degrees (6.3 +/- 5%) through 90 degrees (23.3 +/- 8%). The trend toward decreasing MFV in normal subjects, even while MABP brain remained within the normal limits of cerebral autoregulation, may suggest a downward shift of the cerebral blood flow plateau in the classic cerebral autoregulation curve.