Effect of G-suit protection on carotid-cardiac baroreflex function.

INTRODUCTION: To test the hypothesis that G-suit inflation could increase cardiac chronotropic responses to baroreceptor stimulation and enhance baroreflex buffering of BP, the carotid-cardiac baroreflex response of 12 subjects was measured across two levels of lower body negative pressure (LBNP = 0 and 50 mm Hg) and two levels of G-suit inflation (0 and 50 mm Hg) in random order. METHODS: Carotid-cardiac baroreflex stimulation was delivered via a silastic neck pressure cuff and responsiveness quantified by determination of the maximum slope of the stimulus-response function between R-R intervals (ms) and their respective carotid distending pressures (mmHg). RESULTS: Mean +/- SE baseline control baroreflex responsiveness was 3.8+/-0.4 ms x mm Hg(-1). LBNP reduced the baroreflex response to 2.7+/-0.4 ms x mm Hg(-1), but G-suit inflation with LBNP restored the baroreflex response to 4.3+/-0.6 ms x mm Hg(-1). CONCLUSIONS: These results suggest that, in addition to increased venous return and elevated peripheral resistance, G-suit inflation may provide protection against the debilitating effects of blood distribution to the lower extremities during orthostatic challenges such as standing or high +Gz acceleration by increasing cardiovascular responsiveness to carotid baroreceptor stimulation.

Alterations in the volume stimulus-renal response relationship during exposure to simulated microgravity.

We measured central venous pressure (CVP), plasma volume (PV), urine volume rate (UVR), and circulating hormones (renin activity (PRA), vasopressin (AVP), atrial natriuretic peptide (ANP), and cortisol) before and after acute volume infusion (Dextran-40) to test the hypotheses that head-down tilt bedrest (HDT) caused (1) a resetting of the CVP operating point and (2) attenuated urine excretion. Six rhesus monkeys underwent two experimental conditions (HDT and control, each of 48 hour duration) with each condition separated by nine days of ambulatory activities to produce a cross-over counterbalance design. One test condition was continuous exposure to 10 degrees HDT and the second test condition was a control, defined as approximately 12-14 hours per day of 80 degrees head-up tilt and 10-12 hours prone. Following 48 hours of exposure to either test condition, 20-minute continuous infusion of Dextran-40 was administered. CVP in HDT was lower than the control condition. Similar elevations in CVP occurred 30 min post-infusion in both test conditions, and returned to pre-infusion baseline levels between 22 and 46 h post-infusion in both treatments. The UVR response during infusion was attenuated by HDT despite similar elevation in CVP. Elevation in ANP and reduction in PRA at the end of infusion were greater in Control compared to HDT. No differences between control and HDT were detected for AVP and cortisol responses to infusion. Since CVP returned to its pre-infusion levels following volume loading in HDT and control conditions, it appeared that the lower CVP may reflect a new operating point about which vascular volume is regulated. Further, attenuated ANP and PRA responses during vascular volume loading may contribute to depressed UVR in low gravity exposure.

Evaluation of dual-tip micromanometers during 21-day implantation in goats.

Investigative research efforts using a cardiovascular model required the determination of central circulatory haemodynamic and arterial system parameters for the evaluation of cardiovascular performance. These calculations required continuous beat-to-beat measurement of pressure within the four chambers of the heart and great vessels. Sensitivity and offset drift, longevity, and sources of error for eight 3F dual-tipped micromanometers were determined during 21 days of implantation in goats. Subjects were instrumented with pairs of chronically implanted fluid-filled access catheters in the left and right ventricles, through which dual-tipped (test) micromanometers were chronically inserted and single-tip (standard) micromanometers were acutely inserted. Acutely inserted sensors were calibrated daily and measured pressures were compared in vivo to the chronically inserted sensors. Comparison of the pre- and post-gain calibration of the chronically inserted sensors showed a mean sensitivity drift of 1.0 +/- 0.4% (99% confidence, n = 9 sensors) and mean offset drift of 5.0 +/- 1.5 mmHg (99% confidence, n = 9 sensors). Potential sources of error for these drifts were identified, and included measurement system inaccuracies, temperature drift, hydrostatic column gradients, and dynamic pressure changes. Based upon these findings, we determined that these micromanometers may be chronically inserted in high-pressure chambers for up to 17 days with an acceptable error, but should be limited to acute (hours) insertions in low-pressure applications.

Evidence for increased cardiac compliance during exposure to simulated microgravity.

We measured hemodynamic responses during 4 days of head-down tilt (HDT) and during graded lower body negative pressure (LBNP) in invasively instrumented rhesus monkeys to test the hypotheses that exposure to simulated microgravity increases cardiac compliance and that decreased stroke volume, cardiac output, and orthostatic tolerance are associated with reduced left ventricular peak dP/dt. Six monkeys underwent two 4-day (96 h) experimental conditions separated by 9 days of ambulatory activities in a crossover counterbalance design: 1) continuous exposure to 10 degrees HDT and 2) approximately 12-14 h per day of 80 degrees head-up tilt and 10-12 h supine (control condition). Each animal underwent measurements of central venous pressure (CVP), left ventricular and aortic pressures, stroke volume, esophageal pressure (EsP), plasma volume, alpha1- and beta1-adrenergic responsiveness, and tolerance to LBNP. HDT induced a hypovolemic and hypoadrenergic state with reduced LBNP tolerance compared with the control condition. Decreased LBNP tolerance with HDT was associated with reduced stroke volume, cardiac output, and peak dP/dt. Compared with the control condition, a 34% reduction in CVP (P = 0.010) and no change in left ventricular end-diastolic area during HDT was associated with increased ventricular compliance (P = 0.0053). Increased cardiac compliance could not be explained by reduced intrathoracic pressure since EsP was unaltered by HDT. Our data provide the first direct evidence that increased cardiac compliance was associated with headward fluid shifts similar to those induced by exposure to spaceflight and that reduced orthostatic tolerance was associated with lower cardiac contractility.

Evaluation of transit-time and electromagnetic flow measurement in a chronically instrumented nonhuman primate model.

The Physiology Research Branch at Brooks AFB conducts both human and nonhuman primate experiments to determine the effects of microgravity and hypergravity on the cardiovascular system and to identify the particular mechanisms that invoke these responses. Primary investigative efforts in our nonhuman primate model require the determination of total peripheral resistance, systemic arterial compliance, and pressure-volume loop characteristics. These calculations require beat-to-beat measurement of aortic flow. This study evaluated accuracy, linearity, biocompatability, and anatomical features of commercially available electromagnetic (EMF) and transit-time flow measurement techniques. Five rhesus monkeys were instrumented with either EMF (3 subjects) or transit-time (2 subjects) flow sensors encircling the proximal ascending aorta. Cardiac outputs computed from these transducers taken over ranges of 0.5 to 2.0 L/min were compared to values obtained using thermodilution. In vivo experiments demonstrated that the EMF probe produced an average error of 15% (r = .896) and 8.6% average linearity per reading, and the transit-time flow probe produced an average error of 6% (r = .955) and 5.3% average linearity per reading. Postoperative performance and biocompatability of the probes were maintained throughout the study. The transit-time sensors provided the advantages of greater accuracy, smaller size, and lighter weight than the EMF probes. In conclusion, the characteristic features and performance of the transit-time sensors were superior to those of the EMF sensors in this study.