Central venous pressure-volume relationship with head-down tilt and LBNP.
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Adult↗
Biomedical subjects
D R Northey
Publications and source records attributed to D R Northey.
Heart rate variability and fractal dimension during orthostatic challenges.
Heart rate variability (HRV) spectral analysis has been used as a tool for short-term assessment of parasympathetic (PNS) and sympathetic nervous system (SNS) control of heart rate. However, it has been suggested that the PNS and SNS indicators are superimposed on a broad-band noise spectrum in which the power spectral densities are inversely proportional to their frequency (1/f beta). In this study, we have used coarse-graining spectral analysis to extract the harmonic components for calculation of PNS and SNS indicators and to obtain the slope (beta) of the 1/f beta component to estimate fractal dimension (DF) of a trail of HRV. DF was regarded as an indicator of cardiovascular system complexity. Ten healthy young subjects (6 women and 4 men) were studied in supine rest and with sequential applications of four levels of lower body negative pressure (LBNP; -10, -20, -30, and -50 mmHg) and head-up tilt (HUT; 10, 20, 30, and 70 degrees). In the 20 tests, there were six occurrences of presyncopal symptoms that required the test to be terminated before the planned end point. At low levels of LBNP or HUT, arterial pulse pressure (PP) was not changed from rest, and calculated DF was very high (beta approximately 1.00). At the higher levels of LBNP and HUT, PP decreased. Coincident with this reduction in PP, PNS activity decreased, SNS activity increased, and DF was reduced, each with a significant linear relationship to the change in PP (PNS: r = 0.56; SNS: r = 0.57; DF: r = 0.70, P < 0.01). Each occurrence of presyncope was associated a low PNS indicator as well as DF < 2.50 (beta > or = 1.80). These data indicate that the cardiovascular system is operating at a reduced level of complexity and further suggest that reduced complexity might not be compatible with cardiovascular homeostasis.
Adult↗
Probing heart rate and blood pressure control mechanisms during graded levels of lower body negative pressure (LBNP).
Following space flight or head down tilt bed rest, cardiovascular deconditioning is often observed as a failure to maintain arterial blood pressure with symptoms of presyncope or syncope. LBNP can be used as a stressor of the cardiovascular system to observe the regulatory process. We have recently developed a new method to study cardiovascular control. Coarse graining spectral analysis (CGSA), allows simultaneous extraction of the harmonic components to evaluate sympathetic and parasympathetic nervous activities, and of the underlying complexity of the physiological response as given by the slope (beta) or fractal dimension (DF). The recognition that system complexity plays a major role in maintenance of cardiovascular stability is a relatively new concept. It was the purpose of the present study to examine the underlying complexity of heart rate and systolic blood pressure (SBP) variablities as indicated by the DF and power spectral analysis. Eight healthy men completed a test protocol of 20 min supine rest followed sequentially by 10 min at -5, -15, -25, -40, and -50 mmHg LBNP, and 10 min supine recovery. At rest, DF of R-R interval was 3.57 (beta = 1.56 +/- 0.12). There was a progressive decline in DF with LBNP, until at -50 mmHg, DF decreased to 1.2 (beta = 2.66 +/- 0.09). The DF for SBP was 1.3 (beta = 2.1 +/- 0.18) at rest, and was not significantly changed during LBNP. In the mid- and high-frequency ranges of the spectra, there was a moderately high degree of coherence between the variability in R-R interval and SBP. These findings indicate that short term SBP has a relatively low and unchanged complexity compared to heart rate. The changes in DF of heart rate variability were marked with increasing levels of LBNP. The results of this, and our previous study, indicate that a decline of DF to a critical level (near 1.4) is associated with orthostatic hypotension. These data show the utility of simple, non-invasive methods of data collection in conjunction with sophisticated data analysis techniques to point to possible mechanisms of orthostatic hypotension.
Adult↗
Reduced orthostatic tolerance following 4 h head-down tilt.
The cardiovascular responses to a 10-min 1.22 rad (70 degrees) head-up tilt orthostatic tolerance test (OST) was observed in eight healthy men following each of a 5-min supine baseline (control), 4 h of 0.1 rad (6 degrees) head-down tilt (HDT), or 4 h 0.52 rad (30 degrees) head-up tilt (HUT). An important clinical observation was presyncopal symptoms in six of eight subjects following 4 h HDT, but in no subjects following 4 h HUT. Immediately prior to the OST, there were no differences in heart rate, stroke volume, cardiac output, mean arterial pressure and total peripheral resistance for HDT and HUT. However, stroke volume and cardiac output were greater for the control group. Mean arterial pressure for the control group was less than HDT but not HUT. Over the full 10-min period of OST, the mean arterial pressure was not different between groups. Heart rate increased to the same level for all three treatments. Stroke volume decreased across the full time period for control and HDT, but only at 3 and 9 min for HUT. There was a higher total peripheral resistance in the HDT group than control or HUT. The pre-ejection period to left ventricular ejection time ratio was less in HDT than for control or HUT groups. These data indicate a rapid adaptation of the cardiovascular system to 4 h HDT that appears to be inappropriate on reapplication of a head to foot gravity vector. We speculate that the cause of the impaired orthostatic tolerance is decreased tone in venous capacitance vessels so that venous return is inadequate.
Adult↗
Alignment of ventilation and gas fraction for breath-by-breath respiratory gas exchange calculations in exercise.
The effects of correct alignment of the ventilation and fractional gas concentration during breath-by-breath calculation of oxygen uptake (VO2) have been examined during exercise in four subjects, at each of 50, 100, 150, and 200 W. Data were analyzed using lagtimes in the range of 200 to 400 msec to align ventilation with gas fraction. VO2 was markedly affected by lagtime. Calculated values of VO2 at 200 W ranged from 1953 +/- 55 (mean +/- SEM), to 2583 +/- 29, to 2843 +/- 28 ml.min-1 with lagtimes of 200, 310, and 400 msec, respectively. Mean values from a mixing box system did not differ significantly from the mean of the breath-by-breath data collection with a lagtime of 310 msec. Additional computations have shown that temperature correction can markedly affect calculated ventilatory volumes and N2 balance. VO2 was not changed because of the compensation from the calculated effects of changes in lung gas stores.
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Effect of hypoxia on VO2 kinetics during pseudorandom binary sequence exercise.
The dynamic response characteristics of the oxygen uptake (VO2) response were investigated during upright cycle ergometer exercise in six healthy male volunteers. The exercise test consisted of a pseudorandom binary sequence (PRBS) with 15 units per sequence, each unit 15 s long, for a total period of 225 s. Six identical sequences were completed in a single test session. Each subject exercised under both normoxic and hypoxic (FIO2 = 14%) conditions. VO2 was measured breath-by-breath. The data were analyzed in the frequency domain by Fourier analysis to yield amplitude and phase shift coefficients for the relationship between the input work rate and the output responses of VO2 and heart rate (HR). The amplitude of the VO2/work rate was significantly reduced by hypoxia compared to normoxia over a wide range of frequencies. The mean VO2 was not different between hypoxia and normoxia. The phase shift for the VO2/work rate response was significantly greater for hypoxia than normoxia. The amplitude of the HR/work rate relationship was not significantly altered by hypoxia; however, the mean HR was higher during hypoxia. The phase shift of the HR/work rate response was significantly different between hypoxia and normoxia only at certain frequencies. These data indicate that the effects of hypoxia on the cardiorespiratory response to exercise can be characterized by the use of PRBS exercise and Fourier analysis techniques. A significant reduction in the ability of the cardiorespiratory system to adapt to changes in work rate appears to be caused by a reduction in the arterial O2 content.
Adult↗
Oxygen uptake and heart rate responses to treadmill and water running.
The relationship between heart rate (HR) and oxygen uptake (VO2) was assessed during treadmill running and during running-like motions in water without ground support. Athletes self-selected easy, moderate and hard efforts during water running. While there was a linear relationship between HR and VO2 in both exercise conditions, there was considerable between-subject variability of the slope and intercept of the linear regressions. These data are taken to indicate that HR for a given VO2 during water compared to land running is influenced in large part by the skill of the athlete in the water running activity.
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