John P. Florian

Non-esterified fatty acids increase arterial pressure via central sympathetic activation in humans

Previous studies have shown that acute increases in plasma NEFAs (non-esterified fatty acids) raise SVR (systemic vascular resistance) and BP (blood pressure). However, these studies have failed to distinguish between CNS (central nervous system) mechanisms that raise sympathetic activity and paracrine mechanisms that increase SVR directly, independent of CNS involvement. The aim of the present study was to directly determine whether the sympathetic nervous system contributes to the pressor response to NEFAs. On 2 days separated by at least 2 weeks, 17 lean healthy volunteers (ten male/seven female; age, 22+/-1 years; body mass index, 23+/-1 kg/m2; values are means+/-S.E.M.) received a 4-h intravenous infusion of 20% Intralipid or placebo (in a single-blind randomized balanced order). MSNA (muscle sympathetic nerve activity), HR (heart rate), BP (oscillometric brachial measurement) and Q (cardiac output; acetylene rebreathing) were measured before and throughout infusion. The change in HR (+8.2+/-1.0 and +2.4+/-1.2 beats/min), systolic BP (+14.0+/-1.6 and +3.2+/-2.5 mmHg) and diastolic BP (+8.2+/-1.0 and -0.1+/-1.7 mmHg) were significantly greater after the 4-h infusion of Intralipid compared with placebo (P<0.001). The change in BP with Intralipid resulted from an increase in SVR (Q/mean arterial pressure; P<0.001) compared with baseline, without a change in Q. MSNA burst frequency increased during Intralipid infusion compared with baseline (+4.9+/-1.3 bursts/min; P<0.05), and total MSNA (frequencyxamplitude) was augmented 65% (P<0.001), with no change during placebo infusion. Lipid infusion increased insulin, aldosterone and F2-isoprostane, but not leptin, concentrations. On the basis of the concomitant increase in BP, MSNA and SVR, we conclude that central sympathetic activation contributes to the pressor response to NEFAs.

Sympathetic and haemodynamic responses to lipids in healthy human ageing

Plasma non‐esterified fatty acids (NEFAs) activate the sympathetic nervous system and increase vascular resistance and blood pressure (BP); however, the response with ageing is not known. The objectives of this study were to characterize the cardiovascular, neural and endocrine responses to acute elevation of NEFA concentration. Seventeen healthy older volunteers (7 male and 10 female; age, 69 ± 1 years; body mass index, 24 ± 0 kg m−2; values are means ±s.e.m.) received a 4 h intravenous infusion of the lipid emulsion Intralipid® 20% or placebo (single‐blind, randomized, balanced order) on two different days separated by at least 2 weeks. Muscle sympathetic nerve activity (MSNA), heart rate (HR), BP, cardiac output, leptin, insulin, aldosterone, angiotensin II and F2‐isoprostanes were measured. The change in HR (+8.8 ± 0.9 versus+3.0 ± 0.9 beats min−1), systolic BP (+13.9 ± 2.2 versus+6.6 ± 2.4 mmHg) and diastolic BP (+7.4 ± 1.5 versus+1.3 ± 0.8 mmHg) was significantly greater after Intralipid®versus placebo infusions (P < 0.001). Lipid infusion increased MSNA burst frequency (+6.7 ± 1.6 bursts min−1), total MSNA (+45%; P < 0.001) and concentrations of insulin (+40%), aldosterone (+50%) and F2‐isoprostanes (+80%), but not leptin. Hyperlipidaemia caused directionally opposite responses for insulin (increased) and calf vascular resistance (decreased) in men, whereas insulin and calf vascular resistance responses were severely blunted and non‐existent, respectively, in women. We conclude that direct vascular mechanisms and central sympathetic activation contribute to the NEFA pressor response; though absolute values are higher, the change is not different compared with previous studies in a younger population.

Sex differences in vasoconstrictor reserve during 70 deg head-up tilt.

Women are generally recognized to be less orthostatically tolerant than men. We hypothesized that during head‐up tilt (HUT), women would demonstrate less splanchnic vasoconstriction, leading to splanchnic pooling, lower blood pressure and lower orthostatic tolerance. Mean arterial blood pressure (MAP), heart rate (HR), cardiac output ( , assessed by C2H2 rebreathing), stroke volume, splanchnic blood flow (SpBF, assessed by Indocyanine Green clearance) and vascular conductance (systemic, ; splanchnic, SpVC = SpBF/MAP; non‐splanchnic, non‐SpVC = SVC – SpVC) were measured during supine baseline conditions, 70 deg HUT and recovery in 14 healthy women (23 ± 6 years old; mean ±s.d.) and 16 men (23 ± 5 years old). The proportion of sexes surviving 45 min of HUT trended towards significance (χ2= 2.92, P= 0.09). The MAP was lower in women than in men (supine, 77 ± 5 versus 86 ± 9 mmHg, P < 0.01; tilt, 72 ± 8 versus 83 ± 10 mmHg, P < 0.01), while HR and cardiac index ( /body surface area) were not different between the sexes (heart rate supine, 66 ± 6 versus 64 ± 8 beats min−1; heart rate tilt, 96 ± 13 versus 94 ± 10 beats min−1; cardiac index supine, 3.8 ± 0.9 versus 3.7 ± 0.7 l min−1 m−2; cardiac index tilt, 2.7 ± 0.8 versus 2.3 ± 0.5 l min−1 m−2). The SpBF and SpVC were lower in women at rest but not during tilt (SpBF supine, 1174 ± 243 versus 1670 ± 391 ml min−1, P < 0.01; SpVC supine, 14.83 ± 3.61 versus 19.59 ± 4.95 ml min−1 mmHg−1, P < 0.01; SpBF tilt, 884 ± 300 versus 1094 ± 271 ml min−1; SpVC tilt, 13.14 ± 4.28 versus 14.82 ± 4.16 ml min−1 mmHg−1). However, in the women the SpVC did not decrease from baseline to tilt (ΔSpVC, in women, −1.70 ± 3.19 ml min−1 mmHg−1, n.s.; in men, −4.81 ± 3.44 ml min−1 mmHg−1, P < 0.01), suggesting a blunted vasoconstrictor response. In conclusion, women tended to have lower tilt‐table tolerance associated with a smaller splanchnic vasoconstrictor reserve than men.

A somatostatin analog improves tilt table tolerance by decreasing splanchnic vascular conductance

Splanchnic hemodynamics and tilt table tolerance were assessed after an infusion of placebo or octreotide acetate, a somatostatin analog whose vascular effects are largely confined to the splanchnic circulation. We hypothesized that reductions in splanchnic blood flow (SpBF) and splanchnic vascular conductance (SpVC) would be related to improvements in tilt table tolerance. In randomized, double-blind, crossover trials, hemodynamic variables were collected in 14 women and 16 men during baseline, 70° head-up tilt (HUT), and recovery. A repeated-measures analysis of variance was used to compare changes from baseline with respect to sex and condition. HUT elicited an increase in heart rate and decreases in mean arterial pressure, cardiac index, stroke index, and systemic vascular conductance. Additionally, SpVC and non-SpVC were lower during HUT. Octreotide reduced SpBF and SpVC and increased systemic vascular conductance and non-SpVC. Changes in SpBF and SpVC between supine and HUT were smaller in women (P < 0.05). Tilt table tolerance was increased after administration of octreotide [median tilt time: 15.7 vs. 37.0 min (P < 0.05) and 21.8 vs. 45.0 min (P < 0.05) for women and men, respectively]. A significant relationship existed between change (Δ) in SpBF (placebo-octreotide) and Δtilt time in women (Δtilt time = 2.5-0.0083 ΔSpBF, P < 0.01), but not men (Δtilt time = 3.41-0.0008 ΔSpBF, P = 0.59). In conclusion, administration of octreotide acetate improved tilt table tolerance, which was associated with a decrease in SpVC. In women, but not men, the magnitude of reduction in SpBF was positively associated with improvements in tilt tolerance.

Highly sensitive index of sympathetic activity based on time-frequency spectral analysis of electrodermal activity.

Time-domain indices of electrodermal activity (EDA) have been used as a marker of sympathetic tone. However, they often show high variation between subjects and low consistency, which has precluded their general use as a marker of sympathetic tone. To examine whether power spectral density analysis of EDA can provide more consistent results, we recently performed a variety of sympathetic tone-evoking experiments (43). We found significant increase in the spectral power in the frequency range of 0.045 to 0.25 Hz when sympathetic tone-evoking stimuli were induced. The sympathetic tone assessed by the power spectral density of EDA was found to have lower variation and more sensitivity for certain, but not all, stimuli compared with the time-domain analysis of EDA. We surmise that this lack of sensitivity in certain sympathetic tone-inducing conditions with time-invariant spectral analysis of EDA may lie in its inability to characterize time-varying dynamics of the sympathetic tone. To overcome the disadvantages of time-domain and time-invariant power spectral indices of EDA, we developed a highly sensitive index of sympathetic tone, based on time-frequency analysis of EDA signals. Its efficacy was tested using experiments designed to elicit sympathetic dynamics. Twelve subjects underwent four tests known to elicit sympathetic tone arousal: cold pressor, tilt table, stand test, and the Stroop task. We hypothesize that a more sensitive measure of sympathetic control can be developed using time-varying spectral analysis. Variable frequency complex demodulation, a recently developed technique for time-frequency analysis, was used to obtain spectral amplitudes associated with EDA. We found that the time-varying spectral frequency band 0.08-0.24 Hz was most responsive to stimulation. Spectral power for frequencies higher than 0.24 Hz were determined to be not related to the sympathetic dynamics because they comprised less than 5% of the total power. The mean value of time-varying spectral amplitudes in the frequency band 0.08-0.24 Hz were used as the index of sympathetic tone, termed TVSymp. TVSymp was found to be overall the most sensitive to the stimuli, as evidenced by a low coefficient of variation (0.54), and higher consistency (intra-class correlation, 0.96) and sensitivity (Youdens index > 0.75), area under the receiver operating characteristic (ROC) curve (>0.8, accuracy > 0.88) compared with time-domain and time-invariant spectral indices, including heart rate variability.

Caloric restriction decreases orthostatic tolerance independently from 6° head-down bedrest.

Astronauts consume fewer calories during spaceflight and return to earth with an increased risk of orthostatic intolerance. Whether a caloric deficiency modifies orthostatic responses is not understood. Thus, we determined the effects of a hypocaloric diet (25% caloric restriction) during 6° head down bedrest (an analog of spaceflight) on autonomic neural control during lower body negative pressure (LBNP). Nine healthy young men completed a randomized crossover bedrest study, consisting of four (2 weeks each) interventions (normocaloric bedrest, normocaloric ambulatory, hypocaloric bedrest, hypocaloric ambulatory), each separated by 5 months. Muscle sympathetic nerve activity (MSNA) was recorded at baseline following normocaloric and hypocaloric interventions. Heart rate (HR) and arterial pressure were recorded before, during, and after 3 consecutive stages (7 min each) of LBNP (-15, -30, -45 mmHg). Caloric and posture effects during LBNP were compared using two-way ANOVA with repeated measures. There was a strong trend toward reduced basal MSNA following caloric restriction alone (normcaloric vs. hypocaloric: 22±3 vs. 14±4 burst/min, p = 0.06). Compared to the normocaloric ambulatory, both bedrest and caloric restriction were associated with lower systolic blood pressure during LBNP (p<0.01); however, HR responses were directionally opposite (i.e., increase with bedrest, decrease with caloric restriction). Survival analysis revealed a significant reduction in orthostatic tolerance following caloric restriction (normocaloric finishers: 12/16; hypocaloric finishers: 6/16; χ2, p = 0.03). Caloric restriction modifies autonomic responses to LBNP, which may decrease orthostatic tolerance after spaceflight.

Effects of prolonged and repeated immersions on heart rate variability and complexity in military divers

BACKGROUND The influence of prolonged and repeated water immersions on heart rate variability (HRV) and complexity was examined in 10 U.S. Navy divers who completed six-hour resting dives on five consecutive days. Pre-dive and during-dive measures were recorded daily. METHODS Dependent variables of interest were average heart rate (HR), time-domain measures of HRV [root mean square of successive differences of the normal RR (NN) interval (RMSSD), standard deviation of the NN interval (SDNN)], frequency-domain measures of HRV [low-frequency power spectral density (psd) (LFpsd), low-frequency normalized (LFnu), high-frequency psd (HFpsd), high-frequency normalized (HFnu), low-frequency/ high-frequency ratio (LF/HF)], and non-linear dynamics of HRV [approximate entropy (ApEn)]. A repeated-measures ANOVA was performed to examine pre-dive measure differences among baseline measures. Hierarchical linear modeling (HLM) was performed to test the effects of prolonged and repeated water immersion on the dependent variables. RESULTS Pre-dive HR (P=0.005) and RMSSD (P⟨0.001) varied significantly with dive day while changes in SDNN approached significance (P=0.055). HLM indicated that HR decreased during daily dives (P=0.001), but increased across dive days (P=0.011); RMSSD increased during daily dives (P=0.018) but decreased across dive days (P⟨0.001); SDNN increased during daily dives (P⟨0.001); LF measures increased across dive days (LFpsd P⟨0.001; LFnu P⟨0.001), while HF measures decreased across dive days (HFpsd P⟨0.001; HFnu P⟨0.001); LF/HF increased across dive days (P⟨0.001); ApEn decreased during daily dives (P⟨0.02) and across dive days (P⟨0.001). CONCLUSIONS These data suggest that the cumulative effect of repeated dives across five days results in decreased vagal tone and a less responsive cardiovascular system.

The impact of repetitive long-duration water immersion on vascular function

While physiological responses to water immersion (WI) are well-studied, the vascular responses after WI are less understood. Fifteen male subjects performed six-hour resting thermoneutral water immersions (WI) at 1.35 atmospheres absolute for four consecutive days, with follow-up on the fifth day. Measurements included peripheral endothelial function and augmentation index (PAT, peripheral arterial tonometry), beat-to-beat blood pressure (BP, photoplethysmography), heart rate (HR), and plasma volume (PV) calculated from changes in hemoglobin and hematocrit. The reactive hyperemia index (RHI), a marker of peripheral endothelial function, increased with repeated immersions (p = 0.008). By WI2 and WI3, RHI increased 12% and 16%, respectively, compared to WI1 values, but no significant differences were detected between WI4 and WI1 for either measure. Absolute augmentation index (AI) increased by an average of 33% (p<0.001) and AI normalized for HR (AI@75) by 11% (p = 0.12) following each WI. PV decreased significantly by 13.2% following WI and remained 6.8% lower at follow-up compared to pre-WI. Systolic blood pressure significantly decreased by an average of 2.5% following each WI (p = 0.012). Compared to pre-WI HR, average post-WI HR decreased 4.3% lower (p<0.001), but increased overall by 8.2% over the course of repeated WI (p<0.001). Total peripheral resistance increased by an average of 13.1% following WI (p = 0.003). Thus, peripheral endothelial function increases after two days of WI, and PAT-derived measures of arterial stiffness increase transiently post-WI. Additionally, BP and PAT-derived endothelial function diverge from their usual associations with arterial stiffness (i.e. augmentation index) in the context of WI.

Pulmonary effects of repeated six-hour normoxic and hyperoxic dives

This study examines differential effects of immersion, elevated oxygen partial pressure, and exercise on pulmonary function after series of five daily six-hour dives at 130 kPa (1.3 ATA), with 18 hours between dives. Five cohorts of 10 to 14 divers participated. The exposure phases were resting while breathing O2 or air in the water (“wetO2”, “wetAir”) or O2 in the hyperbaric chamber (“dryO2”), and exercise in the water while breathing O2 or air (“wetO2X”, “wetAirX”). Respiratory symptoms were recorded during and after each dive, and pulmonary function (forced flow-volume) was measured twice at baseline before diving, after each dive both immediately and on the following morning, and three days post diving (“Day+3”). The incidences of symptoms and of flow volume changes from baseline greater than normal limits (“ΔFV”) were assessed, as were mean ΔFV. The parameters examined were forced vital capacity (FVC), forced expired volume in 1 second (FEV1), and forced expired flow from 25% to 75% volume expired (FEF25–75). The phases ranked from greatest to least fraction of diver-days with symptoms were wetO2X (56%) > dryO2 (42%) > wetO2 (13%) > [wetAir (2%) or wetAirX (1%)] (p<0.05). FEV1 and FEF25–75 were depressed in the morning following wetO2 and wetO2X and on Day+3 after and wetO2X, but increased immediately following each wetAirX dive. O2 exposures caused symptoms and ΔFV suggestive of pulmonary oxygen toxicity,exacerbated by exercise. Indices of small airway function showed late (17-hour) post-O2 exposure deficits, but, particularly with exercise, improvement was evident early after exposure with or without O2. FEF25–75 and FEV1 remained depressed on Day+3 after wetO2 and wetO2X.

Time-varying analysis of electrodermal activity during exercise

The electrodermal activity (EDA) is a useful tool for assessing skin sympathetic nervous activity. Using spectral analysis of EDA data at rest, we have previously found that the spectral band which is the most sensitive to central sympathetic control is largely confined to 0.045 to 0.25 Hz. However, the frequency band associated with sympathetic control in EDA has not been studied for exercise conditions. Establishing the band limits more precisely is important to ensure the accuracy and sensitivity of the technique. As exercise intensity increases, it is intuitive that the frequencies associated with the autonomic dynamics should also increase accordingly. Hence, the aim of this study was to examine the appropriate frequency band associated with the sympathetic nervous system in the EDA signal during exercise. Eighteen healthy subjects underwent a sub-maximal exercise test, including a resting period, walking, and running, until achieving 85% of maximum heart rate. Both EDA and ECG data were measured simultaneously for all subjects. The ECG was used to monitor subjects’ instantaneous heart rate, which was used to set the experiment’s end point. We found that the upper bound of the frequency band (Fmax) containing the EDA spectral power significantly shifted to higher frequencies when subjects underwent prolonged low-intensity (Fmax ~ 0.28) and vigorous-intensity exercise (Fmax ~ 0.37 Hz) when compared to the resting condition. In summary, we have found shifting of the sympathetic dynamics to higher frequencies in the EDA signal when subjects undergo physical activity.