FC Changes Research Articles

Linking the patterns of change in composition and function in bacterioplankton successions along environmental gradients

The connections that exist between the composition of bacterial communities and their functional attributes are still a matter of intense debate, despite over a decade of intense studies. Here we explored three different facets of the links that may exist between bacterioplankton compositional and functional successions that occurred along the water flow path in a complex watershed in southern Quebec. We analyzed the correlation between composition and function in terms of their absolute patterns, and in terms of their rates of change relative to transit time in environmental transitions, and relative to shifts in resources along the same transitions. Our results showed that the absolute patterns in bacterial community composition (BCC, using DGGE [denaturing gradient gel electrophoresis] profiles) and functional capacities (FC, using BIOLOG profiles) were not correlated, but that the rates of change in BCC and FC along the transitions were strongly correlated to each other. Further, we observed that the strength and shape of the relationship between the changes in BCC and FC varied relative to the type and intensity of gradient considered. Collectively, these results showed that BCC and FC are strongly related but in a very dynamic manner, such that their absolute patterns do not appear to be connected. This in turn suggests a high level of functional redundancy that occurs both within the existing community and in the meta-community from which phylotypes are selected to occupy the new niches that are created along the transitions.

Scale effects and constraints for sound production in katydids (Orthoptera: Tettigoniidae): correlated evolution between morphology and signal parameters

Male katydids (Orthoptera: Tettigoniidae) produce mating calls by rubbing the wings together, using specialized structures in their forewings (stridulatory file, scraper and mirror). A large proportion of species (ca. 66%) reported in the literature produces ultrasonic signals as principal output. Relationships among body size, generator structures and the acoustic parameters carrier frequency (f(c)) and pulse duration (p(d)), were studied in 58 tropical species that use pure-tone signals. A comparative analysis, based on the only available katydid phylogeny, shows how changes in sound generator form are related to changes in f(c) and p(d). Anatomical changes of the sound generator that might have been selected via f(c) and p(d) are mirror size, file length and number of file teeth. Selection for structures of the stridulatory apparatus that enhance wing mechanics via file-teeth and scraper morphology was crucial in the evolution of ultrasonic signals in the family Tettigoniidae.

Structure and Stability Changes of Human IgG1 Fc as a Consequence of Methionine Oxidation

The Fc region has two highly conserved methionine residues, Met 33 (C(H)3 domain) and Met 209 (C(H)3 domain), which are important for the Fc's structure and biological function. To understand the effect of methionine oxidation on the structure and stability of the human IgG1 Fc expressed in Escherichia coli, we have characterized the fully oxidized Fc using biophysical (DSC, CD, and NMR) and bioanalytical (SEC and RP-HPLC-MS) methods. Methionine oxidation resulted in a detectable secondary and tertiary structural alteration measured by circular dichroism. This is further supported by the NMR data. The HSQC spectral changes indicate the structures of both C(H)2 and C(H)3 domains are affected by methionine oxidation. The melting temperature (Tm) of the C(H)2 domain of the human IgG1 Fc was significantly reduced upon methionine oxidation, while the melting temperature of the C(H)3 domain was only affected slightly. The change in the C(H)2 domain T m depended on the extent of oxidation of both Met 33 and Met 209. This was confirmed by DSC analysis of methionine-oxidized samples of two site specific methionine mutants. When incubated at 45 degrees C, the oxidized Fc exhibited an increased aggregation rate. In addition, the oxidized Fc displayed an increased deamidation (at pH 7.4) rate at the Asn 67 and Asn 96 sites, both located on the C(H)2 domain, while the deamidation rates of the other residues were not affected. The methionine oxidation resulted in changes in the structure and stability of the Fc, which are primarily localized to the C(H)2 domain. These changes can impact the Fc's physical and covalent stability and potentially its biological functions; therefore, it is critical to monitor and control methionine oxidation during manufacturing and storage of protein therapeutics.

Coronary Angiography for Follow-up of Heart Transplant Recipients: Insights From TIMI Frame Count and TIMI Myocardial Perfusion Grade

The aim of this study was to evaluate the course of Thrombolysis in Myocardial Infarction (TIMI) frame count (TIMI FC) and myocardial perfusion grade (TIMI MPG) in heart transplant recipients and whether these parameters could predict mortality. Sixty-two heart transplant recipients were enrolled in this study. All patients had coronary angiography at baseline and at 1 year, which were evaluated for TIMI FC and TIMI MPG. Also, 50 vessels in 35 patients were analyzed with volumetric intravascular ultrasound (IVUS) at baseline and at 1 year. Rejection episodes and mortality were recorded during the follow-up period. The mean follow-up interval was 51.5 +/- 17.2 months (range 12.2 to 78.4 months). TIMI FCs of all three coronary arteries and global TIMI FC (gTIMI FC) significantly increased from baseline during the first year (p < 0.0001). TIMI MPG deteriorated significantly (p < 0.0001 for left anterior descending and circumflex coronary arteries, p = 0.002 for right coronary artery). There was no correlation between changes in TIMI FC and progression of transplant vasculopathy as assessed by IVUS. Episodes of Grade > or = 3A rejection were significantly more frequent in the stable gTIMI FC group (p = 0.03). Mortality rate was significantly higher in the group with increasing gTFC (p = 0.02). gTIMI FCs and TIMI FCs of coronary arteries increase, and TIMI MPG gradually deteriorates during the first year after transplantation. Mortality rate is significantly higher in patients whose gTIMI FC increases from baseline. Change in gTIMI FC is a simple quantitative predictor of long-term mortality in heart transplant recipients.

Study on changes of characteristic parameters of biological tissues impedance spectroscopy in vitro within 5 to 360 min after excision at the frequency range from 1Hz to 1MHz.

We constructed an impedance spectroscopy measurement system based on frequency response analyzer. Using four-terminal method, we measured impedance spectroscopy of rabbit different tissues in vitro within 5 to 360 minutes after excision at the frequency range from 1Hz to 1MHz. According to the measurement results, we analyzed the changes of characteristic parameters (Ro, Rinfin, fc and alpha) of different tissues in the time course. The results show that the Cole-Cole model characteristic parameters change notably in the time course. The value of Ro change great especially. For kidney, liver, spleen and heart, the value of Ro reached 700%, 125%, 170% and 200% respectively at the 6 hour after excision. The change of alpha is less than 30% for all tissues. The changes of fc are various remarkably for different tissues, for liver, kidney and heart, the value of fc decreased continually and the decrement reached 60%, 19%, 33% respectively at the end of the measurement. For muscle (longitudinal, transverse) and heart, the value of fc decreased firstly and then began to increase and reached 145% > 155% > 125% at 6 hour after excision. The results of our reaching show the measurement time plays a remarkable effect on the impedance spectroscopy measurement of tissue in vitro. We hypothesized that the dispersion observed in the impedance spectroscopy measurement in vitro between different researchers was partly due to the effect of measurement time.

Impact of initiating carvedilol before angiotensin-converting enzyme inhibitor therapy on cardiac function in newly diagnosed heart failure

The purpose of this research was to evaluate the therapeutic value of initiating a beta-blocker before an angiotensin-converting enzyme inhibitor (ACEI) in the treatment of heart failure. Although ACEI and carvedilol produce benefits in heart failure, whether the order of initiation of therapy determines the impact on left ventricular (LV) function and New York Heart Association functional class (NYHA FC) has not been determined. A single-center, prospective, randomized, open-label study was performed. We evaluated whether initiation of therapy with carvedilol either before (n = 38) or after (n = 40) perindopril therapy in newly diagnosed patients in NYHA FC II to III heart failure with idiopathic dilated cardiomyopathy, with the addition of the alternative agent after six months, determined subsequent changes in NYHA FC and LV function (echocardiography and radionuclide ventriculography). Study drugs were titrated to maximum tolerable doses. There were no differences in baseline characteristics between the study groups. After 12 months 11 patients died (6 in the group where the ACEI was initiated). At 12 months the group receiving carvedilol as initial therapy achieved a higher tolerable dose of carvedilol (43 +/- 17 mg vs. 33 +/- 18 mg, p = 0.03); a lower dose of furosemide (p < 0.05); and better improvements in symptoms (NYHA FC, p < 0.002), LV ejection fraction (radionuclide: 15 +/- 16% vs. 6 +/- 13%, p < 0.05; echocardiographic, p < 0.01), and plasma N-terminal pro-brain natriuretic peptide concentrations (p < 0.02). As opposed to the conventional sequence of drug use in the treatment of heart failure, initiation of therapy with carvedilol before an ACEI results in higher tolerable doses of carvedilol and better improvements in FC and LV function.

Crystal Structure at 2.8 Å of an FcRn/Heterodimeric Fc Complex: Mechanism of pH-Dependent Binding

The neonatal Fc receptor (FcRn) transports immunoglobulin G (IgG) across epithelia, binding IgG in acidic vesicles (pH ≤ 6.5) and releasing IgG in the blood at pH 7.4. Well-ordered FcRn/Fc crystals are prevented by the formation of “oligomeric ribbons” of FcRn dimers bridged by Fc homodimers, thus we crystallized a 1:1 complex between rat FcRn and a heterodimeric Fc containing only one FcRn binding site. The 2.8 Å complex structure demonstrates that FcRn uses its α2 and β2-microglobulin domains and carbohydrate to interact with the Fc C γ2–C γ3 interface. The structure reveals conformational changes in Fc and three titratable salt bridges that confer pH-dependent binding, and can be used to guide rational design of therapeutic IgGs with longer serum half-lives.

The control of carrier frequency in cricket calls: a refutation of the subalar-tegminal resonance/auditory feedback model.

The subalar-tegminal resonance/auditory feedback hypothesis attempts to explain how crickets control the carrier frequency (f(C)), the loudness and the spectral purity of their calls. This model contrasts with the 'clockwork cricket' or escapement model by proposing that f(C) is not controlled by the resonance of the cricket's radiators (the harps) but is instead controlled neurally. It suggests that crickets are capable of driving their harps to vibrate at any frequency and that they use a tunable Helmholtz-like resonator consisting of the tegmina and the air within the subalar space to amplify and filter the f(C). This model predicts that f(C) is variable, that call loudness is related to tegminal position (and subalar volume) and that low-density gases should cause f(C) to increase. In Anurogryllus arboreus, f(C) is not constant and varied by as much as 0.8 % between pulses. Within each sound pulse, the average f(C) typically decreased from the first to the last third of a sound pulse by 9 %. When crickets called in a mixture of heliox and air, f(C) increased 1.07- to 1.14-fold above the value in air. However, if the subalar space were part of a Helmholtz-like resonator, then its resonant frequency should have increased by 40-50 %. Moreover, similar increases occurred in species that lack a subalar space (oecanthines). Experimental reduction of the subalar volume of singing crickets resulted neither in a change in f(C) nor in a change in loudness. Nor did crickets attempt to restore the subalar volume to its original value. These results disprove the presence of a subalar-tegminal resonator. The free resonance of freshly excised Gryllus rubens tegmina shifted by 1.09-fold when moved between air and a mixture of helium and air. Auditory feedback cannot be the cause of this shift, which is similar to the f(C) shifts in intact individuals of other species. Calculations show that the harp is 3.9-1.8 times more massive than the air that moves en masse with the vibrating harps. Replacing air with heliox-air lowers the mass of the vibrating system sufficiently to account for the f(C) shifts. These results re-affirm the 'clockwork cricket' (escapement) hypothesis. However, as realized by others, the harps should be viewed as narrow-band variable-frequency oscillators whose tuning may be controlled by factors that vary the effective mass.

Central effects of 5-HT on respiratory rhythm in newborn rats in vivo.

The role of 5-HT in inducing apnoeas (a major element in sudden infant death syndrome) is controversial because while 5-HT is a respiratory depressant in vivo, it evokes respiratory analeptic effects when applied to the isolated brainstem of the newborn rat. In decerebrate newborn rats, the electrical activity of the diaphragm and that of a hypoglossally innervated tongue muscle, as well as the cardiac frequency (Fc), were recorded before and after the application of 5-HT and related agents to the floor of the IVth ventricle. To assess the spontaneous variability in inspiratory frequency (Fi) and Fc, a sham group was studied. A decrease in Fi was observed in response to 5-HT. This respiratory depressant effect was associated with an activation of the tongue muscle, but there was no change in Fc. Application of agonists elicited a small increase in Fi linked to activation of 5-HT1A receptors, and decreases in both Fi and the activity of the tongue muscle resulting predominantly from activation of 5-HT2 receptors. The decrease in Fi was much smaller in newborn rats than that reported in newborn kittens. Indeed, in newborn rats, we did not observe long-lasting apnoeas. Our results differ from those obtained from the newborn rat in vitro, inasmuch as in vivo 5-HT essentially depressed the respiratory rhythm generator. The role of the afferent system appears to be crucial in modulating the action of 5-HT.

Cardiovascular effects of hyperbaric oxygen with and without addition of carbon dioxide.

It is commonly believed that during hyperbaric oxygen (HBO) treatment, in spite of the vasoconstriction induced by the increased O2 content in the breathing gas, the elevated carrying capacity of O2 in the arterial blood results in augmented O2 delivery to tissues. The experiments described here tested the hypothesis that HBO treatment would be more efficient in delivering O2 to poorly perfused tissues if the vasoconstriction induced by elevated O2 could be abolished or attenuated by adding CO2 to the breathing gas. Organ blood flow (QOBF), systemic hemodynamics, and arterial blood gases were measured before, during and after exposure to either 300 kPa O2 (group 1) or 300 kPa O2 with 2 kPa CO2 (group 2), in awake, instrumented rats. During the HBO exposure the respiratory frequency (fb) fell (4 breaths x min(-1) x 100 kPa O2(-1)), with no changes in arterial CO2 tension (PaCO2), but when CO2 was added, fb and PaCO2 increased. The left ventricular pressure (LVP) and the systolic arterial pressure (SBP) increased. The maximum velocity of LVP (+dP/dt) rose linearly with LVP whether CO2 was added or not (r2 = 0.72 and 0.75 respectively). Similarly, the cardiac output (Qc) and heart rate (fc) fell, while the stroke volume (SV) was unaltered, independent of PaCO2. There was a general vasoconstriction in most organs in both groups, with the exception of the central nervous system (CNS), eyes, and respiratory muscles. HBO reduced the blood flow to the CNS by 30%, but this vasoconstriction was diminished or eliminated when CO2 was added. In group 2, the blood flow to the CNS rose linearly with increased PaCO2 and decreased pH. After decompression fc and SBP stayed high, while Qc returned to control values by reducing the SV; CNS blood flow remained markedly elevated in group 2, while in group 1, it returned to control levels. We conclude that the changes in fc, Qc, LVP, dP/dt, SBP and most QOBF values induced by HBO were not changed by hypercapnia. Blood flow to the CNS decreased during HBO treatment at a constant PaCO2. Hypercapnia prevented this decline. Elevated PaCO2 augmented O2 delivery to the CNS and eyes, but increased the susceptibility to O2 poisoning. A prolonged suppression of O2 supply to the CNS occurred during the HBO exposure and in air following the decompression in the absence of CO2. This suppression was offset by the addition of CO2 to the breathing gas.

Copper electrodissolution in 1M HCl at low current densities. II. Electrochemical impedance spectroscopy study

Abstract Anodic oxidation of a copper rotating disk in 1 M HCl is studied by electrochemical impedance spectroscopy (EIS). Experimental impedance diagrams show two capacitive loops separated in frequency. These diagrams validated with Kramers–Kronig relationships reveal a change in the frequency fc, at the apex of the low frequency loop, with the electrode potential. The theoretical expression for the impedance is derived and the possible limiting shapes of the impedance diagrams are discussed. The low frequency loop can take different forms as a convective-diffusion loop, a first order semi-circle or a mixed graph. An increase in fc with the electrode potential should be observed. Fitting of the impedance diagrams by the kinetic model gives good agreement with the experimental frequency distribution. A discussion is presented between the theoretical and experimental results.

Analytic approximation of the off-axis modulation transfer function of the eye.

Published experimental measurements of the ocular modulation transfer function (MTF) in the peripheral field are approximated by the expression T(f) = exp[-(f/fc)n], where T(f), f, fc, and n are modulation transfer, spatial frequency, spatial frequency constant and MTF index, respectively. It is shown that n (which describes the shape of the MTF) remains relatively constant at about 0.9 for field angles out to 40 deg but fc (which defines the spatial frequency scaling) declines steeply over this range, depending upon the pupil diameter and conditions of focus. The oblique astigmatism of the eye plays a major role in the off-axis changes in fc at field angles > or = 20 deg. The approximation may be useful in allowing the form of the degraded optical stimulus in studies of the peripheral retinal function to be evaluated.

Abnormal lipoprotein phospholipid composition in patients with essential hypertension

Vascular cell membranes from patients with essential hypertension (EH) and animals with genetic forms of hypertension have been found to have alterations in the content of free cholesterol and negatively charged phospholipids that may modify their function. Since membrane and lipoprotein lipids exchange freely, the lipid composition of lipoproteins may be an indirect measure of the content of vascular and other cells. To determine whether abnormalities are present in the lipid and phospholipid composition of lipoproteins from patients with EH, 30 EH (11 women; 19 men) and 20 normotensive control subjects were studied. Since significant gender differences were present in a number of parameters of lipoprotein composition, male and female data were examined separately. The EH group of both sexes tended to have higher plasma TG and VLDL + LDL and HDL 2 lipid levels than their respective controls. Not only were the calcium-binding phospholipids phosphatidylinositol (PI) + phosphatidylserine (PS), and the membrane fluidizer phosphatidylethanolamine (PE) significantly reduced in their VLDL + LDL, but all phospholipids (L, sphingomyelin (SPH), PI + PS, and PE) were significantly reduced relative to their neutral lipid content in both the HDL 2 and HDL 3 subfractions. These directional changes in lipoprotein FC and phospholipid in the EH women significantly increased the EH FC/PC (mol/mol) ratio in their plasma, a new cardiovascular risk factor, (EH 1.08 ± 0.22 vs. control 0.86 ± 0.08; P < 0.01) and lowered the SPH/PC ratio of HDL 2 and HDL 3 in EH patients of both sexes. These findings show that lipoproteins in normolipidemic EH women are relatively enriched in FC and in EH patients of both sexes depleted in certain phospholipids. If membranes of vascular tissues in EH are deficient in the same negatively charged phospholipids lacking in lipoproteins, their functional properties could be altered and vascular tone increased.

Change in Fc γ II Receptor during Platelet Storage

Change in Fc γ II Receptor during Platelet Storage

Inhibition of Urea Tubular Reabsorption by PGE&lt;sub&gt;1&lt;/sub&gt; Infusion in Man

We have shown that the inhibition of prostaglandin (PG) synthesis in man decreases the fractional clearance of urea (FCurea). To understand the mechanism(s) by which PG affect the renal handling of urea, 6 normal volunteers were randomly studied in maximal antidiuresis (by water deprivation and by administering 1-desamino-8-D-arginine vasopressin) before and during PGE1 infusion, in two separate occasions: (A) after 7 days of normal protein (1 g/kg b.w./day) and water intake (10 ml/kg b.w./day), and (B) after 7 days of low protein intake (0.5 g/kg b.w./day) and high water intake (80 ml/kg b.w./day) to lower the corticomedullary osmotic gradient. During infusion of PGE1 at rates of 0.01, 0.05 and 0.1 micrograms/min/kg, randomly administered, the urinary fluid losses were replaced by infusing equal volumes of hypotonic NaCl (80 mmol/l). To evaluate the time effects of this protocol, control studies were performed in an other 8 subjects receiving vehicle infusion without PGE1. In study A, FCurea rose by 23% (p less than 0.01) at the lowest PGE1 infusion rate (0.01 micrograms/min/kg), in the absence of any simultaneous change in water and salt output, Uosm, PAH and inulin clearance. Higher PGE1 infusion rates (0.05 and 0.1 micrograms/min/kg) were associated with a progressive increase of FCurea (50%, p less than 0.001 and 91%, p less than 0.001, respectively), fractional clearance of water and salt output, inulin and PAH clearance and reduced Uosm from 1,005 (22 SEM; basal value) to 772 (38 SEM; minimum value) mosm/kg (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Inter-relationships between pituitary-adrenal hormones and catecholamines during a 6-day Nordic ski race.

The aim of the study was to investigate the inter-relationships between pituitary-adrenal hormones and catecholamines during a prolonged competition over 6 days. Plasma adrenocorticotropic hormone (ACTH), cortisol (C), beta-endorphin (beta EP), free and sulphated adrenaline (A) and noradrenaline (NA) were measured in 11 volunteer male subjects during a national Nordic-ski race (323 km). Blood samples were obtained before the competition in the evening as control (D0), and before and after each day's racing (D1-D6). The mean daily heart rate (fc) was calculated from fc values recorded every minute during the race. The results showed the following: changes in mean fc [from 147 (SEM 3) to 156 (SEM 3) beats.min-1 according to the day] were not significant during the race. Diurnal variations in ACTH, beta EP and C were no longer apparent after the race: evening levels were higher than their respective D0 values during the race, except on D3 when there was a lack of response to exercise in the three hormones. Unlike ACTH and beta EP, pre- and postexercise C values on D1 and D2 were higher than those on the subsequent days (P less than 0.001). In contrast, there was a progressive accumulation of A and NA in pre- and postrace concentrations which reached a plateau in about 4 days. Positive correlations between exercise responses in ACTH, C and beta EP were found especially on D3 and D6 (P less than 0.001) but there were no significant correlations between catecholamines and the other three hormones. Thus, prolonged competition over 6 days evoked different control mechanisms for hormones of the pituitary-adrenal axis and catecholamines.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic, body temperature and hormonal responses to repeated periods of prolonged cycle-ergometer exercise in men

This study was designed to find out whether rest intervals and prevention of dehydration during prolonged exercise inhibit a drift in metabolic rate, body temperature and hormonal response typically occurring during continuous work. For this purpose in ten healthy men the heart rate (fc), rectal temperature (Tre), oxygen uptake (VO2), as well as blood metabolite and some hormone concentrations were measured during 2-h exercise at approximately 50% maximal oxygen uptake split into four equal parts by 30-min rest intervals during which body water losses were replaced. During each 30-min exercise period there was a rapid change in Tre and fc superimposed on which, these values increased progressively in consecutive exercise periods (slow drift). The VO2 showed similar changes but there were no significant differences in the respiratory exchange ratio, pulmonary ventilation, mechanical efficiency and plasma osmolality between successive periods of exercise. Blood glucose, insulin and C-peptide concentrations decreased in consecutive exercise periods, whereas plasma free fatty acid, glycerol, catecholamine, growth hormone and glucagon concentrations increased. Blood lactate concentrations did not show any regular drift and the plasma cortisol concentration decreased during the first two exercise periods and then increased. In conclusion, in spite of the relatively long rest intervals between the periods of prolonged exercise and the prevention of dehydration several physiological and hormonal variables showed a distinct drift with time. It is suggested that the slow drift in metabolic rate could have been attributable in the main to the increased concentrations of heat liberating hormones.

Effect of pyridostigmine on the exercise-heat response of man

The effect of pyridostigmine on thermoregulatory responses was evaluated during exercise and heat stress. Eight heat acclimated, young adult male subjects received four doses of pyridostigmine (30 mg) or identical placebo tablets every 8 h, in a double blind, randomized, cross-over trial. A 30.3%, SD 4.6% inhibition of the circulating cholinesterase (ChE) activity was induced in the pyridostigmine-treated group. The subjects were exposed to 170-min exercise and heat-stress (dry bulb temperature, 33 degrees C; relative humidity 60%) consisting of 60 min in a sitting position and two bouts of 50-min walking (1.39 m.s-1, 5% gradient) which were separated by 10-min rest periods. No differences were found between treatments in the physiological responses and heat balance parameters at the end of exposure: heart rate (fc) was 141 beats.min-1, SD 16 and 150 beats.min-1, SD 12, rectal temperature (Tre) was 38.5 degrees C, SD 0.4 degrees and 38.6 degrees C, SD 0.3 degrees, heat storage was 60 W.m-2, SD 16 and 59 W.m-2, SD 15 and sweat rate was 678 g.h-1, SD 184 and 661 g.h-1, SD 133, in the pyridostigmine and placebo treatments, respectively. The changes in Tre and fc over the heat-exercise period were parallel in both study and control groups. Pyridostigmine caused a slight slowing of fc (5 beats.min-1) which was consistent throughout the entire exposure (P less than 0.001) but was of no clinical significance. The overall change in fc was similar for both groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac responses to the Valsalva manoeuvre in different body positions

A standardized Valsalva manoeuvre (VM) with a 15-s straining period was repeated in each of four postures by six male subjects. The postures were supine (SUP), sitting leaning back (LB), sitting leaning forward (LF) and standing (ST). During straining, the increase in heart rate (fc) was different between LB and LF (+50% and +23%, respectively P less than 0.05). The decrease in stroke volume (SV), which was monitored by means of impedance cardiography, was different (63%, 68%, 39%, and 72%, P less than 0.001) as well as the decrease in cardiac output (CO) (55%, 53%, 26%, and 61%, P less than 0.001) in SUP, LB, LF, and ST, respectively. Accordingly, after pressure release the smallest changes of SV, fc and CO were found in LF. In conclusion, cardiovascular stability during straining was increased during LF. Consequently, this posture would appear to be superior to other postures during unavoidable VM (weight lifting and defaecation). To perform tests on autonomic function LB would appear to be superior to the other postures because of the large autonomic responses, combined with minimum risk for the subject. The impedance method provided simple and reproducible determinations of SV changes during VM.

A non-invasive method for measuring inspiratory muscle fatigue during progressive isocapnic hyperventilation in man.

Eleven normal adults each performed a ten minute progressive isocapnic hyperventilation (PIHV) test in which ventilatory levels were increased every two minutes. All subjects exhibited mechanical fatigue by failing to maintain the target of 80% of maximum voluntary ventilation (MVV). The mean ventilation at this level was 67.5 +/- 1.4% MVV. This fatigue was accompanied by a fall in transdiaphragmatic pressure. During the test the EMG of the sternomastoid (SM) was monitored by surface electrodes and was analyzed using fast-fourier transform. The centroid frequency (Fc) fell as ventilation increased, and correlated negatively with the inability to achieve target ventilation(r = -0.99, p less than 0.015). Five subjects performed the test while the diaphragmatic EMG was recorded from an oesophageal electrode (DIes) and from surface electrodes (DIs). The Fc of DIes fell with increasing ventilation levels (r = -0.95, p less than 0.05) and there was a correlation between the Fc changes of both DIes and the SM (r = -0.92, p less than 0.001). The Fc of DIs did not correlate with either mechanical performance or the Fc of DIes, because of contamination of surface signals by signals from expiratory muscles. It is concluded that the PIHV along with surface monitoring of EMG activity from the sternomastoid can serve as a non-invasive method for evaluating inspiratory muscle fatigue.