Continuous Blood Withdrawal Research Articles

A CZT-based blood counter for quantitative molecular imaging

BackgroundRobust quantitative analysis in positron emission tomography (PET) and in single-photon emission computed tomography (SPECT) typically requires the time-activity curve as an input function for the pharmacokinetic modeling of tracer uptake. For this purpose, a new automated tool for the determination of blood activity as a function of time is presented.The device, compact enough to be used on the patient bed, relies on a peristaltic pump for continuous blood withdrawal at user-defined rates. Gamma detection is based on a 20 × 20 × 15 mm3 cadmium zinc telluride (CZT) detector, read by custom-made electronics and a field-programmable gate array-based signal processing unit. A graphical user interface (GUI) allows users to select parameters and easily perform acquisitions.ResultsThis paper presents the overall design of the device as well as the results related to the detector performance in terms of stability, sensitivity and energy resolution. Results from a patient study are also reported. The device achieved a sensitivity of 7.1 cps/(kBq/mL) and a minimum detectable activity of 2.5 kBq/ml for 18F. The gamma counter also demonstrated an excellent stability with a deviation in count rates inferior to 0.05% over 6 h. An energy resolution of 8% was achieved at 662 keV.ConclusionsThe patient study was conclusive and demonstrated that the compact gamma blood counter developed has the sensitivity and the stability required to conduct quantitative molecular imaging studies in PET and SPECT.

Delayed Selective Cerebral Hypothermia Decreases Infarct Volume After Reperfused Stroke in Baboons

Hypothermia is known to provide neuroprotection from focal ischemia. However, lethal cardiovascular complications resulting from total body cooling have greatly restricted hypothermia as a therapy for stroke. This study determined whether selective cerebral cooling induced after reversible cerebral artery occlusion would decrease the infarct volume. Under general anesthesia, 8 baboons were subjected to 1-hour simultaneous occlusion of the left internal carotid artery and anterior cerebral arteries by transorbital surgical approach. Four animals were treated with selective cerebral hypothermia to 25°C, initiated 2.5 hours after placement of cerebral artery clips. Selective cerebral hypothermia was achieved, after heparinization, by continuous withdrawal of femoral arterial blood into an extracorporeal closed-circuit pump system, cooling by water bath and perfusion into the right internal carotid artery. Pump flow was adjusted to maintain right internal carotid artery pressure near systemic blood pressure. Cerebral cortical temperature was maintained below 27°C for 12 hours, whereas systemic temperature was preserved near normal by convective air mattresses and warm water blankets. Four control animals were maintained at 36°C. Blood pressure, pH, and blood gases were maintained at normal values for both groups. Forty-eight to 72 hours after cerebral artery occlusion, magnetic resonance imaging brain scans were obtained and infarct volume measured. Normothermic baboons had infarction of 35.4±4.4% (mean±SD) of the left cerebral hemisphere compared with 0.5±1% for baboons treated with cerebral hypothermia (P<0.01). In brain-cooled animals, esophageal temperature was maintained greater than 34°C, despite cerebral temperature less than 27°C. Selective brain cooling initiated 2.5 hours after onset of focal ischemia resulted in marked reduction in infarct volume, without cardiovascular derangement.

Unreliability of Pulse Contour-Derived Cardiac Output in Piglets Simulating Acute Hemorrhagic Shock and Rapid Volume Expansion

Pulse contour-derived cardiac output for continuous hemodynamic monitoring is becoming popular in critical care. However, the data regarding its reliability during acute hemodynamic instability are inconsistent. This study was conducted to determine whether pulse contour-derived cardiac output truly reflects rapid hemodynamic changes. Hemorrhagic shock was created in seven anesthetized piglets by continuous blood withdrawal at a rate of 1 mL . kg . min for 20 minutes. Volume expansion with 10% hydroxyethyl starch 8 mL . kg was then administered for 5 minutes. Pulse contour-derived and thermodilution- derived hemodynamic parameters were compared. Baseline thermodilution-derived cardiac index was 3.2 +/- 0.4 L . min . M. After exsanguination, it decreased to 2.1 +/- 0.3 L . min . M while pulse contour-derived cardiac index increased to 4.4 +/- 0.4 L . min . M (p value <0.001). Thermodilution-derived systemic vascular resistance index (SVRI) increased to 2463 +/- 474 dyne . sec . cm . M while pulse contour-derived SVRI paradoxically decreased to 1047 +/- 368 dyne . sec . cm . M (p value <0.001). Following rapid volume expansion, thermodilution-derived cardiac index increased to 3.5 +/- 0.2 L . min . M while pulse contour-derived cardiac index decreased to 1.8 +/- 0.4 L . min . M (p value <0.001). Thermodilution-derived SVRI decreased to 2215 +/- 323 dyne . sec . cm . M while pulse contour-derived SVRI increased to 4105 +/- 1097 dyne . sec . cm . M (p value = 0.001). Pulse contour-derived hemodynamic parameters do not accurately reflect rapid hemodynamic changes, and the trend may be misleading in piglets. Physicians are advised to interpret pulse contour-derived hemodynamic parameters with caution or to use invasive monitoring to guide treatment strategy therapy.

Effects of extreme hemorrhage in hypoxically preconditioned pigeons, Columba livia

Pigeons tolerate the loss of >90% of their blood by rapidly replacing the lost blood with an equal volume of tissue fluid. This prevents shock but causes extreme anemia. No data exist for birds about the effects of hypoxic preconditioning on responses to hemorrhage. We therefore placed arterially cannulated pigeons (mean mass 0.6 kg) in a chamber at 8% O2 for 3 h (H group) or at 21% O2 for 3 h (N group). A third group of pigeons was not preconditioned (C). Body mass was similar in H, N, and C. All birds were subjected to continuous blood withdrawal of ≤7% body mass at 0.4 ml/min. In H and N, hemorrhage began 24 h after preconditioning. During hemorrhage, hematocrit decreased significantly in all birds, reflecting red-cell dilution. Body temperature and osmolarity, PO2, pH, PCO2, and HCO3 in blood were also maintained. Plasma lactate and glucose increased with blood loss to the same extent in all birds, reflecting increased anaerobic respiration. Plasma succinate was higher in H than in N or C, reflecting additional anaerobic ATP production through pathways activated by hypoxic preconditioning. The results suggest that hemorrhagic hypoxia in H may be compensated by anaerobic metabolism yielding both lactate and succinate, compared to lactate alone in N and C. Anaerobic metabolic pathways other than glycolysis alone have not previously been reported in birds and require further study (NIGMS grants GM61222, GM08136, and GM07667.)

VEGF plasma pattern in ovulation induction: evidence for an episodic secretion and lack of immediate effect of hCG

Vascular endothelial growth factor (VEGF) is a potent stimulator of vascular proliferation and permeability. Ovarian granulosa cells have been identified as a major source of the cytokine and r-hCG was able to stimulate VEGF mRNA expression in vitro. In this study we have investigated the immediate effect of ovulation induction with hCG on peripheral VEGF levels in 6 women with primary infertility enrolled in the IVF/ET program. The patients underwent a 24-hour continuous blood withdrawal with sampling intervals of 15 minutes starting from 5 hours before ovulation induction with 10.000 IU hCG. Ovulation induction with hCG had no significant immediate effect on mean peripheral VEGF levels. However, VEGF plasma levels did exhibit significant episodic fluctuations with rapid increases every 90-120 minutes without any relation to circulating hCG levels. Taken together, the results of this study suggest that VEGF is released episodically and that systemic VEGF levels are not acutely altered by ovulation induction with hCG.

Hypothalamo-Pituitary-Adrenal Axis Activity Is Related to the Level of Central Arousal: Effect of Sleep Deprivation on the Association of High-Frequency Waking Electroencephalogram with Cortisol Release

The temporal and quantitative interrelationships between the hypothalamo-pituitary-adrenal (HPA) axis activity and the level of central arousal were studied in 10 healthy young men during daytime wakefulness. Two experimental sessions were conducted randomly between 09.00 and 18.00 h, once after nocturnal sleep and once after a night of total sleep deprivation. Spectral analysis of serial waking electroencephalography (EEG) from a short target fixation task repeated every 10 min was undertaken, along with an estimation of cortisol secretory profiles by deconvolution of plasma radioimmunoassay measures obtained from continuous blood withdrawal with regular sampling at a 10-min interval. Following nocturnal sleep, a temporal association between the HPA axis activity and the waking EEG activity was found, cortisol secretory rate following changes in frontal gamma (20–45 Hz) band power by 10 min (average R = 0.458, p < 0.001). Although it remained significant (average R = 0.276, p < 0.05), the association strength decreased significantly following total sleep deprivation (p < 0.05, Wilcoxon test). Cortisol plasma level, secretory rate and pulse amplitude were increased as well as waking EEG power in the delta (0.5–5.5 Hz), theta (5.5–8.5 Hz) and gamma frequency bands (all p values <0.05, Student t tests). The sleep deprivation-related increases in cortisol secretory rate and waking EEG gamma activity were quantitatively associated (R = 0.504, p < 0.05). These results support the existence of a common ultradian regulatory mechanism, co-ordinating HPA axis activity to the level of central arousal in man, which seems involved in the sleep deprivation-induced hyperarousal.

Metabolic and behavioral consequences of a snack consumed in a satiety state

In view of the influence of dietary habits on obesity, human eating patterns merit study. We investigated the behavioral and biological consequences of consumption of a 1-MJ snack by subjects in a satiety state. Eleven lean young men were deprived of time cues and subjected to continuous blood withdrawal over each of 4 sessions scheduled 2 wk apart. The first session was a basal session designed to determine the following in each subject: 1) the amount eaten in an ad libitum lunch; 2) the temporal patterns of plasma concentrations of glucose, insulin, fatty acids, and triacylglycerols between lunch and the spontaneous dinner request; and 3) the latency of the dinner request. In the 3 other sessions, each subject ingested the same lunch as in the basal session and a nutritionally well-balanced snack either 5 min before his individual peak of hyperglycemia observed in the first session, 40 min after this peak, or 120 min before the time he had requested his dinner in the first session. There was no significant difference in latency of the dinner request or the energy intake at dinner between sessions. Insulin secretion increased but glucose profiles did not change significantly regardless of the time of snack intake. A snack consumed in a satiety state fails to prolong the intermeal interval and would thus tend to favor storage.

Magnesium reduces free radicals in an in vivo coronary occlusion-reperfusion model

Objective. This study demonstrated that magnesium (Mg) reduces free radicals after a brief coronary occlusion-reperfusion sequence.Background. Magnesium has been shown to reduce infarct size in patients with acute myocardial infarction. We hypothesized that this action of Mg occurs through its action on free radicals.Methods. Eighteen mongrel dogs were studied (nine control, nine receiving Mg). Catheters were placed into the coronary sinus for continuous blood withdrawal. A Varian E-4 electron paramagnetic resonance spectrometer was used to monitor the ascorbate free radical (AFR) signal in the coronary sinus blood; AFR is a measure of total oxidative stress. Occlusion of the left anterior descending coronary artery for 20 min was followed by reperfusion. The study animals received 4 g Mg intravenously starting at 15 min of occlusion (5 min before reperfusion) and continuing during reperfusion.Results. Results are presented as percent change from baseline ± SEM. Magnesium blunted the peak AFR increase: at 4 min of reperfusion there was a 4.7 ± 3.3% increase in AFR signal in the dogs receiving Mg versus an 18.2 ± 3.3% increase in the control animals (p < 0.05). Total radical flux was reduced during reperfusion by 53% in the Mg dogs compared with controls (p < 0.05).Conclusions. Magnesium attenuates AFR increase after an occlusion-reperfusion sequence. To our knowledge this is the first in vivo real-time demonstration of Mg’s impact on free radicals.

Growth hormone secretion during sleep in male depressed patients

1. 1. Growth hormone (GH) secretion during sleep was studied in ten male patients with major depression according to DSM III and eight normal controls. 2. 2. Samples were collected through a continuous blood withdrawal pump while sleep was recorded in the laboratory. 3. 3. The results showed a marked decrease in the GH secretion mainly during the first three hours of sleep in depressed patients as compared to normal controls. DST and TRH tests were also administered to the same patients but no correlation was observed between a positive test and a blunted GH secretion, suggesting that the various neuroendocrinological disturbances do not coexist in all depressed patients. 4. 4. This disturbance in GH secretion during sleep, along with reduced slow wave sleep (SWS), gives support to the theory that GHRH is the common stimulus of SWS and GH release and that the ratio of GHRH and its counterpart CRH plays a major role in the pathophysiology of disturbed endocrine activity during sleep in depression.

Efficacy of intravenous granisetron in suppressing the bradycardia and hypotension associated with a rabbit model of the Bezold-Jarisch reflex.

This study investigated whether granisetron, a 5-HT3 receptor antagonist, can alter the Bezold-Jarisch reflex (i.e., hypotension and inappropriate heart rate slowing). A hemorrhagic rabbit model that has been shown to induce the Bezold-Jarisch reflex was used. In 11 rabbits (3.8 kg), catheters were placed in the carotid arteries one day before experimental hemorrhage. On the day of the study, the rabbits were given intravenous granisetron (50 micrograms/kg) or an equal volume of saline. Five minutes after administration of granisetron or saline, hemorrhage was induced by continuous blood withdrawal at 5 mL/min and blood pressure (BP) and heart rates were obtained at frequent intervals until systolic BP declined to 80 mmHg. Six rabbits received saline and five granisetron. An average of 77.6 mL +/- 16.4 mL of blood was removed in the group receiving granisetron (compared with 56.5 mL +/- 13.1 mL for the saline group) before achieving the target systolic BP of 80 mmHg. The group receiving granisetron demonstrated the same ability to increase their heart rate from baseline as the saline group. However, the granisetron group had a final heart rate that was closer to their maximal heart rate than the saline group. In this animal model, granisetron was significantly more effective at preventing inappropriate heart rate slowing and allowed significantly more blood to be removed before reaching the target blood pressure. This implies that granisetron may be effective in preventing vasovagal syncope, although further study should be carried out to verify these potentially interesting findings.

Modulation of episodic adrenocorticotropin hormone secretion by cadmium in male rats.

The effects of cadmium on adrenocorticotropin hormone (ACTH) secretion are controversial and seem to depend on the dose and duration of the exposure to the metal. This work was undertaken to analyze the effects of acute cadmium administration on the episodic pattern of ACTH release in adult male rats. For this purpose, animals were cannulated 40 h before the experiment to allow a continuous blood withdrawal. Two and a half hours after the administration of a single dose of cadmium chloride (4.5 mg kg-1 bodyweight), the episodic pattern of ACTH was analyzed during three hours (from 10:30 to 13:30, samples being collected every seven minutes) in conscious and freely moving adult male rats. The mean values of ACTH during the bleeding period and the absolute pulse amplitude were decreased by acute cadmium chloride administration (P < or = 0.001, P < or = 0.01, respectively). By contrast, the frequency of ACTH pulses increased (P < or = 0.01). However, no changes in any other parameters of episodic ACTH secretion were observed compared with control animals. These data suggest that cadmium interferes with the regulatory mechanism of ACTH.

Continuous blood withdrawal as a rapid screening method for determining clearance of oral bioavailability in rats.

To develop a methodology for continuous blood withdrawal in rats suitable for drug discovery screening purposes and perform limited validation studies with a series of test compounds. A reliable methodology for continuous blood withdrawal in rats was developed. The method is dependent on continuous heparin infusion during withdrawal and the minimization of constrictive, thrombogenic sites. Plasma drug concentrations from either intermittent sampling or continuous withdrawal experiments were determined with HPLC analysis. The continuous withdrawal method was successfully adapted to rats such that blood samples could be reliably collected over a 6-hr experiment. The clearance and oral bioavailability values for theophylline, atenolol, propranolol, warfarin, BMS-182874 and BMS-A were determined from continuous withdrawal or intermittent sampling experiments. The results from the two methods were comparable, with each compound reliably placed in the same low, medium or high category based on clearance or oral bioavailability characteristics. The continuous withdrawal method proved to be a viable alternative to the classic intermittent sampling technique. The method should prove useful in drug discovery screening, where the evaluation of large numbers of compounds for systemic clearance or oral bioavailability is often necessary.

Cadmium does not inhibit pulsatile prolactin secretion through TRH.

This work was undertaken to analyse the effects of acute cadmium administration on the pulsatile pattern of prolactin release, in adult male rats. For this purpose, animals were cannulated 40 h before the experiment to allow a continuous blood withdrawal. Two hours after the administration of one dose of cadmium chloride (4.5 mg kg1), the pulsatile pattern of prolactin, during three hours, was studied. The effects of two pulses of thyrotropin-releasing hormone (TRH) (1 microgram per rat), given 60 and 120 min after starting the period of blood sampling, were studied. The mean values of prolactin during the bleeding period and the absolute pulse amplitude were decreased by acute cadmium chloride administration. However, no changes in any other parameters of prolactin pulsatility were observed. TRH administration to control rats increased mean prolactin levels, and absolute and relative pulse amplitudes, but decreased the mean half-life of the hormone. In animals pretreated with cadmium, TRH increased the mean levels of prolactin, and absolute and relative amplitudes of the hormone pulses. No other parameter studied was changed by TRH in cadmium pretreated rats. These data suggest that acute administration of cadmium did not inhibit the pulsatile prolactin release through TRH.

Nonclassical secretory dynamics of LH revealed by hypothalamo-hypophyseal portal sampling of sheep.

Continuous withdrawal of hypophyseal portal blood from unrestrained sheep has permitted detailed assessments of the pulsatile secretion of gonadotrophin-releasing hormone (GnRH). To determine if this blood can also be used to characterize the sensory dynamics of pituitary hormones, patterns of luteinizing hormone (LH) in the hypophyseal portal blood of ovariectomized ewes was compared with previous patterns of GnRH and peripheral LH. Hypophyseal portal blood and jugular vein blood were collected every 5 min from six ovariectomized ewes over 6-12 h. Hypophyseal portal blood contained GnRH-associated, sharply defined LH pulses that were much larger than in the periphery. Pulses of secreted LH (hypophyseal portal LH less peripheral LH) showed much faster rates of rise and fall than peripheral and followed pulses of GnRH by an average of 1.26 min. In contrast to pulses in jugular blood, secreted LH pulses often reached a relatively unchanging interpulse nadir-plateau and thereby approached closely algorithm-estimated, extrapolated baselines. The interpulse baseline concentrations of secreted LH (99.6 ng/mL) in hypophyseal portal blood were 31-fold higher than those for jugular LH (3.23 ng/mL). These elevated concentrations also exceeded mean jugular peak concentrations (11.1 ng/mL) and, thus, primarily must represent newly secreted LH. The non-Gaussian profiles of this secreted LH were substantially more complex than the inputs predicted from jugular LH measurements by deconvolution. Furthermore, regardless of the analytical approach, estimations of the mass of secreted LH in each pulse did not correlate well with inputs predicted by deconvolution or Kushler-Brown pulsefit analysis of corresponding pulses in jugular blood (r2 ranging 0.40-0.48). Among alternative explanations is the possibility of heterogeneity in concentrations of GnRH in the portal vessels and variable distribution within the hypophysis. In summary, assay of hypophyseal portal blood obtained directly from the pituitary provides a method for direct assessment of secretory responses to hypothalamic peptides, and thereby serves as an unmatched method for studying the dynamics of LH secretion in vivo. With this approach, LH is revealed to be secreted as complex, non-Gaussian pulses that are far more sharply defined that those in the periphery, include non-GnRH-dependent, secretory components that cannot be predicted by deconvolution and are followed by periods of relatively constant, basal secretion.

Can the microsphere model by applied to cerebral blood flow measurement using N-isopropyl-p-[123I]iodoamphetamine with SPET regardless of washout from brain tissue?

The microsphere model and the continuous withdrawal of arterial blood have commonly been used in clinical studies when measuring regional cerebral blood flow (rCBF) by N-isopropyl-p-[123I]iodoamphetamine (IMP) single photon emission tomography. The method is considered to underestimate rCBF because of the washout of tracer from brain tissue; however, the extent of this underestimation is not known. To assess whether this underestimation can be determined quantitatively, we performed simulation analysis based on the microsphere model and the two-compartment model [influx, K1(rCBF); and outflux, k2(washout)] using the time-activity curves of 123I-IMP in arterial blood [Ca(t)] of 10 subjects. With the microsphere method, rCBF values fell as time post-injection increased. The extent of underestimation of rCBF was 4.7 +/- 0.28% (mean +/- S.D.) at 5 min, 10.2 +/- 0.42% at 10 min and 15.2 +/- 0.55% at 15 min. There was little variation in the extent of underestimation and it was not dependent on the Ca(t) of the subjects. We therefore considered the results to be generally applicable to various studies of the microsphere model. As 4.7% is considered to be negligible in clinical studies, we conclude that the microsphere model can be applied to obtain accurate measurement of rCBF up to 5 min regardless of washout.

Patterns of luteinizing and follicle-stimulating hormone pulsatility in menopausal women: Correlation with plasma testosterone level and vasomotor instability episodes

OBJECTIVE: The purpose of this study was to test the hypothesis that subtle differences among postmenopausal women can be detected by careful analysis of plasma luteinizing hormone and follicle-stimulating hormone pulses. STUDY DESIGN: Twelve postmenopausal women not receiving estrogen therapy were admitted for continuous blood withdrawal at the rate of 1 ml/min. Aliquots of 3 ml of pooled blood were collected every 3 minutes for 3 hours for the measurement of plasma luteinizing hormone and follicle-stimulating hormone. Estradiol, estrone, free testosterone, androstenedione, and dihydrotestosterone were measured from pooled specimens. Concomitantly, continuous recordings of peripheral blood flow and peripheral temperature from the opposite arm were obtained to detect vasomotor instability episodes. RESULTS: Analysis of gonadotropin pulses revealed a single pattern of plasma follicle-stimulating hormone but two distinct patterns of luteinizing hormone frequency and amplitude. One group of six women had a low mean (±SEM) interpulse interval frequency of 0.8 ± 0.26 ( p < 0.005) and a high mean amplitude of 18.4 ± 1.8 IU/L ( p < 0.05). The second group of six women had a high mean interpulse interval frequency of 3.5 ± 0.34 and a low mean amplitude of 12.2 ± 1.8 IU/L luteinizing hormone pulses. Women with this pattern had more inter-vasomotor instability episode intervals (3.5 ± 1.6) than did women with low frequency and high amplitude (1.3 ± 0.5, p < 0.001). Women with low frequency and high amplitude pulses had higher plasma levels of estrone (266 ± 33 nmol/L), testosterone (2.3 ± 0.7 mmol/L), and free testosterone (8.0 ± 0.1 pmol/L, p < 0.05). CONCLUSION: There are subgroups of postmenopausal women with different patterns of luteinizing hormone pulsatility. This difference can be explained by higher steroid levels in women with low frequency and high amplitude pulses.

Quantifying regional cerebral blood flow with N-isopropyl-p[123I]iodoamphetamine by ring-type single-photon emission computed tomography: validity of a method to estimate early reference value by means of regional brain time-activity curve.

A more accurate quantitative method for the measurement of regional cerebral blood flow (rCBF) with the microsphere model and N-isopropyl-p-[123I]iodoamphetamine (123I-IMP) and ring-type single-photon emission computed tomography (SPECT) was developed. Continuous withdrawal of arterial blood was carried out for 5 minutes after the injection. Static SPECT data were acquired from 25 min to 55 min. To estimate reconstructed images at 5 min, total brain count collections and one minute SPECT studies were performed at 5, 20, and 60 min. Quantitative values for rCBF were calculated from short time SPECT images at 5 min (rCBF), static SPECT images corrected by total brain counts (rCBFCt) and those corrected by reconstructed counts on short time SPECT images (rCBFCb). Practically, rCBFCb is calculated by using reconstructed counts of regions of interest placed in the same position as static SPECT and short time SPECT at 5, 20, 60 min. Although there was good correlation between rCBF and rCBFCt (r = 0.69), rCBFCt tended to be underestimated in high flow areas and overestimated in low flow areas. A better correlation was observed between rCBF and rCBFCb (r = 0.92). The overestimation and underestimation observed in rCBFCt was considered to be due to the correction method with a total cerebral time activity curve, because the kinetic behavior of 123I-IMP was different in each region.

Cross-flow membrane plasmapheresis technique for continuous ex vivo plasma sampling

A technique is described for plasma sampling by continuous membrane plasmapheresis performed on blood flowing through an extracorporeal arteriovenous shunt. The plasmapheresis sampler in the shunt employs replaceable commercial planar membranes 2.5 cm in diameter. Validation tests were conducted for 0.6-μm pore diameter microporous membranes with several low-molecular-weight, nonmetabolized solutes that either rapidly equilibrate between plasma and formed elements or remain extracellular. Ex vivo tests were performed for bolus intravenous administration to rabbits. The technique yielded values for time-averaged plasma concentrations comparable to those obtained with serial blood and continuous blood withdrawal methods. The new technique should be particularly advantageous when the distribution of the solute of interest between plasma and formed elements of the blood undergoes significant changes during the sampling interval as a result of binding, exchange, or metabolism in the formed element phase.

Estimation of Growth Hormone Secretory Patterns in Children with Use of a Numerical Deconvolution Technique: Experimental Design with Use of Computer Simulation

Growth hormone (GH) secretion rates were estimated from 24-hour plasma GH concentration profiles using a model-based numerical deconvolution method. The basic kinetic parameters describing GH distribution and elimination were obtained from studies using single intravenous injections in GH-deficient children. Computer simulation techniques were used to produce 24-hour GH secretion profiles, which could serve as reference curves for analysis of the impact of certain practical aspects of the estimation of GH secretion, such as the method of blood sampling (discrete or integrated), sampling frequency and analytical errors. The results show that sampling intervals of 20 min are acceptable even in the presence of analytical errors, and that with this sampling interval, discrete blood sampling does not seem to be preferable to continuous withdrawal of blood.

Plasma Cortisol Levels Increase with Age in Obese Subjects

In order to assess the influence of age, sex, and body mass on plasma cortisol concentrations, we measured the 24-hour Integrated Concentration (IC) of cortisol (F) in 36 obese subjects (16 males, 20 females) aged 5.3-56.4 years, BMI = 35.5 +/- 7.3 kg/m2 and compared with 119 nonobese subjects, body mass indices (BMI) 21.2 +/- 2.7 kg/m2, aged 8.8-66.2 years (55 males, 64 females). Subjects were nondiabetic, normotensive, without history of psychiatric illness, and otherwise in good health. IC studies were performed using a continuous blood withdrawal methodology, and IC-F was assayed from the 24 hour pooled sample by a protein binding method. The effect of age and gender on IC-F was analyzed by multivariate regression. In the nonobese group there was no effect of age or sex on IC-cortisol levels, the mean IC-F = 173.8 +/- 44.1 nmol/L. A statistically significant but weak negative effect of BMI on IC-cortisol (r=-.18, p<0.05) was present. In the obese subjects there was a significant increase in IC-cortisol levels with age IC-F(nmol/L) = 2.76 x age(years) + 85.0 (r2=.36, p<0.0001). IC-cortisol levels tended to be lower in obese males than females when controlled for age (p<0.05). We conclude that in nonobese subjects IC-F levels are independent of age and gender. However, there is a significant increase of IC-cortisol levels with age in obese individuals. The observed increase of IC-cortisol with age may contribute to metabolic complications of obesity.