Effects of mild to moderate sedation on saccadic eye movements.

Sedatives alter the metrics of saccadic eye movements. If these effects are nonspecific consequences of sedation, like drowsiness and loss of attention to the task, or differ between sedatives is still unresolved. A placebo-controlled multi-step infusion of one of three sedatives, propofol or midazolam, both GABA-A agonists, or dexmedetedomidine, an α2-adrenergic agonist, was adopted to compare the effects of these three drugs in exactly the same experimental conditions. 60 healthy human volunteers, randomly divided in 4 groups, participated in the study. Each infusion step, delivered by a computer-controlled infusion pump, lasted 20min. During the last 10min of each step, the subject executed a saccadic task. Target concentration was doubled at each step. This block was repeated until the subject was too sedated to continue or for a maximum of 6 blocks. Subjects were unaware which infusion they were receiving. A video eye tracker was used to record the movements of the right eye. Saccadic parameters were modeled as a function of block number, estimated sedative plasma concentration, and subjective evaluation of sedation. Propofol and midazolam had strong effects on the dynamics and latency of the saccades. Midazolam, and to a less extent, propofol, caused saccades to become increasingly hypometric. Dexmedetedomidine had less impact on saccadic metrics and presented no changes in saccadic gain. Suppression of the sympathetic system associated with dexmedetomidine has different effects on eye movements from the increased activity of the inhibitory GABA-A receptors by propofol and midazolam even when the subjects reported similar sedation level.

[The study of naphthyzin present in material evidence and biological fluids].

The optimal conditions for isolation of naphazoline from naphthyzin preparations and biological fluids with chloroform at pH 9.18 are described. The compound of interest was identified with the use of color and precipitation reactions, IR and UV spectroscopy, thin-layer and gas chromatography, and chemical methods including high performance liquid chromatography, chromatodensitometry, and UV spectroscopy. The results obtained by the three methods are comparable.

Effect of mild hyperisulinemia on conduit vessel endothelial function: role of noradrenergic activation.

OBJECTIVE: The evidence that an exogenously induced modest hyperinsulinemia deteriorates conductance artery endothelial function - flow-mediated dilatation (FMD) - in healthy individuals is in contrast with in-vitro and in-vivo studies that consistently found that insulin facilitates both nitric oxide release and the endothelium-dependent dilatation. The aim of this study was to verify whether this effect is caused by the enhancement of insulin-induced adrenergic tone. METHOD: In 10 healthy male volunteers, endothelium-dependent (FMD) and endothelium-independent (glyceryl trinitrate, GTN) dilatation were evaluated by high-resolution ultrasound of the brachial artery, combined with a computerized edge detection system, at baseline (-60 and 0 min) and after 120 and 240 min during insulin infusion (INS study). In five participants, randomly selected from the initial group, the study was repeated during an isotonic saline (0.9% sodium chloride) intravenous infusion (SAL study). In an additional five participants, insulin infusion was preceded by an intravenous infusion of clonidine started 40 min before insulin and continued throughout the study (INS + CLN study). RESULTS: Plasma norepinephrine concentration increased in the INS study from 260 ±â€Š40 to 333 ±â€Š62 pg/ml (P < 0.05), whereas it remained stable throughout the INS + CLN study. In the INS study, no change in FMD was observed, whereas the response to GTN tended to decrease (P = 0.09). In the INS + CLN study, no significant changes in FMD response were observed, whereas GTN response was completely restored. CONCLUSION: Physiological hyperinsulinemia has no effect on endothelium-dependent vasodilatation in conduit vessels of healthy individuals, but it induces a slight decline in endothelium-independent vasodilatation, which is entirely explained by the insulin-induced noradrenergic activation.

Dried blood spots as a sampling technique for the quantitative determination of guanfacine in clinical studies.

BACKGROUND: Dried blood spot (DBS) technology was evaluated for the quantitative determination of guanfacine in human blood in clinical studies. A very sensitive DBS assay has been developed using HPLC coupled with an AB Sciex 5500 QTRAP® (Applied Biosystems/MDS Sciex, ON, Canada) MS system (LC-MS/MS) with a linear calibration range of 0.05 to 25 ng/ml. High-resolution MS using an Exactive Orbitrap® (ThermoFisher, LLC., CA, USA) was compared with the QTRAP using extracted exact mass ion current profiles for guanfacine and its stable-isotope-incorporated internal standard. The sample preparation employed liquid-liquid extraction with methyl t-butyl ether of 5 mm punched DBS card disks, followed by reversed-phase HPLC separation coupled with either MS/MS or high-resolution MS. Routine experiments were performed to establish the robustness of the DBS assay, including precision, accuracy, linearity, selectivity, sensitivity and long-term stability of up to 76 days. In addition, several factors that potentially affect quantitation were investigated, including blood volume for DBS spotting, punch size and punch location. RESULTS: A sensitive research assay with a LLOQ of 0.05 ng/ml was developed and subjected to several components of a method validation common to a regulated bioanalysis procedure employing DBS. This method development and partial validation study determined that spot volume, punch size or punch location do not affect assay accuracy and precision. The DBS approach was successfully applied to a clinical study (a Phase I, randomized, double-blind, placebo-controlled, crossover study to assess the effect of varying multiple oral doses of guanfacine on the pharmacokinetic, pharmacodynamic, safety, and tolerability profiles in healthy adult subjects). The pharmacokinetic profiles for 12 volunteers generated from the DBS assay and from a previously validated plasma assay were compared and were found to be comparable. DBS incurred samples collected from finger prick blood and directly applied to the DBS cards were also analyzed for comparison. CONCLUSION: From a bioanalytical perspective, DBS sample collection and analysis is a potentially viable alternative for guanfacine determination in clinical studies, utilizing approximately 100 µl of blood per subject profile compared with a few millilitres of blood drawn for conventional plasma bioanalysis.

Buccal administration of dexmedetomidine as a preanesthetic in children.

PURPOSE: The objective of this study was to evaluate the efficacy and safety of buccal dexmedetomidine as a preanesthetic in children, to compare it with diazepam, and to investigate the optimal dosage for buccal dexmedetomidine administration by measuring its serum concentration. METHODS: We performed a prospective study with 40 children who were assigned to two groups. The patients underwent an operation for inguinal or umbilical hernia. Twenty children received dexmedetomidine buccally at 3-4 microg/kg (Dex Group) and 20 received a diazepam suppository at 0.7 mg/kg (Diazepam Group) as preanesthetics 1 h before the operation. Heart rate, systolic blood pressure, SpO2, and respiratory rate were measured 1 h after premedication in all children. Sedation level was preoperatively evaluated, and compared with the Ramsay score, in the ward, at the entrance to the main operating rooms, and at anesthesia induction between the two groups. To investigate the optimal dosage of buccal dexmedetomidine, we compared the mean serum concentration of dexmedetomidine at induction between patients with a Ramsay score of 5 or greater and those with a Ramsay score less than 5. The Mann-Whitney U test was used for statistical analysis. RESULTS: There was no significant difference between the two groups in age or body weight. Furthermore, there was no significant difference between the two groups in heart rate, systolic blood pressure, SpO2, or respiratory rate after administration of either medication. The Ramsay score of the Dex Group was significantly higher than that of the Diazepam Group at all times. The mean serum dexmedetomidine concentration at induction in patients with a Ramsay score of 5 or greater (75 +/- 50 pg/ml) was significantly higher than in those with a Ramsay score less than 5 (34 +/- 36 pg/ml, P < 0.05). CONCLUSION: These results suggest that the buccal administration of dexmedetomidine (3-4 microg/kg) 1 h before the operation can be safely and effectively applied as a preanesthetic in children.

Combined pharmacokinetic and urodynamic study of the effects of oral administration of phenylpropanolamine in female Beagle dogs.

This study investigated the differences in pharmacokinetic, urodynamic and haemodynamic parameters after administration of two dosages of phenylpropanolamine (PPA) in female Beagle dogs. Blood was collected and urethral pressure profiles were performed over 24 h periods following single or three times daily (T(0),T(6h),T(12h)) administration of PPA. The maximal concentration (C(max)) was reached 2 h after PPA administration (T(max)) and the half-life (T((1/2))) was 4 h. Three times daily administration induced an increase in C(max) due to bioaccumulation. A significant increase in urethral resistance, compared to the control group, was observed at T(max) after 1 week of once daily administrations, but not when PPA was administered every 6 h during the day, despite higher plasma concentrations following more frequent dosing. An increase in mean arterial pressure was compensated by a decreased heart rate. Clinical efficacy with the temporary increase in urethral resistance following single daily administration of PPA in dogs suffering from urethral sphincter mechanism incompetence (USMI) needs to be further investigated in a randomised clinical trial.

Toxicity from a clonidine suspension.

BACKGROUND: Clonidine is frequently prescribed to children. Clonidine overdose in children has resulted in major clinical effects and deaths. CASE REPORT: A 3.5-year-old male with a history of a seizure disorder and night terrors presented following difficulty walking, excessive sleeping, agitation when awake, and possible seizure activity. Chronic medications were valproic acid (VPA) and clonidine. On presentation, he alternated between poor responsiveness and agitation, with initial vitals: blood pressure, BP 144/76 mmHg; heart rate, 65 bpm; respiratory rate, 18 bpm; temperature 99.5 degrees F; and pulse oximetry 96% on room air. VPA level was 35 microg/mL. A toxicology consult the next day noted a dry mouth, 2-mm pupils, intermittent gasping, and central nervous system (CNS) depression, with a diagnostic impression of clonidine overdose. The caregiver had been giving 1 mL (0.1 mg) qd of a pharmacy-compounded clonidine suspension by a provided syringe. The pharmacy procedure record agreed with the physicians order. The amount dispensed was a 30-day supply but the bottle was empty on day 19, leading us to suspect a possible accelerated dosing error. The concentration in the bottle thus could not be confirmed. The child slowly returned to his baseline state over 48 hours. A serum clonidine level drawn approximately 18 hours after his last dose later returned at 300 ng/mL (reference range = 0.5-4.5 ng/mL). CASE DISCUSSION: Compounding and liquid dosing errors are common in children and may result in massive overdoses. There was an accelerated dosing error, but whether a compounding or suspension error or even an acute overdose occurred as well is unknown. CONCLUSION: Particular care should be taken with medications that have low therapeutic indices, that are extemporaneously compounded, and are prepared as liquids, where medication errors are more likely.

Endogenous catecholamine stimulates alveolar fluid clearance in rats with acute pancreatitis.

BACKGROUND AND OBJECTIVE: Acute pancreatitis causes pulmonary oedema with the accumulation of fluid in the alveolar spaces, possibly due to reduced clearance. This study tested the hypothesis that acute pancreatitis decreases alveolar fluid clearance in a rat model of pulmonary oedema during acute pancreatitis. METHODS: Acute pancreatitis was induced by a retrograde injection of 5% taurocholate sodium (0.2 mL) into the common bile duct. The lungs were isolated 4, 24 and 48 h after the induction of acute pancreatitis and alveolar fluid clearance was measured in the absence of pulmonary perfusion. RESULTS: Alveolar fluid clearance increased to 31.0 +/- 3.5% of instilled volume/h in rats with acute pancreatitis for 4 h compared with 17.3 +/- 1.0% of instilled volume/h in sham rats (P < 0.01), then returned to the control level 48 h after acute pancreatitis (16.0 +/- 4.1% of instilled volume/h). In contrast, the lung water to dry lung weight ratio decreased maximally 24 h after acute pancreatitis (P < 0.01), then returned to the control level 48 h after acute pancreatitis. The plasma epinephrine levels increased to 25-fold higher in rats with acute pancreatitis for 4 h than in sham rats without acute pancreatitis. Prazosin (an alpha(1)-adrenergic antagonist, 10(-4) mol/L), yohimbine (an alpha(2)-adrenergic antagonist, 10(-4) mol/L) or a bilateral adrenalectomy inhibited the increase in part, a combination of prazosin (10(-4) mol/L) and yohimbine (10(-4) mol/L) completely inhibited the increase in alveolar fluid clearance in rats after acute pancreatitis for 4 h, whereas propranolol (a beta-adrenergic antagonist, 10(-4) mol/L) had no effect. CONCLUSIONS: Endogenous catecholamine stimulates alpha-adrenoceptors and increases alveolar fluid clearance in rats with acute pancreatitis.

Absorption pharmacokinetics of clonidine nasal drops in children.

BACKGROUND: The alpha2 agonist clonidine has become a popular drug for premedication in children. Effects and pharmacokinetics after oral, rectal, and intravenous administration are well known. The aim of this study was to investigate the absorption pharmacokinetics of clonidine nasal drops in children. METHODS: Thirteen ASA I pediatric patients received after induction of anesthesia 4 mcg x kg(-1) of clonidine by the nasal route. Blood samples were taken during a 12-h period and plasma levels of clonidine were analyzed by liquid chromatography-mass spectrometry. Data were calculated by a computer-aided curve-fitting program. RESULTS: Plasma pharmacokinetics following administration of clonidine nasal drops showed a considerable interindividual variability and absorption was delayed and limited. A total of 95% confidence intervals for maximum plasma concentration and time to achieve maximum plasma concentration were 0.4-0.6 ng x ml(-1) and 1.4-3.0 h, respectively. CONCLUSIONS: Clonidine nasal drops are erratically absorbed from the nasal mucosa and, thus, this mode of drug administration is not recommended for premedication purposes.

Clonidine pharmacokinetics in pregnancy.

The objective of this study was to determine the pharmacokinetic parameters of clonidine during pregnancy compared with previously published data in nonpregnant subjects. Serial blood and urine samples were collected in 17 women during mid to late pregnancy over one steady-state dosing interval to determine clonidine noncompartmental pharmacokinetic parameters (n = 17) and creatinine clearance. In six of these pregnant subjects, maternal and umbilical cord (venous and arterial) plasma samples were collected at the time of delivery for measurement of clonidine concentrations. Clonidine apparent oral clearance was found to be 440 +/- 168 ml/min during pregnancy compared with 245 +/- 72 ml/min as previously reported in nonpregnant subjects (p < 0.0001) (Cunningham et al., 1994). There was a strong correlation (r = 0.82, p < 0.001) between clonidine renal clearance, adjusted for variation in glomerular filtration rate, and urine pH. Umbilical cord to maternal plasma clonidine concentration ratios were 1.0 +/- 0.1 (arterial) and 1.0 +/- 0.1 (venous). In conclusion, clonidine is cleared more rapidly in pregnant women than in nonpregnant subjects. At the time of delivery, the fetus is exposed to similar plasma clonidine concentrations as the mother.

Increased sympathetic reactivity may predict insulin resistance: an 18-year follow-up study.

Insulin resistance and sympathetic activity are related by a positive feedback system. However, which precedes the other still remains unclear. The present study aimed to investigate the predictive role of sympathoadrenal activity in the development of insulin resistance in an 18-year follow-up study. We also examined whether reactivity to 2 different stress tests, a cold pressor test and a mental stress test, would differ in their predictive power. The 2 tests are supposed to represent different reactivity mechanisms: alpha- and beta-adrenergic responses, respectively. At entry, arterial plasma epinephrine and norepinephrine concentrations were measured in 99 healthy men (age, 19.3 +/- 0.4 years, mean +/- SD) during rest, a mental stress test, and a cold pressor test. Fasting plasma glucose concentration was measured at entry and at follow-up. Insulin resistance at follow-up was calculated using the homeostasis model assessment of insulin resistance (HOMA-IR). Eighty subjects (81%) were eligible for follow-up after 18.0 +/- 0.9 years (mean +/- SD). The norepinephrine responses to cold pressor test at entry predicted plasma glucose concentration (r = 0.301, P = .010) and HOMA-IR (r = 0.383, P = .004) at follow-up in univariate analyses. In multiple regression analyses, corrected for fasting glucose at entry, family history of diabetes, blood pressure-lowering medication, body mass index at entry, and level of exercise, norepinephrine response to cold pressor test was found to be a positive predictor of future HOMA-IR (P = .010). This is the first long-term follow-up study in white subjects showing that sympathetic reactivity predicts future insulin resistance 18 years later. These findings may provide further insights into the pathophysiologic mechanisms of insulin resistance.

Alpha1-adrenergic receptor mRNA and inflammatory mediator expression in circulating leucocytes after cardiac surgery.

Vasodilation after coronary artery bypass surgery is a common complication. Inflammatory mediators influence the expression of alpha1-adrenergic receptors. Do patients requiring high doses of postoperative inotropic support have down-regulated alpha-adrenergic receptors? Is there a characteristic pattern of preoperative inflammatory mediator expression that could predict a complicated course after the operation? Forty-four patients undergoing cardiac bypass surgery with extracorporeal circulation were prospectively investigated. Five perioperative blood samples were taken (preoperative, two hours, 12 hours, 36 hours and 72 hours postoperative). The leucocyte mRNA-expression of the three alpha1-adrenergic receptor subtypes (A, B and D) and 11 different pro-inflammatory mediators were investigated with the real-time reverse transcriptase polymerase chain reaction. The patients were divided into three groups (No-noradrenaline [No-NA]= 0 microg/min, Low-noradrenaline [Low-NA]=0.1-7 microg/min, High-noradrenaline [High-NA] >7 microg/min), according to their postoperative noradrenaline requirements. Preoperatively, alpha1(A)-receptor expression was 4.9-fold (High-NA) and 18.7-fold (Low-NA) higher than the No-NA group (P=0.005) and plasma noradrenaline levels were higher in the High-NA group (P=0.005). Across all groups at 12 hours after the operation, alpha1(A) -receptor expression decreased to approximately one-fifth of preoperative levels (P=0.01); but with greater duration and magnitude of relative decrease in the High-NA group. Patients in the No-NA group had significant postoperative increases in leucocyte inflammatory mediator expression for IL-1beta, TLR4, TREM, MPO, MMP9 and TNF genes, whereas the changes in the Low-NA and High-NA groups were not significant. Low preoperative levels of noradrenaline and low expression of alpha1(A)-adrenoreceptors in leucocytes was associated with less probability of requiring noradrenaline support after cardiac surgery.

Pharmacokinetics of intravenous dexmedetomidine in children under 11 yr of age.

BACKGROUND: Information has been very limited on the pharmacokinetics of the selective alpha(2)-adrenoceptor agonist dexmedetomidine in children, particularly in children <2 yr of age. METHODS: Eight children aged between 28 days and 23 months and eight children aged between 2 and 11 yr undergoing either elective bronchoscopy or nuclear magnetic resonance imaging were included in the study. Dexmedetomidine 1 microg kg(-1) was infused i.v. over 5 min. Blood samples for the measurement of plasma concentrations of dexmedetomidine were collected for 5 h after starting the infusion. Pharmacokinetic calculations were based on non-compartmental methods. RESULTS: In the two groups of paediatric patients, the median (range) values for total plasma clearance of dexmedetomidine were 17.4 (14.1-27.6) and 17.3 (9.3-22.5) ml kg(-1) min(-1), for volume of distribution at steady state 3.8 (1.9-4.6) and 2.2 (1.3-2.8) litre kg(-1) (P<0.05), and for elimination half-life 139 (90-198) and 96 (69-140) min (P<0.05), respectively. The volume of distribution at steady state was negatively associated with subject age (r=-0.69, P<0.05). CONCLUSIONS: To reach a certain plasma concentration, children younger than 2 yr of age evidently need larger initial doses of dexmedetomidine than the older children, as young children have a larger volume of distribution of the drug than older children and adults. Since the total plasma clearance of dexmedetomidine is independent of age, similar rates of infusion can be used in younger and older children to maintain a steady-state concentration of dexmedetomidine in plasma.

Rapid and sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the determination of clonidine in human plasma.

A rapid and sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been developed and validated for the estimation of clonidine in human plasma. Clonidine was extracted from human plasma by using solid-phase extraction technique. Nizatidine was used as the internal standard. A Hypurity C18 (50 mm x 4.6 mm i.d., 5 microm particle size) column provided chromatographic separation of analyte followed by detection with mass spectrometry. The method involves a rapid solid-phase extraction from plasma, simple isocratic chromatography conditions and mass spectrometric detection that enables detection up to picogram levels with a total run time of 3.0 min only. The method was validated over the range of 50-2500 pg/mL. The absolute recoveries for clonidine (71.86%) and IS (69.44%) achieved from spiked plasma samples were consistent and reproducible.

The effect of clonidine infusion on distribution of regional cerebral blood flow in volunteers.

BACKGROUND: Through their action on the locus coeruleus, alpha2-adrenoceptor agonists induce rapidly reversible sedation while partially preserving cognitive brain functions. Our goal in this observational study was to map brain regions whose activity is modified by clonidine infusion so as to better understand its loci of action, especially in relation to sedation. METHODS: Six ASA I-II right-handed volunteers were recruited. Electroencephalogram (EEG) was monitored continuously. After a baseline H2(15)O activation scan, clonidine infusion was started at a rate ranging from 6 to 10 microg x kg(-1) x h(-1). A sequence of 11 similar scans was then performed at 8 min intervals. Plasma clonidine concentration was measured. Using statistical parametric mapping, we sought linear correlations between normalized regional cerebral blood flow (rCBF), an indicator of regional brain activity, and plasma clonidine concentration or spindle EEG activity. RESULTS: Clonidine induced clinical sedation and EEG patterns (spindles) comparable to early stage nonrapid eye movement sleep. A significant negative linear correlation between clonidine concentration and rCBF or spindle activity was observed in the thalamus, prefrontal, orbital and parietal association cortex, posterior cingulate cortex, and precuneus. CONCLUSIONS: The EEG patterns and decreases in rCBF of specific brain regions observed during clonidine-induced sedation are similar to those of early stage nonrapid eye movement sleep. Patterns of deactivated brain regions are also comparable to those observed during general anesthesia or vegetative state, reinforcing the hypothesis that alterations in the activity of a common network occur during these modified conscious states.

Ethnic and genetic determinants of cardiovascular response to the selective alpha 2-adrenoceptor agonist dexmedetomidine.

The alpha(2)-adrenoceptor agonist clonidine reduces blood pressure more effectively in White than Black Americans despite similar degrees of sympatholysis. Functional genetic variation in receptor signaling mechanisms, for example in the beta 3 G-protein subunit (GNB3 C825T) and in the alpha(2C)-adrenoceptor subtype (ADRA2C del322-325), may affect drug responses. We examined the hypothesis that there are ethnic differences in the responses to the highly selective alpha(2)-agonist, dexmedetomidine, and that these genetic variants contribute to interindividual variability in drug responses. In a placebo-controlled, single-masked study, 73 healthy subjects (37 whites and 36 blacks) received 3 placebo infusions and then 3 incremental doses of dexmedetomidine (cumulative dose, 0.4 microg/kg), each separated by 30 minutes. Blood pressure, heart rate, and plasma catecholamine concentrations were determined after each infusion. We measured dexmedetomidine concentrations after the last infusion and determined ADRA2C del322-325 and GNB3 C825T genotypes. Dexmedetomidine lowered blood pressure and plasma catecholamine concentrations significantly (all P<0.001). There was substantial interindividual variability in the reduction of systolic blood pressure (range, 1 to 34 mm Hg) and plasma norepinephrine concentrations (range, 24 to 424 pg/mL). However, there were no differences between black and white subjects in dexmedetomidine responses (P>0.16 for all outcomes) before or after adjustment for covariates. Neither ADRA2C del322-325 nor GNB3 C825T genotypes affected the responses to dexmedetomidine (all P>0.66). There is large interindividual variability in response to the selective alpha(2)-AR agonist dexmedetomidine, and neither ethnicity nor ADRA2C and GNB3 genotypes contribute to it. Further studies to identify determinants of alpha(2)-AR-mediated responses will be of interest.

Simultaneous enantioseparation and sensitivity enhancement of basic drugs using large-volume sample stacking.

Simultaneous enantioseparation with sensitive detection of four basic drugs, namely methoxamine, metaproterenol, terbutaline and carvedilol, using a 20-mum ID capillary with native beta-CD as the chiral selector was demonstrated by the large-volume sample stacking method. The procedure included conventional sample loading either hydrodynamically or electrokinetically at longer injection times without polarity switching and EOF manipulation. In comparison to conventional injections, depending on the analyte, about several hundred- and a thousand-fold sensitivity enhancement was achieved with the hydrodynamic and the electrokinetic injections, respectively. The simple method developed was applied to the analysis of racemic analytes in serum samples and better recovery was achieved using hydrodynamic injection than electrokinetic injection.

Development and validation of a liquid chromatography-tandem mass spectrometry method for the determination of phenylephrine in human plasma and its application to a pharmacokinetic study.

A sensitive and reliable method was developed to quantitate phenylephrine in human plasma using liquid chromatography-electrospray tandem mass spectrometry. The assay was based on solid-phase extraction with C18 cartridges and hydrophilic interaction chromatography performed on a pentafluorophenylpropylsilica column (50 mm x 4 mm, 3 microm particles), the mobile phase consisted of methanol-10 mM ammonium acetate (90:10, v/v). Quantification was through positive-ion mode and selected reaction monitoring at m/z 168.1-->135.0 for phenylephrine and m/z 182.1-->135.0 for internal standard etilefrin, respectively. The lower limit of quantitation was 51 pg/ml using 0.25 ml of plasma and linearity was observed from 51 to 5500 pg/ml. Within-day and between-day precision expressed by relative standard deviation was less than 12% and inaccuracy did not exceed 8% at all levels. The assay was applied to the analysis of samples from a pharmacokinetic study.