Cerebrovascular carbon dioxide reactivity in sheep: effect of propofol or isoflurane anaesthesia

Propofol and isoflurane are commonly used in neuroanaesthesia. Some published data suggest that the use of these agents is associated with impaired cerebral blood flow/carbon dioxide (CO2) reactivity. Cerebrovascular CO2 reactivity was therefore measured in three cohorts of adult merino sheep: awake (n=6), anaesthetized with steady-state propofol (15 mg/min; n=6) and anaesthetized with 2% isoflurane (n=6). Changes in cerebral blood flow were measured continuously from changes in velocities of blood in the sagittal sinus via a Doppler probe. Alterations in the partial pressure of carbon dioxide in arterial blood (PaCO2) over the range 18-63 mmHg were achieved by altering either the inspired CO2 concentration or the rate of mechanical ventilation. Cerebral blood flow/CO2 relationships were determined by linear regression analysis, with changes in cerebral blood flow expressed as a percentage of the value for a PaCO2 of 35 mmHg. Propofol decreased cerebral blood flow by 55% relative to pre-anaesthesia values (P=0.0001), while isoflurane did not significantly alter cerebral blood flow (88.45% of baseline, P=0.39). Significant linear relationships between cerebral blood flow and CO2 tension were determined in all individual studies (r2 ranged from 0.72 to 0.99). The slopes of the lines were highly variable between individuals for the awake cohort (mean 4.73, 1.42-7.12, 95% CI). The slopes for the propofol (mean 2.67, 2.06-3.28, 95% CI) and isoflurane (mean 2.82, 219-3.45, 95% CI) cohorts were more predictable. However, there was no significant difference between these anaesthetic agents with respect to the CO2 reactivity of cerebral blood flow.

Evidence of poor vitamin status in coeliac patients on a gluten-free diet for 10 years

BACKGROUND

Patients with coeliac disease are advised to keep to a lifelong gluten-free diet to remain well. Uncertainty still exists as to whether this gives a nutritionally balanced diet.

AIM

To assess the vitamin nutrition status of a series of coeliac patients living on a gluten-free diet for 10 years.

METHODS

Thirty adults with coeliac disease (mean age, 55 years; range, 45-64 years; 60% women), in biopsy-proven remission following 8-12 years of dietary treatment, were studied. We measured the total plasma homocysteine level, a metabolic marker of folate, vitamin B-6 and vitamin B-12 deficiency, and related plasma vitamin levels. The daily vitamin intake level was assessed using a 4-day food record. Normative data were obtained from the general population of the same age.

RESULTS

Coeliac patients showed a higher total plasma homocysteine level than the general population, indicative of a poor vitamin status. In accordance, the plasma levels of folate and pyridoxal 5'-phosphate (active form of vitamin B-6) were low in 37% and 20%, respectively, and accounted for 33% of the variation of the total plasma homocysteine level (P < 0.008). The mean daily intakes of folate and vitamin B-12, but not of vitamin B-6, were significantly lower in coeliac patients than in controls.

CONCLUSIONS

Half of the adult coeliac patients carefully treated with a gluten-free diet for several years showed signs of a poor vitamin status. This may have clinical implications considering the linkage between vitamin deficiency, elevated total plasma homocysteine levels and cardiovascular disease. The results may suggest that, when following up adults with coeliac disease, the vitamin status should be reviewed.

The genome of the natural genetic engineer Agrobacterium tumefaciens C58

The 5.67-megabase genome of the plant pathogen Agrobacterium tumefaciens C58 consists of a circular chromosome, a linear chromosome, and two plasmids. Extensive orthology and nucleotide colinearity between the genomes of A. tumefaciens and the plant symbiont Sinorhizobium meliloti suggest a recent evolutionary divergence. Their similarities include metabolic, transport, and regulatory systems that promote survival in the highly competitive rhizosphere; differences are apparent in their genome structure and virulence gene complement. Availability of the A. tumefaciens sequence will facilitate investigations into the molecular basis of pathogenesis and the evolutionary divergence of pathogenic and symbiotic lifestyles.

Social functioning in first- and multiepisode schizophrenia

OBJECTIVE

To compare the social functioning of individuals experiencing their first episodes of schizophrenia with those who have experienced multiple episodes and with nonpsychiatrically ill control subjects.

METHOD

Subjects included 40 patients with first-episode (FE) schizophrenia, 40 patients with multiepisode (ME) schizophrenia, and a control group of 40 nonpsychiatrically ill individuals. Three social-functioning measures were used: the Social Functioning Scale (SFS), the Quality of Life Scale (QOL), and the Assessment of Interpersonal Problem-Solving Skills (AIPSS).

RESULTS

Control subjects significantly outperformed FE and ME participants on all social-functioning measures. FE and ME samples did not differ in their performance on the SFS and the AIPSS. On the QLS, ME participants outperformed FE participants.

CONCLUSIONS

This study demonstrated that deficits in social functioning are present near the onset of schizophrenia.

Biogel Super-Sensitive and Biogel Indicator glove systems

Surgical gloves have been in use since the late 19th century. Initially, gloves were introduced to protect clinical staff from the carbolic acid used to sterilize equipment and prepare skin before surgery. During the mid-20th century emphasis was more towards protecting the patient from the clinician's skin flora and now, at the beginning of the 21st century, the balance lies between protecting the patient from the clinician and the clinician from the patient. The three main types of glove are latex, vinyl and polythene and they can be used singly or as part of a double gloving system. This article discusses how one hospital evaluated the glove systems Biogel Super-Sensitive and Biogel indicator underglove from Regent Medical (a division of SSL International). The results indicate that both systems are acceptable to clinical staff in a variety of specialty areas.

A comparison of effects measured with isotonic and isometric recording: I. Concentration-effect curves for agonists

Concentration-effect curves were obtained with carbachol tested on isolated preparations of guinea-pig ileum taken from adjacent sites in the same animal, one recorded isotonically, the other isometrically: similar experiments were made with histamine as agonist and with carbachol on rat uterus (in oestrus). The position and steepness of the curves was expressed as the values of [EC(50)] and the exponent, P: with carbachol or histamine on guinea-pig ileum the curves were significantly steeper with isotonic recording (P<0.02, sign test) and displaced towards lower concentrations (P<0.005) but there were significant correlations (P<0.05) between values obtained with tissues from the same animal. The curves for carbachol on the rat uterus were very steep: with isotonic recording the exponent (often eight or more) was consistently higher than with isometric (P<0.001): there was no significant displacement but there was a significant correlation (P<0.05) between values of [EC(50)] obtained with tissues from the same animal. Although the results obtained by the two methods are different, they are correlated. These effects are to be expected because with isotonic recording there can be no change in length until the tension exceeds the load and the tissue bulk sets an upper limit to shortening: the range within which an effect can be measured (the "operational window") is smaller. The observed effects on [EC(50)] and P have been reproduced with theoretical data.

A comparison of effects measured with isotonic and isometric recording: II. Concentration-effect curves for physiological antagonists

If one drug, B, antagonizes another, A, by producing the opposite physiological effect, the antagonist concentration-effect curves should be affected by the recording system, which limits the range of agonist responses. With pieces of isolated guinea-pig ileum taken from adjacent parts of the same animal, one recorded isotonically, the other isometrically with the same load, the isotonic IC(50) values for (-)isoprenaline opposing carbachol or histamine were lower than the isometric values (P<0.01) but there was a significant correlation between them (P<0.01): the isotonic curves were steeper (P<0.01) and there were wider shifts in IC(50) before increasing the agonist reduced the maximum relaxation. In similar experiments with pieces of rat uterus in oestrus from the same animal, the concentration-effect curves for carbachol opposed by increasing concentrations of (-)isoprenaline or (-)adrenaline had slightly lower EC(50) values with isometric recording but there was a significant correlation (P<0.01) with isotonic values. The antagonist effect (ratio of the EC(50) relative to that for the control) was higher with isotonic recording (P<0.01 for (-)isoprenaline, P<0.025 for (-)adrenaline) and all (27) curves were steeper than the corresponding isometric curve (P<0.001). The influence of the method of recording on the results is expected from the narrower operational window and smaller upper limit to relaxation with isotonic recording. A way of obtaining measurements of IC(50) against a standard agonist effect is suggested in an Appendix.

A xylazine infusion regimen to provide analgesia in sheep

The efficacy of continuous low-dose xylazine infusion following an initial loading dose in providing analgesia in sheep was examined using an algesimetry method based on a leg lifting response to an electrical stimulus. Sheep received a 5 mg intramuscular injection of xylazine followed by continuous infusion of intravenous xylazine (2mg/h) for 90 min. This treatment resulted in significant increases in the level of current required to elicit a leg lifting response (287% of baseline) and steady state analgesia was maintained from 10 min after the start of the infusion until the end of the experimental period. This protocol appears to be a simple and effective regimen for providing steady state analgesia in sheep.

In vivo cerebral pharmacokinetics and pharmacodynamics of diazepam and midazolam after short intravenous infusion administration in sheep

The cerebral pharmacokinetics and pharmacodynamics of midazolam and diazepam were examined in chronically instrumented sheep via measurements of their arterio-venous concentration difference across the brain during and after 2-min i.v. infusions. Diazepam (30 mg) or midazolam (10 mg) were administered on 5 separate occasions to 4 sheep. For both drugs, rapid cerebral uptake occurred during the infusion, which quickly turned to elution in the postinfusion period. However, this process was more rapid for midazolam than diazepam. The cerebral pharmacokinetics of both was better described by a kinetic model with slight membrane limitation rather than flow limitation. For diazepam, the estimated brain:plasma partition coefficient was 2.67, and the first and second compartments filled with half-lives of 2.2 and 0.5 min, respectively. For midazolam, these values were 0.27, 0.26 and 1.34 min, respectively. In a subset of sheep, pulmonary arterial-arterial gradients were too small to measure suggesting limited metabolism and small distribution volumes for both drugs in the lungs. Simultaneous dynamic measurements of cerebral blood flow and algesimetry lagged behind both the arterial and sagittal sinus blood concentrations. The changes in cerebral blood flow were best described by a previously published a dynamic model that incorporated long half-lives for drug dissociation from the benzodiazepine receptor (13.3 and 5.5 min for midazolam and diazepam, respectively). Effect compartment modeling of the cerebral blood flow data showed apparent effect compartment half-lives (t1/2,keo) that were longer than the half-lives of cerebral equilibration.

The cerebral and systemic kinetics of thiopentone and propofol in halothane anaesthetized sheep

The cerebral and systemic kinetics of intravenous thiopentone (250 mg over 2 minutes, n=5) and propofol (100 mg over 2 minutes, n=6) were determined in sheep anaesthetized with halothane (2.0%) and mechanically ventilated to an end-expired carbon dioxide tension of 40 mmHg. The sheep were previously instrumented with arterial and sagittal sinus (effluent from the brain) blood sampling catheters. Systemic kinetics were inferred from the time-course of the arterial blood concentrations, and cerebral kinetics from the time-course of the arterio-sagittal sinus concentration difference across the brain. Under halothane anaesthesia, the peak arterial concentrations of each drug occurred at the end of the two-minute infusion, and was 42.3 mg/l and 12.3 mg/l for thiopentone and propofol, respectively. Propofol had a significantly larger systemic clearance (3.19 l/min) than thiopentone (0.99 l/min). The brain concentrations of propofol equilibrated more slowly with the arterial concentrations than those of thiopentone. The extraction ratio across the brain near the end of the infusions (1.5 min) were 0.85 and 0.46 respectively. These data were also compared to analogous previously published data for initially conscious sheep. The systemic kinetics of thiopentone were little affected by halothane anaesthesia. For propofol, halothane anaesthesia was associated with a statistically significant reduction in clearance (50% of awake), a slower initial half-life (247% of awake), and the emergence of a second slower half-life in some sheep. The cerebral kinetics of both drugs were subtly altered by halothane anaesthesia.

The anti-nociceptive efficacy of low dose intramuscular xylazine in lambs

The anti-nociceptive effects of 50 microg kg(-1)of intramuscular xylazine were examined in seven lambs of 4-6 weeks of age using an electrical nociceptive testing method. Lamb anti-nociception increased from an average baseline of 5.88+/-0.72 mA to an average peak value of 13.66+/-1.49 mA at 21 minutes (mean+/-SEM) after the dose, and remained above baseline for the duration of the experimental period (60 minutes). All values were significantly above baseline from 5 minutes post-xylazine administration onwards. These data were also compared with previously published data from adult sheep undergoing the same treatment. There were no differences in the analgesic response between the adult or lamb groups suggesting xylazine dose requirements scale with bodyweight and are unaffected by age over this range. These findings support the use of xylazine as an effective analgesic in sheep with comparable effects and consistent dosing requirements per unit body weight between adult sheep and lambs.

Cardiovascular and haemodynamic effects of intramuscular doses of xylazine in conscious sheep

OBJECTIVE

To determine if a commonly used analgesic dose of xylazine has detrimental cardiovascular or haemodynamic effects in sheep.

DESIGN

A physiological study following intramuscular administration of xylazine.

PROCEDURE

Xylazine (50 micrograms/kg) was injected intramuscularly into six healthy Merino ewes. For 60 min heart rate, mean arterial blood pressure and cardiac output were recorded; arterial blood samples for the measurement of blood gas tensions were also collected.

RESULTS

There were no significant changes in heart rate, mean arterial blood pressure, cardiac output or arterial carbon dioxide tension. A slight degree of arterial hypoxaemia was noted with a 10% reduction in arterial oxygen tension values at 30 min.

CONCLUSION

The minimal changes to cardiovascular and respiratory values in this study verify the safety of previously suggested analgesic dosing regimens for sheep. Previously reported hypoxaemic effects in sheep as a result of intravenous xylazine administration appear to be reduced as a result of intramuscular administration.

Epinephrine, norepinephrine and dopamine infusions decrease propofol concentrations during continuous propofol infusion in an ovine model

OBJECTIVE

To determine the effects of exogenous ramped infusions of epinephrine, norepinephrine and dopamine on arterial and effluent brain blood concentrations of propofol under steady state intravenous anesthesia.

DESIGN

Prospective, randomized animal study.

SETTING

University research laboratory.

SUBJECTS

Five adult female merino sheep.

INTERVENTIONS

Induction (5 mg/kg) and continuous infusion of propofol (15 mg/min) with controlled mechanical ventilation to maintain PaCO2 40 mmHg. After 1 h of continuous anesthesia, each animal randomly received ramped infusions of epinephrine, norepinephrine (10, 20, 40 microg/min) and dopamine (10, 20, 40 microg x kg x min) in 3 x 5 min intervals followed by a 30-min washout period.

MEASUREMENTS

Arterial and sagittal sinus whole blood for determination of propofol concentrations using high-pressure liquid chromatography. Cardiac output using a thermodilution method. Level of consciousness using an observational scale.

MAIN RESULTS

All three drugs significantly and transiently increased cardiac output in a dose-dependent fashion to a maximum of 146-169% of baseline. Baseline arterial and sagittal sinus propofol concentrations were not statistically different prior to catecholamine infusions. All three drugs significantly reduced mean arterial propofol concentrations (95 % CI, p < 0.05): epinephrine to 41.8% of baseline (11.4-72), norepinephrine to 63 % (27-99) and dopamine to 52.9 % (18.5-87.3). There were parallel reductions of concentrations in sagittal sinus blood leaving the brain. The lowest blood concentrations were associated with emergence from anesthesia. Arterial concentrations were inversely related to the simultaneously determined cardiac output (r2 = 0.74, p < 0.0001). Comparison of the data with the predictions of a previously developed recirculatory model of propofol disposition in sheep showed the data were consistent with a mechanism based on increased first pass dilution and clearance of propofol secondary to the increased cardiac output.

CONCLUSIONS

Catecholamines produced circulatory changes that reversed propofol anesthesia. These observations have potential clinical implications for the use of propofol in hyperdynamic circulatory conditions, either induced by exogenous catecholamine infusions or pathological states.

The effect of altered cerebral blood flow on the cerebral kinetics of thiopental and propofol in sheep

BACKGROUND

Thiopental and propofol are highly lipid-soluble, and their entry into the brain often is assumed to be limited by cerebral blood flow rather than by a diffusion barrier. However, there is little direct experimental evidence for this assumption.

METHODS

The cerebral kinetics of thiopental and propofol were examined over a range of cerebral blood flows using five and six chronically instrumented sheep, respectively. Using anesthesia (2.0% halothane), three steady state levels of cerebral blood flow (low, medium, and high) were achieved in random order by altering arterial carbon dioxide tension. For each flow state, 250 mg thiopental or 100 mg propofol was infused intravenously over 2 min. To quantify cerebral kinetics, arterial and sagittal sinus blood was sampled rapidly for 20 min from the start of the infusion, and 1.5 h was allowed between consecutive infusions. Various models of cerebral kinetics were examined for their ability to account for the data.

RESULTS

The mean baseline cerebral blood flows for the "high" flow state were over threefold greater than those for the low. For the high-flow state the normalized arteriovenous concentration difference across the brain was smaller than for the low-flow state, for both drugs. The data were better described by a model with partial membrane limitation than those with only flow limitation or dispersion.

CONCLUSIONS

The cerebral kinetics of thiopental and propofol after bolus injection were dependent on cerebral blood flow, despite partial diffusion limitation. Higher flows produce higher peak cerebral concentrations.

Management of menorrhagia in primary care-impact on referral and hysterectomy: data from the Somerset Morbidity Project

OBJECTIVE

To investigate the management of menorrhagia in primary care and its impact on referral and hysterectomy rates.

DESIGN

Prospective observational study.

SETTING

11 general practices from the Somerset Morbidity Project.

SUBJECTS

885 women consulting their general practitioner with menorrhagia over four years.

MAIN OUTCOME MEASURES

Proportions of these women investigated and treated with drugs in primary care, referred to a gynaecologist and undergoing operative procedures. The relation between investigation and prescribing in primary care and referral to and surgery in secondary care.

RESULTS

Less than half of women had a vaginal examination (42%, 95% CI 39% to 45%), or a full blood count (39%, 95% CI 36% to 43%). Almost a quarter of women, 23% (95% CI 20% to 26%), received no drugs and 37% (95% CI 34% to 40%) received norethisterone. Over a third, 38% (95% CI 34% to 40%), of women were referred, and once referred 43% (95% CI 38% to 48%) of women were operated on. Women referred to a gynaecologist were significantly more likely to have received tranexamic acid and/or mefenamic acid in primary care (chi(2)=16.4, df=1, p<0.001). There were substantial between practice variations in management, for example in prescribing of tranexamic acid and/or mefenamic acid (range 16% to 72%) and referral to gynaecology (range 24% to 52%). There was a significant association between high referral and high operative rates (Spearman's correlation coefficient=0.86, p=0.001).

CONCLUSIONS

Substantial differences in management exist between practices when investigating and prescribing for menorrhagia in primary care. Rates of prescribing of effective medical treatment remain low. The decision to refer a woman impacts markedly on her chances of subsequently being operated on. Effective management in primary care may not reduce referral or hysterectomy rates.

Prevalence of iron deficiency in Auckland high school students

AIMS

To determine the frequency of iron deficiency and anaemia in high school students.

METHOD

The survey was carried out at eight Auckland high schools with a high proportion (> or =15%) of Pacific Islands students. All students in Forms 5-7 at these schools were invited to participate, and 1644 students (Pacific Island 765, Asian 350, European 295, Maori 234) had iron assessments (response rate 61%). Iron deficiency was defined as any two (or more) of the following, three: serum ferritin <12 microg/L, iron saturation <14%, or red cell distribution width >14.5%. Anaemia was defined as haemoglobin <120 g/L for females and <130 g/L for males.

RESULTS

Iron deficiency and anaemia were each ten times more common in girls (18.3% and 11.5%, respectively) than boys (1.5% and 1.4%). In females, iron deficiency was two to three times more common in Maori (25.6%), Pacific Islanders (20.9%) and Asians (15.4%) compared with Europeans (8.3 %), while anaemia was three to four times more common in Asians (15.9%), Pacific Islanders (12.1%) and Maori (11.2%) compared with Europeans (4.2%). Iron deficiency and anaemia prevalences were inversely associated with aerobic fitness, but not with age or years since menarche.

CONCLUSION

Prevalences of iron deficiency and anaemia are high in non-European female adolescents in Auckland, for reasons currently unknown.

Aberrant macrophage cytokine production is a conserved feature among autoimmune-prone mouse strains: elevated interleukin (IL)-12 and an imbalance in tumor necrosis factor-alpha and IL-10 define a unique cytokine profile in macrophages from young nonobese diabetic mice

Cytokines derived from macrophages (Mø) play a critical role in the development of type 1 diabetes in the nonobese diabetic (NOD) mouse. Based on earlier findings from lupus-prone strains of inherent cytokine defects in Mø , NOD Mø were evaluated for intrinsically dysregulated cytokine production with the potential to initiate or exacerbate disease. Endotoxin-activated peritoneal Mø from young prediseased NOD mice produced interleukin (IL)-1 and tumor necrosis factor (TNF)-alpha levels similar to those of Mø from a panel of control strains but reduced compared with the congenic diabetes-resistant NOR strain. IL-6 and IL-10 production were similar in NOD and NOR Mø, indicating that reduction in NOD IL-1 and TNF-alpha expression was selective. Nevertheless, the ratio of TNF-alpha and IL-10 production, a stringent index of normal Mø function, distinguished NOD from all normal strains. The most striking feature of NOD Mø, however, was their substantially elevated IL-12 production. This response was induced not only by endotoxin but also by bacillus Calmette-Guerin (BCG) and CD40 ligand and was associated with (and likely caused by) the enhanced and prolonged expression of p40 mRNA. Moreover, NOD Mø IL-12 expression appeared to be near maximally induced by lipopolysaccharide (LPS) alone, because it was only slightly enhanced by the addition of gamma-interferon, a stimulus that substantially elevated LPS-induced IL-12 production in Mø from normal strains. Accompanied by a unique profile of TNF-alpha and IL-10, the dramatic elevation of IL-12 expression by NOD Mø reflects intrinsic defects of the innate immune system with the potential to initiate and propagate the pathogenic autoreactive T-helper type 1 response characteristic of type 1 diabetes.

A randomised controlled trial and economic evaluation of a referrals facilitator between primary care and the voluntary sector

OBJECTIVES

To compare outcome and resource utilisation among patients referred to the Amalthea Project, a liaison organisation that facilitates contact between voluntary organisations and patients in primary care, with patients receiving routine general practitioner care.

DESIGN

Randomised controlled trial with follow up at one and four months.

SETTING

26 general practices in Avon.

PARTICIPANTS

161 patients identified by their general practitioner as having psychosocial problems.

MAIN OUTCOME MEASURES

Primary outcomes were psychological wellbeing (assessed with the hospital anxiety and depression scale) and social support (assessed using the Duke-UNC functional social support questionnaire). Secondary outcomes were quality of life measures (the Dartmouth COOP/WONCA functional health assessment charts and the delighted-terrible faces scale), cost of contacts with the primary healthcare team and Amalthea Project, cost of prescribing in primary care, and cost of referrals to other agencies, over four months.

RESULTS

The Amalthea group showed significantly greater improvements in anxiety (average difference between groups after adjustment for baseline -1.9, 95% confidence interval -3.0 to -0.7), other emotional feelings (average adjusted difference -0.5, -0.8 to -0.2), ability to carry out everyday activities (-0.5, -0.8 to -0.2), feelings about general health (-0.4, -0.7 to -0.1), and quality of life (-0.5, -0.9 to -0.1). No difference was detected in depression or perceived social support. The mean cost was significantly greater in the Amalthea arm than the general practitioner care arm ( pound153 v pound133, P=0. 025).

CONCLUSION

Referral to the Amalthea Project and subsequent contact with the voluntary sector results in clinically important benefits compared with usual general practitioner care in managing psychosocial problems, but at a higher cost.

Development and validation of a recirculatory physiological model of the myocardial concentrations of lignocaine after intravenous administration in sheep

A recirculatory physiological model of the determinants of the myocardial concentrations of lignocaine after intravenous administration was developed in sheep and validated with the intention of analysing and predicting the outcome of altered dose regimens and various pathophysiological states on the initial myocardial concentrations of lignocaine. The structure and parameters of the model were determined by hybrid modelling of the time-courses of the pulmonary artery, arterial and coronary sinus concentrations of lignocaine after the intravenous administration of 100 mg of lignocaine over 5 min to 5 chronically instrumented sheep. The model accounted for the determinants of the myocardial concentrations via compartments for venous mixing, the lung (a single-compartment model with a first-order loss) and the heart (a single flow-limited compartment). Recirculation and the remainder of the body were represented as a single tissue pool with a clearance term. The distribution volume of the heart was 0.42+/-0.009 L, which gave a half-time of myocardium:blood equilibration of 2.37 min. The distribution volume of the lungs was 5.40+/-0.23 L, with an apparent first-order loss of 1.02 L min(-1) representing deep distribution or metabolism. The validity of the model was tested by comparing the predictions of the model with the equivalent data collected in 6 sheep when lignocaine (89 mg) was administered via a complex dose regimen with a faster initial rate of infusion (39.1 mg min(-1)), declining exponentially to basal infusion rate (7.02 mg min(-1)) over 8 min. The predictions of the model were in general agreement with these data. It is concluded that the model was sufficient to account for the effect of altered dose regimens of lignocaine on the time-course of its myocardial concentrations.