Halothane-induced variability in the neuromuscular transmission of patients with myasthenia gravis

The purpose of the present clinical study was to explore the skeletal muscle mechano (MMG)- and electromyographic (EMG) responses during halothane/oxygen/air anaesthesia in patients with myasthenia gravis (MG) compared with patients with normal neuromuscular transmission. The majority of MG-patients had a significant decremental response of the evoked muscle action potentials to a train-of-four (TOF) stimulation during halothane exposure (mean decrease of train-of-four ratio was 33% during the highest mean halothane concentration of 1.9 MAC). An excellent correlation was found between MMG- and EMG-measurements (r2 = 0.878, P less than 0.001). However, marked individual variations in the neuromuscular response to halothane were seen. Neither preoperative muscle fatigability nor acetylcholine receptor antibodies predicted the decremental muscle responses produced by halothane among MG-patients. The increased presence of HLA-B8 among myasthenics with halothane-suppressed muscle responses after TOF stimulation could be demonstrated (P less than 0.01).     

Strabismus as a possible sign of subclinical muscular dystrophy predisposing to rhabdomyolysis and myoglobinuria: a study of an affected family

Administration of succinylcholine to normal individuals results in alterations in muscle membrane integrity expressed as a slight increase in the concentrations of creatine Phosphokinase (CK) in serum and appearance of small amounts of myoglobin in the urine, but without clinical symptoms. Subjects with strabismus due to congenital muscular dystrophy may develop more significant rhabdomyolysis expressed as muscle stiffness and weakness, massive myoglobinuria, marked elevation of serum CK and other enzymes, metabolic acidosis, tachycardia and moderate elevation of body temperature. In some cases grave malignant hyperthermia with significant hypoxia, metabolic acidosis, tachycardia and marked abnormalities in serum electrolyte concentrations may cause irreversible damage to the central nervous system and other vital organs and death. A case of difficult anaesthesia for a six year old boy belonging to family affected with muscular dystrophy is presented. More attention must be given to preoperative examination (anamnesis, serum enzymes) of ophthalmological patients and more careful monitoring during anaesthesia and in the early postoperative period must be instituted to prevent and treat complications induced by succinylcholine and volatile anaesthetic agents.     

Problems of anesthesia for cesarean section in myasthenia gravis

The case of a 27 year old female, being pregnant the second time and suffering from myasthenia gravis since 1974 is reported. The patient was admitted to the hospital for repeat caesarian section. Anaesthesia of the preoxygenated patient during caesarian section was induced with 0.5% halothane and ketamine (2 mg/kg b.w.) intravenously. Muscle relaxing drugs (competitive and depolarizing inhibitors) as well as morphine derivates (e.g. fentanyl) and barbiturates are contraindicated. Therefore anaesthesia was maintained with 0.2:N2O = 1:1 combined with 0.5% halothane up to the point of ligation of the umbilical cord, and up to 1.5% halothane thereafter. Intubation was performed without muscle relaxation. The postoperative somnolence frequently seen after the combined use of ketamine and halothane was antagonized with physostigmine salicylate. The newborn showed all signs of neonatal myasthenia.     

Displacement of warfarin from human serum proteins by halothane anaesthesia

The in vitro effects of the halothane metabolite, trifluoroacetic acid, on the binding of warfarin to human serum proteins have been investigated. An increase in the percentage of free warfarin following incubation with different concentrations of trifluoroacetic acid (1 and 4 mmol/l) was observed. In addition, protein binding of warfarin was studied in serum from patients undergoing halothane anaesthesia (1-2.5%; 2.5 h). After 24 h from the end of the halothane anaesthesia, an increase in the percentage of free warfarin was also detected (0.92 +/- 0.06% at 24 h vs 0.60 +/- 0.02% before halothane administration; P less than 0.005). We conclude that halothane anaesthesia may temporarily potentiate the pharmacological effect of warfarin in the postoperative period following anaesthetic procedures.     

Serum creatine kinase levels after succinylcholine in children with “muscle,eye and brain disease“

Four boys belonging to a group of children affected by a rare form of muscular dystrophy with eye and brain involvement, termed the "muscle, eye and brain disease" (MEB), were anaesthetized for various eye examinations and surgery. On some occasions succinylcholine was used during anaesthesia and the initially elevated serum creatine kinase (CK) values increased from a range of 122 to 1200 units.L-1 to a range of 4350 to 9690 units.L-1 22 hours after anaesthesia. CK values after anaesthesia without succinylcholine remained at the initially elevated levels. Rectal temperatures of the children were normal. These findings suggest that succinylcholine should be avoided in patients with MEB disease.     

Changes in serum potassium, calcium and creatine kinase following suxamethonium administration in children with and without strabismus

The immediate changes in serum potassium and calcium and 24-h changes in creatine kinase (CK) following suxamethonium administration were compared in children undergoing strabismus repair or tonsillectomy following induction of anaesthesia with thiopentone or halothane. A separate group of children were anaesthetized with isoflurane and did not receive suxamethonium. There was a significant increase ( P < 0.05) in serum potassium of 0.26 and 0.56 mmolċ⁻¹ following halothane-suxamethonium induction and a significant decrease ( P < 0.05) of 0.35 and 0.13 mmolċ⁻¹ after thiopentone-suxamethonium induction in the strabismus and tonsillectomy groups respectively. There was an increase in the 24 h CK values of 624 and 694 uċl⁻¹ ( P < 0.05) in patients receiving halothane-suxamethonium induction and of 43 (NS) and 247 uċ⁻¹ ( P < 0.05) in patients receiving thiopentone-suxamethonium induction in the strabismus and tonsillectomy groups respectively. Suxamethonium administration was associated with a small but sometimes significant ( P < 0.05) decrease in total serum calcium concentrations (0.036 to 0.049 mmolċ⁻¹). Changes in all indices were minimal in children anaesthetized with isoflurane. It is concluded that the administration of halothane and suxamethonium is the main cause for the changes in serum potassium and CK and not the presence of strabismus.     

Increased hepatic microsomal activity after halothane anaesthesia in children

The effect of anaesthesia and surgery on microsomal enzyme activity was studied in 19 children aged 4-9 years, scheduled for tonsillectomy. The children were randomly allocated to either halothane or ketamine anaesthesia. Antipyrine clearance was measured before and 4 days after surgery by a salivary one-sample technique. Statistically significant (p less than 0.001) increases in antipyrine clearance was found in children who received halothane anaesthesia. The antipyrine clearance was increased by a mean of 26% 4 days after surgery, compared with a pre-operative control measurement. No significant change in antipyrine clearance was observed in children who received ketamine anaesthesia. There was also a significant difference in antipyrine clearance changes after surgery between the two groups (p less than 0.05). Halothane has enzyme-inducing properties after a single exposure in children, while a single dose of ketamine does not.     

Myocardial damage in coronary artery bypass surgical patients anaesthetized with two anaesthetic techniques: a random comparison of halothane and enflurane

Fifty patients with ischaemic heart disease scheduled for coronary artery bypass surgery received either an enflurane or a halothane anaesthetic. The anaesthetic techniques were randomly assigned to the patients and consisted of induction with diazepam 0.5 mg·kg^-1 and pancuronium 0.1 mg·kg^-1 supplemented by nitrous oxide and oxygen (50:50). Enflurane in dosages of 0.5–1.5 volumes per cent and halothane 0.3–0.7 volumes per cent were administered to Group E (25 patients) and Group H (25 patients), respectively. The inhalation drug dosage was varied to maintain heart rate and systemic blood pressure within predetermined limits. The two patient groups (E and H) were compared with regard to myocardial damage (determined electrocardiographically and enzymatically) as well as by haemodynamic changes and adjuvant cardiovascular drug therapy. One patient in group H sustained a postoperative infarction detected by electrocardiogram and sustained CK MB release. There was no other unequivocal electrocardiographic evidence of myocardial infarction in either group and the myocardial damage estimated from CK MB curves was remarkably low and similar in both anaesthetic groups. Myocardial damage was estimated by CK MB maximum release (CK MB MAX) of 8.1 ± 1.00IU/1 (Group E), 7.8 ± 1.32IU/I (Group H), by area under the CK MB disappearance curve (CK MB AREA) of 144 ± 21.9 IU/1 x hr (Group E), 173 ± 32.9 IU/1 x hr (Group H), and by the accumulated CK MB or CK MB damage size (CK MB DS) of 10.S±1.79IU/1 (Group E), 10.3 ± 2.26 IU/1 (Group H). There was no release of CK MB before cardiopulmonary bypass in either group. There was no statistically significant difference between the two groups for myocardial damage, haemodynamics or adjuvant drug interventions. There was a trend toward greater use of vasodilators in Group H than in Group E. It is concluded that enflurane and halothane are associated with equally low levels of myocardial damage when used for anaesthesia in patients with ischaemic heart disease. The release of CK MB occurred following cardiopulmonary bypass and probably represents imperfect myocardial preservation. Patients with severely impaired ventricular function were not studied, and in these patients enflurane and halothane must be used judiciously.     

Sevofluran in der Kinderanästhesie Maligne Hyperthermie

Inhalational anaesthesia is the most common anaesthesia technique in paediatric anaesthesia worldwide. Up to now the standard anaesthetic used is halothane. Because halothane is tolerated in the upper airways without side effects it is well suited for the inhalational induction of anaesthesia. However, halothane exerts side effects on the hepatic and the cardiovascular system. This review focuses on the replacement of halothane by sevoflurane in paediatric anaesthesia. Apart from its favorable pharmacological properties sevoflurane is also superior because of economical considerations. The following conclusions are drawn: (1) Halothane and sevoflurane do not cause irritations of the airways and are thus suitable for an inhalational induction. Sevoflurane should be administered in oxygen/nitrous oxide during induction of anaesthesia to reduce excitation. (2) The MAC values of sevoflurane are age dependent. In contrast to adult patients the MAC values of sevoflurane are only decreased by 20 to 25% in paediatric patients. The end-tidal concentration of sevoflurane necessary for intubation or insertion of a laryngeal mask is 2 to 4 Vol.%. (3) The blood/gas partition coefficient of sevoflurane is low, resulting in shorter induction times with sevoflurane compared to halothane. The so called priming technique with 8 Vol.% of sevoflurane results in shorter induction times. Consequently, times to recovery and psycho-motor functions are favourable for sevoflurane compared to halothane in paediatric patients. However, shorter recovery times lead to earlier perception of postoperative pain, requiring adequate pain management. (4) The hemodynamic stability after administration of sevoflurane is favourable to that after halothane in paediatric patients, leading to significantly less bradycardia. (5) In paediatric patients no negative effects on kidney function have been observed after administration of sevoflurane. There is no scientific basis for organotoxic effects, thus sevoflurane is suitable for low-flow and minimal-flow anaesthesia. (6) The duration of the action of muscle relaxants is increased to a greater extent in presence of sevoflurane compared to halothane. Consequently, the total dose of muscle relaxants can be reduced using sevoflurane. (7) Similar to the established inhalational anaesthetics sevoflurane triggers malignant hyperthermia (MH) and must not be used in patients in which MH is suspected or in which a predisposition for MH is known.     

AWARENESS FOLLOWING DIFFERENT TECHNIQUES OF GENERAL ANAESTHESIA FOR CAESAREAN SECTION

Using the isolated arm technique, the frequency of awarenesswas evaluated in 50 full-term patients undergoing elective Caesareansection under general anaesthesia. In 20 patients, anaesthesiawas induced with thiopentone 4 mg kg^–1, and in the other30 patients, induction was with ketamine 1.5 mg kg^–1.Following suxamethonium 1.5 mg kg^–1 and tracheal intubation,anaesthesia was supplemented during the induction—deliveryperiod in the thiopentone group with 50% nitrous oxide and 0.5%halothane in oxygen in 10 patients, and with 1% halothane in100% oxygen in the other 10 patients. In the ketamine group,the patients underwent ventilation with 50% nitrous oxide and0.5% halothane in oxygen in 10 patients, with 1% halothane in100% in 10 patients, and with 100% oxygen only in the last 10patients. Awareness was significantly greater after inductionwith thiopentone (14/20) than after ketamine (4/30). There wereno significant differences in Apgar scores or umbilical veinblood-gas values in the newborns.     

Level of consciousness after cadaveric kidney transplant under halothane anaesthesia

Serum and urine bromide levels, serum morphine level, and postoperative awakening times were observed in 24 patients; 16 undergoing cadaveric kidney transplantation under halothane anaesthesia, eight without preanaes-thelic medication (Croup A) and eight with morphine preanaesthetic medication (Group B), and eight patients (control) with normal kidney/unction scheduled for colon resection (Group C). Bromide levels were higher in patients after kidney transplant than in the control group. Transplant patients who did not receive morphine awakened faster after anaesthesia than patients who received 10 mg of morphine 60 minutes before anaesthesia. Morphine levels at the end of anaesthesia were 145 p,g-100 ml^-1 in Group B; significantly higher than those found in patients with normal kidney function (Group C) (5 ± 1.05). The majority of the patients undergoing kidney transplantation and receiving morphine for pre-medication were fully awake 24 hours following surgery despite serum bromide levels of 158μg.m^-1. Control patients had higher awakening scores after surgery than patients in Group A, despite receiving 10 mg of morphine as preoperative medication. Serum bromide levels were far below 480μg.ml^-1, the level required to produce clinical symptoms of bromism. The serum and urine bromide levels found in the patients after kidney transplant, even when higher than the control group, were not high enough to explain the prolonged sleepiness found in these patients. The morphine given as preoperative medication may be one of the factors responsible for this finding. Other factors, such as decreased protein binding, decreased albumin levels, abnormal blood brain barrier and brain function, and the degree of function of the transplanted kidney may account for the prolonged sleepiness found in patients receiving transplants.     

Drive and timing components of respiration in young children following induction of anaesthesia with halo-thane or ketamine

Timing and drive components of respiration were studied in 18 young children following induction of anaesthesia with ketamine and were compared with results from ten children following induction of anaesthesia with halothane. During one minute of quiet breathing, signals from a pneumotachograph attached to the anaesthetic mask were analysed for tidal volume (Vt), respiratory frequency (f), minute volume (Ve), inspiratory and expiratory times (Ti, Te) and flow pattern. Following induction of anaesthesia with ketamine, children breathed more slowly and deeply than children receiving halothane, but there was no significant difference in Ve or in Vt/Ti, suggesting that respiratory drive was similar in the two groups of children. In the children receiving ketamine, Ti was more than twice as long, and thus the ratio Ti/Te was significantly increased, in comparison with the group receiving halothane. In addition to the prolonged Ti in the children induced with ketamine, there was a more rapid increase in volume in early inspiration than in late inspiration, which is an apneustic breathing pattern. There was a slower decrease in volume in early expiration, with occasional early expiratory breath holding lasting up to three seconds, in the ketamine-induced children. The unique breathing pattern demonstrated with ketamine, consisting of large Vt, increased Ti/Te ratio, apneustic inspiratory pattern, and expiratory braking, contributed to an increased mean lung volume above functional residual capacity, of 2.40 ml.kg-1 body weight, in comparison to 1.27 ml.kg-1 in the children receiving halothane.     

Serum Bromide After General Anaesthesia with Halothane

Serum bromide was determined in 30 patients during the first few postoperative days after general anaesthesia with halothane. The material comprised a group of younger patients (19-50 years) and a group of elderly patients (greater than 70 years), neither of whom received thiomebumal induction, and a group of younger patients (19-50 years) who received thiomebumal induction. The changes in serum bromide were independent of age and induction with barbiturate, and a maximum rise in serum bromide was found most often 2-3 days after the anaesthesia. All groups showed a rise in serum bromide to therapeutically sedative concentrations, and a significant correlation was found between MAC-hours halothane exposure and serum bromide.     

Continuous measurement of pulmonary gas exchange during general anaesthesia in man

We report a system for the continuous measurement of oxygen uptake (VO2) and carbon dioxide output (VCO2) during open-circuit anaesthesia. Gas concentrations were measured by a mass-spectrometer, and expired flow by a pneumotachograph. The values measured by the system were compared in vitro to values produced by a nitrogen-dilution technique. Excellent correlations were found. Continuous measurements were performed in 21 patients anaesthetized for abdominal surgery. Compared to pure intravenous anaesthesia (flunitrazepam-fentanyl), anaesthesia including the administration of nitrous oxide or nitrous oxide and halothane led to more pronounced and sustained decreases in VO2 and core temperature, with a better cardiovascular stability. Two hours postoperatively, VO2 was not different from preoperative values. After a transient increase at the onset of anaesthesia, the respiratory exchange ratio (VCO2/VO2) returned to preoperative values, and then remained subsequently unchanged. Based on these observations, the system described provides an accurate approach to noninvasively monitoring the pulmonary gas exchange in the operating room.     

General Anaesthesia with Halothane and Drug Absorption: The Effect of General Anaesthesia with Halothane and Diazepam on Postoperative Gastric Emptying in Man

The effect of general anaesthesia with halothane on postoperative absorption of paracetamol was measured in seven patients, after minor surgery on the extremities. Gastric emptying, estimated indirectly by the rise in serum paracetamol following oral administration of the drug, was significantly delayed after general anaesthesia with halothane compared with gastric emptying when general anaesthesia with halothane was not administered. It was furthermore demonstrated that absorption of paracetamol and therefore gastric emptying were normal or nearly normal when diazepam 15 mg was given orally to the same patients 2 weeks later.     

A randomized prospective controlled study of the metabolism and hepatotoxicity of halothane in humans

In a randomized prospective controlled study in humans, the metabolism and hepatic effects of a single administration of halothane were compared with enflurane and meperidine. Pre- and postoperative antipyrine pharmacokinetics, intraoperative indocyanine green clearance, liver histology, and postoperative liver function tests were determined in 24 patients undergoing abdominal surgery who were randomly allocated to receive either halothane (0.5%, group I), enflurane (0.8%, group II), or meperidine (group III) as a supplement to a common basal anesthetic regimen consisting of thiopental, nitrous oxide/oxygen/muscle relaxant. In addition, end-tidal concentrations of the volatile reductive metabolites of halothane, chlorodifluoroethylene (CDF), and chlorotrifluoroethane (CTF) were determined in group I patients and serum and urinary inorganic fluoride were determined in both group I and II patients. Indocyanine green clearance was measured before anesthesia (stage I), during basal anesthesia (stage II), in the presence of surgical stimuli (stage III), and after introduction of the selected anesthetic agent (stage IV). CDF and CTF were detectable within 20 min of the start of halothane anesthesia in every patient receiving halothane. Peak serum fluoride concentrations occurred at 2 and 24 hr in the enflurane and halothane groups, respectively, whereas urinary fluoride excretion was elevated postanesthesia in the enflurane group only. There was no difference between the pre- and postoperative disposition of antipyrine in group II or III, but after anesthesia, antipyrine clearance was significantly decreased (P less than 0.02) and plasma half-life increased (P less than 0.05) in group I patients (halothane). Concentrations of serum alanine aminotransferase (ALT) and bilirubin were significantly elevated (P less than 0.5) postoperatively in groups I and II but unchanged from preoperative values in group III patients. Three of the 24 liver biopsies taken at the end of stage IV showed several foci of acute liver cell necrosis; of these, two patients were from group I and one from group II. There were no significant differences in liver cell morphology (P greater than 0.5) in biopsies taken at the end of stage IV compared with biopsies at the end of stage III, from groups I and II. The results of this study show that reductive metabolism of halothane occurs routinely in patients undergoing halothane anesthesia under conditions of normoxia. This may be the cause of the changes in antipyrine clearance after halothane anesthesia.     

Increase in serum creatine phosphokinase concentrations after suxamethonium during sevoflurane or isoflurane anaesthesia in children

We have studied whether sevoflurane or isoflurane anaesthesia modulates the effect of suxamethonium on serum concentrations of enzyme markers of skeletal muscle function in paediatric patients. Eighty patients undergoing bilateral tonsillectomy, aged 5-12 yr, were allocated randomly to receive anaesthesia with either sevoflurane and nitrous oxide or isoflurane and nitrous oxide. Serum creatine phosphokinase (CK), aspartate aminotransferase (AST), alanine aminotransferase (ALT) and lactate dehydrogenase (LDH) concentrations were measured before, and at 30 min and 20 h after induction of anaesthesia. Mean CK concentrations increased from 97.0 (SD 17.3) to 478 (170) iu litre-1 in the sevoflurane group and from 86.9 (22.4) to 628 (223) iu litre-1 in the isoflurane group, 20 h after induction of anaesthesia. Mean peak serum CK concentration in the sevoflurane group (478 (170) iu litre-1) was significantly less (P < 0.05) than that in the isoflurane group (628 (223) iu litre-1). Mean serum AST concentration increased from 17.5 (4.9) to 31.7 (3.5) iu litre-1 in the sevoflurane group and from 17.3 (2.4) to 34.8 (5.7) iu litre-1 in the isoflurane group, 20 h after induction of anaesthesia. Mean peak serum AST concentrations in the sevoflurane group were significantly lower (P < 0.05) than those in the isoflurane group. There were no significant differences in serum ALT or LDH concentrations between the groups either before or after anaesthesia. We conclude that administration of suxamethonium during either sevoflurane or isoflurane anaesthesia caused a marked increase in serum CK concentrations in paediatric patients. The clinical significance of this finding is uncertain.      

Hepatocellular integrity during and after isoflurane and halothane anaesthesia in surgical patients

Subclinical disturbance in hepatocellular integrity, indicated by glutathione transferase Alpha (GSTA), has been associated with halothane, sevoflurane and propofol, but not with isoflurane anaesthesia. We anaesthetized 82 patients with isoflurane or halothane at 1 MAC for superficial surgery. GSTA concentration were measured with a sensitive time-resolved immunofluorometric assay in serum samples. GSTA concentrations increased from a baseline value of geometric mean 1.8 micrograms litre-1 (95% confidence intervals 1.4-2.2 micrograms litre-1) to a peak of 4.3 (3.3-5.7) micrograms litre-1 in the isoflurane group and from 2.1 (1.6-2.9) micrograms litre-1 to 6.2 (4.1- 9.5) micrograms litre-1 in the halothane group. The change in GSTA was significant within groups but the difference between groups was not significant. Two patients exhibited an unexpectedly large increase in GSTA (peaks 370 and 620 micrograms litre-1) and a mild increase in alanine aminotransferase after halothane anaesthesia. We conclude that hepatocellular integrity was mildly disturbed after isoflurane and halothane anaesthesia but there was no difference between anaesthetics. Halothane anaesthesia may be associated with more advanced hepatocellular disturbance in some cases.      

Effects of Mannitol and Furosemide on Urinary Fluoride Excretion of Surgical Patients Anaesthetized with Enflurane or Halothane

Urinary excretion of fluoride after enflurane anaesthesia has been found to correlate Mith urinary pH, while the correlation with urinary volume has remained unsettled. We therefore studied the ellert of moderate dobes of mannitol and furosemide on serum fluoride levels and urinary excretion of fluoride in aurgical patients alter controlled doses of enflurane (6+6+6 patients) or halothane (5+5+5 patients). The highest serum fluoride level was 31 μmol/l in an enflurane patient (enflurane dose 1.26 end-tidal vol.%×h) and 8 μmol/l in a halothane patient (halothane dose 0.59 end-tidal vol.%×h). Mannitol caused the greatest mean exmetion of fluoride not significant (n.s.)) in the enflurane patients without any marked rise in urinary pH or volume. Furosemide increased urinary output markedly but did not enhance urinary fluoride excretion or raise urinary pH. Compared with the control groups of both inhalation anaesthetic patients, the diuretics appeared to have no effect on the serum fluoride levels. In the enflurane patients there was a positive correlation between the change in fluoride clearance and the change in urinary pH, but not with the change in urinary volume during the first postoperative hours. On the other hand, in the halothane patients there was a positive correlation between the change in fluoride clearance and the change in urinary volume. A possible "fluoro-uretic" action of mannitol was also seen in the halothane patients, as in the later postoperative period fluoride excretion was greatest when mannitol had been given.     

Effects of halothane on the electrical activity of respiratory muscles in rats

This study was designed to investigate the effects of halothane on the electrical activity of the respiratory muscles in rats. Indeed, halothane is known to reduce end-expiratory lung volume, both in man and in animal; this may be due to altered activity in the respiratory muscles. The electrical activity of the diaphragm, parasternal, intercostal and abdominal muscles were recorded using intramuscular electrodes in ten rats weighing between 400 and 450 g each. The rats were prepared under light halothane anaesthesia (tracheostomy, laparotomy, electrode positioning, plaster of Paris cast to impede leg movements). They were placed prone in a 20 1 plexiglass chamber, and allowed to awake. Thereafter halothane was vaporized in this chamber at a known concentration, until the animals no longer reacted to tail pinching. The measurements were carried out during the awake state, and under 2 vol % halothane. Muscle tone was assessed by the thickness of baseline inspiratory muscle activity at the end of expiration (maximally amplified raw electromyographic signals). The measurement of phasic electrical activity was carried out using peak inspiratory integrated electromyographic signals. Under halothane, phasic activity of the diaphragm was slightly reduced (p less than 0.05), whereas tonic activity remained unchanged. Parasternal intercostal muscle activity, both tonic and phasic, was also decreased during halothane anaesthesia (p less than 0.001). No phasic activity occurred in the abdominal muscles, but muscle tone was reduced during halothane administration (p less than 0.01). In rats, the decrease in intercostal muscle tone under halothane anaesthesia could play a major part in the fall in functional residual capacity.(ABSTRACT TRUNCATED AT 250 WORDS)