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The performance of a model of a subject breathing from a circle system has been examined in relation to nitrogen and helium. The ability of the model to maintain a nitrogen steady-state breathing air, the attainment of a new steady-state after perturbation of an existing nitrogen equilibrium, the washout of nitrogen from the subject model on breathing oxygen, and the estimation of functional residual capacity using a rebreathing method with helium as an indicator have been assessed. The predictable and accurate performance of the model in these studies, together with its ability to reproduce the results of a number of previously published studies in man, suggest that the model can be used to predict the behaviour of circle systems when used with inhaled anaesthetic agents.

A model has been constructed of a subject breathing from a circle system. The subject model is based on the circulation-time model of anaesthetic uptake described by Mapleson. This is a multi-compartmental model of body tissues in which gas exchange in each compartment is calculated at each heart beat. The lung compartment of Mapleson's model has been modified to allow for an unlimited number of gases to be present in the inspired gas mixture. The circle system model assumes total absorption of all expired carbon dioxide and full mixing of all gases within the system. The volume of gas in the system and its composition is calculated for each respiratory cycle. The subject model can be considered as being either attached to a non-rebreathing system, when inspired gas composition is under the control of the operator, or attached to the circle system, when the operator has control of fresh gas flow and composition. The model has been realized as a computer program written in Pascal.

Expressions have been derived to show the dependence of alveolar oxygen and anaesthetic concentrations on fresh gas flow to a circle system, the composition of fresh gas, ventilation and gas uptake. The form of these expressions is influenced by the degree of mixing of fresh and expired gases within the circle system. These expressions assume an equilibrium state within the circle system and the rate at which equilibrium will be approached has been quantified in terms of the time-constants of change of composition of gas within the system. Time-constants approach infinity as fresh gas flow approaches values which just satisfy gas uptake. Whilst simplifying assumptions made in the derivation of the various alveolar gas equations limits their accuracy, the expression can serve as a guide to the likely behaviour of circle systems under any given conditions of use.

Equations have been derived to determine the alveolar gas concentrations which occur when the Magill (Mapleson A) circuit is used with a low fresh gas flow. Alveolar oxygen and carbon dioxide concentrations are determined by the fresh gas flow and composition, carbon dioxide output and oxygen uptake. Gas mixing within the circuit and alterations in the inspired gas concentrations do not affect the final equilibrium. If oxygen uptake and carbon dioxide output are constant, the alveolar gas concentrations are unaffected by alterations in ventilation.

Gas concentrations and ventilation levels have been measured within a conventional Magill circuit when conscious volunteers breathed a non-narcotic gas mixture at varying fresh gas flows. When evidence of rebreathing of alveolar gas was detected, the fresh gas flow was kept constant until a steady state developed. All subjects showed evidence of rebreathing when the fresh gas flow approached the predicted alveolar ventilation levels. A variety of subject-circuit interactions was seen and shown to be precipitated by naturally occurring breath-to-breath variations in ventilation. A single large breath could perturb the system. This could have a temporary effect, when the fresh gas flow was sufficient to wash the increased aliquot of expired carbon dioxide from the circuit. At other times a progressive response occurred as ventilatory stimulation as a result of the increased inspired carbon dioxide concentrations caused alveolar gas to reach the reservoir bag and converted the system behaviour from that of a simple added deadspace to that of a total mixing device. Whilst marked changes occurred commonly in both ventilation and inspired gas concentrations, only slight changes in end-tidal gas concentrations occurred.

Away with the cant of “measures not men” —the idle supposition that it is the harness and not the horses that draw the chariot along. If the comparison must be made, if the distinction must be taken, men are everything, measures comparatively nothing. George Canning, 1801

Three electronic respirometers, two of which have been recently introduced (the Birchover respiratory flowmeter and the Wright respiration monitor), and one a prototype model (Spiroflo) have been laboratory tested in conditions designed to simulate possible modes of clinical use. It was found that the accuracy of the Spiroflo and Wright instruments were acceptable, but the Birchover was limited in its application and became less accurate in the variations of use described.

The cardiovascular effects of pancuronium may be caused partly by an interaction of this drug with the sympathetic nervous system. We examined one possible mechanism of interaction, the effect on the re-uptake processes for noradrenaline. Pancuronium and its closely related steroidal homologues, Org. 6368, Org. 7268 and NC 45, were studied at a high concentration (500 [imol litre−1) for inhibition of the uptake of tritiated noradrenaline into neuronal sites (Uptake2) and extraneuronal sites (Uptake1) in the isolated perfused rat heart. All drugs tested caused almost total inhibition of Uptake1 The bis-quaternary steroids pancuronium and Org. 6368 were selective for Uptaket inhibition, the mono-quaternary steroids Org. 7268 and NC45 also produced significant inhibition of Uptake. Uptake1 inhibition was investigated in detail using lesser concentrations of the compounds. All four steroids were found to cause a concentration-dependent inhibition of Uptake1. It seems likely, therefore, that inhibition of neuronal uptake of noradrenaline plays a significant role in the aetiology of the chronotropic actions of pancuronium in the rat.

An experimental lung model was used, with controlled ventilation, to determine the effect of different circle arrangements and varying ventilatory frequencies on the efficiency of carbon dioxide removal from a circle system without carbon dioxide absorption. Greater efficiency was found when fresh gas entered the system between the unidirectional inspiratory valve and the subject than when the fresh gas inlet was on the ventilator side of this valve. At any fresh gas flow and minute volume, efficiency was greater at low respiratory frequencies. Good correlations existed between carbon dioxide concentration in the model lung, fresh gas flow and minute ventilation when respiratory frequency was constant. Paradoxical results were obtained when minute volume was varied by changes in frequency at a constant tidal volume. The major cause of the various differences in performance has been ascribed to variations in the degree of mixing of fresh and expired gas within the system.

A theoretical analysis has been performed to show the interrelatinships between alveolar carbon dioxide concentration, carbon dioxide output, alveolar ventilation and fresh gas flow during controlled ventilation with the Mapleson D system. The model and forms of equation used have been substantiated by experimetns in which dogs were ventilated through a coaxial version of the Mapleson D system at varying levels of fresh gas flow and alveolar ventilation. By assuming that man ventilated with this system behaves as does the dog, a nomogram has been produced to predict alveolar carbon dioxide concentrations at any levels of fresh gas frow and minute volume.

A mathematical model of a subject breathing from a circle system has been used to follow the course of anaesthetic uptake during the simulated administration of 60% nitrous oxide, 2% halo-thane and 2% methoxyflurane, under non-re-breathing conditions and with fresh gas flows to the circle system of between 8 and 0.25 litre min−. Compared with the non-rebreathing state, the use of a circle system reduced the initial rate of increase of alveolar towards fresh gas anaesthetic concentration, and the rate of increase in body anaesthetic content. The degree of reduction became more marked as fresh gas flow was reduced, and as agents of increasing blood solubility were used. These effects of a circle system were influenced by the volume of the circle system and the composition of gas initially present within the system. When the circle system was in use there were increases in the magnitude of both the concentration effect and the second gas effect which were related to the magnitude of fresh gas flow. The use of a circle system augmented the effects of changes in cardiac output and reduced the effects of changes in ventilation on the alveolar concentrations of the anaesthetic. These influences of a circle system were also dependent on the magnitude of fresh gas flow. The degree of augmentation of the effects of cardiac output decreased with increasing blood solubility of the agent in use, whilst the limitation of the effects of ventilation was greatest with the agent of highest blood solubility. Both under non-rebreathing conditions and with the circle system in use, the effects of cardiac output and ventilation were greater with 2% nitrous oxide than with 60% nitrous oxide, and were also greater when gases were given separately than when administered in combination.

Measurements of ventilation and of inspired gas composition were made while volunteers breathed a non-anaesthetic gas through a Bain anaesthetic system. It was found that rebreathing occurred when the fresh gas flow was between two-and-a-half and three times the minute volume. Fresh gas flows at least three times the minute volume appear to be necessary to prevent rebreathing when using this system.

A paramagnetic oxygen analyser which has a response time allowing oxygen concentration to be followed within a breath is described. Noise generated by the differential pressure transducer precludes the achievement of accuracy comparable with other paramagnetic analysers (e.g., Servomex OA150). The performance of the analyser is not disturbed by the presence of other gases or water vapour. It cannot be used during IPPV as pressure changes produce false deflections in the transducer system for which there is no simple method of compensation.

The acute haemodynamic effects of thiopentone, methohexitone and propanidid have been compared in anaesthetized dogs. All three drugs caused a reduction in arterial blood pressure and minor changes in cardiac output, but whereas thiopentone and methohexitone had similar effects propanidid was found to be three times as potent as thiopentone in its effect upon arterial systolic pressure. Thiopentone and methohexitone had slight effects upon right ventricular pressures, whilst propanidid markedly increased right ventricular systolic pressure.

Duplicate determinations of arterial oxygen and carbon dioxide tensions were made in seventy adult patients of both sexes awaiting elective surgery. No difference in arterial oxygen tension was noted between the sexes but a marked inverse correlation between age and arterial oxygen tension was demonstrated. When this relationship was evaluated from the duplicate estimations the resultant regression equation was such that Pao2 = 102.5–0.22 (age). No statistically significant differences were noted between the Paco2 values of patients of different sexes and age groups.

Twenty-five patients were anaesthetized with chloroform. In 2 instances a partial rebreathing method using nitrous oxide and oxygen as the carrier gases was used. In 6 patients a non-rebreathing technique with oxygen alone as the carrier gas was substituted; the inspired concentration varied between 1.5 and 2.5 per cent. In the remainder a closed circle system was used giving inspired concentrations of up to 4 per cent. Cardiac irregularities were seen in 7 patients before recovery of consciousness. Hypotension below 90 mm Hg occurred in 3 patients. Of 26 cardiac output determinations in 8 patients 18 fell within the range 5 to 7 l./min. A four- to tenfold increase in finger blood flow was observed as well as a less marked increase in forearm blood flow. In 87 determinations of arterial carbon dioxide tension in 18 patients 38 per cent were less than 40 mm Hg and only 20 per cent were above 60 mm Hg. Metabolic acidosis was observed in only 3 of 18 patients investigated. In the closed circle system oxygen consumption in 9 patients ranged from 159 to 261 ml/min. Blood chloroform levels derived from 29 samples from 7 patients gave a mean value of 17.9 ±4.7 mg percent.

The interrelationship between Fio2, true gas exchange ratio (R) and the mathematical slope of the gas R line (RS) is not at all obvious. This paper shows the derivation of an equation relating these quantities and offers both a graphical presentation and an example of a table enabling R to be measured from knowledge of RS and Fio2.

Passive Valsalva manoeuvres were performed on young healthy patients about to undergo elective plastic surgery requiring induction of hypotension. This challenge was applied both in the supine and 30-degree head-up postures, during light anaesthesia, and after administration of beta-adrenergic blocking and ganglion blocking drugs, halothane and methoxyflurane. The resulting changes in blood pressure, pulse rate and systolic time intervals were recorded. During light anaesthesia more than half the responses were found to be unblocked, despite administration of beta-adrenergic blocking and ganglion blocking agents. Deepening the level of anaesthesia with either halothane or methoxyflurane usually resulted in a blocked response. The response to the manoeuvre could be influenced by posture. Marked alterations in the pre-ejection period occurred during every manoeuvre and were the same whether the manoeuvre was blocked or unblocked.