SPONTANEOUS VENTILATION WITH THE BAIN ANAESTHETIC SYSTEM

Measurements of ventilation and of inspired gas compositionwere made while volunteers breathed a non-anaesthetic gas througha Bain anaesthetic system. It was found that rebreathing occurredwhen the fresh gas flow was between two-and-a-half and threetimes the minute volume. Fresh gas flows at least three timesthe minute volume appear to be necessary to prevent rebreathingwhen using this system.     

ACID-BASE CHANGES IN ARTERIAL BLOOD ASSOCIATED WITH SPONTANEOUS AND CONTROLLED VENTILATION DURING ANAESTHESIA

Arterial pH, Pco₂ and whole blood standard bicarbonate weremeasured in surgical patients before, during, and after spontaneousor controlled pulmonary ventilation with nitrous oxide, oxygenand halothane. Relative increases in arterial Pco were accompaniedby significant reductions in standard bicarbonate, indicativeof a non-respiratory acid aemia. This effect may be attributablein part to differences between the log Pco₂ equilibration linesfor blood and for the whole organism. Small reductions in standardbicarbonate occurred over several hours of pulmonary overventilationin neurosurgical patients during induced hypothermia, and inthe early postoperative period.     

COMPARISON OF THE BAIN AND THE ADE SYSTEMS DURING CONTROLLED VENTILATION IN ADULTS

It has been suggested that Humphrey's ADE system may be an improvementon the Bain system. To compare their efficiencies we have studied20 anaesthetized adults who had controlled ventilation establishedwith a circle absorber system at a VE sufficient to maintainnormocapnia. Patients were then randomly allocated to eithera Bain or an ADE system. Fresh gas flow was sufficient to keepPE _CO2 constant for 30 min. The other circuit was then usedfor an additional 30 min. The required FGF was 54 ml kg^–1min^–1 for the Bain system, compared with 67 ml kg^–1min^–1 for the ADE system. We conclude that, during controlledventilation, the ADE system is 25% less efficient than the Bainsystem.     

CLINICAL COMPARISON OF THE BAIN AND MAGILL ANAESTHETIC SYSTEMS DURING SPONTANEOUS RESPIRATION

The Bain and Magill anaesthetic breathing systems were comparedfor spontaneous breathing during nitrous oxide in oxygen andhalothane anaesthesia. A mean fresh gas flow (Vf) of 150mlkg^–1min^–1(SD±30,range 106–250) was required with the Bain system to preventrebreathing sufficient to cause respiratory stimulation; meanfresh gas flow/expired minute volume (JVF/Vfe) was 1.49 (SD±0.32,range 0.86–2.17). Equivalent figures for the Magill attachmentwere a mean VFof 82mlkg^–1min^–1(SD±19,range43–125),while mean VF/Vfc was 0.76)(SD±0.19)range0.38–1.23,P<0.001).Theresultsattest the efficiency of the Magill attachment in termsof gas economics, and indicate the very high flows requiredto avoid respiratory stimulation in some subjects when the Bainsystem is used.     

VALIDATION OF A SYSTEM FOR MEASUREMENT OF METABOLIC GAS EXCHANGE DURING ANAESTHESIA WITH CONTROLLED VENTILATION IN AN OXYGEN CONSUMING LUNG MODEL

An oxygen consuming lung model was used for evaluation and validationof a technique for metabolic gas exchange measurements duringcontrolled ventilation. The technique comprised a Servo 900C ventilator (Siemens) and separate oxygen and carbon dioxideanalysers (Beck-man). Measurements of oxygen consumption andcarbon dioxide production were made either by measuring inspiredand expired ventilation and gas fractions in these volumes orby measuring gas fractions and calculating expired ventilationfrom inspired by transformation (Haldane). Irrespective of theFl_O2, measured values correlated well with lung model settings:measured values were within ±2% of simulated. When Haldanetransformation was used with an Fl_O2 of 0.5 there was a significantunderestimation of oxygen consumption. Carbon dioxide productionvalues correlated well irrespective of the Fl_O2 used or methodof measurement of ventilation volume. Metabolic gas exchangemeasurements by measuring both inspired and expired ventilationvolumes may be used when inert gases are not in equilibrium,for example during nitrous oxide anaesthesia.     

FACTORS INFLUENCING THE ARTERIAL OXYGEN TENSION DURING ANAESTHESIA WITH ARTIFICIAL VENTILATION

Factors influencing oxygenation of the arterial blood were studiedin 26 patients, during routine anaesthesia with artificial ventilation.The mean minute volume was 10 1./min and the arterial Pco₂ 27mm Hg. The mean oxygen consumption was 215 ml/min (STPD) andthe physiological deadspace 36 per cent of tidal volume (noallowance being made for the volume of the air passages bypassedby the endotracheal tube). At high levels of inspired oxygenconcentration, the mean alveolar-arterial Po₂ gradient was 145mm Hg, corresponding to a calculated shunt of 11 per cent. Atan alveolar Po₂, of 155 mm Hg the gradient was 52 mm Hg, correspondingto a calculated venous admixture of 9 per cent. There was noprogressive increase in venous admixture during anaesthesiain patients under the age of 43 but most of the older patientsshowed falls and their mean change was significant. Attemptswere made to reduce the venous admixture by hyperinflation ofthe lungs. This was successful in some patients, when a pressureof 40 cm H₂O was maintained for 40 seconds. Lower pressures,and the use of a sustained expiratory pressure of 5 cm H₂O werenot effective. Indices of disordered respiratory function foundin this study were not significantly different from those ina comparable study of anaesthetized patients breathing spontaneously. Present addresses: ^* Department of Anaesthesia, University ofLeeds, 24 Hyde Terrace, Leeds, 2. Present addresses: Veterans Administration Hospital, Salt LakeCity, Utah, U.S.A. Present addresses: University of Natal, Durban, South Africa.     

STUDIES OF ANAESTHESIA IN RELATION TO HYPERTENSION IH: PULMONARY GAS EXCHANGE DURING SPONTANEOUS VENTILATION

Ventilation, pulmonary gas exchanges and oxygen transport werestudied in a group of treated and untreated hypertensive elderlypatients, before, during and after nitrous oxide-halothane anaesthesiawith spontaneous ventilation. During anaesthesia minute andalveolar ventilation were depressed (— 30 per cent) outof proportion to the decrease in oxygen uptake (-18 per cent)and carbon dioxide production (— 19 per cent), and moderatehypercapnia ensued (mean Paoo3 50.3 mm Hg). All these variablesreturned to the pre-anaesthetic levels within 1 hour of terminatinganaesthesia. VD/VT was increased following induction of anaesthesiaas a result of decreased tidal volumes (— 47 per cent),but did not change progressively during the course of anaesthesia.Total deadspace (VD) was reduced by an average of 44 ml as aresult of intubation. Based on measurements of arterial Poaand the alveolar-arterial Poj difference, mean pulmonary venousadmixture was 10.4 per cent before anaesthesia, 10.9 per centduring anaesthesia but before surgery, 13.1 per cent after surgery,and 15.6 per cent 1 hour after the termination of anaesthesia.Although the average postoperative arterial Poa was slightlylower ( - 7.4 rnm Hg) than before anaesthesia, the differencewas due to many factors, and no evidence of a deteriorationin the over-all pulmonary gas exchange could be found. The conceptof "airway closure" in the supine elderly subject as a causefor the increased pulmonary venous admixture at rest is discussed.Impairment of pulmonary function during anaesthesia in hypertensiveelderly patients causes less concern than the severe changesin cardiovascular function ^*Present address: Hospital Notre Dame, Montreal133, P.Q., Canada     

FACTORS INFLUENCING THE ARTERIAL OXYGEN TENSION DURING HALOTHANE ANAESTHESIA WITH SPONTANEOUS RESPIRATION

Factors influencing oxygenation of the arterial blood were studied,during routine anaesthesia, in thirty-six patients anaesthetizedwith halothane and allowed to breathe spontaneously, with amean minute volume of 5 l./min. There was evidence of an unsteadyrespiratory state during the first hour of anaesthesia. Oxygenconsumption was 87 per cent of basal. Physiological deadspaceamounted to 33 per cent of the expired tidal volume (all patientsintubated). At high levels of inspired oxygen concentration,the mean alveolar-arterial Po, gradient was 184 mm Hg, correspondingto a shunt of 14 per cent of pulmonary bloodflow. At lower levelsof alveolar Po₂, the alveolar-arterial Po, gradient diminishedbut was above the value which would be caused by a shunt of14 per cent. This was probably due to uneven ventilation perfusionratios (maldistribution) corresponding to a calculated venousadmixture rising as high as 30 per cent. It is concluded that,to ensure the maintenance of a normal arterial Poa in the majorityof patients, the alveolar Po, is required to be as high as 200mm Hg and this needs an inspired oxygen concentration of 35per cent under the conditions investigated in this study.     

AN EVALUATION OF REBREATH1NG IN A MODIFIED T-PIECE SYSTEM DURING CONTROLLED VENTILATION OF ANAESTHETIZED CHILDREN

Measurements of minute volumc, airway pressure, and inspiredand end-expired carbon dioxide concentrations were made on anaesthetizedchildren during the use of controlled ventilation with a modifiedT-piece system. The data obtained indicate that a fresh gasinflow as low as 100 ml/lb. (220 ml/kg) body weight per minute,with a minimum total flow of 3 1./min, is sufficient to preventcarbon dioxide retention. The influence of the rate of ventilationupon end-expired carbon dioxide concentration also was assessed. ^* Present address: The London Hospital London, E.1.     

REBREATHING CHARACTERISTICS OF THE BAIN CIRCUIT An Experimental and Theoretical Study

The Bain circuit was studied in a model lung on the assumptionthat, in addition to the ratio of fresh gas flow to total ventilation(FG/E), different time fractions of the respiratory cycle might influencerebreathing. We found that the time fraction for active expiration(FE_t) governed rebreathing for each FG/E value. With FE_t, as an independentvariable, a theoretical formula was derived for rebreathing.Rearranging this formula made it possible to calculate the necessaryincrease in ventilation to keep end-tidal carbon dioxide constantfor each FG/E. Thus, at a fresh gas flow of 70 ml kg^-1 min^-1,I has to be increased 2.6 times. For spontaneously breathing patients inhalation anaestheticsthat do not depress carbon dioxide sensitivity seem to be bettersuited to use in the Bain circuit. The FE_CO2 can then kept constantthrough increased ventilation in spite of the concomitant increasein rebreathing     

METABOLIC GAS EXCHANGE DURING NITROUS OXIDE ANAESTHESIA WITH CONTROLLED VENTILATION

A system for metabolic gas exchange has been used during nitrousoxide-opioid anaesthesia incorporating a Servo Ventilator 900C and external analysers for oxygen and carbon dioxide. Oxygenconsumption and carbon dioxide excretion were calculated asdifferences in content between inspired and expired minute ventilation.Nitrous oxide uptake was calculated similarly, assuming it wasthe only other gas present in addition to oxygen and carbondioxide. The mean value for oxygen consumption was 3.25 ml kg^–1min^–1, declining by 8% during the 2 h of anaesthesia.The formula for the best fit curve of nitrous oxide uptake was18.3.t^–0.48 ml kg^–1 min^–1 when Fl_NO was 0.7.To simplify measurement procedures and avoid measurements ofexpiratory volume, we also calculated metabolic gas exchangewhen expiratory minute ventilation was expressed as a functionof inspiratory minute volume and nitrous oxide uptake. The lattervalue was obtained from the overall best fit curve for nitrousoxide uptake.     

EFFICIENCY OF AN ENCLOSED AFFERENT RESERVOIR BREATHING SYSTEM DURING CONTROLLED VENTILATION

We describe an enclosed afferent reservoir (EAR) breathing systemdeveloped by Ohmeda and designed to operate efficiently in spontaneousand controlled ventilation. The efficiency of the system wasevaluated by calculating the fractional utilization of freshgas in 10 ASA I–III patients during anaesthesia with controlledventilation. Maximum efficiency occurred when the minute ventilationto fresh gas flow ratio was greater than 1.5. Under these conditions,fractional utilization was relatively constant with a valueof 0.73 (95% confidence interval 0.69–0.78). The minimumfresh gas flow for use during controlled ventilation was determinedin another eight ASA I–III patients when the minute volumeto fresh gas ratio was greater than 1.5. In view of an increasedarterial to end-tidal carbon dioxide partial pressure differencein patients in the first part of the study (1.03 kPa), normocapniawas defined as an end-tidal carbondioxide partial pressure of4.3 kPa. Normocapnia was achieved with a mean fresh gas flowof 66 ml kg^–1 min^–1, while 70 ml kg^–1 min^–1produced mild hypocapnia.     

PHYSIOLOGICAL SHUNTING AND DEADSPACE DURING SPONTANEOUS RESPIRATION WITH HALOTHANE-OXYGEN ANAESTHESIA AND THE INFLUENCE OF INTUBATION ON THE PHYSIOLOGICAL DEADSPACE

In thirteen patients the physiological deadspace: tidal volumeratio and the alveolar to arterial oxygen tension differencehave been measured immediately before and during anaesthesiawith halothane in oxygen after 30–70 minutes spontaneousrespiration. The mean values were unchanged and it was concludedthat the physiological shunt was also unaltered except in twoinstances which are discussed. In a further series of ten patientsan average reduction of 27 ml in the physiological deadspacewas measured as a result of endotracheal intubation.     

INTER-RELATIONS OF THE VOLUME AND TIMING COMPONENTS OF VENTILATION DURING CARBON DIOXIDE REBREATHING IN AWAKE AND ANAESTHETIZED SUBJECTS

We studied the relations between the volume and timing componentsof ventilation during carbon dioxide rebreathing in conscioussubjects and patients anaesthetized with propofol. In consciousresponses, breath-by-breath minute ventilation (VI) generallycorrelated better with end-tidal carbon dioxide than did tidalvolume (VT), but VT correlated better than VI in the anaesthetizedresponses. The source of this difference was that, whereas VTand the inspiratory period were both smaller and less variablewhen subjects were anaesthetized rather than conscious, theexpiratory period was no less variable, and this disturbed theusual inverse relation between VT and the duration of the ventilatorycycle. Anaesthesia stabilized the switch from inspiration toexpiration, but not that from expiration to inspiration. Insome patients, it produced a disturbance pronounced enough tosuggest bimodality of the timing of expiration.     

CARBON DIOXIDE PRODUCTION IN CYANOTIC CHILDREN DURING ANAESTHESIA WITH CONTROLLED VENTILATION

Carbon dioxide production and ventilatory efficiency were measuredduring undisturbed anaesthesia with intermittent positive pressureventilation in 34 children about to undergo closed or open cardiacsurgery. Anaesthesia was provided with fentanyl or halothaneand nitrous oxide. There were 15 cyanotic and 19 acyanotic children.Children with cyanotic heart disease produced approximately20% less carbon dioxide per unit body weight than acyanoticchildren, but ventilation was approximately 20% less efficient.Adequate ventilation should therefore be obtained when "normal"ventilation in relation to body weight is used in cyanotic children. Previously presented at the Anaesthetic Research Society [1].     

INTERMITTENT MANDATORY VENTILATION DURING ANAESTHESIA USING THE MANLEY SERVOVENT VENTILATOR

The lungs of 25 patients were ventilared with intermittent mandatoryventilation (IMV) during anaesthesia using the Manley ServoventModel MS. This ventilatory mode is especially suitable for prolongedoperations in which there is no need for muscle relaxation.While incorporating the advantages of spontaneous and mechanicalventilation, it is superior to both in selected cases. The ManleyServovent Model MS ventilator is capable of delivering IMV withoutmodification, using a single source of gas.     

FLOW REQUIREMENTS AND REBREATHING DURING MECHANICALLY CONTROLLED VENTILATION IN A T-PIECE (MAPLESON E) SYSTEM

The influence of fresh gas flow (FGF) setting on rebreathingwas investigated in 15 infants and children (weight 3.5–21.8kg) during balanced anaesthesia with mechanically controlledventilation using a T-piece (Mapleson E) system and a Nuffieldventilator 200. Tidal volume (VT), minute volume (VE), maximalinspired (PlCO₂) and end-tidal (PE'_co2) carbon dioxide tensionsand airway pressure were measured. VE, set to produce a PE'_CO2of about 4.5 kPa and measured at a high FGF (minimal rebreathing),was unchanged throughout the study and the regression equationfor VE and weight was: VE (ml min^–1) = 146x kg ³482, r = 0.92. Measurements were then repeated at FGF: VE ratiosreduced to 1.5 and 1.0. To achieve minimal rebreathing (Pl_CO2less than 0.5 kPa), FGF: VE ratios greater than 1.8 (range 1.8–4.9)had to be used. At FGF: VE ratios of 1.5, some alveolar rebreathingoccurred, indicated by increased inspired (P < 0.001) andend-tidal (P < 0.001) carbon dioxide tensions. At FGF: VEratios equal to 1.0, alveolar rebreathing was more pronouncedand hypercapnoea occurred with a PE'_CO2 (mean ± 1 SD)of 5.89±0.53 kPa. At this FGF setting, change in I: Eratio from 1: 2 to 1: 1 did not influence the level of alveolarrebreathing. A minimal FGF (ml min^–1) setting of 1.5 xVE (that is, 1.5 (146 x kg ³ 482), approximated to theexpression (200 x kg ³ 1000) is recommended for controlledventilation to avoid hypercapnoea when using the T-piece systemin children weighing less than 20 kg.     

INCREASED VENTILATION REQUIREMENTS DURING OBSTETRIC GENERAL ANAESTHESIA

The inspiratory fresh gas flow rate (FGF) required to producean end-tidal carbon dioxide tension (PE' _CO2)of 4kPa duringgeneral anaesthesia, neuromuscular blockade and artificial ventilation,was compared in a group of 46 obstetric patients and a matchedgroup of 50 non-pregnant female patients. The non-pregnant patientsrequired a mean (SD) inspiratory FGF of 77 (10.6) ml kg^–1min^–1, whereas the pregnant patients required a mean FGFof 121 (24.6) ml kg^–1 min^–1 before delivery (inthose who reached a stable state), and 109 (19.3) ml kg^–1min^–1 after delivery. These represent significant (P <0.0001) increases of 57% and 42%, respectively, over the non-pregnantstate. ^*Anaesthetics Unit, The London Hospital, Whitechapel, LondonEl IBB. 335, Southampton Road, Titchfield, Hants PO14 4AX. Northampton General Hospital, Whitechapel, London E1 1BB.     

CONCEPT OF ADEQUATE VENTILATION USING THE APNOEIC THRESHOLD DURING CURARE AND NITROUS OXIDE ANAESTHESIA

This paper describes the use of an absolute filter to removepathogenic organisms from the inspired air during intermittentpositive pressure respiration. The filter used removes all particlesdown to a diameter of 0.5 microns with an efficiency of 99.99per cent, thus ensuring complete sterility from airborne bacteria. ^*This paper was presented in part at the Second World Congressof Anaesthesiologists, Toronto, Canada: September 4–10,1960.