Admixture of Atmospheric Air in the T-piece Used as a Weaning System

Ayre's T-piece may be used for intubated, non-anaesthetized patients requiring an oxygen supplement or humidified gas mixtures. The system has the disadvantage that atmospheric air (V_AD) enters through the expiratory leg when the inspiratory How and therapeutic gas flow (TGF) are not equal. This admixture can be reduced in the simple T-piece by the addition of either a compliant inspiratory reservoir, or a compliant inspiratory reservoir and an expiratory unidirectional valve. These three systems have been studied in a model investigation using different tidal volumes (V_T), respiratory frequencies, and therapeutic gas flows. V_AD was measured using a pneumotachograph and the inspiratory tidal volume was measured with a Wright Respiratory Monitor. The ratio V_AD/V_T was employed as an expression of the admixture of atmospheric air during inspiration. The introduction of a compliant inspiratory reservoir to the simple T-piece results under all conditions in a considerable reduction in V_AD/V_T. A further reduction was obtained by introducing an expiratory unidirectional valve into the system - however, with the result that the previously open system is turned into a closed system. When selecting a weaning system, the risk to the patient of the discontinuation of the gas flow should therefore be weighed against the advantages obtained by a reduction of the admixture.     

Distribution of Expiratory Gas and Rebreathing in a T-piece Modification Combined with a PEEP Valve

T-piece modifications with PEEP valves are often used in weaning from mechanical ventilation or for intubated patients not requiring ventilatory support. Distribution of expiratory gas and the extent of rebreathing in a T-piece modified with an inspiratory reservoir (ICR) and with a PEEP valve were studied in a model with various fresh gas flows (FGF), tidal volumes and frequencies at three valve settings: 0 cmH2O (ZEEP) and PEEP of 5 and 10 cmH2O (0.490-0.981 kPa). Two types of distribution of expiratory gas were delineated: type one with expiratory gas in the inspiratory limb (IL) and a high ratio of the maximum CO2 content and corresponding end-expiratory CO2 concentration in the expiratory limb (EL) (FmaxCO2/FECO2) and a type 2 with no detectable alveolar gas in the IL and a low ratio of FmaxCO2/FECO2. The use of PEEP did not increase the amount of alveolar gas in the system, and no increase occurred in the end-expiratory CO2 concentration. The investigated system is in fact a Mapleson A system. The ratio of FGF to minute ventilation just preventing rebreathing during spontaneous ventilation is approximately 1, in contrast to 3 in other modifications. These advantages minimize the risk of rebreathing, even when the minute ventilation rises to that of the fresh gas flow. The T-system with a compliant inspiratory reservoir and a PEEP valve can, in most clinical weaning situations, satisfy the inspiratory peak flow of different respiratory patterns with a standard FGF of 15 l X min-1.     

Control of Carbon Dioxide in Modified Mapleson A and D (Hafnia) Anaesthetic Systems. An Experimental Model

The effects of varying ventilations (V_e) and fresh gas flows (FGF) on end-expiratory CO₂ (F eco ₂) levels were investigated in an experimental model lung, employing the Hafnia modification of the Mapleson A and D anaesthetic systems during CO₂-absorption and CO₂-wash-out (rebreathing). Identical results were found in both systems: F eco ₂ was constant and independent of FGF with CO₂-absorption and constant V_e, whereas rebreathing resulted in increasing F eco ₂ levels as FGF was decreased. As control of F eco ₂ in the rebreathing systems by regulating FGF could only take place within F eco ₂ levels higher than that determined by V_e at complete CO₂-absorption, e. g. for the Hafnia A and D rebreathing systems, control of FGF necessitates relative hyperventilation. F eco ₂ with constant FGF decreased with increasing V_e during CO₂-absorption, as well as during rebreathing, although this decrease was less in the rebreathing systems. Thus a decrease in F eco ₂ with rising V_e can be avoided and hypocapnia prevented. The results agree with those obtained in clinical studies.     

Responses of respiratory drive and breathing pattern to inspiratory loading during nitrous oxide and isoflurane sedation

Background: Increased inspiratory resistance in combination with mild gas narcosis is common during recovery after a general anesthesia, but there are only few previous studies on inspiratory loading during subanesthetic gas narcosis.
Methods: Responses of respiratory drive (central inspiratory activity P_0.1) and ventilatory pattern to an inspiratory threshold load of –6 cm H₂O were studied in 16 healthy subjects during mild subanesthetic gas narcosis. One group (n=9) was exposed to 13, 26 and 39% nitrous oxide (N₂O) and air control (Group N). Another group (n=7) was exposed to 0.1, 0.2 and 0.3% isoflurane and air control (Group I). Measurements were done after 1 min adaptation to the load.
Results: Nitrous oxide and isoflurane had no effect on respiratory drive and V_T either during unloaded breathing or during inspiratory threshold loading. Across all gas concentrations (including 0% control), inspiratory threshold loading resulted in significant P_0.1 increases, amounting to 62% in group N and 38% in group I. At the same time V_T decreased by 11 and 12%, respectively. A significantly increased end-expired CO₂ and decreased minute volume compared to air control was found during isoflurane inhalation but could be ascribed to normalization of the hyperventilation in the control situation.
Conclusions: It is concluded that the steady-state ventilatory responses to loading, consisting of increased P_0.1 and decreased V_T, are maintained during inhalation of subanesthetic doses of N₂O (0.13–0.38 MAC) and isoflurane (0.09–0.26 MAC).     

Expired Minute Volume Measurements During Anaesthesia Using Non-Rebreathing Valves

Non-rebreathing valves may cause leakage of anaesthetic gases from the inspiratory to the expiratory limb, leacling to errors in measurements of exhaled gas volume and gas composition. The fresh gas flow (FGF 1Xmin^-1) leakage (gas-shunt) over a non-rebreathing valve (AMBU Paedi Anaesthesia System) was experimentally investigated in a conscious volunteer. The gas-shunt depended upon the tension within the breathing balloon, which is placed in the inspiratory limb. At low FGF's, i.e. with the breathing balloon close to collapse, no gas-shunting was registered. At higher FGF's, a large gas-shunt was found. Due to the negligible gas-shunt found at low FGF settings, this AMBU system might be used for analyses and measurements of exhaled gases during Anaesthesia and spontaneous ventilation.     

Respiratory compensation during spontaneous ventilation with the Bain circuit

The volume of carbon dioxide rebreathed by spontaneously breathing patients under halothane anaesthesia at various fresh gas flow rates (FGF) with the Bain modification of the Mapleson "D" breathing circuit is measured. The effect of rebreathing on a heterogeneous patient population is shown to be unpredictable hypercapnia in those patients who cannot respond adequately to this carbon dioxide challenge. All adults rebreathe significant volumes of carbon dioxide at a FGF rate of 100 ml . kg-1 . min-1. This carbon dioxide load is a potential risk to every patient and this hypercapnia is preventable by using high FGF rates. Rebreathing occurs because the inspired carbon dioxide load is unpredictable in a given patient and the patient's response is uncontrolled. Patients respond to this carbon dioxide challenge by increasing inspiratory flow rate (Vt/Ti), which results in increased rebreathing of carbon dioxide from the expiratory limb of the circuit. To prevent potentially dangerous rebreathing of carbon dioxide in all patients the fresh gas flow rate must be much higher than presently recommended.     

THE ADVANTAGES OF A COMBINED T-PIECE AND NON-REBREATHING VALVE: A SIMPLE DEVICE

A combined device is described which is considered to utilizethe advantages of both the T-piece and non-rebreathing valve,while eliminating the disadvantages. The device incorporatesthe reservoir bag and inhalation valve of the non-rebreathingsystem and the expiratory reservoir limb of the T-piece.     

Unexpected intraoperative hypercapnia due to undetected expiratory valve dysfunction--a case report

The normally functioning of anesthetic circle system depends mainly on the integrity of both inspiratory and expiratory unidirectional valves which keep the inspiratory gas will not be contaminated by the expired CO2. In case there is a leakage defect in one or both of these valves, i.e. inability to keep tightly closed during the cycle, retrograde gas flow may happen and the exhaled CO2 may get into the inspiratory limb, resulting in rebreathing and hypercapnia with disastrous aftermath. Here we report a rather rare incident of unrecognized expiratory valve insufficiency that was not detected before anesthesia in a 40-year-old female patient who developed intraoperative hypercapnea during general anesthesia with mechanical ventilation. Discussions on the causes, management, and prevention of hypercapnia due to respiratory valve dysfunction are presented.     

Oxygen enrichment of entrained room air during Venturi jet ventilation of children undergoing bronchoscopy

Intermittent oxygen jet ventilation at an inspiratory:expiratory ratio of 1:3 was used to ventilate 15 children undergoing rigid Storz bronchoscopy for removal of inhaled foreign body. Oxygenation of the patient was continuously monitored by pulse oximetry. In all children S pO₂ was above 95% when the bronchoscope was above the carina. When the bronchoscope was introduced into one of the bronchi, S pO₂ decreased to 70–85% in five children. Delivery of a continuous flow of oxygen via a T-piece attached to the side-arm of the bronchoscope increased the S pO ₂ >95% in the five children. Oxygen jet ventilation during bronchoscopy is based on the Venturi principle; the oxygen jet will entrain room air from the side arm of the bronchoscope which functions as an entrainment orifice. This will decrease the F IO₂. The F IO₂ can be increased by flowing oxygen continuously via the T-piece attached to the side arm of the bronchoscope.     

Defining segments and phases of a time capnogram

The division of a time capnogram into inspiratory and expiratory segments is arbitrary and results in the inability of a time capnogram to detect rebreathing instantaneously. Demarcation of a time capnogram into inspiratory and expiratory components using gas flow signals will not only facilitate prompt detection of rebreathing, but will also allow application of standardized and physiologically appropriate nomenclature for better understanding and interpretation of time capnograms. A Novametrix((R)) CO(2)-SMO plus respiratory profile monitor (Novametrix Medical Systems, Wallingford, CT) was used to obtain a simultaneous display of CO(2) and respiratory flow waveforms on a computer screen during spontaneous and controlled ventilation using a circle system with the inspiratory valve competent (no rebreathing) and with the valve displaced (rebreathing). Because the response time of the CO(2) analyzer was similar to the response time of the flow sensor, a comparison was made between the two waveforms to determine the inspiratory segment (Phase 0) and the expiratory segment of the time capnogram and its subdivisions (Phases I, II, and III). The end of expiration almost coincides with the downslope of the CO(2) waveform in the capnograms when there is no rebreathing. However, in the presence of rebreathing, the alveolar plateau is prolonged and includes a part of inspiration (Phase 0), in addition to the expiratory alveolar plateau (Phase III). Implications: Presently, the division of a time capnogram into inspiratory and expiratory segments is arbitrary. Demarcation of a time capnogram into various components using the gas flow signals facilitates prompt detection of the cause of abnormal capnograms that can widen the scope of future clinical applications of time capnography.     

Resistance to gas flow in the "new" anaesthesia circuits: a comparative study

The resistance to gas flow is measured in the Jackson-Rees modification of the Ayre's T-piece, the "Bain" circuit, and the circle circuit using the conventionally employed rubber tubing, or either of two coaxial circle tubing systems. The Ayre's T-piece has the lowest resistance to gas flow, followed by the circle circuit with conventional rubber tubing, then the "Bain" circuit and the coaxial circle circuits. When the resistance to gas flow in the inspiratory limb is measured with a tracheal tube attached to each circuit the resistances increase in magnitude, but the differences among the circuits become less significant.     

Resistance to gas flow in the “new” anaesthesia circuits: A comparative study

The resistance to gas flow is measured in the Jackson-Rees modification of the Ayre’s T-piece, the “Bain” circuit, and the circle circuit using the conventionally employed rubber tubing, or either of two coaxial circle tubing systems. The Ayre’s T-piece has the lowest resistance to gas flow, followed by the circle circuit with conventional rubber tubing, then the “Bain” circuit and the coaxial circle circuits. When the resistance to gas flow in the inspiratory limb is measured with a tracheal tube attached to each circuit the resistances increase in magnitude, but the differences among the circuits become less significant.     

Hypercapnia due to rupture of the unidirectional valve in the inspiratory limb of the breathing system after induction of general anesthesia--a case report

Malfunction of either inspiratory or expiratory check valve in a breathing circuit system may allow carbon dioxide (CO2) rebreathing and result in hypercapnia. The subsequent increase of PaCO2 may entail increased sympathetic activity which in turn causes serious problems such as tachyarrhythmia and myocardial ischemia, particularly in patients who have history of coronary artery disease (CAD). Here, we report an incident of rupture of the inspiratory valve in the breathing circuit which happened to a patient during induction of general anesthesia and eventuated in markedly heightened end-tidal CO2 (EtCO2) of the patient. The recognition, related complications and management of the inspiratory valve malfunction are discussed.     

AYRE'S T-PIECE: A REVIEW OF ITS MODIFICATIONS

The modifications of Ayre's T-piece system can be divided intothree types. In the first there is no expiratory limb, in thesecond the capacity of the expiratory limb is greater than thetidal volume, and in the third the capacity of the expiratorylimb is less than the tidal volume. On the basis of previousmathematical and laboratory investigations by other authors,the three types are discussed with reference to the resistanceto respiration and minimum fresh gas flow required to preventrebreathing and air dilution during both spontaneous and controlledventilation. The resistance and the fresh gas flow requirementsare related to the expiratory flow rates and respiratory flowpatterns which occur in children. It is suggested that the mostconvenient system is a T-piece of 10 mm diameter with an expiratorylimb greater than the tidal volume and a bag attached to theexpiratory limb. A fresh gas flow of up to 2–3 times theminute volume is required.     

Carbon dioxide distribution in Mapleson A and D systems: an experimental study

The distribution of CO2 in the Mapleson A and D rebreathing systems was investigated experimentally during controlled ventilation and with the expiratory valve closed during inspiration. Maximal and minimal levels of CO2-concentration obtained from capnograms along the tubing were used to construct "gas profiles". For both systems, high tidal volumes and low fresh gas flows resulted in a high degree of gas separation with a pool of alveolar gas near the expiratory valve, and longitudinal gas mixing was minimal. In this manner fresh gas loss was prevented and fresh gas utilization optimized. The end of the tubing nearest the patient was found to act as a reservoir for alveolar gas in the Mapleson A system and fresh gas in the Mapleson D system. Fresh gas utilization in the Mapleson D system was somewhat less efficient than in the Mapleson A system due to the fresh gas admixture to exhaled alveolar gas in the patient-near end of the tubing during expiration. The replacement of the usual expiratory valve of the Mapleson A system by a valve which is closed during inspiration makes the A system an alternative to the D system for controlled ventilation.     

A low-pressure portable anaesthesia system for field use: clinical trials

This is a report of our experience with a portable anaesthesia system that was developed for use under field conditions, when compressed gas supplies are limited. We first assembled and bench-tested a low-pressure plenum system, based upon the Farman entrainer. The entrainer required a low flow of compressed gas, O2 at 1-2 L.min-1, and generated a low-pressure mixture of O2 and air which was directed through an Oxford miniature vaporizer, a non-return valve, and a widebore T-piece circuit. With this system we anaesthetized 24 patients with intermittent positive pressure ventilation (IPPV) and nine patients breathing spontaneously. During IPPV, the circuit resembled a T-piece and provided effective gas exchange with a FGF of 1.2 times minute ventilation. Inspiratory and expiratory valves were arranged so that the spontaneous mode was non-rebreathing, and FGF was adjusted to equal minute ventilation. The system was very economical, using 1-2 L.min-1 O2 and 20-25 ml.hr-1 liquid halothane to produce a FGF of 6-10 L.min-1, an FIO2 of 0.33, and FIhal of 1-1.5 per cent. We have demonstrated that this is a versatile, safe, and economical system, compatible with the practice of modern inhalational anaesthesia under field conditions. It can be readily assembled from commercially available components.     

Effect of Analgesia on Respiratory Muscle Function after Upper Abdominal Surgery

The effects of three methods of analgesia (intravenous morphine, epidural lidocaine and epidural morphine) on vital capacity (VC), forced expiratory volume in 1 s (FEV₁) and maximal expiratory and inspiratory pressures (MEP and MIP) at the mouth were studied in 12 high respiratory risk patients following upper abdominal surgery. VC, FEV¹, MEP and MIP markedly decreased following laparotomy. VC and FEV₁, were partially restored by epidural analgesia and remained unchanged following intravenous morphine. MEP and MIP remained unchanged after each of the three methods of analgesia. This suggests the existence of a non-analgesic dependent dysfunction of inspiratory and expiratory muscles following upper abdominal surgery.     

Le circuit-filtre en mode semi-fermé

In a semi-closed circle system, the inspiratory and expiratory limbs are completely separated and part of the patient's expired air recirculates. CO2 rebreathing is prevented by CO2- absorption with soda lime, which is always incorporated in such a circle. The inspiratory and expiratory valves ensure that gas flow is unidirectional and also prevent rebreathing, even at tidal volumes of 10 ml and ventilation frequencies of 60 c . min-1. This circuit can be used as an universal anaesthetic system for all age groups, simply by changing the hoses and connecting pieces. The values of expiratory resistance are within the recommended limits of the ISO; prewarming and humidification of the inspiratory gas mixture are sufficient without additional equipment. Standard monitoring of the circuit such as measurement of inspiratory O2 concentration and ventilation pressure, including a disconnection alarm, can be used for all age groups; spirometry or end-tidal CO2 measurements ensure normoventilation. The fresh gas flow required in a semi-closed circle system is about 2-4 1 . min-1, so that costs and environmental contamination with anaesthetic gases are relatively low in comparison with a semi-open system.     

Respiratory waveform and rebreathing in T-piece circuits: a comparison of enflurane and halothane waveforms

The effects of respiratory waveform on rebreathing in a modified Mapleson D circuit were studied in 18 healthy adult patients anesthetized with either enflurane or halothane. At high fresh gas flow (FGF) rate, when no rebreathing of CO2 occurred, the duration of inspiration (Ti) with enflurane was 41 per cent greater than that with halothane. With enflurane there was a characteristic long end-expiratory pause, 0.69 s, whereas with halothane it was only 0.196 s. The mean inspiratory flow rate (Vt/Ti) was higher (224 ml/s) when halothane was used than with enflurane (187 ml/s). When the FGF rate was reduced to 100 ml/kg/min in the modified Mapleson D circuit, patients breathing halothane had increases in minute volumes (VE) in response to increases of 53-75 per cent in inspired volumes of CO2. The increases in VE resulted fro increases in Vt/Ti of 34-38 per cent. The volume of CO2 inspired when enflurane was used did not increase until FGF rate was as low as 70 ml/kg/min. The reduced rebreathing was related to the respiratory waveform. The advantage of reduced rebreathing with enflurane is counter-balanced by the more profound respiratory depression it causes. The FGF needed to abolish rebreathing of CO2 is highly variable, and is dependent on respiratory waveform.     

EARLY DETECTION OF "REBREATHING" IN AFFERENT AND EFFERENT RESERVOIR BREATHING SYSTEMS USING CAPNOGRAPHY

Capnography was used to determine the onset rebreathing in afferent(AR) and efferent (ER) reservoir breathing systems in a spontaneousventilation lung model. In the case of the Lack and enclosedAR systems, the best sampling site was found to be in the exhaustlimb of the systems, 5 cm from the Y connector. For the Magillsystem, fitted with a hooded scavenging valve, the best sitewas deep inside the hooded valve. In contrast, the best samplingsite in an ER system (e.g. Bain system) was in the trachealtube. For AR systems, the loss of a fresh gas elimination pattern(carbon dioxide trace failing to reach zero) was shown to occurat the onset of rebreathing. As the sampling site was moveddistally into the exhaust limb, the same pattern was seen atgreater flow rates-that is, before rebreathing was actuallyoccurring. When sampling was within the tracheal tube, usingER systems, a typical "rebreathing wave" occurred at the onsetof established rebreathing.