MODIFICATION OF THE BIRD VENTILATOR FOR USE DURING SPONTANEOUS RESPIRATION

By incorporating an Ayre T-piece system into the circuit forthe Bird ventilator, efficient humidification and nebulizationof drugs can be provided for patients breathing spontaneously.This modification has proved very useful during "weaning" ofpatients from prolonged intermittent positive pressure ventilation,and other advantages of using this circuit are mentioned.     

OXYGEN FAIL-SAFE DEVICE FOR AN ANAESTHETIC APPARATUS

The criteria for an oxygen fail-safe device are designed toprevent the patient breathing an asphyxiating mixture if theoxygen supply should fail, be disconnected, or be turned off.Such a device should operate when the patient is breathing spontaneouslyor being artificially ventilated whatever the circuit arrangementmay be. A fail-safe device which fulfils these criteria is described.     

A METHOD OF STERILIZATION FOR THE EAST-RADCLIFFE VENTILATOR

A method is described in which Resiguard (a mixture of germicidesand a detergent) is cycled through the circuit of an East-Radcliffeventilator. Bacteriological studies show that this method adequatelysterilizes ventilators infected by patients and by deliberatecontamination with organisms. The results also suggest thatResiguard produces a prolonged surface antibacterial activityin the ventilator.     

A VAPORIZING SYSTEM FOR PROGRAMMED ANAESTHESIA

A vaporizing system for closed circuit "programmed" anaesthesiais described. Despite its location within the circuit, the vaporizercontrols directly the input of volatile anaesthetic agents irrespectiveof fluctuations in ventilation of the anaesthetized subject.It is interfaced easily with electronic controllers and hasan accuracy approaching 1.0% under laboratory conditions. Duringexperimental anaesthesia, it maintained stable end-tidal concentrationsof halothane at 1.2 MAC (the intended value) despite wide variationsin ventilation. Present address: Department of Anaesthetics, Alfred Hospital,Prahran, Victoria 3181, Australia     

A PREDICTOR FOR HALOTHANE CONCENTRATIONS DURING CLOSED-CIRCUIT ANAESTHESIA

Reasons of economy make the use of a circle circuit desirableduring halothane anaesthesia. The relationship between the concentrationinhaled by the patient and that delivered from the vaporizerwas expressed by Mapleson in an equation that is too complexfor routine solution in the operating theatre. A nomogram hasbeen prepared which solves this equation accurately enough forpractical purposes, provided assumptions of uptake rates atvarious times are made, and provided an assumption can be madeof how alveolar ventilation depends on total ventilation. Thishas been tested during 64 anaesthetics, more than 200 comparisonsof actual and predicted concentrations having been studied.The results are discussed, and it is suggested that Mapleson'sequation gives results sufficiently in agreement with thosefound that it might prove useful to have a quick method of solvingit in die theatre.     

A SIMPLE SYSTEM FOR ADMINISTERING INTERMITTENT MANDATORY VENTILATION (IMV) WITH THE OXFORD VENTILATOR

A circuit has been devised enabling intermittent mandatory ventilation(IMV) to be delivered from an Oxford ventilator. Two methodsfor delivering oxygen to the system are described. The circuitis constructed from inexpensive components and only one oxygenflowmeter is required. No modifications to the ventilator arenecessary.     

TEMPERATURE ALARM AND CUT-OUT SYSTEM FOR USE WITH HEATED WATER HUMIDIFIERS

The temperature of the inspired humidified gases of an intubatedpatient may be monitored accurately, close to the airway, bya commercially available thermistor probe and alarm/cut-outdevice for use with heated water humidifiers. The apparatusis designed to alarm when the inspired gas temperature exceedsthe pre-set temperature and when the thermistor probe is inopen or short circuit. When the temperature alarm is activated,the humidifier heater is switched off automatically, thus preventingprolonged transmission of excessively hot gases to the airway.The apparatus has been used successfully in this hospital forseveral months, requiring minimal observer attention and maintenance.     

A DEVICE FOR INTRODUCING A PHASE OF SUBATMOSPHERIC PRESSURE INTO MANUALLY PERFORMED IPPR

The apparatus is designed to introduce a phase of subtmosphericpressure into manually performed intermittent positive pressurerespiration. When the breathing bag of a closed circuit apparatusis compressed by the anaesthetist, the pressure produced compressesa weighted Connell bellows. When the pressure is released theweight causes a negative pressure of 10cm H₂O. The two sidesof the circuit are connected via a non-return value allowingthe breathing bag to be kept full in the ordinary way. A negativemean pressure can be produced by the use of this apparatus.     

A COMPUTER-CONTROLLED SYRINGE DRIVER FOR USE DURING ANAESTHESIA: A Modification of the Graseby MS16A

The requirements of a drug infusion device for use in theatreare discussed. A modification of the Graseby MS16A syringe pumpand an interface circuit permitting its remote control by micro-computerare described. The pump is controlled via a standard computerinterface (RS232) which makes it a unique and powerful researchtool. Aspects of safety are considered.     

A METHOD FOR MEASURING TIDAL VOLUME DURING HIGH FREQUENCY JET VENTILATION

A new method has been developed to measure tidal volume duringhigh frequency jet ventilation. A pneumotachograph is placedin the expiratory limb of the breathing circuit. Using a modellung, the results from this method did not differ from the tidalvolume calculated from pressure changes measured within themodel lung. The results from the two methods correlated well(r = 0.99). The method enables the gas volumes entrained andspilled out of the airway during inspiration, true l: E ratiosand mean driving pressures to be determined. The method is non-invasiveand could easily be applied clinically.     

AN ANAESTHETIC TECHNIQUE FOR ADENOTONSILLECTOMY IN CHILDREN

An anaesthetic technique for adenotonsillectomy, successfullyused in 650 children, is described. Premedication includes morphinein addition to trimeprazine and atropine. Following thiopentoneand suxamethonium-aided intubation, anaesthesia is maintainedby controlled hyperventilation with nitrous oxide and oxygenvia a modified Rees circuit, employing a combined T-piece andmodified Worcester connection, to simplify the use of the Doughtytongue-blade, and firm thermoplastic endotracheal tubes to resistblade compression. Continuity of relaxation is provided by intermittentinjections of suxamethonium. The authors' orotracheal assemblyis unobtrusive, secure and ensures a patent airway with minimaldeadspace. Airway resistance changes are readily appreciated.The technique makes for perfect airway control, relaxation forsurgical access, and rapid recovery. ^*A Short 16-mm film of this technique is avilable. Present address: Liverpool Royal Infirmary, Liverpool, Lancashire,England.     

Development of a completely closed circuit using an air filter in a drainage circuit for minimally invasive cardiac surgery

The completely closed circuit system is the future direction of cardiopulmonary bypass because of its compactness and superior biocompatibility. The most serious obstacle for clinical application is the sucking of air bubbles into the drainage circuit. The purpose of this study was to remove the air bubbles from the drainage circuit. Infusing 50 ml/min of air bubbles into the drainage circuit of the usual closed circuit, and infusing 50, 100, and 150 ml/min of air into the drainage circuit of a newly developed closed circuit (drainage circuit using an air filter), the number and size of air bubbles were observed at the outlet of the arterial filter. In the usual closed circuit, many air bubbles of over 40 microm were detected within 5 s at a blood flow of 4 L/min because the centrifugal pump decreased the size of the bubbles, which then passed through the oxygenator and arterial filter. Air bubbles of over 40 micro were not detected in the newly developed closed circuit within 5 min at a blood flow of 4 L/min. The removal of air mixed into the completely closed circuit was possible with a drainage circuit using an air filter that was developed. The clinical use of the completely closed circuit for minimally invasive cardiac surgery (MICS) became possible based on this development.     

A new hybrid anaesthetic circuit for a low–flow rebreathing technique

The coaxial Mapleson D (Bain) circuit is close to an ideal anaesthetic breathing circuit but it requires a high fresh gas flow (V_F). The CO₂ absorber circuit can be used with a low V_F but the patient end is clumsy and hyperventilation is common unless a small tidal volume (V_T) and a low respiratory frequency (f) is used. To overcome these problems a new hybrid anaesthetic circuit has been constructed. The new system has a slender coaxial patient end, connected to a Y–piece and a CO₂ absorber circuit with unidirectional valves. The V_F supply runs inside the coaxial tube to the patient end, similar to the Bain circuit. The new circuit was tested with simulated controlled ventilation in a lung model and compared to an absorber circuit, an absorber circuit with an added 300 ml dead space (V_D) and a Bain circuit. The fraction of end tidal CO₂ (F_ECO₂) was measured as a function of V_F With V_F 1 1 min^-1, V_T 10 ml kg^-1 and f 12 breaths min^-1 a simulated 70 kg patient had an F_ECO₂ of 4.8% and a 40 kg patient 5.5% with V_F 2 1–min^-1. At zero V_Fthe the new system was equal to an absorber circuit with 300 ml V_D. With increasing V_F the F_ECO₂ in the new circuit decrease towards the absorber circuit values. The new system combines the benefits of the Bain circuit with those of a low flow CO₂ absorber circuit and offers flexibility in the choice of combinations of V_D, f and V_F.     

Comparative study of biocompatibility between the open circuit and closed circuit in cardiopulmonary bypass.

The theoretical benefit of a centrifugal pump or heparin coating demonstrated through in vitro or in vivo studies is not recognizable in cardiopulmonary bypass (CPB) during chemical open heart surgery. The objective of this study was to investigate the influence of the interface of air and blood in current CPB with an open circuit system and its relative significance in relationship to the heparin dose and heparin coating. Using the same oxygenator and circuit, an open circuit and closed circuit CPB with the same priming volume were prepared for a 4 h perfusion experiment using diluted and heparinized (3.6 U/ml) fresh human blood. In these experiments, both heparin-coated and noncoated circuits were examined. Blood was sampled before and 2, 30, 60, 120, and 240 min after the start of perfusion, and the platelet and white blood cell counts and beta-thromboglobulin (beta-TG) and C3a levels were measured. The amount of adsorbed protein in the hollow fibers was also measured after retrieval. Although the results demonstrated significantly better biocompatibility of the heparin-coated circuit than the noncoated circuit, the difference between the open and closed circuits was unexpectedly small and insignificant with either the heparin-coated circuit or noncoated circuit. In contrast, the C3a level was higher in the closed circuit than the open circuit. However, the amount of adsorbed protein was markedly lower in the closed circuit (0.7 microgram/cm2) than in the open circuit (11.1 micrograms/cm2). An immunoblot of the adsorbed protein showed a higher density of fibrinogen bands and conversion to fibrin in the open circuit. We speculate that the lower blood C3a level in the open circuit suggests that C3a was taken in by the adsorbed protein. In conclusion, analysis of the adsorbed protein indicates the lower biocompatibility of the open circuit. Similar experiments with less heparin use and more severe conditions will be necessary to elucidate the essential benefit of making a CPB closed circuit.     

Novel Extracorporeal Dialysis Circuit to Improve the Removal Efficiency of High and Middle Molecular Weight Toxins

A new extracorporeal circuit for hemodialysis was designed with the goal of improving the middle and high molecular weight toxins removal. A recirculation pathway was added to the hemodialysis circuit and relevant pressure regulation was performed along the circuit in order to keep the ultrafiltration rate as zero. The influence of increasing the recirculation to dialysate flow rate ratio on the removal of urea, vitamin B12, and hemoglobin was investigated. This removal was also modeled by an analytical method and solved by MATLAB software. A significant increase in removal of vitamin B12 (34%) and especially hemoglobin (228%) was achieved using the recirculation flow in an adjusted hemodialysis circuit. The model showed an acceptable agreement with the experimental results which shows its applicability for prediction of different toxin removal in this circuit.     

Flow resistance of coaxial breathing systems: investigation of a circuit disconnect

A clinical incident involving an undetected disconnection occurred during the use of a CPRAM coaxial breathing circuit. The flow resistance of this circuit was evaluated and compared with that of a Bain circuit to determine the factors involved. A differential pressure transducer was used to monitor the pressure drop across each circuit during simulation of controlled ventilation with a fresh gas flow of 6 L.min-1. An Ohio V5 anaesthesia ventilator was adjusted to different flow rates and tidal volumes and a test lung simulated an airway resistance of 2.7 cm H2O.L-1.sec and a compliance of 0.05 L.cmH2O-1. Absolute pressure at the ventilator was also monitored when the circuit was disconnected from the test lung. The CPRAM circuit displayed a pressure drop from the ventilator to the endotracheal tube of 6.3 cm H2O at 60 L.min-1, about twice that found with the Bain. A disconnection at the tracheal connector produced an absolute pressure at the ventilator of 9.6 cm H2O with the CPRAM and 5.5 cm H2O with the Bain. Since the ventilator low pressure alarm was preset to 9.2 cm H2O, the alarm provided a warning with the Bain but not the CPRAM. The elevated flow resistance of the CPRAM circuit was attributed to a restriction in the flow area at the patient end of the circuit. Capnographs or adjustable low-pressure alarms provide more reliable monitoring for breathing circuit disconnects.     

Anesthetic breathing circuit obstruction mimicking severe bronchospasm: An unusual manufacturing defect

We report an unexpected ventilation difficulty with an anesthetic breathing circuit in a pediatric patient receiving left herniorrhaphy. A manufacturing defect in a limb of the anesthetic breathing circuit caused this problem. This defect induced a high-pitched, wheezing-like sound, which was difficult to differentiate from a hyper-reactive airway, commonly seen in pediatric patients with recent upper respiratory tract infection. We recommend that the patency of the anesthetic breathing circuit should routinely be examined before connecting it to the anesthesia machine.     

The effect of the bain circuit on gas exchange

This is an experimental and theoretical analysis of the Mapleson D (Bain) circuit. A bench model was used to determine the effects of breathing rate, tidal volume, and fresh gas flow on the simulated alveolar gas composition when a commercial Bain breathing circuit is used. in addition, an effort was made to derive mathematical equations that describe the CO₂-profile in the expiratory limb of the Bain circuit, the amount of CO₂ rebreathed, and the effect of this rebreathing on the alveolar gas composition. Data obtained with the bench model and with the equations were compared to data from the literature. The effect of the Bain circuit on gas exchange was compared to that of an equivalent dead space.     

A COMPARISON OF FLUSHING SOLUTIONS FOR LIVER PROCUREMENT USING AN ISOLATED PERFUSED PORCINE MODEL

There is no agreement on the best technique of in situ flushing of livers prior to storage. In order to study this, porcine livers were stored in the University of Wisconsin (UW) solution for 18h and then assessed using an isolated perfused porcine model. Livers flushed in situ with UW solution were compared to livers flushed with a non-preservation solution (Hartmann's solution). No statistically significant differences could be found in bile production (18.7 ± 4.4 vs 17.9 ± 3.8 mL/1000g per 2h), aspartate amino-transferase (AST) levels in the perfusate after 2h of isolated perfusion (687 ± 101 vs 724 ± 114U/L), potassium levels in the perfusate after 2h on the circuit (5.4 ± 1.5 vs 5.5 ± 2.3mmol/L), weight gain (15.2 ± 3.7 vs 17.1 ± 4.0%) or platelet sequestration (41.6 ± 11.7 vs 37.4 ± 9.8%) between livers flushed with UW solution as opposed to those flushed with Hartmann's solution, respectively. Of overriding importance was the solution in which the liver was stored, reconfirming the superiority of UW solution over an extracellular solution for preservation. If extrapolated to the clinical situation, these findings would have substantial cost-saving implications.     

Air contamination of a closed anesthesia circuit

Closed-circuit anesthesia (CCA) has certain advantages such as decreased cost, decreased anesthetic gas pollution, improved in-halational gas humidity and temperature in comparison to conventional inhalational anesthesia using a high fresh gas flow, i.e. more than 2 L. min^-1, with a semi-closed breathing circuit. The main disadvantage of CCA is the possibility of hypoxic anesthetic gas delivery. This potentially lethal situation is caused by an insufficient oxygen flow rate for the body metabolism or by the accumulation of inactive gas, usually nitrogen, within the breathing circuit in spite of a sufficient oxygen concentration in the fresh gas supply to the breathing circuit. In the latter case, the accumulation of inactive gas may also lead an increased risk of awareness because of its dilution effect on the concentrations of inhalational anesthetics. We herein present a case of air contamination of the breathing circuit through a sampling line of an anesthetic gas monitor. The air caused a decrease in the oxygen concentration during closed circuit anesthesia.