Effects of Pulsatile Flow on Gas Transfer of Membrane Oxygenator: MENOX EL-4000 and Gyro C1-E3 Pulsatile Mode

Abstract: It is acknowledged that pulsatile flow enhances the gas exchange performance of membrane oxygenators. However, the data for currently developed oxygenators are limited. In this study, the effect of pulsatile flow was assessed utilizing the MENOX EL-4000 oxygenator. The in vitro test was performed following the Association for the Advancement of Medical Instrumentation (AAMI) standards. Pulsatile flow was produced by the Gyro C1-E3 centrifugal pump with periodical changing of the impeller speed. In Study 1, the following 3 groups were created and examined: nonpulsatile flow, pulsatile flow of 40 bpm, and pulsatile flow of 60 bpm. The blood flow rate was maintained at 3 LImin, and the VIQ ratio was I. In Study 2, four groups were examined, nonpulsatile flow with V/Q = 1, nonpulsatile with V/Q = 2, pulsatile with VIQ = 1, and pulsatile with V/Q = 2. The blood flow rate was maintained at 4 LImin, and the pulse frequency was set at 40 bpm. In study 1, although 0, transfer was not enhanced. CO₂, transfer was significantly improved (40–50%) by pulsatile flow, regardless of pulse frequency. Study 2 demonstrated that pulsatile flow resulted in improved CO₂ transfer as did higher ventilation (VIQ = 2). Furthermore, even after applying higher ventilation, the pulsatile mode enhanced CO₂ transfer more than the nonpulsatile mode. It was considered that the pulsatile mode induced an active secondary flow and enhanced mixing effects, and consequently CO₂ transfer was improved. In conclusion, the pulsatile flow significantly enhanced the CO₂ transfer of the MENOX oxygenator. It is indicated that applying the pulsatile mode is a unique and effective method to improve the gas exchange performance for a current membrane oxygenator.     

Silicone-Coated Polypropylene Hollow-Fiber Oxygenator: Experimental Evaluation and Preliminary Clinical Use

Background. A membrane oxygenator consisting of a microporous polypropylene hollow fiber with a 0.2-μm ultrathin silicone layer (cyclosiloxane) was developed. Animal experimental and preliminary clinical studies evaluated its reliability in bypass procedures.

Methods. Five 24-hour venoarterial bypass periods were conducted on dogs using the oxygenator (group A). In 5 controls, bypass periods were conducted using the same oxygenator without silicone coating (group B). As a preliminary clinical study, 14 patients underwent cardiopulmonary bypass with the silicone-coated oxygenator.

Results. Eight to 16 hours (mean, 12.2 hours) after initiation of bypass, plasma leakage occurred in all group B animals, but none in group A. The O₂ and CO₂ transfer rates after 24 hours in group A were significantly higher than at termination of bypass in group B (p < 0.005 and p < 0.03, respectively). Scanning electron microscopy of silicone-coated fibers after 24 hours of bypass revealed no damage to the silicone coating of the polypropylene hollow fibers. In the clinical study, the oxygenator showed good gas transfer, acceptable pressure loss, low hemolysis, and good durability.

Conclusions. This oxygenator is more durable and offers greater gas transfer capabilities than the previous generation of oxygenators.     

Sweep Gas Flowrate and C02 Exchange in Artificial Lungs

Abstract A simple analysis and graphic result are presented for characterizing the dependence of CO₂ exchange on the sweep gas (ventilating gas) flowrate in artificial lungs. The analysis requires no knowledge of the device-specific mass transfer characteristics of an artificial lung, nor does it require detailed mathematical modeling or computer simulation. Rather, it uses appropriate normalization to establish generic features of the gas flow dependency of CO₂ exchange that are applicable to all artificial lung devices. Principal results are that the transition from relatively gas flow-sensitive to gas flow-insensitive CO₂, exchange occurs at sweep gas flowrates of approximately 40–60 times the CO₂ exchange rate. Achieving a CO₂ exchange rate within 85% of maximal (for a given oxygenator and blood-side conditions) requires a sweep gas flowrate of no less than approximately 50 times the nominal CO₂ exchange rate. When the sweep gas flowrate is less than 20 times the CO₂ exchange rate, CO₂ exchange is highly gas flow dependent and less than one-half the maximal possible rate.     

Initial In Vitro Evaluation of a Pediatric Vortex-Mixing Membrane Lung

Abstract: A new design for a pediatric membrane lung is described in this paper. The lung consists of eight blood compartments, each having six U-shaped blood channels, with microporous PTFE membranes supported on rigid plates in such a way that the membranes form furrowed blood channels. Two rolling diaphragm pumps are attached to the open ends of the U-shaped blood channels; these pumps are operated in antiphase. Mean flow is provided by a roller pump placed at the inlet end of the membrane lung. Pulsatile blood flow within the blood channels produces successive vortex formation and ejection, leading to good blood mixing and high efficiency in gas transport. The design of the rolling diaphragm piston pumps ensures that the blood prime volume is low (280 ml), and the grouping of the pumps at one end of the oxygenator allows the driving mechanism to be simple and compact. The relatively wide blood channels (minimum width 0.5 mm) and vortex mixing make priming the membrane lung particularly easy. The membrane area is 0.39 m². Preliminary performance testing of the pediatric membrane lung was undertaken by pumping blood around a circuit containing a roller pump, the membrane lung, and a bubble oxygenator (to adjust the blood gases at the inlet to the membrane lung). In five such experiments it was shown that the membrane lung transferred 80 ml O₂/min and 120 ml CO₂/min at a blood flow rate of 1.5 L/min.     

From the spinning disc to the membrane oxygenator for open-heart surgery

Gibbon's rotating cylinder could not be enlarged to oxygenate an animal larger than a cat. The spinning disc oxygenator, introduced in 1947, had the capacity to perfuse a dog and the potential to increase oxygenation capacity by addition of more discs. When centers began to do three to four open-heart operations per day, the disposable bubble oxygenator was more practical. Bubble size was optimized to decrease the flow of oxygen relative to the blood flow and reduce trauma to blood. The bubble oxygenator is the type most commonly used today. Use of deep hypothermia with whole blood at an esophageal temperature of 10 degrees C was initially complicated by brain damage due to aggregation of white blood corpuscles and platelets. The introduction of hemodilution permitted safe utilization of hypothermic perfusion. Perfusion of infants should not be carried out at hematocrit below 25 ml/100 m. Early membrane oxygenators used nonporous silicone, or modified silicone membranes. High priming volumes, high pressure drop and marginal gas transfer efficiency characterized these devices. Recent advances in membrane technology have spawned a new generation of membrane oxygenators utilizing microporous polypropylene. In these new oxygenators, with either microporous hollow fibers or sheet membrane, the gas transfer characteristics are far superior to those of types produced in the past. The hollow-fiber devices typically have larger surface areas and higher pressure drop than in the new state-of-the-art flat plate models. An evaluation of one of these new-generation membrane oxygenators gave optimal oxygen and carbon dioxide exchange at a gas flow of 1 l/min of 60% oxygen in air at 30 degrees C and 2 l/min of 80% oxygen in air at normal temperature and rewarming for an adult. Today, after almost 40 years of oxygenator development, these new membrane device can offer better platelet preservation and reduced blood trauma as compared with types developed in the past. The new membrane oxygenators are fast becoming the preferred choice for use in infants and in protracted perfusion.     

Intravascular Membrane Oxygenation and Carbon Dioxide Removal with IVOX: Can Improved Design and Permissive Hypercapnia Achieve Adequate Respiratory Support During Severe Respiratory Failure?

Abstract: The intravenacaval oxygenator and carbon dioxide removal device (IVOX) conceived by Mortensen at CardioPulmonics is a diffusion–limited device capable of removing 30% of CO₂ production of an adult at normocapnia with minimal reduction in ventilator requirements. Through mathematical modeling, an ex vivo venovenous bypass circuit to model the vena cava and animal models of severe smoke inhalation injury, the practice of permissive hypercapnia has been established to enhance CO₂ removal by IVOX. By allowing the blood PCO₂ to rise gradually, the CO₂ excretion by IVOX can be linearly increased in a 1: 1 relationship. Experimental and clinical studies have shown that CO₂ removal by IVOX increased from 30–40 ml/min at normal blood PCO₂ to 80–90 ml/min at PCO₂ of 90 mm Hg. In addition, IVOX with permissive hypercapnia allowed a significant reduction in minute ventilation and peak airway pressure. Design changes could also improve the performance of IVOX. Increased surface area and mixing with more fibers and crimping in new prototypes of IVOX significantly increased CO₂ removal and oxygen transfer. Active mixing in the blood to decrease the boundary layer resistance can further enhance gas exchange of IVOX. In conclusion, gas exchange by the current design of IVOX is limited, and improvements in design are needed for it to become a more clinically applicable device. Permissive hypercapnia can significantly enhance CO, removal by IVOX as well as significantly reduce ventilator requirements.     

Development of an Intravascular Pumping Oxygenator Using a New Silicone Membrane

Abstract: A new intravascular pumping oxygenator (IVPO) was developed for intravascular gas exchange and circulatory assistance in critically ill patients with respiratory and circulatory failure. The IVPO utilizes new silicone hollow fibers (diameter. 1 mm: membrane width, 50 μm) and consists of two driving tubes for the oxygenation and pumping of circulating blood. The performance characteristics of the IVPO were studied using an experimental ex vivo model. With a mean hemoglobin concentration of 10.5 ± 2.3 g/dl, total oxygen transfer was 5.6 ± 1.5 ml/min at a blood flow of 200 ml/min and 6.3 ± 2.2 ml/min at a blood flow of 250 ml/min. Total CO₂ transfer was 3.8 ± 1.4 ml/min at a blood flow of 200 ml/min and 4.2 ± 1.6 ml/min at a blood flow of 250 ml/min during IVPO pumping. This preliminary experiment demonstrated that the IVPO has the capacity to function both as a circulatory assist pump and as an intravascular hollow fiber oxygenator.     

Halothane Anaesthesia with Spontaneous Respiration for Tonsillectomy in Children

Bain's anaesthetic circuit was used in 22 children undergoing tonsillectomy under halothane anaesthesia with spontaneous respiration. End-tidal CO₂ was monitored by capnography. The median maximum end-tidal CO₂ was 7%, and during surgery nine patients had an end-tidal CO₂ higher than 7%, corresponding to a Paco₂ close to 8 kPa when the arterial to end-tidal CO₂ difference is taken into consideration. Increase in fresh gas flow or change to a non-rebreathing system had virtually no effect on end-tidal CO₂. However, following discontinuation of halothane or during controlled respiration, acceptable values of end-tidal CO₂ were reached, leading to the conclusion that respiratory depression was responsible for the high values of end-tidal CO₂ rather than properties of Bain's circuit or too low gas flow rates.     

A Newly Developed Silicone-Coated Membrane Oxygenator for Long-Term Cardiopulmonary Bypass and Cardiac Support

Abstract: The surface of polypropylene hollow fiber was successfully coated with a very thin (0.2 μm) silicone layer. Experimental studies were performed in long-term (6 h) normothermic cardiopulmonary bypass (CPB) using 10 goats. A conventional membrane oxygenator (Mera Exce-lung HPO-lSH, MERA, Tokyo, Japan) was used for 5 goats as a control (Group C) and a new silicone-coated membrane oxygenator, which is of the same construction as that of the one used for Group C, for 5 (Group S). The O₂transfer and CO₂removal functions showed the same ranges. In the other parameters, there were no differences between the 2 groups. As for hemolysis, however, the plasma free hemoglobin of Group S was lower than that of Group C. Currently, 3 chronic percutaneous cardiopulmonary support (PCPS) experimental models have been conducted, and there has been no evidence of thromboembo-lism or deterioration of the oxygenator. In conclusion, this new oxygenator is suitable not only for CPB, but also for long-term cardiac support.     

Spontaneous gas bubbling in microporous oxygenators

During operation of the microporous membrane oxygenators at some conditions, gas microbubbles penetrate into the blood. This effect, so-called spontaneous bubbling, takes place even when the blood pressure is higher than the gas pressure. This phenomenon was confirmed experimentally both in a model cell with hydrophobic microporous hollow fibers being used in the oxygenators and in in vitro tests on the actual microporous hollow fiber oxygenator. We proposed a mechanism of spontaneous gas bubbling into liquid that contains dissolved gases. Because of a partial pressure gradient, the dissolved gases and water vapors are transported from blood into the gas pore. This causes Stefans gas flow directed from the liquid-gas interface. Because of the high hydraulic resistance of the micropores, gas pressure at the meniscus increases up to gas bubbling. A mishandled priming of the oxygenator as well as the blood pressure pulsation caused by the roller pump operation contribute to spontaneous gas bubbling in the microporous oxygenators. The flow and pressure in the hydrophobic pores were calculated for various gases.     

Gas Transfer Performance of a Hollow Fiber Silicone Membrane Oxygenator: Ex Vivo Study

Based on the results of in vitro studies of many experimental models, a silicone hollow fiber membrane oxygenator for pediatric cardiopulmonary bypass (CPB) and extracorporeal membrane oxygenation (ECMO) was developed using an ultrathin silicone hollow fiber with a 300 microm outer diameter and a wall thickness of 50 microm. In this study, we evaluated the gas transfer performance of this oxygenator simulating pediatric CPB and ECMO conditions. Two ex vivo studies in a pediatric CPB condition for 6 h and 5 ex vivo studies in an ECMO condition for 1 week were performed with venoarterial bypass using healthy calves. At a blood flow rate of 2 L/min and V/Q = 4 (V = gas flow rate, Q = blood flow rate) (pediatric CPB condition), the O2 and CO2 gas transfer rates were maintained at 97.44 +/- 8.88 (mean +/- SD) and 43.59 +/- 15.75 ml/min/m2, respectively. At a blood flow rate of 1 L/min and V/Q = 4 (ECMO condition), the O2 and CO2 gas transfer rates were maintained at 56.15 +/- 8.49 and 42.47 +/- 9.22 ml/min/m2, respectively. These data suggest that this preclinical silicone membrane hollow fiber oxygenator may be acceptable for both pediatric CPB and long-term ECMO use.     

In Vivo Uptake and Elimination of Isoflurane by Different Membrane Oxygenators during Cardiopulmonary Bypass

Background: Volatile anesthetics are frequently used during cardiopulmonary bypass (CPB) to maintain anesthesia. Uptake and elimination of the volatile agent are dependent on the composition of the oxygenator. This study was designed to evaluate whether the in vivo uptake and elimination of isoflurane differs between microporous membrane oxygenators containing a conventional polypropylene (PPL) membrane and oxygenators with a new poly-(4-methyl-1-pentene) (PMP) membrane measuring isoflurane concentrations in blood.

Methods: Twenty-four patients undergoing elective coronary bypass surgery with the aid of CPB were randomly allocated to one of four groups, using either one of two different PPL-membrane oxygenators for CPB or one of two different PMP-membrane oxygenators. During hypothermic CPB, 1% isoflurane in an oxygen-air mixture was added to the oxygenator gas inflow line (gas flow, 3 l/min) for 15 min. Isoflurane concentration was measured in blood and in exhaust gas at the outflow port of the oxygenator. Between-group comparisons were performed for the area under the curve (AUC) during uptake and elimination of the isoflurane blood concentrations, the maximum isoflurane blood concentration (Cmax), and the exhausted isoflurane concentration (FE).

Results: The uptake of isoflurane, expressed as AUC of isoflurane blood concentration and a function of FE, was significantly reduced in PMP oxygenators compared to PPL oxygenators (P < 0.01). Cmax was between 8.5 and 13 times lower in the PMP-membrane oxygenator groups compared to the conventional PPL-membrane oxygenator groups (P < 0.01).     

In vivo uptake and elimination of isoflurane by different membrane oxygenators during cardiopulmonary bypass

BACKGROUND: Volatile anesthetics are frequently used during cardiopulmonary bypass (CPB) to maintain anesthesia. Uptake and elimination of the volatile agent are dependent on the composition of the oxygenator. This study was designed to evaluate whether the in vivo uptake and elimination of isoflurane differs between microporous membrane oxygenators containing a conventional polypropylene (PPL) membrane and oxygenators with a new poly-(4-methyl-1-pentene) (PMP) membrane measuring isoflurane concentrations in blood. METHODS: Twenty-four patients undergoing elective coronary bypass surgery with the aid of CPB were randomly allocated to one of four groups, using either one of two different PPL-membrane oxygenators for CPB or one of two different PMP-membrane oxygenators. During hypothermic CPB, 1% isoflurane in an oxygen-air mixture was added to the oxygenator gas inflow line (gas flow, 3 l/min) for 15 min. Isoflurane concentration was measured in blood and in exhaust gas at the outflow port of the oxygenator. Between-group comparisons were performed for the area under the curve (AUC) during uptake and elimination of the isoflurane blood concentrations, the maximum isoflurane blood concentration (C(max)), and the exhausted isoflurane concentration (F(E)). RESULTS: The uptake of isoflurane, expressed as AUC of isoflurane blood concentration and a function of F(E), was significantly reduced in PMP oxygenators compared to PPL oxygenators (P < 0.01). C(max) was between 8.5 and 13 times lower in the PMP-membrane oxygenator groups compared to the conventional PPL-membrane oxygenator groups (P < 0.01). CONCLUSIONS: The uptake of isoflurane into blood via PMP oxygenators during CPB is severely limited. This should be taken into consideration in cases using such devices.     

Development of a New Hollow Fiber Silicone Membrane Oxygenator: In Vitro Study

An experimental silicone hollow fiber membrane oxygenator for long-term extracorporeal membrane oxygenation (ECMO) was developed in our laboratory using an ultrathin silicone hollow fiber. However, the marginal gas transfer performances and a high-pressure drop in some cases were demonstrated in the initial models. In order to improve performance the following features were incorporated in the most recent oxygenator model: increasing the fiber length and total surface area, decreasing the packing density, and modifying the flow distributor. The aim of this study was to evaluate the gas transfer performances and biocompatibility of this newly improved model with in vitro experiments. According to the established method in our laboratory, in vitro studies were performed using fresh bovine blood. Gas transfer performance tests were performed at a blood flow rate of 0.5 to 6 L/min and a V/Q ratio (V = gas flow rate, Q = blood flow rate) of 2 and 3. Hemolysis tests were performed at a blood flow rate of 1 and 5 L/min. Blood pressure drop was also measured. At a blood flow rate of 1 L/min and V/Q = 3, the O2 and CO2 gas transfer rates were 72.45 +/- 1.24 and 39.87 +/- 2.92 ml/min, respectively. At a blood flow rate of 2 L/min and V/Q = 3, the O2 and CO2 gas transfer rates were 128.83 +/- 1.09 and 47.49 +/- 5.11 ml/min. Clearly, these data were superior to those obtained with previous models. As for the pressure drop and hemolytic performance, remarkable improvements were also demonstrated. These data indicate that this newly improved oxygenator is superior to the previous model and may be clinically acceptable for long-term ECMO application.     

Similarity of Clinical and Laboratory Results Obtained with Microporous Teflon Membrane Oxygenator and Bubble-Film Hybrid Oxygenator

For 80 elective clinical cardiopulmonary bypasses we alternately used either a commercial microporous Teflon membrane oxygenator or a commercial hybrid bubble-film oxygenator. Setup time was a little longer with the membrane unit (20 minutes), but priming volume (2,250 ml) was the same. No problems were encountered with the hybrid oxygenator. However, despite our monitoring of additional variables, including shim and inlet pressure and recirculation flow, gas exchange abnormalities were encountered in 5 patients on whom the membrane oxygenator was used; in 4 of these cases the abnormalities were encountered prior to our recognition of the potential for occasional internal shunting with this device.There were no hospital deaths. When the two groups, matched except for oxygenator selection, were compared, there were no significant differences clinically or hematologically. For cardiopulmonary bypass of 2 hours or less, both oxygenators studied are definite improvements over previous silicone membrane and high-gas-flow bubble oxygenators. However, lower cost and reduced complexity favor the hybrid oxygenator.     

Development of an All-in-One Percutaneous Cardiopulmonary Support System

Abstract: This paper deals with development of an all-inone percutaneous cardiopulmonary support (PCPS) system. In recent years, PCPS has been used for the treatment of acute myocardial infarction. A prototype of a compact all-in-one PCPS system was developed. The system contains a centrifugal pump and an extra-capillary flow-type membrane lung in one body. The system has a priming volume of 250 ml, which allows for PCPS with no additional blood. The in vitro tests and an ex vivo test were conducted. The system produces 1.6–5 L/min of flow in the experiments. The O₂ transfer rate was 310 ml/min, and the CO₂ transfer rate was 300 ml/min at a blood flow rate of 5 L/min. This device is compact, requires less priming volume than a standard system, and is easy-to-handle in the experiments. The system is considered applicable to percutaneous cardiopulmonary support.     

Nitric oxide administration after the ventilator: evaluation of mixing conditions

Background: Because of the potential toxicity of nitric oxide (NO) and its oxidising product nitrogen dioxide (NO₂), any system for the delivery of inhaled NO must aim at stable and predictable levels of NO and as low concentrations as possible of NO₂.
Methods: In a laboratory set-up, we have evaluated mixing conditions in a system where NO is added after the ventilator with continuous flow. Mixing was studied by using carbon dioxide (CO₂) as a tracer gas since capnography has a short response time (360 ms) in comparison with measurements of NO with electrochemical fuel cells (response time of 18s). CO₂ (in volumes corresponding to an ideal mixture of 1,3 and 6%) was fed, after the ventilator, either into plain breathing tubing, into one or two soda lime absorbers, or into an empty and a soda lime-filled canister, at different ventilatory rates and different I: E ratios. Samples were drawn from the inspiratory limb close to the Y-piece. NO was added in the same way and in the same volume as the highest concentration of CO₂.
Results: CO₂ added to plain tubing resulted in peak levels up to five times the set levels, while addition to a mixing box with an empty and a soda lime-filled canister resulted in even mixing with gas concentrations close to the ideal. When NO was fed into plain tubing, low levels were measured at the Y-piece, indicating poor mixing. Gas supply to a mixing chamber resulted in even concentrations.
Conclusions: Even and predictable levels of NO can be obtained with continuous flow of NO to the inspiratory limb, after the ventilator, if a mixing chamber is used. To obtain adequate mixing, the volume of the mixing box should be greater than the tidal volume.     

Development of a new silicone membrane oxygenator for ECMO.

Two types small and efficient ECMO oxygenators were developed utilizing the most up to date hollow fiber technology. Newly silicone hollow fibers possess sufficient mechanical strength while maintaining ultra thin walls of 50 micro meter. Two types of oxygenators were made with this fiber. The fiber length for the type 1 module is 150mm with a priming volume 194 cc (surface area 1.3 m(2)) and type 2 has a fiber length of 100 mm with a 144 cc priming volume (the surface area 0.8 m(2)). The studies were performed at 0.5, 1.0 and 2.0 L/min of blood flow and these oxygenators demonstrated. O(2) gas transfer rate of 69+/-4 ml/min/L for type 1 and 68+/-6 ml/min/L for type 2. The CO(2) gas transfer rate was 25+/-2 ml/min/L for type 1 and 32+/-2 ml/min/L for type 2. These results demonstrate type 2 oxygenator has similar gas exchange capabilities to those of Kolobows' oxygenator which has about 2.0 times larger surface area. Additionally, comparative hemolysis tests were preformed with this new oxygenator and the Kolbow. The NIH value was 0.006 (g/100 L) for the type 1 oxygenator and 0.01 (g/100 L) for the Kolbow oxygenator. These results suggested that this ECMO oxygenator had sufficient gas exchange performance in spite of being smaller and induced minimal blood damage.     

Reliability of CO2 measurements from the airway by a pharyngeal catheter in unintubated, spontaneously breathing subjects

Although several short communications have appeared describing attempts to record the concentrations of carbon dioxide (CO₂) from the unintubated airway by a catheter placed in the nose, so far only few reports have documented the reliability of the method. To evaluate the reliability of CO₂ measurements by a catheter in the open, unintubated airway during spontaneous respiration, a 12 CH PVC catheter was forwarded through the nostril to the hypopharynx and connected to a capnograph in nine healthy volunteers. Another capnograph was connected to a tightly fitting face mask and simultaneous CO₂ recordings were attained from the two parts of the airway during normoventilation, hyperventilation and rebreathing. A corresponding blood sample was drawn from the radial artery for blood gas analysis. The configurations of the capnograms recorded from the pharyngeal catheter were similar to those recorded from the face mask. The results were analysed by a multifactor analysis of variance. The carbon dioxide tension ( p CO₂) was significantly influenced by degree of ventilation ( P <0.0001), subject ( P <0.0001), measurement site ( P =0.030) and interaction subject-ventilation ( P =0.015). In spite of the significant influence of the measurement site, the difference between end tidal carbon dioxide tension ( P CO₂(ET)) and carbon dioxide tension in arterial blood ( P CO₂(a)) was small. The mean differences between paired measurements ( p CO₂(ET)- p CO₂(a)) were -0.10 kPa±0.41 kPa (mean±SD) for the catheter and -0.20 kPa ±0.43 kPa for the face mask. The study demonstrates that reliable recordings of CO₂ concentrations during spontaneous respiration can be obtained by a thin catheter positioned in the hypopharynx.     

The Venturi Anaesthesia Circuit II Carbon Dioxide Production and Gas Flow Requirements

Carbon dioxide production was measured in 20 adult patients undergoing alloplastic operation of the hip. Body weight ranged from 40 to 81 kg. Anaesthesia consisted of lumbar plexus block, i. v. diazepam, pethidine, pavulon and N₂O/O₂ under controlled ventilation. CO₂ production was 2.13 ml kg^-1 min^-1 (interquartile range 2.09-2.23). A fresh gas flow rate of about 30 ml kg^-1 min^-1 was required for the elimination of CO₂ produced when using the Venturi system for inhalation anaesthesia.