Medical gases

Understanding the complex process of production, storage and delivery of medical gases is vitally important to ensure safe and efficient practice by anaesthetists. This article discusses the medical gases commonly used in anaesthesia and intensive care and details the journey of the commonly used medical gases from production to patient delivery. It includes core knowledge for the FRCA.     

Medical gases

Understanding the complex process of production, storage and delivery of medical gases is vitally important to ensure safe and efficient practice by anaesthetists. This article discusses the medical gases commonly used in anaesthesia and intensive care, and details the journey of the commonly used medical gases from production to patient delivery. It includes core knowledge for the FRCA.     

Nitric oxide: description of a pipeline delivery system

We describe a pipeline system suitable for the delivery of nitric oxide gas to an 18-bed intensive care unit. The pipeline was developed and installed according to the current UK regulations HTM 2022, which relates to the supply of piped medical gases. Where HTM 2022 did not specify the appropriate standard, we consulted widely to achieve a safe solution. We continue to monitor all aspects of the performance of the pipeline to ensure safe operating practices and recommend changes to the standards.     

A new humidifier.

A new heated water-bath humidifier operates on a new basic principle which overcomes the practical disadvantages of existing systems. A heated hose is used to control a temperature drop along the whole length of the delivery line instead of raising the temperature of the gases in the delivery line as in previous systems. Therefore the tank does not have to fully saturate the gases and a simple, totally cleanable design is possible. A temperature sensor at the delivery point controls the hose heater, constituting a rapid response, low thermal inertia system and delivery temperature is displayed on the unit. Compensation for varying gas flows and ambient temperature changes is inherent and the unit is suitable for neonatal/paediatric and adult use without special adjustment. The tank has a separate reservoir chamber which feeds an evaporation chamber via a cleanable float valve, conferring many advantages including clearly visible water levels, easy filling without breaking the circuit and constant compression volume. Full fail-safe alarm systems are incorporated.     

A SYSTEM OF DIFFERENTIAL DELIVERY PRESSURES AND HOSE DIAMETERS FOR USE IN HOSPITAL MEDICAL GAS INSTALLATIONS

Medical gases can be identified simply if they are suppliedat different pipeline pressures. Routine maintenance errors,accidental pipeline cross-connections, or gas failures, canbe automatically detected by incorporating a pneumatic unitwith pressure-sensitive valves into the anaesthetic machine.A fall-iafe method of ensuring the delivery of the correct gasesto the patient is described.     

The use of a uniquely designed anesthetic scavenging hood to reduce operating room anesthetic gas contamination during general anesthesia

Numerous studies have suggested that chronic exposure to trace levels of anesthetic gas is harmful to operating room (OR) personnel. In the delivery of pediatric general anesthesia, an uncuffed endotracheal tube (ETT) is normally used which can result in considerable volatile anesthetic and nitrous oxide contamination of the OR. In this report, we present a method to reduce exposure to these anesthetic gases by means of an anesthetic scavenging hood (ASH). The ASH was used on six pediatric patients undergoing general endotracheal anesthesia via an uncuffed ETT. Measurements of all ambient gas levels were made 6 in. horizontally from the patient's ear and 6 in. from the table surface. The application of the vacuum source to the ASH resulted in a very significant (P < 0.01, paired t-test) decrease in levels of ambient anesthetic gas, with no measurable change in ventilatory variables or changes in body temperature (P > 0.05, paired t-test). Discontinuation of the vacuum force to the ASH resulted in a marked increase in ambient levels of anesthetic gas. We conclude that the ASH is extremely effective in reducing waste anesthetic gas associated with anesthesia administered via an uncuffed ETT. The ASH may be a valuable and cost-effective addition in the OR for both reducing ambient anesthetic waste gas levels and conserving patient heat. IMPLICATIONS: Chronic exposure to trace levels of anesthetic gas is harmful to operating room personnel, especially in the delivery of pediatric general anesthesia via an uncuffed endotracheal tube. The anesthetic scavenging hood is a cost-effective and efficient method to reduce these waste anesthetic gases, and it offers patient heat conservation.     

A coaxial circle circuit: Comparison with conventional circle and bain circuit

A coaxial system to be used for gas delivery to patients in a closed or low fresh gas flow anaesthetic system is described. The resistance to gas flow, humidity of inspired gases, and static compliance of the circuit are provided and compared with the circle tubing customarily employed or the coaxial Mapleson D (“Bain”) circuit, The resistance to gas flow is highest in the coaxial circle and “Bain” circuits; the resistance of the conventional circle is approximately 40 per cent less. Static compliance of this coaxial circle is 50 per cent greater than the conventional circle. During artificial ventilation humidity of inspired gases is maintained at levels recommended in the literature for all circuits, but during spontaneous breathing only the conventional rubber circle maintains appropriate levels. Advantages of this coaxial circle over the conventional circle include light weight and small size. Advantages of this coaxial circle over the “Bain” circuit include lower fresh gas flows and improved humidity during spontaneous breathing. These advantages make this coaxial circle useful for routine use.     

Preparation of modern anesthesia workstations for malignant hyperthermia-susceptible patients: a review of past and present practice

Patients with malignant hyperthermia experience an exaggerated metabolic response when exposed to volatile anesthetic gases and succinylcholine. The minimum concentration of anesthetic gas needed to trigger a malignant hyperthermia crisis in humans is unknown and may remain so because of the inherent risks associated with studying the complex nature of this rare and lethal genetic disorder. The Malignant Hyperthermia Association of the United States provides specific instructions on purging anesthesia machines of volatile agents to reduce the risk of exposure. However, these recommendations were developed from studies of older generation machines. Modern anesthesia workstations are more complex and contain more gas absorbing materials. A review of the literature found the current guidelines inadequate to prepare newer generation workstations, which require more time for purging anesthetic gases, autoclaving or replacement of parts, and modifications to the gas delivery system. Protocols must be developed to prepare newer generation anesthesia machines.     

The effect of pH and blood gas correction in DPG and plasma potassium content of stored blood.

During storage of CPD preserved blood, red cell DPG decreases and plasma potassium increases, the changes becoming more marked with increasing duration of storage. Correction of the blood gas and pH alterations of stored blood decreases plasma potassium and increases red cell DPG, but they do not return to normal except in blood stored less than seven days. There is a highly significant correlation between increasing red cell DPG and decreasing plasma potassium as the pH and blood gases of stored blood are restored to physiological range.     

Severe local hypothermia from laparoscopic gas evaporative jet cooling: a mechanism to explain clinical observations.

BACKGROUND AND OBJECTIVES: Explanations for laparoscopic-induced hypothermia fail to explain clinical observations. It is possible that water evaporation occurs from the jet stream of gas inflation resulting in tissue surface super-cooling leading to tissue damage and drying. METHODS: Theoretical calculations based on thermal conductivity, mass transfer effects and heat flux considerations correlated closely with synthetic and tissue experiments. Thermocouple measurements at a rate of 15 data points per second were performed. RESULTS: Cooling rates of 10 to 25 degrees centigrade per second for high flow rates were found based on gas flow rate and effective size of gas delivery site. These rapid temperature drops extended beyond a 2 cm2 diameter. CONCLUSIONS: Evaporative cooling accounts for significant hypothermia. The cooling is dependent on the lack of water vapor in the gases currently used during laparoscopy. Cooling rates are independent of height from tissue and geometry of delivery port. Heating and hydrating the gas to a physiologic condition eliminates hypothermia and tissue dessication.     

Preservation of ischemically injured canine kidneys by retrograde oxygen persufflation

Canine kidneys were briefly perfused with Ross and Marshall's hypertonic citrate solution and stored at O C. This study concerns the effect, during such storage, of insufflating various gases via the renal vein and allowing the gas to escape through needle perforations of the renal surface. We were able to confirm the finding of Ross and Escott that kidneys that have suffered 30 min of warm ischemia prior to preservation, will, if oxygen is so "persufflated" during 24 hr storage, provide life-supporting function when subsequently auto-grafted. Moreover, we were able to extend the preservation period to 48 hr after 30 min warm ischemia, and to achieve 24-hr preservation after 60 min of warm ischemia. Oxygen was essential: our results suggest that air is less effective than pure oxygen, and we found inert gases to be completely ineffective. Uniformly high oxygen tensions were measured throughout the kidneys during storage, but we were unable to demonstrate any resynthesis of adenosine triphosphate and adenosine diphosphate. The mechanism responsible for the effectiveness of retrograde oxygen persufflation remains obscure.     

Special article: general anesthetic gases and the global environment

General anesthetics are administered to approximately 50 million patients each year in the United States. Anesthetic vapors and gases are also widely used in dentists' offices, veterinary clinics, and laboratories for animal research. All the volatile anesthetics that are currently used are halogenated compounds destructive to the ozone layer. These halogenated anesthetics could have potential significant impact on global warming. The widely used anesthetic gas nitrous oxide is a known greenhouse gas as well as an important ozone-depleting gas. These anesthetic gases and vapors are primarily eliminated through exhalation without being metabolized in the body, and most anesthesia systems transfer these gases as waste directly and unchanged into the atmosphere. Little consideration has been given to the ecotoxicological properties of gaseous general anesthetics. Our estimation using the most recent consumption data indicates that the anesthetic use of nitrous oxide contributes 3.0% of the total emissions in the United States. Studies suggest that the influence of halogenated anesthetics on global warming will be of increasing relative importance given the decreasing level of chlorofluorocarbons globally. Despite these nonnegligible pollutant effects of the anesthetics, no data on the production or emission of these gases and vapors are publicly available. The primary goal of this article is to critically review the current data on the potential effects of general anesthetics on the global environment and to describe possible alternatives and new technologies that may prevent these gases from being discharged into the atmosphere.     

Model predictions of gas embolism growth and reabsorption during xenon anesthesia

BACKGROUND: It is not readily obvious whether an intravascular bubble will grow or shrink in a particular tissue bed. This depends on the constituent gases initially present in the bubble, the surrounding tissue, and the delivered gas admixture. The authors used a computational model based on the physics of gas exchange to predict cerebrovascular embolism behavior during xenon anesthesia. METHODS: The authors estimated values of gas transport parameters missing from the literature. The computational model was used with those parameters to predict bubble size over time for a range of temperatures (18 degrees -39 degrees C) used during extracorporeal circulation. RESULTS: Bubble size over time is highly nonlinearly dependent on multiple factors, including diffusivity, solubility, gas partial pressures, magnitude of concentration gradients, vessel diameter, and temperature. Xenon- and oxygen-containing bubbles continue to grow during xenon delivery. Bubble volume doubles from 50 to 100 nl in approximately 3-68 min, depending on initial gas composition and bubble shape. Bubble growth and reabsorption are relatively insensitive to temperature in the physiologic and surgical range. CONCLUSIONS: Xenon anesthesia results in gas exchange conditions that favor bubble growth, which may worsen neurologic injury from gas embolism. The concentration gradients can be manipulated by discontinuation of xenon delivery to promote reabsorption of xenon-containing bubbles. Estimated growth and reabsorption rates at normothermia can be applied to temperature extremes of cardiopulmonary bypass.     

Effects of Minute Volume Increases on the Rebreathing Characteristics of the Bain Anaesthesia Circuit During Controlled Ventilation

In the course of a study on the carbon dioxide rebreathing characteristics of the Bain anaesthesia circuit, it was noted that raising the minute volume without changing the fresh gas inflow invariably led to increased rebreathing of expired gases. Altering tidal volume and rate in order to reproduce a given minute volume had the same effect on rebreathing. A nomogram constructed to quantitate increases in rebreathing in function of carbon dioxide production per minute, fresh gas flow from the anaesthesia machine, and minute volume is produced. It can be used to assess the amount of fresh gas flow necessary to mantain a steady and inspired gas composition.     

European normative recommendations for medical gas pipeline systems

Several recent decisions have been made in order to increase the security of medical gases delivery in French hospitals. These different changes affect: 1) the hospital itself with the creation of working groups in charge of both monitoring and maintenance of gases networks; 2) the pharmaceutical regulation with promotion of several gases to the status of drugs or need of CE marking for the whole gas network. European rules onset required to give up French former norms (NF) to the profit of "NF EN" rules. Nevertheless, the new norm NF EN 737-3 which concerns medical gases distribution systems does not affect principal clauses of the previous NF S 90-155. It introduces new elements allowing to deploy two types of medical gases networks: the double pressure level used in France and the single pressure level used in the rest of Europe. This new norm, which attempts to harmonize alarm control systems in both types of networks, suffers from important limitations describing the double pressure level systems. Lastly, the final checking proposed by this new norm is very different from the previous one, and is likely to be problematic for the final users within the hospital.     

Inspired oxygen concentration during general anaesthesia for caesarean section

The effects on maternal oxygen saturation, foetal wellbeing and umbilical blood gases were compared when parturients received either 30 or 50% oxygen prior to delivery by Caesarean section under general anaesthesia. Maternal arterial oxygen saturation was significantly increased in the group receiving 50% oxygen. There was no difference between the two groups in terms of Apgar score minus colour, time to sustained respiration or umbilical cord blood gas estimations. The use of 30% inspired oxygen during uncomplicated Caesarean section is advocated.     

Present-day and future anesthesia equipment from the viewpoint of the clinician

With mainly clinical needs in mind, the technical and functional details of current anaesthesia equipment are discussed, as well as potential future developments. The following topics are discussed in detail: delivery of narcotic gases, dosimetry of both gaseous and volatile anaesthetics, the patterns of anaesthesia systems, mechanical ventilation during anaesthesia and intra-operative monitoring. As for the narcotic gases, plain air might become more important than ever before. The importance of helium has so far not been clearly assessed. The safety measures of the German Industry Norm (DIN) 13252 should become more widely known and be fully accepted: colour codes, non-interchangeable dimensions, oxygen bypass and oxygen-failure alarms, and automatic cut-off and alarm if the concentrations of nitrous oxide are too high. The delivery of a hypoxic gas mixture should be detectable whatever the cause might be and not only in cases when the oxygen-supply pressure is too low. Special attention should be given to the interactions between man and machines to exlude difficulties arising from "inhuman engineering". Concerning the rotameters, uniform serial attachment of these controllers of gaseous flow should be aimed at. The ranges (low flow, high flow), accuracy of the meter (+/- 3%) and DIN safety measures (different shape of button switches, etc.) should be kept in mind; a gas proportioning and mixing device that would strictly exclude the possibility of too-low oxygen concentrations would be very advantageous. The vaporizers for volatile anaesthetics should work independently of external physical variables and also fulfil the DIN safety programme.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nitric oxide delivery to the lung: a model

Dedicated nitric oxide equipped ventilators are now available commercially but are not yet common in clinical practice. With other ventilators, there is no standardized procedure for the administration or monitoring of nitric oxide. We describe the use of nitric oxide in conjunction with a simple time-cycled, pressure regulated, flow generating ventilator attached to a model infant-sized lung. The measured nitric oxide concentrations were always less than calculated. Infusion site, minute ventilation and sampling port all affected nitric oxide concentration (P < 0.05). Increasing minute ventilation lowered measured nitric oxide concentration exponentially. Mixing of gases improved when nitric oxide was infused closer to the ventilator. Acid contamination was found in water samples from humidifier, water trap and ventilator gas outlet. Acidification was reduced, without change in measured nitric oxide delivery, when infused prehumidifier. We recommend, when used as therapy, nitric oxide levels in inspired gases should always be measured.     

Model Predictions of Gas Embolism Growth and Reabsorption during Xenon Anesthesia

Background: It is not readily obvious whether an intravascular bubble will grow or shrink in a particular tissue bed. This depends on the constituent gases initially present in the bubble, the surrounding tissue, and the delivered gas admixture. The authors used a computational model based on the physics of gas exchange to predict cerebrovascular embolism behavior during xenon anesthesia.

Methods: The authors estimated values of gas transport parameters missing from the literature. The computational model was used with those parameters to predict bubble size over time for a range of temperatures (18[degrees]-39[degrees]C) used during extracorporeal circulation.

Results: Bubble size over time is highly nonlinearly dependent on multiple factors, including diffusivity, solubility, gas partial pressures, magnitude of concentration gradients, vessel diameter, and temperature. Xenon- and oxygen-containing bubbles continue to grow during xenon delivery. Bubble volume doubles from 50 to 100 nl in approximately 3-68 min, depending on initial gas composition and bubble shape. Bubble growth and reabsorption are relatively insensitive to temperature in the physiologic and surgical range.