[Low flow anaesthesia with isoflurane in the dog].
This article in german compares the safety of high and low flow anesthesia in dogs preparing for surgery. Three groups of animals were divided into two low flow and one high flow groups. Vital signs were kept well controlled during the procedures. Notes the highest decrease in body temperature was in the high flow group, and a particularly cooled gas used for that group may have caused this.
At least in the context of dogs, this article seems to treat the question of flow rate and core body temp. as a function of gas cooling the body.
[Comparison of the effects of low-flow and high-flow inhalational anaesthesia with nitrous oxide and desflurane on mucociliary activity and pulmonary function tests.]
This study was primarily involved in lung functions. However, they also recorded body temperature data. Fifty healthy adults ranging from 18-70 years old were recruited and randomly assigned to one of three groups: two low flow and one high flow group. Ultimately, they concluded that low flow rate was better from the mucociliary perspective but noted a statistical difference in saccharin clearance time, and temperature.
[Heated breathing tubes affect humidity output of circle absorber systems]
This study compared the effects of heating breathing tubes in regulating the physiology. 26 patients were assigned to one of three low flow gas groups groups: no heating, heated to 30 degrees centigrade, and heated to 36 degrees centigrade. They found that the temperature of the gas in the no heating group reached 34.5 C after 120 minutes.
[Special Aspects of Pharmacokinetics of Inhalation Anesthesia]
This is an overview of the pharmacokinetics of gases. It is specifically looking at the kinetics of the gases at low flow rate and will discuss properties inherent to the gasses. It’s possible that the temperature rise you’re seeing is a result of some property of the gas. Something related to boyle’s law.
[Low and minimal flow anesthesia: Angels dancing on the point of a needle]
This article discusses the resurgence in low flow gas delivery. “At a gross level, the idea of low and minimal flow anesthesia is appealing. The use of minimal flows will necessarily decrease the loss of body temperature and drying of mucus membranes in the ventilatory tract, which occurs with the use of higher flows.”
[The climatisation of anesthetic gases under conditions of high flow to low flow.]
This German study was looking to control the climatisation of anesthetic gases to something closer to the nasophyrangeal climate. 6, 3, and 1.5 l/min flows rates were concluded to be inadequate in prolonged anesthesia for maintaining the environment. 0.5 l/min was found to improve the temperature greatly from 28 to 32 degrees.
[Decreased Fresh Gas Flow Cannot Compensate for an Increased Operating Room Temperature in Maintaining Body Temperature During Donor Hepatectomy for Living Liver Donor Hepatectomy]
This study sought to see if reducing the flow rate could compensate for increased operating room temperatures. Retrospectively reviewing studies, they found that warmer environments and low flow anesthesia resulted in higher body temperatures than high or low flow rates at a lower operating room temperature.
[Low flow, minimal flow and closed circuit system inhalational anesthesia in modern clinical practice]
This review discusses many aspects of low flow benefits. It reduces the issue of temperature and flow rate management to one of anatomy. By bypassing a significant portion of the pharynx, the physiological heating and humidifying is not occurring. Cold dry gasses are placed directly into the bronchi. Body temperature falls as the heat loss occurs in the lungs due to the large volume of cold gas; the hypothermia is exacerbated by the muscle paralysis. They were able to control temperature using 0.6 l/min flow to 30 degrees after one hour. In flow rates any higher than 1.5 l/min they conclude that it’s impossible to maintain heating and humidity. Basically, this article reduces to the problem of the high volume of gas bypassing the pharynx lowering and cooling the lungs directly. Less gas flow means that less cold gasses are introduced.
[Tracheobronchial consequences of the use of heat and moisture exchangers in dogs.]
This German study divided dogs undergoing surgery into four groups. A pair of high flow rate groups with and without a heat and moisture exchanger, and a pair of low flow rate groups similarly with and without a heat and moisture exchanger. Temperature and humidity was compared between the groups. The groups with the exchangers had the highest ambient humidity and the high flow without exchanger group had the lowest humidity. There was no difference in tympanic temperature or inhaled gas temperature between the groups.
[Temperature and humidity of the Dräger Cato anesthetic machine circuit.]
This study reviewed the efficacy of a specific heat and moisture exchanger. It divided patients into 8 equal groups. 4 groups received Aestiva and 4 received Cato. Similarly, the groups were equally divided into having the exchanger in place or not, and low and high flow rate groups. Cato produced higher temperatures in both the high and low flow groups without the exchanger. The exchangers significantly increased temperatures in the high flow rate groups, but not in the low flow rate groups. This study eludes to an increased body temperature resulting from decreased flow rate.
[The temperature and humidity in a low-flow anesthesia workstation with and without a heat and moisture exchanger.]
This study looking into anesthesia systems divided 30 female patients into two groups. One receiving anesthesia using an exchanger and one without. There were 2 main findings in this study: (1) the Primus anesthesia workstation partially humidifies the inspired gas when a low FGF is used; (2) insertion of an Heat and Moisture Exchanger increases the humidity in inhaled gas, bringing it close to physiological values. At low flow without the exchanger, after 60 minutes the temperature of the gas was about 25 degrees. With the exchanger the temperature was approximately 30 degrees.
[The temperature-humidity profile of the PhysioFlex. Studies on a model].
This is a German case series using another exchanger device. The system relies on low gas flow rates (less than 0.5 l/min) and in the study temperature and humidity was measured sequentially in four distinct locations: 1) in the soda-lime canister; 2) at the outlet of the anaesthesia machine; (3) at the inlet of the anaesthesia machine; and (4) in the inspiratory limb close to the Y-piece. The physioflex system was able to create a suitable operative climate in as little as ten minutes.
[Climatization of anesthetic gases using different breathing hose systems].
This study looked at attempts to appropriately climatize the patient using different breathing hoses and different gas flow rates. Using various hoses, humidity and temperature were measured at flow rates of 1, 2, and 4.4 l/min. Ultimately, the researchers found that gas temperatures were independent of the flow and remained from 28-32 degrees C, even at 4.4 l/min.
Works Cited:
Baum, J., et al. (2000). Climatization of anesthetic gases using different breathing hose
systems. Anaesthesist, 49(5), 402-411.
Bilgi, M., et al. (2011). Comparison of the effects of low-flow and high-flow inhalational
anaesthesia with nitrous oxide and desflurane on mucociliary activity and pulmonary function tests. Eur J Anaesthesiol, 28(4), 279-83. doi: 10.1097/EJA.0b013e3283414cb7.
Bisinotto, B.F.M., et al. (1999). Tracheobronchial consequences of the use of heat and
moisture exchangers in dogs. Can J Anaesth., 46(9), 897-903.
De Castro, J., et al. (2011). The temperature and humidity in a low-flow anesthesia
workstation with and without a heat and moisture exchanger. Anesth Analg., 113(3), 534-538. doi: 10.1213/ANE.0b013e31822402df
Cheng, K.W., et al (2010). Decreased Fresh Gas Flow Cannot Compensate for an
Increased Operating Room Temperature in Maintaining Body Temperature
During Donor Hepatectomy for Living Liver Donor Hepatectomy. Transplant Proc, 42(3), 703-4. doi: 10.1016/j.transproceed.2010.02.065.
Hendrickx, J.F., De Wolf, A. (2008). Special Aspects of Pharmacokinetics of Inhalation
Anesthesia. Handb Exp Pharmacol.,182, 159-86. doi: 10.1007/978-3-540-74806-9_8.
Kleemann, P.P., et al. (1993). Heated breathing tubes affect humidity output of circle
absorber systems. J Clin Anesth., 5(6), 463-7.
Kleemann, P.P. (1990). The climatisation of anesthetic gases under conditions of high
flow to low flow. Acta Anaesthesiol Belg., 41(3), 189-200.
Kramer, S., Alyakine, H., Nolte, I. (2005). Low flow anaesthesia with isoflurane in the
dog. Berl Munch Tierarztl Wochenschr, 118(3-4), 164-74.
Mychaskiw, G. (2012). Low and minimal flow anesthesia: Angels dancing on the point of
a needle. J Anaesthesiol Clin Pharmacol., 28(4), 423–425. doi: 10.4103/0970-9185.101883
Vecil, M., et al. (2008). Low flow, minimal flow and closed circuit system inhalational
anesthesia in modern clinical practice. Signa Vitae, 3(1), 33-36.
Wada, H., et al. (2003). Temperature and humidity of the Dräger Cato anesthetic machine
circuit. J Anesth., 17(3), 166-170.
Wissing, H., et al. (1997). The temperature-humidity profile of the PhysioFlex. Studies
on a model. Anaesthesist., 46(3), 201-206.
