For optimal gas exchange, the lungs require the inspired gas to be at a temperature of 37 degrees C and approx. 44 mg H2O/l of absolute humidity (100% relative humidity). Under normal conditions, the respiratory tract warms the air during passage and the conditions described above are already met 5 cm below the carina.

During anesthesia, however, the upper respiratory tract is bypassed by endotracheal intubation or the placement of a laryngeal mask, leaving the lower respiratory tract potentially overcharged with the task of adequately acclimatizing the cold and dry gas applied during high flow anesthesia^5,6,12.

Mechanical ventilation with cold and dry gas may result in:

• Damage to the respiratory epithelium and reduced mucous transport, and may ultimately lead to complications such as infections and atelectasis^12-14  
• Drops in body core temperature^7-11    
• Increased risk for inflammation and subsequent harm^11,15-18

Low-flow anesthesia is considered to be achieved at a fresh gas flow of 1 L / min, any higher fresh gas flow settings are considered high-flow. A general anesthetic with fresh gas flows of approx. 0.5 L / min is called minimal-flow anesthesia.

An absolute humidity of 30-35 mg H2O/l is described as a target value for prolonged mechanical ventilation and a minimum of 15-20 mg H2O/l to mitigate the risk of the negative effects that perioperative ventilation with unconditioned gas has on the airways^6,7,11,12,14. In clinical studies on low- and minimal-flow techniques, values from 20 mg H2O/l up to 30 mg H2O/l and 32degrees C could be well achieved^7,9,11,12,19.

General anesthesia using low fresh gas flows presents a variety of benefits, including:

• Resource and cost savings: The potential to save anesthetic gases, resulting in potential economic and ecological benefits¹⁹
• Ease of use: It is an effective and easy way to humidify respiratory gas^6,10,11,19
• Positive clinical effects: Respiratory gas conditioning can be a component to avoid some of the potentially negative effects of ventilation with cold and dry gas

It is important to note that low- and minimal-flow techniques place demands on the technology deployed. A list of technical requirements for the effective application of low- and minimal-flow technology can be found in our paper Technology Insights. For a more in-depth look at low- and minimal-flow anesthesia, you can also view our white paper on the Clinical Benefits of Low- and Minimal- Flow Anesthesia.

¹Canet et al., Prediction of postoperative pulmonary complications in a population-based surgical cohort. Anesthesiology. 2010 Dec;113(6):1338-50. doi: 10.1097/ ALN.0b013e3181fc6e0a
²Güldner et al., Intraoperative protective mechanical ventilation for prevention of postoperative pulmonary complications: a comprehensive review of the role of tidal volume, positive end-expiratory pressure, and lung recruitment maneuvers. Anesthesiology. 2015 Sep;123(3):692-713. doi: 10.1097/ALN.0000000000000754
³Rock P et al. Postoperative pulmonary complications. Curr Opin Anaesthesiol. 2003;16:123-31.
⁴ Hemmes et al., High versus low positive end-expiratory pressure during general anesthesia for open abdominal surgery (PROVHILO trial): a multicentre randomized controlled trial. Lancet. 2014 Aug 9;384(9942):495-503. doi: 10.1016/S0140-6736(14)60416-5.
⁵Al Ashry et al., Humidification during mechanical ventilation in the adult patient. Biomed Res Int. 2014;2014:715434. doi: 10.1155/2014/715434
⁶Gross et al., Humidification of inspired gases during mechanical ventilation. Minerva Anestesiol. 2012 Apr;78(4):496-502.
⁷Brattwall et al., Brief review: theory and practice of minimal fresh gas flow anesthesia. Can J Anaesth. 2012 Aug;59(8):785-97. doi: 10.1007/s12630-012-9736-2
⁸Hönemann et al., Inhalational anesthesia with low fresh gas flow. Indian J Anaesth. 2013 Jul;57(4):345-50. doi: 10.4103/0019-5049.118569
⁹de Oliveira et al., The humidity in a low-flow Dräger Fabius Anesthesia Workstation with or without thermal insulation or a heat and moisture exchanger: A prospective randomized clinical trial. PLoS One. 2017 Jan 27;12(1):e0170723. doi: 10.1371/journal.pone.0170723
¹⁰Stone et al., Adult body temperature and heated humidification of anesthetic gases during general anesthesia., Anesth Analg. 1981 Oct;60(10):736-41.
¹¹Branson et al., Humidification for patients with artificial airways. Resp Care. 1999 Jun;44(6).
¹²Bilgi et al., Comparison of the effects of low-flow and high-flow inhalational anesthesia with nitrous oxide and desflurane on mucociliary activity and pulmonary function tests. Eur J Anaesthesiol. 2011 Apr;28(4):279-83. doi: 10.1097/EJA.0b013e3283414cb7
¹³Branson et al., Anesthesia circuits, humidity output, and mucociliary structure and function. Anaesth Intensive Care. 1998 Apr;26(2):178-83.
¹⁴Kilgour et al., Mucociliary function deteriorates in the clinical range of inspired air temperature and humidity. Intensive Care Med. 2004 Jul;30(7):1491-4.
¹⁵Ranieri at al., Effect of mechanical ventilation on inflammatory mediators in patients with acute respiratory distress syndrome: a randomized controlled trial. JAMA. 1999 Jul 7;282(1):54-61.
¹⁶Hernández-Jiménez et al., Respiratory mechanics and plasma levels of tumor necrosis factor alpha and interleukin 6 are affected by gas humidification during mechanical ventilation in dogs. PLoS One. 2014 Jul 18;9(7):e101952. doi: 10.1371/journal.pone.0101952.
¹⁷Jiang et al., Airway Humidification Reduces the Inflammatory Response During Mechanical Ventilation. Respir Care. 2015 Dec;60(12):1720-8. doi: 10.4187/respcare.03640
¹⁸Song et al., Zhonghua Wei Zhong Bing Ji Jiu Yi Xue. 2014 Dec;26(12):884-9. doi:10.3760/cma.j.issn.2095-4352.2014.12.008.
¹⁹Hönemann C., Mierke B., Low-flow, minimal-flow und metabolic-flow anesthesia, Published by Drägerwerk AG & Co. KGaA