Pediatric Mask Induction – What is Success?
Jeffrey Feldman, MD, MSE, FASA and Diane Gordon, MD
As we all know, the answer to the question posed in the title is not just “When the Patient is Asleep!” The art of pediatric mask induction is honed through experience and accomplished with a variety of techniques. When Sevoflurane replaced Halothane, it became easier to achieve a successful result with a greater margin of safety. Despite different approaches, all practitioners seek an efficient process where the patient transitions from awake to anesthetized quickly without emotional stress and/or physical resistance. While we may know a successful mask induction when we see one, defining success and studying the practice variations that influence the outcome is challenging. What are objective measures of success? Mask acceptance? Time from mask application to loss of lid reflex? Degree of combativeness? Environmental exposure to anesthetic agent? Patient satisfaction score?
This article was prompted by a request from Myron to review a recent study comparing the rate of change of anesthetic concentration during pediatric mask induction between the AISYS (GE Healthcare, Madison, WI) and Apollo (Draeger Medical, Lubeck, Germany) anesthesia workstations. The article sets the stage for commentary on strategies for mask induction and definitions of a successful mask induction.
Original article
Lerman J, Correa AMR. Sevoflurane Washin With the Dräger Apollo and GE Datex Ohmeda Aisys Workstations in Healthy Children. Paediatr Anaesth. 2025 Jul;35(7):535-541. doi: 10.1111/pan.15106. Epub 2025 Apr 3. PMID: 40178391.
In this paper, Lerman and Correa1 performed a standardized mask induction for 12 patients using a GE AISYS anesthesia workstation and 12 patients using a Draeger Apollo. All patients were healthy outpatients between 2 and 7 years of age. The mask induction was standardized to use 4 L/m N2O and 2 L/m Oxygen. The Sevoflurane vaporizer was turned on to 8% when the end-tidal N2O concentration reached 50%. Inspired and expired concentrations of Sevoflurane were recorded every 30 seconds for 5 minutes and every minute thereafter up to 10 minutes. Vital signs and “notable patient responses” were also recorded. Spontaneous ventilation was favored with positive pressure applied only if apnea/hypopnea occurred.
The primary objective of the study was to evaluate the rate of rise of inspired sevoflurane concentration in the AISYS and Apollo machines during mask induction; this study was prompted by an observed difference in sevoflurane concentration during induction between Apollo and AISYS machines. The data indicated that inspired sevoflurane concentrations approached the 8% vaporizer setting within the first minute when using AISYS whereas for the Apollo, the concentration peaked around 6.6%. This finding is not surprising and is easily explained, as the authors observe, by differences in the architecture of the two machines. The important question for clinical practice is whether or not the observed difference in anesthetic washin has any impact on the success of mask induction. The authors concluded that the Apollo “underperformed” compared with the AISYS but the paper did not offer any data on mask induction success. Therefore, we do not know from the results if the concentration differences observed had a clinical impact on induction time. The findings do however create an opportunity to reflect on the details of the mask induction process in an effort to understand the equipment and practices most likely to lead to consistent success.
Do the washin times and peak anesthetic concentrations of different anesthesia workstations make a difference to the success of mask induction? We are not aware of any published data indicating how rapidly sevoflurane concentration needs to rise for a successful mask induction and the ideal rate of rise will vary between patients. The key difference between the AISYS and Apollo workstations impacting the internal circuit volume, and therefore the rate of rise of anesthetic concentration, is the ventilator technology. When using the AISYS for mask induction, the selector switch is in the manual/spontaneous ventilation position and the ventilator’s internal volume is not in the circuit. For the Apollo, the ventilator’s internal volume is always in the circuit, adding 1.4 L of gas that dilutes the incoming fresh gas during mask induction.
In the current anesthesia machine market, there are 4 different ventilator technologies available in various models of machines – bellows, piston, turbine and volume reflector. The details of these architectures are beyond the scope of this article but available in other references.2,3 While the rate of rise of anesthetic concentration has not been studied methodically for all of these workstations during mask induction, architectural differences allow us to predict differences in the rate of washin. For example, the turbine ventilator machines3 are likely to be the most rapid since the ventilator does not add any volume to the circuit (lowest internal volume) and the turbine augments the mixing of circuit gases with fresh gas. If anesthetic washin was the most important determinant of successful mask induction, we should all use machines with turbine ventilators. Given the variety of machines used for mask induction globally on a daily basis, while washin time may be a consideration, there are other factors that determine which machine is ultimately selected and used by a given practice.
As we seek to understand the importance of washin time to mask induction, it is useful to be more specific about what determines success.
Assuming the overall objective of mask induction is an efficient process where the patient transitions from awake to anesthetized quickly without emotional stress and/or physical resistance, we can break down the process into a series of clinical goals.
· Mask Acceptance: Placing the mask on the patient’s face is a moment when the pediatric patient can quickly become afraid and combative. Easing that process is a primary goal of mask induction. Many different strategies are employed alone or together including preoperative preparation, premedication, distraction, parental presence, fragrances, nitrous oxide and limiting the potent anesthetic concentration in the initial breaths. The desired anesthetic concentration in the first breath is relevant to the discussion of this article. While sevoflurane is much better tolerated than halothane, high concentrations are unpleasant, and mask acceptance is facilitated by a steady increase in concentration over several breaths. From that perspective, practices for adjusting the vaporizer to control the rate of rise of anesthetic concentration are likely to vary depending upon the anesthesia workstation and the patient needs. When a gradual increase in anesthetic concentration is desired, several stepwise increases in sevoflurane concentration over the first minute of induction may be preferable for workstations with rapid washin, whereas a single increase to 8% sevoflurane may be preferable for workstations with a slower rate of rise.
· Efficient transition from awake to anesthetized: Efficiency is a surrogate term for “rapid” since there is time pressure to start the procedure and no patient benefit to a prolonged induction process. Single breath induction is the most rapid method for mask induction, but it is not routinely used since it requires a priming process to increase the circuit concentration to 8% sevoflurane, and a cooperative patient since it is unpleasant to inhale 8% sevoflurane in the first breath. Achieving an efficient (rapid) induction does require an immediate increase in anesthetic concentration, but the desired maximum concentration and how quickly it needs to be achieved are matters for debate. Data on time from mask application to loss of lid reflex is not included in the study reviewed here.1 The rate at which anesthetic is taken up by the blood and ultimately brain compartments increases as the concentration in the alveolus increases, but the minimum concentration required to achieve the desired effect is not objectively known. The Apollo workstation is widely used in pediatric anesthesia practice. At the Children’s Hospital of Philadelphia where one of us practices (JF), the Apollo was the standard workstation from 2008-2023 with tens of thousands of mask inductions performed during that time. Experience with the Apollo confirms that the peak anesthetic concentration during mask induction is typically between 6% and 7%. The extensive experience of successful mask induction with the Apollo suggests that the peak concentration achieved, which is well above the desired MAC level, is acceptable.
· Reduce environmental impact of anesthetic gases: Until recently, the first two bullet points would have been sufficient to describe the goals of mask induction. We are now keenly aware of the environmental impact of inhaled anesthetics and strategies to minimize that impact should be utilized as long as the desired clinical goals are achieved. It is very easy to use high fresh gas flows well in excess of that needed to increase the inspired anesthetic concentration sufficiently to achieve the desired speed of induction. Fresh gas flow that exceeds minute ventilation wastes vaporized anesthetic without speeding up mask induction. In a recent review, we offered several strategies for minimizing the environmental impact of mask induction including limiting maximum fresh gas flow to a weight-based estimate of minute ventilation, eliminating N2O, priming the circuit in a specific manner and reducing fresh gas flow as soon as the circuit and lung anesthetic concentrations are adequate.4
Mask induction with sevoflurane is a fundamental aspect of pediatric anesthesia practice. Honing that skill and teaching the next generation requires defining the objectives by which we would measure success. The characteristics of the anesthesia machine are only one aspect of an overall strategy for achieving successful mask induction that begins before the patient enters the operating room. Once the objectives for successful mask induction are clear, we can implement the techniques needed to achieve success for every patient, regardless of the practice setting and equipment available.
Do you, as the authors of today’s article, still use nitrous oxide prior to initiating sevoflurane? And/or have you completely eliminated nitrous oxide from your practice? Do you use a single breath induction technique? Have you switched to low gas flows during induction and during maintenance? Send your thoughts and comments to Myron (myasterster@gmail.com) who will post in a Friday reader response.
PS from Myron: When sevoflurane was first introduced into the U.S. market, the original vaporizers allowed up to 12-15% inspired concentrations to match the 5% halothane vaporizer. It was reduced to the current 8% to minimize hypotension. Additionally, when myoclonus and EEG changes were first noticed when inducing with sevoflurane, even 8% was initially considered dangerous and slow up titration was recommended.5 Finally, I would highly recommend the Open Anesthesia article by Alexander and Feldman for all of you as a review! I learned a lot by reading it for today’s PAAD.
References
1. Lerman J, Correa AMR: Sevoflurane Washin With the Dräger Apollo and GE Datex Ohmeda Aisys Workstations in Healthy Children. Paediatr Anaesth 2025; 35: 535–541
2. Hendrickx JFA, De Wolf AM: The Anesthesia Workstation: Quo Vadis? Anesth Analg 2018; 127: 671–675
3. Alexander A, Feldman, J. M.: Pediatric Ventilation: Anesthesia Ventilators, Pediatric Ventilation: Anesthesia Ventilators. In Open Anesthesia. https://www.openanesthesia.org/keywords/pediatric-ventilation-anesthesia-ventilators/?search_term=pediatric%20ventilation. Accessed June 27, 2025.
4. Gordon D, Feldman J: Environmentally responsible mask induction. Best Practice & Research Clinical Anaesthesiology 2024; 38: 321–331
5. Lerman J: Induction of anesthesia with sevoflurane in children: Curiosities and controversies. Paediatr Anaesth 2022; 32: 1100–1103