There is a potential need for airway management in microgravity because astronauts are at an increased risk of cardiopulmonary arrest, and surgery may be required during extended spaceflight, such as a mission to Mars (1,2). In 1978, LeJeune (3) hypothesized that laryngoscope-guided tracheal intubation (LG-TI) would be difficult in microgravity because the anterior force exerted by the laryngoscope causes the head and neck to move out of the field of view, and the hand not holding the laryngoscope cannot synchronously stabilize the head and neck and insert the tracheal tube. In 2000, our group tested this hypothesis using a deep pool to simulate microgravity and found that the success rate for anesthesiologists in the free-floating condition was 15%, increasing to 92% if the manikin was strapped to a surface using restraints (4). A major limitation of restraints is the time taken to apply them during a cardiac arrest. Potentially faster options include the use an extraglottic airway device, a self-retaining laryngoscope, or to grip the patient’s head between the anesthesiologist’s knees. Also, there are no data about the feasibility of airway management in true as opposed to simulated microgravity. In the following manikin study, we determined the feasibility of LG-TI in microgravity during parabolic flight and tested the hypothesis that LG-TI is similarly successful in the free-floating condition, with the head gripped between the knees, and in the restrained condition, with the torso strapped to a surface.
Research approval was obtained from the University of Innsbruck, the European Space Agency, and the French Ministry of Defense. One woman and 2 men (age, 26–29 yr; weight, 62–80 kg; height, 174–185 cm) without airway management or SCUBA diving experience participated in the study. Training was in five stages. Stage 1 involved 8 h of theory about airway anatomy and physiology. Stage 2 involved 100 supervised LG-TIs of a standard advanced life support (ALS) manikin (Laerdal International A/S, Kopenhagen, Denmark). Stage 3 involved two supervised LG-TIs of anesthetized patients. Stage 4 involved a certified course in SCUBA diving. Stage 5 involved 10 supervised LG-TIs of a full-sized manikin submerged in a freshwater pool using neutrally buoyant equipment and personnel: five in the free-floating condition, in which the manikin was held for intubation by gripping its head between the knees (Fig. 1), and five in the restrained condition, in which the manikin was held for intubation by a Velcro restraint attached to the inner surface of the aircraft similar to that attached to the inner surface of the International Space Station and Space Shuttle.
All investigators were given 0.4 mg of oral scopolamine to prevent motion sickness. The manikin was a customized, full-body, 50-kg ALS manikin that contained thoracic wall sensors that measured the efficacy of ventilation. The parabolic flights took place within 1 wk of completion of the training program. The parabolic flights were performed on three consecutive days in June 2004 on an Airbus 300 flown over the North Atlantic Ocean. The parabolic flight cycle lasted approximately 3 min and comprised (a) rapidly pulling up at 1.8 g from 6100 to 8500 m at an angle of 47 degrees, (b) reducing the throttle so that the plane was in free fall for 23 s, (c) pulling out when the descent angle was 42 degrees, and (d) flying level for 1.5 min before beginning the cycle again.
The airway management equipment, which consisted of a size 7.5-mm tracheal tube (Mallinckrodt Medical, Lo-Contour, Athlone, Ireland), a 20-mL syringe, a self-inflating bag, and a size 3 Macintosh laryngoscope, was in a sealed box adjacent to the manikin. All investigators attempted LG-TI on seven occasions in each of the 2 conditions in random order. During level flight, the investigators positioned themselves above the head of the manikin. The attempt began at the end of the pull-up phase when microgravity commenced. Once inserted, the cuff was inflated with air 10 mL and the self-inflating bag attached. The tracheal tube and self-inflating bag were then held in place during the pull-out phase to prevent dislodgement. The efficacy of ventilation was assessed by a trained observer during level flight by squeezing the bag and noting whether the manikin sensors indicated a tidal volume ≥300 mL. All insertions were recorded using a video camera placed in a fixed position. The cause of failed insertion and the time taken to insert were assessed on the ground by analyzing video recordings taken from a fixed position during the parabolic flight sequence. Insertion time was from when the investigator opened the sealed box to attachment of the self-inflating bag.
Statistical analysis was with paired t-test for the time to successful insertion and χ2 test for the ventilation success rate. Unless otherwise stated, data are presented as mean ± sd. Significance was taken as P < 0.05.
There were no differences in ventilation success or time to successful insertion between the free-floating condition and the restrained condition (Table 1). More than 90% of failures were caused by an inability to insert the tracheal tube within 23 s. There were no differences in performance among investigators.
The success rate for LG-TI was similarly infrequent for inexperienced personnel in the free-floating condition, with the manikin’s head gripped between the knees, and in the restrained condition, with the manikin strapped to a surface. The percentage rate for failed attempts is not surprising because the average time taken to perform LG-TI in simulated microgravity in a deep pool is approximately 35 seconds (4). We suspect that success rates would have been improved if microgravity had lasted a minute. We suggest that gripping the patient’s head between the knees should be adopted by astronauts attempting LG-TI during cardiopulmonary resuscitation in microgravity and that this be included in their training program.
Alternative strategies for single-handed tracheal intubation in the free-floating condition include the use of a self-retaining laryngoscope blade or the use of an airway intubator, such as the intubating laryngeal mask airway (4). Neither of these techniques has been tested in true or simulated microgravity. An alternative strategy is to use an extraglottic airway device, and our group found that the classic and intubating laryngeal masks and cuffed oropharyngeal airway had more frequent success than LG-TI in simulated microgravity (4). Perhaps LG-TI is too difficult for the infrequent user. Trips to Mars may take as many as three years.
Our study has two limitations. First, we did not compare LG-TI in the free-floating condition with or without the knee grip. However, our previous study showed that the free-floating condition without the knee grip had much less success than the restrained condition (4). Second, in the restrained condition, the straps were applied before microgravity commenced, making the time comparisons biased against the free-floating condition. We attached the straps before microgravity because of the limited time frame. Data from our previous study in simulated microgravity indicated that strap application takes 5–10 seconds, making it likely that the free-floating position would be quicker than the restrained condition (4).
Finally, our study highlights the time difficulties of assessing airway management techniques during parabolic flight. Nonetheless, given the limitations of simulated microgravity environments (such as the deep pool and neutrally buoyant equipment) and the staggering expense of flying higher parabolas (such as could be achieved in SpaceShipOne (5)), we feel further studies are worthwhile.
We conclude that LG-TI is feasible in microgravity obtained during parabolic flight, but success is infrequent because of severe time restrictions. There were no differences in success rate between the free-floating condition, with the manikin’s head gripped between the knees, and in the restrained condition, with the torso strapped to a surface (Fig. 1).
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