Training Non-Anesthesia Personnel to Perform Nerve Blocks in Microgravity

With the burgeoning human presence in space, the need to prepare for medical emergencies in microgravity environments becomes increasingly critical. Space is an inherently hazardous environment, with 21 known fatalities. Most injuries occur during lift off or re-entry, but in-flight incidents have also been reported, including fires, blunt injuries, vehicular collisions, and loss of environmental controls. Extraction times vary significantly, with the potential for hours in low-earth orbit (LEO), days from the Moon, and months from Mars. In this case or during more distant space exploration, damage control procedures may be necessary.

If astronauts are injured, physiologic and logistic complexities necessitate a different approach from hospital-based care on Earth. In addition, due to ergonomic challenges of microgravity. Systemic pain medication is not optimal in that it can cause sedation, confusion, and even delirium. This could compromise safety for the crew given the small numbers of personnel who play critical roles. If a surgical intervention was required, general anesthesia is problematic for multiple reasons.

The benefits of regional anesthesia (nerve blocks) have been recognized in remote settings, including regions with fewer resources and trained providers. Regional anesthesia limits airway management, ventilation, and hemodynamic changes. The equipment is portable, offers stable hemodynamics, effective analgesia, minimal need for supplemental oxygen, and improved alertness. As each crew member's role is pivotal for mission success, regional anesthesia ensures their cognitive participation. Our group has shown that nerve blocks are feasible in a simulated microgravity environment when performed by anesthesiologists and anesthesia residents.

In specialized teams such as astronauts, rigorously trained for space exploration, there might not be the depth of personnel to allow physicians on crews. At the current time, most missions have a single Crew Medical Officer (CMO) who is usually not a physician. Space travel with its associated communication and logistical delays further emphasizes the need for such medical training so they can act without real-time guidance.

Aim Recognizing that non-physician medical technicians may be integral to future space missions, our study aims to ascertain the feasibility of training such personnel to perform simple nerve blocks in microgravity.

Hypothesis The investigators hypothesize that despite training about nerve block mechanics and ultrasound imaging, non-physician personnel have worse accuracy, time to injection, and ease of needle placement in a simulated microgravity environment than compared to normal gravity conditions. Furthermore, the investigators hypothesize that accuracy, time to injection, and ease of needle placement will be inferior to anesthesia providers in a simulated microgravity environment.

Methods This will be a prospective cohort study using within- and between-subjects comparisons. This study will assess the ease and success of ultrasound-guided injections ("nerve blocks") by non-anesthesia personnel comparing normal gravity to microgravity conditions using a meat model. Secondly, the ease and success rate of non-anesthesia participants will be compared to anesthesia participants.

Recruitment and Allocation The investigators will recruit 20 non-anesthesia military personnel and 5 anesthesia providers who are trained scuba divers. Inclusion criteria for the non-anesthesia group would be adult (age >18 years) certified scuba divers and those who have not had prior nerve block technique training. Inclusion criteria for the anesthesia group are that they are staff anesthesiologists or senior residents and certified scuba divers since nerve block training is part of basic anesthesia training.

Training session Recruited participants would be given a 4-hour training session on ultrasound imaging theory and practical optimization, needle visualization and control, basics of local anesthetics, basic sono-anatomy, and complications of nerve blocks. This session will be developed and led by regional anesthesia fellowship trained team members. There will be opportunities to practice image acquisition and needle placement with feedback. This session will take place the day before the study.

Meat model creation All injections will use a meat model. This meat model using beef muscle, beef tendon, and ballistics gel has been previously validated for accessing accuracy of US-guided injections.

Gravity conditions In the normal gravity condition, participants will be in a seated position with the meat model positioned in the chest wall of a mannikin, supported on a table. The ultrasound, phone, and dive case will be the same as in the microgravity condition. Participants will use hand signals to communicate with their block assistant since verbal communication will not be possible underwater.

In the microgravity condition, the participants will be scuba diving in a pool. The mannikin will be secured to a table at the bottom of the pool and they will empty their buoyancy control device (causing them to sink), effectively removing the added complexity of buoyancy control. An investigator will assist each participant with the block by positioning the meat model in the mannikin, passing equipment, and injecting the dye through a syringe once they are signalled to do so. However, the investigator will not give the participant guidance on the accuracy of the needle position or the quality of their ultrasound image.

Statistical analysis and power calculation Demographics will be collected (age, gender, prior medical training, experience with nerve blocks) and reported in aggregate for the purposes of external validity and generalizability. Outcomes will be compared between normal gravity and microgravity for non-anesthesia participants (within subjects, paired data). Outcomes will also be compared between anesthesia and non-anesthesia participants (between subjects, independent data).

Data will be analyzed using generalized estimating equations to adjust for the clustered nature of the data (i.e., blocks nested within participants). For binary outcomes (success vs. failure), a binomial outcome with a logit link will be used. For numerical outcomes, a normal distribution will be assumed, though loglinear models for skewed distributions will also be explored if they fit the data better.

Power analyses adjust sample sizes for the clustered design using the design effect method described by Bland (2004). Moreover, the design assumes 14 blocks per participant. The first a-priori power analysis is for comparing normal gravity and microgravity for non-anesthesia participants with the outcome of success vs. failure. Assuming a one-tailed alpha of 0.05, power of 80%, a normal gravity success rate of 90% and a microgravity success rate of 70%, and an intraclass correlation of 0.10, a total of 19 non-anesthesia participants will be required.

The second a-priori power analysis is for comparing normal gravity and microgravity for non-anesthesia participants for time to block, which has been used as a measure of ease in learning curve studies. Assuming a one-tailed alpha of 0.05, power of 80%, a mean difference of 12 seconds, a standard deviation of 23 seconds (based on data from our prior study), and an intraclass correlation of 0.10, a total of 15 participants will be needed.