George Beck

En-route care in the air: snapshot of mechanical ventilation at 37,000 feet.

OBJECTIVE En-route care necessitates the evacuation of seriously wounded service members requiring mechanical ventilation in aircraft where low light, noise, vibration, and barometric pressure changes create a unique clinical environment. Our goal was to evaluate ventilatory requirements, oxygenation, and oxygen use in flight and assess the feasibility of a computer interface in this austere environment. METHODS A personal computer was integrated with the pulse oximeter and ventilator data port used in aeromedical evacuation from Iraq to Germany. Ventilator settings, inspired oxygen (FiO2), tidal volume (VT), respiratory rate (RR), minute ventilation (VE), monitored values, heart rate (HR), and oxygen saturation (SpO2), were recorded continuously. Oxygen use was determined using the equation ([FiO2 - 21]/79) x (MVE). Additional data were obtained through the United States Air Force (USAF) Transcom Regulation and Command/Control Evacuation System (TRAC2ES) and the United States Army Institute of Surgical Research Joint Theater Trauma Registry databases. RESULTS During a 4 month time frame 117 hours of continuous recording was accomplished in 22 patients. Mean age was 27 +/- 9.83 and injury severity score military was 31.75 +/- 20.63 (range, 9-75). All patients survived transport. Mean values for ventilator settings were FiO2 (24-100%) of 49% +/- 13%, positive end-expiratory pressure of 6 +/- 2.5 (range, 0-17 cm H2O), RR of 15 +/- 2.4 (range, 10-22 breaths/min), and VT of 611 +/- 75 (range, 390-700 mL). Delivered VT in mililiter per kilogram was 6.9 +/- 1.30 and VE was 9.1 L/min +/- 1.4 L/min. Oxygen requirements for desired FiO2 and VE resulted in a mean oxygen usage of 3.24 L/min +/- 1.87 L/min (range, 1.6-10.2 L/min). There were 32 changes to FiO2, 18 changes to PEEP, 26 changes to RR, and 20 changes to VT during flight. Five patients under-went no recorded changes in flight. Three desaturation events (<90%) were recorded lasting 35, 115, and 280 seconds. Recorded ventilatory changes averaged less than 1 (0.82) per hour of recorded flight with FiO2 being the most common. CONCLUSIONS A computer interface is feasible in the austere aeromedical environment. Implications to military operations and civilian homeland defense include understanding casualty oxygen requirements for resource planning in support of aeromedical evacuation. Portable oxygen generation systems may be able to provide adequate oxygen flow for transport, reducing the need for compressed gas. Future studies of oxygen conservation systems including closed loop control of FiO2 are warranted.

Autonomous Control of Inspired Oxygen Concentration During Mechanical Ventilation of the Critically Injured Trauma Patient

BACKGROUND Transport of mechanically ventilated patients in a combat zone presents challenges including conservation of resources. In the battlefield setting, the provision of adequate oxygen supplies remains a significant issue. Autonomous control of oxygen concentration may allow a reduction in mission load. METHODS Trauma patients requiring ventilation and inspired oxygen concentration (FIO2) greater than 0.35 were evaluated for study. Patients were randomized to consecutive 4-hour periods of autonomous control or standard care. The system for autonomous control consisted of a ventilator, oximeter, and a portable computer. The portable computer housed the control algorithm and collected ventilator and oxygen saturation (SpO2) data every 5 seconds. The controller goal was to maintain SpO2 at 94% +/- 2% via discrete changes of 1% to 5%. Ventilator settings and SpO2 were recorded every 5 seconds for analysis. RESULTS Fifteen patients were enrolled in this study. Oxygen saturation was maintained in the 92% to 96% saturation range 33% +/- 36% of the time during clinician control versus 83% +/- 21% during autonomous control. Oxygen usage was reduced by 44% during autonomous control. There was a slight difference in the total duration of SpO2 episodes less than 88% between groups (6.02 +/- 0.83 vs. 6.87 +/- 0.46 minutes, p < 0.05). There were no differences in the number of episodes of SpO2 <88%. CONCLUSION Autonomous control of FIO2 offers the opportunity for a reduction in oxygen usage, allowing a weight and resource reduction, without increasing risk of hypoxemia in ventilated trauma patients.

Closed-Loop Strategies for Patient Care Systems

Military operations, mass casualty events, and remote work sites present unique challenges to providers of immediate medical care, who may lack the necessary skills for optimal clinical management. Moreover, the number of patients in these scenarios may overwhelm available health care resources. Recent applications of closed-loop control (CLC) techniques to critical care medicine may offer possible solutions for such environments. Here, feedback of a monitored variable or group of variables is used to control the state or output of a dynamic system. Some potential advantages of CLC in patient management include limiting task saturation when there is simultaneous demand for cognitive and active clinical intervention, improving quality of care through optimization of the titration of medications, conserving limited consumable supplies, preventing secondary insults in traumatic brain injury, shortening the duration of mechanical ventilation, and achieving appropriate goal-directed resuscitation. The uses of CLC systems in critical care medicine have been increasingly explored across a wide range of therapeutic modalities. This review will provide an overview of control system theory as applied to critical care medicine that must be considered in the design of autonomous CLC systems, and introduce a number of clinical applications under development in the context of deployment of such applications to austere environments.

Cardiopulmonary resuscitation during spaceflight: examining the role of timing devices.

INTRODUCTION The majority of International Space Station (ISS) astronauts represent nonmedical professions. In order to serve as Crew Medical Officers (CMO), future crewmembers receive 40-70 h of medical training within 18 mo before missions, including cardiopulmonary resuscitation (CPR) per the Guidelines of the American Heart Association. CPR compliance with the Guidelines is known to vary even among trained clinicians, let alone minimally trained caregivers (e.g., bystanders, nonphysician astronauts). The purpose of this study was to evaluate the effect of timing devices, including audible metronomic tones, on CPR performed by nonmedical personnel, specifically 40 astronaut analogues trained in a fashion and within a timeframe similar to an ISS astronaut. METHODS Twenty bystander pairs performed two-person CPR for 4 min on a simulated cardiac arrest patient using three interventions: 1) CPR with no timing devices; 2) CPR with metronomic tones for chest compressions; and 3) CPR with a timing device and metronome for coordinating ventilation and compression rates, respectively. Each CPR performance was evaluated for compliance with the (then current) 2000 AHA Guidelines. RESULTS Numbers of breaths and compressions significantly deviated from target values in the first two interventions (38 and 42 breaths vs. target of 32 breaths; 282 and 318 compressions vs. target of 240 compressions); the use of timing devices for both components of CPR resulted in significant improvement (32 breaths and 231 compressions). CONCLUSIONS CPR timing devices that coordinate both breaths and compressions improve compliance of astronaut analogue rescuers with CPR guidelines, and may improve overall CPR performance and outcome.