Improved anesthesia support of the forward surgical team: A proposed combination of drawover anesthesia and the life support for trauma and transport

The Ohmeda Portable Anesthesia Complete is the apparatus available for delivery of general anesthesia in the forward surgical teams in the U.S. Army. The Life Support for Trauma and Transport is being field tested for use as a single patient critical care transport bed. An effective circuit was created which linked the currently fielded draw-over anesthesia machine with the patient ventilator (Impact 754 Eagle) mounted in the Life Support for Trauma and Transport, with bench testing indicating that the anesthesia levels were accurate and that it was a useful system for field resuscitation and surgery. Others should be able to utilize this information for the benefit of their patients in field environments, especially forward surgical teams and others working in austere health care locations.

Introduction

Provision of resuscitative surgery near the battlefield in the U.S. Army is the prime mission of the forward surgical teams (FST). Anesthesia providers may administer general anesthesia in these units with the Ohmeda Portable Anesthesia Complete (PAC), which is the only apparatus presently available in the FST for this purpose. Currently, the table of organization and equipment maintained by FST does not contain a ventilator, which can be used with the Ohmeda PAC. Therefore, during the administration of anesthetics, either the patient spontaneously ventilates or the provider must squeeze the self-inflating bag for positive pressure ventilation. A ventilator, which works with the Ohmeda PAC, would allow the anesthesia provider the opportunity to conduct a full range of resuscitative interventions while providing the patient positive pressure ventilation in a consistent and reliable manner.

The Life Support for Trauma and Transport (LSTAT) is a rugged single patient intensive care bed being field tested for deployment to units in the U.S. Army. This platform contains an integrated system of capabilities, including ventilation with onboard oxygen, fluid and drug infusion pumps, suction, defibrillation, blood chemistry analysis, physiological monitoring as well as data accumulation and communications connectivity. The LSTAT is compatible with standard evacuation vehicles in the U.S. Army inventory. The Department of Resuscitative Medicine in the Division of Military Casualty Research at Walter Reed Army Institute of Research developed the LSTAT for the minority of patients being evacuated from the battlefield who require continuing critical care during transport.

The Impact model 754 ventilator was integrated into the LSTAT. The Impact ventilator is primarily a stand-alone transport ventilator. When external compressed gases are not available, as they will not typically be for FST, the Impact ventilator uses an internal compressor to provide appropriate volumes for positive pressure ventilation. Low-pressure oxygen can supplement the minute ventilation provided by the 754 ventilator. The LSTAT does have on-board compressed oxygen, but this is designed for short legs of the evacuation, primarily as a bridge from one source of compressed gas to another.

The other capabilities present on the LSTAT allow for monitoring a variety of hemodynamic and physiologic parameters. These capabilities match those required by the American Society of Anesthesiologists in their guidelines for intraoperative monitoring. Therefore, when the patient on the LSTAT is cared for by an anesthesiologist or nurse anesthetist, all of the American Society of Anesthesiologists guidelines for anesthesia care can easily be provided.

The goal of this study was to examine the feasibility of use of the LSTAT and the incorporated ventilator as an anesthetizing location. Specifically, the object of this bench study was to document the output of the inhalational anesthetic gas concentrations from the Ohmeda PAC when connected to the ventilator on board the LSTAT. We expected clinically acceptable and titratable anesthetic concentrations produced from this combination of equipment.

Methods

The Ohmeda PAC may be connected to the ventilator on the LSTAT, as well as the Impact model 754 ventilator, using an H-type circuit which allows for spontaneous ventilation, hand-- controlled ventilation, and ventilator-controlled ventilation (Figs. 1-3). A single check valve in the interlinking limb of the H circuit allows flow of gas from the drawover device to the patient when the patient is spontaneously ventilating or when the patient's respirations are controlled with use of the self-inflating bag of the Ohmeda PAC. Similarly, the check valve prevents flow back to the anesthesia apparatus when the ventilator is used to control ventilations.

A bench study was performed to determine output characteristics of the conjoint system of the Ohmeda PAC and the LSTAT. Anesthetic gas concentrations were measured at the test-lung/ patient connection with an Ohmeda respiratory gas monitor model 5250. This side-stream sampling respiratory gas monitor had been calibrated according to the operator's manual prior to use.

To approximate the level of inspired oxygen delivered by an oxygen concentrator found in a FST-pressurized oxygen routed through a flowmeter at 4 L/min was attached to the supplementation nipple of the Ohmeda PAC. A 750-mL oxygen reservoir was used in this study.

Typical minute ventilations were used to test the output of the system (Table I). The Impact ventilator was set and the tidal volumes were checked with a Wright's spirometer.

Clinically similar and acceptable ranges of oxygen were attained with each system (Fig. 4).

Discussion

The clinical applicability of the combination of the Ohmeda PAC and the LSTAT in terms of the anesthetic delivery is obvious to anesthesiologists and nurse anesthetists. The vapor concentrations produced by this system are within clinically appropriate ranges. Although low-end delivery is much less than expected based on the disk dial setting, the incremental increase in percentage delivered allows for titrated administration of anesthetic agent. The smooth transition of isoflurane produced across the clinical ranges used in the operating room are clearly evident in Figures 5 and 6. This output is most desirable in any anesthetic delivery system.

The variation between vaporizer settings and system output underscores the authors' concern that a field-worthy respiratory gas monitor is also needed in FST and field hospitals. The Impact model 754 ventilator will need to be fully tested by the manufacturer to ensure that the Federal Food and Drug Administration concerns are allayed before clinical testing and use of this technique. Additionally, as this circuit was engineered from circuits and connections primarily used for nonanesthetic uses, it too will need to be further investigated before Food and Drug Administration approval for use in this manner.

Guarantor: COL Paul C. Reynolds, MC USA

Contributors: COL Paul C. Reynolds, MC USA*; MAJ Mark Calkins, MC USA^; Timothy Bentley, PhD;^ MAJ Christian Popa, MC USA^^

*SHAPE Healthcare Facility, Avenue d'Oslo, Batiment 401, Casteau, Belgium 7010.

^Department of Resuscitative Medicine, Walter Reed Army Institute of Research, Silver Spring, MD 209 10.

^^Department of Critical Care Medicine, Walter Reed Army Medical Center, Washington, DC 20307-5001.

This manuscript was received for review in June 2002. The revised manuscript was accepted for publication in December 2001.