Reducing the incidence of surgical fires: supplying nasal cannulae with sub-100% O2 gas mixtures from anesthesia machines

In June 2003, the Joint Commission on Accreditation of Healthcare Organizations (JCAHO) recommended: "As a general policy, use air or FiO2 at < or =30% for open delivery (consistent with patient needs)" to prevent surgical fires. One way to interpret JCAHO's recommendation is that 100% O2 should not be indiscriminately used, and anesthesia providers should have the ability, consistent with patient needs and their clinical judgment, to deliver sub-100% O2 with nasal cannulae. An auxiliary O2 flowmeter has a barbed outlet connector that offers a convenient means to connect a cannula to an anesthesia machine and is routinely used for open delivery of 100% O2. The auxiliary O2 flowmeter provides only 100% O2 and thus does not allow titration of the O2 concentration to patient needs and may increase the risk of surgical fires. This report clarifies the JCAHO recommendation and describes different means of addressing it that are based primarily on using the anesthesia machine to blend a sub-100% O2 gas mixture and delivering it via a nasal cannula. The options presented depend on the model and manufacturer of the anesthesia machine and allow delivery via nasal cannula of O2 concentrations that range from 21% to 100%.     

Factors influencing capnography in the bain circuit

The Bain circuit provides continuous fresh gas flow near the airway. The potential mixing of this fresh gas with expired gas may prevent reliable analysis of expired gas. We therefore investigated the interaction of sampling site, fresh gas flow rate, expiratory flow rate, and sampling flow rate on expiratory capnography. Sampling near the fresh gas outlet yielded inaccurate results under several of these conditions. The magnitude of the error was related to the fresh gas and expiratory flow rates. A reliable sampling region near the endotracheal tube was identified.     

Twitcher: A Device to Simulate Thumb Twitch Response to Ulnar Nerve Stimulation

Objective. To design and fabricate a device to simulate evoked thumb adduction in response to ulnar nerve stimulation. Methods. We implemented a computer-controlled, motorized thumb (TWITCHER) that responds to ulnar nerve stimulation by an unmodified peripheral nerve stimulator. Clinically realistic response patterns are generated for both depolarizing and non-depolarizing muscle relaxants and three modes of stimulation (single twitch, train-of-four, tetanus). Results. The device has been used in a full-scale patient simulator for the last six years. Discussion. TWITCHER has been well received by participants in simulation exercises including the use of neuromuscular blocking drugs.     

Automatic endotracheal tube position confirmation system based on image classification--a preliminary assessment

Endotracheal intubation is a complex medical procedure in which a ventilating tube is inserted into the human trachea. Improper positioning carries potentially fatal consequences and therefore confirmation of correct positioning is mandatory. This paper introduces a novel system for endotracheal tube position confirmation. The proposed system comprises a miniature complementary metal oxide silicon sensor (CMOS) attached to the tip of a semi rigid stylet and connected to a digital signal processor (DSP) with an integrated video acquisition component. Video signals are acquired and processed by a confirmation algorithm implemented on the processor. The confirmation approach is based on video image classification, i.e., identifying desired expected anatomical structures (upper trachea and main bifurcation of the trachea) and undesired structures (esophagus). The desired and undesired images are indicators of correct or incorrect endotracheal tube positioning. The proposed methodology is comprised of a continuous and probabilistic image representation scheme using Gaussian mixture models (GMMs), estimated using a greedy algorithm. A multi-dimensional feature space, which consists of several textural-based features, is utilized to represent the images. The performance of the proposed algorithm was evaluated using two datasets: a dataset of 1600 images extracted from 10 videos recorded during intubations on dead cows, and a dataset of 358 images extracted from 8 videos recorded during intubations performed on human subjects. Each one of the video images was classified by a medical expert into one of three categories: upper tracheal intubation, correct (carina) intubation and esophageal intubation. The results, obtained using a leave-one-case-out method, show that the system correctly classified 1530 out of 1600 (95.6%) of the cow intubations images, and 351 out of the 358 human images (98.0%). Misclassification of an image of the esophagus as carina or upper-trachea, which is potentially fatal, was extremely rare (only one case when in the animal dataset and no cases when in the human intubation dataset). The classification results of the cow intubations dataset compare favorably with a state-of-the-art classification method tested on the same dataset.     

Effects of expiratory flow resistance on inspiratory work of breathing

To minimize work of breathing, airway pressure should not fluctuate during spontaneous breathing with continuous positive airway pressure (CPAP). However, flow resistance in the inspiratory limb of the breathing circuit and an inadequate continuous gas flow rate result in airway pressure fluctuation and increased work of breathing. Flow resistance of the expiratory pressure/exhalation valve also directly affects the level of airway pressure during spontaneous inhalation with CPAP (the greater the resistance of the valve, the greater the decrease in airway pressure and work of breathing). We compared this effect with three types of expiratory pressure valves: a threshold resistor with low resistance to flow, an inflatable balloon (mushroom) valve with moderate resistance to flow, and a variable-orifice flow resistor with a high resistance to flow. Work increased up to threefold with the balloon valve and more than tenfold with the flow resistor compared with the threshold resistor. To apply CPAP, expiratory pressure valves with low resistance to flow should be used to minimize fluctuations in airway pressure and, thus, in the work of spontaneous breathing.     

Capnography and the bain circuit II: Validation of a computer model

Validation of a computer model is described. The behavior of this model is compared both with mechanical ventilation of a test lung in a laboratory setup that uses a washout method and with manual ventilation. A comparison is also made with results obtained from a volunteer breathing spontaneously through a Bain circuit and with results published in the literature. This computer model is a multisegment representation of the Bain circuit and connecting tubing. For each segment, gas pressure, gas volume flow, and partial pressure of carbon dioxide are calculated for any number of breaths wanted. As a result, the time course of these variables can be generated for any location or, conversely, the carbon dioxide distribution in the system can be calculated for any time instant. A test lung, the human lungs, the ventilator bellows, and the reservoir bag are each represented by a single segment. The shapes of pressure and flow curves and of the capnograms taken at different locations in the Bain tubing are in good agreement. The washout study permits measurement of the time delay between the first expiration and the arrival of carbon dioxide at a particular location. The carbon dioxide level in the test lung decreases during inspiration and is stable during expiration. Quantitative agreement between model and experimental transport delays and carbon dioxide levels is such that the differences can be explained by the inaccuracy of the measurement. This is concluded from a sensitivity analysis. The study of the effect of segment size shows an almost optimal agreement between model behavior and experimental results for a 36-segment model. Execution of a thorough validation is imperative before such models can be used for clinical management and decision making or for teaching.     

Magnetron-based inline microwave fluid warmer

Objective. To develop an in-line microwave fluid warming system that eliminates the difficulties of uneven heating that are characteristic of batch-mode microwave fluid warmers.Methods. Using a commercial microwave oven, we developed a method for warming fluid as it flowed through tubing along a defined path in the oven's cavity. Algorithms utilizing either proportional or adaptive control were used to control microwave heating cycles by varying the heating pulse-width during 3-second epochs. Methods of fluid entry and exit were devised to minimize microwave leakage. Heating performance was tested using icewater at multiple flow rates from 18 mL/min to 105 mL/min.Results. In all warming tests, the system achieved temperature control without exceeding the maximum temperature allowable based on American Association of Blood Banks requirements. The adaptive control maintained the set temperature, with peak-to-peak oscillations of 2°C or less. Microwave leakage was below the commercially required limit for home microwave appliances.Conclusions. The combination of proportional and adaptive control is successful in controlling the permanent magnet magnetron microwave energy to heat the icewater tested. The in-line microwave warmer has the potential to become a successful medical fluid warmer. More study is needed to determine the stability of the control system under clinical conditions, and to evaluate its utility for warming blood.     

The effect of a bellows leak in an Ohmeda 7810 ventilator on room contamination, inspired oxygen, airway pressure, and tidal volume

We investigated the effect of a small bellows leak (bellows full at end-expiration) on inspired oxygen fraction (Fio(2)), exhaled tidal volume (Vt), airway pressure, and room contamination in an oxygen-driven anesthesia ventilator (Ohmeda 7810, Madison, WI). CO(2) concentration at the ventilator exhalation valve, Fio(2), Vt, and airway pressure were measured (n = 3) while ventilating a CO(2)-producing test lung at 8 breaths/min and an inspiratory/expiratory ratio of 1:2, with and without a bellows leak (4-mm-long tear). Set Vt was 400, 600, 800, and 1000 mL. Fresh gas flow (FGF) was 0.3 L/min O(2) and (a) 5.0 L/min air, (b) 2.0 L/min air, and (c) 0.2 L/min nitrogen. There was no clinical difference in Fio(2), Vt, PIP (peak inspiratory pressure) and PEEP (positive end-expiratory pressure), with and without a 4-mm bellows tear, at all FGFs and Vt settings. CO(2) at the ventilator exhalation valve was always nonzero with a bellows leak, indicating that CO(2)-laden circuit gas was contaminating the drive gas via the bellows leak. A 4-mm bellows tear in an Ohmeda 7810 ventilator allows anesthetic gases to contaminate ambient air but does not cause clinically significant changes in Fio(2), exhaled Vt, PIP, or PEEP.