Add-on MixingChamber forMechanical VentilatorTeam Members: Missy Haehn, LauraSheehan, Ben Sprague, AndreaZeliskoClient: Matt O’Brien, RPTAdvisor: Professor WebsterMay 5, 2004 2Abstract:The metabolic rates of critically ill patients can vary drastically from typical metabolic rates. This can cause problems for health care providers when determining the correct amount to feed the patient. Matt O’Brien, RRT, performs metabolic tests on these critically ill patients to determine their metabolic rate and food intake needs. This is done with indirect calorimetry, measuring the concentration of oxygen of the inspired and exhaled air of the patient. In order to stabilize the inspired oxygen level produced by the ventilator, our final design is an air mixing chamber placed within the series of sir flow from the ventilator to the patient. Problem Statement: To develop a mixing chamber to help stabilize oxygen percentage delivered from mechanical ventilators to critically ill patients; this chamber would allow for increased accuracy of metabolic measurements. Background: Metabolism is defined as the amount of energy one uses throughout the course of a day (1). The body’s metabolism is similar to a scale balance in that the calories fed to a person must equal the amount of heat energy produced by the patient’s body (Figure 1).Contrary to popular belief, critically-ill patients receiving supplemental oxygen via a mechanical ventilator “resting” in bed actually have a much greater metabolic rate than healthy resting individuals (2). Due to the higher metabolic rate of a ventilated patient, a different caloric intake needs to be calculated tailored to the patients needs. This is very important because of the incidence of malnutrition among critically ill patients on mechanical ventilators (3). This malnourishment can lead to lengthened hospital stays and unnecessary ventilator support (4). The vital task of determining the caloric needs of a patient falls to the Pulmonary Respiratory Technician. Calculating this need is done by a method called “indirect calorimetry.” According to The American Association for Respiratory Care, metabolic measurements use indirect calorimetry “to reduce the incidence of overfeeding and underfeeding and to decrease costs3Figure 1: The balance of caloric intake and energy output (6).associated with total parenteral nutrition (5).” Indirect calorimetry is performed by measuring the dimensionless respiratory quotient, RQ, which is the CO2 produced divided by the O2 consumed (4). From this value, one can determine if the patient is receiving too few or too many calories, because what someone eats affects the level of CO2 production. For example, a patient consuming a diet consisting of primarily carbohydrates produces a significantly higher amount of CO2 from metabolism. Because ofthis high level of CO2, it is harder to maintain a sufficient level of ventilationand can result in either respiratory failure or difficulty in weaning the patient from the ventilator due to a reduction in respiratory muscle strength(2). Ideally, a reasonable adjustment in the patient’s diet is to reduce the carbohydrates and increase the lipids consumed, thus reducing the amount of CO2 produced by the patient’s metabolism and making ventilation easier. The patient’s diet is modified accordingly and metabolic measurements are repeated shortly afterward to determine the effectiveness of the new nutritional regimen. The validity of the metabolic measurement can be assessed by lookingto see if the RQ is appropriate for the patient’s diet and if it is within the “normal” physiological range, this being 0.67 to 1.3 (5). Generally, overfeeding increases this value while underfeeding decreases it (3). Occasionally, a technician finds him or herself with an RQ outside the “normal” range and must determine why this unusual data is occurring. Recalling that RQ is the ratio of carbon dioxide expired to oxygen inspired, one would expect to theoretically find the percent oxygen inspired by the patient (also called the fractional inspirational oxygen or FIO2) equal to what percent oxygen the ventilator is delivering. However, often the FIO2 value is inconsistent due to the varying concentrations of oxygen delivered to a patient from breath to breath. This can be caused by one of two factors: the mixing of the gasses or changes in gas pressure between one end to another (pressure drops). Therefore, the FIO2 must be measured on a breath-to-breath basis during these metabolic tests and the inconsistencies from gas concentration and pressure must be made as smallas possible in order to have appropriate RQ values. The Association of American Respiratory Care states that this can be done by either a blender for high-pressure gas or “an inspiratory mixing chamber between the ventilator main flow circuit and the humidifier (5).” Lastly, to ensure complete understanding of how the testing of these metabolic measurements is performed, the set-up of the hardware involved should be explained in more detail.The critically ill patient is breathing via a mechanical ventilator. This ventilator has two scissors valves, which, from the percent oxygen entered by the technician or physician on the ventilator display, calculate the respective amounts of air and oxygen to be mixed and subsequently delivered to the patients. This mixture of gas leaves the mechanical ventilator and heads to the humidifier device where it is heated and humidified. From the humidifier the gas travels down a tube to the 4patient’s lungs. However, before it reaches the patient it enters a pneumotach during metabolic measurements. The pneumotach is a device that determines the rate of flow of the gas. In order to measure the FIO2, there is a gas sample line that constantly is sucking out a tiny amount of thegas mixture to be delivered to the patient (figure 2). This gas sample is then analyzed on a breath-by-breath basis by the metabolic measurement software (our client’s is made by MedGraphics, Figure 3) and from this data, the RQ is determined and the necessary nutritional adjustments can be made. The mixing chamber, which is recommended above to create a more uniform gas mixture, would be placed between the ventilator and humidifier. The improvement made from the addition of this chamber wouldbe seen from the sample taken at the site of the pneumotach and its
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