Volumetric Airflow Gauge M. Chakan, J. Kiswardy, M. Nilo Introduction The United States 911 emergency call center receives an average of 500,000 calls daily; of these approximately 35% involve individuals with some type of cardiopulmonary failure. Most common method of initiating breathing in an individual suffering from cardiac arrest or pulmonary failure, aside from CPR, is through the use of a manual resuscitator or bag-valve-mask (BVM) system. Efficacy of resuscitation using a manual BVM is highly dependant on the training and skill level of the user and, as a result, many potential complications exist, the most significant involving over/under inflation of the patients airway. Well documented side-effects include: Lung tissue damage Decreased lung compliance Gastric distension Regurgitation These side-effects frequently lead to co-morbidities involving increased hospital stay and cost incurred by the patient. A solution needs to be presented that would allow the user to better estimate and thereby control the amount of air that is introduced to the patients airway while using a manual BVM system. Project objective was to design a gauge that could be easily incorporated into a manual resuscitation device, providing the user with a constant volumetric display of air introduced to the patient. The device will potentially minimize the undesired side-effects associated with under/over inflation mentioned previously. Materials and Methods The overall design involved two separate components: electrical circuitry (Figure 1), which measures and displays the inspiratory air volume of the BVM to the user, and a casing that houses the circuitry and provides a streamlined product that easily incorporates into existing manual resuscitators. Circuit Design Objectives: 1) Measure an airflow between 0 and 1.6 L/s 2) Sample, calculate, and display the air volume in near real-time 3) Display the volume with a precision of 10 mL 4) Calculate the effective volume of air that entered into the patient’s lungs, by compensating for temperature expansion: V 2 =V 1 *[T 2 /T 1 ] 5) Warn the user when to replace the batteries 6) Provide a rescue breathing rate metronome to aid proper ventilation technique Solutions: 1) Use a mass-flow sensor (AWM720P1, Honeywell) which measures an airflow between 0 and 3.3 L/s 2) Use a microcontroller (PIC18F2321, Microchip Technology) with 512 bytes RAM, 8 kbytes ROM, and a 40 MHz oscillator 3) Use a 3-digit LED display (LDT-C514RI, Lumex) to display volumetric readings 4) Incorporate a thermistor (MCP9700A, Microchip Technology) to measure ambient air temperature 5) Independently sample the battery voltages and display low battery voltages with LEDs 6) Include an independent timing program to control an LED Results A prototype of the Volumetric Airflow Gauge has been developed and constructed (Figure 2 for circuit; Figure 3 for casing). Initial battery-life tests indicated that the use of one 9V battery permitted a circuit life of 10 minutes, while the use of two 9V batteries increased the circuit life to 120 minutes. Discussion A prototype of the Volumetric Airflow Gauge has been successfully developed and constructed. Upon successful completion of the accuracy verification tests, the volumetric airflow gauge will be a proven, practical device that will provide the user with an accurate display of the volume of air introduced to the patient during resuscitation. An accuracy of +/- 50 mL will help minimize the potential for over/under inflation of patients’ airways during manual resuscitation with a BVM. In addition, the simple design of the device allows rapid user comprehension and ease of use. Further validation of the device will be performed by specialists in the Emergency Medicine field. These voluntary specialists will have an opportunity to test the device and provide feedback based on their observations, thus providing additional confirmation of the benefits of the device. Acknowledgments The authors thank Dr. Hal Wrigley, Dr. Linda Baker, the BioEngineering Department, Guy Guimond, and Eric Reiss for their generosity and support. Following completion of circuit assembly (Figure 2), a battery-life test was performed to determine the functional device time before battery warnings were indicated. A test was performed that powered the entire circuit from one battery, while another test was performed that used two batteries: one powering the airflow sensor and one powering the remaining components. Additionally, accuracy tests are planned to compare the volume measured by our device with the volume measured by a human simulator. The difference in volume will undergo statistical testing (i.e. a t-test) to determine the device error. Casing Design Objectives: 1) Interface the casing with existing BVM devices 2) Eliminate air leaks from the flow path 3) House and protect the circuitry from damaging environments and treatments (e.g. rain & device dropping) 4) Minimize device weight Solutions: 1) Input/output ports were designed to fit existing BVM hardware 2) The mass-flow sensor input/output ports were snugly fit into the input/output ports of the casing, and reinforced with rubber gaskets 3) A robust casing was designed to snugly fit the circuitry, while rubber gaskets were added between joints to repel water 4) The prototype was first developed in SolidWorks, where the casing mass was determined and altered through continual redesign Abstract Individuals suffering from cardiopulmonary failure and other types of respiratory distress are commonly treated with a manual resuscitation device in order to initiate breathing. In many instances, excessive or insufficient air volume delivery occurs as a result of the user incorrectly estimating the amount of air administered to the patient. As a result, side-effects including lung tissue damage, gastric distension, and regurgitation have been observed. A volumetric airflow gauge was developed that provides the user with an accurate display of the volume introduced to the patients’ airway with each inflation/deflation cycle of the resuscitator device. The gauge is comprised of an electric circuit board that directly measures the airflow volume powered through the use of two external 9V batteries. A non-reactive plastic was used to construct the housing for the electrical components, permitting the device to be incorporated with relative ease into most standard manual resuscitators. The device will provide a cost-effective method for the user to accurately determine and control the amount of air he/she is administering to the traumatized patient. Figure 2. Completed circuitry of the Volumetric Airflow Gauge. Front: Numerical volumetric display and LED indicators; Back: Mass airflow sensor; Top left: Thermistor Figure 3. SolidWorks model of the Volumetric Airflow Gauge casing Left: Battery compartment and door; Center: Void for the completed circuit; Right: Cap to enclose circuit and direct flow through airflow sensor Figure 1. Volumetric Airflow Gauge circuit schematic.