Design of a Pressure Measuring Syringe

Endotracheal intubations are performed on thousands of patients each day. Intubation is achieved by inserting a small plastic tube down a patient’s trachea, allowing oxygen and anesthetics to be delivered directly to the lungs. The tube is held in place by inflating a small cuff on the distal tip, which also serves to seal the trachea. The use of a manometer to measure the pressure within the cuff is essential to keep the practice safe. Hyperinflation of the cuff can put too much pressure on the trachea, leading to tissue death and post-procedure patient discomfort. A hypo-inflated cuff results in a poor seal within the patient’s airway and can lead to ineffective positive pressure ventilation, or gastro-inflation, which can in turn lead to vomiting, putting the patient at risk for asphyxiation. The latter complication can cause hypoxia and death. Manometers used to measure cuff pressure are costly, cumbersome, and potentially inaccurate. A pressure measuring syringe has been designed, tested, and verified to meet physicians’ needs for a simple, low-cost pressure measurement device. New data suggest that overblown cuffs are very common during surgery (2009, Abstract 3AP1-1, presented at the European Society of Anaesthesiology, Milan, Italy). In fact, most are inflated to a pressure greater than the recommended 25 cm $H2O$⁠, and past studies on patients in critical care settings corroborate these observations (Jaber, S., et al., 2007, “Endotracheal Tube Cuff Pressure in Intensive Care Unit: The Need for Pressure Monitoring,” Intensive Care Med., 33, pp. 917–918). A pressure-sensing device that gives physicians a tool to help avoid over- and underinflation of the endotracheal tube (ETT) cuff was able to provide an accurate, repeatable measurement of the intracuff pressure. A deterministic design process was used to develop a set of functional requirements for a pressure measuring device that accomplishes both inflation of the cuff and a simultaneous measurement of the cuff pressure. A silicone bellow inside the body of the plunger acts as a single elastomechanical measurement device, permitting a highly repeatable measurement of the intracuff pressure. The design also maintains most of the traditional syringe design in that only the plunger is modified to accommodate the bellows. The components of the syringe are also scalable in order to allow the design to be utilized for other pressure sensitive procedures. The current iteration of the syringe can accurately measure pressure within a range of 0–40 cm $H2O$⁠. Prototypes for the syringe were 3D printed and tested, and silicone rubber bellows were outsourced. In the final prototoype, the plunger is injection molded. The total estimated final cost of the syringe is about $1.50, which is comparable to the cost of a typical syringe. Because of this, the pressure measuring syringe is a viable candidate for low-cost mass production. The calculated pressure-deflection relationship of the bellows was experimentally verified, further demonstrating the scalability of the design. In conclusion, a simple and cost-effective syringe manometer has been developed, which controls and measures air pressure in ETT cuffs.