The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

An alternate method to safely monitor a High Humidity NAsal cannula (HHNC)

* Eric Hurd, RRT-NPS, t Brian K. Walsh, RRT-NPS, RPFT, ^Douglas Petsinger, RRT-NPS,

^Jan D. Fernandez BS RRT-NPS, CPFT

*Georgetown University Hospital ^Children's Healthcare of Atlanta, t Univ. of Virginia Children's Hospital

Background: In the last couple of years, the use of HHNC has rapidly grown in neonatal intensive care units (NICU) across the country. Arguably, the leading HHNC system has been the Vapotherm (Vapotherm, Stevensville, MD, USA) which unfortunately is currently on voluntarily recall for enhancements. This has led to a surge of replacement systems from other manufactures as well as some expert ingenuity by Respiratory departments abroad. However, this new modality of gas therapy has introduced new areas of concern that require additional levels of safety. For instance, the small inner diameter of these small NC's can generate back pressures > 400 cm H20. For this reason, we have reviewed a method that not only monitors the system pressures (and indirectly flow), but also increases safety by providing an alarm system that is tied into the patients cardiac monitor.

Method:
We conducted several groups of studies. The purpose of the first was to establish a level of reproducibility using a typical invasive (vascular) pressure monitor setup in conjunction with a Sensor Medics 3100A HFOV (Viasys Healthcare, CA, USA). The vital sign monitor system consisted of a GE Solar 8000I Monitor (GE Healthcare Waukesha, WI), Marquette Tram-Rac 4 (GE Healthcare Waukesha, WI) and a Transpac® Transducer (IV Monitor Kit #42588-05; Hospira, Inc, Lake Forest, IL).  A heated wire HFOV circuit was set up and calibrated per the manufacture specifications. An endotracheal tube (ETT) was connected to the distal end of the circuit (as done with a patient) and the pressure transducer (PT) was inserted into the other end of the ETT. The PT was calibrated to 0 pressure (zero procedure). The ventilator was set at various pressure levels (i.e. MAP of 20 cm H20, Bias flow of 20 L/min, delta P 20, and a hertz of 15) and the corresponding pressure (mean) readings on the monitor recorded for a total sample size of 90. The second study group consisted of the same pressure monitor system in conjunction with a F&P Low-Flow Single Limb circuit (Cat 329; Fisher & Paykel East Tamaki, New Zealand), a 60 ml Luer-LokTM syringe (BD, Franklin Lakes, NJ), and a 2-inch piece of Argyle Universal Bubble Tubing (Tyco/Healthcare Kendall). The circuit was assembled in its normal fashion and the 60 ml syringe was connected to the gas delivery side of the humidification port while the distal end of the circuit was capped (where the NC would normally connect). The PT was directly connected to the male luer-lock adaptor (provided with the circuit) and the entire circuit filled with 490 ml of H20. The PT was calibrated to 0 pressure (zero procedure) and H20 was injected in 5 ml increments up to 30 ml. Prior to recording the corresponding pressure readings, a time of 60 seconds ( ± 20 seconds) for pressure equalization throughout the circuit. A total sample size of 155 recordings was obtained.

Results:
The first experiment using the HFOV showed results of: HFOV with 2.5 ETT & set MAP of 20 cm H20 = PT MAP 19 ± 0.0 cm H20; HFOV with 2.5 ETT & set MAP of 10 cm H20 = PT MAP 9.5 ± 0.0 cm H20; HFOV with 3.5 ETT & set MAP of 10 cm H20 = PT MAP 9.5 ± 0.18 cm H20. The results of the 2nd portion of the study involved a much wider range in pressures, so H20 was used instead of a compressible gas to reduce any variance in readings: the low pressure was an average of 76 cm H20 (95% CI 74-78) with a SD of ± 5 cm H20 and the highest pressure had an average of 449 cm H20 (95% CI 442-455) with a SD of ± 16 cm H20. Initially, there was a total of 31 sets (5mL-10mL-15mL-20mL-25mL-30mL), of which 6 sets had to be dropped during the statistical analysis of the results due to one or more results lying outside 2 SD.

Conclusion:
We have proven that a typical "arterial line" pressure setup can reliably measure gas pressure within a HHNC setup such as that manufactured by F&P. With further studies, this may prove to be a valuable alarming mechanism for things such as accidental disconnection, tubing occlusion, or any other undesired change in the gas flow setting.

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