Prototype Oxygen Flowmeter Delivers Supplemental O2 at Variable Rate

Although supplemental oxygen is an essential element of patient care for a wide range of clinical conditions, it compromises both carbon dioxide and peripheral capillary O₂ saturation monitoring methods, making it difficult to detect respiratory depression. However, that issue may be resolved by the development of a prototype intelligent O₂ flowmeter, which delivers supplemental O₂ at a variable rate, and only during inhalation.

As Joseph A. Orr, PhD, research professor of anesthesiology at the University of Utah, in Salt Lake City, noted, O₂ is often delivered during sedation through a nasal cannula, and at a constant flow rate. When delivered at a constant, high flow rate, supplemental O₂ interferes with end-tidal CO₂ measurement by diluting exhaled CO₂. Supplemental O₂ also maintains high arterial O₂ levels during minimal flow, preventing pulse oximetry from notifying clinicians of respiratory depression.

To address this issue, the researchers developed the flowmeter, which is designed to not dilute end-tidal CO₂.

The researchers enrolled 30 healthy volunteers, all of whom were fitted with a nasal cannula containing ports for both O₂ delivery and sampling pressure. The cannula was modified to measure intranasal pressure and sample CO₂. A semiautomated system administered nasal O₂ through the cannula at flow rates of 0 to 10 L per minute, using either continuous or pulsed inspiratory flow. Each flow rate and mode combination was delivered for 30 seconds.

At the start of the testing period, an anesthesia gas analyzer was used to collect baseline end-tidal CO₂ values with no O₂ flowing. CO₂ waveforms were collected from the gas analyzer.

The prototype device incorporated a simple threshold algorithm to analyze patients’ breath cycles, where the start of expiration occurred when CO₂ rose above 20 mm Hg and the start of inspiration occurred when CO₂ fell below 5 mm Hg. End-tidal CO₂ was considered the maximum CO₂ value during a breath cycle. The average end-tidal CO₂ values were calculated.

“Our demand oxygen system uses a pressure sensor in the naris to measure when the patient is inhaling,” said Dr. Orr, who originally presented his findings at the 2016 annual meeting of the American Society of Anesthesiologists (abstract A1104). “When this happens, it delivers the oxygen in proportion to the depth of their breathing.”

The observed end-tidal CO₂ value when using the intermittent mode on the device was within ±0.57 mm Hg for all flow rates. When using constant flow, on the other hand, the error increased as the supplemental O₂ flow rate increased. Indeed, at a constant flow rate of 2 L per minute, the end-tidal CO₂ measurement was 5.97 mm Hg below baseline. Not surprisingly, the largest error occurred when delivering a constant flow of 10 L per minute, where the end-tidal CO₂measurement was 21.45 mm Hg below baseline. Although statistical analysis revealed no significant difference when monitoring end-tidal CO₂ with the prototype flowmeter, a statistically significant (P<0.05) difference was observed for all rates during constant flow.