Critical comparisons of the clinical performance of oxygen-conserving devices

Abstract

Rationale: Clinical testing of oxygen-conserving devices is not mandated before marketing. Consequently, little is known about individual or comparative therapeutic effectiveness.

Objectives: To relate oxygen delivery from prototypical instruments to physiological performance.

Methods: Thirteen subjects with obstructive lung disease performed progressive treadmill exercise while inhaling either room air, 2 L O(2)/min, or bolus oxygen from four commercially available conserving devices at regulator settings of 2, 5, and continuous. The devices were studied blindly in random order after first being tested to determine performance characteristics. Pulse oximetry, oxygen delivery, and nasal and oral ventilations were monitored at rest and with exertion.

Measurements and main results: At a setting of 2 at rest, all conservers maintained saturation greater than 90%, but there were significant differences in oxygenation between systems. Only one equaled 2 L O(2)/min. With exertion, saturation decreased with all conservers but not with 2 L O(2)/min. One device did not perform any better than room air. Two systems provided less oxygen than predicted, one more, and in one the expected and actual amounts were equal only at rest. Breath-by-breath performance was highly variable, with irregular activation and inconsistent oxygen bolus size delivery. Increasing oxygen pulse volume to the point of eradicating conservation with the continuous setting did not eliminate all disparities.

Conclusions: The mechanical and clinical performances of current oxygen conservers are highly variable and in some instances actually contribute to limitations in exercise ability. Seemingly equivalent technical features do not guarantee equivalent therapeutic functionality.

Figure 1.

Representative example of the findings in an individual subject during each trial. The labeling at the top of each column denotes the separate trials; that at the bottom indicates the Naughton stage achieved. The nasal and mouth airflow signals are unidirectional because only inspiration is displayed. In the Device 4 experiment, the polarity was reversed. Note the decreases in percent arterial saturation as measured by pulse oximetry (Sp_O2) with exertion with each conserver, the fluctuating breathing pattern, and the inconsistent conserver actuations and bolus sizes. Nasal V·e = the minute ventilation with nose breathing in L/min. Mouth V·e = the minute ventilation with mouth breathing in L/min; O₂ cylinder flow = the flow rate of O₂ from each conserver in L/min.

Figure 2.

Changes in pulse oximetry during rest and exercise in all experiments. The ordinate displays arterial saturation as measured by pulse oximetry in percent (Sp_O2) and the abscissa displays Naughton work stages. The insert indicates the studies performed with room air (RA), O₂ at flows of 2 L/min (2LO₂), and devices 1 through 4 (D1–D4). The data points are mean values and the brackets indicate 1 SEM. The zero values represent resting observations. A, B, and C depict the experiments performed at regulator settings of 2, 5, and continuous, respectively.

Figure 3.

Minute ventilations during all experiments. The ordinate displays minute ventilation (V·e) in L/min and the abscissa displays Naughton work stages. The insert indicates the studies performed with room air (RA), O₂ at flows of 2 L/min (2LO₂), and devices 1 through 4 (D1–D4). The data points are mean values and the brackets indicate 1 SEM. The zero values represent resting observations. A, B, and C depict the experiments performed at regulator settings of 2, 5, and continuous, respectively.

Figure 4.

Comparison of the predicted and delivered quantities of O₂ provided by each conserver. The abscissa displays the predicted values derived from ex vivo testing and the ordinate the amounts actually provided during in vivo use. The data points are mean values and the brackets 1 SEM. The solid lines are the lines of identity. The solid symbols indicate data obtained at rest; open symbols depict exercise. The prefixes 2, 5, and cont refer to the 2, 5, and continuous regulator settings studied. The notations D1, D2, D3, and D4 indicate the various conservers. The insert in the D2 graph presents the findings with the continuous setting.

Figure 5.

Comparison of conserver firing and respiratory frequency for each instrument. The ordinates indicate conserver actuation per minute (apm). The abscissa indicates breaths per min (bpm). The data points are individual values obtained at the 2 and 5 regulator settings. The notations D1 through D4 represent each device. The solid lines are the lines of identity.

Figure 6.

The patterns of oxygen cylinder flow associated with each conserver at a regulator setting of 2. These data represent individual O₂ flow profiles at rest and during exercise in each subject for each device. D1 to D4 indicates device number. S1 to S13 indicates subject number. The ordinates are O₂ flow in L/min and the abscissas are the Naughton work stages completed. The zero time point indicates resting values. The termination of exercise is marked by diamonds in each graph. The prominent features are the sporadic pattern of firing and irregular flow profiles. Note that Device 2 went for long periods without activating.

Figure 6.

The patterns of oxygen cylinder flow associated with each conserver at a regulator setting of 2. These data represent individual O₂ flow profiles at rest and during exercise in each subject for each device. D1 to D4 indicates device number. S1 to S13 indicates subject number. The ordinates are O₂ flow in L/min and the abscissas are the Naughton work stages completed. The zero time point indicates resting values. The termination of exercise is marked by diamonds in each graph. The prominent features are the sporadic pattern of firing and irregular flow profiles. Note that Device 2 went for long periods without activating.