Testing limits to airflow perturbation device (APD) measurements.

BACKGROUND: The Airflow Perturbation Device (APD) is a lightweight, portable device that can be used to measure total respiratory resistance as well as inhalation and exhalation resistances. There is a need to determine limits to the accuracy of APD measurements for different conditions likely to occur: leaks around the mouthpiece, use of an oronasal mask, and the addition of resistance in the respiratory system. Also, there is a need for resistance measurements in patients who are ventilated. METHOD: Ten subjects between the ages of 18 and 35 were tested for each station in the experiment. The first station involved testing the effects of leaks of known sizes on APD measurements. The second station tested the use of an oronasal mask used in conjunction with the APD during nose and mouth breathing. The third station tested the effects of two different resistances added in series with the APD mouthpiece. The fourth station tested the usage of a flexible ventilator tube in conjunction with the APD. RESULTS: All leaks reduced APD resistance measurement values. Leaks represented by two 3.2 mm diameter tubes reduced measured resistance by about 10% (4.2 cmH2O.sec/L for control and 3.9 cm H2O.sec/L for the leak). This was not statistically significant. Larger leaks given by 4.8 and 6.4 mm tubes reduced measurements significantly (3.4 and 3.0 cm cmH2O.sec/L, respectively). Mouth resistance measured with a cardboard mouthpiece gave an APD measurement of 4.2 cm H2O.sec/L and mouth resistance measured with an oronasal mask was 4.5 cm H2O.sec/L; the two were not significantly different. Nose resistance measured with the oronasal mask was 7.6 cm H2O.sec/L. Adding airflow resistances of 1.12 and 2.10 cm H2O.sec/L to the breathing circuit between the mouth and APD yielded respiratory resistance values higher than the control by 0.7 and 2.0 cm H2O.sec/L. Although breathing through a 52 cm length of flexible ventilator tubing reduced the APD measurement from 4.0 cm H2O.sec/L for the control to 3.6 cm H2O.sec/L for the tube, the difference was not statistically significant. CONCLUSION: The APD can be adapted for use in ventilated, unconscious, and uncooperative patients with use of a ventilator tube and an oronasal mask without significantly affecting measurements. Adding a resistance in series with the APD mouthpiece has an additive effect on resistance measurements, and can be used for qualitative calibration. A leak size of at least the equivalent of two 3.2 mm diameter tubes can be tolerated without significantly affecting APD measurements.

Human subject testing of leakage in a loose-fitting PAPR.

Leakage from loose-fitting PAPRs (powered air-purifying respirators) can compromise the safety of wearers. The Martindale Centurion MAX multifunction PAPR is a loose-fitting PAPR that also incorporates head, eye, and ear protection. This respirator is used in mines where coal dust usually is controlled by ventilation systems. Should the respirator be depended on for significant respiratory protection? Ten human volunteers were asked to wear the Centurion MAX inside a fog-filled chamber. Their inhalation flow rates were measured with small pitot-tube flowmeters held inside their mouths. They were video imaged while they breathed deeply, and the points at which the fog reached their mouths were determined. Results showed that an average of 1.1 L could be inhaled before contaminated air reached the mouth. As long as the blower purges contamination from inside the face piece during exhalation, the 1.1 L acts as a buffer against contaminants leaked due to overbreathing of blower flow rate.

Exercise-induced respiratory resistance changes as measured with the airflow perturbation device.

Respiratory resistance (Rrs) changes during physical labor can modify the work of breathing and thus influence the time that the labor can be performed. Ideal in vivo human respiratory performance measurements should have fine temporal resolution and minimally impact the respiratory system itself. The airflow perturbation device (APD) provides respiratory resistance measurements in this manner. Pre-exercise Rrs was compared with post-exercise Rrs, continuously monitored for 6 min in 12 non-asthmatic subjects. Rrs following exercise at 70-75% VO2max was below pre-exercise levels for 40 s into recovery (p < 0.05) and inhalation resistance differed more markedly than exhalation resistance. The same post-exercise Rrs decline was found when Rrs measurements were compared to those taken during flow rates comparable to those found late in recovery. Results indicate that (1) Rrs indeed declines following exercise in non-asthmatic subjects, probably due to bronchial dilation, (2) Rrs changes can occur on short time scales, (3) Rrs changes are not related primarily to flow rate.

Inward leakage in tight-fitting PAPRs.

A combination of local flow measurement techniques and fog flow visualization was used to determine the inward leakage for two tight-fitting powered air-purifying respirators (PAPRs), the 3M Breathe-Easy PAPR and the SE 400 breathing demand PAPR. The PAPRs were mounted on a breathing machine head form, and flows were measured from the blower and into the breathing machine. Both respirators leaked a little at the beginning of inhalation, probably through their exhalation valves. In both cases, the leakage was not enough for fog to appear at the mouth of the head form.

Using CO(2) to determine inhaled contaminant volumes and blower effectiveness in several types of respirators.

This experiment was conducted to determine how much contaminant could be expected to be inhaled when overbreathing several different types of respirators. These included several tight-fitting and loose-fitting powered air-purifying respirators (PAPRs) and one air-purifying respirator (APR). CO(2) was used as a tracer gas in the ambient air, and several loose-and tight-fitting respirators were tested on the head form of a breathing machine. CO(2) concentration in the exhaled breath was monitored as well as CO(2) concentration in the ambient air. This concentration ratio was able to give a measurement of protection factor, not for the respirator necessarily, but for the wearer. Flow rates in the filter/blower inlet and breathing machine outlet were also monitored, so blower effectiveness (defined as the blower contribution to inhaled air) could also be determined. Wearer protection factors were found to range from 1.1 for the Racal AirMate loose-fitting PAPR to infinity for the 3M Hood, 3M Breath-Easy PAPR, and SE 400 breath-responsive PAPR. Inhaled contaminant volumes depended on tidal volume but ranged from 2.02 L to 0 L for the same respirators, respectively. Blower effectiveness was about 1.0 for tight-fitting APRs, 0.18 for the Racal, and greater than 1.0 for two of the loose-fitting PAPRs. With blower effectiveness greater than 1.0, some blower flow during the exhalation phase contributes to the subsequent inhalation. Results from this experiment point to different ways to measure respirator efficacy.

How far can one walk wearing a self-contained self rescuer?

Self-contained self rescuers (SCSRs) are used for emergency escapes in oxygen-deficit atmospheres. Certified by NIOSH to last 1 hour, SCSRs generate oxygen by chemical reaction and must meet a minimum oxygen generation time of 60 min when used according to instructions. Wearers must walk or crawl at a controlled rate to conform to the limits of the devices. The aim of the is study was intended to determine the distance that can be walked when using SCSRs as intended. Fourteen volunteer subjects walked on a treadmill at moderate rates that they controlled. They were instructed to walk as far as possible without using oxygen at a rate higher than the SCSR could produce. Distances walked ranged from 2.1 km to 9.2 km with an average distance of 6.0 km. SCSR operating times ranged from 30 min to 94 min. If the required walking distance is farther than this, multiple units will have to be available to allow safe escape.

The correlation between personality type and performance time while wearing a respirator.

Many studies have shown that individuals perform better if not wearing a respirator compared with wearing a respirator. This study examined the degree of performance reduction attributable to specific dominant character traits. The subjects performed on a treadmill at a constant speed and grade resulting in 80-85% VO(2)max. A modified M40 respirator was used to create three levels of inspiratory resistance: 2.8, 16.8, and 27.3 cmH(2)O*(sec/L). The 31 subjects were tested using a Myers-Briggs Type Indicator and State-Trait Anxiety Inventory. Multiple regressions and an ANOVA were used to test for correlation. When air intake is very constricted, the only multiple regression equation that was found to be statistically significant was sensing-intuition (how one takes in information) and thinking-feeling (how one makes a decision) vs. performance time for the highest value of inhalation resistance. A simple linear regression between trait anxiety level and performance time was not found to be statistically significant for the same highest value of inhalation resistance.

Exercise performance while wearing a tight-fitting powered air purifying respirator with limited flow.

Sixteen subjects exercised at 80-85% of maximal aerobic capacity on a treadmill while wearing a tight-fitting, FRM40-Turbo Powered Air Purifying Respirator (PAPR). The PAPR was powered by a DC power supply to give flow rates of 0%, 30%, 66%, 94%, and 100% of rated maximum blower capacity of 110 L/min. As flow rate was reduced, so was performance time. There was a 20% reduction in performance time as blower flow changed from 100% to 0% of maximum. Significant differences in breathing apparatus comfort and facial thermal comfort were found as flow rate varied. It was concluded that inadequate blower flow rate decreases performance time, facial cooling, and respirator comfort.