To investigate BTPS correction, five different experiments were conducted. Both the flow sensors and associated electronics were purchased (Tamarac Systems, Denver) and used with data acquisition software written specifically for our particular experiments. To calibrate the ceramic spirometry system, five runs of 30 different constant flows (6 L of volume injected at a constant flow) from 0.4 to 12 L/s were injected through each of the flow sensors (150 flow tests for each sensor). The resulting flows were measured and a calibration equation for flow was determined by using a quadratic function least squares fit to the 30 flow values. http://asthma-inhalers-online.com itat on Volume was determined by integrating the calibrated flow signal.
Because of the inherent variability of spirometric parameters in human subjects, we decided to use a mechanical pump to simulate subjects performing an FVC maneuver. In experiments 1, 2, 3, and 4, we tested several ceramic sensors, filling the mechanical pump with either room air or air heated to 37°C and saturated with water vapor. The dead space between the pump and the ceramic flow sensor was equal approximately to that of the mouthpiece used when testing subjects. The temperature inside the pump was measured before each test (thermocouple temperature probe, Doric model 412A) and was maintained between approximately 35° and 38°C. The temperature of the air as it left the flow sensor (exit temperature) was measured with a temperature sensor (National Semiconductor model LM34DZ) mounted to measure the downstream air temperature as shown in Figure 2.
Since a difference is being calculated, the uncorrected (no flow calibration or BTPS correction) values from the flow sensor were used in equation 1. The pump injected the identical volume or waveform but under two different conditions: (1) room air and (2) heated-humidified air. Therefore, any differences in results should be due to differences in the condition of the air passing through the sensor. The room air tests were conducted first, with the pump unheated for several days before the testing. Three repeated injections through the flow sensor, using room air, were conducted and the results were averaged for each of the waveforms to provide the FEV i room air results used in equation 1, for experiments 1 through 4.
In experiment 1, the first four ATS standard waveforms (FVC=6.00, 5.00, 3.50, and 1.50 L, representing a range of expiratory volumes) were forced through the flow sensor. Each waveform was repeatedly forced through the flow sensor (ten times) as quickly as the simulator could be filled with heated-humidified air (less than 2 min between maneuvers). This experiment was conducted to simulate a subject performing ten consecutive FVC maneuvers within 2 min of each other.
Figure 2. Diagram of ceramic flow sensor showing placement of air temperature sensor used to measure exit air temperature.