This method can help answer key questions in pulmonary physiology, particularly about how the structure-function relationship in the lung contributes to disease pathology. The main advantage of this technique is that it allows measures to be made repeatedly, within the same subject, without expensive imaging equipment or technically advanced analysis algorithms. The implications of measuring the lung structure-function relationship extend toward understanding the development of lung diseases, using classic, well-established physiological tools to evaluate novel interventions.
Though this method provides insights into human lung physiology, it can also be applied to animal models of lung disease. Generally, individuals new to this method may struggle with the correct coaching of participants to follow the outlined maneuvers. Several practice runs are often necessary with a first-time participant.
Visual demonstration of this method is critical, as the capnographic maneuver can be difficult to learn and the flow and volume of the maneuver must be controlled carefully. Procedures involving human subjects has been approved by the Institutional Review Board of the University of Iowa. Before beginning the calibration, use a standard barometer to measure the temperature, barometric pressure, and relative humidity, and enter these values into the plethysmograph software as correction factors.
To calibrate the flow sensor, use a calibrated three liter syringe at variable flow rates and use a 50 milliliter pump to calibrate the box pressure. Immediately prior to the plethysmography measurement, have the participant enter the whole-body plethysmograph and close the door. After 30 to 60 seconds of thermal equilibration, instruct the participant to place their mouth on the mouthpiece, put on the nose clips, and place their hands on their cheeks.
Instruct the participant to breathe normally, allowing at least four tidal breaths to be acquired, and a functional residual capacity to be established. At the end of a normal exhalation, close the shutter and coach the participant to pant lightly at 0.5 to one breaths per second for three to four seconds. Evaluate the relationship between the mouth pressure and plethysmograph pressure to ensure that it is a series of overlapping, straight lines, without thermal drift, and open the shutter and allow the participant to take a normal breath.
Then coach the participant to exhale to the residual volume. Followed by a maximal inspiratory maneuver to total lung capacity. Before the participant arrives, address and modify the variables in the table, as necessary.
To calibrate the gas analyzer, attach the drying tube to the mixing chamber and flush the bag with inert gas at a rate of at least 10 liters per minute, taking care not the pressurize the system. Once the displayed concentrations of the displaced carbon dioxide and oxygen have stabilized, adjust the zero knobs until they both read zero. Next, repeat the flush with 6%carbon dioxide and room air containing 20.93%oxygen as the calibration gases, matching the concentration of the calibration gases with the span knob when the concentration of the gases stabilizes.
Then, recheck the inert gas and calibration gas concentrations and adjust the zero and span knobs until both are accurate to plus or minus 0.1%To calibrate the heated pneumotach, allow the pneumotach to warm to 37 degrees Celsius for at least 20 minutes, and open the Flow Channel menu in the system software. Select Spirometer and click Zero to zero the pneumotach. Then select OK.Use a flow head adaptor to connect a three liter syringe to the pneumotach and highlight the calibration breath.
Under the Flow Channel menu, select Spirometer Flow, and Calibrate, and enter three liters. Click OK.And use the syringe the inject three liters of room air into the pneumotach at varying flow rates. The difference from three liters should be less than 5%When the system is ready, coach the participant to perform a single maneuver consisting of two pairs of breaths:a coaching breath and a breath for analysis.
Because we have inspiration and expiration, with inspiration. During the maneuver, coach the participant to follow the flow guide on the computer monitor. Consider adding a resistor in line with the mouthpiece to make the exhaled flow easier to control.
To measure the functional residual capacity, instruct the participant to sit straight with both feet on the floor, put on nose clips, and place their mouth on the mouthpiece. Coach the participant to complete at least one minute of tidal breathing to measure the metabolic function and to allow the participant to become familiar with the mouthpiece. Stop the data collection after one minute.
Before beginning the maneuver, participants should vary their tidal volume, taking either normal, smaller, or larger than normal tidal breaths, to ensure that capnograms are obtained at different lung volumes. Coach the participant to transition to performing a capnogram maneuver as soon as the flow tracings appear on the screen, and resume the data collection at a random point in the participant's respiratory cycle, to allow measurements to be obtained at different lung volumes. Finally, coach to perform a sigh at the end of each maneuver, so that the muscles of respiration are completely relaxed, to allow the functional respiratory capacity to be determined.
The single most important step, is the correct measurement and completion of the maneuver performed with volumetric capnography. Then, stop the data collection and have the participant repeat the capnogram maneuvers at least six to eight more times, to obtain 12 to 16 pairs of breaths for analysis. Here, a representative single capnogram used in an analysis, and the raw data for the entire sequence of the maneuver, are shown.
In the raw data, the capnogram and the flow tracing were not aligned to account for the time delay. In these representative cases, the dead space and the slope were significantly correlated to the lung volume, suggesting that the dead space and airway homogeneity increase as the lung volume increases. While attempting this procedure, it's important to remember that it is highly dependent on the participant's exhaled flow rate and the delay between the analyzer and the spirometer.
The accuracy of these vales should be checked carefully. After its development, this technique paved the way for researchers in the field of pulmonology to explore the structure-volume relationship of the lung in various patient populations, and this could potentially be integrated into the standard bedside care of respiratory diseases.
