The spirometer with mouthpiece and noseclips (MPC/NC) provides a simple, precise technology for the measurement of maximal pulmonary volumes and flow rates. The MPC/NC provides an artificial milieu for the measurement of resting ventilation (Ve), however. As such, MPC/NC stimulate change in the breathing pattern, for Vt measured by spirometer exceeds that measured noninvasively by 15 percent in normal subjects. One might expect that determination of Vt in disease states, and most important to the clinician, the determination of change in Vt induced by therapy, will be affected by MPC/NC as well.
Alternate techniques quantify Ve by the summation of rib cage and abdominal volume displacements. Two such devices, the two-channel magnetometer (which measures the change in anteroposterior diameter of each compartment during tidal breathing) and the respiratory inductance plethysmograph (which measures change in cross-sectional area) were used in this study. To calibrate these devices, it is assumed that change of either diameter or cross-sectional area measured in any one plane is representative of the diameter or area change of the entire rib cage and abdominal compartments during quiet breathing. Stated in a different way, it is assumed that neither compartment changes shape during respiration. When body position is fixed, both compartments do maintain a constant shape during quiet breathing in normal subjects. One may therefore quantify Vt by estimation of chest wall volume displacement in normal subjects.
By contrast, the shape of either compartment may change either during change in position (eg, from seated to supine) or during tidal breathing in patients with airway obstruction. This study was designed to examine the precision with which Vt can be measured by chest wall volume displacement methods in such patients. The three methods we selected for comparison included two standard calibration techniques (isovolume, least squares) and the two most available apparatuses. One might postulate that rip, which measures area, would provide a more accurate approximation of volume displacement than would magne-tometry. Change in anteroposterior diameter as measured by the magnetometer would appear to provide a tenuous estimate of volume displacement if either rib cage or abdominal compartment were to change shape during the ventilation cycle. Are you ready to know more about medicine and pharmacy? Go to the page using the link – Issues on Canadian Health&Care Mall diigo group.
Error is inherent in both calibration techniques employed. The 1 sq calibration requires that the patient breathe quietly in two positions. Vt is measured in one of the calibrating positions. The 1 sq is valid, therefore, only if the shapes of both rib cage and abdominal compartments remain constant throughout quiet breathing in both calibrating positions. This may not be so, however. The volume-to-motion (V-M) coefficients of both compartments change with position change in normal subjects. These physiologic events must affect the precision of Vt measurement by the 1 sq method. Conceivably, such changes in V-M coefficients with change in position do not occur in patients with airway obstruction. The observation that end-expiratory lung volume decreases when a normal subject (but not a patient with airway obstruction) assumes the supine position supports the possibility that neither the shape of rib cage or abdomen nor their V-M coefficient change with position change in airway obstruction patients.
The isovolume technique, unlike the least squares method as used in this and other studies to date, requires only one position for both calibration and Vt measurement. The major disadvantage of the isovolume technique is the patient cooperation requirement. The subject must close his mouth and nose (or glottis), and permitting only minimal change in pressure within both rib cage and abdominal cavities, shift an equal volume back and forth between the two compartments. This maneuver is often difficult for patients with severe airflow obstruction to perform and to reproduce. Bellia et al have suggested that the precision of any calibrating technique depends upon the subject s ability to produce a wide range of randomly occurring Vt, rib cage and abdominal relationships during the calibrating procedure. It would appear difficult for the airway obstruction patient to produce an average of these relationships during such a brief and difficult calibrating procedure.
Rib Cage Shape
A change of rib cage shape during respiration would likely affect the calibration by magnetometer, and possibly rip, as well. Ringel et al have shown in asthma, the anteroposterior diameter of the thorax, when measured high on the rib cage, increases at a constant rate throughout inspiration. Thus, placement of the anterior magnetometer coil in the second intercostal space, as in our study, maximizes the possibility that the magnetometer signal will be proportional to rib cage volume change during tidal breathing in the asthmatic subjects. Retraction of intercostal spaces on inspiration, seen in some chronic airway obstruction patients, might also affect the precision of volume calibration.
To minimize both asynchrony between anteroposterior and lateral rib cage motions and discoordination between rib cage and abdominal motions during breathing, we selected a comfortable position for the study. Patients were seated, in a slightly forward position, arms and trunks stabilized with pillows as necessary. Patients with either chronic airflow obstruction or asthma often choose this position to minimize both asynchrony of chest wall movements, and, therefore, breathing work. Coincidentally, this position should minimize error when Vt is measured by chest wall displacement methods.
Precision of Volume Measurements
We found little difference in the precision with which Vt could be measured by each of the three methods. Comparison of Vt measured with (Table 3) to that without (Table 2) MPC/NC demonstrates that each method identifies the augmentation of Vt by the respiratory apparatus. Similarly, each method identifies the increment of Vt induced by addition of dead space to the circuit. The errors for each method when expressed without regard to direction were similar and small, when presented as pooled averages of many breaths, as shown in Figure 1. By contrast, perusal of
Figure 2 suggests that for individual breaths, the differences between volume-displacement and pneumotachygraph methods are greater than implied by comparison of the mean values.
Two recent studies indicate that Vt may be accurately measured in airway obstruction patients when the inductive plethysmograph is calibrated by either two-body position least-squares or isovolume method. This study extends those observations by stimulating Vt with addition of an external dead space to the mouthpiece. We show that during quiet tidal breathing and when respiration is stimulated so as to increase Vt to approximately one and one-half times that measured without mouthpiece (as with mouthpiece, noseclips, and added dead space), each of three methods measures Vt with, on the average, less than 10 percent error when compared to simultaneous flow meter volume estimates. Measurement of chest wall volume displacement provides a valid semiquan-titative determination of tidal volume in patients with airflow obstruction.
Frequency was not affected by the addition of mouthpiece or dead space. The fact that neither intervention influences frequency has been demonstrated previously in asthmatic subjects when studied during episodes of acute airway obstruction but not in chronic airway obstruction patients.