Measurements of ventilation in freely ranging subjects.
Both the level of ventilation and breathing pattern (breathing frequency, inspiratory time, and tidal volume) have an important influence on particle deposition and gas uptake in the lungs. Accordingly, a description of these measures is needed to assess better the dose of particulate deposit and gas uptake in the lungs during varied activities. The long-term objectives of this study were to develop a means of measuring minute ventilation in the field by using body surface displacements, and to evaluate the utility of heart rate as an index of minute ventilation. By using respiratory inductance plethysmographic belts and magnetometers placed on the rib cage and abdomen, ventilation and breathing pattern can be noninvasively measured in mobile individuals. Our specific aims were (1) to validate measurements of ventilation using body surface displacement; (2) to describe breathing patterns in subjects performing a variety of daily activities in the laboratory; (3) to analyze relationships between minute ventilation and heart rate; and (4) to measure ventilation in the field with one technique utilizing body surface displacements and another based upon heart rate. We found that values of tidal volume, inspiratory time, and breathing frequency derived from body surface displacement measurements correlated well with those determined spirometrically during a variety of activities. The coefficient of determination for tidal volume was 0.97 +/- 0.2 for cycling, 0.93 +/- 0.07 for arm cranking, 0.91 +/- 0.05 for pulling, and 0.84 +/- 0.12 for lifting. Our experiments showed that the breathing pattern was altered by the use of a mouthpiece and varied according to the type of activity. The use of a mouthpiece increased tidal volume by 34%, decreased the breathing frequency by 10%, and increased minute ventilation by 16%. There was more variability of these parameters during lifting and pulling activities than during cycling. The ventilation-heart rate relationship varied from subject to subject and was altered by the use of a mouthpiece. We found that ventilation measured in the field from body surface displacement correlated well with ventilation measured using the pneumotachograph (R2 = 0.89). However, measurements of ventilation derived from heart rate were not as accurate as those derived from body surface displacements. We concluded that minute ventilation can be measured accurately using body surface displacements in the laboratory and in the field. Heart rate can also be utilized, but factors affecting the minute ventilation-heart rate relationship, such as the use of a mouthpiece and range of heart rate, must be addressed to obtain more accurate estimates of minute ventilation.