The respiratory inductive plethysmograph is a noninvasive device that has been used to measure tidal volume (V(T)) in humans from changes in self-inductance of wire coils excited by an oscillator circuit placed about the rib cage and abdomen. We investigated its accuracy in conscious sheep utilizing a new calibration procedure during quiet breathing and breathing associated with bronchospasm provoked by aerosolized carbachol. Seven sheep were intubated with a nasotracheal tube and an esophageal balloon placed for determination of transpulmonary pressure. Base-line mean pulmonary flow resistance (R(L)) in the sheep was 1.5 ± 0.7 (SD) cmH2O.l-1.s. After carbachol inhalation, mean R(L) increased to a maximum of 8.8 ± 2.8 cmH2O.l-1.s (P<0.002). At base line, mean (V(T) estimated by respiratory inductive plethysmography over a 20-s period fell within ±6% of spirometry. After carbachol V(T) in five of the sheep remained close to the initial validation, but in two, it deviated ±11% from spirometry. Analysis of the continuous recording of timing and volume components of the breaths revealed that bronchoprovocation did not significantly alter mean V(T) or frequency. However, there was a slight increase in both parameters resulting in an increase in minute ventilation from 7.6 ± 2.4 to 9.6 ± 2.8 l/min (P<0.02). Similarly, a slight decline in inspiratory time coupled with the slight rise in V(T) produced an increase in mean inspiratory flow from a base-line value of 0.35 ± 0.12 to 0.44 ± 0.17 l/s (P< 0.05). These results indicate that the respiratory inductive plethysmograph accurately monitors breathing pattern in conscious sheep even during severe bronchospasm.
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