The goal of this study was to compare the relative performance of two noninvasive ventilation sensing technologies on adults during artifacts. We recorded changes in transthoracic impedance and cross-sectional area of the abdomen (abd) and rib cage (rc) using impedance pneumography (IP) and respiratory inductance plethysmography (RIP) on ten adult subjects during natural breathing, motion artifact, simulated airway obstruction, yawning, snoring, apnea, and coughing. We used a pneumotachometer to measure air flow and tidal volume as the standard. We calibrated all sensors during natural breathing, and performed measurements during all maneuvers without changing the calibration parameters. No sensor provided the most-accurate measure of tidal volume for all maneuvers. Overall, the combination of inductance sensors [RIP(sum)] calibrated during an isovolume maneuver had a bias (weighted mean difference) as low or lower than all individual sensors and all combinations of sensors. The IP(rc) sensor had a bias as low or lower than any individual sensor. The cross-correlation coefficient between sensors was high during natural breathing, but decreased during artifacts. The cross correlation between sensor pairs was lower during artifacts without breathing than it was during maneuvers with breathing for four different sensor combinations. We tested a simple breath-detection algorithm on all sensors and found that RIP(sum) resulted in the fewest number of false breath detections, with sensitivity of 90.8% and positive predictivity of 93.6%.

The goals of this study are: 1) to characterize the structure of the skin by measuring impedance variations with a suction microelectrode; 2) to correlate the observed impedance variations with dc current pathways through the skin; 3) to characterize the breakdown phenomenon. We constructed a suction microelectrode with a 200-microns internal diameter and performed several tests on two male subjects. Skin impedance measured from different locations on the forearm and palm varied considerably. We found that the average skin impedance on the forearm was larger than the average impedance on the palm and that the ratio between the maximal and minimal skin impedance is larger for the forearm than for the palm. For both the forearm and the palm the magnitude and variance of skin impedance decrease with increasing stimulus frequency. The density of low impedance points observed on the forearm and palm are consistent with the density of dc current pathways through the skin as indicated by traces left on 1-cm2 Ag electrodes when we passed dc current through the skin. The ratio between the highest and lowest impedances decreased as temperature decreased--at low temperatures the skin displayed mostly high impedances. We were not able to break down the skin using the suction microelectrode. The tests with dry and wet electrodes suggest that breakdown is of thermal nature, and that the thermal capacitance of the saline in the suction microelectrode prevents the temperature of the underlying skin from increasing very rapidly. In conjunction with the larger impedance values, this would tend to increase the breakdown voltage.

We have designed an inductive plethysmograph to obtain a non-invasive measure of ventilation. Two elastic bands containing insulated wires encircle the chest and abdomen--the inductance of each band depends on the enclosed cross sectional area. Each inductive band forms an element in a tank circuit, which determines the resonant frequency of a Colpitts oscillatory. By measuring the oscillatory frequency, we indirectly measure the changes in cross sectional area that occur during breathing. Independent measures of chest and abdominal cross sectional area provide a way to detect both normal breathing and airway obstruction. Magnetic coupling due to the mutual inductance between chest and abdominal bands modulates the desired oscillation frequencies. When modulation is excessive, frequency locking occurs and we cannot make independent measures of chest and abdominal area. We have performed simulations that show that, as the chest and abdominal band oscillator frequencies are sufficiently separated, we decrease modulation and avoid frequency locking. We have compared simulataneous recordings of ventilation using our inductive plethysmography and a commercial impedance pneumograph and spirometer. Recordings of normal ventilation by all methods appear similar; however, our inductive device is less prone than the impedance pneumography to artifacts caused by applied pressure and body movements. In addition, during simulated airway obstruction, signals from the chest and abdominal bands are out of phase--suggesting that the inductive technique may be useful for detecting airway obstruction.

We measured transthoracic impedance in nine presumed healthy adult subjects with a two-frequency plethysmograph at 57 kHz and 185 kHz. The measurement protocol included periods of normal breathing without motion and periods of motion without breathing. We analyzed the cross-correlation and the ratio between the signals at both frequencies for all the different maneuvers. The correlation coefficient was between 0.97 and 1 for breathing, the minimal cross-correlation (0.81) was for simulated obstructive apnea. We found that the amplitude ratio between the two-frequency signals was different for normal breathing and for motion. Based on these results, we designed and tested an adaptive filter to increase the signal-to-artifact ratio (SAR). The increase in SAR (mean +/- standard deviation) compared with the signal at 57 kHz was: 183% +/- 117% for arm movement, 133% +/- 93% for leg movement, and 34% +/- 62% for simulated obstructive apnea.