Measurement of Tidal Volume during High Frequency Ventilation by Impedance Plethysmography

Abstract

ABSTRACT. Electrical impedance plethysmography was evaluated in lambs as a method of measuring tidal volume (VT). Over tidal volumes ranging from 15 to 414% of estimated dead space, and frequencies of 300 to 1000 breathslmin, correlation between VT measured by the impedance technique and VT measured by whole-body plethysmography was 0.98. Above 600 breathdmin, the correlation between the two methods was 0.94. Independent calibration of the impedance technique using a pneumotachograph at conventional rates of ventilation yielded absolute values of VT which closely corresponded to values obtained with the whole-body plethysmograph (slope = 1 &, baseline transthoracic impedanceDirect measurement of VT during HFV is difficult if not impossible in most clinical circumstances due to constraints resulting from limited frequency response and high gas velocity. These difficulties are compounded in newborn and premature infants because of their small size, critical dependence on a closely regulated thermal environment, and inability to cooperate.Variations in transthoracic electrical impedance have been shown to correlate closely with VT during conventional ventilation (1). This investigation was undertaken to determine if impedance plethysmography could also be used during HFV to measure VT. Inasmuch as the technique is noninvasive and unobtrusive, it would be well suited to meet the requirements of safe application in newborn infants. METHODSFour lambs weighing 4.0 to 10.2 kg were studied on 6 days between 1 and 7 wk after birth. During each study session, the lamb was sedated with chloralose, but spontaneous breathing was not abolished. After placement of a cuffed endotracheal tube through the mouth, the lamb was placed in a pressure-compensated, integrated flow, whole-body plethysmograph having a volume of 165 liters. The endotracheal tube extended through a sealed port in the plexiglass wall of the plethysmograph. VT was measured by pressure compensating the integrated pressure signal from the plethysmograph. The plethysmograph had been tuned to eliminate phasic distortion up to 20 Hz.Transthoracic electrical impedance was measured by a DCcoupled SAAB Respimeter operating at a frequency of 50 kHz anda current of 0.2 mA peak to peak (2). A tetrapolar electrode system was used, with one pair of electrodes attached to each side of the lamb's chest wall after shaving. Each electrode had a circular silver-silver chloride surface 0.8 cm in diameter and each electrode pair was housed in a soft rubber casing that maintained a constant interelectrode distance of 2.25 cm center to center. Within each electrode pair, one electrode was used for current injection and the other was used to detect voltage drop. This type of impedance bridge minimizes the effect of resistance variations at the skin-electrode interface. The frequency response of the impedance bridge was found to be flat below 20 Hz. HFV was provided by a Bird VDR. The Bird VDR is a flowintermvter tvve of high frecluency ventilator that has a pneumatic inspira~ory/exp...

An optimized method for estimating the tidal volume from intracardiac or body surface electrocardiographic signals: implications for estimating minute ventilation

“…Spirometer (4), Pitot tube (13), and ultrasonic airflow meter (26) directly measure air flow into and out of the lungs. Respiratory inductance plethysmography (32), computed tomography (31), electrical impedance plethysmography (25,28), and differential pressure pneumotachograph (1) have been used to indirectly measure TV by monitoring body volume changes.…”

Four methods of measuring tidal volume during high-frequency oscillatory ventilation

“…These techniques have included whole body plethysmography (animal studies) (12,30,31), jacket plethysmography (32), impedance plethysmography (33), and differential pressure pneumotachometers (11)(12)(13)(14)30). However, none of these approaches are readily adaptable for use in the intensive care unit, and all are challenging to calibrate for use during HFOV.…”

Electrical impedance plethysmography technique in estimating pulmonary function status

“…They found that transthoracic impedance in an open chest condition was greater in the expiratory state than in the inspiratory state. Researchers measured tidal volume (V T ) using transthoracic impedance variations in animals and found a linear relationship between change in impedance and V T [6,7]. Guha and Anand [8] investigated the EIP technique in comparison with electrical field plethysmography for evaluating cardiac and respiratory EIP pulses.…”