A new correction technique for measuring respiratory impedance through an endotracheal tube

Abstract

Measurement of respiratory impedance (Zrs) in intubated patients requires corrections for flow-dependent resistance and air compression inside the endotracheal tube (ET). The purpose of this study was to test a new correction technique for these effects.We therefore studied 110 patients in two conditions: breathing normally (C1), or breathing through an ET placed at the mouth (C2). In C1, we measured pressure and flow signals at the mouth, and in C2, at the ET inlet, during application of a pseudorandom forced excitation (4-32 Hz). In C1, respiratory impedance was calculated directly as Z1. In C2, pressure data were first corrected for the flow-dependent resistance of the ET, and respiratory impedance was then corrected both for gas compression inside the set-up and ET inertance (impedance Z2).Strong linear relationships were found between the reference and corrected estimates of the resistance at 6 Hz, the frequency dependence of resistance and the resonant frequency. The mean normalized distance between Z1 and Z2 observed in the patients over the 4-32 Hz frequency range was about 14% for resistance and 12% for reactance (-9% and -4%, respectively, when considering the algebraic value of the distance). This slight underestimation of both components of impedance might be due to an overcorrection of pressure for the flow-dependent resistance of the ET.We conclude that, in intubated patients, newly tested corrections for the mechanical contribution of the endotracheal tube may yield a fair estimate of respiratory impedance when pressure is measured at the inlet of the endotracheal tube. Eur Respir J., 1996Respir J., , 9, 1079 TECHNICAL NOTETo monitor respiratory mechanics of intubated patients in intensive care units, it is highly desirable to use a measurement technique which requires no special cooperation from the patient. The forced oscillation technique (FOT) allows the estimation of respiratory impedance (Zrs) by simply applying a pseudorandom airflow excitation at the airway opening of the patient. This method of Zrs measurement is based on the assumption that the mechanical behaviour of the respiratory system remains fairly linear within the flow amplitude range of the forced oscillations applied. In this respect, the presence of an endotracheal tube (ET), which introduces a nonlinear resistive component [1], makes it problematical to measure Zrs by the standard FOT, i.e. with proximal pressure and flow recorded at the ET inlet.The effects of the nonlinearities due to the flow-dependent component of the ET resistance can be limited by measuring Zrs during periods of apnoea obtained after either hyperventilation or curarization [2][3][4][5]. However, from an ethical point of view, it is not always possible to obtain periods of apnoea in spontaneously breathing patients. Theoretically, the effect of the flow-dependent ET resistance could be totally removed by measuring a distal pressure [6,7], either by using a special and rather expensive ET incorporating a lumen with an opening near the distal ...

Respiratory mechanics in ventilated COPD patients: forced oscillation versus occlusion techniques

“…On the other hand, the interest in the forced oscillation technique for monitoring respiratory mechanics in intubated patients has recently been pointed out [29]. Its feasibility has been facilitated by the solution of the main methodological problems concerning this technique (generators capable of operating in parallel with the ventilator [7,8,10] and ways of overcoming the nonlinearity of the endotracheal tube [7,[30][31][32]. An additional feature of the forced oscillation technique is that it may provide automatic and on-line assessment of respiratory mechanics over the breathing cycle [7,[33][34][35].…”

A comparison of endotracheal tube compensation techniques for the measurement of respiratory mechanical impedance at low frequencies

“…Equations 2 and 3 assume that gas compression occurring within the ETT is negligible [31]. Each measurement lasted 180 s. The first 60 s was discarded to minimize the impact of transient effects, leaving about 13 overlapping data segments for the calculation of Z tot and Z rs .…”

“…Frequency-domain compensation is also feasible [28,29], either during discrete sinusoidal forcings [16,17] or pseudorandom broadband excitation [3,4]. Methods that incorporate both time-and frequency-domain compensation techniques have also been proposed [13,30,31], and have been applied in infants [14] and adults [15,32]. However, direct comparisons of the performances of these various compensation approaches have not been performed, especially during measurements of Z rs at low frequencies (i.e., < 10 Hz).…”

Targeted Versus Continuous Delivery of Volatile Anesthetics During Cholinergic Bronchoconstriction

“…The sampled P tp , V , and V̇ waveforms were fitted to a single-compartment model using multiple linear regression [30]: Ptp=RLV.+ELV+P0 where R L and E L are lung resistance and elastance, respectively, and P 0 is the transpulmonary pressure at end-expiration. Prior to each regression, airway pressure measurements were corrected for nonlinear effects of the endotracheal tube [33].…”