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We found that under our test conditions two of the three ventilators delivered adequate tidal volumes at the usual frequency of 15 Hz, regardless of the size of the endotracheal tube and of the mechanical properties of the respiratory system. When lung compliance increased or mean airway pressure decreased, both of which are common events during the recovery phase of hyaline membrane disease, we found that the intrinsic properties of two of the ventilators tested were responsible for a decrease in tidal volume. This decrease may account for some cases of heretofore unexplained hypercapnia.

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This study was designed to determine the responses of lung volume and respiratory resistance (Rrs) to decreasing levels of continuous negative airway pressure (CNAP). Twenty normal subjects were studied in the basal state and under CNAP levels of -5, -10, and -15 hPa. Rrs was measured by the forced oscillation technique (4-32 Hz). End-expiratory lung volume (EELV) and tidal volume (VT) were measured by whole body plethysmography. Rrs was extrapolated to 0 Hz (R(0)) and estimated at 16 Hz (R(16)) by linear regression analysis of Rrs vs. frequency. Specific Rrs, SR(0) and SR(16), were then calculated as R(0) (EELV + VT/2) and R(16) (EELV + VT/2), respectively. EELV significantly decreased, whereas R(0), R(16), SR(0), and SR(16) significantly increased, as the CNAP level decreased (P < 0.0001 for all). At the lowest CNAP level, R(0) and R(16) reached 198 +/- 13 and 175 +/- 9% of their respective basal values. The CNAP-induced increase in R(0) was significantly higher than that in R(16) (P < 0.004). Our results demonstrate that the CNAP-induced increase in Rrs does not result from a direct lung volume effect only and strongly suggest the involvement of other factors affecting both intrathoracic and extrathoracic airway caliber.

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 ...

In posterior rhinomanometry (PRM), oropharyngeal pressure is measured using a tube placed between the tongue and the hard palate. For valid results the patient must position the tongue and soft palate so that both the oropharynx and nasopharynx remain open. A high rate of failure of conventional PRM has been reported in normal individuals. In patients with obstructive sleep apnoea syndrome (OSAS), upper airway abnormalities may further increase the failure rate.This study proposes a modification of the technique in which protrusion of the tongue enhances pressure transmission between the nasopharynx and the mouth.In eight normal subjects, resistance was similar when measured by both methods. Of 24 OSAS patients, conventional PRM was unsuccessful in 11. In the remaining 13 patients, a significant correlation between the two methods was found, but resistance was lower by "tongue-out" than by conventional PRM, consistent with a decrease, during tongue protrusion, in retropalatal resistance, which is a component of the "nasal" resistance measured by PRM. In 26 OSAS patients, unilateral nasal resistance values measured by "tongue-out" PRM were similar to those measured by anterior rhinomanometry. When the "tongue-out" method was used routinely in 541 snorers, failure rates were 1.1% in the 272 non-OSAS patients and 3.7% in the 269 OSAS patients.These results indicate that posterior rhinomanometry with tongue protrusion is a highly effective tool for measuring nasal resistance in snorers.