INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES APPENDIX

WHEN THE IMPEDANCE of the respiratory system (Zrs) is determined by applying forced oscillations at the mouth (8), the upper airway acts not only as an impedance in series with the lungs but also as a shunt for the oscillatory flow. Motion of the upper airway wall results in loss of flow (), leading to an underestimation of the impedance of the downstream respiratory system. In 1956, DuBois et al. (8) already recommended support of the cheeks with the palms of the hands to reduce the motion of the cheeks. In an attempt to correct for upper airway wall shunt impedance (Zuaw) in adults, Michaelson et al. (14) performed measurements during a Valsalva maneuver and subtracted the obtained impedance value from the Zrs. This correction overestimates the influence of the upper airway on Zrs. Indeed, during the Valsalva maneuver the upper airway behaves as a single shunt impedance, with the series element of the upper airway being negligible (1, 16). Besides, performance of a Valsalva maneuver requires good cooperation, which cannot be obtained in infants. As an alternative, Peslin et al. (15) tried to reduce the influence of the upper airway wall shunt on respiratory impedance measurements by using a head generator in which pressures are varied around the head rather than directly at the mouth (conventional technique), thereby minimizing transmural pressure across the upper airway wall. Comparison of respiratory impedance measured with the head generator or by the conventional technique in adults yielded values of upper airway impedance that were similar to those measured directly with a head plethysmograph (16).

In 1989, Marchal et al. (12) compared the use of the head generator with the conventional technique in 24 infants. Because infants are nearly obligatory nose breathers, the assumption that upper airway wall motion is eliminated because oscillations are applied around the head is not entirely valid. Indeed, during nose breathing the nose induces a pressure difference across the upper airway wall, resulting in upper airway wall motion. This problem was recognized by the authors, and an adaptation of their initial model was introduced in the discussion, enabling them to simulate the influence of the upper airway in nose-breathing infants. No attempt thus far has been made to directly estimate Zuaw in infants.

The aim of the present study was to estimate Zuaw in infants by means of a model combining impedance measurements obtained with the conventional forced oscillation technique and with the head generator and pressure measurements obtained with a head plethysmograph. Estimation of Zuaw allowed us to evaluate the validity of results of conventional and head generator techniques.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES APPENDIX

Nine infants, hospitalized with bronchiolitis, were recruited for the study. Their ages varied between 4 and 17 mo. The study was approved by the local hospital ethics committee, and oral informed consent was obtained from the parents.

Conventional technique combined with head plethysmograph. A schematic representation of the experimental setup is presented in Fig. 1. A volume-constant head plethysmograph was made of a 35 × 20 × 24-cm wooden box. Two walls consisted of Plexiglas to ensure visualization of the baby. One sidewall consisted of two parts, which delimited an orifice 10 cm in diameter. After sedation with oral choral hydrate in a dose of 80 mg/kg, the infant was nursed supine, and the two sidewall parts were assembled around the lower part of the neck, thus closing the box. A reasonably good seal with minimal compression of the airways was obtained by covering the space between the orifice and the neck with a rim of silicone putty. A 90-W loudspeaker, mounted in the wall of a chamber, generated pseudorandom noise containing all harmonics of 4 up to 52 Hz. The oscillations were conducted to the infant through a 60-cm-long and 2-cm-bore flexible tube, and a well-fitting rigid face mask with a silicone border was adapted to each infant individually (5). Differential pressure across a Fleisch no. 1 pneumotachograph, yielding , airway opening pressure (Pao) measured as close as possible to the face mask, and box pressure (Pbox) were measured with identical differential pressure transducers (±2 hPa, Validyne MP45-1). Pbox, Pao, and were digitized with a sampling rate of 128 Hz and fed into a computer. The ratio of Pao to (Pao/) was calibrated by using two reference loads and according to the two-point calibration procedure (6).

View larger version (28K): [in this window] [in a new window]   Fig. 1.   Conventional technique combined with head plethysmograph. Computer (1) generating pseudorandom noise drives the loudspeaker (2); bias flow of 6 l/min flushes tubing between side tubes (3) and (4). See text for details.

Head generator technique. The experimental setup was easily changed into a head generator (15) by disconnecting the tube from the pneumotachograph, thus allowing the infant to breathe into the box (Fig. 2). Oscillations produced by the loudspeaker were now applied around the head, and the impedance values obtained were called Z₂ (resistance, R₂; reactance, X₂).

View larger version (26K): [in this window] [in a new window]   Fig. 2.   Head generator technique. Setup is similar to Fig. 1, but tube was disconnected from pneumotachograph.

Estimation of Zuaw. The experimental setup allows for the determination of four variables: Z₁, Z₂, Z₃, and TF. In the conventional technique measuring Z₁, Zuaw is assumed to be in parallel with the impedance of the lungs, airways, and chest wall (which will be referred to as ZL), whereas, when oscillations are applied around the head, yielding Z₂, Zuaw is in parallel with nasal (Zn) and pneumotachograph impedance (Zp). Figure 3 depicts electrical analogs for the three measurement conditions, which can be described by the following equations

(1)

(2)

(3)

Combining Eqs. 1-3, we can solve for

(4)

(5)

(6)

where  = (Zbox  TF · Z₁)/(TF · Z₁)

View larger version (20K): [in this window] [in a new window]   Fig. 3.   Electrical circuits for the three measurement conditions: A, conventional technique; B, conventional technique with baby disconnected, showing estimation of box impedance (Zbox); C, head generator technique. Zp, impedance of pneumotachograph; Zn, nasal impedance; Zuaw, upper airway wall impedance; ZL, impedance of chest, airways, and chest wall; P1, P2, P3, and P4: pressures at different points in the circuits; 1, 2, and 3: flows at different points in the circuits. See text for details.

(7)

(8)

1

1

1

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES APPENDIX

Measurements were extremely difficult to perform because the sedated baby had to be nursed supine with his or her head in a tightly closed box with minimal leakage. Reproducible impedance values for the three experimental conditions were obtained in seven infants (Table 1). The data of two infants were excluded: in one patient leakage of the mask resulted in abnormally low resistance values, and in another patient Z₃ measurements showed major variations between frequencies. Because these variations were reproducible and the coherence function was 0.95, this phenomenon was probably due to cross talk between frequencies because of marked alinearities of Zrs (4). Mean SDs were lower with the conventional technique than with the head generator, especially at higher frequencies.

When Z₁, Z₂, Z₃, and TF were substituted into Eqs. 4-6, values of Zn were obtained nearly equal to Z₁, whereas ZL and Zuaw were close to zero. Similar results were obtained with the physical analog. These results are not physiologically meaningful, and thus the assumption that Z₃ = Zbox is invalid.

On the other hand, the results were different from zero and made sense when the exact relation for Zbox was used (Figs. 4 and 5; see DISCUSSION).

View larger version (49K): [in this window] [in a new window]   Fig. 4.   Resistance (A) and reactance (B) of upper airway in 7 infants (each represented by different symbol).

View larger version (45K): [in this window] [in a new window]   Fig. 5.   Resistance (A) and reactance (B) of respiratory system in 7 infants (each represented by different symbol).

Varying the values of Rn or Xn did not result in major differences in Zuaw or ZL (Figs. 6 and 7). In most infants, Zn did not show a frequency dependence, with the maximum range of Rn and Xn being, respectively, 4.6 and 4.0 hPa · l¹ · s at 12 Hz and 1.2 and 1.6 hPa · l¹ · s at 52 Hz. Introduction of this frequency dependence of Zn in the model did not result in marked differences in the estimates of Zuaw or ZL.

View larger version (42K): [in this window] [in a new window]   Fig. 6.   Influence of different values of Zn on mean values of upper airway resistance (A) and reactance (B). Symbols representing nasal resistance and reactance values (in hPa · l1 · s), respectively, are as follows: , 0.5 and 4.6; , 0.5 and 0.9; , 0.5 and +2.8; , 2.5 and 4.6; , 2.5 and 0.9; , 2.5 and +2.8; , 4.5 and 4.6; , 4.5 and 0.9; +, 4.5 and +2.8.

View larger version (38K): [in this window] [in a new window]   Fig. 7.   Influence of different nasal resistance and reactance values (in hPa · l1 · s) on mean values of respiratory resistance (A) and reactance (B). See Fig. 6 legend for explanation of symbols.

When estimated ZL was compared with Z₁ and Z₂, two different patterns were observed: lung resistance (RL) independent of frequency (n = 4) and RL increasing with increasing frequency (positive frequency dependence) (n = 3). The values of lung reactance (XL) initially increased and then decreased at the higher frequencies. An example of both patterns is shown for Z₁, Z₂, and ZL (Figs. 8 and 9). There was a close parallelism between ZL and Z₂, with the values of RL and XL being slightly less than the corresponding values of R₂ and X₂. The difference between ZL and Z₁ was larger for both RL and XL. As shown in Fig. 10, this difference depends on the ratio of upper airway wall resistance (Ruaw) to RL (Ruaw/RL).

View larger version (36K): [in this window] [in a new window]   Fig. 8.   Pattern of respiratory resistance (A) and reactance (B) observed in 4 infants. , conventional technique; , head generator technique; , estimated lung impedance.

View larger version (37K): [in this window] [in a new window]   Fig. 9.   Pattern of respiratory resistance (A) and reactance (B) observed in 3 infants. See Fig. 8 legend for explanation of symbols.

View larger version (23K): [in this window] [in a new window]   Fig. 10.   Difference between respiratory resistance, measured with conventional technique, and estimated respiratory resistance as function of ratio of upper airway resistance to estimated respiratory resistance.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES APPENDIX

By combining measurements obtained with the conventional forced oscillation technique, the head generator, and previously determined Zn values, it was possible to estimate Zuaw and ZL in infants.

Previously, consecutive measurements of the conventional (without head plethysmograph) and head generator techniques have been performed by Marchal et al. (12) in 24 infants, aged 2-49 mo. Data were analyzed between 6 and 20 Hz only because the reproducibility of the measurements was poor >20 Hz. In our measurements, a satisfactory reproducibility was obtained up to 52 Hz, although SDs were higher in the higher frequency range with the head generator. Resistance values obtained with the head generator were higher than with the conventional technique, whereas reactance values were on the average less negative. Our data are in agreement with the findings of Marchal et al. and with the previous observations of our laboratory (5).

Several investigators have reported on the mechanical characteristics of the upper airway in adults (1-3, 9, 11, 14). Ruaw varied between 4 and 10 hPa · l¹ · s, whereas upper airway wall reactance (Xuaw) ranged between 2.1 and 5.3 hPa · l¹ · s for frequencies between 4 and 28 Hz, depending on whether or not the cheeks were supported. Cauberghs and Van de Woestijne (1, 2) found Ruaw values of ~20 hPa · l¹ · s at 4 Hz, decreasing to 10 hPa · l¹ · s at 20 Hz, with a Xuaw of 51 and 11 hPa · l¹ · s at 4 and 20 Hz, respectively, in adults. These data were confirmed by the measurements with the head plethysmograph (16).

In children, few data are available. In 15 children, aged 5-15 yr, Ruaw with support of the cheeks was found to be 46 hPa · l¹ · s at 4 Hz, decreasing to 21 hPa · l¹ · s at 20 Hz, whereas the reactance was 65 and 20 hPa · l¹ · s at 4 and 20 Hz, respectively (1). There are no data available on the upper airway impedance in infants. Our data show still higher resistance and more negative reactance values in some infants than in children. This is to be expected from the comparison of data between adults and children.

Because infants breathe through the nose, it seemed that the head generator did not correctly take the upper airway shunt into account. Marchal et al. (12) recognized this problem and estimated the error that results from either techniques. Two assumptions were made: first, the nose was assumed to represent 40 and 20% of total respiratory system resistance and inertance, respectively; and, second, Ruaw and Xuaw were chosen to be 80 and 77.1 hPa · l¹ · s, respectively. With the use of these hypothetical values for Zn and Zuaw, the Zrs without any shunt effects was calculated. Marchal et al. found that, with the head generator technique, resistance was overestimated by 6%, increasing to 8% in airway obstruction, whereas with the conventional technique the underestimation was 13% and was 23% in airway obstruction. In the present study, we used previously determined Zn values to estimate the upper airway shunt. Alternatively, one nostril was occluded, and measurements were performed through the other. By combining these two measurements with impedance values obtained with the unoccluded nose, Zn was calculated (7). These measurements were performed in a group of children other than those in the present study. Indeed, it was practically not possible to perform these occlusions in the setup of the present study: the infant was lying with his or her head fixed in a box, which remained closed during the measurement of Z₁, Z₂, Z₃, and TF. However, because the study of Zn showed that the range of values was small as long as the nose was not obstructed by secretions, we felt that these values could be used to solve the set of equations in the present study. This resulted in estimations of ZL closer to Z₂ (head generator) than to Z₁ (conventional technique), with the underestimation of Z₁ being larger with smaller Ruaw/RL. If Zuaw is high, less flow goes to the shunt pathway and thus Z₁ reflects more closely ZL. The overestimation of RL with the head generator varied between 6 and 18% (mean 10%) at 20 Hz, whereas with the conventional technique the underestimation ranged from 13 to 57% (mean: 35%). Errors in reactances, expressed in absolute values, were greater for the conventional than for the head generator technique.

Comparative studies of the head generator and the conventional technique have been performed in adults (2, 10) and in school-age children (13). In normal adult subjects, measurements obtained with both techniques are very similar. In patients with moderate airway obstruction, a negative frequency dependence is observed with the conventional technique, whereas resistance is higher without frequency dependence with the head generator. In patients with severe obstruction, both techniques show resistance values with negative frequency dependence, although resistance values are higher with the head generator. Accordingly, a negative frequency dependence of resistance may be an artifact due to the upper airway shunt only in patients with moderate airway obstruction. Moreover, this artifact is not necessarily an inconvenience for diagnostic purposes, because it allows a clear-cut separation between healthy subjects and patients (10). In 75 children, aged 5.5-15 yr, the power of the conventional and head generator technique vs. forced expiratory volume in 1 s was evaluated in detecting airway response to bronchodilators (13). The head generator technique improved specificity of resistance at 20 Hz from 65 to 78% without a change in sensitivity (76%). Resonant frequency had larger sensitivity with the conventional than with the head generator technique (91 vs. 53%) but slightly lower specificity (70 vs. 78%). Changes in reactance were more specific and more sensitive with the conventional than with the head generator technique.

In conclusion, Zuaw was estimated by using a combination of measurements obtained with the conventional forced oscillation and head generator techniques and previously determined Zn values. Upper airway resistance and reactance at 12 Hz were between 40 and 120 and 0 and 150 hPa · l¹ · s, respectively. Head generator impedance values are closer to the estimated ZL values than are those of the conventional technique. Still, the head generator technique slightly overestimates the Zrs, because it does not completely suppress the pressure gradient across the upper airway. Because the head of the infant has to be enclosed in a box, the technique is unpractical and nearly impossible to perform routinely.