Effect of upper airway shunt and series properties on respiratory impedance measurements

Because of the contradictory statements published about the influence of the shunt properties of the upper airway on the measurements of the respiratory impedence by means of the forced oscillation technique, this influence has been reevaluated. In healthy adults and children and in patients with obstructive lung disease, the total respiratory impedance was measured by applying oscillations at the mouth (conventional technique) or around the head (head generator technique), with the cheeks either supported by the hands or not. In healthy adults the two techniques (conventional cheeks supported and head generator) yield similar results for respiratory resistance (Rrs) and a more pronounced increase of respiratory reactance (Xrs) with frequency with the head generator. In children and in patients with moderate airway obstruction, the negative frequency dependence of Rrs observed with the conventional technique tends to disappear with the head generator. This is not observed in patients with severe airway obstruction. The differences between the two techniques can be explained by the influence of the shunt impedance of the upper airway on Rrs and Xrs. Correction for this influence by subtracting the impedance measured during a Valsalva maneuver is not satisfactory, since the Valsalva maneuver itself modifies the upper airway shunt. The head generator technique reduces the influence of the upper airway shunt but does not suppress it altogether; the residual error is small, however.     

Influence of upper airway shunt on total respiratory impedance in infants.

When input impedance is determined by means of the forced oscillation technique, part of the oscillatory flow measured at the mouth is lost in the motion of the upper airway wall acting as a shunt. This is avoided by applying the oscillations around the subject's head (head generator) rather than at the mouth (conventional technique). In seven wheezing infants, we compared both techniques to estimate the importance of the upper airway wall shunt impedance (Zuaw) for the interpretation of the conventional technique results. Computation of Zuaw required, in addition, estimation of nasal impedance values, which were drawn from previous measurements (K. N. Desager, M. Willemen, H. P. Van Bever, W. De Backer, and P. A. Vermeire. Pediatr. Pulmonol. 11: 1-7, 1991). Upper airway resistance and reactance at 12 Hz ranged from 40 to 120 and from 0 to -150 hPa. l(-1). s, respectively. Varying nasal impedance within the range observed in infants did not result in major changes in the estimates of Zuaw or lung impedance (ZL), the impedance of the respiratory system in parallel with Zuaw. The conventional technique underestimated ZL, depending on the value of Zuaw. The head generator technique slightly overestimated ZL, probably because the pressure gradient across the upper airway was not completely suppressed. Because of the need to enclose the head in a box (which is not required with the conventional technique), the head generator technique is difficult to perform in infants.     

Early detection of upper airway obstructions by analysis of acoustical respiratory input impedance

Repetitive occurrence of partial or total upper airway obstruction characterizes several respiratory dysfunctions such as the obstructive sleep apnea syndrome (OSAS). In OSAS patients, pharyngeal collapses are linked to a decrease in upper airway muscle activity during sleep which causes decreased upper airway wall stiffness. Continuous positive airway pressure (CPAP) is recommended as the treatment of choice. Advancements in CPAP therapy require early detection of respiratory events in real time to adapt the level of the applied pressure to airway collapsibility. The forced oscillation technique (FOT) is a noninvasive method which reflects patients' airway patency by measuring respiratory impedance. The aim of this study was to evaluate by a mathematical model of the respiratory system if FOT can provide an early detection index of total or partial upper airway obstruction. Furthermore, the simulation should suggest which characteristic features are relevant for early apnea detection in measured clinical data. The respiratory system has been treated as a series of cylindrical segments. The oropharynx analog of the model allows simulation of upper airway collapse, mimicking the situation in patients with OSAS. We calculated the input impedance for different degrees of upper airway obstruction ranging from unobstructed airways to total occlusion. Furthermore, we simulated different upper airway wall compliances. We compared the simulation with real data. The results of the study suggest that FOT is a valuable tool for assessing the degree of upper airway obstruction in patients with OSAS. Especially, the phase angle of the impedance seems to be a potentially useful tool for early apnea detection by assessing the upper airway wall collapsibility. Received: 23 July 1998 / Accepted in revised form: 26 January 1999     

Respiratory impedance measured with head generator to minimize upper airway shunt

A new method for measuring total respiratory input impedance (Zrs), which ensures minimal motion of extrathoracic airway walls, was tested over frequencies of 4-30 Hz in 14 normal subjects and 10 patients with airway obstruction. It consists of applying pressure variations around the head, rather than at the mouth, so that transmural pressure across upper airway walls is equal to the small pressure drop across the pneumotachograph. Compared with reference Zrs values obtained by directly measuring airway wall motion with a head plethysmograph and correcting the data for it, the investigated method provided similar values for respiratory resistance at all frequencies (30 Hz, 3.67 +/- 2.24 cmH2O X 1(-1) X s compared with 3.55 +/- 2.00) but slightly overestimated respiratory reactance at the largest frequencies (30 Hz, 2.82 +/- 1.28 cmH2O X 1(-1) X s compared with 2.52 +/- 1.22, P less than 0.01). In contrast, when the data were not corrected for airway wall motion, resistance was largely underestimated, especially in patients (-48% at 30 Hz, P less than 0.001), and the reactance-frequency curve was shifted to the right. The investigated method is almost as accurate as the reference method, provides equally reproducible data, and is much simpler.     

Contribution of large airway to the input impedance of the respiratory system

To evaluate the contribution of the large airway to total respiratory impedance, we develop a one-dimensional model of pressure and flow in these airways by coupling conservation of mass and momentum equations with the geometric information obtained by the acoustic reflection technique. We use this model to calculate the impedance of the respiratory system distal to the carina from impedance data estimated at the airway opening by the forced oscillation technique. Simulations show that the real part of the impedance distal to the carina is uniformly decreased from the impedance at the airway opening, indicating a resistive loss, while the imaginary part is increased as a function of frequency. We estimate parameter values for a six-parameter two-compartment lung model and for a three-parameter reduction of this model before and after the application of the upper airway data to the impedance spectrum. Although compliance terms seem to be minimally affected by the manipulation of the data, resistance and inertance terms are influenced in a fashion that suggests that the resistive contribution of the upper airway to total respiratory impedance is significant. Furthermore it appears that the elastic nature of the walls of the upper airway also impact on estimates of total respiratory impedance at the airway opening.     

Inability to separate airway from tissue properties by use of human respiratory input impedance

Respiratory input impedance (Zrs) from 2.5 to 320 Hz displays a high-frequency resonance, the location of which depends on the density of the resident gas in the lungs (J. Appl. Physiol. 67: 2323-2330, 1989). A previously used six-element model has suggested that the resonance is due to alveolar gas compression (Cg) resonating with tissue inertance (Iti). However, the density dependence of the resonance indicates that is associated with the first airway acoustic resonance. The goal of this study was to determine whether unique properties for tissues and airways can be extracted from Zrs data by use of models that incorporate airway acoustic phenomena. We applied several models incorporating airway acoustics to the 2.5- to 320-Hz data from nine healthy adult humans during room air (RA) and 20% He-80% O2 (HeO2) breathing. A model consisting of a single open-ended rigid tube produced a resonance far sharper than that seen in the data. To dampen the resonance features, we used a model of multiple open-ended rigid tubes in parallel. This model fit the data very well for both RA and HeO2 but required fewer and longer tubes with HeO2. Another way to dampen the resonance was to use a single rigid tube terminated with an alveolar-tissue unit. This model also fit the data well, but the alveolar Cg estimates were far smaller than those expected based on the subject's thoracic gas volume. If Cg was fixed based on the thoracic gas volume, a large number of tubes were again required. These results along with additional simulations show that from input Zrs alone one cannot uniquely identify features indigenous to alveolar Cg or to the respiratory tissues.     

Dual assessment of airway area profile and respiratory input impedance from a single transient wave.

This report concerns the inference of geometric and mechanical airway characteristics based on information derived from a single transient planar wave recorded at the airway opening. We describe a new method to simultaneously measure upper airway area and respiratory input impedance by performing dual analysis of a single pressure wave. The algorithms required to reconstruct airway dimensions and mechanical characteristics were developed, implemented, and tested with reference to known physical models. Our method appears suitable to estimate, even under severe intensive care unit conditions, the respiratory system frequency response (above 10 Hz) in intubated patients and the patency of the endotracheal tube used to connect the patients to the ventilator.     

Hydraulic input impedance measurements in physical models of the arterial wall

A white noise method was used to measure the hydraulic input impedance and transmission characteristics in physical models of an arterial system made of single, unbranched latex tubes. The experimentally obtained impedance curves show a rise in modulus and a positive phase at high frequencies in the absence of wave reflections. Using the impedance moduli in the presence of wave reflections, wave velocity and attenuation were calculated. The influence of wall nonlinearity on hydraulic impedance was also examined. It is concluded that, in the model used neither wave reflections nor wall nonlinearity can account for the deviations of the experimental impedance curves from the theoretically predicted ones. Impedance moduli in the presence of reflections may be used to study transmission characteristics (wave velocity and attenuation) of the model.     

Changes of respiratory input impedance during breathing in humans.

Changes of total respiratory resistance (Rrs) and reactance (Xrs) were studied between 8 and 32 Hz at five moments during the respiratory cycle in healthy adults (group A) and children (group B) and in patients with chronic obstructive lung disease (group C) and with upper airway obstruction (group D). Two forced oscillation techniques were used: the conventional one and the head generator, with the oscillations applied at the mouth and around the head of the subject, respectively. Both techniques yielded similar results. Rrs is lowest during the transition from inspiration to expiration and highest in the course of expiration, except in group D. Mean Xrs is highest at the transitions from inspiration to expiration or vice versa and lowest during expiration, except in group D. In groups C and D, the increases of Rrs are accompanied by a more pronounced negative frequency dependence of Rrs. The variations of Rrs and Xrs appear to be markedly flow dependent and may be a consequence of the interaction of breathing with oscillatory flows.     

Upper airway negative-pressure effects on respiratory activity of upper airway muscles

Influence of upper airway negative-pressure change on the respiratory activity of various upper airway muscles was investigated in 13 anesthetized rabbits. Phasic inspiratory activity increased or appeared during virtually all negative-pressure trials in nasolabial, cricothyroid, and posterior cricoarytenoid muscles. No phasic inspiratory activity was seen in the sternothyroid (ST) and sternohyoid (SH) muscles before negative-pressure applications but appeared during 80% of trials in ST and 62% of trials in SH. During maintained negative pressure, a gradual decline in activity was often observed in the nasolabial and laryngeal muscles, whereas a rapid decline in activity was seen in the cervical strap muscles. Reflex effects of negative pressure was markedly reduced or abolished by sectioning the internal branch of the superior laryngeal nerve bilaterally. Reflex augmentation of upper airway muscle activity reported here may have functional significance in the maintenance of upper airway patency. It could prevent upper airway collapse when negative pressure swings in the upper airway increase or facilitate recovery when large negative pressure swings are produced by obstructed inspiratory efforts.     

Effects of continuous positive airway pressure on upper airway and respiratory muscle activity

To study the effects of continuous positive airway pressure (CPAP) on lung volume, and upper airway and respiratory muscle activity, we quantitated the CPAP-induced changes in diaphragmatic and genioglossal electromyograms, esophageal and transdiaphragmatic pressures (Pes and Pdi), and functional residual capacity (FRC) in six normal awake subjects in the supine position. CPAP resulted in increased FRC, increased peak and rate of rise of diaphragmatic activity (EMGdi and EMGdi/TI), decreased peak genioglossal activity (EMGge), decreased inspiratory time and inspiratory duty cycle (P less than 0.001 for all comparisons). Inspiratory changes in Pes and Pdi, as well as Pes/EMGdi and Pdi/EMGdi also decreased (P less than 0.001 for all comparisons), but mean inspiratory airflow for a given Pes increased (P less than 0.001) on CPAP. The increase in mean inspiratory airflow for a given Pes despite the decrease in upper airway muscle activity suggests that CPAP mechanically splints the upper airway. The changes in EMGge and EMGdi after CPAP application most likely reflect the effects of CPAP and the associated changes in respiratory system mechanics on the afferent input from receptors distributed throughout the intact respiratory system.     

Influence of respiratory drive on upper airway resistance in normal men

The variations in nasal and pharyngeal resistance induced by changes in the central inspiratory drive were studied in 10 normal men. To calculate resistances we measured upper airway pressures with two low-bias flow catheters; one was placed at the tip of the epiglottis and the other in the posterior nasopharynx, and we measured flow with a Fleisch no. 3 pneumotachograph connected to a tightly fitting mask. Both resistances were obtained continuously during CO2 rebreathing (Read's method) and during the 2 min after a 1-min voluntary maximal hyperventilation. The inspiratory drive was estimated by measurements of inspiratory pressure generated at 0.1 s after the onset of inspiration (P0.1) and by the mean inspiratory flow (VT/TI). In each subject both resistances decreased during CO2 rebreathing; these decreases were correlated with the increase in P0.1. During the posthyperventilation period, ventilation fell below base line in seven subjects; this was accompanied by an increase in both nasal and pharyngeal resistances. These resistances increased exponentially as VT/TI decreased. Parallel changes in nasal and pharyngeal resistances were seen during CO2 stimulus and during the period after the hyperventilation. We conclude that 1) the indexes quantifying the inspiratory drive reflect the activation of nasopharyngeal dilator muscles (as assessed by the changes in upper airway resistance) and 2) both nasal and pharyngeal resistances are similarly influenced by changes in the respiratory drive.     

Activity of respiratory pump and upper airway muscles during sleep onset

Ventilationdecreases at sleep onset. This change is initiated abruptly at -electroencephalographic transitions. The aim of this study was todetermine the contributions of reduced activity in respiratory pumpmuscles and upper airway dilator muscles to this change. Surfaceelectromyograms over the diaphragm (Di) and intercostal muscles andfine-wire intramuscular electrodes in genioglossus (GG) and tensorpalatini (TP) muscles were recorded in nine healthy young men. It wasshown that phasic Di and both phasic and tonic TP activities were lowerduring  than during  activity. Breath-by-breath analysis of thechanges at - transitions during the sleep-onset period showed anumber of changes. At - transitions, phasic activity of Di,intercostal, and GG muscles fell and rose again, and phasic and tonicactivities of TP fell and remained at low levels during . With astate transition from  to , the phasic and tonic activities ofthe Di, GG, and TP increased dramatically. It is now clear that thefall in ventilation that occurs with sleep is related to a fall inactivities of both upper airway dilator muscles and respiratory pumpmuscles.