Confidence intervals of respiratory mechanical properties derived from transfer impedance

Short-term intraindividual variability of the parameters derived from respiratory transfer impedance (Ztr) measured from 4 to 32 Hz was studied in 10 healthy subjects. The corresponding 95% confidence intervals (CIo) were compared with those computed from a single set of data (CIL) according to Lutchen and Jackson (J. Appl. Physiol. 62: 403-413, 1987). Ztr was analyzed with the six-coefficient model of DuBois et al. (J. Appl. Physiol. 8: 587-594, 1956), which includes airway resistance (Raw) and inertance (Iaw), tissue resistance (Rti), inertance (Iti), and compliance (Cti), and alveolar gas compressibility (Cg). The lowest variability was seen for Iaw (CIo = 11.1%), closely followed by Raw (14.3%) and Cti (14.8%), and the largest for Rti and Iti (24.6 and 93.6%, respectively). Using a simpler model, where Iti was excluded, significantly decreased the variability of Iaw (P less than 0.01) and Rti (P less than 0.05) but was responsible for a systematic decrease of Raw and Iaw and increase of Rti. Except for Raw with both models and Iaw with the simpler model, CIL was greater than CIo. Whatever the model, a high correlation between both sets of confidence intervals was found for Rti and Iaw, whereas no correlation was seen for Raw. This suggests that the variability of the former coefficients mainly reflects experimental noise, whereas that of the latter is largely due to biological variability.     

Airway resistance and tissue elastance from input or transfer impedance in bronchoconstricted monkeys.

Ascaris suum (AS) challenge in nonhuman primates is used as an animal model of human asthma. The primary goal of this study was to determine whether the airways and respiratory tissues in monkeys that are bronchoconstricted by AS inhalation behave similarly to those in asthmatic humans. Airway resistance (Raw) and tissue elastance (Eti) were estimated from respiratory system input (Zin) or transfer (Ztr) impedance. Zin (0.4-20 Hz) and Ztr (2-128 Hz) were measured in anesthetized cynomolgus monkeys (n = 10) under baseline (BL) and post-AS challenge conditions. Our results indicate that AS challenge in monkeys produces 1) predominantly an increase in Raw and not tissue resistance, 2) airway wall shunting at higher AS doses, and 3) heterogeneous airway constriction resulting in a decrease of lung parenchyma effective compliance. We investigated whether the airway and tissue properties estimated from Zin and Ztr were similar and found that Raw estimated from Zin and Ztr were correlated [r(2) = 0.76], not significantly different at BL (13.6 +/- 1.4 and 13.1 +/- 0.9 cmH(2)O. l(-1). s(-1), respectively), but significantly different post-AS (20.5 +/- 4.5 cmH(2)O. l(-1). s(-1) and 18.5 +/- 5.2 cmH(2)O. l(-1). s(-1)). There was no correlation between Eti estimated from Zin and Ztr. The changes in lung mechanical properties in AS-bronchoconstricted monkeys are similar to those recently reported in human asthma, confirming that this is a reasonable model of human asthma.     

Airways impedance during single breaths of foreign gases.

The changes in airways resistance (Raw) and inertance (Iaw) during single inspirations of pure methane, helium, neon, and ethane at a flow of 0.1 l/s were measured in six healthy subjects by use of a forced-oscillation technique. Raw and Iaw were computed from respiratory transfer impedance obtained at a frequency of 20 Hz by applying pressure oscillations at the chest and measuring flow at the mouth with a bag-in-box system. Compared with the air data, the changes of Iaw after inhalation of 500 ml of gas averaged -41.1% with methane, -82.8% with helium, -25.8% with neon, and +4.8% with ethane. These changes were slightly less than the changes in gas density (-45%, -86%, -31%, and +5%, respectively). The inhaled volumes at which 50% of the changes had occurred (V50) did not differ significantly among gases and were approximately 100 ml. For Raw the data were more noisy than for Iaw; they were discarded in two subjects because of a strong and irreproducible volume dependence in air. Consistent differences were seen between the remaining subjects, one of whom exhibited a predominant viscosity dependence of Raw, one a predominant density dependence, and two an intermediate pattern. V50s were larger for Raw than for Iaw, indicating a more peripheral distribution of Raw. For Raw, V50s were lower with helium than with methane, in agreement with the notion that density-dependent resistance is located mainly in the large airways. The results suggest that some information on the serial distribution of Raw and Iaw may be derived from impedance measurements with foreign gases.     

Respiratory transfer impedances with pressure input at the mouth and chest

Two methods of measuring respiratory transfer impedance (Ztr) were compared in 14 normal subjects, from 4 to 30 Hz, 1) studying the relationship between transrespiratory pressure (Prs) and flow at the chest when varying pressure at the mouth (Ztrm) and 2) studying the relationship between Prs and flow at the mouth when varying pressure around the chest wall (Ztrw). The similarity of the two relationships was expected on the basis of a T-network model. Almost identical phase responses were obtained from the two methods. Pressure-flow ratios were slightly larger for Ztrw than for Ztrm, but differences did not exceed 2% on average in 11 of 14 subjects. When the data were analyzed with the six-coefficient model proposed by DuBois et al. (J. Appl. Physiol. 8: 587-594, 1956), similar values were found for tissue compliance and tissue inertance but slightly different values for gaseous inertance in the airways (1.97 +/- 0.35 X 10(-2) cmH2O X l-1 X s2 for Ztrw vs. 1.73 +/- 0.26 for Ztrm; P less than 0.01). Similar results were also found for total respiratory resistance but with a slightly larger contribution of airway resistance for Ztrw (64 +/- 14 vs. 57 +/- 10%; P less than 0.05). As a practical conclusion it is recommended to measure Ztrw, which is technically much easier.     

Flow and volume dependence of respiratory mechanical properties studied by forced oscillation

The influence of inspiratory and expiratory flow magnitude, lung volume, and lung volume history on respiratory system properties was studied by measuring transfer impedances (4-30 Hz) in seven normal subjects during various constant flow maneuvers. The measured impedances were analyzed with a six-coefficient model including airway resistance (Raw) and inertance (Iaw), tissue resistance (Rti), inertance (Iti), and compliance (Cti), and alveolar gas compressibility. Increasing respiratory flow from 0.1 to 0.4 1/s was found to increase inspiratory and expiratory Raw by 63% and 32%, respectively, and to decrease Iaw, but did not change tissue properties. Raw, Iti, and Cti were larger and Rti was lower during expiration than during inspiration. Decreasing lung volume from 70 to 30% of vital capacity increased Raw by 80%. Cti was larger at functional residual capacity than at the volume extremes. Preceding the measurement by a full expiration rather than by a full inspiration increased Iaw by 15%. The data suggest that the determinants of Raw and Iaw are not identical, that airway hysteresis is larger than lung hysteresis, and that respiratory muscle activity influences tissue properties.     

Lung and chest wall mechanics in rabbits during high-frequency body-surface oscillation

In eight anesthetized and tracheotomized rabbits, we studied the transfer impedances of the respiratory system during normocapnic ventilation by high-frequency body-surface oscillation from 3 to 15 Hz. The total respiratory impedance was partitioned into pulmonary and chest wall impedances to characterize the oscillatory mechanical properties of each component. The pulmonary and chest wall resistances were not frequency dependent in the 3- to 15-Hz range. The mean pulmonary resistance was 13.8 +/- 3.2 (SD) cmH2O.l-1.s, although the mean chest wall resistance was 8.6 +/- 2.0 cmH2O.l-1.s. The pulmonary elastance and inertance were 0.247 +/- 0.095 cmH2O/ml and 0.103 +/- 0.033 cmH2O.l-1.s2, respectively. The chest wall elastance and inertance were 0.533 +/- 0.136 cmH2O/ml and 0.041 +/- 0.063 cmH2O.l-1.s2, respectively. With a linear mechanical behavior, the transpulmonary pressure oscillations required to ventilate these tracheotomized animals were at their minimal value at 3 Hz. As the ventilatory frequency was increased beyond 6-9 Hz, both the minute ventilation necessary to maintain normocapnia and the pulmonary impedance increased. These data suggest that ventilation by body-surface oscillation is better suited for relatively moderate frequencies in rabbits with normal lungs.     

Reliability of parameter estimates from models applied to respiratory impedance data

Many previous studies have fit lumped parameter models to respiratory input (Zin) and transfer (Ztr) impedance data. For frequency ranges higher than 4-32 Hz, a six-element model may be required in which an airway branch (with a resistance and inertance) is separated from a tissue branch (with a resistance, inertance, and compliance) by a shunt compliance. A sensitivity analysis is applied to predict the effects of frequency range on the accuracy of parameter estimates in this model obtained from Zin or Ztr data. Using a parameter set estimated from experimental data between 4 and 64 Hz in dogs, both Zin and Ztr were simulated from 4 to 200 Hz. Impedance sensitivity to each parameter was also calculated over this frequency range. The simulation predicted that for Zin a second resonance occurs near 80 Hz and that the impedance is considerably more sensitive to several of the parameters at frequencies surrounding this resonance than at any other frequencies. Also, unless data is obtained at very high frequencies (where the model is suspect), Zin data provides more accurate estimates than Ztr data. After adding random noise to the simulated Zin data, we attempted to extract the original parameters by using a nonlinear regression applied to three frequency ranges: 4-32, 4-64, and 4-110 Hz. Estimated parameters were substantially incorrect when using only 4- to 32-Hz or 4- to 64-Hz data, but nearly correct when fitting 4- to 110-Hz data. These results indicate that respiratory system parameters can be more accurately extracted from Zin than Ztr, and to make physiological inferences from parameter estimates based on Zin impedance data in dogs, the data must include frequencies surrounding the second resonance.     

Effect of airway impedance on CO2 retention and respiratory muscle activity during NREM sleep

We hypothesized that a sleep-induced increase in mechanical impedance contributes to CO2 retention and respiratory muscle recruitment during non-rapid-eye-movement (NREM) sleep. The effect NREM sleep on respiratory muscle activity and CO2 retention was measured in healthy subjects who increased maximum total pulmonary resistance (RLmax, 1-81 cmH2O.l-1.s) from awake to NREM sleep. We determined the effects of this sleep-induced increase in airway impedance by steady-state inhalation of a reduced-density gas mixture (79% He-21% O2, He-O2). Both arterialized blood PCO2 (PaCO2) and end-tidal PCO2 (PETCO2) were measured. Inspiratory (EMGinsp) and expiratory (EMGexp) respiratory muscle electromyogram activity was measured. NREM sleep caused 1) RLmax to increase (7 +/- 3 vs. 39 +/- 28 cmH2O.l-1.s), 2) PaCO2 and/or PETCO2 to increase in all subjects (40 +/- 2 vs. 44 +/- 3 Torr), and 3) EMGinsp to increase in 8 of 9 subjects and EMGexp to increase in 9 of 17 subjects. Compared with steady-state air breathing during NREM sleep, steady-state He-O2 breathing 1) reduced RLmax by 38%, 2) decreased PaCO2 and PETCO2 by 2 Torr, and 3) decreased both EMGinsp (-20%) and EMGexp (-54%). We concluded that the sleep-induced increase in upper airway resistance accompanied by the absence of immediate load compensation is an important determinant of CO2 retention, which, in turn, may cause augmentation of inspiratory and expiratory muscle activity above waking levels during NREM sleep.     

Tracking of airway and tissue mechanics during TLC maneuvers in mice.

A tracking impedance estimation technique was developed to follow the changes in total respiratory impedance (Zrs) during slow total lung capacity maneuvers in six anesthetized and mechanically ventilated BALB/c mice. Zrs was measured with the wave-tube technique and pseudorandom forced oscillations at nine frequencies between 4 and 38 Hz during inflation from a transrespiratory pressure of 0-20 cmH2O and subsequent deflation, each lasting for approximately 20 s. Zrs was averaged for 0.125 s and fitted by a model featuring airway resistance (Raw) and inertance, and tissue damping and elastance (H). Lower airway conductance (Glaw) was linearly related to volume above functional residual capacity (V) between 0 and 75-95% maximum V, with a mean slope of dGlaw/dV = 13.6 +/- 4.6 cmH2O-1. s-1. The interdependence of Raw and H was characterized by two distinct and closely linear relationships for the low- and high-volume regions, separated at approximately 40% maximum V. Comparison of Raw with the highest-frequency resistance of the total respiratory system revealed a marked volume-dependent contribution of tissue resistance to total respiratory system resistance, resulting in the overestimation of Raw by 19 +/- 8 and 163 +/- 40% at functional residual capacity and total lung capacity, respectively, whereas the lowest frequency reactance was proportional to H; these findings indicate that single-frequency resistance values may become inappropriate as surrogates of Raw when tissue impedance is changing.     

Inverse modeling of dog airway and respiratory system impedances

Mechanical parameters of the respiratory system are often estimated from respiratory impedances using lumped-element inverse models. One such six-element model is composed of an airway branch [with a resistance (Raw) and inertance (Iaw)] separated from a tissue branch [with a resistance (Rt), inertance (It), and compliance (Ct)] by a shunt compliance representing alveolar gas compression (Cg). Even though the airways are known to have frequency-dependent resistance and inertance, these inverse models have been composed of linear frequency-independent elements. In this study we investigated the use of inverse models where the airway branch was represented by a frequency-independent Raw and Iaw, a Raw that is linearly related to frequency and an Iaw that is independent of frequency, and a system of identical parallel tubes the impedance of which was computed from the tube radius and length. These inverse models were used to analyze airway and respiratory impedances between 2 and 1,024 Hz that were predicted from an anatomically detailed forward model. The forward model represented the airways by an asymmetrically branched network with a terminal impedance representative of known Cg, Rt, It, and Ct. For respiratory impedances between 2 and 128 Hz, all models fit the data reasonably well, and reasonably accurate estimates of Cg, Rt, It, and Ct were extracted from these data. For data above 200 Hz, however, only the multiple-tube model accurately fitted respiratory impedances (Zrs). This model fitted the Zrs data best when composed of 27 tubes, each having a radius of 0.148 cm and a length of 16.5 cm.     

Deep breath reversal and exponential return of methacholine-induced obstruction in asthmatic and nonasthmatic subjects.

A deep breath (DB) during induced obstruction results in a transient reversal with a return to pre-DB levels in both asthmatic and nonasthmatic subjects. The time course of this transient recovery has been reported to be exponential by one group but linear by another group. In the present study, we estimated airway resistance (Raw) from measurements of respiratory system transfer impedance before and after a DB. Nine healthy subjects and nine asthmatic subjects were studied at their maximum response during a methacholine challenge. In all subjects, the DB resulted in a rapid decrease in Raw, which then returned to pre-DB levels. This recovery was well fit with a monoexponential function in both groups, and the time constant was significantly smaller in the asthmatic than the nonasthmatic subjects (11.6 +/- 5.0 and 35.1 +/- 15.9 s, respectively). Obstruction was completely reversed in the nonasthmatic subjects (pre- and postchallenge mean Raw immediately after the DB were 2.03 +/- 0.66 and 2.06 +/- 0.68 cmH2O.l-1.s, respectively), whereas in the asthmatic subjects complete reversal did not occur (2.29 +/- 0.78 and 4.84 +/- 2.64 cmH2O.l-1.s, respectively). Raw after the DB returned to postchallenge, pre-DB values in the nonasthmatic subjects (3.78 +/- 1.56 and 3.97 +/- 1.63 cmH2O.l-1.s, respectively), whereas in the asthmatic subjects it was higher but not significantly so (9.19 +/- 4.95 and 7.14 +/- 3.56 cmH2O.l-1.s, respectively). The monoexponential recovery suggests a first-order process such as airway wall-parenchymal tissue interdependence or renewed constriction of airway smooth muscle.     

Human respiratory input impedance from 4 to 200 Hz: physiological and modeling considerations

Recent studies on respiratory impedance (Zrs) have predicted that at frequencies greater than 64 Hz a second resonance will occur. Furthermore, if one intends to fit a model more complicated than the simple series combination of a resistance, inertance, and compliance to Zrs data, the only way to ensure statistically reliable parameter estimates is to include data surrounding this second resonance. An additional question, however, is whether the resulting parameters are physiologically meaningful. We obtained input impedance data from eight healthy adult humans using discrete frequency forced oscillations from 4 to 200 Hz. Three resonant frequencies were seen: 8 +/- 2, 151 +/- 10, and 182 +/- 16 Hz. A seven-parameter lumped element model provided an excellent fit to the data in all subjects. This model consists of an airway resistance (Raw), which is linearly dependent on frequency, and airway inertance separated from a tissue resistance, inertance, and compliance by a shunt compliance (Cg) thought to represent gas compressibility. Model estimates of Raw and Cg were compared with those suggested by measurement of Raw and thoracic gas volume using a plethysmograph. In all subjects the model Raw and Cg were significantly lower than and not correlated with the corresponding plethysmographic measurement. We hypothesize that the statistically reliable but physiologically inconsistent parameters are a consequence of the distorting influence of airway wall compliance and/or airway quarter-wave resonance. Such factors are not inherent to the seven-parameter model.     

Frequency response of the chest: modeling and parameter estimation.

The frequency response of the respiratory system was studied in the range from 3 to 70 Hz in 15 normal subjects by applying sinusoidal pressure variations around the chest and measuring gas flow at the mouth. The observed input-output relationships were systematically compared to those predicted on the basis of linear differential equations of increasing order. From 3 to 20 Hz the behavior of the system was best described by a 3rd-order equation, and from 3 to 50 Hz by a 4th-order one. A mechanistic model of the 4th order, featuring tissue compliance (Ct), resistance (Rt) and inertance (It), alveolar gas compressibility (Cg) and airway resistance (Raw), and inertance (Iaw) was developed. Using that model, the following mean values were found: Ct = 2.08-10(-2)1-hPa-1 (1 hPa congruent to 1 cm of water); Rt = 1.10-hPa-1(-1)-s; It = 0.21-10(-2)hPa-1(-1)-s2; Raw = 1.35-hPa-1(-1)-s; Iaw = 2.55-10(-2)hPa-1(-1)-s2. Additional experiments devised to validate the model were reasonably successful, suggesting that the physical meaning attributed to the coefficients was correct. The validity of the assumptions and the physiological meaning of the coefficients are discussed.     

Respiratory input impedance in anesthetized paralyzed patients

Respiratory impedance (Zrs) was measured between 0.25 and 32 Hz in seven anesthetized and paralyzed patients by applying forced oscillation of low amplitude at the inlet of the endotracheal tube. Effective respiratory resistance (Rrs; in cmH2O.l-1.s) fell sharply from 6.2 +/- 2.1 (SD) at 0.25 Hz to 2.3 +/- 0.6 at 2 Hz. From then on, Rrs decreased slightly with frequency down to 1.5 +/- 0.5 at 32 Hz. Respiratory reactance (Xrs; in cmH2O.l-1.s) was -22.2 +/- 5.9 at 0.25 Hz and reached zero at approximately 14 Hz and 2.3 +/- 0.8 at 32 Hz. Effective respiratory elastance (Ers = -2pi x frequency x Xrs; in cmH2O/1) was 34.8 +/- 9.2 at 0.25 Hz and increased markedly with frequency up to 44.2 +/- 8.6 at 2 Hz. We interpreted Zrs data in terms of a T network mechanical model. We represented the proximal branch by central airway resistance and inertance. The shunt pathway accounted for bronchial distensibility and alveolar gas compressibility. The distal branch included a Newtonian resistance component for tissues and peripheral airways and a viscoelastic component for tissues. When the viscoelastic component was represented by a Kelvin body as in the model of Bates et al. (J. Appl. Physiol. 61: 873-880, 1986), a good fit was obtained over the entire frequency range, and reasonable values of parameters were estimated. The strong frequency dependence of Rrs and Ers observed below 2 Hz in our anesthetized paralyzed patients could be mainly interpreted in terms of tissue viscoelasticity. Nevertheless, the high Ers we found with low volume excursions suggests that tissues also exhibit plasticlike properties.     

Determination of airway and tissue resistances after antigen and methacholine in nonhuman primates

Madwed, Jeffrey B., and Andrew C. Jackson.Determination of airway and tissue resistances after antigen andmethacholine in nonhuman primates. J. Appl.Physiol. 83(5): 1690-1696, 1997.Antigen challenge of Ascaris suum-sensitiveanimals has been used as a model of asthma in humans. However, noreports have separated total respiratory resistance into airway (Raw)and tissue (Rti) components. We compared input impedance (Zin) andtransfer impedance (Ztr) to determine Raw and Rti in anesthetizedcynomolgus monkeys under control and bronchoconstricted conditions. Zindata between 1 and 64 Hz are frequency dependent during baselineconditions, and this frequency dependence shifts in response toA. suum or methacholine. Thus itcannot be modeled with the DuBois model, and estimates of Raw and Rticannot be determined. With Ztr, baseline data were much less variablethan Zin in all monkeys. After bronchial challenge withA. suum or methacholine, the absoluteamplitude of the resistive component of Ztr increased and its zerocrossing shifted to higher frequencies. These data can estimate Raw and Rti with the six-element DuBois model. Therefore, in monkeys, Ztr hasadvantages over other measures of lung function, since it provides amethodology to separate estimates of Raw and Rti. In conclusion, Ztrshows spectral features similar to those reported in healthy andasthmatic humans.

     

Inertance of the respiratory system in ponies

The purpose of the present work was to measure the pulmonary inertance (IL) in ponies and to analyze its potential influence on the mechanics of breathing and on their aptitude to increase ventilation during exercise. Five healthy ponies 2.4-4 yr old [mean wt 255 +/- 15 (SE) kg] were used. On the one hand, inertance of the respiratory system (Irs) was computed from the value of the resonant frequency (fr) measured by the forced oscillation technique. On the other hand, respiratory airflow, tidal volume (VT), and transpulmonary pressure (PL) changes were recorded while the ponies were performing a light treadmill exercise, and IL was calculated as the ratio of the associated differences in inertial pressure (delta Pin) to volume acceleration (delta V). Respiratory airflow and VT were measured with a Fleisch pneumotachograph (no. 5) and PL with an intraesophageal balloon catheter. First, the protocol was carried out with the ponies breathing air and He-O2, second, while the ponies breathed through two additional tubes (100 cm long, 3 cm ID), then one, and finally none, fixed on the Fleisch pneumotachograph. Finally, the contribution of the extra- vs. the intrathoracic airways to IL was estimated by measuring the lateral midtracheal pressure recorded simultaneously with the aforementioned parameters. The values of Irs calculated with fr and of IL calculated on the basis of the delta Pin-delta V ratio were 29.8 +/- 0.4 and 19.8 +/- 1.0.10(-4) kPa.l-1.s-2, respectively. During He-O2 breathing IL decreased about three times; this result was similar to the predicted decrease based on gas density only.(ABSTRACT TRUNCATED AT 250 WORDS)     

Forced oscillatory impedance of the respiratory system at low frequencies

Respiratory mechanical impedances were determined during voluntary apnea in five healthy subjects, by means of 0.25- to 5-Hz pseudo/random oscillations applied at the mouth. The total respiratory impedance was partitioned into pulmonary (ZL) and chest wall components with the esophageal balloon technique; corrections were made for the upper airway shunt impedance and the compressibility of alveolar gas. Neglect of these shunt effects did not qualitatively alter the frequency dependence of impedances but led to underestimations in impedance, especially in the chest wall resistance (Rw), which decreased by 20-30% at higher frequencies. The total resistance (Rrs) was markedly frequency dependent, falling from 0.47 +/- 0.06 (SD) at 0.25 Hz to 0.17 +/- 0.01 at 1 Hz and 0.15 +/- 0.01 kPa X l-1 X s at 5 Hz. The changes in Rrs were caused by the frequency dependence of Rw almost exclusively between 0.25 and 2 Hz and in most part between 2 and 5 Hz. The effective total respiratory (Crs,e) and pulmonary compliance were computed with corrections for pulmonary inertance derived from three- and five-parameter model fittings of ZL. Crs,e decreased from the static value (1.03 +/- 0.18 l X kPa-1) to a level of approximately 0.35 l X kPa-1 at 2-3 Hz; this change was primarily caused by the frequency-dependent behavior of chest wall compliance.     

Effects of CO2 rebreathing on pulmonary mechanics in premature infants

The effects of hypercapnia produced by CO2 rebreathing on total pulmonary, supraglottic, and lower airway (larynx and lungs) resistance were determined in eight premature infants [gestational age at birth 32 +/- 3 (SE) wk, weight at study 1,950 +/- 150 g]. Nasal airflow was measured with a mask pneumotachograph, and pressures in the esophagus and oropharynx were measured with a fluid-filled or 5-Fr Millar pressure catheter. Trials of hyperoxic (40% inspired O2 fraction) CO2 rebreathing were performed during quiet sleep. Total pulmonary resistance decreased progressively as end-tidal PCO2 (PETCO2) increased from 63 +/- 23 to 23 +/- 15 cmH2O.l-1.s in inspiration and from 115 +/- 82 to 42 +/- 27 cmH2O.l-1.s in expiration between room air (PETCO2 37 Torr) and PETCO2 of 55 Torr (P less than 0.05). Lower airway resistance (larynx and lungs) also decreased from 52 +/- 22 to 18 +/- 14 cmH2O.l-1.s in inspiration and from 88 +/- 45 to 30 +/- 22 cmH2O.l-1.s in expiration between PETCO2 of 37 and 55 Torr, respectively (P less than 0.05). Resistance of the supraglottic airway also decreased during inspiration from 7.2 +/- 2.5 to 3.6 +/- 2.5 cmH2O.l-1.s and in expiration from 7.6 +/- 3.3 to 5.3 +/- 4.7 cmH2O.l-1.s at PETCO2 of 37 and 55 Torr (P less than 0.05). The decrease in resistance that occurs within the airway in response to inhaled CO2 may permit greater airflow at any level of respiratory drive, thereby improving the infant's response to CO2.     

Interrupter technique for measurement of respiratory mechanics in anesthetized humans

Flow (V), volume (V), and tracheal pressure (Ptr) were measured throughout a series of brief (100 ms) interruptions of expiratory V in six patients during anesthesia (halothane-N2O) and anesthesia-paralysis (succinylcholine). For the latter part of spontaneous expiration and throughout passive deflation during muscle paralysis, a plateau in postinterruption Ptr was observed, indicating respiratory muscle relaxation. Under these conditions, passive elastance of the total respiratory system (Ers) was determined as the plateau in postinterruption Ptr divided by the corresponding V. The pressure-flow relationship of the total system was determined by plotting the plateau in Ptr during interruption against the immediately preceding V. Ers averaged 23.5 +/- 1.9 (SD) cmH2O X l-1 during anesthesia and 25.5 +/- 5.4 cmH2O X l-1 during anesthesia-paralysis. Corresponding values of total respiratory system resistance were 2.0 +/- 0.8 and 1.9 +/- 0.6 cmH2O X l-1 X s, respectively. Respiratory mechanics determined during anesthesia paralysis using the single-breath method (W.A. Zin, L. D. Pengelly, and J. Milic-Emili, J. Appl. Physiol. 52: 1266-1271, 1982) were also similar. Early in spontaneous expiration, however, Ptr increased progressively during the period of interruption, reflecting the presence of gradually decreasing antagonistic (postinspiratory) pressure of the inspiratory muscles. In conclusion, the interrupter technique allows for simultaneous determination of the passive elastic as well as flow-resistive properties of the total respiratory system. The presence of a plateau in postinterruption Ptr may be employed as a useful and simple criterion to confirm the presence of respiratory muscle relaxation.