Lung and chest wall impedances in the dog: effects of frequency and tidal volume

Simulating ventilation distribution in heterogenous lung injury using a binary tree data structure

To determine the impact of mechanical heterogeneity on the distribution of regional flows and pressures in the injured lung, we developed an anatomic model of the canine lung comprised of an asymmetric branching airway network, which can be stored as binary tree data structure. The entire tree can be traversed using a recursive flow divider algorithm, allowing for efficient computation of acinar flow and pressure distributions in a mechanically heterogeneous lung. These distributions were found to be highly dependent on ventilation frequency and the heterogeneity of tissue elastances, reflecting the preferential distribution of ventilation to areas of lower regional impedance.

Changes in the mechanical properties of the respiratory system during the development of interstitial lung edema

Background

Pulmonary edema induces changes in airway and lung tissues mechanical properties that can be measured by low-frequency forced oscillation technique (FOT). It is preceded by interstitial edema which is characterized by the accumulation of extravascular fluid in the interstitial space of the air-blood barrier. Our aim was to investigate the impact of the early stages of the development of interstitial edema on the mechanical properties of the respiratory system.

Methods

We studied 17 paralysed and mechanically ventilated closed-chest rats (325–375 g). Total input respiratory system impedance (Zrs) was derived from tracheal flow and pressure signals by applying forced oscillations with frequency components from 0.16 to 18.44 Hz distributed in two forcing signals. In 8 animals interstitial lung edema was induced by intravenous infusion of saline solution (0.75 ml/kg/min) for 4 hours; 9 control animals were studied with the same protocol but without infusion. Zrs was measured at the beginning and every 15 min until the end of the experiment.

Results

In the treated group the lung wet-to-dry weight ratio increased from 4.3 ± 0.72 to 5.23 ± 0.59, with no histological signs of alveolar flooding. Resistance (Rrs) increased in both groups over time, but to a greater extent in the treated group. Reactance (Xrs) did not change in the control group, while it decreased significantly at all frequencies but one in the treated. Significant changes in Rrs and Xrs were observed starting after ~135 min from the beginning of the infusion. By applying a constant phase model to partition airways and tissue mechanical properties, we observed a mild increase in airways resistance in both groups. A greater and significant increase in tissue damping (from 603.5 ± 100.3 to 714.5 ± 81.9 cmH₂O/L) and elastance (from 4160.2 ± 462.6 to 5018.2 ± 622.5 cmH₂O/L) was found only in the treated group.

Conclusion

These results suggest that interstitial edema has a small but significant impact on the mechanical features of lung tissues and that these changes begin at very early stages, before the beginning of accumulation of extravascular fluid into the alveoli.

Effects of heterogeneities on the partitioning of airway and tissue properties in normal mice

We measured the mechanical properties of the respiratory system of C57BL/6 mice using the optimal ventilation waveform method in closed- and open-chest conditions at different positive end-expiratory pressures. The tissue damping (G), tissue elastance (H), airway resistance (Raw), and hysteresivity were obtained by fitting the impedance data to three different models: a constant-phase model by Hantos et al. (Hantos Z, Daroczy B, Suki B, Nagy S, Fredberg JJ. J Appl Physiol 72: 168-178, 1992), a heterogeneous Raw model by Suki et al. (Suki B, Yuan H, Zhang Q, Lutchen KR. J Appl Physiol 82: 1349-1359, 1997), and a heterogeneous H model by Ito et al. (Ito S, Ingenito EP, Arold SP, Parameswaran H, Tgavalekos NT, Lutchen KR, Suki B. J Appl Physiol 97: 204-212, 2004). Both in the closed- and open-chest conditions, G and hysteresivity were the lowest and Raw the highest in the heterogeneous Raw model, and G and H were the largest in the heterogeneous H model. Values of G, Raw, and hysteresivity were significantly higher in the closed-chest than in the open-chest condition. However, H was not affected by the conditions. When the tidal volume of the optimal ventilation waveform was decreased from 8 to 4 ml/kg in the closed-chest condition, G and hysteresivity significantly increased, but there were smaller changes in H or Raw. In summary, values of the obtained mechanical properties varied among these models, primarily due to heterogeneity. Moreover, the mechanical parameters were significantly affected by the chest wall and tidal volume in mice. Contribution of the chest wall and heterogeneity to the mechanical properties should be carefully considered in physiological studies in which partitioning of airway and tissue properties are attempted.

Dysanaptic growth of conducting airways after pneumonectomy assessed by CT scan

In immature dogs after pneumonectomy (PNX), pulmonary viscous resistance is persistently elevated predominantly as a result of a high airway resistance (Raw). We examined the anatomical basis for this observation by using computerized tomography scans obtained from foxhounds 4-10 mo after right PNX. Airways of the left lower lobe were followed from generations z = 0 (trachea) to z = 12. By 4 mo post-PNX, airway length increased significantly relative to sham-operated dogs, but airway cross-sectional area (CSA) did not. By 10 mo post-PNX, average airway CSA was 24% above that in controls. Theoretically, the increased airway length and CSA should reduce lobar Raw by 50%. However, post-PNX airway dilatation did not normalize total CSA, and estimated resistance due to turbulence and convective acceleration increased threefold; i.e., the 50% reduction in lobar Raw would be offset by the loss of four of seven lobes. Thus the expected reduction in work of breathing in the whole animal is only ~30%, consistent with previously measured work of breathing in pneumonectomized dogs. We conclude that airway structure adapts slowly and incompletely, resulting in limited functional compensation.

Effect of compliant intermediate airways on total respiratory resistance and elastance in mechanical ventilation

Total respiratory resistance and elastance were estimated off-line in a sample of 60 patients undergoing mechanical ventilation by means of two regression models in order to analyse and understand a possible physiological mechanism determining differences in inspiration and expiration. The first model considered a single value for resistance and elastance over a whole breathing cycle, whereas the second model considered separate values for inspiratory and expiratory resistance and a single value for elastance. Inspiratory resistance was found to be lower than expiratory resistance, and intermediate values were obtained for resistance estimated over the whole breathing cycle. Student's t-test showed a highly significant difference between these resistance estimates, and principal components analysis demonstrated a significant increase in information when both inspiratory and expiratory resistances were used. Minor differences were found between values of elastance calculated with the two approaches. In an attempt to interpret these experimental results, a lung model incorporating the non-linear viscoelastic properties of the intermediate airways was considered. This model suggested that changes in intermediate airway volume play a significant role in breathing mechanics during artificial ventilation and indicated that inspiratory and expiratory resistance could be useful parameters for locating airway obstruction.

Adaptation of respiratory muscle perfusion during exercise to chronically elevated ventilatory work

Pneumonectomy (PNX) leads to chronic asymmetric ventilatory loading of respiratory muscles (RM). We measured RM energy requirements during exercise from RM blood flow (Q) using a fluorescent microsphere technique in dogs that had undergone right PNX as adults (adult R-PNX) or as puppies (puppy R-PNX), compared with dogs subjected to right thoracotomy without PNX as puppies (Sham) and to left PNX as adults (adult L-PNX). Ventilatory work (W) was measured during exercise. RM weight was determined post mortem. After adult and puppy R-PNX, the right hemidiaphragm becomes grossly distorted, but W and right costal muscle mass increased only after adult R-PNX. After adult L-PNX, the diaphragm was undistorted; W and left hemidiaphragm RM Q were elevated, but muscle mass did not increase. Mass of parasternal muscle did not increase after adult R-PNX, despite increased Q. Thus muscle mass increased only in response to the combination of chronic stretch and dynamic loading. There was a dorsal-to-ventral gradient of increasing Q within the diaphragm, but the distribution was unaffected by anatomic distortion, hypertrophy, or workload, suggesting a fixed pattern of neural activation. The diaphragm and parasternals were the primary muscles compensating for the asymmetric loading from PNX.

Postpneumonectomy alveolar growth does not normalize hemodynamic and mechanical function

Immature foxhounds underwent 55% lung resection by right pneumonectomy (n = 5) or thoracotomy without pneumonectomy (Sham, n = 6) at 2 mo of age. Cardiopulmonary function was measured during treadmill exercise on reaching maturity 1 yr later. In pneumonectomized animals compared with Sham animals, maximal oxygen uptake, ventilatory response, and cardiac output during exercise were normal. Arterial and mixed venous blood gases and arteriovenous oxygen extraction during exercise were also normal. Mean pulmonary arterial pressure and resistance were elevated at a given cardiac output. Dynamic ventilatory power requirement was also significantly elevated at a given minute ventilation. These long-term hemodynamic and mechanical abnormalities are in direct contrast to the normal pulmonary gas exchange during exercise in these same pneumonectomized animals reported elsewhere (S. Takeda, C. C. W. Hsia, E. Wagner, M. Ramanathan, A. S. Estrera, and E. R. Weibel. J. Appl. Physiol. 86: 1301-1310, 1999). Functional compensation was superior in animals pneumonectomized as puppies than as adults. These data indicate a limited structural response of conducting airways and extra-alveolar pulmonary blood vessels to pneumonectomy and suggest the development of other sources of adaptation such as those involving the heart and respiratory muscles.

Effects of lung volume on lung and chest wall mechanics in rats

To investigate the effect of lung volume on chest wall and lung mechanics in the rats, we measured the impedance (Z) under closed- and open-chest conditions at various positive end-expiratory pressures (0-0.9 kPa) by using a computer-controlled small-animal ventilator (T. F. Schuessler and J. H. T. Bates. IEEE Trans. Biomed. Eng. 42: 860-866, 1995) that we have developed for determining accurately the respiratory Z in small animals. The Z of total respiratory system and lungs was measured with small-volume oscillations between 0.25 and 9.125 Hz. The measured Z was fitted to a model that featured a constant-phase tissue compartment (with dissipation and elastance characterized by constants G and H, respectively) and a constant airway resistance (Z. Hantos, B. Daroczy, B. Suki, S. Nagy, and J. J. Fredberg. J. Appl. Physiol. 72: 168-178, 1992). We matched the lung volume between the closed- and open-chest conditions by using the quasi-static pressure-volume relationship of the lungs to calculate Z as a function of lung volume. Resistance decreased with lung volume and was not significantly different between total respiratory system and lungs. However, G and H of the respiratory system were significantly higher than those of the lungs. We conclude that chest wall in rats has a significant influence on tissue mechanics of the total respiratory system.

Effect of hypoxia on respiratory system impedance in dogs

The effects of hypoxia on lung and airway mechanics remain controversial, possibly because of the confounding effects of competing reflexes caused by systemic hypoxemia. We compared the effects of systemic hypoxemia with those of unilateral alveolar hypoxia (with systemic normoxemia) on unilateral respiratory system impedance (Z) in intact, anesthetized dogs. Independent lung ventilation was obtained with a Kottmeier endobronchial tube. Individual left and right respiratory system Z was measured during sinusoidal forcing with 45 ml of volume at frequencies of 0.2-2.1 Hz during control [100% inspired O2 fraction (FIO2)], systemic hypoxemia (10% FIO2), and unilateral alveolar hypoxia (0% FIO2 to left lung, 100% FIO2 to right lung). During systemic hypoxemia, there was a mean Z magnitude increase of 18%. This change was entirely attributable to a decrease in the imaginary component of Z; there was no change in the real component of Z. Administration of atropine (0.2 mg/kg) did not block the increase in Z with systemic hypoxemia. In contrast, there was no change in Z in the lung subjected to unilateral alveolar hypoxia. We conclude that alveolar hypoxia has no direct effect on lung mechanical properties in intact dogs. In contrast, systemic hypoxemia does increase lung impedance, apparently through a noncholinergic mechanism.

Chest wall and lung mechanics during acute hemorrhage in anesthetized dogs

OBJECTIVES

In trauma and in surgical patients, respiratory mechanics may change because of many factors, including the hypotension induced by hemorrhage. The effects of acute hemorrhage on elastic and resistive characteristics of the respiratory system were studied.

DESIGN

Prospective study.

SETTING

Anesthesia research laboratory.

INTERVENTIONS

Acute hemorrhagic shock was induced in 24 supine anesthetized/paralyzed, mechanically ventilated dogs by blood withdrawal over a 12-minute period to decrease systolic arterial pressure to 50 mmHg; additional blood was subsequently withdrawn to maintain this pressure for 2 hours. Total respiratory system dynamic compliance and resistance and lung and chest wall compliances and resistances were measured.

MEASUREMENTS AND MAIN RESULTS

Total respiratory system dynamic compliance decreased from control (0.03 +/- 0.002 L/cmH2O) by the first 10 minutes of shock (p < 0.05) and was 9.8 +/- 2% lower than control 2 hours after the induction of shock because of decreases in both lung (9.6 +/- 3%) and chest wall (7.7 +/- 3%) compliances. Total respiratory resistance increased 12.8 +/- 3% from control (3.08 +/- 0.19 cmH2O/L/s) after 2 hours of shock (p < 0.05) because of an increase in chest wall resistance (21.6 +/- 8%, p < 0.05). Pulmonary resistance was not significantly increased (p > 0.05). In six control dogs, prepared similarly but not hemorrhaged, chest wall compliance and resistance did not change, but lung compliance gradually decreased by 17.8% during 150 minutes of anesthesia/paralysis. Lung resistance increased only after 100 minutes (p < 0.05).

CONCLUSIONS

(1) Hemorrhagic shock caused slight changes in the chest wall, but effects on lung mechanics were a consequence of prolonged mechanical ventilation during anesthesia/paralysis, and (2) changes in respiratory mechanics caused by hemorrhagic shock are small and, unless other deleterious factors are present, would probably have little clinical significance.

Airway and tissue constrictions are greater in closed than in open-chest conditions

We measured lung impedance (ZL) before and after four doses of methacholine (Mch) infusion in five intact chest (with esophageal balloon) and six open-chest dogs from 0.2 to 8 Hz with an optimal ventilator waveform. From ZL, we estimated airway resistance (R(aw)) and inertance (Iaw) and tissue viscance (GL) and elastance (HL). Two-way analysis of variance revealed that: (1) Mch had a strong influence on all parameters (p < 0.001), but small effect on hysteresivity, nL = GL/HL; (2) closed-chest GL and HL were significantly higher and Iaw lower than their open-chest values (p < 0.002, p < 0.05 and p < 0.0001); and (3) at the highest Mch dose, the relative increase in R(aw) was six times higher in the closed-chest condition. The reduced impact of Mch on open-chest mechanics may be due to constrictions superimposed on grossly different lung configurations and/or some humoral effects initiated by the thoracotomy. We conclude that Mch doses that elicit mild constriction in open-chest condition can cause a severe constriction in intact animals.

Changes in lung and chest wall properties with abdominal insufflation of carbon dioxide are immediately reversible

Previously we have reported that large increases in lung and chest wall elastances as well as lung resistance occur with abdominal insufflation of carbon dioxide during laparoscopic surgery. To examine whether these effects were reversible with abdominal deflation, we calculated lung and chest wall elastances and resistances from measurement of airway flow and pressure and esophageal pressure in 17 anesthetized/paralyzed patients undergoing laparoscopic surgery. Measurements were made immediately prior to abdominal insufflation and after deflation. Lung and chest wall elastances and resistances were not changed from baseline (P > 0.05), although total respiratory elastance remained slightly increased compared to baseline (P < 0.05). The change in total respiratory elastance did not correlate with abdominal insufflation time, surgical site, smoking history, or physical characteristics of the patients. There were no differences in frequency and tidal volume dependences of the elastances and resistances before and after abdominal insufflation (P > 0.5). We conclude that residual changes in respiratory mechanics caused by carbon dioxide insufflation during laparoscopic surgery are minor, and that the reported compromise of respiratory function indicated by pulmonary function tests after laparoscopy does not appear to be due to changes in passive mechanical properties of the lungs or chest wall.

Automated system for detailed measurement of respiratory mechanics

OBJECTIVE

The mechanical properties of the respiratory system (i.e., elastance and resistance) depend on the frequency, tidal volume, and shape of the flow waveform used for forcing. We developed a system to facilitate accurate measurements of elastance and resistance in laboratory and clinical settings at the frequencies and tidal volumes in the physiologic range of breathing.

METHODS

A personal computer (PC) is used to drive a common clinically used ventilator while simultaneously collecting measurements of airway flow, airway pressure, and esophageal pressure from the experimental subject or animal at different frequencies and tidal volumes. Analysis analogous to discrete Fourier transform at the fundamental frequency (i.e., ventilator setting) is used to calculate elastances and resistances of the total respiratory system and its components, the lungs and the chest wall. We have shown that this analysis is independent of the high-frequency harmonics that are present in the waveform from clinical ventilators.

RESULTS

The system has been used successfully to make measurements in anesthetized/paralyzed dogs and awake or anesthetized human volunteers in the laboratory, and in anesthetized human volunteers in the laboratory, and in anesthetized humans in the operating room and intensive care unit. Elastances and resistances obtained with this approach are the same as those obtained during more controlled conditions, e.g., sinusoidal forcing.

CONCLUSIONS

Accurate, standardized measurements of lung and chest wall properties can be obtained in many settings with relative ease with the system described. These properties, and their frequency and tidal volume dependences in the physiologic range, provide important information to aid in the understanding of changes in respiratory function caused by day-to-day conditions, clinical intervention and pathologies.

Influence of flow pattern on the parameter estimates of a simple breathing mechanics model

The first-order model of breathing mechanics is widely used in clinical practice to assess the viscoelastic properties of the respiratory system. Although simple, this model takes the predominant features of the pressure-flow relationship into account but gives highly systematic residuals between measured and model-predicted variables. To achieve a better fit of the entire data set, an approach hypothesizing deterministic time-variations of model parameters, summarized by information-weighted histograms was recently proposed by Bates and Lauzon. The present study uses flow and pressure data measured in intensive care patients to evaluate the real potential of this approach in clinical practice. Information-weighted histograms of the model parameters, estimated by an on-line identification algorithm, were first constructed by taking into account the parameter percentage standard deviations. Then, the influence of the respiratory flow pattern on the calculated histograms was evaluated by the Kolmogorov-Smirnov statistical test. The results show that the method gives good reproducibility under stable experimental conditions. In addition, for a given airflow waveform, an increase in respiratory frequency shifts the histograms representing time-varying viscous properties strongly versus lower values, whereas it shifts the histograms representing time-varying elastic properties slightly versus higher values. On the other hand, the same histograms were highly dependent on the airflow waveform, especially for the viscous properties. Even in a limited experimental work, in all the conditions considered, the method provides results which agree well with the physiological knowledge of nonlinear and multicompartment behavior of respiratory mechanics.

Assessment of time-domain analyses for estimation of low-frequency respiratory mechanical properties and impedance spectra

Time-domain estimation has been invoked for tracking of respiratory mechanical properties using primarily a simple single-compartment model containing a series resistance (Rrs) and elastance (Ers). However, owing to the viscoelastic properties of respiratory tissues, Rrs and Ers exhibit frequency dependence below 2 Hz. The goal of this study was to investigate the bias and statistical accuracy of various time-domain approaches with respect to model properties, as well as the estimated impedance spectra. Particular emphasis was placed on establishing the tracking capability using a standard step ventilation. A simulation study compared continuous-time versus discrete-time approaches for both the single-compartment and two-compartment models. Data were acquired in four healthy humans and two dogs before and after induced severe pulmonary edema while applying sinusoidal and standard ventilator forcing. Rrs and Ers were estimated either by the standard Fast Fourier Transform (FFT) approach or by a time-domain least square estimation. Results show that the continuous-time model form produced the least bias and smallest parameter uncertainty for a single-compartment analysis and is quite amenable for reliable on-line tracking. The discrete-time approach exhibits large uncertainty and bias, particularly with increasing noise in the flow data. In humans, the time-domain approach produced smooth estimates of Rrs and Ers spectra, but they were statistically unreliable at the lower frequencies. In dogs, both the FFT and time-domain analysis produced reliable and stable estimates for Rrs or Ers spectra for frequencies out to 2 Hz in all conditions. Nevertheless, obtaining stable on-line parameter estimates for the two-compartment viscoelastic models remained difficult. We conclude that time-domain analysis of respiratory mechanics should invoke a continuous-time model form.

Influence of waveform and analysis technique on lung and chest wall properties

To test an approach for measuring respiratory system resistance (R) and elastance (E) during non-sinusoidal forcing, we measured airway and esophageal pressures and flow at the trachea of 9 anesthetized-paralyzed dogs during sinusoidal forcing (SF) and 4 types of non-sinusoidal forcings at 0.15 and 0.6 Hz and 300 ml tidal volume. During SF, calculations of E and R of the lungs, chest wall or total system from discrete Fourier transform (DFT) and two other widely used methods (multiple regression and volume-pressure loop analysis) did not differ from each other (P > 0.05). During forcing with sinusoidal or step inspiration with passive expiration (inspiratory to expiratory ratio, I/E, = 1:1), Es from any analysis method were within 10% of values during SF. Although Rs of the lungs, chest wall or total system were not affected by waveform shape with DFT (P > 0.05), the other analysis methods gave values for R during non-SF that differed (P < 0.05) from those during SF by up to 77%. If I/E was changed to 1:2, with or without an added 10% inspiratory pause, values for E and R differed least from values during SF if DFT was used. During severe pulmonary edema induced by infusion of oleic acid in the right atrium, results for lung properties were similar to controls, despite large increases in E and R of the lungs. We conclude that E and R of the lungs and chest wall can be measured by DFT using nonsinusoidal forcing waveforms available on most clinical ventilators, incurring only modest error.

Low-frequency vs. high-frequency respiratory mechanics after methacholine challenge in artificially ventilated rabbits

Respiratory system resistance (Rrs) and elastance (Ers) were estimated by two methods, before and after methacholine in six anesthetized, paralyzed, and artificially ventilated rabbits. Rrs and Ers were obtained (1) by multiple linear regression analysis of the relationship between tracheal pressure and tidal volume and flow [Rrs(mlr)], Ers(mlr), and (2) by analysis of the Fourier transforms of tracheal pressure and flow resulting from 4 to 30 Hz pseudorandom pressure oscillations delivered by an Infant Star respirator [Rrs(os), Ers(os)]. Rrs(os) was significantly lower than Rrs(mlr). For instance, Rrs(os) at 20 Hz [Rrs(os)20] was (mean +/- SD) 17.3 +/- 3.5 vs 21.4 +/- 3.6 cm H2O x L-1 x s (P < 0.01) for Rrs(mlr). Ers(os) was significantly higher than the respective value obtained by multiple linear regression (718.2 +/- 81.0 vs 403.7 +/- 43.0 cm H2O L-1; P < 0.01). After methacholine, the changes of respiratory mechanics were similar with both methods. Rrs(mlr) and Rrs(os)20 increased respectively by 131 +/- 45 and 134 +/- 76%, and Ers(mlr) and Ers(os) increased respectively by 63 +/- 7 and 54 +/- 13%. A significant correlation was observed between Rrs(mlr) and Rrs(os)20 (r = 0.97) and between Ers(mlr) and Ers(os) (r = 0.96). We conclude that positive response to methacholine may be detected by forced oscillation as well as by multiple linear regression. However, the identified physiological components of the lung response (alteration in lung viscoelastic properties, increased lung inhomogeneity or increased intrathoracic airway shunt) are likely to be different with each method.

Pseudorandom signals to estimate apparent transfer and coherence functions of nonlinear systems: applications to respiratory mechanics

There is an increasing need in physiology to estimate nonparametric linear transfer functions from data originating from biological systems which are invariably nonlinear. For pseudorandom (PRN) input stimuli, we derive general expressions for the apparent transfer (Z) and coherence (gamma 2) functions of nonlinear systems that can be represented by a Volterra series. It is shown that in the case of PRN signals in which the frequency components are integer multiples of other components the estimates of Z are seriously biased due to harmonic distortion and crosstalk among frequency components of the input. When the PRN signal includes components that are not integer multiples of other components harmonic distortion is avoided, but not necessarily cross talk. Here the estimates of Z remain poor without a noticeable influence on gamma 2. To avoid the problems associated with harmonic distortions and minimize the influence of crosstalk, a family of pseudorandom signals is proposed which are especially suited for the estimation of Z and gamma 2 in mechanical measurements of physiological systems at low frequencies. The components in the signals cannot be reproduced as linear combinations of two or more frequency components of the input. In a second-order system, this completely eliminates the bias, while in higher-order, but not strongly nonlinear systems, the interactions among the components are reduced to a level that the response can be considered as if it was measured with independent sine waves of an equivalent amplitude. It is also shown that the values of gamma 2 are not appropriate to assess linearity of the system. The theory is supported by simulation results and experimental examples brought from the field of respiratory mechanics by comparing the input impedance of the respiratory system of a dog measured with various PRN signals.