Effect of lung volume on lung resistance and elastance in awake subjects measured during sinusoidal forcing

Respiratory system impedance in different decubitus evaluated by impulse oscillometry in individuals with obesity

BACKGROUND AND OBJECTIVE

The body posture can influence gas exchange, respiratory mechanics, and mucociliary clearance and different positions can be used as a therapeutic strategy to improve in gas exchange and can also help physiotherapists to assist patients who have difficult or restrictions to stay seated or the ones who stay in the same position for a long period. The objective of this study was to evaluate the effect of different positions on respiratory system impedance in obese and eutrophic subjects, using Impulse Oscillometry System (IOS).

METHODS

The IOS parameters were evaluated in seated (Se), right lateral decubitus (RL), left lateral decubitus (LL), and supine (Su).

RESULTS

Sixty two volunteers were allocated in obese group (OG) or eutrophic group (EG) according to BMI. In seated position, OG showed higher impedance than EG for R5: 0.55 (0.31; 0.93) and 0.33 (0.24; 0.52); R20: 0.39 (0.23; 0.54) and 0.32 (0.03; 0.41); R5-R20: 0.13 (0.02; 0.47) and 0.01 (-0.08; 0.27); X5: -0.20 (-0.51; 0.16) and -0,10 (-0.016; -0.04); Fres: 20.59 (11.54; 36.45 and 10.69 (7.56; 24.7) (p<0.05) and the impedance were higher in the Su for both groups. Compared to Se, there were differences with Su (R5, R5-20, X5), with RL (R20), and with LL (R5, R20) for OG; and with Su (R5, R5-20, X5, Fres), with RL and LL (X5) for EG. Compared to Su, there were differences with RL and LL (R5-20, X5) for OG; and with RL (R5, R5-20, X5, Fres), and LL (R5-20, X5, Fres) for EG. There were no differences between RL and LL for OG and EG.

CONCLUSION

The respiratory system impedance is increased in OG, with greater contribution of peripheral resistance. The higher values of resistance and reactance were obtained in the supine position, in both groups, with lower differences obtained in the right and left lateral decubitus.

Technical standards for respiratory oscillometry

Oscillometry (also known as the forced oscillation technique) measures the mechanical properties of the respiratory system (upper and intrathoracic airways, lung tissue and chest wall) during quiet tidal breathing, by the application of an oscillating pressure signal (input or forcing signal), most commonly at the mouth. With increased clinical and research use, it is critical that all technical details of the hardware design, signal processing and analyses, and testing protocols are transparent and clearly reported to allow standardisation, comparison and replication of clinical and research studies. Because of this need, an update of the 2003 European Respiratory Society (ERS) technical standards document was produced by an ERS task force of experts who are active in clinical oscillometry research.The aim of the task force was to provide technical recommendations regarding oscillometry measurement including hardware, software, testing protocols and quality control.The main changes in this update, compared with the 2003 ERS task force document are 1) new quality control procedures which reflect use of "within-breath" analysis, and methods of handling artefacts; 2) recommendation to disclose signal processing, quality control, artefact handling and breathing protocols (e.g. number and duration of acquisitions) in reports and publications to allow comparability and replication between devices and laboratories; 3) a summary review of new data to support threshold values for bronchodilator and bronchial challenge tests; and 4) updated list of predicted impedance values in adults and children.

Prestrain and cholinergic receptor-dependent differential recruitment of mechanosensitive energy loss and energy release elements in airway smooth muscle

We tested the hypothesis that oscillatory airway smooth muscle (ASM) mechanics is governed by mechanosensitive energy loss and energy release elements that can be recruited by prestrain and cholinergic stimulation. We measured mechanical energy loss and mechanical energy release in unstimulated and carbachol-stimulated bovine ASM held at prestrains ranging from 0.3 to 1.0 L_o (reference length) and subjected to sinusoidal length oscillation at 1 hz with oscillatory strain amplitudes ranging from 0.1 to 1.5% L_o. We found that oscillatory ASM mechanics during sinusoidal length oscillation is governed predominantly by one class of nonlinear mechanosensitive energy loss element and one class of nonlinear mechanosensitive energy release element with differential mechanosensitivities to oscillatory strain amplitude. The greater mechanosensitivity of the energy loss element than energy release element may explain the bronchodilatory effect of deep inspiration. Prestrain, an important determinant of ASM responsiveness, differentially increased energy loss and energy release in unstimulated and carbachol-stimulated ASM. Cholinergic stimulation, an important cause of bronchoconstriction and airway inflammation, also differentially increased energy loss and energy release. When prestrain and cholinergic stimulation were combined, we found that prestrain and cholinergic stimulation synergistically increased energy loss and energy release by ASM. The relationship between recruitment of energy loss elements and recruitment of energy release elements was nonlinear, suggesting that energy loss and energy release elements are not coupled in ASM cells. These findings imply that large lung volume and cholinergic ASM activation would synergistically increase mechanical energy expenditure during inspiration and mechanical recoil of ASM during expiration. NEW & NOTEWORTHY We report for the first time that oscillatory airway smooth muscle mechanics is governed predominantly by one class of nonlinear mechanosensitive energy loss element and one class of nonlinear mechanosensitive energy release element with differential mechanosensitivities to oscillatory strain amplitude. Prestrain and cholinergic stimulation synergistically and differentially recruit energy loss and energy release elements. The greater mechanosensitivity of the energy loss element than the energy release element may explain the bronchodilatory effect of deep inspiration.

Constant-phase descriptions of canine lung, chest wall, and total respiratory system viscoelasticity: effects of distending pressure

The dynamic mechanical properties of the respiratory system reflect the ensemble behavior of its constituent structural elements. This study assessed the appropriateness of constant-phase descriptions of respiratory tissue viscoelasticity at various distending pressures. We measured the mechanical input impedance (Z) of the lungs, chest wall and total respiratory system in 12 dogs at mean airway pressures from 5 to 30 cm H(2)O. Each Z was fitted with a constant-phase model which provided estimates tissue damping (G), elastance (H), and hysteresivity (η=G/H). Both G and H sharply increased with increasing distending pressure for the lungs and chest wall, while η attained a minimum near 15-20 cm H(2)O. Model fitting errors for the lungs and total respiratory system increased for distending pressures greater than 20 cm H(2)O, indicating that constant-phase descriptions of parenchymal and respiratory system viscoelasticty may be inappropriate at volumes closer to total lung capacity. Such behavior may reflect alterations in load distribution across various parenchymal stress-bearing elements.

Oscillation mechanics of the respiratory system: applications to lung disease

Since its introduction in the 1950s, the forced oscillation technique (FOT) and the measurement of respiratory impedance have evolved into powerful tools for the assessment of various mechanical phenomena in the mammalian lung during health and disease. In this review, we highlight the most recent developments in instrumentation, signal processing, and modeling relevant to FOT measurements. We demonstrate how FOT provides unparalleled information on the mechanical status of the respiratory system compared to more widely used pulmonary function tests. The concept of mechanical impedance is reviewed, as well as the various measurement techniques used to acquire such data. Emphasis is placed on the analysis of lower, physiologic frequency ranges (typically less than 10 Hz) that are most sensitive to normal physical processes as well as pathologic structural alterations. Various inverse modeling approaches used to interpret alterations in impedance are also discussed, specifically in the context of three common respiratory diseases: asthma, chronic obstructive pulmonary disease, and acute lung injury. Finally, we speculate on the potential role for FOT in the clinical arena.

Measuring lung function in mice: the phenotyping uncertainty principle

Measuring lung function in mice is essential for establishing the relevance of murine models to human lung disease. However, making such measurements presents particular technical challenges due to the small size of the animal, particularly with regard to the measurement of respiratory flows. In this review, we examine the various methods currently available for assessment of lung function in mice and contrast them in terms of a concept we call the phenotyping uncertainty principle; each method can be considered to lie somewhere along a continuum on which noninvasiveness must be traded off against experimental control and measurement precision. Unrestrained plethysmography in conscious mice represents the extreme of noninvasiveness and is highly convenient but provides respiratory measures that are so tenuously linked to respiratory mechanics that they cannot be considered as meaningful indicators of lung function. At the other extreme, the measurement of input impedance in anesthetized, paralyzed, tracheostomized mice is precise and specific but requires that an animal be studied under conditions far from natural. In between these two extremes lie methods that sacrifice some precision for a reduction in the level of invasiveness, a promising example being the measurement of transfer impedance in conscious, restrained mice. No method is optimal in all regards; therefore, the appropriate technique to use depends on the application.

Respiratory impedance response to continuous negative airway pressure in awake controls and OSAS

The aim of the study was to determine whether the response of respiratory impedance (Zrs) to decreasing levels of continuous negative airway pressure (CNAP) during wakefulness, differs in controls and subjects with obstructive sleep apnoea syndrome (OSAS). Zrs was measured by the forced oscillation technique (4-32 Hz) in 15 controls and 21 patients with OSAS (apnoea/hypopnoea index >20 per sleep hour) with normal lung function, in the basal state and with application of decreasing CNAP of -5, -10, and -15 hPa. Respiratory resistance was extrapolated to 0 Hz (R0) and estimated at 16 Hz (R16) by linear regression analysis of respiratory resistive impedance versus frequency. Respiratory elastance (Ers) and inertance (Irs) were estimated by multilinear regression analysis of respiratory reactance versus frequency, and resonance frequency (RF) was determined as RF=(1/2pi)(Ers/Irs)0.5. In both groups, R0, R16, Ers and RF significantly increased as the CNAP level decreased (p <0.0001 for all). R0, Ers, and RF increased significantly more in OSAS than in controls (p < 0.01, 0.001, and 0.0001, respectively), independently of the severity of obesity. Receiver operator characteristic curves showed that the parameter which best detected OSAS was RF, with a sensitivity of 81% and 93% specificity for the 13.6 Hz cut-off point. The results of the present study suggest that the response of respiratory impedance to decreasing continuous negative airway pressure levels, might allow detection of obstructive sleep apnoea syndrome in subjects with normal lung function.

Single-breath CO2 analysis as a predictor of lung volume change in a model of acute lung injury

OBJECTIVE

To examine the utility of single-breath CO2 analysis as a measure of lung volume change in a model of acute lung injury.

SETTING

Animal laboratory in a university-affiliated medical center.

DESIGN

Prospective, animal cohort study comparing 21 variables derived from single-breath CO2 analysis with lung volume measurements determined by nitrogen washout.

SUBJECTS

Seven lambs with saline lavage-induced acute lung injury.

METHODS

Animals were treated with repetitive saline lavage to achieve a uniform degree of acute lung injury (PaO2 < 100 torr [13.32 kPa] on FiO2 of 1.0). Twenty-one derived components of the CO2 expirogram were evaluated as predictors of lung volume change. Lung volume was manipulated by 3-cm H2O incremental increases in positive end-expiratory pressure from 0 to 21 cm H2O and ranged between 90 and 765 mL.

MEASUREMENTS AND MAIN RESULTS

Fifty-five measurements of lung volume were available for comparison with derived variables from the CO2 expirogram. Stepwise linear regression identified five variables that were most predictive of lung volume change: a) dynamic lung compliance; b) the slope of phase III; c) the slope of phase II divided by the mixed expired CO2 concentration; d) airway deadspace; and e) PaO2/FIO2 ratio. The multivariate equation was highly statistically significant and explained 94% of the variance (adjusted r2 = .94, p < .0001). The bias and precision of the calculated lung volume were 10.9 and 55.9, respectively. The mean percentage difference for the lung volume estimate derived from the single-breath CO2 analysis station was 3.3%.

CONCLUSIONS

Our data indicate that analysis of the CO2 expirogram can yield accurate information about lung volume in animals with saline lavage-induced acute lung injury. Specifically, five variables derived from a plot of expired CO2 concentration vs. expired volume predict changes in lung volume in healthy lambs with an adjusted coefficient of determination of 0.94. We hope to further define the utility of this technique by prospective application of this methodology in the clinical setting.

Volume dependence of respiratory impedance in infants

We previously studied low-frequency respiratory impedance (Zrs) data at an elevated lung volume to separate airway and tissue mechanical properties in normal infants (Am. I. Respir. Crit. Care Med. 1996; 154:161-166). The aim of the present study was to determine the volume dependence of the airway and tissue mechanics by extending Zrs measurements to lower lung volumes. Zrs spectra between 0.5 and 21 Hz were measured in supine sleeping infants (n = 8; 7 to 26 mo of age) at mean transrespiratory pressures (Ptr[mean]) of 20, 10, and 0 cm H2O, during periods of apnea induced by inflating the infants' lungs to a pressure of 20 cm H2O through a face mask. At each inflation pressure, a model containing airway resistance (Raw) and inertance (law) and tissue damping (G) and elastance (H) was fitted to Zrs data. At FRC, the values of Raw, law, G, and H were 20.6+/-4.9 (SD) cm H2O x s/L, 0.037+/-0.014 cm H2O x s2/L, 39.6+/-10.3 cm H2O/L, and 147+/-35 cm H2O/L, respectively. Increase of Ptr(mean) caused a monotonous decrease in Raw (42+/-7% of the value at FRC), while law remained constant. The tissue parameters were minimal at a Ptr(mean) of 10 cm H2O (68+/-10% and 78+/-6% in G and H, respectively) and significantly higher at both 0 and 20 cm H2O. Although Zrs measurements can be made in most infants at lung volumes as low as FRC, an inflation pressure of 20 cm H2O provides a higher success rate and is therefore a more suitable condition for general use.

Effects of PEEP on respiratory mechanics are tidal volume and frequency dependent

How the effects of frequency, tidal volume (VT) and PEEP interact to determine the mechanical properties of the respiratory system is unclear. Airway flow and airway and esophageal pressures were measured in ten intubated, anesthetized/paralyzed patients during mechanical ventilation at 10-30 breaths/min and VT of 250-800 ml. From these measurements, Fourier transformation was used to calculate elastance (E) and resistance (R) of the total respiratory system (subscript rs), lungs (subscript L) and chest wall (subscript cw) at 5, 10 and 0 cm PEEP. As PEEP increased from 0-5 cmH2O, all elastances and resistances decreased (P < 0.05). Increasing PEEP to 10 cmH2O decreased EL, Rrs, and RL further (P < 0.05). The changes in Ers, EL, Rrs and RL caused by PEEP were less (P < 0.05) as VT increased, while changes in Rrs, RL and Ers were less (P < 0.05) as frequency increased. VT dependences in Ers and Rrs were enhanced (P < 0.05) at 0 cmH2O PEEP. The ratio of EL to chest wall elastance was not affected by PEEP (P > 0.05), but increased (P < 0.05) with increasing VT at 5 and 10 cmH2O PEEP. We conclude that it is critical to standardize ventilatory parameters when comparing groups of patients or testing clinical intervention efficacy and that the differential effects on the lungs and chest wall must be considered in optimizing the application of PEEP.

The airways and anaesthesia II. Pathophysiology

We pointed out in the first of these two articles that the commonest cause of an anaesthetic disaster in young healthy patients is a loss of airway patency then a failure to intubate occurring unexpectedly in the absence of head or neck pathology. Upper airway obstruction is a very common complication of general anaesthesia and all anaesthetists must be trained in the management of this problem. Less obvious are the changes that can occur in the lower airways which can impair gas exchange by increasing ventilation-perfusion mismatch. This article is concerned with these pathophysiological changes that occur during general anaesthesia.

Effects of Trendelenburg and reverse Trendelenburg postures on lung and chest wall mechanics

STUDY OBJECTIVE

To test whether the Trendelenburg ("head-down") or reverse Trendelenburg ("head-up") postures change lung and chest wall mechanical properties in a clinical condition.

DESIGN

Unblinded study, each patient serving as own control.

SETTING

University of Maryland at Baltimore Hospital, Baltimore, Maryland.

PATIENTS

15 patients scheduled for laparoscopic surgery.

INTERVENTIONS

Patients were anesthetized and paralyzed, tracheally intubated and mechanically ventilated at 10 to 30 per minute and at a tidal volume of 250 to 800 ml. Measurements were made before surgery in supine, head-up (10 degrees from horizontal) and head-down (15 degrees from horizontal) postures.

MEASUREMENTS AND MAIN RESULTS

Airway flow and airway and esophageal pressures were measured. From these measurements, discrete Fourier transformation was used to calculate elastances and resistances of the total respiratory system, lungs, and chest wall. Total respiratory elastance and resistance increased in the head-down posture compared with supine due to increases in lung elastance and resistance (p < 0.05); but chest wall elastance and resistance did not change (p > 0.05). Lung elastance also exhibited a negative dependence on tidal volume while head-down that was not observed in the supine posture. The change in lung elastance compared with supine was positively correlated to body mass index (weight/height2) and negatively correlated to tidal volume. Lung and chest wall elastance and resistance were not affected by shifting from supine to head-up (p > 0.05).

CONCLUSIONS

The Trendelenburg posture increases the mechanical impedance of the lung to inflation, probably due to decreases in lung volume. This effect may become clinically relevant in patients predisposed with lung disease and in obese patients.

Respiratory mechanics in the open chest: effects of parietal pleurae

To understand how the parietal pleurae affect the mechanical behavior of the human respiratory system after the chest wall is opened by median sternotomy, we studied 18 anesthetized/paralyzed patients immediately before coronary artery bypass grafting surgery. Elastances and resistances of the total respiratory system (ETr, Rrs) were calculated from measurements of airway pressure and flow during mechanical ventilation in the frequency and tidal volume ranges of normal breathing. Elastances and resistances of the lungs (EL, RL), chest wall (Ecw, Rcw) were also estimated from measurements of esophageal pressure. Data were collected in the closed chest, after median sternotomy with the parietal pleurae intact and after the left parietal pleura was opened for internal mammary artery harvest. After sternotomy with pleurae intact (n = 14), Ers did not change but Rrs decreased (p < 0.05). Ecw (including the contribution of the pleurae) was higher than in the closed chest (p < 0.05) while EL and RL were lower (p < 0.05); Rcw did not change. Opening the left pleura (n = 10) decreased Ers (p < 0.05), but Rrs did not change. We conclude that the chest wall/pleurae compartment offers significant impedance to lung expansion after sternotomy and rib retraction, unless one pleura is opened.

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.

The effects of increased abdominal pressure on lung and chest wall mechanics during laparoscopic surgery

We tested the hypothesis that increases in pressure in the abdomen (Pab) exerted by CO2 insufflation during laparoscopy would increase elastance (E) and resistance (R) of both the lungs and chest wall. We measured airway flow and airway and esophageal pressures of 12 anesthetized/paralyzed tracheally intubated patients during mechanical ventilation at 10-30/min and tidal volume of 250-800 mL. From these measurements, we used discrete Fourier transformation to calculate E and R of the lungs and chest wall. Measurements were made at 0, 15, and 25 mm Hg Pab in the 15 degrees head-down (Trendelenburg) posture and at 0 and 15 mm Hg Pab in the 10 degrees head-up (reverse Trendelenburg) posture. Lung and chest wall Es and Rs while head-down increased at Pab = 15 mm Hg, and both Es increased further at Pab = 25 mm Hg (P < 0.05). Both Es and Rs also increased while head-up at Pab = 15 mm Hg (P < 0.05), but increases in lung E and R were less than while head-down (P < 0.05). The increase in lung E and R at Pab = 15 mm Hg in either posture were positively correlated to body weight or body mass index, whereas the increases in chest wall E and R were negatively correlated to the same factors (P < 0.05). Lung and chest wall mechanical impedances increase with increasing Pab; the increases depend on body configuration and are greater while head-down.(ABSTRACT TRUNCATED AT 250 WORDS)

Pendelluft is not the major contributor to respiratory insufficiency in dogs with flail chest: a mathematical analysis

"Pendelluft", or out-of-phase movement of the airway gas between the intact and flait-chest-side lungs has long been believed to be the major contributor to respiratory dysfunction in patients with flail chest. However, conflicting findings have also been reported mainly from animal studies. The aim of this study was to provide a mathematical projection on this classical problem. We measured respiratory impedance (ZRS) of dogs with flail chest using a pseudorandom forced oscillation method. A mathematical model implementing flail chest was fitted toZRS. The fitted results were used in simulating the mechanical behavior of a respiratory system with flail chest during spontaneous breathing. Our results suggest that the paradoxical movement of breathing between the flail segment and the intact chest wall does not create substantial pendelluft and that alveolar hypoventilation is created by the wasting movement of the flail segment which interferes with effective thoracic expansion.

The effect of changing end-expiratory pressure on respiratory system mechanics in open- and closed-chest anesthetized, paralyzed patients

The decrease in functional residual capacity (FRC) with anesthesia may cause lung volume to decrease below closing volume, thereby impairing oxygenation. Increasing end-expiratory pressure (EEP) reexpands atelectatic areas in anesthetized, ventilated patients, but its effect on pulmonary mechanics is less well understood. We studied the effect of varying EEP on the mechanical behavior of the respiratory system in patients undergoing either closed (Group 1) or open-chest (Group 2) surgical procedures. We measured airway opening pressure (PaO), flow (V), and esophageal pressure (Pes) (in Group 1 only) at EEPs of 0, 2.5, 5, and 10 cm H2O. Dynamic elastance (E) and resistance (R) for the respiratory system (RS), the lung (L), and the chest wall (CW) were estimated by fitting the equation P = RV + EV + K to the measured data by multiple linear regression where P was either Pao, Pes, or Pao-Pes. Group 1 EL decreased with increases in EEP to 5 cm H2O and then began to increase with EEP above this level. The same occurred in Group 2 before opening the chest. After opening the chest in Group 2, EL increased as EEP increased at all values above 0 cm H2O. The magnitudes of RRS and RL were similar in both groups of subjects and in each group these quantities decreased with increases in EEP. Dynamic EL responded differently to changes in EEP in subjects with open-chest and closed-chest procedures. We attribute this difference to overdistension of the remaining ventilable lung tissue at all levels of EEP in open-chest patients.