SOME EFFECTS OF RESPIRATORY FREQUENCY ON PULMONARY MECHANICS

Oscillation mechanics of the respiratory system

The mechanical impedance of the respiratory system defines the pressure profile required to drive a unit of oscillatory flow into the lungs. Impedance is a function of oscillation frequency, and is measured using the forced oscillation technique. Digital signal processing methods, most notably the Fourier transform, are used to calculate impedance from measured oscillatory pressures and flows. Impedance is a complex function of frequency, having both real and imaginary parts that vary with frequency in ways that can be used empirically to distinguish normal lung function from a variety of different pathologies. The most useful diagnostic information is gained when anatomically based mathematical models are fit to measurements of impedance. The simplest such model consists of a single flow-resistive conduit connecting to a single elastic compartment. Models of greater complexity may have two or more compartments, and provide more accurate fits to impedance measurements over a variety of different frequency ranges. The model that currently enjoys the widest application in studies of animal models of lung disease consists of a single airway serving an alveolar compartment comprising tissue with a constant-phase impedance. This model has been shown to fit very accurately to a wide range of impedance data, yet contains only four free parameters, and as such is highly parsimonious. The measurement of impedance in human patients is also now rapidly gaining acceptance, and promises to provide a more comprehensible assessment of lung function than parameters derived from conventional spirometry.

Low-frequency assessment of airway and tissue mechanics in ventilated COPD patients

Low-frequency forced oscillations have increasingly been employed to characterize airway and tissue mechanics separately in the normal respiratory system and animal models of lung disease; however, few data are available on the use of this method in chronic obstructive pulmonary disease (COPD). We studied 30 intubated and mechanically ventilated patients (COPD, n = 9; acute exacerbation of COPD, n = 21) during short apneic intervals at different levels of positive end-expiratory pressure (PEEP), with small-amplitude forced oscillations between 0.4 and 4.8 Hz. In 16 patients, measurements were made before and after inhalation of fenoterol hydrobromide plus ipratropium bromide (Berodual). Newtonian resistance and coefficients of tissue resistance (G) and elastance (H) were estimated from the respiratory system impedance (Zrs) data by model fitting. Apart from some extremely high Zrs data obtained primarily at relatively low PEEP levels, the model yielded a reasonable partitioning of the airway and tissue parameters, and the inclusion of further parameters did not improve the model performance. With increasing PEEP, Newtonian resistance and the ratio G/H decreased, reflecting the volume dependence of the airway caliber and the improved homogeneity of the lungs, respectively. Bronchodilation after the administration of Berodual was also associated with simultaneous decreases in G and H, indicating recruitment of lung units. In conclusion, the measurement of low-frequency Zrs can be accomplished in ventilated COPD patients during short apneic periods and offers valuable information on the mechanical status of the airways and tissues.

The effect of doxapram-induced hyperventilation on respiratory mechanics in horses

To investigate the influence of increased respiratory frequency on respiratory mechanics in the horse, measurements were made in two groups of seven tracheostomized horses before and after the administration of doxapram. The horses in group I had normal base line values for respiratory mechanics, whereas the horses in group II had significantly lower values of dynamic compliance (Cdyn), higher respiratory resistance (R), and a higher total change in pleural pressure (delta P). The administration of 0.3 mg kg-1 doxapram intravenously resulted in a significant increase in respiratory frequency (fR), R, delta P, tidal volume (VT), and peak to peak respiratory flow (V), and a decrease in Cdyn in both groups of horses. The group II horses had significantly greater increases in R and delta P than the horses in group I.

Respiratory mechanics by the passive relaxation technique in conscious healthy adults and patients with restrictive respiratory disorders

The passive relaxation single-breath technique has been used primarily in anesthetized human subjects to measure total respiratory system elastance and resistance. This method was used to assess the pressure-flow characteristics in 32 relaxed, conscious patients with restrictive respiratory disorders (20 with neuromuscular disease, 12 with sarcoidosis) and 27 similarly aged control subjects free of cardiothoracic disease. Using Rohrer's pressure-flow relationship during passive expiration, P/V = K1 + K2V, considerable curvilinear pressure-flow characteristics were found in both groups. These can be attributed to a combination of the upper airway and viscoelastic and elastoplastic behavior of the respiratory system. Despite the greater elastic recoil pressure (and respiratory elastance) of the restrictive patients, their pressure-flow characteristics were similar to those of the control subjects. These findings imply structural similarities in at least the lower airways, or in the effects of retractile forces along airways compensating for reduced lung volumes.

Dependencies of respiratory system resistance and elastance on amplitude and frequency in the normal range of breathing

We calculated respiratory system resistance (Rrs) and elastance (Ers) from pressure and flow at the mouth in six seated subjects relaxed at FRC (cheeks tightly compressed) during sinusoidal volume forcing (250, 500, and 750 ml) at 0.2, 0.4, and 0.6 Hz. Dependencies of Rrs and Ers on frequency and tidal volume were generally the same in each subject; Rrs tended to decrease with frequency and tidal volume, whereas Ers tended to increase with frequency and decrease with tidal volume. Multiple linear regression of combined data indicated that the frequency and tidal volume effects on Rrs and Ers were significant (p less than 0.05), and that the effects on Rrs decreased at higher flows. Average Rrs was highest (4.43 cm H2O/L/s +/- 0.21 SE) at 0.2 Hz-250 ml, and lowest (3.07 cm H2O/L/s +/- 0.37) at 0.6 Hz-750 ml. Average Ers was highest (12.1 cm H2O/L +/- 1.1) at 0.6 Hz-250 ml, and lowest (7.1 cm H2O/L +/- 0.6) at 0.2 Hz-750 ml. We conclude that frequency and tidal volume dependencies in Rrs and Ers in the normal range of breathing should be considered when interpreting measurements of respiratory system impedance or developing models to describe the mechanical behavior of the respiratory system.

Measurements of components of resistance to breathing

Total respiratory resistance (Rrs), pulmonary resistance (RL), and airway resistance (Raw) of 12 male patients with chronic obstructive lung disease were measured by the partial occlusion method, intraesophageal balloon technique, and body plethysmography, respectively. Chest wall resistance (Rew) and lung tissue resistance (Rti) were computed. Percentages of Rew/Rrs, RL/Rrs, Raw/Rrs, Rti/Rrs, Raw/RL, and Rti/RL were calculated. The magnitude of the components of resistance to breathing of this study and the data appearing in the literature are compared. Wide variation between the data reported by various authors was observed. The possible causes of these variations are discussed.

Amplitude dependency of regional chest wall resistance and elastance at normal breathing frequencies

Current methods for measurement of chest wall properties assume that resistance (R) and elastance (E) are independent of the volume breathed. In six healthy subjects relaxed at functional residual capacity, we measured total and regional R and E of the chest wall within the range of normal breathing frequencies (0.2 to 0.6 Hz) and tidal volumes (250 to 750 ml), using volume forcing at the mouth as previously described. With these methods, esophageal and gastric pressures are compared with surface displacements measured with inductance plethysmographic belts to calculate R and E of rib cage and diaphragm-abdomen "pathways." Rib cage R and E were 25 to 30% higher than that of the total chest wall at each frequency and volume, whereas diaphragm-abdomen R and E were at least five times higher. R of the chest wall and each of the pathways decreased by about 70% with increasing frequency and by about 30% with increasing tidal volume. E of the chest wall and each of the pathways also decreased by about 30% with increasing tidal volume but was independent of frequency in this range. These results are consistent with nonlinear, viscoplastic models presented elsewhere. We conclude that: (1) despite the great structural differences between the rib cage and diaphragm-abdomen, each exhibits nonlinear behavior similar to that of the total chest wall; (2) chest wall R and E depend importantly on frequency and tidal volume.

A nonlinear model of respiratory mechanics in emphysematous lungs

Existing mechanical models of chronic obstructive lung disease have failed to explain a number of experimental findings of airway obstruction, e.g., the varying manners of frequency dependence of resistance (FDR). Departing from the parallel-unit concept and attempting to account for the "check-valve" mechanism in the emphysematous lung, we proposed a single-compartment lung model with a nonlinear pressure-flow relationship: P + P* = LV + K1V + K3(V +/- V*)3 + V/c, where P* = K3V*, V* is a constant. The plus and minus signs in the cubic term indicate the expiratory and inspiratory check valves, respectively. The choice of an asymmetric P - V relation reflects several properties of emphysematous lungs such as airflow limitation and higher expiratory resistance. Implementation of the above equation using sine wave, white noise, and step inputs resulted in various forms of FDR at frequencies between 0 and 40 Hz depending on the type of input used. Resistance was most sensitive to changes in input pressure amplitude. The model's results suggest that the P - V nonlinearity can have a significant influence on the impedance construct in obstructed lung disease.

Pulmonary mechanics during treadmill exercise in race ponies

Exercise-induced variations in their ventilatory mechanics were studied in 8 healthy ponies 4.2 +/- 1.4 years old and weighing 282 +/- 11 kg. Airflow (V), tidal volume (VT), esophageal pressure, mask pressure and electrocardiogram were simultaneously recorded before, during and after a treadmill (incline 8.3 degrees) exercise which consisted of 2 min walking (1.5 m.sec-1), 3 min slow trotting (3.0 m.sec-1) and 3 min fast trotting (3.5 m.sec-1). The results of three consecutive daily measurements were averaged for each pony. Heart rate, minute volume (Ve), respiratory frequency (f) and peak inspiratory and expiratory V, mean inspiratory and expiratory V, and peak to peak changes in transpulmonary pressure (maxdPtp) increased linearly and significantly with increasing velocity (v) (R2 = 0.99). Tidal volume and the inspiratory time to total breathing time ratio showed a curvilinar relation with v (R2 = 0.99). Minute volume, maxdPtp, total pulmonary resistance (RL) and VT increased from rest to fast trot 6.7, 5.7, 1.5 and 1.6 times respectively. When the ponies stopped all these values decreased significantly. After 5 min recovery, the Ve was approximately doubled, VT and max dPtp unchanged and RL 30% smaller than their respective resting values. The exercise-induced increase in Ve was achieved by an increase in f at both low and high intensity of work.

A critical assessment of pulmonary function testing in exercising ponies

Pulmonary function measurements during exercise were tested for accuracy and reproducibility in 5 saddle ponies weighing 267 +/- 9 Kg. Airflow (V) and tidal volume (VT) were measured with a Fleisch pneumotachograph mounted on a face mask. The linearity of the response and the symmetry of this device were carefully checked. Pleural pressure changes were measured by pleural puncture (Ppl) and with an esophageal balloon catheter (Pes). The elastance of the esophageal wall and the effect of the position of the esophageal catheter tip on Pes were also investigated. Airflow, VT, Ppl, Pes, mask pressure, an electrocardiogram and limb movements were simultaneously recorded before, during and after exercise. These recordings were used to assess the validity of some pulmonary function measurements and to evaluate the influence of the breathing apparatus on the respiratory pattern. Maximal intrathoracic pressure changes and total pulmonary resistance values did not differ significantly when calculated on the basis of the Ppl and the Pes curves respectively. Although the absolute Ppl values were significantly different from the absolute Pes values, both pressures recorded at different workloads were closely correlated (R = 0.99). The mean specific elastance of the esophagus was 1.56 +/- 0.24 kPa.cm.ml.-1. Changes in the position of the esophageal catheter tip induced significant differences in the recorded Pes values. The pressure/flow relationship of the pneumotachograph pressure transducer system was linear within the range of the V measured during exercise. The mask had a significant influence on respiratory frequency and maximum difference in Pes, but did not modify the exercise-induced changes in these parameters. It was concluded that the technique and methods used in this study can allow accurate pulmonary function measurements in exercising ponies.

Total respiratory impedance in healthy children

Impedance of the total respiratory system was measured in 121 healthy children aged 4-16 years during spontaneous breathing by pseudo-random forced oscillations between 3 and 10 Hz. Total respiratory resistance (Rrs), inertance (Irs) and compliance (Crs) were determined by least-mean-squares fitting. Estimates for inertance were reliable only for the larger children, where the values of Irs (0.0127 +/- 0.0034 SD) were similar to those reported for normal adults. Rrs correlated significantly (P less than 0.001) with height (r = -0.868), age (r = -0.865), and, in a subpopulation of the 6- to 16-year-old children, with forced vital capacity (r = -0.803). The corresponding correlation coefficients for Crs were 0.873, 0.844, and 0.853, respectively. Crs amounted to about a third of the static total compliance values of Sharp et al. (J Appl Physiol 1970; 29: 775-779) over the same interval of heights. In these relationships no significant difference was found between boys and girls.

Pulmonary mechanics by spectral analysis of forced random noise

The magnitude (Zrs) and phase angle (thetars) of the total respiratory impedance (Zrs), from 3 to 45 Hz, were rapidly obtained by a modification of the forced oscillation method, in which a random noise pressure wave is imposed on the respiratory system at the mouth and compared to the induced random flow using Fourier and spectral analysis. No significant amplitude or phase errors were introduced by the instrumentation. 10 normals, 5 smokers, and 5 patients with chronic obstructive lung disease (COPD) were studied. Measurements of Zrs were corrected for the parallel shunt impedance of the mouth, which was independently measured during a Valsalva maneuver, and from which the mechanical properties of the mouth were derived. There were small differences in Zrs between normals and smokers but both behaved approximately like a second-order system with thetars = 0 degree in the range of 5--9 Hz, and thetars in the range of +40 degrees at 20 Hz and +60 degrees at 40 Hz. In COPD, thetars remained more negative (compared to normals and smokers) at all frequencies and crossed 0 between 15 and 29 Hz. Changes in Zrs, similar in those in COPD, were also observed at low lung volumes in normals. These changes, the effects of a bronchodilator in COPD, and deviations of Zrs from second-order behavior in normals, can best be explained by a two-compartment parallel model, in which time-constant discrepancies between the lung parenchyma and compliant airway keep compliant greater than inertial reactance, resulting in a more negative phase angle as frequency is increased.

Distribution of ventilation and frequency-dependence of dynamic lung compliance

Lung components were analysed and dynamic pulmonary compliance was determined in 10 young healthy female subjects and seven adult male patients with bronchial obstruction. In normal subjects with a single ventilatory component (by multiple breath nitrogen washout method) a change of respiratory frequency did not affect dynamic lung compliance. Other normal subjects had two ventilatory components; in them and in the patients with bronchial obstruction, an increase of respiratory frequency decreased dynamic pulmonary compliance. A change of respiratory rate caused a greater change of dynamic lung compliance in the patients with bronchial obstruction than in normal subjects with two-component lungs. The results indicate that frequency-dependence of compliance and non-uniform distribution of inspired gas are caused by a similar mechanism. Inequality of regional time constants may be an important factor in this mechanism. The data also show that a decrease of dynamic lung compliance by more than 20% at a respiratory rate of 80 to 100 breaths/minute may be indicative of lung disease with obstruction.

Frequency dependence of compliance as a test for obstruction in the small airways

We selected five bronchitics and four asthmatics in remission, whose routine lung function tests were not significantly abnormal. Dynamic compliance was measured at different respiratory frequencies and the results compared with those obtained from a normal control group. In all patients compliance was frequency dependent and remained so after the administration of bronchodilator aerosols. Compliance was frequency dependent in only one normal subject, and this was completely reversed by bronchodilators. Because the elastic properties of the patients' lungs were normal, and because pulmonary resistance was normal or only minimally increased, we interpret these results as indicating obstruction in peripheral airways.