Frequency and amplitude effects during high-frequency vibration ventilation in dogs

High-frequency external body vibration, combined with constant gas flow at the tracheal carina, was previously shown to be an effective method of ventilation in normal dogs. The effects of frequency (f) and amplitude of the vibration were investigated in the present study. Eleven anesthetized and paralyzed dogs were placed on a vibrating table (4-32 Hz). O2 was delivered near the tracheal carina at 0.51.kg-1.min-1, while mean airway pressure was kept at 2.4 +/- 0.9 cmH2O. Table vertical displacement (D) and acceleration (a), esophageal (Pes), and tracheal (Ptr) peak-to-peak pressures, and tidal volume (VT) were measured as estimates of the input amplitude applied to the animal. Steady-state arterial PCO2 (PaCO2) and arterial PO2 (PaO2) values were used to monitor overall gas exchange. Typically, eucapnia was achieved with f greater than 16 Hz, D = 1 mm, a = 1 G, Pes = Ptr = 4 +/- 2 cmH2O, and VT less than 2 ml. Inverse exponential relationships were found between PaCO2 and f, a, Pes, and Ptr (exponents: -0.69, -0.38, -0.48, and -0.54, respectively); PaCO2 decreased linearly with increased displacement or VT at a fixed frequency (17 +/- 1 Hz). PaO2 was independent of both f and D (393 +/- 78 Torr, mean +/- SD). These data demonstrate the very small VT, Ptr, and Pes associated with vibration ventilation. It is clear, however, that mechanisms other then those described for conventional ventilation and high-frequency ventilation must be evoked to explain our data. One such possible mechanism is forcing of flow oscillation between lung regions (i.e., forced pendelluft).     

Minimization of lung pressure swings during high-frequency ventilation: a model

The goal of this theoretical study was to develop a simple computational model for determining the lung pressure excursions that accompany the maintenance of adequate gas transport through high-frequency airway oscillations applied via the trachea (HFAO) and by transthoracic means (HFTO). Respiratory mechanics and gas transport parameters estimated from the preceding companion study (J. Appl. Physiol. 67: 985-992, 1989) were used in the model for computing tracheal, alveolar, pleural, and transpulmonary pressure swings. Comparison of model predictions with corresponding data obtained in dogs showed close agreement. The specification of eucapnia as a constraint led to results that were significantly different from previous findings which had assumed constant airflow. We used the model to identify "quasi-optimal" strategies for HFAO and HFTO application in which all pressure excursions were kept below the corresponding levels produced by conventional mechanical ventilation operating at 15 breaths/min. The model suggests the application of both HFAO and HFTO at frequencies substantially lower than the settings commonly employed in high-frequency ventilation. Application of HFAO at frequencies ranging from 1 to 4 Hz is recommended, whereas for HFTO the quasi-optimal range lies between 1 and 1.7 Hz. In patients with chronic obstructive pulmonary disease, pressure costs during HFAO or HFTO are minimized in the vicinity of 1 Hz.     

Effect of lung inflation on diaphragmatic shortening

The effect of lung inflation on chest wall mechanics was studied in 11 vagotomized pentobarbital sodium-anesthetized dogs. Diaphragmatic shortening (percent change from initial length at functional residual capacity, %LFRC) and transdiaphragmatic pressure swings (delta Pdi) were compared with control values over a range of positive-pressure breathing that produced a maximum increase in lung volume to 40% of inspiratory capacity. There was no change in the electromyogram of the diaphragm or parasternal intercostals during positive-pressure breathing. delta Pdi and tidal volume (VT) fell to 52 +/- 3.3 and 42.5 +/- 5% (SE) of control. This was associated with a reduction in the initial resting length of 13 +/- 1.9 and 21 +/- 2.2%LFRC (SE) in the costal and crural diaphragms, respectively. Tidal diaphragmatic shortening, however, decreased to 66 +/- 7 and 57 +/- 7 and the mean velocity decreased to 78 +/- 10 and 63 +/- 8% (SE) of control for the costal and crural diaphragms, respectively. We conclude that the reduction in diaphragmatic shortening is the main determinant of the reduced delta Pdi and VT during lung inflation and relate this to what is currently known about diaphragmatic contractile properties.     

Validation of esophageal balloon technique at different lung volumes and postures

The esophageal balloon technique for measuring pleural surface pressure (Ppl) has recently been shown to be valid in recumbent positions. Questions remain regarding its validity at lung volumes higher and lower than normally observed in upright and horizontal postures, respectively. We therefore evaluated it further in 10 normal subjects, seated and supine, by measuring the ratio of esophageal to mouth pressure changes (delta Pes/delta Pm) during Mueller, Valsalva, and occlusion test maneuvers at FRC, 20, 40, 60, and 80% VC with the balloon placed 5, 10, and 15 cm above the cardia. In general, delta Pes/delta Pm was highest at the 5-cm level, during Mueller maneuvers and occlusion tests, regardless of posture or lung volume (mean range 1.00-1.08). At 10 and 15 cm, there was a progressive increase in delta Pes/delta Pm with volume (from 0.85 to 1.14). During Valsalva maneuvers, delta Pes/delta Pm also tended to increase with volume while supine (range 0.91-1.04), but was not volume-dependent while seated. Qualitatively, observed delta Pes/delta Pm fit predicted corresponding values (based on lung and upper airway compliances). Quantitatively there were discrepancies probably due to lack of measurement of esophageal elastance and to inhomogeneities in delta Ppl. At every lung volume in both postures, there was at least one esophageal site where delta Pes/delta Pm was within 10% of unity.     

Heterogeneity of mean alveolar pressure during high-frequency oscillations

Mean alveolar pressure may exceed mean airway pressure during high-frequency oscillations (HFO). To assess the magnitude of this effect and its regional heterogeneity, we studied six excised dog lungs during HFO [frequency (f) 2-32 Hz; tidal volume (VT) 5-80 ml] at transpulmonary pressures (PL) of 6, 10, and 25 cmH2O. We measured mean pressure at the airway opening (Pao), trachea (Ptr), and four alveolar locations (PA) using alveolar capsules. Pao was measured at the oscillator pump, wherein the peak dynamic head was less than 0.2 cmH2O. Since the dynamic head was negligible here, and since these were excised lungs, Pao thus represented true applied transpulmonary pressure. Ptr increasingly underestimated Pao as f and VT increased, with Pao - Ptr approaching 8 cmH2O. PA (averaged over all locations) and Pao were nearly equal at all PL's, f's, and VT's, except at PL of 6, f 32 Hz, and VT 80 ml, where (PA - Pao) was 3 cmH2O. Remarkably, mean pressure in the base exceeded that in the apex increasingly as f and VT increased, the difference approaching 3 cmH2O at high f and VT. We conclude that, although global alveolar overdistension assessed by PA - Pao is small during HFO under these conditions, larger regional heterogeneity in PA's exists that may be a consequence of airway branching angle asymmetry and/or regional flow distribution.     

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

Dependences of the mechanical properties of the respiratory system on frequency (f) and tidal volume (VT) in the normal ranges of breathing are not clear. We measured, simultaneously and in vivo, resistance and elastance of the total respiratory system (Rrs and Ers), lungs (RL and EL), and chest wall (Rcw and Ecw) of five healthy anesthetized paralyzed dogs during sinusoidal volume oscillations at the trachea (50-300 ml, 0.2-2 Hz) delivered at a constant mean lung volume. Each dog showed the same f and VT dependences. The Ers and Ecw increased with increasing f to 1 Hz and decreased with increasing VT up to 200 ml. Although EL increased slightly with increasing f, it was independent of VT. The Rcw decreased from 0.2 to 2 Hz at all VT and decreased with increasing VT. Although the RL decreased from 0.2 to 0.6 Hz and was independent of VT, at higher f RL tended to increase with increasing f and VT (i.e., as peak flow increased). Finally, the f and VT dependences of Rrs were similar to those of Rcw below 0.6 Hz but mirrored RL at higher f. These data capture the competing influences of airflow nonlinearities vs. tissue nonlinearities on f and VT dependence of the lung, chest wall, and total respiratory system. More specifically, we conclude that 1) VT dependences in Ers and Rrs below 0.6 Hz are due to nonlinearities in chest wall properties, 2) above 0.6 Hz, the flow dependence of airways resistance dominates RL and Rrs, and 3) lung tissue behavior is linear in the normal range of breathing.     

Liquid-filled esophageal catheter for measuring pleural pressure in preterm neonates

The precise measurement of esophageal pressure (Pes) as a reflection of pleural pressure (Ppl) is crucial to the measurement of lung mechanics in the newborn. The fidelity of Pes as a measurement of Ppl is determined by the occlusion test in which, during respiratory efforts against an occlusion at the airway opening, changes in pressure (delta Pao) (Pao is assumed to be equal to alveolar pressure) are shown to be equal to changes in Pes (delta Pes). Eight intubated premature infants (640-3,700 g) with chest wall distortion were studied using a water-filled catheter system to measure Pes. During the occlusion test, all patients had a finite region of the esophagus where delta Pes equaled delta Pao, which corresponded to points in the esophagus above the cardia but below the carina. In conclusion, even in the presence of chest wall distortion, a liquid-filled catheter with the tip between the cardia and carina can provide an accurate measurement of Ppl, even in the very small premature infant with chest wall distortion.     

Clapping or percussion causes atelectasis in dogs and influences gas exchange

We examined the effects of 10 min of lower lateral chest wall percussion with a mechanical percussor or hand clapping in groups of anesthetized, paralyzed, and ventilated supine dogs. Mechanical percussion was applied at 10-16 Hz and caused an esophageal pressure swing (delta Pes) of 10-17 cmH2O. Hand clapping was applied at 4-7 Hz and caused a delta Pes of 6-17 cmH2O. At necropsy there were large reddened areas on the lateral surface of the underlying lung as well as smaller reddened areas on the hilar surfaces of both lungs and on the lateral surface of the opposite lung. These reddened regions were demonstrated to be atelectatic by postmortem lung inflation (which caused the reddened areas to disappear) and by microscopic examination. Despite the atelectasis, gas exchange improved toward the end of the percussion or clapping period. In four dogs that were ventilated for an additional 20 min after percussion, there was a tendency for gas exchange initially to worsen and then to gradually improve.     

Relationship between inspiratory effort and breathlessness in pregnancy.

Using open-magnitude scaling, we compared the relationships between breathlessness, inspiratory esophageal pressure swing (delta Pes), and ventilation in pregnancy and postpartum. Thirteen healthy women performed progressive cycle exercise tests at 33 +/- 2 wk gestation and 12 +/- 3 wk postpartum. Pulmonary function and maximal transdiaphragmatic pressure did not change. Minute ventilation (VE) was greater in the third trimester. This increase was entirely due to the increase in tidal volume (VT; 0.74 +/- 0.18 vs. 0.54 +/- 0.18 liters at rest, P less than 0.01; 1.56 +/- 0.3 vs. 1.24 +/- 0.24 liters at 48 W, P less than 0.001). delta Pes (15.3 +/- 3.0 vs. 11.9 +/- 3.5 cmH2O at 48 W, P less than 0.01) and breathlessness (1.8 +/- 1.4 vs. 1.0 +/- 0.9 at 48 W, P less than 0.05) were greater in the third trimester. However, the relationships between VT and delta Pes and between delta Pes and breathlessness were identical in the two conditions. The VT-tidal abdominal volume (Vab) and Vab-tidal gastric pressure swing (delta Pga) relationships were similar in the two conditions. In conclusion, the relationship between delta Pes and breathlessness is the same in the third trimester and postpartum. The increased VE is responsible for the breathlessness in the third trimester. Despite progressive abdominal distension by the gravid uterus, the VT-Vab and Vab-delta Pga relationships were the same in the two conditions.     

Effects of mean airway pressure on gas transport during high-frequency ventilation in dogs

In 10 anesthetized, paralyzed, supine dogs, arterial blood gases and CO2 production (VCO2) were measured after 10-min runs of high-frequency ventilation (HFV) at three levels of mean airway pressure (Paw) (0, 5, and 10 cmH2O). HFV was delivered at frequencies (f) of 3, 6, and 9 Hz with a ventilator that generated known tidal volumes (VT) independent of respiratory system impedance. At each f, VT was adjusted at Paw of 0 cmH2O to obtain a eucapnia. As Paw was increased to 5 and 10 cmH2O, arterial PCO2 (PaCO2) increased and arterial PO2 (PaO2) decreased monotonically and significantly. The effect of Paw on PaCO2 and PaO2 was the same at 3, 6, and 9 Hz. Alveolar ventilation (VA), calculated from VCO2 and PaCO2, significantly decreased by 22.7 +/- 2.6 and 40.1 +/- 2.6% after Paw was increased to 5 and 10 cmH2O, respectively. By taking into account the changes in anatomic dead space (VD) with lung volume, VA at different levels of Paw fits the gas transport relationship for HFV derived previously: VA = 0.13 (VT/VD)1.2 VTf (J. Appl. Physiol. 60: 1025-1030, 1986). We conclude that increasing Paw and lung volume significantly decreases gas transport during HFV and that this effect is due to the concomitant increase of the volume of conducting airways.     

Transmission of airway pressure to pleural space during lung edema and chest wall restriction

To investigate the influence of positive end-expiratory pressure (PEEP) on hemodynamic measurements we examined the transmission of airway pressure to the pleural space during varying conditions of lung and chest wall compliance. Eight ventilated anesthetized dogs were studied in the supine position with the chest closed. Increases in pleural pressure were similar for both small and large PEEP increments (5-20 cmH2O), whether measured in the esophagus (Pes) or in the juxtacardiac space by a wafer sensor (Pj). Increments in Pj exceeded the increments in Pes at all levels of PEEP and under each condition of altered lung and chest wall compliance. When chest wall compliance was reduced by thoracic and abdominal binding, the fraction of PEEP sensed in the pleural space increased as theoretically predicted. Acute edematous lung injury produced by oleic acid (OA) did not alter the deflation limb pressure-volume characteristics of the lung, provided that end-inspiratory volume was adequate. With the chest and abdomen restricted OA was associated with less than normal transmission of airway pressure to the pleural space, most likely because the end-inspiratory volume required to restore normal deflation characteristics was not attained. Together these results indicate that the influence of acute edematous lung injury on the transmission of airway pressure to the pleural space depends importantly on the peak volume achieved during inspiration.     

Estimation of alveolar pressure during forced oscillation of the respiratory system.

A method for obtaining a continuous estimate of alveolar pressure (PAlv) during periodic flow is described; it was developed to improve the precision of measurements of airway and respiratory tissue impedance using the improved resolution of relatively high-frequency (approximately 5 Hz) singlas. The respiratory system was modulated with a piston pump, and lung volume and the volume change due to compression and expansion of alveolar gas were measured plethysmorgraphically; these signals and an analog divider were used to obtain a continuous solution of Boyle's law during flow. The plethysmorgraph was of the "flow" type; with it volume changes at frequencies up to 10 Hz and with rates of change up to 6 l/s were measured without amplitude or phase distortion. The method permits control of frequency and flow amplitude during PAlv measurement and calibration of PAlv in the absence of an active chest wall. However, it is technically complex.     

High-frequency oscillations via the pleural surface: an alternative mode of ventilation?

We applied high-frequency oscillatory ventilation (HFOV) of low amplitude to the pleural surface of the isolated rat lung (IPL) perfused at 10 ml X min-1 with Krebs bicarbonate containing 4.5% albumin (hematocrit 34%). Lung volume was held constant by a continuous positive airways pressure (CPAP) of 5 cmH2O. Varying CPAP from 2 to 15 cmH2O did not affect O2 uptake. Tidal volume (VT) was estimated with an impedance pneumograph, and it bore a direct linear relationship to the amplitude of both the loudspeaker input signal and the pressure change in the chamber up to 30 Hz; VT was inversely proportional to the frequency (f). However, at a constant loudspeaker input of 10 V, minute expired ventilation (VE) remained constant (mean 104 ml X min-1) as f increased from 5 to 30 Hz. Hemoglobin saturation increased by more than 80% during HFOV of 5-40 Hz and amplitude of 10 V, the maximum O2 uptake being 14.6 ml O2 per 100 ml perfusate. Whereas dead space was approximately 335 microliters, a VT of less than 40 microliters could effect normal O2 uptake, suggesting that bulk flow is playing only a minor role in gas exchange. HFOV for 60 min (CPAP 5 cmH2O) did not affect the amount of alveolar surfactant compared with conventional ventilation at the same mean airway pressure. We conclude that normal O2 uptake can be maintained by applying HFOV to the pleural surface of the IPL held at constant volume.     

Preferential axial flow during high-frequency oscillations: effects of gas density

Allen et al. (J. Clin. Invest. 76: 620-629, 1985) reported that regional phasic lung distension during high-frequency oscillations (HFO) is substantially and systemically heterogeneous when both frequency (f) and tidal volume (VT) are large. They hypothesized that this phenomenon was attributable to central airway geometry and preferential axial flow induced therein by the momentum flux of the inspiratory gas stream. According to that hypothesis, the observed distribution of phasic lung distension would depend on the ratio VT/VD* (where VD* is an index of anatomic dead space), independent of gas density (rho), when f is scaled in proportion to lung resonant frequency, fo. To test this hypothesis, we used the methods of Allen et al. (ibid.) to study six excised dog lungs during HFO (f = 2-32 Hz; VT = 5-80 ml) using gases of different densities. Alveolar pressure excursions (PA) were measured as rho spanned a 12-fold range using He, air, and SF6. The apex-to-base and right-to-left ratios of PA were used as indexes of regional heterogeneity of phasic lung distension. For each gas at low f, distension of the lung base was favored slightly independent of VT, but at higher f distension of the lung apex was favored when VT was small, whereas distension of the lung base was favored when VT was large. In addition, we observed substantial right-to-left differences in apical lobes during oscillation at high f not seen before.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory dynamics during laughter.

Lung and chest wall mechanics were studied during fits of laughter in 11 normal subjects. Laughing was naturally induced by showing clips of the funniest scenes from a movie by Roberto Benigni. Chest wall volume was measured by using a three-dimensional optoelectronic plethysmography and was partitioned into upper thorax, lower thorax, and abdominal compartments. Esophageal (Pes) and gastric (Pga) pressures were measured in seven subjects. All fits of laughter were characterized by a sudden occurrence of repetitive expiratory efforts at an average frequency of 4.6 +/- 1.1 Hz, which led to a final drop in functional residual capacity (FRC) by 1.55 +/- 0.40 liter (P < 0.001). All compartments similarly contributed to the decrease of lung volumes. The average duration of the fits of laughter was 3.7 +/- 2.2 s. Most of the events were associated with sudden increase in Pes well beyond the critical pressure necessary to generate maximum expiratory flow at a given lung volume. Pga increased more than Pes at the end of the expiratory efforts by an average of 27 +/- 7 cmH2O. Transdiaphragmatic pressure (Pdi) at FRC and at 10% and 20% control forced vital capacity below FRC was significantly higher than Pdi at the same absolute lung volumes during a relaxed maneuver at rest (P < 0.001). We conclude that fits of laughter consistently lead to sudden and substantial decrease in lung volume in all respiratory compartments and remarkable dynamic compression of the airways. Further mechanical stress would have applied to all the organs located in the thoracic cavity if the diaphragm had not actively prevented part of the increase in abdominal pressure from being transmitted to the chest wall cavity.     

Effect of the chest wall and blood volume on pulmonary distensibility.

To determine the reason for increased pulmonary distensibility in excised lungs, we performed deflation pressure-volume (PV) studies in 24 dogs. Exponential analysis of PV data gave K, an index of distensibility. Lung volume was measured by dilution of neon. Compared with measurements obtained in the supine position, with the chest closed, and with esophageal pressure (Pes) to obtain transpulmonary pressure, K was not changed significantly with the chest strapped, with pleural pressure to obtain transpulmonary pressure, or with the chest open. From displacement of PV curves obtained in the supine position and with the chest closed or open, we estimated that Pes was 0.18 kPa greater than average lung surface pressure. An increase in K in the prone and head-up positions was attributed to a traction artifact decreasing Pes. Exsanguination increased K and produced a relative increase in gas volume. These results show that overall pulmonary distensibility is unaffected by an intact chest wall. An increase in K and gas volume after exsanguination probably reflects a decreased pulmonary blood volume, with collapse of capillaries increasing the alveolar volume-to-surface ratio.     

Regional mapping of gas transport during high-frequency and conventional ventilation

The effects of changing tidal volume (VT) and frequency (f) on the distribution of ventilation during high-frequency ventilation (HFV) were assessed from the washout of nitrogen-13 by positron emission tomography. Six dogs, anesthetized and paralyzed, were studied in the supine position during conventional ventilation (CV) and during HFV at f of 3, 6, and 9 Hz. In CV and HFV at 6 Hz, VT was selected to achieve eucapnic arterial partial pressure of CO2 (37 +/- 3 Torr). At 3 and 9 Hz, VT was proportionally changed so that the product of VT and f remained constant and equal to that at 6 Hz. Mean residence time (MRT) of nitrogen-13 during washout was calculated for apical, midheart, and basal transverse sections of the lung and further analyzed for gravity-dependent, cephalocaudal and radial gradients. An index of local alveolar ventilation per unit of lung volume, or specific ventilation (spV), was calculated as the reciprocal of MRT. During CV vertical gradients of regional spV were seen in all sections with ventral (nondependent) regions less ventilated than dorsal (dependent) regions. Regional nonuniformity in gas transport was greatest for HFV at 3 and 6 Hz and lowest at 9 Hz and during CV. During HFV, a central region at the base of the lungs was preferentially ventilated, resulting in a regional time-averaged tracer concentration equivalent to that of the main bronchi. Because the main bronchi were certainly receiving fresh gas, the presence of this preferentially ventilated area, whose ventilation increased with VT, strongly supports the hypothesis that direct convection of fresh gas is an important mechanism of gas transport during eucapnic HFV. Aside from the local effect of increasing overall lung ventilation, this central area probably served as an intermediate shuttle station for the transport of gas between mouth and deeper alveoli when VT was less than the anatomic dead space.     

Effects of tonic vagal input on breathing pattern in newborn rabbits

Respiratory effects of positive and negative pressure breathing were studied in 1- and 4-day-old rabbit pups anesthetized with ketamine (50 mg/kg, im) and acepromazine (3 mg/kg, im). We recorded tidal volume (VT), tracheal pressure (Ptr), and integrated diaphragmatic EMG (DiEMG). Inspiratory (TI) and expiratory time (TE) were measured from the records of DiEMG. During breathing with increased Ptr by 1 or 2 cmH2O, VT, minute ventilation (VE), and respiratory rate (f) decreased. Changes in f relied on a TE prolongation. Neither DiEMG nor its rate of rise (DiEMGt) were affected. Except for VT decrease during positive Ptr, all other effects disappeared after vagotomy. Our results indicate that an increase in tonic vagal activity interacts with the mechanisms controlling TE and has no effect on depth and duration of inspiration. When Ptr decreased by 1 and 2 cmH2O, VE increased due to an increase in f. Increase in f relied on shortening of both TI and TE; the TE effect being more pronounced. DiEMG and DiEMGt also increased. Adverse effects of lung deflation and vagotomy strongly suggest that the respiratory reflex stimulation due to decrease in Ptr does not rely on inhibition of the slowly adapting stretch receptor activity. Therefore other excitatory vagal inputs must be responsible for this response. We propose two vagally mediated inputs: the irritant and/or the cardiac receptors.     

Relationship for gas transport during high-frequency ventilation in dogs

Alveolar ventilation during high-frequency ventilation (HFV) was estimated from the washout of the positron-emitting isotope (nitrogen-13-labeled N2) from the lungs of anesthetized paralyzed supine dogs by use of a positron camera. HFV was delivered at a mean lung volume (VL) equal to the resting functional residual capacity with a ventilator that generated tidal volumes (VT) between 30 and 120 ml, independent of the animal's lung impedance, at frequencies (f) from 2 to 25 Hz, with constant inspiratory and expiratory flows and an inspiration-to-expiration time ratio of unity. Specific ventilation (SPV), which is equivalent to ventilation per unit of compartment volume, was found to follow closely the relation: SPV = 1.9(VT/VL)2.1 X f. From this relation and from arterial PCO2 measurements we found an expression for the normocapnic settings of VT and f, given VL and body weight (W). We found that the VL was an important normalizing parameter in the sense that VT/VL yielded a better correlation (r = 0.91) with SPV/f than VT/W (r = 0.62) or VT alone (r = 0.8).     

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.