Effect of mean airway pressure on gas exchange during high-frequency oscillatory ventilation

We studied the effect of mean airway pressure (Paw) on gas exchange during high-frequency oscillatory ventilation in 14 adult rabbits before and after pulmonary saline lavage. Sinusoidal volume changes were delivered through a tracheostomy at 16 Hz, a tidal volume of 1 or 2 ml/kg, and inspired O2 fraction of 0.5. Arterial PO2 and PCO2 (PaO2, PaCO2), lung volume change, and venous admixture were measured at Paw from 5 to 25 cmH2O after either deflation from total lung capacity or inflation from relaxation volume (Vr). The rabbits were lavaged with saline until PaO2 was less than 70 Torr, and all measurements were repeated. Lung volume change was measured in a pressure plethysmograph. Raising Paw from 5 to 25 cmH2O increased lung volume by 48-50 ml above Vr in both healthy and lavaged rabbits. Before lavage, PaO2 was relatively insensitive to changes in Paw, but after lavage PaO2 increased with Paw from 42.8 +/- 7.8 to 137.3 +/- 18.3 (SE) Torr (P less than 0.001). PaCO2 was insensitive to Paw change before and after lavage. At each Paw after lavage, lung volume was larger, venous admixture smaller, and PaO2 higher after deflation from total lung capacity than after inflation from Vr. This study shows that the effect of increased Paw on PaO2 is mediated through an increase in lung volume. In saline-lavaged lungs, equal distending pressures do not necessarily imply equal lung volumes and thus do not imply equal PaO2.     

Lung pressures and gas transport during high-frequency airway and chest wall oscillation

The major goal of this study was to compare gas exchange, tidal volume (VT), and dynamic lung pressures resulting from high-frequency airway oscillation (HFAO) with the corresponding effects in high-frequency chest wall oscillation (HFCWO). Eight anesthetized paralyzed dogs were maintained eucapnic with HFAO and HFCWO at frequencies ranging from 1 to 16 Hz in the former and 0.5 to 8 Hz in the latter. Tracheal (delta Ptr) and esophageal (delta Pes) pressure swings, VT, and arterial blood gases were measured in addition to respiratory impedance and static pressure-volume curves. Mean positive pressure (25-30 cmH2O) in the chest cuff associated with HFCWO generation decreased lung volume by approximately 200 ml and increased pulmonary impedance significantly. Aside from this decrease in functional residual capacity (FRC), no change in lung volume occurred as a result of dynamic factors during the course of HFCWO application. With HFAO, a small degree of hyperinflation occurred only at 16 Hz. Arterial PO2 decreased by 5 Torr on average during HFCWO. VT decreased with increasing frequency in both cases, but VT during HFCWO was smaller over the range of frequencies compared with HFAO. delta Pes and delta Ptr between 1 and 8 Hz were lower than the corresponding pressure swings obtained with conventional mechanical ventilation (CMV) applied at 0.25 Hz. delta Pes was minimized at 1 Hz during HFCWO; however, delta Ptr decreased continuously with decreasing frequency and, below 2 Hz, became progressively smaller than the corresponding values obtained with HFAO and CMV.     

Effect of high-frequency ventilation on gas exchange and pulmonary vascular resistance in lambs

We studied the effects of conventional mechanical ventilation (CMV) (15 ml/kg tidal volume delivered at 18-25 breaths/min) and high-frequency oscillatory ventilation (HFOV) (less than or equal to 2 ml/kg delivered at 10 Hz) on pulmonary hemodynamics and gas exchange during ambient air breathing and hypoxic gas breathing in 10 4-day-old lambs. After instrumentation and randomization to either HFOV or CMV the animals breathed first ambient air and then hypoxic gas (inspired O2 fraction = 0.13) for 20 min. The mode of ventilation was then changed, and the normoxic and hypoxic gas challenges were repeated. The multiple inert gas elimination technique was utilized to assess gas exchange. There was a significant increase with HFOV in mean pulmonary arterial pressure (Ppa) (20.1 +/- 4.2 vs. 22 +/- 3.8 Torr, CMV vs. HFOV, P less than 0.05) during ambient air breathing. During hypoxic gas breathing Ppa was also greater with HFOV than with CMV (29.5 +/- 5.7 vs. 34 +/- 3.1 Torr, CMV vs. HFOV, P less than 0.05). HFOV reduced pulmonary blood flow (Qp) during ambient air breathing (0.33 +/- 0.11 vs. 0.28 +/- 0.09 l . kg-1 . min-1, CMV vs. HFOV, P less than 0.05) and during hypoxic gas breathing (0.38 +/- 0.11 vs. 0.29 +/- 0.09 l . kg-1 . min-1, P less than 0.05). There was no significant difference in calculated venous admixture for sulfur hexafluoride or in the index of low ventilation-perfusion lung regions with HFOV compared with CMV.(ABSTRACT TRUNCATED AT 250 WORDS)     

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)     

Intensification of gas exchange in high-frequency artificial ventilation

Jet high-frequency artificial ventilation produces oscillations of some parts of the chest wall, which in its turn transmits oscillations to the lung parenchyma. It results in the mix-up of the gas in the alveolar space, which leads to the increase in the gradient of oxygen concentration on the alveolar membranes, thus, augmenting oxygen saturation of the blood. The effect is the same when oscillation artificial ventilation is performed, owing to the provocation of the oscillations amplified by the resonance in the natural acoustic circuit, formed by the adjacent parts of the chest and lung parenchyma. Derangement of the exudative adhesion to the bronchi epithelial tissue intensifies gas exchange, when the oscillations are generated in the lungs. It facilitates the removal of the exudate and lets the air into the previously obstructed parts of the lungs. Clinical studies confirm the effect of the increase in the blood oxygenation (by average 20%) at the feeding air column by pneumatic oscillations in the range of 65 Hz, when traditional artificial ventilation is performed.     

Effect of cardiogenic oscillations on gas mixing during tracheal insufflation of oxygen

In a previous study using tracheal insufflation of O2 (TRIO) at a rate of 2 l/min, we showed that anesthetized paralyzed dogs could be adequately oxygenated for up to 5 h, albeit with hypercapnia (mean arterial PCO2 approximately 160 Torr). To examine the contribution of cardiogenic oscillations in producing this gas exchange, we studied seven anesthetized paralyzed dogs weighing between 19.6 and 25.5 kg and quantified gas transport by analyzing continuous N2-washout curves in vivo and postmortem. We found that cardiogenic oscillations increase gas mixing roughly fourfold and that this value was independent of insufflation flow rate (0.2-10.0 l/min). Our results lend indirect evidence that, with regard to gas exchange, there are two mechanistically different zones in the lung during TRIO. One zone, located in the more peripheral areas of the lung, is dominated by the effects of cardiac oscillations and molecular diffusion and accounts for the increase in gas mixing found in the alive vs. dead dog. A second zone, close to the insufflated jet of O2, uses convective streaming to produce greater gas mixing at higher flows.     

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.     

Effect of local pulmonary blood flow control on gas exchange: theory

The effect of local pulmonary blood flow control by local alveolar O2 tension on steady-state pulmonary gas exchange is analyzed with techniques derived from control theory. In a single homogeneous lung unit with normal inspired and mixed venous blood gas composition, the homeostatic effect on local ventilation-perfusion ratios (VA/Q) regulation occurs over a restricted range of VA/Q. The homeostatic effect is maximal at a moderately low VA/Q (about 0.4) due to the slope of the O2 dissociation curve. In a multicompartment lung with a lognormal distribution of VA/Q, regulation of arterial O2 tension varies with the extent of inhomogeneity. At mild degrees of inhomogeneity where local pulmonary blood flow (Q) control acts predominantly on the lower VA/Q of the Q distribution, the regulatory effect is best. At severe degrees of inhomogeneity where local Q control acts mainly on the higher VA/Q of the Q distribution, the regulatory effect is worse, and positive-feedback behavior may occur. Local Q control has the potential of reducing the deleterious effects of lung disease on pulmonary gas exchange particularly when it operates in association with other regulatory mechanisms.     

Intra-airway gas mixing during high-frequency ventilation

We examined the intra-airway gas transport mediated by high-frequency oscillations (HFO) in 10 nonintubated healthy volunteers using a method based on comparisons of single-breath N2-washout curves obtained after various durations of breath hold or high-frequency oscillations. With a mathematical analysis based on Fick's law of diffusion we computed the local transport parameter, effective diffusivity, during oscillations of frequency 2-24 Hz and tidal volume 10-120 ml and during breath hold alone. Local effective diffusivity increased with both oscillatory frequency and tidal volume at all levels in the tracheobronchial tree; the enhancing effect of tidal volume on local effective diffusivity was more pronounced than that of frequency so that effective diffusivity was greater with larger tidal volume at fixed frequency-tidal volume product (f . VT). The greatest enhancement of gas mixing within the lung during HFO (over breath hold) was seen in the central airways. In previous studies examining CO2 removal rate during HFO (J. Clin. Invest. 68: 1475, 1981), we found that CO2 output was also greater with larger tidal volume at fixed f . VT, and we attributed this to an end constraint imposed by a fresh gas bias flow. Results of the current study, performed without a bias flow, indicate that bias flow end constraint does not solely account for the observed dependence of CO2 output on frequency and tidal volume.