Intraregional gas mixing in humans during high frequency oscillations

Effect of high-frequency oral airway and chest wall oscillation and conventional chest physical therapy on expectoration in patients with stable cystic fibrosis

STUDY OBJECTIVE

To compare the effect of high-frequency oral airway oscillation, high-frequency chest wall oscillation, and conventional chest physical therapy (CPT) on weight of expectorated sputum, pulmonary function, and oxygen saturation in outpatients with stable cystic fibrosis (CF).

DESIGN

Prospective randomized trial.

SETTING

Pediatric pulmonary division of a tertiary care center.

PATIENTS

Fourteen outpatients with stable CF recruited from the CF center.

INTERVENTIONS

Two modes of oral airway oscillation (1: frequency 8 Hz; inspiratory to expiratory [I:E] ratio 9:1; 2: frequency 14 Hz; I:E ratio 8:1), two modes of chest wall oscillation (1: frequency 3 Hz; I:E ratio 4:1; 2: frequency 16 Hz; I:E ratio 1:1, alternating with frequency 1.5 Hz, I:E ratio 6:1), and CPT (clapping, vibration, postural drainage, and encouraged coughing) were applied during the first 20 min of 4 consecutive hours.

MEASUREMENTS AND RESULTS

Sputum was collected on an hourly basis for a total of 6 consecutive hours. During the first and the last hour, patients collected sputum without having any treatment and underwent pulmonary function tests (PFTs). Oxygen saturation was measured at 30-min intervals during hours 1 to 6. For the first 20 min of the second to the fifth hour, patients received one of the treatments. To assess the effect of the intervention, the weight of expectorated sputum during hours 2 to 6 was averaged and expressed as percentage of the weight expectorated during the first hour (baseline). For the five treatment modalities, mean sputum dry and wet weights ranged between 122% and 185% of baseline. There was no statistically significant difference among the treatment modalities. As measured by sputum wet weight, all oscillatory devices tended to be less effective than CPT (p=0.15). As measured by dry weight, oral airway oscillation at 8 Hz with an I:E ratio of 9:1 and CPT tended to be more effective than the other treatment modalities (p=0.57). None of the treatment modalities had an effect on PFTs and oxygen saturation and all were well tolerated.

CONCLUSION

In outpatients with stable CF, high-frequency oscillation applied via the airway opening or via the chest wall and CPT have comparable augmenting effects on expectorated sputum weight without changing PFTs or oxygen saturation. In contrast to CPT, high-frequency oral airway and chest wall oscillations are self-administered, thereby containing health-care expenses.

Sloping alveolar plateaus of He and SF6 measured in excised cat lungs ventilated at constant volume by pressure changes

Single-breath washout experiments with He and SF6 were performed in excised cat lungs placed in a closed, liquid-filled reservoir, where lung volume was clamped by the surrounding liquid and breathing was accomplished by hyperbaric pressure changes (pressure breathing) produced by a piston pump. Under these conditions the flow into each lung unit was assumed to be proportional to its volume, and sequential filling and emptying of lung units by convection probably did not occur. Thus, implicitly, gravity-dependent patterns of sequential filling and emptying of lung regions were also excluded. Different lung volumes (VL = 50%, 75%, 100% TLC, where TLC is total lung capacity), tidal volumes (VT = 21%, 34%, 47% TLC) and durations of post-inspiratory apnea (tA = 0,1,2,4,8 sec) were applied. The expirograms showed that the slopes of the alveolar plateau (S) were significantly positive for both He and SF6. For tA = 0 sec SHe ranged from 8.7 to 62.8% and SSF6 ranged from 24.4 to 87.8% (S is expressed in % per unit VE/TLC, where VE is expired volume). The ratio SSF6/SHe was larger than unity for each combination of VL and VT. Further, for tA = 0 sec both SHe and SSF6 showed a tendency to decrease with increasing VL and with increasing VT. For tA = 8 sec both SHe and SSF6 were close to zero. Additional single-breath washout experiments were performed with the same cat lungs by applying normal breathing where lung volume was not clamped and asynchronous unequal ventilation might have occurred. For comparable values of VL and VT, there were no clear differences between the slopes obtained at normal breathing and those obtained at pressure breathing. We conclude that asynchronous unequal ventilation plays only a minor role in the sloping alveolar plateau during normal breathing, and that the mechanism underlying the sloping alveolar plateau is diffusion dependent.

High-frequency oscillatory ventilation may increase airway closure

In five seated, normal subjects, we measured closing volumes using 133Xe boluses inhaled at residual volume. High frequency oscillatory ventilation (HFOV) (15 Hz, 2 cc/kg) was applied during either inspiration to total lung capacity or the subsequent expiration. Closing volume was increased (P less than 0.001) when HFOV was applied during the latter half of expiration, but not when HFOV was applied during inspiration or the first half of expiration. Subsequently, in seven subjects, we measured the regional distributions of 133Xe boluses delivered during open-glottis breath-hold at 14% vital capacity after equilibration with N2O. HFOV was applied during bolus delivery for about 16 sec. These distributions were compared with those achieved by intravenous injections of 133Xe in saline. Regional perfusion (injected isotope) exceeded regional N2O uptake at the lung bases and this was significantly accentuated by HFOV, compatible with increased basal closure. We conclude that in normal subjects at low lung volumes, HFOV may enhance airway closure, though other explanations are possible.