H.K. Chang

A model study of flow dynamics in human central airways. Part I: Axial velocity profiles

We measured detailed steady inspiratory and expiratory velocity profiles in a 3:1 scale model of the human central airways. The model was constructed out of acrylic plastic, mounted vertically, and connected to a specially designed steady-flow system. Laterally introduced hot-wire anemoneter probes were used to record axial velocities along 4 diameters at each of the 12 pre-drilled stations of measurement; the flow distribution among the five lobar bronchi was controlled by distally positioned linear resistors. Whether with a flat entrance profile or entering as a narrow jet, the inspiratory flow velocity profiles in the frontal plane showed a high degree of asymmetry in all branches, with peak velocities near the inner wall of the bifurcation. In the sagittal plane the velocity profiles were nearly symmetric, exhibiting a single peak near the center in the frontal plane and almost flat in the sagittal plane. Overall, the velocity profiles were more sensitive to airway geometry than to flow rate. The only site of flow separation was observed in the right upper lobar bronchus. The most evident modification of axial velocity profiles in a single branch was found in the left main bronchus during expiratory flow.

A Model Study of Flow Dynamics in Human Central Airways, Part II : Secondary Flow Velocities

Secondary velocity components perpendicular to the tube axis were measured in a 3 : 1 scale model of the human central airways. Slanted hot-wire probes were introduced axially in order to measure the secondary velocities at about 12 points for each of the 7 stations investigated. Secondary velocities in the inspiratory direction never exceeded a mean value of 18% of the mean axial velocity. Secondary velocities in the expiratory direction reached a mean value of 21.5% of the mean axial velocity. In the inspiratory direction, two unequal eddies were formed in the left main bronchus and in the right upper lobe. Moreover, maximum velocities were observed near the wall and the decay of secondary velocities in the left main bronchus was observed. The secondary flow patterns observed in the left upper and lower lobes after the secondary bifurcation were difficult to recognize, although they seemed to be more influenced by the second bifurcation. The complexity of the flow pattern was reinforced by viscous effects acting near the wall. In the expiratory direction, only two stations in the trachea were measured; four uneven eddies seemed to have existed, with the ventral eddies appearing to be predominant. Overall, the secondary velocity magnitudes as well as the patterns of eddies were very dependent on the geometry of the model used.

High-frequency ventilation: a review

As a new mode of assisted ventilation, high-frequency ventilation (HFV) embodies several types of devices, all of which employ tidal volumes much smaller and frequencies much greater than conventional mechanical ventilation (CMV). Due to the smaller swings of airway pressure during HFV, it is thought that some of the drawbacks of CMV may be overcome. Besides the obvious clinical implications, considerable interest has been generated concerning the physiological effects of HFV. In this review, the effects of HFV on gas exchange, lung mechanics, mucociliary transport, cardiovascular function and control of breathing will be examined. Although the role of HFV in the management of different lung diseases is still unclear, it has proved to be both a strong stimulus and a useful tool in the study of physiology.

Model study of flow dynamics in human central airways. Part III: Oscillatory velocity profiles

Measurements of oscillatory velocity were made in a 3:1 model of the human central airways. The model was built of acrylic plastic and mounted vertically. A reciprocating pump connected to the upper end of the model privided oscillatory flow frequencies of 0.25, 1, 2 and 4 Hz (equivalent to 2.25, 9, 18 and 36 Hz in the actual airways) and tidal volumes of 300, 500 and 1500 ml. A hot-wire anemometer probe was used to measure velocities along two perpendicular diameters and at six stations distributed through the model. The flow distribution through the five lobar bronchi was controlled by distally positioned linear resistors . The measurements indicate that the entry flow profile into the model during oscillatory flow was essentially flat. At low frequencies, the velocity profiles attained at peak flow rate resemble the profiles seen under steady flow conditions at the corresponding Reynolds number. In the frontal plant these profiles are asymmetric with a maximum in velocity directed towards the outer wall of the bend. In the sagittal plane the velocity profiles are symmetric and have the characteristic bi-peak (M-shaped) structure seen in the steady flows. However, as the frequency increases the velocity profiles throughout most branches tend to flatten except in the right upper lobar bronchus where the skewed velocity profiles persist even at the highest frequencies studied. As in steady flows the nature of the velocity profile is strongly influenced by the airway geometry. Furthermore, the peak velocity profiles resemble steady flow profiles at comparable Reynolds numbers up to a Womersley number of 16.

Effect of ventilation with different gas mixtures on experimental lung air embolism.

In six anesthetized, curarized and mechanically ventilated dogs, air was infused via a jugular vein at 0.1 cm3/kg/min for 25 min. This induced a progressive increase in pulmonary artery pressure (Pap) while arterial PO2 (PaO2) and end tidal PCO2 (PETCO2) decreased. Systemic arterial pressure, dynamic lung compliance and total pulmonary resistances were not affected. Changes tended to plateau by 20 min with a peak increase in Pap of 80 +/- 13% and decrease in PaO2 and PETCO2 of 22.2 +/- 2.8% and 14.5 +/- 2.1% respectively. When embolization was stopped these values returned to control levels within 30 min. During air infusion (at 20 min) some dogs were switched from ventilation with air to ventilation with the following gas mixtures: SF680%-O220%, He80%-O220%, N2O80%-O220%. During the final 5 min of air infusion. He and, to a greater extent, N2O breathing results in an immediate and marked further increase in Pap and decrease in PaO2 and PETCO2. In contrast SF6 produced rapid improvement in these parameters with return to near control levels. The recovery time after stopping infusion was greatly shortened with SF6 but was unaffected by He or N2O. These results are explained by different rates of gas transfer between the intravascular bubbles and the various alveolar gases. These findings show that ventilation with SF6 results in marked improvement in the gas exchange abnormalities produced by air embolism.

Mechanical ventilation with superimposed high frequency oscillation in the normal rat

The gas exchange efficacy of high frequency oscillations superimposed on conventional mechanical ventilation (CMV-HFO) was assessed in 24 normal rats. These animals were anesthetized, paralyzed, tracheotomized and placed in a body box in order to measure the magnitude of the CMV tidal volume and that of the superimposed oscillations. The frequency of oscillations was 20 Hz and the mechanical ventilator delivered a tidal volume of 5 ml/kg at a rate of 50 min-1 which corresponded to a slight alveolar hypoventilation. Four groups of animals were studied with two magnitudes of oscillation (0.75 and 1.25 ml/kg) and with two different volumes of instrumental dead space. Blood gases were measured during CMV-HFO and during CMV alone from blood samples taken from a carotid artery. There were no significant differences in arterial PCO2 during these two modes of ventilation except a decrease in the group with large amplitude oscillations and a small dead space in which the oscillations alone could ensure quasi-normal gas exchange. By contrast in 20 out of 24 animals there was a decrease of alveolar - arterial oxygen difference with CMV-HFO even in the case of small oscillations and a large dead space. These results suggest that VA/Q homogeneity is improved by interregional and/or intraregional redistribution of ventilation due to the high frequency oscillations superimposed on conventional mechanical ventilation.

Effects of unequal pressure swings and different waveforms on distribution of ventilation: A non-linear model simulation

In an attempt to understand the role of unequal pleural pressure swings and of different waveforms of pleural pressure variation in the distribution of ventilation during cyclic breathing, a mathematical model simulation was performed. The computer model which incorporates non-linear resistances and compliances as well as sinusoidal, square, and triangular waveforms of pleural pressure variations indicates that the distribution of ventilation is insensitive to the waveform of the pleural pressure. The distribution is also little changed by the depth of breathing (amplitude), but it is affected significantly by the pattern of different pressures over the regions of the model. For sinusoidal, triangular, and low amplitude square wave pleural pressures with equal amplitudes on both compartments, air was distributed preferentially to the lower compartment under the influence of the static pressure difference. With unequal amplitudes, more air flowed to the compartment experiencing the larger pressure swing. This was virtually independent of the waveform and of the amplitudes of the pleural pressure variation. Comparison of the present results with a constant flow model reveals that the overall distribution of tidal air during cyclic breathing is very different from the results obtained in constant rate inspiration experiments or in bolus distribution experiments. New experiments performed under cyclic breathing conditions are thus indicated.

On the boundary conditions used in calculations of gas mixing in alveolar lungs

The lung boundaries exhibit a tight barrier for any insoluble gas; hence boundary conditions for lung gas mixing have to account for the absence of both diffusive and convective fluxes across the lung walls. Scrimshire et al. (1978) have, in contrast, used the less rigid boundary condition that only the net flux be zero. As we believe this boundary condition to be inappropriate for the study of insoluble gases, the results derived appear to have no physiological significance.