Effect of gas physical properties and flow on lower pulmonary resistance

Abstract

We measured lower pulmonary resistance (Rlp) in eight dogs and three men breathing gas mixtures having different densities (p) and similar viscosities (mu). Rlp increased with gas density and with flow rate (V). In the dogs, these effects were not observed in lung segments subtended from 4-mm-ID bronchi; in more central airways, resistance varied approximately as (mup V)0.5. These results are compatible with Poiseuille flow in peripheral airways, and, in central airways, with flow resistance described by the equation of boundary layer growth. Rather than two discrete flow regimes, it is likely that flow patterns undergo a continual metamorphosis as Reynolds' numbers (Re) decrease between trachea and alveoli. Accordingly, the airways pressure-flow relationship is not described by any single fluid dynamics equation, but may be explained by the general equation, P = Kmu2-apa-1Va, where a reflects the proportion of inertial to viscous pressure losses and varies between 1 and 2 according to Re. Rohrer's equation described the observed pressure-flow relationships and predicted the change in Rlp with gas physical properties, suggesting a physical basis underlying this adequate mathematical description.

Pathophysiology and Classification of Respiratory Failure

“…The partial pressure of O 2 in the alveolus is further affected by the partial pressure of CO 2 in the pulmonary capillary blood. 7,8 Ventilation and perfusion are further influenced by variation in distribution of ventilation and perfusion. The major determinants of distribution of pulmonary blood flow include cardiac output, pulmonary artery pressure, gravity, posture, and interaction of pulmonary artery pressure with airway pressure and pulmonary venous pressure.…”

Effects of Loading and Unloading the Respiratory System on Respiratory Control and Muscle Function

“…Alteration of inhaled gas density by changing gas composition or ambient pressure has been a useful means to exper imentally alter lung mechanics [3,[10][11][12][13][14], Increasing gas density will increase airway resistance in the downstream segment and probably the upstream segment as well at lung volumes greater than 25% of vital ca pacity [12][13][14], As a result both inspiratory and expiratory flows are decreased [10,11,14], work of breathing and oxygen con sumption are increased [11], and exercise performance can be impaired and asso ciated with alveolar hypoventilation [13], Indeed, normal subjects breathing dense gas, especially at high levels of ventilation, behave mechanically like patients with diffuse obstructive lung disease [22], Thus, the use of dense gas to induce airways ob struction should provide a more realistic model of obstructive lung disease than using a fixed external flow resistance. Breathing gas of decreased density, by de creasing respiratory impedance, has the opposite effect on lung mechanics and ventilatory behavior [3,15,16], Utilizing different gas densities to study hyperoxic hypercapnia, the SF6 mixture was more than three times denser than that of the control (N2) gas, while the He mix ture was half as dense (table 1).…”

Helium-oxygen mixtures in acute respiratory failure

“…Results of the various studies aimed at describing airway flow conditions have shown that the latter are turbulent in the larynx and trachea, even during resting breathing, and become laminar as the proximal conducting airways are reached (13,14,16,17). The transition between turbulent and laminar flow occurs between the first and second generation of bronchi (main stem and lobar) during quiet breathing (flow rate of 0.5 l/s) (13).…”

“…Basically, the relationship between driving pressure (DP) and flow (V ) takes into account viscosity (l) and density (q) and can be described by the following equations, depending on the nature of flow (16) (Fig. 2):…”

“…3. In both cases, the effects are equivalent to a decrease in airway resistance (2,3,16,20). Relationship between driving pressure (DP) and flow (V ) in laminar and turbulent conditions.…”