Effect of position and lung volume on upper airway geometry

The occurrence of upper airway obstruction during sleep and with anesthesia suggests the possibility that upper airway size might be compromised by the gravitational effects of the supine position. We used an acoustic reflection technique to image airway geometry and made 180 estimates of effective cross-sectional area as a function of distance along the airway in 10 healthy volunteers while they were supine and also while they were seated upright. We calculated z-scores along the airway and found that pharyngeal cross-sectional area was smaller in the supine than in the upright position in 9 of the 10 subjects. For all subjects, pharyngeal cross-sectional area was 23 +/- 8% smaller in the supine than in the upright position (P less than or equal to 0.05), whereas glottic and tracheal areas were not significantly altered. Because changing from the upright to the supine position causes a decrease in functional residual capacity (FRC), six of these subjects were placed in an Emerson cuirass, which was evacuated producing a positive transrespiratory pressure so as to restore end-expiratory lung volume to that seen before the position change. In the supine posture an increase in end-expiratory lung volume did not change the cross-sectional area at any point along the airway. We conclude that pharyngeal cross-sectional area decreases as a result of a change from the upright to the supine position and that the mechanism of this change is independent of the change in FRC.     

Effect of lung volume on plateau response of airways and tissue to methacholine in dogs.

We have recently shown in dogs that much of the increase in lung resistance (RL) after induced constriction can be attributed to increases in tissue resistance, the pressure drop in phase with flow across the lung tissues (Rti). Rti is dependent on lung volume (VL) even after induced constriction. As maximal responses in RL to constrictor agonists can also be affected by changes in VL, we questioned whether changes in the plateau response with VL could be attributed in part to changes in the resistive properties of lung tissues. We studied the effect of changes in VL on RL, Rti, airway resistance (Raw), and lung elastance (EL) during maximal methacholine (MCh)-induced constriction in 8 anesthetized, paralyzed, open-chest mongrel dogs. We measured tracheal flow and pressure (Ptr) and alveolar pressure (PA), the latter using alveolar capsules, during tidal ventilation [positive end-expiratory pressure (PEEP) = 5.0 cmH2O, tidal volume = 15 ml/kg, frequency = 0.3 Hz]. Measurements were recorded at baseline and after the aerosolization of increasing concentrations of MCh until a clear plateau response had been achieved. VL was then altered by changing PEEP to 2.5, 7.5, and 10 cmH2O. RL changed only when PEEP was altered from 5 to 10 cmH2O (P < 0.01). EL changed when PEEP was changed from 5 to 7.5 and 5 to 10 cmH2O (P < 0.05). Rti and Raw varied significantly with all three maneuvers (P < 0.05). Our data demonstrate that the effects of VL on the plateau response reflect a complex combination of changes in tissue resistance, airway caliber, and lung recoil.     

Influence of lung volume on oxygen cost of resistive breathing

We examined the relationship between the O2 cost of breathing (VO2 resp) and lung volume at constant load, ventilation, work rate, and pressure-time product in five trained normal subjects breathing through an inspiratory resistance at functional residual capacity (FRC) and when lung volume (VL) was increased to 37 +/- 2% (mean +/- SE) of inspiratory capacity (high VL). High VL was maintained using continuous positive airway pressure of 9 +/- 2 cmH2O and with the subjects coached to relax during expiration to minimize respiratory muscle activity. Six paired runs were performed in each subject at constant tidal volume (0.62 +/- 0.2 liters), frequency (23 +/- 1 breaths/min), inspiratory flow rate (0.45 +/- 0.1 l/s), and inspiratory muscle pressure (45 +/- 2% of maximum static pressure at FRC). VO2 resp increased from 109 +/- 15 ml/min at FRC by 41 +/- 11% at high VL (P less than 0.05). Thus the efficiency of breathing at high VL (3.9 +/- 0.2%) was less than that at FRC (5.2 +/- 0.3%, P less than 0.01). The decrease in inspiratory muscle efficiency at high VL may be due to changes in mechanical coupling, in the pattern of recruitment of the respiratory muscles, or in the intrinsic properties of the inspiratory muscles at shorter length. When the work of breathing at high VL was normalized for the decrease in maximum inspiratory muscle pressure with VL, efficiency at high VL (5.2 +/- 0.3%) did not differ from that at FRC (P less than 0.7), suggesting that the fall in efficiency may have been related to the fall in inspiratory muscle strength. During acute hyperinflation the decreased efficiency contributes to the increased O2 cost of breathing and may contribute to the diminished inspiratory muscle endurance.     

Comparison of volume changes in the upper airway and thorax

To describe the mechanical cycles of the upper and lower portions of the respiratory system, we measured volume change in and out of the isolated upper airway in 13 anesthetized dogs and compared volume changes in the upper airway with tidal volume change during spontaneous respiratory efforts. During inspiration the onset and peak increase in volume into the upper airway preceded the onset and peak of inspiratory tidal volume by 84 +/- 8 and 638 +/- 47 ms, respectively. The volume cycle of the upper airway was nearly complete by the end of inspiratory airflow into the thorax. With progressive hypercapnia there was an increase in the change in both upper airway volume and tidal volume but the temporal sequence was preserved. End-expiratory tracheal occlusion increased the volume change in the isolated upper airway at any level of CO2; however, the effect was disproportionately greater at low rather than at high levels of CO2. Following hyperventilation-induced apnea, a change in volume in the upper airway and thorax occurred on the first inspiratory effort. In most animals at lower levels of CO2, the percent change in upper airway volume with inspiration was relatively less than tidal volume, but the reverse was true at higher levels of CO2. These differences represent dissimilarities in the mechanical forces occurring as the result of upper airway and chest wall muscle contraction during inspiration.     

Influence of lung volume on pulmonary microvascular pressure-volume characteristics.

The pressure-volume (P-V) characteristics of the lung microcirculation are important determinants of the pattern of pulmonary perfusion and of red and white cell transit times. Using diffuse light scattering, we measured capillary P-V loops in seven excised perfused dog lobes at four lung volumes, from functional residual capacity (FRC) to total lung capacity (TLC), over a wide range of vascular transmural pressures (Ptm). At Ptm 5 cmH(2)O, specific compliance of the microvasculature was 8.6%/cmH(2)O near FRC, decreasing to 2.7%/cmH(2)O as lung volume increased to TLC. At low lung volumes, the vasculature showed signs of strain stiffening (specific compliance fell as Ptm rose), but stiffening decreased as lung volume increased and was essentially absent at TLC. The P-V loops were smooth without sharp transitions, consistent with vascular distension as the primary mode of changes in vascular volume with changes in Ptm. Hysteresis was small (0.013) at all lung volumes, suggesting that, although surface tension may set basal capillary shape, it does not strongly affect capillary compliance.     

Influence of acute lung volume change on contractile properties of human diaphragm

The effect of stimulus frequency on thein vivo pressure generating capacity of the human diaphragm is unknownat lung volumes other than functional residual capacity. Thetransdiaphragmatic pressure (Pdi) produced by a pair of phrenic nervestimuli may be viewed as the sum of the Pdi elicited by the first (T1Pdi) and second (T2 Pdi) stimuli. We used bilateral anteriorsupramaximal magnetic phrenic nerve stimulation and a digitalsubtraction technique to obtain the T2 Pdi at interstimulus intervalsof 999, 100, 50, 33, and 10 ms in eight normal subjects at lung volumesbetween residual volume and total lung capacity. The reduction in T2Pdi that we observed as lung volume increased was greatest at long interstimulus intervals, whereas the T2 Pdi obtained with short interstimulus intervals remained relatively stable over the 50% ofvital capacity around functional residual capacity. For all interstimulus intervals, the total pressure produced by the pair decreased as a function of increasing lung volume. These data demonstrate that, in the human diaphragm, hyperinflation has a disproportionately severe effect on the summation of pressure responseselicited by low-frequency stimulations; this effect isdistinct from and additional to the known length-tension relationship.

     

Influence of endothelial volume on kinetics of reacting indicators in the lung

The purpose of this work is to show mathematically the relationship between the classical maximum velocity of reaction, Vmax, for enzyme kinetics and an analogous parameter, Vmax, derived by Linehan and Dawson (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 47:404-411, 1979) for the analysis of tracers which disappear by saturation kinetics from the lung circulation during the passage of indicators after bolus injection. Rederivation of the original equation for the combination of flow and reaction in a capillary showed that Vmax is equal to the product of enzyme Vmax and the volume of endothelium, Ve, in which the enzyme resides. This implies that Vmax interpreted from multiple-indicator curves in the lung by the Linehan-Dawson method is a combination of an enzyme characteristic Vmax and a measure of functioning capillary surface during passage, Ve. Lung injury could change Vmax, functioning surface (Ve), or both.     

Inductance plethysmography measurement of CPAP-induced changes in end-expiratory lung volume

The respiratory inductance plethysmograph (RIP) has recently gained popularity in both the research and clinical arenas for measuring tidal volume (VT) and changes in functional residual capacity (delta FRC). It is important however, to define the likelihood that individual RIP measurements of VT and delta FRC would be acceptably accurate (+/- 10%) for clinical and investigational purposes in spontaneously breathing individuals on continuous positive airway pressure (CPAP). Additionally, RIP accuracy has not been compared in these regards after calibration by two commonly employed techniques, the least squares (LSQ) and the quantitative diagnostic calibration (QDC) methods. We compared RIP with pneumotachographic (PTH) measurements of delta FRC and VT during spontaneous mouth breathing on 0-10 cmH2O CPAP. Comparisons were made after RIP calibration with both the LSQ (6 subjects) and QDC (7 subjects) methods. Measurements of delta FRC by RIPLSQ and RIPQDC were highly correlated with PTH measurements (r = 0.94 +/- 0.04 and r = 0.98 +/- 0.01 (SE), respectively). However, only an average of 30% of RIPQDC determinations per subject and 31.4% of RIPLSQ determinations per subject were accurate to +/- 10% of PTH values. An average of 55.2% (QDC) and 68.8% (LSQ) of VT determinations per subject were accurate to +/- 10% of PTH values. We conclude that in normal subjects, over a large number of determinations, RIP values for delta FRC and VT at elevated end-expiratory lung volume correlate well with PTH values. However, regardless of whether QDC or LSQ calibration is used, only about one-third of individual RIP determinations of delta FRC and one-half of two-thirds of VT measurements will be sufficiently accurate for clinical and investigational use.     

Capillary-elastic instabilities of liquid-lined lung airways

To model the competition between capillary and elastic forces in controlling the shape of a small lung airway and its interior liquid lining, we compute the equilibrium configurations of a liquid-lined, externally pressurized, buckled elastic tube. We impose axial uniformity and assume that the liquid wets the tube wall with zero contact angle. Non-zero surface tension has a profound effect on the tube's quasi-steady inflation-deflation characteristics. At low liquid volumes, hysteresis arises through two distinct mechanisms, depending on the buckling wavenumber. Sufficient compression always leads to abrupt and irreversible collapse and flooding of the tube; flooding is promoted by increasing liquid volumes or surface tension. The model captures mechanisms whereby capillary-elastic instabilities can lead to airway closure.     

Effect of volume history on changes in DLcoSB-3EQ with lung volume in normal subjects.

The purpose of this study was to determine the relationship between the three-equation diffusing capacity for carbon monoxide (DLcoSB-3EQ) and lung volume and to determine how this relationship was altered when maneuvers were immediately preceded by a deep breath. DLcoSB-3EQ maneuvers were performed in nine healthy subjects either immediately after a deep breath or after tidal breathing for 10 min. The maneuvers consisted of slow inhalation of test gas from functional residual capacity to 25, 50, 75, or 100% of the inspiratory capacity and, without breath holding, slow exhalation to residual volume. After either a deep breath or tidal breathing, we found that DLcoSB-3EQ decreased nonlinearly with decreasing lung volume. At all lung volumes, DLcoSB-3EQ was significantly greater when measured after a deep breath than after tidal breathing. This effect increased as lung volume decreased, so that the greatest difference between DLcoSB-3EQ after a deep breath and that after tidal breathing occurred at the lowest lung volume. We conclude that a deep breath or spontaneous sigh has a role in reestablishing the pathway for gas exchange during tidal breathing.